TFE blog

Optimum weight distribution

Posted by on Jun 16, 2012 in Current projects and Tech Notes | 2 comments

Optimum weight distribution

Every car has some percentage of its weight that is carried by the front tires, and the rest by the rears. Because the cornering balance of a car is a strong function of that weight distribution, it has a major influence on how well the car will handle, and how fast it can go around a race track. The optimum number for this critical handling influence is missing from most racing technology books, but Think Fast has some guidance on it.

It’s tempting to make the usual snap judgment that 50% front is ideal, just because each tire carries the same amount of weight when it’s sitting still. But is that really the optimum?

Numerous details about a particular car have an influence on the optimum weight distribution, but there are 3 key parameters:

  • Front, rear, or 4 wheel drive
  • Difference in front to rear tire sizes
  • Acceleration thrust to grip ratio

Using spring and bar rate adjustments, it’s possible to produce a good cornering balance over a wide range of front to rear weight distributions and variations in each of those 3 parameters. So why worry about the optimum? Because your competitors who got closer to the optimum than you did will drive away from you.

This question has remained unresolved in my mind for many years. After considerable reflection, I think I have the solution in hand, and also a rationale for that solution:

The optimum weight distribution is driven by tire heating rates.

The 3 key parameters listed above are primary drivers of tire heating rates, and they will generate differences in tire heating rates between the front and rear tires. Those who use IR tire temp sensors know that the surface temperature of a racing tire changes at an astonishing rate, both up and down, depending how much work that tire is doing. The grip that a racing tire produces is a strong function of tread surface temperature, and when the tread rubber is either colder or hotter than its optimum temperature range, it produces significantly less grip. Road car tires are much less temperature sensitive than racing tires, so a car on street tires is less sensitive to weight distribution. That weight distribution has a strong influence on the rate of heating of each tire while it is producing braking, cornering, and accelerating forces. In an ideal world, the tread surfaces of all 4 tires would heat up and cool down at the same rate, so that the grip that each end of the car can produce stays constant through a complete brake/corner/accelerate sequence. Of course no car does that, but a car with the optimum weight distribution can get pretty close.

OK, so what can we do about it? Fortunately, I also thought of a simple test to guide you toward the optimum weight distribution for your car. This test takes absolutely every influence into account, including your driving technique. Here it is:

 If you have done everything that can be done with springs, bars, and dampers, and your car still oversteers on entry and understeers on exit, your car is too tail-heavy. Conversely, if your car understeers on entry and oversteers on exit, it is either nose-heavy or it has a vast excess of yaw inertia.

Unfortunately, you can’t know that at the design stage unless the car that you are designing is similar to a car that you can test extensively. If you don’t have access to a car you can test, a reasonable goal for the design stage is to make the weight distribution about 3% heavier on the drive axle if the front and rear tires are the same. If they aren’t, both the differences in tread width and tire diameter will affect the optimum weight distribution. A car that has a huge power to weight ratio or very short gearing needs more weight on the drive axle for optimum performance. If that describes your car, the optimum weight distribution may be closer to 5% heavier on the drive axle. That’s not much different, is it? That’s because a 1% change in weight distribution makes a very noticeable difference in the behavior of a race car. A 1% change can either transform or destroy the handling of a car.

I have seen and felt a lot of evidence that supports this theory. The DB-1 that I drove many years ago had an inherent oversteer on entry and understeer on exit that I never got rid of, but one thing that I didn’t try was moving ballast forward or aft to try to tune it out. The DB-6 that I’m racing now has had the opposite problem so far, but that’s because all of the ballast was located in the nose as part of the skinny rear tire quest. That quest is on hold for now, so I have moved the ballast aft and split it into separate blocks in several locations so that I can tune the weight distribution easily.

Because the engine is the heaviest item in the car, race cars usually end up with too much of their weight on the axle nearest the engine. Building the car really under weight so that it needs ballast that you can locate to your advantage is one solution to that problem. Relocating the engine toward the middle of the wheelbase is another. Moving the axles forward or aft is more expensive, but it’s another way to fix the problem.

Several road cars have front engine/rear transaxle layouts, which at first seems like an ideal solution. However, there are two problems with this layout: First, the yaw inertia of the car is quite large because the two major weights are at opposite ends of the car, so the handling responsiveness is degraded. Second, all of the road cars with this configuration have the clutch mounted on the engine and a long prop shaft to the transaxle. The resulting rotary inertia on the input side of the transmission slows the shift time noticeably.

Sonic Raptor

Posted by on Jun 13, 2012 in Current projects and Tech Notes | Comments Off on Sonic Raptor

I’m officially impressed by this Win:

It’s not the photo that I’m impressed with, but what the Lockheed Martin F-22 Raptor is doing that amazes me. The white triangle extending from the wingtips actually is a cloud that is between two shock waves. The leading edge of the triangle is evidence of a shock wave where the local airspeed is accelerated above the speed of sound. The near-straight trailing edge of the triangle is evidence of another shock wave where the local airspeed drops below the speed of sound again. So, the F-22 is obviously flying at an airspeed that is very near Mach 1, or the speed of sound.

The really amazing thing about this photo is the combination of near-sonic flight and a very large load factor, or “pulling g’s”. The two white streaks extending downstream from the wing tips are vortex cores that provide very clear evidence that the wing is creating a LOT of lift, far more than the weight of the aircraft.

The aerodynamic flow field around an airplane flying very near Mach 1 is so highly non-linear and rapidly changing that the behavior of the airplane becomes almost completely unpredictable. It’s the most dangerous speed that a pilot can fly, and it’s also an extremely high drag environment. This photo clearly shows that Lockheed Martin’s aerodynamicists and CFD engineers have a solid understanding of the transonic flow field, so they were able to compensate for all of the aerodynamic weirdness that goes on there. Without that, the high g load that caused those strong wingtip vortices could easily cause the airplane to tumble out of control and break apart.

Since the advent of high speed fighter aircraft, maneuvering near Mach 1 has been a suicidal no-man’s land. One of the first encounters with that death zone happened in another Lockheed aircraft, the World War II era P-38 Lightning. The issue was coined “compressibility” then. The normal method to accelerate through Mach 1 is to get it over with as quickly as possible while minimizing the aerodynamic forces on the aircraft, which means pointing the nose down into a dive. That procedure was even part of the highly optimized record-setting F-15 Streak Eagle flight profiles for minimum time to speed and minimum time to altitude.

That the Raptor can do what this photo shows is a clear indication that it embodies a giant technological leap forward compared to every fighter that preceded it. Air dominance is a prerequisite to nearly everything that all of the other war fighters do. I thank God that America has the heart, the head, and the wallet that are required to make air dominance a certainty.

It is counter-intuitive to discover that the optimum flight profile for the fastest time to altitude or time to speed involves climbing to 40,000 feet, then diving to 30,000 feet in order to get through the transonic speed range as quickly as possible. And yet that is in fact the case. Here is the graph for a different supersonic fighter jet:

The light gray curve shows the optimum speed vs altitude profile to fly for minimum time to altitude. This is a great indication of just how huge the differences in the airflow field are in the transonic speed range.

Deep Fried Donuts

Posted by on Jun 12, 2012 in Current projects and Tech Notes | 2 comments

Deep Fried Donuts

If only I had taken my own advice…

In Think Fast, I made this bold claim: “New tires are awesome, but they are only awesome once. The performance drop after the first race is more than any driver can overcome. There is no point in showing up for a race that you hope to win if new tires are not in the plan. You really don’t have a choice here. There are ways to budget-down every part of the event except for tires.”

Yes, I made that mistake, and I won’t make it again.

Racing tires are made from very soft rubber, so they grip the pavement amazingly well when they are new. But soft rubber gets harder with exposure to heat and mechanical strain. The tire rule for the Pacific Formula F Super Series is that you have to run the same set of tires both days. There is at least one qualifying session and a race each day. I made a brilliant last-minute decision at Buttonwillow to buy a set of conventional medium compound FF tires instead of running the front tire size all around. That decision transformed the car into a stable, predictable, confidence inspiring, and much faster racing machine. However, since I had two simultaneous reliability issues on Saturday, I was only able to run the Sunday qualifying and race there. The tires felt just as good at the end of the Sunday race as they did at the beginning, and my lap times in that race were very consistent. So, I concluded that the Hoosier R35A compound was durable enough to go at least one more full day, maybe even two. Plus I had spent a small fortune that weekend, so I needed to get through the next event without spending as much money. A set of FF tires costs just over $1000, so it’s the biggest single expense in a racing weekend.

Only 3 weeks after the Buttonwillow weekend, the pro series ran the infield Sports Car track at Auto Club Speedway in Fontana, California. Kenton Koch volunteered to crew for me, even though we had only met once briefly at the Laguna Seca MX-5 Cup event. Thanks Kenton!

The superspeedway is the one that NASCAR competes on once a year, and the one that Gil de Ferran ran a 241.426 mph qualifying lap on in 2000 in a Champ car. It’s a for-real, prestigious, high speed professional race track. The sports car track uses the front straight and turns 1 and 2 of the superspeedway, then zigzags back and forth through the infield. My car accelerated to 134-136 mph on the front straight, and the tire drag from 1.6g of lateral acceleration through turns 1 and 2 pulled the speed down to 130-132 mph while at full throttle the whole time. It was a real thrill to drive on a real superspeedway near the top speed of my car, pull some real g’s in the corners, and pretend to be a real race car driver.

Fortunately, there are some good in-car videos of the ACS race track on YouTube, so I was able to learn a lot about the track by studying them closely. I learned most of the track fairly well right off the bat, but there was a slow right hand hairpin turn just after a flat-out left/right lane change that I never got close to driving right. Part of that is inexperience on that particular track, and part of it is rusty driving skills. I think I’m down to about 40 lb of rust on my skills, and improving from my current state is going to be much slower and much harder. Still, my skills are coming back a lot faster than they did the last time I re-learned how to drive, so I’m not too disappointed by my current state. I have a whole 3 races under my belt now. That’s not much. The key advance that I need to make is pushing the braking points far enough downstream. That is doubly important with slicks on a light car because hard braking is required to re-heat the tires back up into their best-performing temperature range. Tires gain and lose surface temperature at a very high rate. I will have a lot more to show you about that when I finally make the mounts for a set of 6 IR tire temp sensors.

From the first lap of the first practice session, the car had a terrible understeer in the slow corners. It was close to neutral in the 2nd gear corners and loose in the 3rd gear corners, but the oversteer was easily controllable. If I backed off the throttle or steering, the car would recover from the slide. Knowing that, I was able to drive it with the tail hung out in the faster corners. Still, the major problem was understeer in the 1st gear corners. It was really, really bad. I couldn’t get close to the apexes in those corners unless I slowed the car dramatically.

I started the weekend with a normal spring and anti-roll bar combination, with 250 lb/in front springs, 350 rears, and both bars in the middle of their adjustment range. I made both the front and rear anti-roll bars cockpit adjustable during the car build, so I adjusted the front bar to full soft and the rear bar to full stiff on separate laps during the first practice. That helped the understeer problem, but not much. I’m disappointed by how little balance change results from the bar adjustments. Perhaps they will be more useful when the balance is close to neutral to start with.

OK, so I had a setup challenge to tackle. I can do that. I chose to attack that challenge in small steps since changing the setup to reduce understeer in the slow corners could make the car dangerously loose in the faster ones. After nearly killing myself a couple of times on the skinny rear tires at Buttonwillow, I had no desire to risk crashing at ACS.

After the first practice session, we swapped to 450 lb/in rear springs. During the Saturday qualifying session, the low speed push was just as bad, and the higher speed oversteer wasn’t any worse. In hindsight, it’s clear to me that the front tires were used up during the Buttonwillow race, and they just continued to get worse every session at Auto Club Speedway. The setup changes were just chasing the deteriorating front tires.

So, before the Saturday race we swapped the rear springs again to 550 lb/in. Those are the stiffest springs I own. The FF field shrank to 4 cars for the race when Ethan Shippert bent a wishbone and didn’t have enough time to fix it for the race. So, I started 3rd and only had to build and maintain a gap to Sage Marie to get another podium finish. Here is the video of the Saturday start and the first lap. The behavior of the car was about the same during the race. Sage got a good jump on me at the start, but I was on the inside going through NASCAR 1 and 2, and was able to brake later before diving into the infield. I was able to keep Ed Erlandson and Conner Ford in sight during the first lap, but I fell back a bit through each corner. I fell back a lot through the critical corner leading onto the front straight, and I had the track to myself for the rest of the race. I was able to drive the full race distance off the pace but consistently, with one exception. I fell off the track in the hairpin that I never learned, but kept it running and rejoined without losing a position. It was a lonely race, but a podium finish! So, I got a nice trophy, got to pose for photos with the trophy girl, then got a dud bottle of champagne so there wasn’t any spraying from me.

Since the incremental spring changes had not fixed the low speed understeer, and the higher speed oversteer was no worse, I decided to make a bigger spring change for Sunday qualifying by swapping the front springs from 250 to 170 lb/in, and I left the bars at full soft front and full stiff rear. I also lowered both the front and rear ride heights since the car wasn’t bottoming anywhere, even over the really rough final corner. By that time, I knew for sure that the front tires were dead, and I wanted to preserve whatever was left of them for the Sunday race. So, I ran the first 12 minutes of the 30 minute qualifying session, then sat in the pit lane until there were 7 minutes left. During that outing, the tires were significantly worse, so I didn’t improve my qualifying time.

Ed, Conner, and Ethan qualified very close to each other, with Sage and I about 2.5 seconds back. After qualifying, my front tires were gone, gone, gone. It was going to be tough to get through the race. The last possible thing I could do to make life easier on the front tires was to disconnect the front bar, so I did that. Surprise! The car was nicely balanced during the Sunday race. It had a serious grip deficit, but it was more fun to drive. The decent balance was a result of hobbling the rear tires, not improving the fronts. Here is the video of Sunday’s first lap. The car had power oversteer coming out of the slow corners, which I reduced by softening the rear bar. The left/right lane change that had been flat easy wasn’t any more, and I had to really fight to keep the car down in the bottom lane through NASCAR 1 and 2. About midway through the race, I tried driving the second lane on the banking to take it easier on the tires. Big mistake. By then, the buildup of marbles made it really slick. I had to lift big time to keep the car from sliding up into the wall. After that, I was able to keep it in the low lane, but it was only just barely flat because the tires were so degraded. So, I can claim to have actually driven a superspeedway corner at the limit. I’m pleased with myself that I have the confidence required to do that.

Late in the race, I passed Ed while he was getting back on the track after an agricultural excursion. Due to my deficits of grip and skill, he chased me down in 2.5 laps and just barely got by me going into the infield. He slowly pulled out a 10 car length lead over the following lap, then fell off again. Back into the podium positions! Right up until I saw that black DB-6 in my mirrors at the beginning of the last lap, that is. He got by me going into the corner that I never figured out, but got a bad launch onto the following straight so I caught up to him. We had a rather lame late braking contest going into the final sequence of turns, which was settled in his favor since I never got my nose ahead of his. So, I followed Ed to the checker and a 4th place finish.

After the maintenance thrashfest that was Buttonwillow, it was a giant relief that the car ran all day for two days in a row. It was great to have the reliability issues behind us and be able to focus on performance, what little there was of it.

Lesson learned: New tires for every pro event, no matter what.

And one more thing: The last remaining vestige of adapting the car for skinny rear tires is the 40 pounds of lead in the tip of the nose box. That will come out and be replaced with a series of ballast plates that can be located in several places along the length of the car. With that, I will be able to fine tune the front to rear weight distribution in fine increments, and hopefully arrive at the optimum weight distribution.

It’s a Newww Traaack Recorrrrd!

Posted by on May 29, 2012 in Current projects and Tech Notes | 4 comments

It’s a Newww Traaack Recorrrrd!

It’s a rare thrill to drive an ultra-high-performance exotic car car up to its top speed. It’s something altogether different to maintain that insane speed for any length of time, and it takes absolute confidence in yourself, the car, and the road to blaze down 90 miles of public highway while holding high warp speed. Only very rare circumstances allow anyone to attempt that without the guarantee of jail time: nobody outruns police radios.

The Silver State Classic Challenge provides that opportunity, but only to highly experienced and qualified competitors. In addition to the thrill of the attempt, the Silver State is a very special event because it offers unlimited-class competitors the chance to take two Guinness World Records home with them. The records are for the world’s fastest road race, and the world’s fastest speed on a public highway. Many have tried, many times over, to beat Chuck Schafer’s 207.78 mph average speed record on the 90 miles of Nevada Highway 318 between Ely and Hiko. That record was set in May 2000, so it stood for 12 long years.

Jim Peruto completely demolished both records on May 20, 2012.

I introduced you to Jim here along with his massive-overkill Hells Charger project. Driving a somewhat modified ex-Kurt Busch NASCAR Dodge Charger superspeedway car, Jim maintained an average speed of 217.557 mph and blew through the speed trap at 243.7 mph. Here is what his car looked and sounded like from the sidelines. Warning: NSFW comments. The Doppler shift at 240 mph is a full octave: listen carefully and you will hear it.

Now that‘s a road rocket!

Both of those numbers are stunning achievements that Jim earned by approaching the challenge in exactly the right way. He started with a logical, practical approach, he worked his way toward his goal in a spectacularly patient, multi-year quest, he learned the right lessons each step along the way, he applied those lessons correctly to each successive attempt, he maintained a clear focus on safety as the highest priority, and he developed the driving skills required to maintain control and composure at speeds that very few racers will ever experience. Congratulations, Jim! Well done. This is an excellent example for anyone to follow.

OK, so what’s the plan for Hells Charger, now that Jim has the Guinness records for open road racing? Jim thinks that its calculated top speed of 349 mph is way too fast for Highway 318, and I’m sure he is right. There is one intruiging possibility out there: the world closed course speed record. The current holder is AJ Foyt, driving the Oldsmobile Aerotech at the Firestone test track in Fort Stockton, TX. That record is 257.123 mph. Is that record within reach?

Turnaround

Posted by on May 23, 2012 in Current projects and Tech Notes | 3 comments

Turnaround

Teamwork at the race track is a stunningly beautiful thing to behold, especially when the stakes are huge. I am forever indebted to Jay Messenger, John Fabijanic, and Dave Frietas for giving me far more help than I ever imagined I would need, and for saving me in the process. Without those golden sacrifices, my road racing adventure would have come to a bitter end instead of propelling a meteoric advance. While zigzagging toward the green flag, I told myself “I’m doing this for them, not for me.”

How does this grab you for ambition: Going straight from driver school to professional open wheel racing, with only two test days in between?

With some encouragement from the guys at Honda Performance Development and Les Phillips, I entered the Formula F Super Series event at Buttonwillow Raceway Park. Since I had not driven any of the big track configurations, the weekend started with the Friday test day. I ran that day as a one man band since Jay and John had other committments that day.

My goals for the test day were just as ambitious as going pro: to learn the line, start developing the speed profile for that line, and continue the car setup development. I started with the basic setup from the driver school: skinny tires all around, 350 lb/in front springs and 250 rears. This time, the car had a real suspension alignment instead of the eyeball job that was all I had time to do during the final build thrash before the driver school. I expected the car to behave better than it did during the school because the alignment was proper instead of wacky.

That turned out to be wishful thinking, with another Buttonwillow dirt shower thrown in to make it obvious. The basic problem I had was that any time I let the rear end slide even slightly, the car wanted to depart into a spin instead of recover on its own. Between sessions, I changed the front springs to 450, then 550 lb/in. That improved the balance considerably, but that behavior remained. During the last track session of the test day, I did a decent job of using all that the car could do while keeping it inside those limits. My best lap was a 1:56. I was making progress with the setup, but that progress wasn’t coming fast enough, and I was sure that the basic behavior of the car would eventually bite me really hard. The friction circle showed the limitations: it looked like an ice cream cone instead of an ellipse. I couldn’t do much trail braking: Beyond Here There Be Dragons.

Friday friction circle

Part of the preparations for the pro race weekend was buying a new set of tires. While pondering the basic behavior of the car on the very short drive to the tire shop at the track, I literally had a last minute thought: Did they have a pair of cantilever FF rear tires that wasn’t spoken for? Happily they did, so I went for them. So for now my quest to tame the skinny tire dragon is on hold.

After picking up the new tires, I changed the setup to a conventional FF setup. I wanted to make sure that the car would understeer because I can deal with that. So, I put 300 front springs on it and 350 rears, then reset the ride heights, tilt, and crossweight. I looked at the gearing consequences compared to my inventory, and concluded that I would just have to live with gears that were too tall for the track.

The last task of the day was downloading the data logger, but the electrical system was suddenly dead. That led to 2 hours of debugging the basics, after which I asked the guys at Dave Frietas Racing for some help. We stepped through the basics again, and this time it worked. The car came back to life, and I got the data. The suspect was a short in the DC to DC converter that powered my video camera.

I scrubbed in the normal FF sticker tires carefully, ran one medium-pace lap to get a feel for them, and pitted so that Jay and John could check the temps and pressures. What a transformation! There was so much more grip, stability, and controllability on the normal FF tires that I could tell it would take a while to crank up my level of effort to match the performance capability of the car. When it comes to tires, fat beats skinny! After the temp and pressure check, the car developed a misfire at about 5300 RPM, accompanied by the tachometer bar flashing. Then the link between the engine and the tires broke somewhere. It would make grinding noises, but it wouldn’t accelerate. So, I parked it at the next flag station and got a wrecker ride back to the paddock.

Two simultaneous reliability problems at my first race, after running 6 hours in the school, a bunch of autox runs, and a track test day flawlessly? Yep. When we got back to the paddock, I dropped it in gear and rolled the car back and forth, and saw that the left outboard tripod joint had jumped out of its housing. That would do it! Jay and John set about pulling the halfshafts while I went back to work on electrical. Those two had only met that morning, but they immediately clicked and worked together like a highly experienced team. It was a delight to see two kindred spirits from different parts of the world work together so well on a common goal.

Rob with Honda Performance Development was on hand for track support, so I called on him to help diagnose the stutter. The data showed that the battery voltage floated up with revs, up to 17.5 volts. One thing we found was that the Z terminals on the master switch weren’t particularly tight, so I fixed that and the car ran fine, with the voltage steady at 14.2 like usual. We spaced out the outboard tripod housings using spacers borrowed from DFR, and got it all buttoned back together. We missed the qualifying session, but we were allowed to start the Saturday race at the back. That didn’t work out so well. The misfire and overvoltage was still there, and both outboard tripods jumped out at the same time after 3 laps. That was not the pro racing debut that I had hoped for. We really got our character built on Saturday.

The halfshafts came back out, and we spent quite a while test fitting bolts of various lengths inside the inboard plungers to act as plunge stops to prevent the tripods from jumping out of the housings again. Both the inboard and outboard tripods had been running right on the edges of their housings because the outboard housings weren’t wide enough. The inboard plunge stops turned out to be an effective fix for the weekend, but of course I’ll replace the housings with ones that are wide enough and set the plunge stops to keep the tripods in the housings.

On the electrical front, we concluded that either the battery or the alternator died, which took out the other one. For Sunday, once again we called on DFR to borrow a big battery, and disconnected the alternator. That turned out to be effective enough to run the qualifying and race on Sunday, but it kept us there until 9:30 pm.

After the nonstop Saturday thrash and on-track humiliation, we really needed a good day on Sunday. And we delivered. What a turnaround! We ran the whole qualifying session with no issues at all. Given my driving technique rust, the fact that the track/car combination was new to me, and the fact that this was my first race day in 16 years, I deliberately under-drove the car. I made no attempt to drive a qualifying lap: I just drove. I qualified 5th of 6 FFs, and I was happy with that. Even driving conservatively, I was 6 seconds faster than my best lap on Friday!

Finally, the work between sessions consisted of things that we should be doing: final fine tuning, a wipe down, setting the fuel level precisely, and having a bit of fun.

OK, here we go: game time. Here’s the video. On lap 1, I got a better launch onto the back straight than Sage Marie and eased by just before the entry to Riverside. I had a better transition from the sweeper to the esses than Ed Erlandson and got inside him going into Sunset. Two passes on lap 1! I must not be that far off the pace after all. I closed the gap to Al Salvo and squeezed through the tiny gap he left me going into Riverside, but he got me back going into Sunset. Sage was very close behind the whole time. A lap or two later, I got by Al again going into Sunrise, and he got me right back going into Cotton corners. I passed him again going into Riverside, then both Al and Sage got by me coming out of Sunset. I dropped back a bit to let the F2000 leader by, but closed the gap back within a lap. I got back by Sage on the back straight and closed the gap to Al again. I almost got enough of a run on him several times, but none of them were enough to make a clean pass. Each time a F2000 car came up to lap me, I gave them a point by and lifted since I wanted to be the nice guy of the day. Each time I was able to close the gap to Al back down, and we finished that way. A podium finish in my first pro race! Spraying champagne was a first for me.

Conner Ford won the FF race by pulling away from us on lap 1 and running 3 seconds a lap faster than Al, Sage, and me. The good news for me is that I know where I was leaving those seconds on the table, and I know how to go get them. With more seat time, I have absolute confidence that I’ll close that gap as well.

Of course I wouldn’t be anywhere close to knowing that if not for the amazing work that Jay Messenger and John Fabijanic put in on my behalf, and the endless patience of Dave Frietas who offered anything we needed every time we asked.

Gentlemen, Thank You.

2012 Buttonwillow FFSS Podium

Smooth is Fast!

Posted by on May 11, 2012 in Current projects and Tech Notes | Comments Off on Smooth is Fast!

Smooth is Fast!

Elliott is showing us all how to do it.

You probably haven’t heard of Elliott Skeer yet, but you will. I was very fortunate to witness his performance in the MX-5 Cup race at Mazda Raceway Laguna Seca. It was a revelation. Elliott has the whole race car driving thing completely sorted, even though is car number is also his age.

We met because he has chosen engineering as his career path, and I’m glad to share my experience as a long time motorsports engineer and driver.

The Friday race began with a standing start. Elliott qualified 2nd in a field of 23, dropped to 3rd at the start, then 4th after a few laps. He just kept doing his thing, he stayed composed, and he had the pace to glue his bumper onto anyone else’s. One by one, each driver in front of him made a small mistake and he breezed right by. He led the last half or so with a small gap. Through the binoculars, I saw that he was clearly smoother and therefore easier on his tires than everyone else in the lead pack of 5. As the race neared its end, there was a threat from behind by a late charging driver, and the front of the field began lapping the back of the field. Either of those could have derailed Elliott’s great drive, but he worked through the lappers easily and the late charger ran out of fuel. This was his first win in the series, and it was very well deserved.

The take-home lesson for me is that when you drive harder, you go slower. Smooth is Fast! Because my natural tendency is to over-drive, it was extremely valuable for me to see that demonstrated so clearly. I hope that this lesson will improve my driving. Thanks Elliott!

I feel the need to type that over and over to drive it into my head. Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast! Smooth is Fast!

Sliding outside the window

Posted by on May 7, 2012 in Current projects and Tech Notes | Comments Off on Sliding outside the window

Sliding outside the window

My second event in Freddie the Formula Ford was an autocross test day in a parking lot inside Auto Club Speedway in Fontana, CA. This was an ideal opportunity to get more familiar with the car in a safe, non-competitive environment with lots of quality seat time available. As you will see, I did a lot with the day and learned several important things, but ultimately found the car to be too far outside the tuning window to be competitive for autocross. The short story is that skinny rear tires aren’t viable for autocrossing a Formula Ford.

This was my first event with Speed Ventures, and I’m extremely impressed. The usual process hassles were completely absent, it started and ended on time, they did an extremely efficient job of cycling cars through the course, and it was an enjoyable, low stress environment. I’ll be back for more of that!

The lot was fairly small, so the course was short and tight. Here’s a map of the autocross course from the GPS logger that is part of my AIM EVO4 data system:

The lower right corner shows a 200 foot long scale reference. The blue cross is at the starting line and the color coding is speed, calculated from GPS data. My top speed was only 54 mph. The surface was slick and a bit gravelly, so the car pulled 1.3 g lateral compared to 1.5 g at Buttonwillow on the same tires. Here’s the friction circle from my best run:

That shows that I did a decent job of using all of the combined lateral/longitudinal grip available, which is surprising since I spent so much of the run sideways. Seriously, big-time-drifting sideways. The setup, even at the end of the day, was crazy loose except in the low speed twisties.

I’ll overload you with data some other time, so let’s get to the play by play account of the day. What follows are my notes from each outing and the changes I made for the next outing. You may find the pace and magnitude of the changes I made to be far larger than most people would do. Since it’s early days for setup development, and since the environment was low risk, I went for it. My policy at the track is to never run exactly the same setup twice. Every time the car comes to a stop, I change something. That makes every outing a development opportunity. My high school band program taught me a lot about the value of cumulative progress, and I apply that lesson to car development whenever I can.

2 runs: Low grip, lively balance, braking understeer, 2nd gear course. Changes: FARB full stiff after 1st run, bias 1 turn to the rear, pressure from 17/17 to 15/15 (front/rear). Didn’t look at lap times.

2 runs: 38.9, 38.7 sec. Less grip, off course in 1st turn because I drove through gravel. FARB full stiff for 2nd run, still loose. Need a cooling fan for grid: water 232°F. Steering lock and steering ratio are problems for autox. Changes: 17/17 psi, rear springs from 250 to 170 lb/in, disconnect RARB, R spring seats up 3 turns.

3 runs: 38.9, 38.9, 38.6 sec. Very strange: big oversteer 1st run, push 2nd run, either depending on driving technique 3rd run. Car or driver? Probably increasing tire pressure. Still huge power-induced oversteer. Changes: 18/18 psi, rear pushrods 3 turns longer for bottoming.

2 runs: 39.4, 38.6 sec. Better grip, worse driving. Tried to keep it stuck down, just coasted a lot. Still steps out to oversteer easily. Seems to work better when being over-driven. Changes: R pushrods +4 turns, press 19/19 psi.

2 runs: 38.5, 38.6 sec. Better driving 1st run, loose but driveable now. Over-drove 2nd run, got behind the course. Changes: front rebound + 10 sweeps, press 20/20 psi.

2 runs: 38.5, 38.2 sec. Better! Smoother reaction to braking to power transition, better grip. Still loose but driveable. Changes: F rebound +5 sweeps, 21/21 psi.

1 run: 38.9 sec. Worse driving, too loose early and pushing late in the run. Grip didn’t seem as good, especially forward traction. Reaction to brake-power transition seemed harsher, less controllable. Changes: F rebound -3 sweeps, 20/20 psi.

1 run: 38.3 sec. Better driving. Brake to power transition better. Still very easy to get into huge oversteer. Grip feels better. Press 20/20 is good. Changes: front springs 350 to 450 lb/in, front spring seats down 2 turns.

2 runs: 38.6, 38.4 sec. Behavior was the same – stiffer front springs didn’t change the behavior. Fronts still lock first. Changes: Rear toe in + 2 turns, bias 1 turn rear.

1 run: 38.2 sec. Worse driving, better car.

This was my first autocross in 16 years. Considering that, my run times were stunningly consistent, but slower than they should have been because of the very loose balance. I was surprised that I had no problems acclimating myself to autocross driving or car control, given the fact that I spent almost the whole time sideways. The lower speed range of autocross calls for much shorter gear ratios than road courses, so the car felt like it had infinite power. The data showed significant wheelspin with any significant throttle application.

The car was reasonably balanced with a bit too much low speed understeer at Buttonwillow with 350 lb/in front springs and 250 rears. Changing them to 450 front and 170 rear with the rear bar disconnected still wasn’t enough to tame the driftitude for autox. So, I have to conclude that running the front tire size on the rear as well just doesn’t work for autocross. I put all of the ballast as far forward as I could arrange, so the car currently sits at 46.6% front with driver but no fuel. I located the ballast forward to accommodate the same size tires all around, and I can’t go any farther. Given that limitation and the short gearing for autocross, my primary conclusion from this test day is that the skinny rear tires are not viable for autocross.

FARBalls

Posted by on Apr 28, 2012 in Current projects and Tech Notes, RB6 Formula Ford | Comments Off on FARBalls

FARBalls

Implementing the innovations that I have developed over the years into the front anti-roll bar on my DB-6 turned it into a bizarre looking science project. This is the story of why it looks like that and why it does the things that it does.

An anti-roll bar adds roll stiffness to the end of the car that it is attached to. The reason we care so much about bars is that changing the relative roll stiffnesses of the front and rear suspensions is the most effective way to change the cornering balance of the car. The ideal cornering balance is neutral, with the slip angles of the front and rear tires the same while cornering. A neutral balance is faster, easier to control, and easier on the tires than understeer or oversteer.

Anti-Roll Bar Basics

Here is an illustration of a typical race car anti-roll bar. It consists of 3 springs in series: two blade-like lever arms and a torsion bar. The end of each blade is usually connected to one side of the suspension through a link with a spherical bearing on each end. When the suspension rolls, the blades are loaded in bending and the torsion bar is loaded in torsion. The spring rates of the three springs determine the stiffness of the assembly.

Racing anti-roll bars are usually made so that the blades can be rotated easily, so that the stiffness of the assembly can be changed quickly. This is even easier when the blade angles are connected through a push-pull cable to a lever in the cockpit, so that the driver can change the blade angles while underway. Standard DB-6es have a cockpit adjustable rear bar. I made my front bar also cockpit adjustable so that I will hopefully have enough adjustability to get the car balanced and keep it that way as the tires change. The illustration above shows the blade angles at the full stiff position. Rotating the blades 90° would put them in the full soft position.

Here are illustrations of one blade by itself, showing the two extremes of blade angles and blade stiffness. Ross Stringham optimized the blade geometry for me through the course of several FEA iterations. Thanks again Ross! Edge Engineeering made the blades for me, and did a superb job as usual.

Blade at Full Soft Angle

Rotating the blade so that the link loads it in the thin direction makes the blade deflect a lot for a given load, so the anti-roll bar assembly stiffness is low.

Blade at Full Stiff Angle

Likewise, rotating the blade so that the link loads it in the thick direction makes the blade, and therefore the anti-roll bar assembly, much stiffer. Intermediate angles produce intermediate stiffnesses, but not in a way that is as useful as we would like. The blade acts somewhat like an on-off switch as the blade angle is varied. If the cockpit adjuster has 5 positions, the first 3 give us almost the same stiffness, then the 4th and 5th make the bar much stiffer, but there isn’t much stiffness difference between those two positions. Here is a graph of stiffness versus blade angle for the 4 possible combinations of bar and blade stiffness.

Anti-Roll Bar Stiffness Graph

One way to make a bar act less like an on-off switch, and provide something closer to uniform stiffness increments between adjustment angles, is to tie the two ends of the blades together with another link. That link makes the shape of the curve in the graph above closer to a straight line than a sine wave. David Bruns did that on most of the DB-series cars. The tip link forces each blade tip to deflect about an arc, which reduces the blade deflection in its soft direction. The blade tip link is shown as a green line in this illustration, and the tip deflection arcs are shown in black.

Link Arc with Conventional Adjuster Linkage

However, unless the blades are full soft or full stiff, there is a subtle flaw in that plan: The tip link makes the bar assembly stiffness asymmetric. It’s stiffer when turning one direction than the other. That makes the cornering balance of the car different when turning left compared to turning right. The stiffness asymmetry is because the bar assembly is asymmetric.

Where did that asymmetry come from? It’s because the usual blade angle adjustment linkage rotates the blades in the same direction, as shown above. When cornering one direction, the blades are deflected toward their stiff angle, and when cornering the other direction, the blades are deflected toward their soft angle. I realized that and pointed it out to Bruns after it was too late to revise the bar design on the Swift 007.i Champ car. On that car, the link between the blade tips was a robust I-beam with a bearing at its center that was the mount point for the 3rd spring.

OK, what can we do about that? It’s easy to design a blade angle adjustment linkage that rotates the blades in opposite directions. That makes the anti-roll bar assembly symmetric. Problem solved, right?

Link Arc with Symmetric Adjuster Linkage

Wrong. With that configuration, the link between the blades wouldn’t do anything to reduce blade deflection toward its soft direction. The link would just move laterally with the blade deflection, and it would move a long way. On the 007.i, there wasn’t room to add anything that would restrain the blade tip link from moving laterally. So, the engineers at Newman Haas used that bar stiffness asymmetry to their advantage, with track-specific blade orientations that produced a cornering balance that was closer to neutral through the critical turn in each direction. Clever!

However, I don’t intend to be that clever. On my DB-6, I added a triangular support that prevents the blade tip link from moving laterally. Here is an illustration of the complete assembly, except that the fasteners are not shown. The blade tip link is shown in dark red.

FARBalls Assembly

Like the 007.i, my blade tip link is a robust part because it presses on a polyurethane bump stop mounted in the middle of the frame. That gives me a front bump stop that distributes the excess tire load due to bump stop contact equally between the two front tires. So, it won’t have as much influence on cornering balance as conventional bump stops mounted on the damper shafts. This is yet another salvo in my war against handling interactions.

An additional feature of my blade tip link is a set of 3 holes on each side for the drop link fasteners. Mounting the links in the outboard pair of holes makes the bar assembly a bit softer than standard. Moving them to the middle set adds 25% to the stiffness of the assembly. Moving them to the inboard pair of holes adds another 25% to the stiffness of the assembly. So, I don’t need a big inventory of bars and blades to cover a large range of bar assembly stiffness, and I can change the bar stiffness easily between track sessions.

I would have done all of this to the rear bar as well, but packaging constraints confounded all of my attempts. So, the only change I made to the rear bar is the optimized blades.

ThinkFast Lab Science Team Roster

Posted by on Apr 4, 2012 in Current projects and Tech Notes | Comments Off on ThinkFast Lab Science Team Roster

ThinkFast Lab Science Team Roster

Getting back in the saddle

Posted by on Apr 3, 2012 in Current projects and Tech Notes, RB6 Formula Ford | 1 comment

Getting back in the saddle

Here are my thoughts after a 2 day SCCA road racing driver school that ended a 16 year gap in my competition driving. Hopefully this will help others who are considering getting back into racing.

1. Be humble, and be patient with yourself. You will have to accept the fact that your early days back in competition driving will be really embarrassing, and that grinding the rust off your skills will take serious effort and focus. The alternative to this is major frustration, because you will remember how good you used to be, but you won’t be able to do anything like picking up where you left off. You are starting almost from scratch. Honing your driving skills back to where they were will take many, many months, so don’t think that a one day recovery is possible. Re-learning how to drive will not be fun, even if you fully embrace your status as a rookie.

2. Start with autocross. You will make some giant mistakes, and their consequences will be much less serious there. The next logical steps are open track days, competition schools, then club races. Of course I didn’t do that. In this case, do as I say, not as I did.

3. Keep running autocrosses. All of the skills and much of the car development that you can do there will transfer directly to road course running.

4. If you drove a formula car or sports racer before, start back in another one. FA, FB, and FC cars are not meant to be training wheels, so start in something less demanding. Formula cars and sports racers are precision instruments, so there is no hiding any flaw in your technique. That makes them excellent platforms for re-learning how to drive because the car will instantly tell you when you got everything exactly right, and when there is any weakness at all in your technique. I really hope that this will shorten my learning curve.

5. Over-drive, then under-drive. You need to briefly get a feel for the full spectrum of car responses, including beyond the limits. Slide recovery should be very high on your priority list of skills to re-develop, and you need to choose a safe place and time to hang it over the edge. I didn’t do this either, and it could have been disastrously expensive and painful. I got lucky. Again, don’t do what I did.

6. The under-driving part of the process is where the real skill development begins. Keep the car stuck down and drive at 7 or 8 tenths. That should free up your brain enough to put the car on something close to the right line and drive consistently. If you look at your lap times at all, which I don’t reccomend, only examine them for consistency, and don’t compare them to anyone else’s.

If you can’t drive consistently, back down your level of aggression another notch. If you are getting complements for your consistency, take it up a half-step and work on improving your line and your speed profile toward the optimum. You will know where you are leaving speed on the table.

7. Run an event every other weekend, carefully evaluate your performance after each event, and go to the next event with a clear development plan for yourself. I started with two school days consecutively, and that was too much, too fast. I got a LOT of track time, but the quality of the track time wasn’t as good as it could have been because the schedule was so compressed that I didn’t have time to fully process what happened.

For me, the skill that I miss the most is the ability to read the track and visualize the right line. If I had started with autocrosses, this issue might not have been the biggest one.