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.

2 Comments

  1. The optimum weight distribution can also be track-specific. If there will be a lot of slow corners, which gives the engine more ability to accelerate the car, you will want more weight on the drive wheels. On a dragstrip you want 99.44% drive wheel weight. Old steam locomotives had what was called “compensated springing” mechanisms to try to keep the weight on the driving axles constant while on the road. For starting some of these mechanisms could be overridden to dump as much weight on the drivers as mechanically possible.

    If you’ve either got no minimum weight or a minimum weight below the car weight, deciding whether to reduce weight at one end or not gets interesting…

    • Very true. The scale of the course should be reflected in the gearing. That’s why I defined the key acceleration parameter as thrust instead of power. Shorter gearing produces more thrust from the same engine.
      Thanks for the train anecdote. Since the coefficient of friction of steel wheels on steel rails is only 0.05, I can see why it’s a big issue.
      Formula SAE teams face the weight balancing issue since there is no minimum weight. I saw two very light cars that were designed to the 60″ minimum wheelbase, and they had 51% and 52% front axle weight. Moving the front axle forward would have been a beneficial tweak despite the longer wheelbase. Such things are possible with scratch-built open wheelers.