June 2010
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RELATIONSHIP OF TIRE SIZE AND WEIGHT DISTRIBUTION
Question:
Many car owners say that because their car has 50/50 weight distribution, they inherently have the best handling possible. However, many owners install wider tires at the rear to improve traction. My opinion is that by doing so, while their car may still have 50/50 weight distribution – at rest – the dynamic balance will be a car that understeers. True?
On a related note, let’s say we have a mid-engine car with 40% front / 60% rear weight distribution. If the rear tires are made 50% wider than the front tires, wouldn’t this negate the static weight distribution, essentially creating a car with more balanced traction front and rear (granted, on a
constant-speed circle?) I understand there may be an issue with getting enough heat into the tires to generate traction proportional to their width. This assumption aside, is this thinking correct?
It is definitely correct that, at least up to a point, we can compensate for weight distribution with unequal size tires. We can also compensate with roll resistance distribution, camber, tire pressure, and aerodynamics.
So it is safe to say that if the car has 50% rear weight, rear drive, and bigger tires in back, it will probably understeer in steady-state cornering if camber properties and settings, overall roll resistance, tire pressure, and downforce are equal at both ends, and if speed is low enough so that the rear tires do not have to transmit a huge amount of power just to maintain steady speed. If any of these conditions are not present, all bets are off – and usually not all of these conditions are present.
We can at least say that if we take an existing car, and add tire size at the rear only, leaving all else unchanged, the tire size change will move the car toward understeer.
Answer:
In many forms of racing, we do not have free choice of tire sizes; the rules impose a maximum size. This may be the same for both ends, or not. The rules often also impose restrictions on other aspects of the car's design, which limit what sort of weight distribution we can have, and what sort of downforce distribution we can have.
Nonetheless, it is interesting to consider what we should want in terms of weight distribution and tire size in a rear-drive car, given a free hand.
It is useful to note what course car evolution took in F1 and in sportscar racing, before tire sizes were limited. The cars were decidedly tail-heavy – around 60% rear – and the rear tires were about a foot and a half wide. Front tires had about 2/3 the tread width of the rears. The cars had more aero downforce at the rear than at the front.
Why would this be better than 50% rear, and equal-size tires? There are various reasons, but probably the main one is that the car doesn't just have to corner; it also has to brake and put power down. It brakes with all four wheels, and propels itself with only the rears.
Even without downforce, in straight-line limit braking about 20% of the wheel loading transfers from the rear to the front. Even with 60% static rear, the car does 60% or more of its braking with the front wheels. A car with 50% static rear and modest downforce does about 70% of its braking with the front wheels. It is possible to get the brake bias needed, but it becomes difficult to sustain it over the length of a race. The front brakes tend to overheat and go away.
As for putting power to the ground, there is no mystery as to why more rear percentage is an advantage with rear wheel drive.
Finally, in most cases it is difficult to accommodate foot-and-a-half wide tires on the front of the car, especially without power steering. The scrub radius ends up being really large, and the car gets difficult to steer.