I've been pondering the need for different size tires on a car with 50/50 weight distribution. I can't understand it. Of course, there are at least two other reasons for different tire sizes, packaging concerns (space for turning wheels,) and solving an oversteer problem with wheels/tires. But the dramatic difference in our tires and wheels baffles me. Any thoughts?

 

It's not only for lateral grip, but for longitudinal grip as well, i.e. to reduce the likelihood of wheelspin. Take a look at just about any powerful RWD car, and you'll see a lot of rubber in back.

 

I can only relate the info from when I used to own Corvettes. Originally, Corvettes had the same size tires front and back. As the cars became faster and more engineering went into them, they became somewhat unstable at high speed. That is to say, they had a tough time holding a straight line without constant steering input. One of the things they did that remedied the instability was to make the front tires narrower than the back OR the back wider than the front.

 

Donut got it right, I think. A more interesting question is why on a car with 50/50 distribution you need different spring (and consequently shock) rates.

The first answer is that wheel rates are different depending on suspension geometry. That is, if the spring is attached to the suspension arm on a point that is further away from the vertical plane running through the middle of the tyre (or in other other words, closer to the arm/chassis attachment point), then the wheel rate goes down and you need stiffer springs and shocks to compensate.

However (major however!) I have looked at (but not measured, so take it with a grain of salt) the S2000 suspension and it looks like wheel rate would be lower at the front if the same springs were used back and front. Not withstanding this, the springs are stiffer at the back.

What gives?

 

I did measure the suspension a while back when designing new front swaybar. If the stock spring rates stated on King web site are correct, then front wheel rate is 116 lb/inch, rear is 134, which is closer than the bare spring rates. Incidently the front and rear swaybar wheel rates are almost identical at 106 and 107 (I calculated the bar rates from dimensions. Did not measure directly).

Why did Honda make spring rear rate higher? Two obvious ones are that driver and passengers will shift the weight balance rearward given rearward seat position and to allow for fully loaded trunk. There are so many other suspension variables (CG height, roll centers, static and dynamic suspension alignment, ...) that there is not much point to speculating. Final spring and shock selection was likely from drive testing anyway, rather than calculation.


[QUOTE]Originally posted by Luis
[B]Donut got it right, I think. A more interesting question is why on a car with 50/50 distribution you need different spring (and consequently shock) rates.

 

Front tyres need to cater mainly for 'lateral' grip while the back tyres need to divide the grip into 'lateral' (cornering) as well 'forward' (acceleration) - they are 'pushing' the car as well as holding you stuck to the road while cornering. Concidering how much work the rear tyre on the S2000 needs to do, it's probably not wide enough.

 

I also imagine the maximum G-loads are different front/rear. You can brake harder than you can accelerate. And the front may need more compliance to steer well while running through rough pavement.

 

[QUOTE]Originally posted by Gregg Lee
[B]I did measure the suspension a while back when designing new front swaybar.