Tires, Friction, and Grip
Let's focus for a moment on the peak grip a tire is capable of. This happens when the tire is at its ideal temperature, perpendicular to the ground, at it's ideal slip angle, and many other ideals. Not all of those ideals are going to happen at once, but there are a few important concepts that will help make the most of your tires. This particular article discusses tires, friction, and weight transfer, which is perhaps the most important subject in all of suspension tuning.
A graph of this potential grip might look like this:
This is a graph showing the peak coefficient of friction for a tire over its range of load, and it is perhaps the absolute most critical relationship to understand when examining vehicle dynamics. While this graph might not represent the tires on your car, one rule governing all tires remains: the relationship between load on a tire and the friction (grip) it creates is [b]not[/b] linear!
What does this mean? It means the more weight we put on a tire, the lower its coefficient of friction.
Look more closely: a tire carrying 400lbs will be able to resist 600lbs of lateral force. Since 600/400 = 1.5, this is enough for 1.5g's of cornering force! On the other hand, a tire carrying 1400lbs resists 1400lbs of lateral force- enough for only 1g of cornering force. Obviously, 1.5g's is a much more impressive figure!
We can graph this change in coefficient of friction, which is essentially the ratio of the X and Y axis from the previous chart. Here's what we get:
So, while putting more load on a tire does increase the total grip of that one tire, it does so disproportionately with the amount of vertical load on it. Thus, grip decreases [i]relative[/i] to the vertical load on it. Knowing this, we also know the traction gained at that tire will not be greater than the traction lost from the tires that weight has been transferred from! This means:
* Peak grip exists when all four tires are evenly loaded. * Reducing weight transfer (via a wider track or lower center of gravity) can increase the mechanical grip of your tires. * A lighter car will have more total grip than a heavier car when on the same set of tires.
Using this graph, we can predict how much grip a car will have, on which end it has the most grip, and finally whether a car will understeer or oversteer. We just need to know how much load will be on each of the car's tires while cornering.
Notice how installing a larger rear sway bar increases load transfer in the rear but reduces load transfer in the front. This is why a rear sway bar creates oversteer; it helps evenly load the front tires (for optimal grip) while unevenly loading the rear tires (for less grip). There are many reasons why this would be beneficial. Take, for example, the typical front-wheel drive car with 60% of its weight over the two front tires. In a front heavy car, the rear tires are less saturated to begin with, and the overall grip of the car increases if a rear sway bar is used to reduce the burden on the front tires. Increasing the front bar size has the opposite effect (though can still increase grip in camber-challenged cars, a separate topic), while installing firmer but equal sized front and rear bars keeps things pretty similar to stock (though again, will still greatly benefit camber challenged cars).
This rule is applicable all over the place, so I'll state it one more time for emphasis: your car will have the most total grip when your four tires are as evenly loaded as possible. This keeps your average coefficient of friction at its highest.
Even load distribution is difficult to achieve in a front-heavy car which is why mid-engine cars enjoy so much success. It's also why so few cars can even come close to stopping as quickly as a Porsche 911: since the car has most its weight in the rear, the tires become more evenly loaded under braking than its comparatively front-heavy rivals. It's not the brake pads or discs; it's the weight distribution that gives Porsches the ability to stop so short. Evenly loaded tires produce more overall grip.
Ever wonder why most cars understeer if already heavy on the brakes? It may come from the front tires being overwhelmed under braking: all the weight shifts forward, and the tires don't like this.
If we could use a tire that does not saturate as easily, that would be very beneficial. If a tire could handle the heavy loads present when driving at the limit without a substantial drop-off in its coefficient of friction, there would be numerous performance benefits. Braking would improve, cornering, and predictability would be removed while under-and-oversteer characteristics would be minimized.
Unfortunately, tire manufacturers do not provide graphs detailing their grip versus load. So, it's left to the buyer to experiment and learn the characteristics of their tires and tune for them based on feel. However, I've found a few guidelines that seem to work and I'd like to share them. These are merely clues as to which tires will resist a drop in their coefficient of friction longer.