Tire Pressures - Tuning Car Balance
When to Adjust
In the last installment, we covered how to find the optimal air pressure settings for the contact patches of your tires. At this point, we should be getting the maximum traction available from both ends of the car. But that doesn’t mean we’re happy. The car probably still handles like a dump truck, because we haven’t addressed the problem of balance.
If you have a Street-class car, that usually means you’re experiencing a fair amount of understeer, since manufacturers typically design the car to understeer as a safety precaution.
So what do we do next?
If your immediate answer is to add or remove air pressure to alter the balance, then you might be in for a surprise. We just spent all this time getting the maximum amount of traction out of each of our four contact patches. If your next instinct is to change tire pressures, then you’re giving up precious grip directly at the contact patch.
Let’s put this another way, and introduce a new rule: in a perfect world where you can change anything you want, the LAST thing you want to do is compromise your tire’s contact patch. Tire pressures (along with suspension alignment, which we’ll get to in a future post), are what determine your contact patch. So once you’ve found the perfect contact patch, don’t mess with it!
Unless you have to.
The reason we mess with tire pressures so much is because it is so gosh darn easy. It is an adjustment that you can do in 10 seconds, on any car, with dramatic results. If your only goal is to improve balance (reduce understeer or oversteer), tire pressures are an incredibly effective way to do it.
But air pressure changes only work by compromising the contact patch, which is a Bad Thing (TM). So only do it when you’ve got no other option, or no time to perform a better change.
Of course, “no other option” and “no time” applies most of the time you are in grid at an event. Or if you have bone-stock car and shock or alignment changes aren’t the answer. In any of these cases, there’s really nothing wrong with adjusting tire pressures. A slight compromise to the contact patch is almost always better than a car that handles like a dump truck! Just remember that you’re giving up grip every time you move away from the sweet spot we found earlier.
What Change To Make
OK, so we’ve decided to change tire pressure to cure an oversteer/understeer problem. Now what? There are two questions to ask yourself:
- Which end of the car do you change?
- Do you go up in pressure, or down?
Which end of the car to change is usually a pretty easy decision. Remember that our sweet spot pressures give us the maximum grip possible at the contact patch. So all we can really do is remove grip. If the car is understeering, it means you are lacking grip at the front, so the last thing you want to do is change front tire pressure. Make your adjustment to the rear tires. Conversely, if your car is oversteering, you’ll want to adjust front tire pressures to remove traction and balance the car.
The decision to go up in pressure or down is more difficult, and probably something worthy of experimentation. Again, any change away from the sweet spot will remove traction, so either option is in play. They will both remove traction, but they will usually result in a different “feel” to the driver. And the exact nature of the change in feel will depend on the tire, the car, and the surface.
Generally-speaking, it seems that most people will ADD tire pressure to balance the car. So that’s probably the best place to start. Adding pressure will stiffen the sidewall, which typically provides for quicker transitions, better feedback to the driver, and a more predictable break-away characteristic.
However, if you are running a tire with a really stiff sidewall, or a car with really stiff suspension, you might be in a situation where the break-away characteristics are too sudden. If adding pressure makes the car too nervous, try REMOVING pressure to take the tire away from the sweet spot. The softer sidewall will flex a little more slowly, allowing you a little more time to feel what the tire is doing.
And Then it Gets Tricky
We’ve covered the basics of finding the right tire pressures and then tuning the balance of the car based on that. In theory, we’d now be done. Ah, if it were only so simple! Here are some "confounding factors” to keep in mind.
THE MOVING SWEET SPOT
The sweet spot of the tire is a perfect Goldilocks scenario, where that tire pressure needs to be “just right.” 2psi one way or the other can really matter.
When we found our sweet spot, that was on a particular surface at a particular temperature. But rubber responds to temperature, and different surfaces have different grip. In my experience, the sweet spot is always the right place to start, but if you encounter a large change in temperature or a large change in surface grip, you might need to adjust your sweet spot.
For example, if you have the (ahem) “opportunity” to run autocrosses where the temperature is below 50 degrees Fahrenheit, you’ll want to move your sweet spot down. The same holds true for rain, which obviously would be a large traction difference. On a cold, rainy day, you might find running as much as 6psi lower will help preserve at least a little bit of the traction you’re accustomed to.
In addition to optimal tire pressures, every tire has an optimal tire temperature. We’ll cover that topic another time, but make sure that you keep the temperature of the rubber in mind as you test.
FEEL VS GRIP
Typically, you can feel when the tires are happy. But there are a few tires out there that seem to be fastest at pressures (or temperatures) that don’t feel as good to the driver. As you play with pressures and balance, be sure to use as much objective data in your decision-making as possible. Even if you don’t have a fancy data logger, you can look at your times and or chalk like we discussed last time.
Test, Test, Test
There are simply too many variables for a “one size fits all” solution to tuning a car, and tire pressures are no exception. But it will always be useful to have a starting point and a method for testing. Use these tips as a plan of attack, but there is no substitute for doing your own testing and coming to your own conclusions.