Why does car understeer when going downhill?

[Resolute]

While lateral traction increases with additional load on a tire, it does not increase at a linear rate. As an example, if your front outside tire has 1000lbs of load on them from weight transfer in a turn on flat asphalt, and this produces 900lbs of lateral traction as determined by the tire's characteristics, this would result in a maximum lateral force of .9g before they lose grip. When you turn in while heading downhill, even more weight is transferred to the front outside tire, so it now has 1200lbs of load, but this may only produce 975lbs of lateral traction, which now limits you to a maximum lateral force of .8g in the turn before they lose grip. So you're right, traction does increase with additional weight transferred onto a tire, but not at the same rate, which is why the maximum lateral force the tire can produce actually decreases when turning in downhill.

[HSUBLU]

Originally Posted by hpde_addict

Intuitively, I thought that when the car is going downhill, more weight is transferred to the front tires, leading to the front tires gaining grip and rear tires losing grip. However at the Thunderhill skid pad, I noticed that when going downhill, it is much more difficult to get the car to oversteer compared to level and uphill. And when I'm already drifting in the downhill portion, my car tends to go outward like in understeer. The test was just driving around in a constant-ish circle, progressively adding throttle. What's mechanics are happening that's leading to this behavior?

Another great place to get some practice on this would be Sears Point, on turn 2 for uphill, and the Carousel for downhill - they're both drastic enough that on a relatively traffic-free session you can get some really good practice on the control into and out of the turn.

[ROADRACER]

Originally Posted by TommyW

You're overloading the front outside tire downhill and overloading the outside rear tire on uphill. Those are the tires responsible for the most grip in those circumstances

Yes, I thought that's what I just said. Understeer on downhill turns and oversteer on uphill turns. Exceeding the tires static coefficient of friction. Same as the OP. I don't know of a way to setup the car to lessen this effect. Im not sure there is a way.

[ROADRACER]

Originally Posted by Resolute

While lateral traction increases with additional load on a tire, it does not increase at a linear rate. As an example, if your front outside tire has 1000lbs of load on them from weight transfer in a turn on flat asphalt, and this produces 900lbs of lateral traction as determined by the tire's characteristics, this would result in a maximum lateral force of .9g before they lose grip. When you turn in while heading downhill, even more weight is transferred to the front outside tire, so it now has 1200lbs of load, but this may only produce 975lbs of lateral traction, which now limits you to a maximum lateral force of .8g in the turn before they lose grip. So you're right, traction does increase with additional weight transferred onto a tire, but not at the same rate, which is why the maximum lateral force the tire can produce actually decreases when turning in downhill.

Very nice explanation!!! I think we have a winner.

[TommyW]

Originally Posted by ROADRACER

. I don't know of a way to setup the car to lessen this effect. Im not sure there is a way.

Better to drive those corners a bit differently rather than modifying the car.

[TommyW]

Originally Posted by ROADRACER

Yes, I thought that's what I just said. .

Sorry didn't see your post however yes correct.

[nico_rsx]

Originally Posted by Resolute

While lateral traction increases with additional load on a tire, it does not increase at a linear rate. As an example, if your front outside tire has 1000lbs of load on them from weight transfer in a turn on flat asphalt, and this produces 900lbs of lateral traction as determined by the tire's characteristics, this would result in a maximum lateral force of .9g before they lose grip. When you turn in while heading downhill, even more weight is transferred to the front outside tire, so it now has 1200lbs of load, but this may only produce 975lbs of lateral traction, which now limits you to a maximum lateral force of .8g in the turn before they lose grip. So you're right, traction does increase with additional weight transferred onto a tire, but not at the same rate, which is why the maximum lateral force the tire can produce actually decreases when turning in downhill.

Best answer by far!
Thanks for taking the time to explain what actually happens.