Tire pressures and understeer/oversteer

[pothiawala786]

I've seen different places say different things.

Lowering front tire pressure does what? Does it cause understeer or oversteer?
How about raising them?

Lowering rear tire pressure does what?
How about raising them?

[Gairloch]

You've seen different places say different things because it's all relative.

If your tires are under inflated then increasing the pressure will increase the grip. Conversely, if your tires are overinflated then decreasing pressure will increase grip.

A better way of putting it is this.

Increasing grip in the front will increase the tendancy to oversteer. Increasing the grip in the rear will increase the tendancy to understeer.



The tire pressure thing is about increasing or decreasing the size of the traction circle. If in relation to the rear you have a larger traction circle in front then the car will oversteer. If you have more available traction in back then the car will push. This can be effect by a lot of different things such as tire compound, tire pressures, tire size, suspension loading and/or suspension effectiveness.

[pothiawala786]

Common sense tells me raising rear tire pressure causes oversteer. Raising front tire pressure causes understeer.

[pothiawala786]

Understandable. Well basically, by chalking the tires, I got around 32 front/34 rear. It still felt like it understeered a bit too much for my liking, so how would I adjust tire pressures to help me oversteer a bit more?

I figure lowering my front tire pressures will cause less understeer/more oversteer, and raising my rear pressures will do the same.

[kbrew8991]

Originally Posted by pothiawala786

Common sense tells me raising rear tire pressure causes oversteer. Raising front tire pressure causes understeer.

well that might be true, unless you're below optimum inflation - the extra air might increase grip at that end

thats why its all relative. Its all relative to optimum which is hard to find without a pyrometer and a really well developed feel of the car

[Gairloch]

Originally Posted by pothiawala786

Common sense tells me raising rear tire pressure causes oversteer. Raising front tire pressure causes understeer.

Did you read my post? It's not that simple.

Originally Posted by pothiawala786

Understandable. Well basically, by chalking the tires, I got around 32 front/34 rear. It still felt like it understeered a bit too much for my liking, so how would I adjust tire pressures to help me oversteer a bit more?

I figure lowering my front tire pressures will cause less understeer/more oversteer, and raising my rear pressures will do the same.

Good on ya for using the chalk trick. I love that one.

Just so you know, the road surface you're on can change the grip characteristics of the tires enough to throw off the dynamics that you're working so hard to get. That being said, based on my experience yes, from the pressures you're at now you are likely correct in your interpretation.

[Wheelman_13]

I never chalked the tires, that would probably have helped in hindsight, but oh well.

I found it was productive, with stock 14" street tires to run a little more pressure all around. IIRC 34 front and 38 rear when I was still using the street tires for Autocross. The car still had mild initial understeer if I went in too hot. Honestly, I fixed that by just changing my driving style to focus on exit speed (which is now common sense) rather than corner entry speed. Getting back to pressure, I ran less than that in the wet, but in the dry it helped keep the tires from deforming too much under stress with the relatively tall sidewalls. Keep in mind that I also have the largest front swaybar I could find as well as a good spot of negative camber on the car. If you feel you have the tires chalked-in pretty well, you might dial in a little negative camber at the front (careful of clearance if you go more than 2degrees or have larger wheels).

[Gairloch]

By chalked what the OP means is to have a line of chalk starting on the tread then extending down the sidewall in three places on each tire. The chalk gets warn away where it contacts the pavement, of course, so you can see how far over the tire is rolling and if you need to add or remove pressure based on the contact patch.

[Wheelman_13]

I know about the trick, sorry if my post sounds clueless

When I started fiddling with it though, I didn't know about it. I went mainly by feel, and some really high-res pictures that a friend took at an autocross. I noticed that the tires were rolling over pretty bad when I was running factory pressure, hence the increase. More pictures, and a video later, also precipitated the camber change. It was going pretty positive under autocross conditions with the "factory" alignment. I haven't felt like the car had a "problem" with understeer since I learned how to drive it correctly and set the camber.

[pothiawala786]

Yea, with the chalk on all 4 tires, 32/34 seemed to be just right. When I was feeling a bit too much understeer, I figured dropping the front tires would help it to oversteer a bit. I did read your post Gairloch, but I believe you edited it when I posted that statement. I was talking about those changes after chalking though.

My setup is currently a bit towards understeer. With the bigger front bar and larger stagger than the stock 195/205 to 205/225, and running stock camber settings, I really need to balance out the suspension a bit more. I believe I'm running -1.5 camber up front and -1.1 or so camber in the rear. I'm going to raise (or lower) camber up front to around -2, and then take it for another event.

If I can figure out my DIY alignments, I definately dont mind doing my alignment once a week after every event to just get it setup the way I like it.

[kbrew8991]

again, there is no cut and dry way to say raising always reduces grip and lowering increases and be correct about it all the time

[Wheelman_13]

I run the 205/225 combo with the race tires, but have kept the 195/205 for street tires.
If you have camber bolts you will be able to go up to (I think) ~3 in the front before things start rubbing too much (if you've got 14" or 15" wheels so the O.D. of the wheel/tire is the same or close to stock). Otherwise, if you haven't been at it for years, you might get a really experienced driver to ride with you (if you autocross) just to make sure it's not a driving issue. I learn something new every time I have one of the pros in my region ride along.

I'll be honest, I don't do near as much fiddling with tire pressure as most of the folks in my region. I put the V710s at about 30/35 cold pressure and then let it ride. I generally don't bleed them down more than once during the day, and I don't spray them. That's partially due to never having a problem with the tires getting hot to the point that they go off, and partially due to laziness