Thread: Tire Pressure

I am going to be running 205/50/15 on AutoX tomorrow, what tire pressure what i run at?

I usually run 38 psi front and 36 psi rear on my 195-55-15, so I say play with it around there, keeping the front slightly more inflated than the rears.

thanks i will do that

What's your suspension setup? Those are decent starting pressures, mark your tires (chalk or white shoe polish).

For example, I ran a course Sunday with longer straights, sharp 180's, TP's were 37psi F, 44 psi R. I needed the rotation and reduction in understeer by substantially encouraging oversteer. It was what I'd call a "Speed Change" course - doing well required broad changes in speed and very good late trail braking, strong exit power. Sometimes courses are more "Speed Maintenance" or similar where conservation of speed is key.

Check this out for some great discussion of same:
Brad And Todd's Driving Adventure

I run the same size tire. Where you run your TP's depends A LOT on what you have on the car for sways, etc. and what kind of course you're looking at.

With a narrower tire, taller profile, you don't have to try to hard to get good rotation, and your main concern is not rolling over the fronts so higher F to R in this case generally is a good idea. Lower profile and wider tire, other issues start to work there way in. Also most cars like ours stock will have some front brake bias, which again potentially overworks the front tires.

Mine has a slight rear bias when turning I find works well for helping induce some rotation. It's pretty neutral and balanced for a FF driver, so I tend to change TP based more on what I need out of the car for a course. You also don't need a pyrometer to tell if you're close, just feel the contact tread and sidewalls right after your run, F and R. Excessive rolling, your sidewall may even feel warmer than the contact patch tread!

It's all good fun, just check often and remember esp at this time of year, temp swings can really change your TP's cold to warm. Check often and get one of those handy little cig lighter compressors, they are super cheap and save your butt when you're running too low. Plus they're a lot smaller than air tanks.

Now when it's raining, it's a real fun process to figure out your TP's

I was running H&R springs with GR2 struts and front and rear swap bars. I ran 39 psi in the front and 36 in the rear. The car did good the 1st three runs. But as soon the the temp went up the last 2 runs where bad. The course was a go-cart coruse some good speed and lots of cornners

i usually run 36 psi f and 40-44 psi r.

You may have been getting some really high pressures from cold to warm. If the "bad" was loss of traction on a really tight course on both ends, the TP may have risen too high. I check every run. Sometimes I've seen 10 psi from when I left the house in the AM. Usually when it's hot out, and runs are far apart, TP stays pretty steady where you left it.

If they were bad because the front end wasn't holding (understeer), drop the TP in the front relative to the rear (or raise the rear relative to the front if you're not having oversteer issues already).

It's likely that you needed to induce more oversteer, with that low of a TP in the rear relative to the front, you weren't getting what you wanted.

Bottom line is:

Front tires - Understeer is your enemy, you do need higher than normal pressure, but usually +5 psi or less on a low profile stiff sidewall tire. Feel the sides of the fronts if you don't mark them - if they're rolling on you, they'll feel warm, sometimes almost as warm as the tread. If you go much higher than +5 psi then you're not going to get the traction you want both in corners and a straight line - most likely the center 2/3 of the tire will feel warm, but not the outside edge tread blocks. If so, drop TP front 1-2 psi. Make small changes, you don't get an infinite number of runs to screw around with car setup (unless you can find an event which is a "tune" day first half, that is awesomely beneficial - check around).

Rear tires - again you need more than standard TP, but this is where things get tricky. I suggest next outing start with 40 psi warm, same apprx front pressure as you used (37-39 psi). If the rear is loose on you, or you have "skidpad" turns on your course an the car won't hold overall (not just understeer) back off the TP. If you're still getting bad US up front, then just back off your entry speed and make time on the exit.

Here's a good example of late corner trail-braking with high rear TP (44 psi) and lower front (~37-38 psi). The fronts have enough air to hold up in the turn, the rears are high enough to allow rotation. If I didn't brake up and through the initial turn-in (note the puffs still coming off the rear wheels, but the fronts are already turning) I'd never make the turn coming in that fast that close to the cone - which cut the distance quite a bit.

Look at both pics and try to see where hth the car went from parallel to your frame of reference to almost 90 deg turned in such a short distance. That was a ball-breaker turn that made or hosed your run. Worse, on entry in #1 pic I'm right foot braking heel-toe to blip the revs so I can manage a downshift to 1st gear. The exit was sharp, uphill into a longish straight for that course where I went from next to no speed to almost 50 mph banging the 1-2 shift at 6500+ rpm. Shifting isn't usually too helpful with our cars during autocross, if the course is open enough. But if you get into a go-kart course like you describe, you have to decide if the course gives you enough room for the speed on exit to matter.

Here, it did. Also notice despite how close the car is to a wall of 2-second cones, I am not even looking at them any longer - the car is set, there's no point - I'm looking at the next gate after the entire box 180 deg turn, because that's when I really have to hug a cone to cut the shortest line. Don't be afraid to knock down a few cones, once you can stay on course consistently, 12" from a cone is about 10" too far. I'm not a great driver, the really good ones shave that space where it saves them time to the point of nearly running over the base of the cone.

Look really closely at the tires, just Kuhmo MX street tires but the TP is keeping them looking like they have a shot at working:

Pic one of two

Look at the driver's side tire - same size exactly as yours, seeing about as much load as it ever will (granted I have about -2 deg of camber) -

Pic two of two

HTH, stay with it, you will love it. Getting killed at first when you know you're a decent driver is hard, but it gets better

thanks for the info bro

Resurrecting a dead thread but is fits. I have hankooks rs2 205/50/15. Started at 36-33 moved to 38-33 due to roll over (i chalked). My main question is the higher vs lower tire pressure to rotate the rear. Kojima says less psi will rotate many others say more will help. Only done 2 autox so very much trying to find my way. Had a blast but always looking for more. I know i need more -camber in front (-1.2 now). Any rec for a bit more rotation with tp chamges alone?