Tire Pressure Woes – Autocross Tuning Tips

Steven Johnson is a serious autocrosser.  He has learned much as an HPDE (High Performance Driver Education event) participant.  He also serves as the coordinator of the Harris Hill Road Chrono-X Time Trial Series.  In addition, Steven is looking for a different career direction.

In the meantime, he asked me if he could share his knowledge & observations with the Racing Ready community.  Knowing how thorough a competitor & helpful instructor Steven is, I did not hesitate to welcome him as Racing Ready’s newest guest blogger.

Here is his first entry…thanks Steven!

Steven's 350Z at SPOKES autocross @ SAR in July 2008

Anyone who tells you that tire pressures are not important has obviously never seen the limits of their own car.  When I first started autocross, I would simply add pressure and hope for the best.  How naive was I?  As I found out at the last SPOKES/SASCA autocross, tire pressures can mean the difference between a smooth run and absolute chaos!  My rear pressure used to be too low, giving me too much rollover.  Then I tried too much pressure and the car was drifting all over the place.  It was so bad that I wasted 3 runs just because I added 5 psi.  After that day, I learned to respect the pressure of your tires.

Steven's 350Z at SPOKES autocross @ SAR in 11/2008

I run a set of Dunlop Direzza Starspec Z1s in the STR class. The class allows for a 140 treadwear with a 255 width maximum.  Unfortunately, the Starspecs are not considered the best tire for the class.  The Toyo R1-R at a 140 treadwear seems to be the tire of choice (Car Tyres).  So why did I choose the Dunlop?  Is it because I like a challenge?  Maybe because I enjoy loosing?  No, the reason is because the autocross gods frown upon heavy cars like my 350Z.  My car requires a larger and taller tire than the Miata’s, Mr2’s, and S2000’s.

Steven's 350Z ready to compete at SPOKES autocross @ SAR in 11/2008

Simply put:

  • The Toyo R1-R is better in a smaller, shorter tire
  • The Dunlop Direzza is better in a larger, taller tire

So while smaller cars can run the best tire in the class, I am forced to use the underdog Direzza.  That being said, I love these tires!  They stick great and heat up quickly.  In fact, if you ask some drivers they will tell you that treadwear doesn’t  tell the whole story.  They will tell you that the Direzza is just as good, if not better, than the R1-R and it depends on the course conditions.  For instance, the Direzza is said to have a little bit better dry course handling.  In the rain, the R1-R cannot be touched.  So which tire should you choose? Don’t ask me, ask your car.  How heavy is it, what tire size does it need, and what type of driving do you plan on doing?

Steven @ speed (picture courtesy of Jerry C, my autocross mentor)!

Back to tire pressure:
What difference can a couple of psi make?  More than you would think! I used to just guess at what was right and put in around 28-30 psi front and rear.  I would just go out with whatever handling characteristics it gave me and deal with it.  Then if the pressure jumped up a few pounds (which it will if you drive right) I would bleed off enough to get back to where I started. This is the WRONG way to setup a car’s tire pressures.  I asked around and found out a few key tips to setting tire pressure:

  1. You can use tire pressure to actually add or subtract traction in front or rear
  2. More pressure = less traction  Less pressure = more power can be transferred
  3. Finding the right balance between front and rear makes a huge difference
  4. You can induce understeer just by having more pressure in the front than in the back
  5. Don’t be afraid to have a 10 psi spread to find out what works.

I found out that my front tires actually worked best if the tire pressures were pumped up very high to around 38-40 psi.  My rear tires at that pressure couldn’t get traction AT ALL!  I ended up with the rear pressure at 32-33 which gave me the balance I was looking for.  I thought that there was some rule out there that said don’t let your pressure be over 5 psi different from front to rear.  As tip #5 above states:  Don’t be afraid to have a dramatic difference from front to rear.  It can teach you a lot if you are not afraid to try out extremes.

Slime - White Tire Marker from AutoZone

Marking your tires is a great way to gauge what pressures to use.  I use a Slime tire marker from AutoZone (see picture above).

Marked tire sidewalls on Steven's Drag brand wheels w/distinctive red stripe

Make three marks equidistant on the outside of the tire.  If you look closely at your tires, you will find a small triangle shape at the edge of the tread.  This is to help you in finding your perfect pressure, so use the tire marker here. You should mark your tire next to these triangles from the top of the tread down to the sidewall.

Tire sidewall triangle mark - click to enlarge, to better see the triangle...

After your run, the marker will rub off to a certain place.  You want the marker to be rubbed off at the tip of the triangle for optimum pressure.  If there is still markings above the triangle, your pressure is too high.  If the markings are rubbed off past the tip of the triangle, your pressure is too low.  You are looking for the maximum usage of tread without rolling over into the side wall.

I could go on all day about tire pressure, but you won’t know what works best for your car until you get out there and play with the extremes.  You will be happy you did.

Racing Ready welcomes Steven’s experience-driven, autocross tuning advice.  I look forward to future editions of this kind of well sorted out knowledge, in his easy to read conversational-style.  More to follow, down the road!


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11 thoughts on “Tire Pressure Woes – Autocross Tuning Tips”

  1. Great article! I once cured a pronounced push at an HPDE by dropping my front pressure until the car’s balance came back. Gives us something to think about.

  2. Noticed the picture shows the Bridgestone – not the Dunlap tires and size is 245/45/17 ? Any recommendations if you ran a set of staggered Track V1 wheels 18 x 8 and 18 x 8.5 ? Was thinking of 285s ?

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