Would high tire pressures all around make a care feel very floaty and squirmy? I had my first autocross of the season and it was a warm day. I am running Dunlop ZII tires. My tires started out at about 40lbs in the morning at about probably 70 degrees. At the end of the day temps were near 90 and I had 4 runs in the afternoon session. The car started to feel different. Pretty soon after that I packed up and headed home. On the highway the car felt floaty and squirmy. I drove 1 hour (60 miles) home and when I got home checked my tire pressures. They were at 45 lbs all around after having time to cool down with the highway drive.

Its my assumption that the tire pressures on the last run were a lot higher than even 45lbs. If they were really up near 50lbs at that last run would that make the car feel floaty and squirmy when doing a slight side to side slalom movement? Even on the highway home with slight side to side movements the backend felt really squirmy.

I have yet to check all the suspension components but did look at the rear swaybar and it seemed intact and snug.

 

Yup, 10 degF = 1 psi of change.

Hot brakes, track temps... You were near 50 by the end of the day.

50 psi = less contact patch, so yes you will squirm and bounce more.

Next time try checking tires with a non-contact pyrometer and adjust as needed. Or fill with nitrogen for a more stable pressure..

 

Next time you can try-

38 psi in front, 33 psi in the rear cold in the morning.
Chalk your sidewall tread and watch to see if you are using enough of the treads. Adjust your pressures after 1-2 runs then leave alone. Usually as the day goes on I let air out once the tire temperatures stabilize.

The pyrometer can be used after run 2, to check the inner, middle and outer edges of the tread for temperatures. If the mid tread reading is highest then let some air out of that tire. It's hard to match all three readings to be the same, if you can get two of the readings very close that would be good. Front and rear tire temperatures will be very different. A probe style pyrometer allows you to check the temperature of the rubber under the surface which is better than an Infrared pyrometer which reads temperatures on the surface only.

 

You guessed it right - but the reason the pressures went way up has little to do with ambient temperature and everything to do with the friction and heat build up from autoXing.

Step 1: Get decent tire gauge (pencil ones will do in a pinch, but the ones that allow you to let out pressure without removing)
Step 2: Start at about 35 psi, and keep checking after every run. They will keep increasing throughout the day (unless it rains). I've found that on both RE-11s and RS-3's, my best times are achieved with 36/38 front/rear HOT pressures.
Step 3: Your tires may prefer different optimal pressure levels, but usually, you want to max out below 45 psi on all street tires. Some street and track R-tires grip best in low 30s, so don't over-inflate !

HTH,
a

 

Pyrometers are fun, and can be used to psych-out the competition, but unless you have fully adjustable suspension, they are not very useful.

You WILL always get highest temperature readings on the outside front wheels because stock cars don't have nearly enough camber for competitive autoX. And you can't fix that with tire pressure changes.

Having said that, pyrometers are great for collecting data for the next autoX (or for prioritizing the next suspension upgrade), and for messing with your competition. If you are a few tens behind (or ahead) with the last run on the line - try this:
Step 1: Take pyrometer readings, record them on paper
Step 2: Whip out a graphical calculator (or iPad) in full view of your main competitor
Step 3: Slap yourself on the head and say out loud something along the lines of "that's it, that's where I am loosing that full second!"
Step 4: Go and fidget under the hood, or with the tires.

On the last run, your competitor will try too hard to find that "extra full second" and fail to improve his time.


a