I did a search for variations of this topic and came up with 'nuthin.

I checked all four pressures before I left on the drive today, and all four were at 32psi. I drove for about an hour or so through some winding roads at speeds up to 120mph. The outside air temp was only 55 degrees (yes, thats summer in Alaska!), and some of the roads still had snow on the shoulder. In other words, it wasn't hot.

I had a valve stem replaced a week ago in the right rear tire, which seemed to fix a slow leak (2psi per day).

I pulled over at a gas station to check that right rear tire was indeed fixed, and to my suprise it read 37.5psi. I'm sure the warm tire could do that, but what REALLY got my attention was the other three tires. Right front was 34 psi, left front was 36, and the left rear was 38psi.

So the question is: Is this normal? I have to admit that I've never checked the tire pressures mid-drive before, but it seems normal as the tires heat.

How do you adjust your pressure (I have the stock tires) for different driving conditions, and is there a "hedge" to prevent uneven heating (or uneven pressures?).

And finally, is a +/- 3psi difference in tire pressure enough to make the handling sketchy?

By the way, this drive is great! A wide seventeen mile park service road (spelled - smoooth paving) that dead ends up a glacial valley and no straight sections. I'll take a camera next time, it was awesome!

Cheers -

 

Uneven pressures after a drive really doesn

 

Uneven 'hot' pressures do not sound too odd to me. I'm not sure about driving on the streets, but Ironwedge did mention that he was pushing the car through the twisties ... if the corners were mostly in one direction (ie mainly left handers) then the outside tyre pressures should be hotter.

On the track, if you really want to be finicky, you end up putting higher pressures on the 'outside' tyres.


[Edited by DavidM on 05-18-2001 at 06:40 AM]

 

Also interesting that the greater rise was in the rear, which I wouldn't expect unless I'm doing more stop-n-go's that twisties. I mean, the front tires are doing most of the turning work and the rear heat from acceleration forces.

Like other posters have observed, excess PSI in the rear can make it oversteer more on the road.

 

<Like other posters have observed, excess PSI in the rear can make it oversteer more on the road.>

Yes the handling DID get interesting as the rear pressures increased. Is there a technique to avoid this, like starting with a lower pressure, if so - how much lower given outside airtemp, elevation, etc?

Cheers -

 

a few items to research - but of course, conclusive answers and what you do with them are up to you.

search for the following items:
"Jason Saini" - he made an excellent point that you want to air up the fronts for control in the steering, and then you adjust the rears for handling - therefore adjusting the balance of understeer / oversteer

"Rick Hesel" - purchased and had installed (immediately after delivery of the car) tire pressure monitors. Slick setup for ~$200 (maybe more) + installation. This item can be found on http://www.tirerack.com

also, there was a thread sometime ago about filling your tires with nitrogen - it does not vary in temparature as much and therefore you will always have a constant pressure in cold and hot, when it comes to tires. Of course, its been argued about how much it actually fluxuates, but its a tip that some racers use.

 

I have found that the stock S02's have best adhesion and handling balance (at least for me) with a pressure of about 36psi when at operating temps. So, if my operating temps (ie at the track or autocross or on a hot day) cause it to go higher and I want consistent at the limit behaviour, then I will adjust the pressures accordingly. To me, cold pressures don't mean a lot because it really depends on what I am going to do with the car.

 

I have the SmarTire system, so I can monitor tire pressures from the car at any time. I've found that the pressures typically increase about 3 pounds after a few miles and stabilize there, even with a lot of high speed driving. I've never seen them go more than 4 pounds over cold pressures. BTW, I suspect that your tire guage is not very accurate. I have one that's accurate to a ten of a pound and the pressures from one tire to another never vary by more than half a pound when the tires warm up (again, readings while driving on the SmarTire system).

[Edited by Rick Hesel on 05-18-2001 at 06:10 PM]

 

As Rick alluded to, GET A CALIBRATED GUAGE!

Be careful when setting the cold pressures that the tires on one side of your car have not been sitting in the hot sun while the other tires have been in the shade. It will throw the pressures off.

 

Thanks for the great advice everyone, all excellent points.

I'll keep watch to see if can spot more of a trend between driving habits/temps/pressures. Since I keep the car in the garage, the cold temps should be the same all the way around. Also, I've been using a digital gauge advertised to be within .5 psi. I have to admit that it was a cheapo gauge, so that would be the best place to start in terms of low cost.

Either way, 32 psi or 37 psi...I couldn't stop grinning!

Cheers -