hi guys i like to run my tire pressures at around 40 front 32 rear and im confused about somthing after each run they go up about 1.5 psi so after my first run i bleed them down some, then they go up again after my second so i bleed them down again, then they went up alittle after my third so i bleed them again.

is this bad it feels weird to just keep letting air out of my tires?

sorry for the newb question.

 

I take it you're not familiar with Charles/Boyles (spelling?) Law?


EDIT: I speeled it correct dammit!!

 

sorry im really tired

 

You didn't stay awake in Phyiscs did you

Heat expands. So when you heat up your tires the air inside expands increasing pressure. So if you want your car to be consistent you need to make sure your air pressures are consistent so you get repeatable results.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by CivicSiRacer &raquo;</TD></TR><TR><TD CLASS="quote">You didn't stay awake in Phyiscs did you

Heat expands. So when you heat up your tires the air inside expands increasing pressure. So if you want your car to be consistent you need to make sure your air pressures are consistent so you get repeatable results.</TD></TR></TABLE>

Heat Expands???? The air inside your tire does not expand!!! The air in your tire stays approximately the same volume (since the tire keeps it that volume) But the PRESSURE of that air goes up.

If you are going to say he didn't go to physics class how about getting your physics right?

 

Hahah I was schooled I think was too tired myself

 

Oh woow...
Front 40 psi
Rear 32 psi

I'm running mine at F 36psi, R 35psi.

Since where on the psi subject. What happen if the front have more psi then the rear and what happen if the rear have more psi in the the front.

Sorry the thread hijacking.

 

thats ok man im kinda wondering that myself

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kaiba &raquo;</TD></TR><TR><TD CLASS="quote">
Since where on the psi subject. What happen if the front have more psi then the rear and what happen if the rear have more psi in the the front.

Sorry the thread hijacking.
</TD></TR></TABLE>

Im always thrown for a loop on what most autoX guys run but, i know for road racing. this is what most people do.

the fronts work harder then the back, so naturally the front will gain pressure easier then the rear. So most people start the fronts off lower then the rear so that they will come to about the same temputer when coming off track.

But thats is not always the case, the other weekend i was messing with tire pressures and i started all four equally, and the car felt great. (this was mainly becuase the car was feeling alittle loose so i fixed it with a pressure adjustment because the rears were seeing to high of temputure)

some times when in a stock class though, tire pressure is the only adjustment you have, so it is used alot more.

in general, autoX guys run much higher pressures then road racers.

 

generally the higher the rear pressure the easier it will rotate

 

I been doing a lot of thinking.

If the rear tire have more presure than the front. Doesn't it oversteer more. In other words, less understeer. Like adding a larger rear swaybar.

 

going either way in the rear with pressures will induce oversteer.

 

For a given tire, car, surface, average speed and air temperature there is an ideal pressure where the tire will have the most grip. If you go either above or below that pressure the tire will grip less.

What most of us with FWD do for autocross is try to set the front pressures for best grip and then set the rear pressures either above or below that to make the rear rotate a little.

So there is no simple answer to the question of "should I run higher or lower in the back then the front?"

regards,
alan

 

And usually if the rears are above their ideal pressure, the breakaway from "rotation" will be a little more abrupt than if the pressure is lower than ideal.