Jump to content
Sign in to follow this  
dunhamr1

Tire Pressure Adjustment for Handling Balance

Recommended Posts

Now, this may be my ignorance, but the tire rack article as presented below indicates that to decrease understeer one should INCREASE  front tire pressure or decrease tear tire pressure...

https://m.tirerack.com/tires/tiretech/techpage.jsp?techid=58

This seems backwards or incomplete to me. I see the tire similar to a spring where decreasing pressure adds grip (and heat) until the tire is overheating, rolling over too much, or in danger of debeading.

I guess my gripe with the article is it doesn't specify where you started, but it still seems backwards to me. Am I just ignorant?

Edit: there was a chart near the back of Caroll Smith's "Tune to Win" which listed a bunch of things that could be adjusted to change balance, I should go back and reference that to start.

Share this post


Link to post

Without a pyrometer to back up your driving style and it’s effect on your setup... I think that entire thing is bunk. I’m with you though, it does seem backward as well. In autocross however it might make sense as you prime the tire for peak temp for its short run.

Share this post


Link to post

I think it would apply mostly to of your standard car with sloppy suspension and weak sidewalled tires. In that scenario, adding pressure results in less tire/tread rollover for more grip. But that's putting salve on an open wound, and limited relevance. E.g. stock class rules and/or SEVERE budget limits.

Better MO is to adjust tire pressures to make tire happiest  in conjuction with suspension adjustments for grip and balance. 

Share this post


Link to post

It goes both ways for multiple reasons.   sidewall stiffness/ rollover as well as contact patch management.

I've had a car that was undriveably loose the first few laps because I'd start the race with tire pressures very low due to the heat rise.  the rear tires would take 2-3 laps to come up to temp and pressure and then the car was good.  but keeping it on the track the first couple laps was a bear.  car would slide all over the place- mostly mid corner and out because the sidewalls would collapse..  Raising start pressures a bit reduced the sidewall flex and gave more response and increased traction, but also helped the tires get "glassy" toward the end of a race when the pressures got higher and less contact patch on the ground.  There's a sweet spot somewhere in the middle which I've had a hard time consistently hitting.

Some things are also a little different in tuning when comparing FWD and RWD/AWD chassis.

The point is, the statement has merit *IF* you're talking about the extreme end of the scale and a particular situation.  I would not use that as a reference case for track tuning.

Share this post


Link to post
1 hour ago, Matt93SE said:

The point is, the statement has merit *IF* you're talking about the extreme end of the scale and a particular situation.  I would not use that as a reference case for track tuning.

Agreed, I was just pointed to that link by a friend when I mentioned dropping front tire pressure to reduce push (understeer). We were having some fairly high temperature increases and were NOT rolling over the edge of the tire too much. (Re-71r on a Miata) Didn't make sense to me, and no one adjustment is a be-all end-all for any situation. 

Found out later that we were probably 5-7 psi above the recommended pressures for that car, which makes sense to me.

I just wanted a reality check from some folks who've been there, done that (you guys) since I'm relatively new at this.

Share this post


Link to post
3 hours ago, El_Tortuga said:

I think it would apply mostly to of your standard car with sloppy suspension and weak sidewalled tires. In that scenario, adding pressure results in less tire/tread rollover for more grip. But that's putting salve on an open wound, and limited relevance. E.g. stock class rules and/or SEVERE budget limits.

Better MO is to adjust tire pressures to make tire happiest  in conjuction with suspension adjustments for grip and balance. 

Makes sense to me. I would agree that the article is steered towards stock /budget limited classes on street tires. 

Share this post


Link to post
20 hours ago, Matt93SE said:

...

I've had a car that was undriveably loose the first few laps because I'd start the race with tire pressures very low due to the heat rise.  the rear tires would take 2-3 laps to come up to temp and pressure and then the car was good.  but keeping it on the track the first couple laps was a bear.  car would slide all over the place- mostly mid corner and out because the sidewalls would collapse..  Raising start pressures a bit reduced the sidewall flex and gave more response and increased traction, but also helped the tires get "glassy" toward the end of a race when the pressures got higher and less contact patch on the ground.  There's a sweet spot somewhere in the middle which I've had a hard time consistently hitting.

 

I definitely fight this with the heavyweight. 1st session of the day the tires go up 10-14 psi!! I'm a hot mess and am trying to protect the tires for a couple of laps, especially in the cold. Much less between sessions, and in the summer temps, but still significant. E.g. coming back from extended break for lunch means I better be extra careful for a lap or two.

Ideal M.O. is to let them normalize while you are finding your rhythm. But if playing TT game, you may have to adjust starting pressure and play for early flyer lap. Tough part is to guess out what the other cars are doing and traffic management. Lots of moving pieces keeps this game 

Share this post


Link to post

Yep..  in my case, I'm going for overall shortest time to hit the checkered flag 15-20 laps later.  so if that means peddling it for a couple laps until temps and pressures are right, then hammering it for 10 laps and holding on to the finish, then I do so.   Ideally, I'd have a consistent tire from green to checker, but that doesn't happen on slicks.   I had much better consistency on DOT-Rs in a lower power car with (minor) aero where I couldn't abuse the tires quite so much under corner exit.  

another thing that would help all of us on that aspect is going to nitrogen purge/fill in our tires.  houston (TX in general) air is stupidly humid and causes lots of pressure rise as the tires get hot.  I also see easily 10+PSI change in a session, but I'm using compressed air from my tiny compressor at the track with no filter or dryer.  http://www.longacreracing.com/technical-articles.aspx?item=70794&article=Tire Pressures - Cold to Hot

I've had plans to install a nitrogen bottle in the trailer for years, but I've just been too lazy to get it done.  it's on the list, but stays near the bottom as a nice-to-have.

 

 

Share this post


Link to post
15 hours ago, Matt93SE said:

Yep..  in my case, I'm going for overall shortest time to hit the checkered flag 15-20 laps later.  so if that means peddling it for a couple laps until temps and pressures are right, then hammering it for 10 laps and holding on to the finish, then I do so.   Ideally, I'd have a consistent tire from green to checker, but that doesn't happen on slicks.   I had much better consistency on DOT-Rs in a lower power car with (minor) aero where I couldn't abuse the tires quite so much under corner exit.  

another thing that would help all of us on that aspect is going to nitrogen purge/fill in our tires.  houston (TX in general) air is stupidly humid and causes lots of pressure rise as the tires get hot.  I also see easily 10+PSI change in a session, but I'm using compressed air from my tiny compressor at the track with no filter or dryer.  http://www.longacreracing.com/technical-articles.aspx?item=70794&article=Tire Pressures - Cold to Hot

I've had plans to install a nitrogen bottle in the trailer for years, but I've just been too lazy to get it done.  it's on the list, but stays near the bottom as a nice-to-have.

 

 

Nitrogen was my first thought when you were talking about the race and the huge pressure swings. A small bottle is fairly inexpensive, like a small welding gas bottle would be enough for a few events, refills are stupid cheap after that ($25?)

Share this post


Link to post

a refill on a 125cuft bottle is under $20-- at least last time I had my C25 welding tank filled up, and nitrogen is cheaper than that.

a friend of mine has a large 300cuft bottle in his trailer and he doesn't even bother with an air compressor for his impact wrench..  he just plugs his hose into the tank and uses it for his impact wrench, "air" source, etc..   said it's been in the trailer for years and he hasn't had to refill it yet.  I'm thinking of the same philosophy, but with a 150/200cu ft bottle to reduce weight in the trailer...

Share this post


Link to post

I’ve got the large nitrogen bottle in the trailer, it lasts at least a year for airing up severely leaking bias ply tires. Works well!

Share this post


Link to post

I'm having a hard time wrapping my little brain around this.  I had no idea a decent size bottle of nitrogen fill could last so long. I have to assume it's able to be contained at a very high pressure level for that to work.  

 

Share this post


Link to post

Yep.  typical fill on a bottle is 2500-3000PSI. so that's a LOT of cubic feet of gas at 14.5psi..

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×
×
  • Create New...