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Tire Pressure Adjustment for Handling Balance

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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.

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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.

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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. 

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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.

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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.

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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. 

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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 

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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.

 

 

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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?)

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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...

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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!

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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.  

 

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

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First post here from a UK based racer (Hey!) Setting (hot) tyre pressures perfectly is a challenge we all face, whether you're using nitrogen, dry air or straight air.

If you don't have specific tyre data then you might be interested to use the following table as a starting guide:

These are target hot pressures i.e. what to aim for when the car is on track.

Vehicle Weight Target (psi) Target (bar)
Kart 14 – 18 1.0 – 1.25
Very Light Racing Car < 800kg 22 – 29 1.5 – 2.0
Light 800kg – 1000kg 24 – 32 1.6 – 2.2
Heavy 1000kg – 1400kg 28 – 40 2.0 – 2.75
Very Heavy > 1400kg 37 – 40 2.5 – 2.75

As you can only set the pressures when they are (cold) in the pits, you typically have to set them lower so that when out on track you hit the target pressures.

Each track of course has its own corner characteristics. That mean tyres warm up on each corner differently. 

So it then becomes a kind of double guess - guess what hot pressures you want and then guess what cold pressures to set them at prior in the pits, given the characteristics of the track.

It is however entirely possible to set your pressure correctly and reliably each time - even considering changes in air and track temperature if you really get into it.

I've found tuning for a scaling factor, rather than a fixed pressure offset, works pretty well.

So say your target pressure was 27 psi front and 26 psi rear. Then say your track is clockwise, with some low speed right hand corners. You might feel that layout may put a lot of energy through the rear tyres, and more than the fronts, and therefore increase the pressures more at the rear.

Typically people might try a 1 or 2 psi stagger around the car. Instead you might try applying the same logic to the scaling factor.

Scaling factors are typically between 1.2 and 1.5.

So in the example above, you might choose scaling factors of:

Front Left: 1.25  |  Front Right: 1.3

Rear Left: 1.2  |  Rear Right. 1.25

You then divide the target pressure (27 psi) by the scaling factors for each corner. This then gives you your target initial pressures. So:

Front Left: 21.6  |  Front Right: 20.8

Rear Left: 21.7  |  Rear Right. 20.8

The next step is you simply go out, do a run. Record what the hot pressures actually end up at. Then tweak the scaling factors accordingly so that next time you go out you can guarantee hitting 27 psi front & 26 psi rear. Or in fact, target any pressure you like. 

I've actually written a bit more about this on my blog (link below) that includes how to cope with environmental changes and a free calculator too but really this is the core principal.

Hope that helps.

https://www.yourdatadriven.com/how-to-set-your-racing-car-tyre-pressures-perfectly-every-time/

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Nice stuff...but you lost me at kg and bar... every time we yanks cross the scales the unit is lbs....

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2 hours ago, Captain Buddha said:

Nice stuff...but you lost me at kg and bar... every time we yanks cross the scales the unit is lbs....

not everything is in football fields per fahrenheit & lbs per bbq brad 😂

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10 hours ago, Captain Buddha said:

Nice stuff...but you lost me at kg and bar... every time we yanks cross the scales the unit is lbs....

Understood. Sorry about that 🙂 ... At least you guys have everything consistent. In the UK we have this strange mix all the time - even with tools we are all doubled up! 

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15 hours ago, Captain Buddha said:

Nice stuff...but you lost me at kg and bar... every time we yanks cross the scales the unit is lbs....

4 hours ago, ydd said:

Understood. Sorry about that 🙂 ... At least you guys have everything consistent. In the UK we have this strange mix all the time - even with tools we are all doubled up! 

You would think that a person who travels the world dealing with manufacturing machinery would understand how to use feeters and bars.   and I KNOW Brad likes bars. 😉

 

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2 hours ago, Matt93SE said:

You would think that a person who travels the world dealing with manufacturing machinery would understand how to use feeters and bars.   and I KNOW Brad likes bars. 😉

 

Remember though, I put the "duh" in Ed Duh Tor (editor)... LOL!

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