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PostPosted: January 26, 2021, 4:36 pm 
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The biggest unknown is the sidewall stiffness of the tire. I loved the R888 I used for a couple off seasons, then had to switch to the R888s a totally different tire with a far stiffer sidewall, I tried using caulk and a tire temp probe. I was down to 10 front a 12 rear and still could not get the right readings. In the end I talked to Johnny Miller the ex-racer, now runs Track Side Tire Services. He listened to my story and said add 10 pounds to each and your good, you will never get the number you want because the side walls are too stiff for the weight of you vehicle He was right the car felt like crap on a 10/12, it was better on 10/22. I later in the year has real problems getting heat into the tires, so they will be replaced before the first event of the year.

Graham


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PostPosted: January 27, 2021, 10:08 am 
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FastG wrote:
The biggest unknown is the sidewall stiffness of the tire. I loved the R888 I used for a couple off seasons, then had to switch to the R888s a totally different tire with a far stiffer sidewall, I tried using caulk and a tire temp probe. I was down to 10 front a 12 rear and still could not get the right readings. In the end I talked to Johnny Miller the ex-racer, now runs Track Side Tire Services. He listened to my story and said add 10 pounds to each and your good, you will never get the number you want because the side walls are too stiff for the weight of you vehicle He was right the car felt like crap on a 10/12, it was better on 10/22. I later in the year has real problems getting heat into the tires, so they will be replaced before the first event of the year.

Graham


This starts us into the question of can you ever have too much tire or too big of brakes. The answer to both is yes. If you don't have the mass or inertia to get either up to operating temp, you can't get the best performance from them.

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PostPosted: May 11, 2021, 4:22 pm 
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The tire psi x the tire contact area = the weight of the car

So to first order, if you want the same contact area with half the car weight, cut the psi in half.

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PostPosted: May 20, 2021, 1:53 pm 
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NoahKatz wrote:
The tire psi x the tire contact area = the weight of the car

So to first order, if you want the same contact area with half the car weight, cut the psi in half.


I've seen this posted on many car forums - do you know the origin or the science behind it?

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PostPosted: May 20, 2021, 4:40 pm 
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it might follow across a very small change. That formula would mean they would approach an infinite tire patch before being mounted to a wheel. But tires have belts and sidewalls etc. that create form and stiffness. I can guarantee the tires on my DD are not twice the tire patch area at 15 PSI than when I run 30 PSI.

My buddy always used a similar formula for tire pressure on his trailer. If the tire is rated 2,000 lbs @50PSI he figured a 1,000 lb tire load should be run @ 25 PSI. Totally bogus, I say.

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PostPosted: May 24, 2021, 10:31 am 
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rx7locost wrote:
That formula would mean they would approach an infinite tire patch before being mounted to a wheel. But tires have belts and sidewalls etc. that create form and stiffness. I can guarantee the tires on my DD are not twice the tire patch area at 15 PSI than when I run 30 PSI.


This is always where my thought process has ended up but I've seen the foot print vs. psi comment posted enough that I'd be curious to know if its origin had any science behind it.


rx7locost wrote:
My buddy always used a similar formula for tire pressure on his trailer. If the tire is rated 2,000 lbs @50PSI he figured a 1,000 lb tire load should be run @ 25 PSI.


By chance is he a firm believer that all modern trailer tires are garbage? Asking for a friend.... :mrgreen:

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PostPosted: May 24, 2021, 12:57 pm 
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NoahKatz wrote:
The tire psi x the tire contact area = the weight of the car



It just doesn't work that way...

I worked in a vintage race shop professionally, and virtually everything on slicks ran 12-14psi hot. This runs the gamut from 1970's F1 cars at 1200lbs, to Can Am cars at ~2000 lbs, to a 996 GT3RS at ~3000lbs. The F1/Can Am cars ran similar sized tires, the 996 were smaller. Even my IKF sprint kart ran similar pressures.

Street tires are all over the board, the only way to know what works best is testing.


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PostPosted: May 24, 2021, 7:21 pm 
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a.moore wrote:
NoahKatz wrote:
The tire psi x the tire contact area = the weight of the car

So to first order, if you want the same contact area with half the car weight, cut the psi in half.


I've seen this posted on many car forums - do you know the origin or the science behind it?



The science is simple A[lb/sq in.] x B sq in. = AB lb, but only applies to a first order approximation, i.e. not accounting for other factors.

The tire carcass contributes its own (fixed) stiffness, so in reality you wouldn't need to fully halve psi with half the weight.

It looks like from what others have said that the carcass stiffness is so great that the approximation is worthless.

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