Rear tires

[ChipBeck]

Size matters.

.....This is a simple physics problem. It is the same as if you sat on different size balls, it doesn't matter how big the balls are.......

From my observations at the Texas Mile, this is incorrect. A couple of our members appeared to be sitting on significantly bigger balls and they went a lot faster than the rest of us!

Chip

 

[Thugboat]

Chip!


Thats Funny!!!!

Larry

 

[Indy GT]

LOL

Good one Chip!!!!! And very astute observation!

 

[fjpikul]

Hey Chipper, you got that one right.

 

[soroush]

From my observations at the Texas Mile, this is incorrect. A couple of our members appeared to be sitting on significantly bigger balls and they went a lot faster than the rest of us!

Chip

post of the day!:lol:lol:lol:lol:lol:lol:lol:lol that was funny

 

[Hammer]

I actually am laughing out loud. And I don't think anyone can argue with you logic, Chip.

 

[Superfly]

From my observations at the Texas Mile, this is incorrect. A couple of our members appeared to be sitting on significantly bigger balls and they went a lot faster than the rest of us!

Chip

:rofl:lol:rofl. To paraphrase an old surf slogan, "most sports just require one ball........."

 

[FLY GT40]

The size of the contact patch is solely a function of sidewall stiffness (which is almost negligible) and tire pressure. If you have 35psi and 1400lbs of load, your contact patch will be 40 sq in (very slightly less since the sidewall will take part of the load). BUT...It turns out that the SHAPE of the contact patch is important and you want it to be long in the direction of force (hence wide tires for cornering). There are other related issues like the fact that as the area of tread gets larger the rubber can be made softer with the same wear rate.

See the "Unified Theory of Rubber and Tire Friction" for great bedtime reading. One of the few remaining things I recall from studying engineering many years ago.

Yukonranger

You sound like a good teacher so here are my questions.

1. This sound like lower tire pressure would increase contact patch and hense increased traction. If this is the case then if you are having launch problems would a lower tire pressure help?

2. So if the first is true and the warning sticker on the door says "when driving at speeds in excess of 150mph increase tire pressure to 40psi ". Would that give a smaller contact patch and less traction? :confused

FLY GT40

 

[fjpikul]

I'm not sure, but I think I remember the Ford engineers telling us at Rally I in Detroit that higher pressures at higher speeds were needed to keep the tire on the rim because of deforming of the bead.

 

[BlackICE]

I'm not sure, but I think I remember the Ford engineers telling us at Rally I in Detroit that higher pressures at higher speeds were needed to keep the tire on the rim because of deforming of the bead.

I believe it is to have the tire deform less per revolution, thus generating less heat. The temperature gain at a sustained 150-200 mph would to too much at lower pressures.

 

[STORMCAT]

FYI, I was informed my a few guys at TWS that going from 315 to 345 may not get you additional rubber on the ground. They challenged me to take my car on dirt road with 315 and then again 345... then measure the contact patch. I'd be glad to get the figure on a 345 Bridgestone if someone else will get the 315 Goodyear measurement.

Here'a some photos of the Bridgestone 345's on stock rims after a drive thru my dusty garage. I tried various tire pressures and the contact patch measurement was less that the Goodyears. The Good Year F1's seam to be more square at the shoulders. These are at 28 psi. Stock suspension settings.

 

[STORMCAT]

Yukonranger

You sound like a good teacher so here are my questions.

1. This sound like lower tire pressure would increase contact patch and hense increased traction. If this is the case then if you are having launch problems would a lower tire pressure help?

2. So if the first is true and the warning sticker on the door says "when driving at speeds in excess of 150mph increase tire pressure to 40psi ". Would that give a smaller contact patch and less traction? :confused

FLY GT40

At higher speeds the extra air pressure makes the tires harder and reduces the rolling resistance and heat build up. Traction is not really an issue once you are rolling. But note the hard tires will spin more easily when you launch so it's best to take off easy and let the tires hook at the start of a high speed run

 

[STORMCAT]

FYI, I was informed my a few guys at TWS that going from 315 to 345 may not get you additional rubber on the ground. They challenged me to take my car on dirt road with 315 and then again 345... then measure the contact patch. I'd be glad to get the figure on a 345 Bridgestone if someone else will get the 315 Goodyear measurement.

Because of the smaller diameter you are giving up some area not from the width stand point but in the circumference contact area. I posted some photos of my wider Scudi tires. note dust imprint.. :biggrin

 

[Empty Pockets]

At higher speeds the extra air pressure makes the tires harder and reduces the rolling resistance and heat build up.

...it also helps keep the tire from spinning on the rim.

 

[timcantwell]

If you decide you want to widen your stock rims, contact James at Weldcraft Wheels in Plymouth, MI:

http://www.weldcraftwheels.com/index.html

These guys can widen, narrow, or repair just about any rim. I have had them modify steel, aluminum,
and magnesium wheels for me. They do a great job. Here is an example, a magnesium Pantera
Campagnolo wheel that they narrowed down to 5" width so I could mount a collapsible spare tire on it:


Jeff

Thanks Jeff, I have heard good things about them as well.

 

[timcantwell]

Here'a some photos of the Bridgestone 345's on stock rims after a drive thru my dusty garage. I tried various tire pressuses and the contact patch measurement was less that the Goodyears. The Good Year F1's seam to be more square at the shoulders. These are at 28 psi. Stock suspension settings.

The wider widths may call for less negative camber to improve your contact patch. (Although road-course set-up and drag-race set-up will differ) Wider wheels will also shift your geometry and merit a double check of your aligment settings to make sure that your chassis is optimized for the new tire/wheel set-up.

 

[Yukonranger]

Lower tire pressure will indeed increase the size of the contact patch but it turns out that this is not a case where more is always better. Load distribution across the contact patch is important too. As you lower tire pressure you get to a point where the loading on center of the contact patch is lower than the edges. Generally, in racing, a lap or two is run and then camber is set so that the temperature on the inside and outside of the tread are roughly equal and then tire pressure is set so that the center tread temperature is the same as the edges or roughly in the middle of the edge temps.

When you think about this, it becomes apparent that different tracks and conditions have different camber and tire pressure optimums. If a track has a higher grip surface then the car will roll more and more negative camber will be optimum as the goal is to have the tread roughly flat and evenly weighted. Naturally the more camber you have the worse your straight line traction will be (both braking and accelerating) as you are now running mostly on the inside edge of the tread. You can see that camber and tire pressure are compromises between lateral and longitudinal acceleration as well as tire wear. In the end, for street or non-racing track use the best approach is to use settings from someone like T&A Shocks and accept that they represent a good compromise reached through testing and experience.

As others have pointed out, the reason for a higher recommended tire pressure at high speeds is to limit tire deformation which results in heat build up. Heat build up can eventually lead to tread separation. Under Inflation is why there are truck tire treads on the highways...

I don't want to hold myself out as some kind of tire guru. I know enough to know that it is a complex subject and may not be completely understood by anyone. It is important to know that there are a lot of variables and that most changes and adjustments are compromises and you have to choose between different performance characteristics, you can't have them all at the same time.

Yukonranger

You sound like a good teacher so here are my questions.

1. This sound like lower tire pressure would increase contact patch and hense increased traction. If this is the case then if you are having launch problems would a lower tire pressure help?

2. So if the first is true and the warning sticker on the door says "when driving at speeds in excess of 150mph increase tire pressure to 40psi ". Would that give a smaller contact patch and less traction? :confused

FLY GT40

 

[FLY GT40]

Lower tire pressure will indeed increase the size of the contact patch but it turns out that this is not a case where more is always better. Load distribution across the contact patch is important too. As you lower tire pressure you get to a point where the loading on center of the contact patch is lower than the edges. Generally, in racing, a lap or two is run and then camber is set so that the temperature on the inside and outside of the tread are roughly equal and then tire pressure is set so that the center tread temperature is the same as the edges or roughly in the middle of the edge temps.

When you think about this, it becomes apparent that different tracks and conditions have different camber and tire pressure optimums. If a track has a higher grip surface then the car will roll more and more negative camber will be optimum as the goal is to have the tread roughly flat and evenly weighted. Naturally the more camber you have the worse your straight line traction will be (both braking and accelerating) as you are now running mostly on the inside edge of the tread. You can see that camber and tire pressure are compromises between lateral and longitudinal acceleration as well as tire wear. In the end, for street or non-racing track use the best approach is to use settings from someone like T&A Shocks and accept that they represent a good compromise reached through testing and experience.

As others have pointed out, the reason for a higher recommended tire pressure at high speeds is to limit tire deformation which results in heat build up. Heat build up can eventually lead to tread separation. Under Inflation is why there are truck tire treads on the highways...

I don't want to hold myself out as some kind of tire guru. I know enough to know that it is a complex subject and may not be completely understood by anyone. It is important to know that there are a lot of variables and that most changes and adjustments are compromises and you have to choose between different performance characteristics, you can't have them all at the same time.

Thanks Yukonranger

You are a good teacher with understandable answers.

FLY GT40:cheers:thumbsup

 

[STORMCAT]

Lower tire pressure will indeed increase the size of the contact patch but it turns out that this is not a case where more is always better. Load distribution across the contact patch is important too. As you lower tire pressure you get to a point where the loading on center of the contact patch is lower than the edges. Generally, in racing, a lap or two is run and then camber is set so that the temperature on the inside and outside of the tread are roughly equal and then tire pressure is set so that the center tread temperature is the same as the edges or roughly in the middle of the edge temps.

When you think about this, it becomes apparent that different tracks and conditions have different camber and tire pressure optimums. If a track has a higher grip surface then the car will roll more and more negative camber will be optimum as the goal is to have the tread roughly flat and evenly weighted. Naturally the more camber you have the worse your straight line traction will be (both braking and accelerating) as you are now running mostly on the inside edge of the tread. You can see that camber and tire pressure are compromises between lateral and longitudinal acceleration as well as tire wear. In the end, for street or non-racing track use the best approach is to use settings from someone like T&A Shocks and accept that they represent a good compromise reached through testing and experience.

As others have pointed out, the reason for a higher recommended tire pressure at high speeds is to limit tire deformation which results in heat build up. Heat build up can eventually lead to tread separation. Under Inflation is why there are truck tire treads on the highways...

I don't want to hold myself out as some kind of tire guru. I know enough to know that it is a complex subject and may not be completely understood by anyone. It is important to know that there are a lot of variables and that most changes and adjustments are compromises and you have to choose between different performance characteristics, you can't have them all at the same time.

great info. Thanks for sharing ...:thumbsup

 

[Triheart7]

Great info. I was wondering if a standing mile was long enough to get the temp up enough to cause deformation?