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Old 2007-07-14, 02:24 AM   #1
Doug
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Patch Size

There have been many questions about and recommendations for tire pressures on this forum. The best answer I've seen is to just experiment and find a pressure with your own gauge that gets you the best performance. I had an idea for a secondary method to get to a starting point for the above. Air pressure recommendations for a specific tire depend greatly on the weight the tire is carrying. Air pressure is force (weight) divided by area. One version is pounds divided by square inches (lbs/in²). Rather than recommending a tire pressure for a specific tire and type of riding, it might be better to recommend the area involved. The patch in contact with the ground is a close match with the theoretical area calculated from weight / pressure. This theoretical patch size could be used as a recommendation of how much to pump up a specific tire. Tread patterns will cause the true patch size to be different from the calculated one.

IMHO a patch size is optimum when the side wall is slightly flexing. Any bump encountered will be absorbed easier by the further collapse of the wall and tread around the rock edge, root, etc. Patch size also affects turning. My tire is a Specialized S-Works Evil Twin Roller 26 x 2.6. My combined weight of rider, uni, pack, clothes, weapons, etc. is around 230 pounds. My current favorite pressure is 35psi. This calculates to be a patch size of 6.6 square inches. (230 / 35 = 6.6)

My recommended patch size for a Specialized S-Works Evil Twin Roller 26 x 2.6 is 6.6in² for muni.

6.6in² might also work for any 2.6 inch wide tire.

Any opinions of this? What are some other patch sizes associated with different types of tires and riding?


Patch Size = Total Weight / Pressure
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Old 2007-07-14, 05:54 AM   #2
feel the light
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You seem to have given this some thought

It seems hard to beat the simplicity of the tire gauge though. My weight is sorta constant, so pumping it up to the preferred psi, derived by experiment, seems an ok system. Measuring the tire to ground contact area seems hard. I think we have all done it that way sometimes. The old eye ball the weighted tire cause we don't have a gauge tech.
A few times I have heard the gunshot of awareness, as someone goes a bit to hard. That has sold a few gauges. Bike riders like em hard. Pow !
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Old 2007-07-14, 07:10 AM   #3
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I think it's a good idea, a good way to normalize the tire pressure relative to weight and style.

For my trials, just over 20 psi, and 155 lbs total, so about 7 in^2.

For muni, about 8.25 in^2.

For touring, about 1.25 in^2.
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Old 2007-07-14, 08:19 AM   #4
Klaas Bil
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It's an interesting approach and one that deserves some further thinking. Rightly, you write 'for muni', implying that your patch size depends on riding type (discipline):
Quote:
Originally Posted by Doug
My recommended patch size for a Specialized S-Works Evil Twin Roller 26 x 2.6 is 6.6in² for muni.

6.6in² might also work for any 2.6 inch wide tire.
Incidentally, the 6.6 sq inch is very close to the square of the width, as 2.6 x 2.6 = 6.76 sq inch. If you think of the shape of the contact patch, that intuitively makes sense. I think that for a wider tyre (same rider, same total weight, same style of riding), the best patch size would be larger. It might well scale with the square of the width. That might generalise your patch size preference for MUni to 98% of width squared. Which implies that for a 3" tyre, your 'equivalent' tyre pressure would be 26.2 psi. Sounds good, I think.

Indeed, for road riding with a 2.6 tyre (people do that you know), the best patch size would be smaller.

I road-ride both with a 28 x 2.35 and a 26 x 1. Usual pressures for me are 60 psi and 125 psi, respectively. That translates in my case to 54% and 132% (rider weight and different weights of unis accounted for). So the narrow tyre pressure should be higher still? Indeed I feel it should but 125 psi is the recommended max for the tyre. To match, it should be 300 psi!


Quote:
Originally Posted by Doug
The patch in contact with the ground is a close match with the theoretical area calculated from weight / pressure.
You might be right but how do you know?


Quote:
Originally Posted by Doug
IMHO a patch size is optimum when the side wall is slightly flexing.
That is an alternative way of reasoning, obviating somewhat the need for your calculation.

Another factor that comes into play is jumping and dropping. The tyre pressure should be at least such that you don't pinch out on high hops or drops. So people who drop higher or use more explosive power in their legs might need to set pressure higher even if everything else (weight, tyre etc) is the same.

All in all, there's more to it than just your theory, but yet you're on to something.

Last edited by Klaas Bil; 2007-07-14 at 08:22 AM.
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Old 2007-07-14, 04:55 PM   #5
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It sounded like you were well on the way to coming up with a useful formula for calculating tire pressure. But then at the end you tossed in that fixed tire width, which kind of killed it...
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Old 2007-07-14, 06:43 PM   #6
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Quote:
Originally Posted by johnfoss
It sounded like you were well on the way to coming up with a useful formula for calculating tire pressure. But then at the end you tossed in that fixed tire width, which kind of killed it...
I only meant that as an example
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Old 2007-07-14, 10:41 PM   #7
Klaas Bil
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Quote:
Originally Posted by Doug
I only meant that as an example
A fixed patch size, i.e. when tyre width is not taken into account, translates to the same pressure for any tyre (for a given total weight). That doesn't sound reasonable to me.
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Old 2007-07-15, 02:53 AM   #8
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Quote:
Originally Posted by Klaas Bil
A fixed patch size, i.e. when tyre width is not taken into account, translates to the same pressure for any tyre (for a given total weight). That doesn't sound reasonable to me.
I communicated poorly. Yes, a fixed patch size is unreasonable for different tires and riding styles. It would be interesting to see many other's patch sizes and their type of riding. I was only trying to have a method for top riders (not me) to have a way to pass on how they pump up their tire.
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