Patch Size

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.:eek: That has sold a few gauges. Bike riders like em
hard. Pow !

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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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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):

Doug said:

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! :eek:

Doug said:

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?

Doug said:

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.

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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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johnfoss said:

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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Doug said:

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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Klaas Bil said:

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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