Tire pressure/recommended psi and rider's weight

wrote:
41 wrote:
Joss Winn wrote:
Thanks for everyone's advice. I'm running them at 110psi and will try
to get used to the softer feel. They are generally more comfortable
than my old 120psi 23mm tyres but I kind of miss the sound of rock hard
tyres rolling on the road and the feel of smooth tarmac underneath the
tyre. These new tyres dampen this experience.

For fun & revelation, why not time yourself, over several attempts,
over a fixed course at say 100, 110, and 120psi.

Dear 41,

Unfortunately, it's unlikely that a series of real-world test-runs will
show any speed difference between 100, 110, and 120 psi inflation on
high quality tires.


Why would that be unfortunate? You seem to have missed the point of the
suggestion.


This calculator uses Jobst's rolling resistance data to predict exactly
that kind of effect:

http://www.analyticcycling.com/ForcesTires_Page.html


The range of difference in a 1-hour 40 km ride at about 25 mph is less
than thirteen seconds for inflations from 120 to 100 psi.

Th is kind of tiny change is why drum spin-down tests are necessary to
provide rolling-resistance data and why our impromptu roll-outs are so
unreliable.

The poster did not ask about rolling resistance, he asked what pressure
he should run his tires at. These are completely different questions.
Furthermore, the question of which pressure will produce the fastest
time over a particular course is also different from which pressure
produces the lowest rolling resistance, as it is considered classically
and as tested on JB's drum apparatus.

In real life one does not ride on a polished drum but on a road surface
with irregularities. The lower the pressure, the less these
irregularities retard the motion, giving an effect opposite to that of
pressure on rolling resistance. This is traditionally not called
rolling resistance because variously deforming or lifting off over
obstacles is not precisely rolling. The roads in England can be very
rough and a lower pressure may very well produce a significantly faster
time.

Potentially large changes of this kind are why systematic "roll-outs"
are useful and interesting to provide pressure effectiveness data and
why drum spin-down tests are so irrelevant for this purpose.

ê

On 9 Jul 2005 21:33:29 -0700, "41"
wrote:



wrote:
41 wrote:
Joss Winn wrote:
Thanks for everyone's advice. I'm running them at 110psi and will try
to get used to the softer feel. They are generally more comfortable
than my old 120psi 23mm tyres but I kind of miss the sound of rock hard
tyres rolling on the road and the feel of smooth tarmac underneath the
tyre. These new tyres dampen this experience.

For fun & revelation, why not time yourself, over several attempts,
over a fixed course at say 100, 110, and 120psi.

Dear 41,

Unfortunately, it's unlikely that a series of real-world test-runs will
show any speed difference between 100, 110, and 120 psi inflation on
high quality tires.


Why would that be unfortunate? You seem to have missed the point of the
suggestion.


This calculator uses Jobst's rolling resistance data to predict exactly
that kind of effect:

http://www.analyticcycling.com/ForcesTires_Page.html


The range of difference in a 1-hour 40 km ride at about 25 mph is less
than thirteen seconds for inflations from 120 to 100 psi.

Th is kind of tiny change is why drum spin-down tests are necessary to
provide rolling-resistance data and why our impromptu roll-outs are so
unreliable.

The poster did not ask about rolling resistance, he asked what pressure
he should run his tires at. These are completely different questions.
Furthermore, the question of which pressure will produce the fastest
time over a particular course is also different from which pressure
produces the lowest rolling resistance, as it is considered classically
and as tested on JB's drum apparatus.

In real life one does not ride on a polished drum but on a road surface
with irregularities. The lower the pressure, the less these
irregularities retard the motion, giving an effect opposite to that of
pressure on rolling resistance. This is traditionally not called
rolling resistance because variously deforming or lifting off over
obstacles is not precisely rolling. The roads in England can be very
rough and a lower pressure may very well produce a significantly faster
time.

Potentially large changes of this kind are why systematic "roll-outs"
are useful and interesting to provide pressure effectiveness data and
why drum spin-down tests are so irrelevant for this purpose.


Dear 41,

Tom Compton's calculator predicts less than 13 seconds per
hour if we reduce a premimum-quality clincher from 120 to
100 psi--0.36%.

If his prediction is off by an order of magnitude, that
would be about two minutes and ten seconds per hour--3.6%.

What do you predict? (You cut most of the figures from the
calculator without providing any of your own.)

And how would you demonstrate this difference on rough
English roads?

Where would such an effect not be obscured by the effects of
wind, rider, posture, temperature, barometric pressure, and
traffic, which vary considerably between 25-mile rides in
England?

Curiously,

Carl Fogel

wrote:

Tom Compton's calculator predicts less than 13 seconds per
hour if we reduce a premimum-quality clincher from 120 to
100 psi--0.36%.

If his prediction is off by an order of magnitude, that
would be about two minutes and ten seconds per hour--3.6%.

As I said the problem is an effect different from rolling resistance
that varies with tire pressure in the opposite sense. If you think
about it you will realize that this effect has a very large potential
for variation, much greater than that typical for rolling resistance.


What do you predict? (You cut most of the figures from the
calculator without providing any of your own.)

The problem is undefined and therefore there is no prediction to be
made. Predict what? I've never met him and I don't know where he rides.
Besides, why shouldn't he just find out for himself?


Where would such an effect not be obscured by the effects of
wind, rider, posture, temperature, barometric pressure, and
traffic, which vary considerably between 25-mile rides in
England?

That is the reason for multiple attempts, if he is sensible under
reasonably similar conditions, perhaps over a much shorter circuit. If
in fact the rider cannot discern any difference in this way for his
typical riding, then he has the information he needs and should feel
free to make the pressure whatever he finds most comfortable or
entertaining. e

On 9 Jul 2005 23:56:38 -0700, "41"
wrote:



wrote:

Tom Compton's calculator predicts less than 13 seconds per
hour if we reduce a premimum-quality clincher from 120 to
100 psi--0.36%.

If his prediction is off by an order of magnitude, that
would be about two minutes and ten seconds per hour--3.6%.

As I said the problem is an effect different from rolling resistance
that varies with tire pressure in the opposite sense. If you think
about it you will realize that this effect has a very large potential
for variation, much greater than that typical for rolling resistance.


What do you predict? (You cut most of the figures from the
calculator without providing any of your own.)

The problem is undefined and therefore there is no prediction to be
made. Predict what? I've never met him and I don't know where he rides.
Besides, why shouldn't he just find out for himself?


Where would such an effect not be obscured by the effects of
wind, rider, posture, temperature, barometric pressure, and
traffic, which vary considerably between 25-mile rides in
England?

That is the reason for multiple attempts, if he is sensible under
reasonably similar conditions, perhaps over a much shorter circuit. If
in fact the rider cannot discern any difference in this way for his
typical riding, then he has the information he needs and should feel
free to make the pressure whatever he finds most comfortable or
entertaining. e

Dear 41,

I suppose that I should have predicted your answer.

Cheers,

Carl Fogel

"41" wrote:
For fun & revelation, why not time yourself, over several attempts,
over a fixed course at say 100, 110, and 120psi.
snip
You might be very surprised at which produces the fastest ride average.

Another experiment might be to ride a century at each of those tire
pressures. The comfort factor might turn out to be more significant than any
slight differences in rolling resistance.

Art Harris

Joss

How did you manage to get some Avocets imported?! I haven't had a reply
(so far) from either Avocet or Harris Cycles about getting them sent to
the UK.

I would happily act as a distributor as I'm sure many others would!

cccrider

wrote:

How did you manage to get some Avocets imported?! I haven't had a reply
(so far) from either Avocet or Harris Cycles about getting them sent to
the UK.

We're happy to send tyres to the U.K. or anywhere else, we'll even spell
it with a "y" for your benefit!

Not sure how you tried to contact us, but we send stuff to the U.K. all
the time.

Before I got my anti-spam filters set up I was averaging 4,000 emails
daily, so I really need to have filters...however, sometimes they
intercept legitimate email. This might have happened to you.

Safest way to make sure you get through is to use one of the encrypted
email links on my Website.

For actual orders, it's best to use our secure Web based shopping cart
system.

http://harriscyclery.com/tires

Sheldon "Avocet Advocate" Brown
+------------------------------------------------+
| Every normal man must be tempted, at times, |
| to spit on his hands, hoist the black flag, |
| and begin slitting throats. -- H.L. Mencken |
+------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
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