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PostPosted: Thu Mar 15, 2012 2:57 pm 
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Recommended tire pressures are based on "cold" pressure, with the assumption that the tire will heat up, with increase in pressure, as they are ridden.

To a rough approximation, the total pressure is proportional to the absolute temperature (e.g., in Kelvin, not Celsius), where total pressure is gauge pressure + 1 bar.


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PostPosted: Thu Mar 15, 2012 3:51 pm 
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erickB wrote:
i put 34kg on front wheel and 48 kg on the rear, my tires are suposly 700x23 but are really 700x24 (challlenge criterium) so i put 75-80 psi for the front tire and 105-110 for the rear. this formula works much better than when i put 120 front and rear.
here is the actual formula that i use now
http://www.bikequarterly.com/images/TireDrop.pdf" onclick="window.open(this.href);return false;



+1 this is a great starting point..


Why is it always the 65kg guys you ride with who wana run 120psi?
After 8 months of trying to get one of my scrawny buddies to stop putting 120psi in his tires it's finally worked.. It took a few slides in the dry, riding my bike and getting front wheel chatter while trying to hold my wheel on a fast technical descent to get him to accept that he would get better performance out of his tires with a lot less pressure in them.

I'm 85kgs and I run 95ish up front and 100ish in the back.. A little less if I know it's gona be wet

A tire that is fighting the road is not faster than one that is not... The other upsides are better comfort and a massive increase in grip :thumbup:

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Posted: Thu Mar 15, 2012 3:51 pm 


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PostPosted: Thu Mar 15, 2012 4:46 pm 
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kbbpll wrote:
I'd like to see better science. There are only two data points above 120 psi in the graph presented in http://www.slowtwitch.com/Tech/What_s_i ... _1034.html" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false; referenced above, and it implies that 130 psi has the same Crr as 75 psi (and ~150 = 60). I don't believe it. Two data points measured with a PowerTap do not make a statistical conclusion. Even so, the delta Crr in the graph between 115 and 130 psi is about .001. If somebody farted in front of him it would have skewed the results more than that.

Is there a better scientific study on this? I fully agree there is a sweet spot. 130 psi on Vittoria CX clinchers felt "bouncy" on certain stretches of chip & seal, and I could feel it was inefficient, but that versus the entire ride at 75 psi and I would think 130 would still prevail.


I could probably help you with that, especially since I took that data and wrote the article :wink:

If you read the article carefully, you'll see that what is actually measured in a field test like that is NOT the tire's Crr, but the entire "bike + rider" system's "resistance to forward motion". That's a subtle difference. Although the tire's inherent Crr is reducing with increasing pressure, at some point the tire basically becomes so "stiff" that it starts transmitting undue amounts of vibrational energy THROUGH the tire (instead of being absorbed by the "air spring" of the tire and mostly returned to the road surface) where it is dissippated in the rider's "squishy bits" <to use a technical term> 8)

THAT'S why (for a particular tire, rider weight, pressure, and speed) there's a "breakpoint" pressure above which the resistance to forward motion (i.e. the thing we REALLY care about) starts increasing...and apparently dramatically so.

People seem to forget that the pneumatic tire is the MAJOR source of suspension on a road bike. Don't "lock out" that suspension by pumping your tires up too high.

BTW, those data points are the result of literally hundreds to thousands of individual power readings taken during the course of a field test run :) Oh...and a delta Crr of .001 is ~equivalent to a 10W difference in power at TT race speeds. That's a strong fart :shock:

Here's the funny thing I remember about that test...if I had to choose what felt faster in a blind test, I'd pick the highest pressures. But, that just shows you how fallible human perceptions can be. Luckily, the stopwatch and power meter don't lie 8)


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PostPosted: Thu Mar 15, 2012 4:52 pm 
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Discoverspeed wrote:
My wrench told me that tire pressures increase when the tires roll and get warmed up. Intuitively, this sounds right judging from motorsports - but those tires have higher air volumes, different compounds etc. I wonder if anybody has conducted a test on bicycle tire pressures when cold and hot? If there is a significant difference, this could mean that we should actually inflate our tires to a slightly lower pressure when the tires are cold.... or this could be nothing to consider about.


I've taken IR temp readings of tires when riding outside. The tires tend to only heat up a few degrees C over ambient conditions. Not a LOT of energy is dissipated in tires (the good ones anyway) and riding outside there is a fair amount of convective heat transfer off of the tires due to the bike travelling through the air. The "ambient" temperature that the tires follow tends to be the air temp, and not the road surface temp BTW...although high road surface temps will tend to increase the air temp near the road surface.

Secondly, a little bit of study with the ideal gas law (i.e PV=nRT) will show you that tire pressures don't rise very much at all even with large increases in temperature.

So...I'd say that your final statement is the most accurate (i.e. "this could be nothing to consider about) :)


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PostPosted: Thu Mar 15, 2012 5:08 pm 
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kbbpll wrote:
Even so, the delta Crr in the graph between 115 and 130 psi is about .001. If somebody farted in front of him it would have skewed the results more than that.


Did you graduate gradeschool? Take any algebra or math? Do you really think because a number is small (relative to what I wonder?) means it is insignificant?

The crr value is analogous to slope... ie a .001 increase is the same resistance as a 0.1% slope increase. There are plenty of calculators online that you can use to quantify just how much that is... you don't even need to know arithmetic.

But here is one example. You attack in a race (flat road) with 4 miles to go. You get a gap and settle in to TT to the finish. With the tires inflated to 115 psi you have a crr of .0038, and your speed is 28.00 mph. With the tires inflated to 130 psi your crr is .0048 and your speed is 27.73 mph. In the 115 psi case, you barely hold on to win. In the 130 psi case you get swallowed up well before the line and are spit out the back... finishing >200ft behind the winner. Still think .001 is trivial?


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PostPosted: Fri Mar 16, 2012 4:28 am 
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A major variable (i looked through SOME of the posts but didn't see it) is road surface. In my experience (76kg/170lbs), sweet spot is anywhere from 95-110 depending on how rough the road is. I tend to run tubulars 5psi higher w similar results.

Frankly I would never imagine any road surface that you could comfortably use 140psi (don't know about track). Or even 130.

I do hear bike shop talk of 160psi, etc., but the same chubby 45 year old also claims an average speed of 40mph....

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PostPosted: Fri Mar 16, 2012 2:12 pm 
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elviento wrote:
Frankly I would never imagine any road surface that you could comfortably use 140psi (don't know about track). Or even 130.

The problem is that it's ingrained in some people. I know some guys who run 130 to 140 because that's what they've always done.
They've moved from 18-20mm tyres where higher pressures were needed to 23s and kept the same pressures or even upped them because it "feels" faster. The roads aren't that great either.


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