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 Post subject: Re: Tire pressure advice
PostPosted: Mon Mar 12, 2012 12:35 am 
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Location: Bay Area, CA
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taken from here

High pressures work well on smooth drums, but as you can see, when high pressures meet the road you very quickly end up significantly increasing your rolling resistance.


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 Post subject: Re: Tire pressure advice
Posted: Mon Mar 12, 2012 12:35 am 


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 Post subject: Re: Tire pressure advice
PostPosted: Mon Mar 12, 2012 12:36 am 
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Joined: Sat May 21, 2011 3:30 am
Posts: 77
Having had personal conversations with team mechanics from Bissel and BMC, they almost never run over 120psi.

Just because you can inflate a tubular to 150psi +, doesn't mean it will perform better. Do a little google search and there are a number of optimal tires pressure articles. IF higher pressure=faster rolling tire, then why do riders at Paris Roubaix run around 80psi??????

Think of this...if you ever drive in snow, watch rock crawlers, drive trails with 4x4 trucks, etc, they run very low pressures so they tire can deform over the obstacles and provide better traction. (it don't drive a truck...by the way :) )

Counter-intuitive doesn't mean its false.


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 Post subject: Re: Tire pressure advice
PostPosted: Mon Mar 12, 2012 4:06 am 
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Location: Canada
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 Post subject: Re: Tire pressure advice
PostPosted: Mon Mar 12, 2012 8:47 am 
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Posts: 562
go4custom wrote:
Having had personal conversations with team mechanics from Bissel and BMC, they almost never run over 120psi.

Just because you can inflate a tubular to 150psi +, doesn't mean it will perform better. Do a little google search and there are a number of optimal tires pressure articles. IF higher pressure=faster rolling tire, then why do riders at Paris Roubaix run around 80psi??????

Think of this...if you ever drive in snow, watch rock crawlers, drive trails with 4x4 trucks, etc, they run very low pressures so they tire can deform over the obstacles and provide better traction. (it don't drive a truck...by the way :) )

Counter-intuitive doesn't mean its false.


Well, you are a little bit confused, I quess.

ad1) Paris Roubaix: rides have such low pressure because of compliance on the rough roads NOT rolling resistance
ad2) yeah, if you ride snow, you run low pressure because of better traction, it means you look for the highest rolling resistance as possible because snow is slippery


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 Post subject: Tire Pressure sweet spot
PostPosted: Wed Mar 14, 2012 5:22 pm 
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Posts: 16
Tire pressure...again...

There was a time (1990's when I raced a lot) when higher was better. It even carried over into mountain. Guys were running 60+ psi and bouncing all over the place. I never liked that. I rode around 30 and did great. I did, however, see how on road higher pressure could be better. I've run fairly high pressures, especially in races, and never felt hampered.

Now lower is better. There has to be a sweet spot. A point where rider weight, tire pressure, tire qualities, glue/tape, etc. all converge to be optimum. And, I guess, this changes with the road surface, too. How can I semi-scientifically determine my sweet spot? My nerves are not attuned enough to "feel" a significant difference between 95-130psi. That's a big range! Higher and I feel a harsher ride. Lower "feels" mushy.

I weigh 150 and on race day I ride Vittoria Crono's. Races are on good pavement, no cobbles, avoidable potholes, etc. I am fairly light on the bike, try to pay attention to the bike, and do not abuse equipment.

Also, I don't care about this issue for training. More resistance will only make me stronger.
Trainer wheelset is heavy and durable. As long as I'm around 100, I'm good.

So, any advice? (other than "just ride between 95 and 130, duh!")

Thanks!


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PostPosted: Wed Mar 14, 2012 6:07 pm 
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Location: California's country side
if you run the pressure too low, you can get a pinch flat if you hit a stone or lip on the road. usually the back tire will suffer from this.


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PostPosted: Wed Mar 14, 2012 7:36 pm 
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http://www.vittoria.com/tech/recom-tyre-pressure/


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PostPosted: Wed Mar 14, 2012 7:46 pm 
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For what it's worth, many tire manufacturer's publish recommended tire pressure charts taking into consideration the load and tire width, given their particular tire's characteristics. Just like there are tire pressure recommendations for your car (given it's weight and tire size). Of course, there's front/rear differences in weight, therefore tire pressure, if you go with that sort of system.

I've always done that and with my 23mm Michelin PR2's, I go ~100-105 in the back and 90-95 in the front and have never had a pinch flat (185 lbs total weight on the tires... I'm about 165, my bike's about 17 fully equipped, and I'm never naked). I don't know if these pressures exactly correspond to the Michelin charts or some other formula I've seen, but years ago, I looked at this stuff (the charts and formulas) and just settled on these pressures, and they've worked.

Is it the sweet spot? It is for me since my criteria are no pinch flats and minimal harshness in the ride (based on "feel" of course), and it achieves both. I have no way of knowing if I could go lower or if what pressure is actuallly optimal for rolling resistance. I would think that the only criteria anyone could use without scientific measuring tools is (1) lack of pinch flats and (2) feel - which is in turn based at least a little on personal preference .... some may actually like the "feel" of a hard tire and may "feel" it's faster.

I do know that I rented a decent road bike last year and the shop pumped the tires up max (~120 lbs on a 23mm tire) and all I knew was that it just seemed to ride "hard" which I can see how some would feel faster. I couldn't tell you if it was actually faster or slower, but was not as comfortable.


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PostPosted: Wed Mar 14, 2012 8:43 pm 
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camilo, Thanks for the insight. I agree that hard tires often make you think "fast." May not be the case, tho.

jdp, Thanks for the great site. I had not looked there. Especially liked their ideas at the bottom of page.

jdp211 wrote:
http://www.vittoria.com/tech/recom-tyre-pressure/" onclick="window.open(this.href);return false;


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PostPosted: Thu Mar 15, 2012 1:03 am 
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xyzgeoman wrote:
So, any advice? (other than "just ride between 95 and 130, duh!") Thanks!


On a smooth surface, higher pressure always has lower resistance. On a real road there is point where the energy lost due to vibration negates the gains you'd get from increasing pressure. This point will depend on your weight and the surface and the tire. Traction and handling are other things to consider.

You need to experiment and see... if there is a point where the ride becomes "harsh" you've probably exceeded the optimal pressure for speed as well as comfort.


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PostPosted: Thu Mar 15, 2012 1:25 am 
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Sorry, saw no reason to run 2 threads on the same thing, when the first was on the 2nd page.

Please 'xyzgeoman' :search:

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PostPosted: Thu Mar 15, 2012 3:07 am 
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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


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PostPosted: Thu Mar 15, 2012 3:38 am 
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Location: Belgium
Hi,

There is no such thing as a formula to define tyre pressure but you could use one as a starting point, I suppose.

There are a number of factors to consider, for example:

Load as seen by the tyre, both by the state of the road surface and the weight across the bike (front and rear).

Suppleness of the tyre; tpi cpount is a good indication.

Vertical stiffnes of the wheels and frame, etc. , etc....

Ciao, ;)

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Last edited by fdegrove on Thu Mar 15, 2012 12:09 pm, edited 1 time in total.

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PostPosted: Thu Mar 15, 2012 7:24 am 
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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; 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.


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PostPosted: Thu Mar 15, 2012 7:34 am 
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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.

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Posted: Thu Mar 15, 2012 7:34 am 


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