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Road bikes: Tyres and pressures


Capri Wheeler

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Does the saying "a chain is as strong as it's weakest link" apply to tyre pressure.

 

I race on Easton Tempest II Tubbies. The wheels can be pumped to 11 bar i think and the tyre I use (schwalbe something) says it can be pumped to like 7 or something.

 

Obviously I want to pump the tyre to as high as I can. What would you recommend. WHat diff will it make. Im sure you guys are all getting my feel here

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Tire pressure and the width of the tire is linked to each other..

As example MTB tire pressures is much lower than that of a road bikes.

 

Essentially it has to do with the surface contact aria between the raod and the tyre.

Generally, the area of contact will be directly proportional to the weight load on the tire, and inversely proportional to the inflation pressure.

The tire construction also has a great impact on the inflation and handling at pressures.

A wheel load of 50kg and a width of 50 mm should be pressurized at 3 bar

From this you can extrapolate the required tire pressure  for your wheels

Ie.

At 50 kg and width of 20 mm pressure should be 8.66 bar

At 35 kg and width of 50 mm pressure should be 2.33 bar

etc, etc, etc.

Effectively the tyre inflation should give you adequate support and enough comfort.

the issue if tire has more or less rolling resistance at the same pressure is another story.

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On a simple smooth drum test, tyres with higher pressure show a lower rolling resistance.

There was, however, another test (I think I read it on the Zipp site) that used a rough drum that better approximated a real road surface. This test showed that, above pressures of about 110psi, rolling resistance starts to increase, regardless of the type of road tyre.

 

Edit: here's the link: http://www.zipp.com/support/askjosh/tires.php#

Edman2009-05-20 01:13:26

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Ask the guy who pumped his Continetal Giro tubby (rated to 10bar) to 14 bar in EP Champs 2 weeks ago if it's a good idea to go over the pressure rating smiley18.gif

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Tyres shouldn't be pumped to maximum just because they can.

 

When riding on hot days the pressure will increase due to the extra heat, and if your tyres are pumped to maximum pressure they will eventually burst.

 

Pressure should also be regulated to a person's personal choice. I weigh 72kg and pump my tyres to app 7,5bar, as I find that I have a very hard ride at pressures higher than that.

 

When riding in a bunch and you are being dragged along in the slip stream, rolling resistance will mean absolutely nothing. The only time that rolling resistance will help is in individual events like TT, hillclimbs and on track etc.

 

I would suggest you find a pressure that you feel comfortable on and ride like that.
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sorry Speed Devil but I disagree. Even in a bunch you still have to pedal to overcome drag/ resistance and RR.

 

BUT...RR is not reduced with increased pressure past a surtain point. You get to a point where the tyre start "bouncing" on the road surface resulting in increased energy loss rather than reducing losses.

 

That's why it is not correct to look at those RR tests done on different brands of tyres on a smooth drum - no road surface is smooth and it is rather the interaction between the rough surface and tyre which results in a specific RR number i.e. tyre rolling over each rough edge/ tyre width/ pressure/ rider's weight/ tubby or clincher etc.

 

Also remember that RR gains/ losses are thrown out of the window as soon as you puncture - any gains made due to a low RR type of tyre (but easy to puncutre) won't make up the time (by 10-100 times!!!) 
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overinflated your tire will bounce on surface roughness.

This causes interruptions in traction, particularly during cornering so it is not a good idea...

 

That said the marketing/lawyers departments will allow for a "safe" zone on your tires maximum inflation and in the same voice they would want to give you the maximum allowable tyre pressure

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That said the marketing/lawyers departments will allow for a "safe" zone on your tires maximum inflation and in the same voice they would want to give you the maximum allowable tyre pressure

Don't blame the lawyers. It's sound engineering practice to include a factor of safety. I wouldn't be too surprised it you can get to at least 20% or more above max recommended pressure before something nasty happens.

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My experience - fully (but not over inflated) tyres are much less prone to puncture.

 

Buy for a MTB, where traction is most important, I run at much lower than the maximum pressure.
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  • 2 years later...

At Msunduzi I put a cheapie tubbie rated at 7.5 bar max on my Zipp 404. As I weigh 84kg and through bitter experience I know 7.5 bar is asking for a puncture, I pumped it to around 8 bar and raced on it - no issues (legs a different story though).

 

Race done, I put said Zipp on the plane, but neglected to deflate it pre-flight (dumbass). When I took the wheel out on Saturday for WP Champs the tire had an ominous kink in it at the valve. At this point I should have put my training wheel on the back (dumbass again). The tire didn't last 10k's before it blew

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A quick question for all the master roadies out there.unsure.gif

 

I'm new to road cycling, started cycling on a MTB a couple of years back and still continue to do that as my main discipline.

 

However I have started to dangle on the french side of things and bought myself a Scott 2011 CR1 comp with the new 105 group set coming in just over 8kg's stock.

 

I must admit, from now on I have some serious respect for all you roadsters out there.thumbup1.gif

 

To get to the point, my weight is +-69 kg's and am currently running my tires at 100 psi front and back.

 

What would be the recommended tire pressure and difference for front and back for my weight considering the same rules applies to MTB.

 

I have found 110psi is a bit too bumpy, and on 90psi it feels like I'm floating in the smoothness.

 

 

Any suggestions / comments would be appreciated.

 

,Dub

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There is a reason why tyre manufacturers cleary state the min. and max. tyre pressures on each tyre. Veloflex tubbies for example suggest a max. pressure of 9-10 bars on their 22mm tyres regardless of what you weigh. Some pros run their tubbies as low as 6-7 bars.... according to their body weight. I cannot believe that some nutters go as high 12 -14 bars, especially considering the kind of road conditions we have in this country.

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I have always used 7 to 7.5 bar pressure for my road tubbies in dry weather. Harder than that made the ride feel too bumpy imo. Went down to 6 bar if a race started in the rain.

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Ektually, tyre pressure is determined by a dynamic process called "sag" (also known as tyre drop), similar to the way you set sag in a MTB suspension. So the tyre manufacturer will have an optimal sag for each tyre they produce (typically 15%) and it is up to the user to attain that sag percentage. So rider weight is crucial as well as wheel loading (front/rear bias). There are ways of measuring wheel load to assist with getting the right tyre pressures, but front is always lower than rear.

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