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Old 02-17-11, 08:45 PM   #1
uprightbent
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How can wider-lower psi tires REALLY be that fast?

I respect Bicycle Quarterly and love the technical articles. Their recent study of rolling resistance for wider lower psi tires seems to make sense, as many have embraced the 650 concept running 38's or 42's.

But on my rollers last night I pumped the tires (Pasela 28's) up to 90 after sitting for several weeks and was shocked by this simple change from my last ride. The resistance difference was huge, almost where I thought of dropping some air to increase my workout. I know this is no big discovery, but doesn't this simple "experiment", all sceince and technical jargon aside, demonstrate or prove that only narrower, higher psi's offer the least rolling resistance?
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Old 02-17-11, 09:00 PM   #2
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On a smooth surface, higher pressure equals more speed (given the same tire). That's why track tires are pumped to over 140psi. Shape of the contact patch and continuous contact with the ground is critical for speed over a rough surface.
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Old 02-17-11, 09:17 PM   #3
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As you said, it's all about rolling resistance. A stiffer tire casing resists changing shape more than a soft casing and so will take more leg power to constantly change its shape as it rolls. A soft casing will be less resistant to changing shape and therefore roll easier. But the less you change ANY tire's shape the less you'll have to worry about that and the easier it'll roll. The interesting part is the difference between skinny tires and wide ones. At an arbitrary 100psi per tire and a 200lb rider you're going to have a contact patch of 1 square inch each. On a skinny tire that contact patch is going to be elongated, effectively deflecting a greater area of the sidewall and compounding the rolling resistance of the casing over what you'd experience with a shorter contact patch on a wide tire. So higher pressure and soft tire casings will always have less rolling resistance than lower pressure and hard casings, when comparing the same widths. The reality of road conditions, though, dictates that I only ride my 170psi silk tubulars to the hospital... for a kidney transplant.
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Old 02-17-11, 10:25 PM   #4
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Rolling resistance is largely a function of tire sidewall deflection. At any given tire pressure, a wider tire will have less sidewall deflection than a narrow tire. You can achieve the same sidewall deflection in a narrow tire simply by increasing the pressure, but at the cost of rider comfort.
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Old 02-17-11, 10:42 PM   #5
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Roilling resistance is only partly about the tire. The road surface plays a HUGE part in it. To get max benefit from a 19mm tire at 160 PSI, you'd also need a freshly sanded velodrome surface. Given that tiny contact patch and correspondingly high surface pressures on the skinny tire, it kind of makes sense that a larger tire, with its lower contact pressure, rolls faster than you'd think; particularly on a rougher road. I *think* the narrower and stiffer tire is suffering energy losses due to being bounced around. Vibration equals energy loss.
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Old 02-17-11, 11:00 PM   #6
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A 4" diameter roller cylinder is going to deflect the tire shape completely differently than a flat road surface. Some roller manufacturers use even smaller diameter rollers to INCREASE the resistance and give you a harder workout. For making tire decisions for a bike I'm going to be riding on real roads I would go with the tests that most closely resembles my riding experience.
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Old 02-17-11, 11:19 PM   #7
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I just use take-offs and whatever's on sale, but this is interesting.
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Old 02-17-11, 11:30 PM   #8
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I ride Serfas Survivor 28's on my centurion, and Kenda Kwest 28's (which are bigger) on my schwinn, and the schwinn goes like a bat out of hell
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Old 02-17-11, 11:38 PM   #9
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I don't believe anyone's done a really academically rigorous, quantitative/qualitiative analysis of rolling resistance vs. tire size vs. contact surface quality. I would have no idea where to start.
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Old 02-18-11, 12:06 AM   #10
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there's some good data over in the long distance forum, but i'm too lazy to look for it.
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Old 02-18-11, 12:23 AM   #11
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"Wider tires roll faster than narrower ones: Riders have argued for years that narrower tires – especially on the road – roll faster and are more efficient than wider ones when in fact, the opposite is true. According to Wheel Energy, the key to reducing rolling resistance is minimizing the energy lost to casing deformation, not minimizing how much tread is in contact with the ground. All other factors being equal, wider casings exhibit less 'bulge' as a percentage of their cross-section and also have a shorter section of deflected sidewall." By James Huang, tech editor, in Boulder, USA

Very informative tire article by bikeradar.com.
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Old 02-18-11, 12:50 AM   #12
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Also you must take into account the tire compound.
Comparing wider touring tires vs race tires (even negating tread pattern adjustments) are not eliminating enough variables to make a conclusion.
Proof? A ceramic tube can be filled to 100 just like a fabric one could. Would they respond the same in contact with the road? Same weight and same pressure do not equal same contact patch as material deflection will differ.
I appreciate the article but its as much a scientific study as mythbusters. Conceptually strong but not scientifically conclusive.
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Old 02-18-11, 02:04 AM   #13
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Hey everyone,
Some of these points have been covered already, but here are some points for thought:

Rolling resistance is essentially the tire having to constantly "climb" the "bulge" and the imperfections on the ground.

The energy is dissipated when the rubber and the casing has to flex to climb the bulge.

as abarth pointed out, a wider tire has less of a bulge than a skinny tire.

Not only is there a "bulge" caused by the weight of the rider, but also, there are bumps that the tire has to climb when it travels over a bumpy road.

A lower psi tire has the ability to float over the bumps in the road. Imagine a soft tire hitting a rock. It will basically conform to the rock, and energy is not wasted trying to climb over the rock. Now imagine an incompressible tire. When it hits the rock, it has climb over it.

A softer compound rubber will have a very very small deflection. This probably adds to the rolling resistance.

Common misconception: The tire does not actually slide on the ground! I (unless you skid) The tire merely touches the ground at single point, then the point rotates out, and another point contacts the ground.


So basically, on an ideal perfectly smooth surface, an incompressible tire would have the lowest rolling resistance.

But in real world conditions, the resistance is a function of the unevenness of the surface, tire pressure, tire casing flexibility, and tire width.
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Old 02-18-11, 02:11 AM   #14
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Originally Posted by RobbieTunes View Post
I just use take-offs and whatever's on sale, but this is interesting.
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Old 02-18-11, 06:57 AM   #15
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Yet another article supporting wide tire = lower resistance.

http://www.cyclingnews.com/features/...f-wheel-energy
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Old 02-18-11, 08:41 AM   #16
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Quote:
Originally Posted by Captain Blight View Post
I don't believe anyone's done a really academically rigorous, quantitative/qualitiative analysis of rolling resistance vs. tire size vs. contact surface quality. I would have no idea where to start.
I was a liberal arts student, know nothin' 'bout this science stuff, but I have been riding for nearly 40 years as an adult. I feelthe 38mm Conti tires on my Hunqapillar roll just as fast as the 25mm Panaracer tires on my Trek. Like I said, that's just how it feels.

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Old 02-18-11, 09:21 AM   #17
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Next years TDF winner will be riding "Fat Franks". The year after that, the world changes.
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Old 02-18-11, 09:28 AM   #18
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I believe you're missing the weight of tire/wheel when comparing wider tires to racing tires. I imagine dragging that fat azz 650X42 plus attendant hardware around the French countryside for 3 weeks would take it's toll.
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Old 02-18-11, 09:33 AM   #19
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I believe you're missing the weight of tire/wheel when comparing wider tires to racing tires. I imagine dragging that fat azz 650X42 plus attendant hardware around the French countryside for 3 weeks would take it's toll.
The weight makes acceleration harder.

Bicycle Quarterly did rolling tests. There wasn't a big difference,
and what difference there was seemed to be mostly a question of how stiff the tire was.
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Old 02-18-11, 09:52 AM   #20
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Next years TDF winner will be riding "Fat Franks". The year after that, the world changes.
Last night I rode with the front group on my Thursday night ride -- on my Retrovelo. Even passed some people on the hill. When they asked me how I did that I said "It's a fast bike." "How can it be with those huge tires?" Then I told them I had 35psi in those Fat Franks.

I'm not a fast rider, and most of them can easily leave me in the dust when they want to. But the only advantage that I *feel* riding my Shogun with 27" 1 1/4" higher psi tires is the lower center of gravity, which has nothing to do with the tires and everything to do with the frame geometry.

I love the Retrovelo -- it makes the roadies think I am superwoman.
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Old 02-18-11, 09:58 AM   #21
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I believe you're missing the weight of tire/wheel when comparing wider tires to racing tires. I imagine dragging that fat azz 650X42 plus attendant hardware around the French countryside for 3 weeks would take it's toll.
Awesome, more opportunity to fuel up on cassoulet, Brie and red wine!
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Old 02-18-11, 10:17 AM   #22
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on my rollers last night I pumped the tires (Pasela 28's) up to 90 after sitting for several weeks and was shocked by this simple change from my last ride. The resistance difference was huge, almost where I thought of dropping some air to increase my workout. I know this is no big discovery, but doesn't this simple "experiment", all sceince and technical jargon aside, demonstrate or prove that only narrower, higher psi's offer the least rolling resistance?
You haven't demonstrated what the B~Q article was getting at. You took 28mm Paselas, tried them at 90psi, and tried them at something less than 90psi. Its the same width tire, same casing, same everything except you tried different pressures.

The B~Q article is talking about a comparison between a narrower tire run at a higher pressure versus a wider tire run at a lower pressure than the narrow tire. Here's the example they cite in the article:

Quote:
An alternative to sturdier casings is
running the wider tires at lower pressures.
This solution has been adopted
by some track riders, who run their 25
mm tires at 10 bar (150 psi) instead of
the 14 bar (200 psi) used for narrower
track tires.
those 25mm tires they're talking about are in comparison to what they claim is the "standard" racing tire width of 23mm.

If you want to do the same comparison at home you'd want to try a pair of 28mm Paselas at Some psi as a reference (call it 90 if you like), and then try a pair of 32mm Paselas at the Same psi, and at a lower Psi and see what you think the difference is.
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Old 02-18-11, 10:28 AM   #23
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Quote:
Originally Posted by Zaphod Beeblebrox View Post
You haven't demonstrated what the B~Q article was getting at. You took 28mm Paselas, tried them at 90psi, and tried them at something less than 90psi. Its the same width tire, same casing, same everything except you tried different pressures.

The B~Q article is talking about a comparison between a narrower tire run at a higher pressure versus a wider tire run at a lower pressure than the narrow tire. Here's the example they cite in the article:



those 25mm tires they're talking about are in comparison to what they claim is the "standard" racing tire width of 23mm.

If you want to do the same comparison at home you'd want to try a pair of 28mm Paselas at Some psi as a reference (call it 90 if you like), and then try a pair of 32mm Paselas at the Same psi, and at a lower Psi and see what you think the difference is.
B Q has done tests looking at things both ways. Tires can have too much, or too little inflation (Captain Obvious would be proud of me ). A lot more needs to be done to be able to quantify all the variables, but the old notion that "fat tires are slower" does not always hold true.
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Old 02-18-11, 11:08 AM   #24
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When I see Andy Schleck or Mark Cavendish line up with 700c x 32's, then maybe I'll consider it. My wife does with her Trek FX but I'm rolling along with 25's or 23's. Or, in the case of the Paramount, 7/8's
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Old 02-18-11, 11:59 AM   #25
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When I see Andy Schleck or Mark Cavendish line up with 700c x 32's, then maybe I'll consider it. My wife does with her Trek FX but I'm rolling along with 25's or 23's. Or, in the case of the Paramount, 7/8's
When Andy and Mark start doing the kind of riding I do, I'll start paying attention to what they use. I don't cruise at 30mph, weigh 130lb or have a team car following my every move. Based on experience, I'm riding 650x38b's (35mm really) at 50 and 60 psi, and they're plenty fast, exceedingly comfortable on what passes for pavement around here, and feel much more secure than hard skinny tires. But what do I know? I've only been doing this since before the Schleck bros, Contador, et al were born. And I stopped drinking the "racerboy" kool-aid a long time ago.

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