Tyre Pressure / Rolling Resistance

Discussion in 'Cycling Equipment' started by Russ Reynolds, Jun 15, 2008.

  1. Russ Reynolds

    Russ Reynolds New Member

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    Have any of you blokes come across a graph that gives the tyre pressure verses speed tradeoff ie: the higher the tyre pressure the greater the speed due to less rolling resistance. I know there would be alot of variables like road surface, tyre compound, load etc but I'm sure Sheldon or someone must plotted a graph that shows that for every 10psi decrease in tyre pressure, you loose 'X' mph for the same amount of energy.
     
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  2. dhk2

    dhk2 Active Member

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    Not a graph, but here's some tire test data done on smooth rollers, with pressures at 120 psi.

    http://www.biketechreview.com/tires/images/AFM_tire_testing_rev7.pdf

    Towards the end on page 3 of the spreadsheets, the effect of increasing pressure to 140 psi is shown on a couple of tires. For the Velo Flex Record, the RR watts per tire drop from 12.1 to 11.8. Even for two tires, the savings is less than a watt, which just isn't going to make much difference at all in speed. According to the test results, going to latex tubes will save you a lot more, something like 2 watts per tire.
     
  3. alienator

    alienator Well-Known Member

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    A graph like that would be difficult, if not impossible, to make.

    First to cRR: cRR goes up with increasing pressure. That's always true. cRR will be dependent on road surface. Overall energy losses with increasing tire pressure are the result not of a decrease in a tire's cRR but are the result of tire/road surface impacts that cause the tire to either lose contact with the surface or to have a decrease in the resultant force at the centroid of the contact patch (for a given lean angle). This is distinctly different from losses due to increasing cRR because energy losses from cRR are the result of hysteresis in the tire carcass.

    Characterizing a road surface would require more than just defining roughness.
     
  4. Russ Reynolds

    Russ Reynolds New Member

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    Thanks fellas. In the table, I was suprised that Latex would make that much difference. Although I haven't really gotten into tyre science yet, I am starting to understand the cRR and energy losses. I haven't had to change tyres yet so haven't really experienced different tyre performances. A couple of questions though, what tyres and pressures do you both run and if I were to sneak into your bike shed in the night and let some air out of your tyres, how much would I have to let out before you would notice a difference.
     
  5. alienator

    alienator Well-Known Member

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    In my clinchers (23mm), I run 105 front, 110 rear. In my tubs (22mm), I run 100 front, 105 rear.
     
  6. Russ Reynolds

    Russ Reynolds New Member

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    So if I was to let say 5psi out of each on my midnight commando mission, do you reckon you would notice ?
     
  7. Phill P

    Phill P New Member

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    I don't pump my tyres for once week or so, and they drop to 80psi fairly consistantly if left over a week. Tend to pump on weekend rides and not week night rides. Often feel slugish on the week night rides, and just put it down to a bit of wind or being tired.
    I don't think the human body doesn't seem to be that well tuned to notice a difference as small as 5psi, but doesn't mean there isn't a difference.

    I have heard that pumping a tyre up too high actually decreases the real world rolling resistance. The ability to roll over small bumps and stones etc effects the real rolling resistance. The higher pressure means the tyre is forced up and backwards more when hitting a crack or bump, and if too high that and can cost more energy than what is gained else where. Measuring rolling resistance on rollers is not the same as chip seal roads!

    It also seems people of different weights prefer different pressures. I prefer to pump front and rear to about 110psi, but I havne't experiemented much.

    On the track They run super high pressures I've heard. More of a controlled environment than the road.
     
  8. Solanog

    Solanog New Member

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    I run both tubulars on 100lbs. I used to run them on around 120 psi, I think I get more punctures with the higher psi. No scientific evidence for this, it's just my impression. It seems to me that at higher pressure debri will go through the tread easier.
     
  9. ScienceIsCool

    ScienceIsCool New Member

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    @Phill P:

    It's not that the tire gets pushed up and backwards. It's a bit more subtle. Imagine a hypothetical wheel at infinite psi. Rock hard and will not deform.

    When that wheel encounters any surface irregularity, say a one millimeter bump, what happens? The entire wheel/bike/rider gets pushed upwards by one mm. The forward kinetic energy goes into lifting you up and is converted to potential energy. Unfortunately, not all that potential energy gets converted back into forward momentum as you roll off the bump. Especially if the bump is rather non-symmetrical and is followed by even more bumps in the road. So you lose speed. Deflate the tires a bit and the tire will deform around the bump rather than push you and the bike upwards. And you lose less energy.

    Compare that to a wheel that is pumped up to a more reasonable, but sluggish 70 psi or so. As you go over the same bump, the tire deforms around the bump and no energy is lost that way. However, the tire casing and tube are flexing and this also has a hysteresis. I.e., energy is lost. So you lose speed. Higher pressure and the tire deforms less and you lose less energy.

    So you have two competing mechanisms. One gets better with higher tire pressure and one gets worse. At some point there is an optimum based on the tire, the road surface, and the weight of the bike/rider.

    I think Continental put together some charts showing their results for optimal pressure...

    John Swanson
    www.bikephysics.com
     
  10. dhk2

    dhk2 Active Member

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    I run 23 clinchers, at exactly the same pressures as Alienator. Doubt I'd notice if you dropped the inflation by 5 or even 10 psi. From what I've read, reducing pressure all the way down to 85 psi only adds a few % to RR, and RR is a good bit less than aero drag at speeds above 10 mph.

    Have never tried latex tubes either, but may get some when I mount the EVO CX tires I've got in the garage. According to the charts, switching from my "slow" GP4000 with butyl tubes to the CX w/latex should save ~14 watts, a very nice power savings.

    However, must add that a tire test engineer I know cautioned me that testing tires on "small diameter" smooth rollers will exaggerate the differences in rolling resistance vs real roads, meaning the savings on the charts from running thin race tires may not pan out on the road.
     
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