4

My bike originally came with 700x26c tires so I assume I can easily go to either 25 or 28

Actually the Shimano WH-R540 is 19mm which supposedly doesn't go below 28 but there is 25 on there now anyway and it seems to fit properly.

I assume 28 will be slightly more comfortable, especially at "only" 100psi vs the 125psi on the 25 (though that's max, so it could also be run at 100psi)

Is there any way to calculate the performance difference for the same brand of tires?

ie. Are we talking 0.5 mph loss or 1mph loss?

It looks like 28's also need their own width tube because the 25 tubes are not recommended to stretch that far, so I assume there is slightly extra weight there.

  • You will not notice the difference between 25 and 28. Tire pressure and tread pattern are far more significant in terms of rolling resistance, and the difference in diameter/gear ratio is too small for you to feel. – Daniel R Hicks Jan 1 '17 at 22:28
  • As to tube sizes, brands differ in terms of what size range they span, so there's no way to say whether the same tube will "officially" fit both a 25 and a 28. And even if the limits are "violated" slightly I wouldn't expect real problems. And the weight difference between the different sizes is less than a few swallows of water. – Daniel R Hicks Jan 1 '17 at 22:34
  • Also, and this is more for racers, pairing widths with rim shapes makes a big difference in areodynamics. – JohnP Jan 2 '17 at 0:09
6

Rolling resistance is complex and not easily calculated. Fortunately, the people at bicyclerollingresistance.com did actual measurements for different width versions of same tire. The short summary is that

  • Wider tire has lower rolling resistance at same pressure
  • The maximum and useful pressures are lower on wider tires
  • When each width is inflated to correct pressure (which is far above 60 psi for 23mm tires), there is very little difference in rolling resistance between different widths.
  • EDIT: At their test setup, 18 mph and 94 lbs load, the power lost to rolling resistance per tire is 12.9W for 25mm@100psi and 12.7W for 28mm@80psi (pressures picked based on what I use). The total difference, 0.4W for both tires together, makes approximately 0.02Mph difference in speed if everything else (including air resistance, which isn't exactly true) is kept the same.
| improve this answer | |
  • 3
    The OP's question asked about speed difference. Can you provide a range for the difference in Crr for 25mm vs. 28mm tires, and then translate that difference into a range of speed differences? – R. Chung Jan 1 '17 at 18:34
  • Answered. To simplify things, I used the rolling resistance watts directly and plugged numbers to bikecalculator.com. – ojs Jan 1 '17 at 18:48
  • 2
    All of these tests are on drum rollers. The rolling resistance that results from vibrating a large bag of water (people) on top of the bike is not quantified. For rougher roads the benefit of larger tires at lower pressure will be under represented. As such, potential speed gains/losses from a switch to 28c cannot be answered without additional information. That said, the tests are great to see which tire will have lower hysteresis losses at lower pressures (i.e., good for comparing similar tires), but real world results will likely differ because of the rider/bike interaction. – Rider_X Jan 1 '17 at 19:21
  • I am aware of the claim. Are there any measurements or even plausible calculations about the effect? – ojs Jan 1 '17 at 19:23
  • 1
    @R.Chung It also depends on the roads, the Silca Journal rolling resistance study inspired by Tom Anhalt findings which showed a substantial break from the theoretical Crr curve when "impedance losses" (aka rider/bike interaction) overwhelms hysteresis losses (tire flexing that drum rollers measure). Suggesting that you have to know what conditions the tire will be used under before you can predict rolling resistance. – Rider_X Jan 1 '17 at 21:48
4

You have a tiny change in gearing, which you can calculate by measuring the outer diameter and plugging it into a calculator like this one -- the larger size will give you higher gearing (i.e. for a given cadence (# of times you turn the pedal per time unit), you'll go faster). You can use this calculator to calculate the difference in speed for a given cadence and gear combination and compare across the two tire sizes.

As for width, its a bit narrow for the rim, but you can technically get any 700c tire onto any rim. Running a too narrow tire though increases risks of pinch flats/rim damage. Running too wide a tire increases sidewall/rim failure risk, plus sloppy handling. That being said, this isn't an egregious violation, so you're probably fine. Make sure your frame+fork clear this bigger tire size with an appropriate margin (depending on your preferences, this could be 3-5 mm as the minimum distance between the tire and the fork/frame).

As for tube sizes, you can violate the marked sizes by a few sizes. A tube is a balloon -- you're just inflating the balloon a bit more than it was designed for, so it will be fine. If you go a lot more (say a 23c tube in a 5 inch tire), you'll have problems. You can probably put your existing tube in the new tire and have it work fine.

As for tire pressure, the thing written on the sidewall is an ad-hoc number (drawn jointly by the legal and marketing departments). Most people should be running a significantly lower pressure than what is marked for a properly inflated tire -- the tire deflects a bit, but has good rolling resistance and hazard+shock absorption properties. Even if you're ~250 lbs with bike+yourself, you're not going to need 100 PSI on a 700x28. And if you're getting near the max pressure, chances are you need to move to a bigger tire size anyway.

Finally, the real answer: If you swap between 25 and 28 and set the tire pressures appropriately, most people (esp. non-racers) will not notice a difference in performance. The 28 will be a bit more comfortable since its running at a lower pressure. For a 50 x 11, at 80 rpm, you're looking at 28.5 (25c) vs 28.9 mph (28c). For a 50 x 23 at 80 rpm, you're looking at 13.6 mph vs 13.8 mph. In any case, unless you're spinning out on top gear, you're not going to notice.

| improve this answer | |
  • 1
    Do you have source for the ad hoc claim? – ojs Jan 1 '17 at 18:20
  • 3
    @ojs - Its in the Sheldon Brown link for "properly inflated tire". Search for "Lawyer" on that page. – Batman Jan 1 '17 at 19:15
  • 1
    @Batman so, you are relying on Sheldon, who does not have a source. – ojs Jan 1 '17 at 19:24
  • 4
    What do you want? An admission from manufacturer X saying "We set this number based on legal and marketing advice?" Sheldon is generally a pretty authoritative source. At the end of the day, pretty much everyone agrees that you should set your tire pressures via trial and error, not on what the markings are. – Batman Jan 1 '17 at 19:33
  • 3
    @CrazieDaizee - negligible. If you want to save 200g on the bike, eat a bit less food or something -- your body weight variation on a decent length drive (along with water bottles) will be far over 200g (1 cup of water is about 225 grams of water). The other option if you really want to save 100 g is to buy a lighter tire / wheel/ other components. – Batman Jan 1 '17 at 20:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.