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Someone suggested recently that rather than using a fatter tyre in my search for fewer flats, I could use bigger wheels. Specifically, that switching from 700c to 27" (622 to 630) would make a detectable difference. Since I currently use 406 wheels on most of my bikes, switching to even 622 wheels would obviously make a huge difference... if this is correct.

What's the mechanism for the larger wheel getting fewer flats?


To clarify: I can't think of a mechanism, and I doubt that this theory is correct. So answers that talk about other causes are off topic, and if you want to show that this is wrong you need to demonstrate that using research or logic. I already think this theory is wrong, what I want are facts. And FWIW, changing from ISO 406 wheels to 622 or 630 means a complete change of bike.

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I can't see that it would make any difference. Did they happen to say why they thought this? –  PeteH Jun 5 at 22:40
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No, which is why I'm asking here. It's not obvious to me, put it that way. –  Mσᶎ Jun 5 at 23:23
    
Seems to me that the larger tire has a larger diameter and would have that much more area to pick up glass and nails. ;) (I can't see how it would make any substantial difference.) –  Daniel R Hicks Jun 5 at 23:50
    
The outer diameter of most wheels someone would run reasonably is nearly the same (but the width of the tire may be different). A lower pressure tire can avoid some types of flats, but increases likelihood of pinch flats, for example. The flat protection from external factors is mostly due to road conditions and the material in the tire and its construction (which is partially why racing tires pick up more flats than touring/commuting tires on the same rim). –  Batman Jun 6 at 4:19
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4 Answers 4

One possible mechanism that occurs to me is that the larger tyre may hit fewer small pieces of glass because it rides higher over corrugations on the road.

Using my well-renowned skills with paint.net I give you the following conceptual diagram:

two wheel sizes on road

The black is the road surface, magnified and exaggerated to show the normal corrugations. Not to scale (or in proportion) are two different sized bike wheels. Note that the green one hits the red shard of glass while the larger one doesn't.

The puncture mechanism being affected is that contact with the glass adheres it to the tyre either through moisture on a wet road or just a tiny point entry to the rubber as shown below. Once adhered every further contact with the road might push it further into the tyre, eventually resulting in a puncture. But the larger wheel rides higher over small bumps, so comes in contact with fewer small shards since they tend to sit at the bottom of the corrugations.

But the size difference is small. Assuming 23mm tyres on both the ISO 622 and 630 wheels the outside diameters will be approximately 668mm and 676mm (via BikeCalc), so at best the larger wheel will be 668/676 = 98.8% as likely to get a puncture by this mechanism. But compared to a 406 wheel at 497mm OD (with a 23mm tyre, unlikely though that is) the ratio is 73.5% (less than 3/4 as many punctures... of the ones that happen by this mechanism).

I doubt that a 1% reduction in frequency of one particular mechanism would be detectable, even if that was the dominant mechanism for getting punctures. I ride about 1.5 hours a day, 5 days a week, and get 2-5 punctures a year. Dropping that by 1% would mean I'd get one fewer puncture every 20 years (at best) making it 39-99 punctures instead of 40-100. The effect is so far down in the noise that it would be undetectable.

Further minimising the effect, most of my punctures are via bigger objects. Recently they've been the shaft of a pop rivet (622x28 non-puncture resistant tyre) and a carpet tack (406 marathon plus), with the last glass shard puncture being late last year (406 marathon). So in real life it's might only be a 1% reduction in 1/3 of the punctures.

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It get on the order of a couple flats a year, and most of them have nothing to do with actual punctures by objects through the tread. It's either misaligned rim tape, bad valve, badly mounted tube (twisted), hitting a rock at high speed, or any number of other things. When I used really cheap tires, I got a lot more flats, but using quality tires, I almost never get an actual puncture. –  Kibbee Jun 6 at 13:00
    
I don't think your picture is accurate. It is not a contact point it is a contact patch. If both tires have the same pressure (and I see no reason they should have different pressure) both tires will have the same size contact patch. –  Blam Jun 6 at 19:06
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I seriously doubt wheel size has anything to do with puncture or flat prevention.

Common causes for flats are sharp objects, or in the case of snake bites, a pressure too low for the activity (type of riding).

In the case of sharp objects, they penetrate the tire casing because, for a moment, all the weight applied to the wheel is pressed against a tiny surface (for example a nail's tip), and the pressure surpasses the tire's structural resistance. Remember pressure is measured in force per area unit, so lets say you apply 50 pounds of weight to the front tire, and that the tire is a slick one inflated to 50 psi (pounds per square inch). Now the contact patch on a smooth surface will have roughly one square inch. Now roll the tire over a sharp nail tip, of surface (1/32 x 1/32 = 0.00098) square inch. This is a pressure equivalent to 51,200 psi (50/((1/32)^2). This tremendous pressure is enough to break the tire casing structure, so the object punctures it.

Now, if you change the size of the wheel, or only the tire size and apply the same 50 pounds of weight over the same nail, the pressure on the contact point will still be the same, since the nail's size is unaffected.

Instead of wheel size, many other variables can make a tire more or less puncture prone, for example thickness, thread count, rubber grade, structure. Some tires are purposely made to be puncture resistant, others are just like a thick(ish) balloon.

Some tires can escape puncture situation just because circumstances, for example a knobby tire that is penetrated by a sharp but short object. The object is so small that it doesn't get to the tube.

In the case of snake bites (a.k.a. Pinch flats), thick and hard casings tend to resist a little more, but they are not invincible. Always check tire air pressure before riding, and if you ride low pressure intentionally, ride accordingly: don't hit sharp edges and absorb shock with your arms and legs.

If you are suffering constant punctures, consider buying puncture proof tubes or specially belted tires, but also examine your path choices. If you are riding on the street, don't ride the gutter, because there is where all the trash accumulates (shredded glass, wire from overly worn car tires, etc.)

By the way: the effective size difference between 700c and 27" is only 4 millimeters, check Sheldon Brown's article on tire sizing: enter link description here. Changing a bike wheel size may just be a hassle, with no benefit at all.

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I don't think that a larger wheel will reduce flats. But I can think of one possible reason that it might work. For a given distance, a smaller circumference wheel will rotate more times than a larger circumference wheel. So, if one wheel has twice the circumference as another, each part of the wheel's surface will touch the road surface half as many times as the smaller wheel. therefore, that larger wheel will wear less than the smaller.

That being said, I don't think that tread wear has much impact on flats for bicycle tires (as it does for automobiles). But this is the only theoretical explanation I can provide.

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Your logic is completely valid for tire wear, but if a sharp object was in the way of a tire, the tire will be punctured no matter how many revolutions the wheel made, it would only change the puncture location. –  Jahaziel Jun 8 at 23:59
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I now get that I answered the wrong (opposite question)

The two most common types of flats are
1) pinch
2) puncture

Can also have the spoke puncture the tube from the inside but nothing a wheel diameter can do about that.

Not sure about puncture but I can think of a situation of pinch flat.

I know wheels don't come in a radius of 30 cm and 50 cm but for simplicity lets assume they do.

Assume you are going 10 mph and hit 10 cm curb

The tire / wheel must absorb the sine of the angle
The 30 cm tire will have a larger angle

Take it to extremes

  • If the wheel is infinitely large the
    sine of 0 is 0

  • If the tire has only a 9 cm radius it will hit the curb straight on and must absorb all the force.
    sine of 90 degrees is 1

If you rode a 30 cm and 50 cm over same curb at increasing faster speeds the 30 cm would bottom out at a lower speed.
Since sine is not linear the difference would not be 3/5 but somewhere around that.

Mountain bikers moved to larger tires as they roll over rocks and roots with less effort.
Less effort equates to less force.

Another answer noted more revolutions so more wear.

There is also a sharper angle on the contact patch.
This puts more stress on the rubber and casing (and more rolling resistance.)

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