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Identical tyres, identical rims, minimal braking on the back wheel, no skids, properly inflated at least once a fortnight to manufacturer's recommended pressure.

Why does the back one wear out more quickerly than the front?

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Have you seen the size of your calf muscles? –  Moab Jun 30 '11 at 1:55
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4 Answers

up vote 2 down vote accepted

I suspect that "squirm" has something to do with it. As you pedal, the rear tire is forced first left, then right, slightly out of line with the direction of travel. (The front tire, OTOH, gets to pick its direction and force the rest of the bike to follow.)

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@Daniel R Hicks you are onto something with this answer, 'squirm' is a brilliant word for what you describe. The more popular answer is complete 'red herring' and shows how that 'research can be misunderstood', 'simple questions can be misunderstood', 'the crowd defers to the citation to authority' and how 'the crowd wisdom can be completely wrong'. –  ʍǝɥʇɐɯ Jun 30 '11 at 15:27
    
The tire moves because it's propelled by the drivetrain, regardless of which pedal one is using at that exact instant. Not seeing how the tire can move in any direction other than forward (or backwards, if you have a fixie). Unless you mean that the bike leans side-to-side, which is a different thing--but the front and rear tires lean together in that case. –  Neil Fein Jun 30 '11 at 17:24
    
When you exert downward force on the pedal it forces the rear tire sideways. You instinctively turn the front wheel in line with the force (to maintain a "straight" trajectory), but the rear wheel can't do that. –  Daniel R Hicks Jun 30 '11 at 18:49
    
I think this is the same thing, inasmuch as the fact that the tire deforms under the force introduced by your pedaling. The primary direction the tire is forced as you pedal is backwards, and the "equal and opposite" force from the roadway is wearing away bits of the rubber. –  lantius Jun 30 '11 at 20:35
    
If you observe another bike from the rear while the cyclist is pedaling hard on relatively soft tires, you'll see the tire deform first to one side, then the other. The thing is that this "squirm" produces an uneven "tearing" force on the tread, whereas simple forward motion produces a force that is evenly distributed across the contact area and much less inclined to produce the bits of slippage of rubber against asphalt that causes wear. –  Daniel R Hicks Jun 30 '11 at 22:32
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Kerry Irons put together an impressive breakdown of tire wear data.

His read on it is the fact that the rear tire is the primary driving force and thus the main point of power dissipation is the reason for the increased wear:

Force per unit area grinds off the rubber, so higher rider power and lower contact area increase the rate of wear.

This makes more sense than the weight ratio, when you consider the weight distribution is perhaps 20% higher on the rear while the ratio of wear is significantly larger.

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...'squirm' could do with a mention? Seems your answer is a 'false dichotomy' without it. –  ʍǝɥʇɐɯ Jun 30 '11 at 13:38
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Thus, consistent tire rotation is key to tire longevity.

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The only form of rotation I'd recommend is swapping the half-worn front tire to the rear and putting the new tire on the front when replacing a worn-out rear tire. You always want the better tire on the front since front tire blowouts are much riskier than rears. –  jefferee Jun 30 '11 at 13:45
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I rotate my tires every time I ride my bike. –  Daniel R Hicks Jun 30 '11 at 15:59
    
Vote down??...guess I assumed too much commonsense here!! –  GuyZee Jul 11 '11 at 17:29
    
What's good for a car doesn't necessarily work on a bike. As jeffree stated, rotating the tires - as in swapping the front and back tires - is a bad idea, and here's why. –  Neil Fein Jul 20 '11 at 4:47
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OMG...alright guess a detailed explanation is necessary!! As the rear tire reaches the point it where it needs to be replaced, typically the front tire is no longer offering peak handling. What I do is move the front tire to the rear and replace the front!! By doing this, I have found that I am able to get the absolute maximum miles out my rubber and still maintain optimal handling!! Rubber side down –  GuyZee Jul 20 '11 at 13:49
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The typical tire wear on a unicycle can provide some illumination here.

Despite there only being one wheel, and the mass bearing down upon it being continuous, a unicycle tyre will wear down unevenly.

This is because on a standard unicycle (i.e. not a "giraffe" unicycle, with a chain), the same part of the tire is touching the ground each time you reach the same phase of the pedalling.

Much more wear occurs in the two places where the pedals are mostly level - i.e. at the point where the legs can give the strongest downstroke.

It is clear that the acceleration being applied to the road is a significant cause of the wear.

A second factor is that unicyclists tend to "idle" (i.e. rock back and forth on the spot, in order to remain stationary) more often (generally exclusively) with their dominant leg down. So, it isn't surprising that one of the two sides wears more frequent than the other.

The result is that unicyclists need to, counter-intuitively, periodically rotate their tyre - move the tyre relative to the rim, so a different section is area of heaviest wear.

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most interesting! What I do not know is whether you can get 'slip angle' on a unicycle (because I don't ride one or probably ever will). In your riding style, when you wobble about (as happens on unicycle) is that more pronounced as certain parts of your pedal stroke? Does the wheel go in an exact straight line in the direction of your travel or does it weave? –  ʍǝɥʇɐɯ Jun 30 '11 at 17:05
    
Beginner: Very wide weave. Typically riding: prominent weave. Amazing riders: can ride along very thin poles, with negligible weave. –  Oddthinking Jun 30 '11 at 17:09
    
As you push down, the wheel has a tendency to turn around your leg - i.e. weave to the right as you push down with the right. –  Oddthinking Jun 30 '11 at 17:33
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