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Author Topic: Why do my bike tyres go flat when I don't use it?  (Read 33419 times)

paul.fr

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I have noticed that when i use my bike daily, the tyre pressure is constant and does not need any extra inflating. Yet if i leave it for more than a week, when i get the bike out, the tyres need inflating as they are almost flat.

Curious, Anyone have an explanation for this?
« Last Edit: 29/07/2007 22:21:09 by chris »


 

Offline neilep

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Re: Why do my bike tyres go flat when I don't use it?
« Reply #1 on: 25/07/2007 17:06:58 »
yes I do.

use of the wheel heats up the air inside and stops it deflating, maintaining the status quo....when not in use the extra pressure caused by the heat is absent and therefore the tyre slowly deflates as normal...

........well..that's my amateurish guestimate anyway.
 

another_someone

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Re: Why do my bike tyres go flat when I don't use it?
« Reply #2 on: 27/07/2007 23:41:38 »
I think the pressure within the tyres help seal the tyre against the rim of the wheel.  So long as the pressure remains high (by maintaining the heat, as Neil suggested, and by sitting on the bike to further increase the pressure), it helps maintain the seal, and so keeps the air in better.  If the pressure reduces, the seal between the tyre and the rim is less efficient, and so more air escapes, and as this happens, the seal becomes ever more ineffective, so creating a downward spiral.

Well that is my guestimate, anyway.
 

Offline eric l

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Re: Why do my bike tyres go flat when I don't use it?
« Reply #3 on: 28/07/2007 09:47:59 »
If the pressure reduces, the seal between the tyre and the rim is less efficient, and so more air escapes, and as this happens, the seal becomes ever more ineffective, so creating a downward spiral.

So far, bicycle tyres are not tubeless, and there is no seal between tyre and rim.
One possibility I see is that as Paul's bicycle remains standing over a longer period (e.g. a week) the local deformation where your wheel is in contact with the floor may cause some local porosity in the rubber of the tube.  (I suppose you leave your bicycle standing, not hung up).
Anyway, I use my bicycle every day for short distance in town, and have to inflate my tyres about every 10 days because I want to keep the pressure high as I have to ride over cobbled streets and so on.  A tyre that could stay inflated simply by riding would come in very handy.
 

Offline lightarrow

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Re: Why do my bike tyres go flat when I don't use it?
« Reply #4 on: 28/07/2007 17:35:34 »
I think the pressure within the tyres help seal the tyre against the rim of the wheel.  So long as the pressure remains high (by maintaining the heat, as Neil suggested, and by sitting on the bike to further increase the pressure), it helps maintain the seal, and so keeps the air in better.  If the pressure reduces, the seal between the tyre and the rim is less efficient, and so more air escapes, and as this happens, the seal becomes ever more ineffective, so creating a downward spiral.

Well that is my guestimate, anyway.
Maybe your reasoning is correct but with the difference that it's the sealing of the air valve.
 

paul.fr

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Re: Why do my bike tyres go flat when I don't use it?
« Reply #5 on: 28/07/2007 22:23:10 »
  (I suppose you leave your bicycle standing, not hung up).


You could be on to something here, Eric. When i leave the bike stood up, IE back wheel on the floor and the front in the air, the back wheel does lose more pressure than the front.
 

Offline chris

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Re: Why do my bike tyres go flat when I don't use it?
« Reply #6 on: 29/07/2007 22:20:02 »
I don't agree with the pressure hypothesis. Increasing the pressure will encourage more gas to escape, not less. The amount of heat generated in the tyre is not sufficient to account for this, and dissipates rapidly. Plus you spend more time not on the bike than on it (unless you're a Tour de France professional, or a courier or something), so the effect would be minimal.

Instead I think it's to do with tyre deformation. There have been studies showing that bikes left standing on the ground develop flat tyres, whilst the same bike hung up maintains its tyre pressure in the long term.

Engineers think that the deformation (flattening) of the rubber where the tyre meets the ground when the wheel is "parked" for an extended period in the same position leads to the opening up of tiny pores in the rubber allowing the gas to escape.

Using the bike regularly, on the other hand, means a different part of the tyre is at the bottom of the wheel and therefore contacting the ground each time the bike is parked. This prevents channels opening up in the rubber and so the tyre stays inflated for longer.

Chris
 

Offline qazwart

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Why do my bike tyres go flat when I don't use it?
« Reply #7 on: 30/07/2007 04:23:23 »
All tires leak air. Rubber is composed of long elastic fibers, and there are gaps between the fibers where air escapes. That's why a rubber helium balloon will quickly lose its ability to float after a day or two while a mylar balloon will stay up for a week or more.

When you put a bike in the garage for a few months, the tires leak air, and no one is topping up the tire when they notice it is low. By the time you take it out of the garage, it is completely flat. If you constantly are riding the bike, you're constantly topping off the tire whenever you notice it is a bit low, and that tire never goes flat.

Therefore, when you constantly ride a bike, the tire never seems to go completely flat. But, when you don't ride it after a while, it goes completely flat.
 

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Why do my bike tyres go flat when I don't use it?
« Reply #7 on: 30/07/2007 04:23:23 »

 

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