What factors influence the maximum tire pressure other than the tire itself?

Question

On the tires of a bike, there is a label which shows the maximum allowed pressure for it. However, I am wondering if there are any other factors that affect the maximum allowed pressure in a tire?

• Does the innertube also has a "maximum allowed pressure"?
• I need a rim tape to cover the spoke holes in my rim. Does this lower the allowed pressure?
• Any other factors?

_{Background: In the last week, two innertubes on two different bikes exploded. They were both under the maximum allowed pressure. I suppose one was old and the other had a manufacturing error. One was exploding during cycling, the other one during inflating the tire. I am now quite afraid of inflating tires, so I want to make sure that this will not happen again.}

Answer 1

The usual cause of inner tubes exploding while being inflated is that part of the tube is pinched under the rim, or was damaged by a tyre lever while you were fitting the tyre (but manufacturing flaws can happen - I once had a puncture in a tube next the stem where it was difficult to patch, and my spare tube failed at the same place as soon as I inflated it (and there wasn't a sharp edge on the hole in the rim)).

But apart from that, if the tube and tyre are properly seated, and the tube is the right size, the tube shouldn't limit the pressure.

Rim tape can make a difference if the wrong sort is used, but shouldn't otherwise, see http://www.schwalbe.co.uk/_webedit/uploaded-files/All%20Files/Technical%20Info.pdf and search for "Which rim tape should I use".

Tyre sizes and rim sizes vary slightly, and a combination that is a slightly loose fit will let the tyre blow off at a lower pressure. The label allows for that, at least to a certain extent (see http://sheldonbrown.com/tires.html#pressure and scroll down to "Pressure Recommendations").

Using a tyre on a rim much narrower than it is intended for can make the rim fail and let a tyre blow off: http://www.sheldonbrown.com/tire-sizing.html#width. So can using a badly worn rim, of course.

Answer 2

Innertube failure can be caused by a variety of factors, and it is usually possible to determine the cause by examining the failed innertube.

You mentioned: "In the last week, two innertubes on two different bikes exploded". If it literally exploded into many small pieces, then you were probably the target of a saboteur. :) More likely, the innertube has a single tear or puncture, and its size and location will help you track down the problem:

• A small puncture along the outer side is usually caused by road debris. Since you ask about tire pressure, it's worth mentioning that high pressure can increase the chance of puncture.

• A tear at the valve can be caused by the age of the innertube or by poor inflating technique--using a hand pump without holding the valve steady can easily tear an innertube. This problem is not directly affected by tire pressure (but trying to reach a really high pressure with a hand pump could increase the risk).

• A tear on the inner side of the tube can be caused by spokes and may indicate a problem with rim tape or low-quality rims. Note that low-quality rims can limit the maximum tire pressure. I had a mountain bike (used for city riding) which had high-quality tires with a maximum pressure around 90 psi. However, the rim left the spokes very exposed, and inflating above about 45 psi would cause the innertube to stretch near each spoke and eventually lead to a tear. There's only so much rim tape can do.

• Two small holes in the tube are usually pinch flats, which are caused by tire pressure being too low.

The main factor limiting tire pressure is the tire itself, but some problems can be caused or exacerbated by tire pressure.

Answer 3

The limiting factor for tire pressure is the tire . The manufacturer designs the tire with a maximum pressure the tire case will be able to safely control. Exceeding the recommended pressure can result in the bead being forced off the rim or the the tire case itself could fail.