Tyre pressures

[maximillion]

I have recently been told that the cause of my constant punctures is down to too low tyre pressure. I bought a new pump with a pressure gauge.
My tyres are Continental Ultra Race 700x 25c and they have 120psi written on the wall. I have got to about 60psi and then the tube explodes.
Any idea what I am doing wrong?

Cheers
Max

[gilesjuk]

Not sure of the cause. But I've never had any luck running that kind of pressure with repaired tubes, the patches come unstuck.

Where are the tubes failing?

[thirdcrank]

Is the tube nipped under the tyre bead? That would cause a real bang.

It is possible, with sometypes of pressure guage, to measure the pressure you have added, rather than the actual pressure. So, if your tyre was quite hard at a true pressure of say 90 PSI and you pumped it up to 60, it would then actually be 150. Even then, I don't think that would be high enough to cause the problem.

If you were grossly exceeding the right pressure for the tyre, it would be the tyre that was being damaged. If the tyre is OK but the tubes are being damaged, it is something to do with the tubes. The consensus seems to be that if you get a bang from the tube, it is caused by a bit expanding outside the tyre.

The way low pressure most obviously causes punctures is 'snakebites' - the tube being nipped between the rim and a hard object such as a kerb or a stone, when the tyre pressure is inadequate to prtect the innertube.

[CREPELLO]

You might want to check the condition and seating of the rim tape. 60psi does seem very low for a ripped tube. Have you tried one from a different batch/brand? It would be useful to know where the rip is occurring though.

[glueman]

What everyone else said. Modern 25mm tyres should be well capable of high pressures. Athough I use a gauged track pump at home the 'slight indentation of tread with thumbs' method of 35 years usually comes out at 85-95psi which I find sufficient with narrow tyres.

[prm]

Fist port of call would be make of tubes. Not unknown to inflate some brands to 70-80% of rated tyre pressure, park up, and then explode at random.
Cut through damaged tubes to check if wall thickness varies. Noticed several failures occur within 6 inches of valve core after initial inflation.

To rule out any irregularities with rim, bead, tyre wall, spokes/nipples and spoke tension.. Mark or place small strips on insulation tape on inside of rim, tube and tyre.
Mount tyre and tube, roughly aligning markings, and check to see if flat occurs at the sane position.

Checking with an alternative gauge may confirm inflation pressure

[Deckie]

Before you worry about pressure guages, I'd check the basics:

1. Have you pinched the tube in the tyre rim while fitting? You can tell this f the tube has split along its side. It may also have attempted to lift the tyre off the rim before exploding.

2. Are the spoke ends and eyes all completely covered by a good rim tape? Remember the tube will be exerting as much pressure against the rim as the tyre and any exposed edges will cut the tube. If in any doubt get some new tapes.

[Ivor Tingting]

You can of course use double rim tapes but be careful the second one doesn't foul the rim edge thus preventing the tyre beading from bedding down properly. At one time I used to always use 2 rim tapes on each wheel but have now reverted to using just one with handbuilt wheels.

[thirdcrank]

Exposed spoke heads are a very obvious cause of punctures - even a split plastic rim tape around a spoke hole can cause one, but if by explosion you mean a loud bang, I think it is something else. Another source of repeated punctures can be not removing the cause of an earlier puncture from the tyre. A tiny shard of glass can go undetected to all but the most thorough of examinations, but again, no explosion. If the tyre is undamaged and you are getting a loud bang, it's hard to imagine anything else but the tube being nipped under the tyre bead.

[rogerzilla]

A tyre can blow off the rim even if the tube isn't trapped, but this is typically at about twice the maximum recommended pressure on the sidewall. Alternative causes include a non-hook bead rim (rare) or a rim that has been splayed by a pothole such that it can no longer retain the bead properly.

[maximillion]

Hi
Thanks for all the advice.
I have checked the rim tape and for foriegn bodies inside the tyre.
The tube I bought last was a slime tyre which the guy at Halfords advised to pump to 100psi.
When I get to 50psi the pump seems unable to push anymore air into it. The tyre feels really hard so I am confused as to how I can possibly get any more pressure in there. What with the last same tube exploded a big hole close to the valve.
Cheers
Max

[quiksilver]

I had a similar problem with a 27" wheel, it would take air but then suddenly pop or puncture when the bike was moved. After checking I found it to be a sharp edge on one of the spoke nipples inside the rim. I filled down the rough edge and put a small dab of silicone on the nipple, after that it was fine.

[thirdcrank]

If you are using a typical modern pump with a push-on adaptor as part pf the pump, it may be that as you are riving it about to try to get more air in you are dislodging the valve. That would obviously cause all the air to escape but not with a bang. In a 25mm tyre, 60 psi is nothing. It may feel quite hard but it is inadequate for such a narrow tyre. It may be that your gauge is way out and that you are actually getting a lot higher pressure but in spite of claims made by manufacturers, few normal pumps could achieve enough pressure to burst a tyre, as opposed to a tube.

[DaveP]

You said your efforts were peaking at 50psi - a figure which could be significant.
A lot of modern compact pumps fall into one of two categories: mountain bike pumps, characterised by short fat bodies and good at filling a fat tyre to about 50psi, after which things get very physical, and longer slimmer pumps designed to make it easier to reach higher pressures - smaller piston face therefore less force required.
One of the consequences of struggling to reach high pressures with an unsuitable pump is that the valve can get yanked about a bit, damaging the tube near its mounting point...