Tyre failures and inflationary pressure

September 2, 2013 at 22:17 | Posted in bicycles | 2 Comments
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tyrebulge I have, in the past, been a vocal supporter of Conti SportContact tyres.  I’ve been running the 32mm wire bead version of this tyre on the fixie (and also a tourer, whilst I had one) for many years, with huge success. They are grippy, highly puncture resistant, they roll well and last for ever. I’ve had examples of these tyres that have lasted well over 10,000km, and even when the kevlar was starts to show through the rubber they just keep going.

Interestingly though, others have voiced a dissenting view – that they have found they just didn’t last long, and disintegrated on them rather quickly. Caveat emptor, and all that.

I have to say, my own faith in them was temporarily shaken recently. I was riding to work, and could hear this ‘tink tink’ noise every time I used the front brake. It was clearly something on the wheel, but even though I stopped a few times to have a look there was nothing obvious – no bit of gunge stuck to the tyre than might catch on the brake callipers, and nothing on the rims. When I got to work I had a proper look, and was somewhat alarmed by what I found. The tyre was bulging off the rim, and the sidewall of the tyre seemed to be splitting. Along the split there was threads hanging off the tyre, and it must have been these I could hear hitting the brakes. Even more worryingly, when I looked around the rest of the tyre, it seemed as if similar threads were coming away all around.

tyreshreddingThis was very very strange. This was the first tyre I had mounted since the bike got a new front rim. Could it be that the tyre was not sitting on the new rim very well, and it was somehow putting pressure on the sidewall? I took a look at the rear tyre, and was even more alarmed to see that it too seemed to be shedding threads. This is the original rim that came with the bike, which made the rim hypothesis rather less likely.

I hotfooted it down to the LBS to get their opinion – and a new tyre, so I could safely ride home that evening. I showed it to the mechanic, and he quickly said, ‘I’ll just let the air out of that tyre before it blows up in our faces…’ He examined the rim, checked the wheel for trueness and so on, but there was nothing obviously wrong. I showed him the back tyre, and he was also a bit nonplussed, although he did point out that the tyre was pretty worn. It hadn’t really occurred to me that they were getting worn out. I guess I’ve probably ridden about 5,000km on those tyres since they were new, so compared to my previous SportContact experiences they are just broken in. I mean, you can even see a glimmer of tread pattern in a few places!

I got a replacement tyre on the front (another SportContact – I’m not ready to give up on them yet). The LBS guy didn’t think the rear one was a problem right now, although he did recommend I keep a close eye on it. And then I rode home, contemplating whether getting 5Ks out of a tyre was reasonable, and/or if the stories I have heard about Conti’s quality control being a bit variable were true, and the experiences of some of my correspondents with these tyres.

And then I had another thought. Around the time I got those tyres, I also got a new floor pump, as my old one gave up the ghost. It was very cheap, but seems to work just fine. However, I have always had a nagging doubt about the accuracy of the pressure gauge.  At the shop, the guy had asked me what pressure I usually inflated it to, and accordingly pumped it up to 85psi using his fancy shop pump – the maximum recommended for the tyre, and what I usually do it to at home.

When I got back, I attached my cheapo pump to the valve, fairly confident that the tyre was at or around 85. I put in one small stroke of air to stabilise the reading, and…   my gauge only read ’60psi’. Ooops. So it seems that I have been significantly over-inflating the tyre for it’s whole life. Who knows what it was getting up to, but I guess it could easily have been in excess of 100psi. Could it be that this is the problem, and that the sides of the tyres are literally cracking under the pressure?

Time will tell if the new tyre lasts better now I have inflation under control, but I suppose the moral of this story is if you have a cheap pump, don’t rely on the gauge!

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