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I bought new wheels for my roadbike (shimano wh-rs80 which have c24 alu/carbon rims). I was mounting tires on them yesterday and inflated them to 8 bar (~116 psi). After a few moments (30 seconds or so), the tire started to slide off of the rim at a certain place, resulting in the inner tube to expand more than it could handle, and blow out.

I tried again with a new inner tube, checked that no part of the inner tube was stuck between the rim and the tire, inflated to 4 bar or so, checked again, inflated to 8 bar, BOOM. Same problem.

The tires are almost new continental GP4000s 23mm (a few 100 kms on a fulcrum 7 wheelset this summer) which were stored for a few months without using them. The inner tubes are also continental.

I checked the inside of the rim and can not see any damage or anything that looks odd. There is a factory installed rim tape in place, all brand new.

I had this twice with the front wheel and the same tire. I could try the rear wheel and/or the other tire to see if there is any difference, but I am a bit reluctant to potentially destroy yet another inner tube at this point.

Am I doing something wrong? Did I miss something given that these are new wheels?

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    I'm not authoritative enough on the subject to answer, but I've had this happen when the bead looked good, but really wasn't seated well enough on the rim. I found that putting a little air in and then reseating helped. But your situation might be something else.
    – Ritch Melton
    Commented Dec 19, 2012 at 20:23

4 Answers 4

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Especially when seating a new tire for the first time (and probably more so on a "virgin" rim) it's wise to inflate to about 1/3 or 1/2 final pressure and then roll/bounce the wheel around quite a bit, to work the bead into position. Then deflate, break the bead loose all around, and repeat the process.

Then, as you're doing the final inflation, watch the joint between tire and rim all around. Usually there will be ribs in the side of the tire that should be a constant distance from the rim. If you see the ribs moving up or down relative to the rim, stop inflating and manually work that section of tire back even.

Generally once the tire has been ridden a few miles it takes a "set" and will then reseat relatively easily if you have to repair the tube.

But do note that there are variations in tires and rims, some due to spec differences and some due to manufacturer differences. It's possible in several scenarios to have a tire and rim that are simply not compatible.

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    Thanks, I did everything you said and even rode them on rollers a little bit a few times with half inflated tires to help working the bead into position. I have them inflated at 9bar now (just to see if it holds), and so far it seems to be alright. I did notice that the tires get on the rims much easier than on my other wheels, so probably this combination of tires/wheels needs more care than what I was used to.
    – Jan Van Besien
    Commented Dec 20, 2012 at 7:19
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I had a rim with similar behaviour. Every tire was very easy to install and remove by hand on this rim, but it had a problem: while inflating, there were always a spot where the bead "tried to escape" from the rim, and I had to reseat everything by hand, cautiously, while slowly pumping more and more air.

The final result was rideable, but quite unstable, and once during a race the tire bead popped at a certain spot, and I had to stop, lower the pressure, etc, losing precious time. There were no explosions, fortunately.

In the end, the problem was a MANUFACTURING DEFECT: the total rim diameter was about 3mm smaller than it should, due to some quality control mistake. I gave up using that rim, and it now lies around, useless...

I don't know if this is your problem, but maybe you could compare it side-by-side with other rims just to check it out.

Hope this helps.

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Sounds to me like the bead (the part of the tire which seats in the rim) has seen better days. This is like a thin steel cable molded in the tire. Cut a cross-section of an old tire which you would otherwise throw away with a hacksaw to see the bead.

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judge according to the tire you have used,the most possible is the carbon rim sidewall problem,the sidewall is not strong enough to withstand such high pressure so it enlarge and the problem happen,it should be the rim defect.

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    First, it’s not true that carbon sidewalls inherently can’t withstand high pressures. It’s all about the design, although some modern hookless rims are indeed only rated up to 72psi. Second, the rim in question actually had aluminum sidewalls.
    – Weiwen Ng
    Commented Jun 24, 2022 at 14:57

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