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I'm restoring my dad's old Jeunet racing bike from the seventies.

First thing I did was buy two new tires. The old ones looked still OK, but wanted to take no risk with 40 year old rubber. The new tires have exactly the same size as the old ones, and a pressure limit of 6.0 bar.

To make sure, I also bought two new inner tubes.

Then I started pumping the rear tire. The aim was 4.5 bar. It exploded at about 4.3 bar! The tire was thrown off the rim, but undamaged. The inner tube showed a large gash at the valve. I checked the rim for any spokes or other sharp objects that might be protruding into the tube, but couldn't find any.

Then I tried pumping the front tire, this time I put in earplugs. SAME THING! Explosion.

Could someone please tell me what's going on?

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  • Did the damage happen in the same spot?
    – L.Dutch
    Commented Jul 31, 2018 at 15:46
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    Was the valve a close fit in the hole in the rim, or loose? If loose it could be the wrong valve type, and the edge of the hole could cause damage
    – Chris H
    Commented Jul 31, 2018 at 15:50
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    I've experienced older rims letting go of the tire bead at well below the tire's max rated pressure - resulting in a noisy blowout of the tube, but no tire damage. Pic of the rim inner profile may help here. I assume your rims are 27". What width tire are you using?
    – Argenti Apparatus
    Commented Jul 31, 2018 at 16:11
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    I would have though a 70s bike would have 27" rims. Obviously they are 622 as you would not be able to get the tires on (27" wheels have 630mm rims). May be that your new tires just don't like the rims.
    – Argenti Apparatus
    Commented Jul 31, 2018 at 16:27
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    An actual racing bike from 70s would have tubulars. The "27 inch" tubular rim is 622 mm, but you should notice that something is wrong if you tried to mount normal tires on one. I suspect it might be a non hook edge rim, something that used to exist and does not work with modern high pressure tires. A photo of the rim would help a lot.
    – ojs
    Commented Jul 31, 2018 at 19:05

2 Answers 2

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You say that you have a large gash in the inner tube. And you say that the tire was thrown off the rim. I assume that the thrown off tire was the cause, not the result of the explosion.

The point is that it's only the tire that is expected to counteract the force of the pressurized air, not the inner tube. The inner tube is for sealing only. Thus, when the bead of the tire jumps out of the rim, this will instantly kill the inner tube: it's simply not designed to withstand the resulting forces. So, the question is: How can the tire jump out of the rim?

You say that you are refurbishing an old 70s bike. Afaik, there have been different ways that tires attach to the rim. I think I recall reading something about rims which were flat on the insides of their sides. Such rims would require tires with very strong beads. There also was a system where the bead was somehow clamped into the rim.

Thus, I would suggest to check the form of the rim. I may be wrong, but if you don't find a small bulge on the inside of the rims edges that holds the bead in place, that's most likely your problem.

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  • These would be called straight or non hook edge rims.
    – ojs
    Commented Aug 1, 2018 at 15:52
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Are you putting presta valve tubes in a rim designed for schrader valves? A presta valve tube could force itself down through the valve hole and rip open on the metal edge.

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  • You're correct that this could be the cause, but something has flagged this answer as too short. Can you please use Edit to add more info? Mention presta-schrader grommits, or any hacks that might help. Or, check the rim hole for burrs.
    – Criggie ♦
    Commented Aug 1, 2018 at 9:05
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    At least explain why valve hole could cause the tire to be thrown off the rim.
    – ojs
    Commented Aug 1, 2018 at 9:11

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