Moth76

Very much a noob here, so apologies if I frustrate anyone. My nephew just got his first mountain bike - Ridgeback 27.5". After a few flats in the rear tyre he bought a 26" thorn resistant tube hoping that would last longer.
Now I was not present when he installed the tube, but he claims that when he was inflating the tube with a hand pump, the tube burst causing a long tear in the tyre and denting the rim.

From what I have read in other posts, it sounds like the new tube may have been pinched in the rim causing it to burst during inflation. But would this usually cause damage to both the tyre and rim? Is this even possible? I have not been able to find any posts on damaging a rim this way. Would the 26" tube be an issue on the slightly larger wheel? I would like to arm myself with some information before approaching the LBS (and to balance my nephew's version of events...).

jimc101

Highly unlikely, and not at the pressures that MTB tires are inflated to. Same for the tire, MTB tires just don't need enough pressure to cause the damage claimed.

Why was he using a 26" tube on a 27.5 (650B ) rim? they are different sizes, the 27.5 is larger, this could have caused the split in the innertube.

Would be looking at why the rider was getting punctures, it's not the season to get lots of thorn punctures yet in the UK (Taking it your from here due to the bike brand being a UK one). Before going back to the shop, would be getting him to demonstrate to you exactly how he caused the damage.

Airburst

I suspect the order of events is actually the other way around - the tyre split, allowing the inner tube to expand out through the tear and burst. Inner tubes that fail inside the tyre generally don't burst, they just go down.

For a new tyre to fail like that it would probably have to be damaged, and given that fact and the damage to the rim, I'd almost suspect the bike had been ridden with a flat tyre or without enough pressure in the tyre. The latter would also explain the frequent punctures, pinch-flats, or "snakebites", due to the tube being pinched between the rim and the ground are common when the tyre pressure is too low, and a severe enough impact could also result in damage to the tyre at the same time.

I doubt putting a 26" inner tube in a 27.5" tyre would cause a tube failure anyway, the local BMXers round here use 16" tubes on 20" rims all the time, and the rubber that inner tubes are made from is pretty stretchy.

nfmisso

I have to disagree with Jim; to blow out a thorn resistant tube take a LOT of pressure. Typical walls are 3mm vs 1mm for regular tubes. An enthusiastic teen with a hand pump can easily get to the limit of the pump. That is enough to cause damage to a rim. A tube for 559mm BSD will easily expand to work on a rim with 584mm BSD without any damage. I would be there is a good chance that he just over inflated it. Wide tires put more force per unit pressure on the rim than do narrow tires.

IF it blew out due to over pressure, it is highly likely that the rim would be damaged beyond use.

10 Wheels

Be so nice to see a pic of the Rim Damage.

jimc101

#nfmisso , re the pressure, for a MTB, if correctly inflated, your only looking at 40psi, to a max of say 60psi -70psi if running slicks (some do go higher, but given the OP makes no mention of it, would guess it still fitted with low pressure knobby tires), that's not a lot of pressure;the pressure is determined by 2 factors, the rim, not normally quoted, but some brands like Stans do have max pressure listed, and more common (standard), the tire. As you note, the enthusiastic teen may have disregarded all the info printed on the tire, which would render any warranty invalided as user error, maybe incorrect seating, or #Airburst suggestion.

Wanderer

The tube and tire did NOT cause damage to the rim! The rim was damaged in some other way.(PERIOD)

nfmisso

Jim - the big IF is "if correctly inflated" That is what I do not believe; the kid kept pumping until it blew. Think back to you were an excited teen, 1st solo, adrenalin pumping. Without watching, it would be easy for him to go way past 100 psi.

FBinNY 

Like some others here, I suspect that, if the tire is ruptured and not simply blown off the rim, overinflation was the cause. This has nothing to do with the tube or rim tape, simply a question of air pressure, which exceeds the strength of the tire to contain it.

As for the rim, it really depends on the nature of the dent. Air wouldn't cause a local dent of any kind, which is more likely to come from a road hazard, or if very local, tools used to install the tire.

I have seen air implode a rim (ends up looking like packman) but that's very rare, and in this case not likely because of the rule of first failure. Once either the rim or the tire give, the air pressure would drop or go to zero, so any other part that hasn't failed yet isn't likely to. Two different, unrelated failures from a single incident, therefore stretches credulity too far.

fietsbob

rim hit the ground when the blowout happened , perhaps .. was this the edge?

brake track wear thins the rim then it comes apart letting the tube push the tire off.

Moth76

sorry, pics attached

My nephew is 14, but already 5'10. Pump did not have gauge.
The rip starts about 30cm away from valve.

tomacropod

Might want to retract that with the pics above!

The tyre may not have been seated properly, but that rim damage is consistent with a tyre blowing off, not impact damage.

- joel

dabac

Agreed.
Unlikely as it is to damage a new rim through overpressure, being able to get it to flare out like that by accidentally catching it on a corner while riding is even more unlikely.

Wanderer

I still don't think that the tire did the damage - I think the damage helped the tire fail. MHO

FBinNY 

Anything is possible. But, without another explanation for how a rim would spread this way over a relatively long section, it's more reasonable to blame hoop stress from the over inflated tire. We know the tire was over-inflated because it didn't just jump off a rim, but it failed structurally with the cord tearing from the bead.

IMO, the sequence of events was; with increasing tire pressure, the hoop stress eventually reached the yield strength of the rim which then spread under the load. Then, as the pressure was raised yet higher it reached the burst strength of the tire, tearing the cord from the bead.

There could be alternate possibilities, but applying Occam's Razor, I choose this one because it's the simplest one that explains both the rim and tire failure.


However, the one element that we can't blame is the tire liner, who's only failure was to hold up and not allow the tube to spring a leak at a spoke hole.

Wanderer

If the rim failed from overpressure, there would be no need for the tire to tear, as the tube could easily slip out of a tire off the rim. I still think the rim was damaged, and the torn tire is a result of the previous damage.

It's hard to speculate accurately, without first hand knowledge of the rim itself, and the previous damage which caused the flat in the first place.

Moth76

As I understand then, no one is blaming the inner tube as being the problem? ie bursting due to being defective and causing the damage? much more likely to be a combination of user error and possible previous damage?

FBinNY 

Yes, the tube simply could not have caused this damage. It's nothing more than a shaped rubber balloon, which on it's own would inflate to busting at less than 5psi. It's sole function is to keep the air from escaping through gaps in seals.

User error in the form of over inflation is the cause, whatever the actual sequence of failure was.

dabac

Don't blame the tube. In comparison with the tire casing, the tube doesn't have any important strength at all.
Along with FB's statement, it's only there to provide an air-tight layer. All the "heavy lifting" is being done by the tire and the rim.
Think of it as lining a basket with a plastic bag to create a makeshift bucket and filling it with water.
You wouldn't blame the bag if the handle came off the basket, would you?