Failing inner tubes mystery

[billabitbaffled]

Hi Forum,
Your help would be much appreciated....
My wife had her push-bike adapted to an e-bike by Swych about a year ago, and she loves it dearly - or would do, if only the inner tubes in the rear tyre didn't keep failing. We haven't actually kept track of the number of failures, but we guess that it's 5 or 6 in the last 6 months or so. As someone who has been a (low-intensity) cyclist and bike maintainer for about 60 years, I'm completely baffled by the repeated failures.
Points to note:
1. I assume that the e-bike element is probably irrelevant, as the electric equipment is all in the front wheel, which (so far, touch wood) has been unaffected by failing tubes. All of our problems have been with the back wheel.
2. The failures have not been straightforward punctures - no thorns, no nails, etc - they have all been ruptures of the rubber of the inner tube immediately adjacent to the base of the valve stem. In some cases the stem has started to pull out of the tube.
3. There are no snags or protrusions or sharp edges on the wheel rim anywhere near the valve entry point.
4. In all cases, the failures have occurred in new - or nearly new - tubes. Also in all cases the valve stem nut has been tightened onto the rim with gentle help from a pair of pliers.
5. On two of these occasions, while removing the tyre casing to investigate the cause, I have noticed that the no-longer-inflated inner tube is bunched-up / folded over onto itself, immediately adjacent to the valve stem. On further investigation on the latest occasion, having removed the tyre from the wheel, I noticed that the non-inflated inner tube is not a snug fit onto the wheel rim, but appears to be about 40-50mm over-long. This seems odd - I have never noticed anything similar before, although I haven't actually carried out the same check before, either.
5. Dimensions - The wheel is marked as 622 x 18 6061H-T6, and the tyre is SPECIALIZED INFINITY 700 x 38C (38 - 622). I have been careful to buy inner tubes to match by taking the wheel plus tyre to a reputable local cycle shop, which has then supplied inner tubes sized as 700 x 38c / 28-38 x 622, with a 40mm valve stem.
6. Inflation pressure - My habit and preference over my 60 years of cycling has been to inflate bike tyres to a pressure level rather lower than is recommended by the manufacturer, and I have done the same with the tyre in this case. I have not noticed any harmful effects of this practice in the past, except possibly some accelerated cracking of the rubber in the tyre walls.

Can anyone help with an explanation of what I'm doing wrong? All suggestions very welcome.

[freeflow]

Don't do 4. Finger tight is all that is needed if you need to fit to use the vale stem nut.

Also, underinflation is OK when the tyre has low rolling resistance. If its a stiff tyre then inflating to pressure or even slightly overinflating is might be better. The difference in rolling resistance might not be notices when using an electric bike.

[Jdsk]

Don't do 4. Finger tight is all that is needed if you need to fit to use the vale stem nut.

Yes. The nut can be useful when fitting and inflating, but I run it slightly away from the rim once that's done.

Jonathan

PS: Welcome.

[Brucey]

......... Inflation pressure - My habit and preference over my 60 years of cycling has been to inflate bike tyres to a pressure level rather lower than is recommended by the manufacturer, and I have done the same with the tyre in this case. I have not noticed any harmful effects of this practice in the past....

the syptoms are 100% consistent with the tyre being ridden on when it is too soft. This is much more likely with an e-bike, because the rider is less likely to feel the attendant extra drag of the underinflated tyre. I suspect that if you simply inflate the tyres to the recommended pressure, the 'mystery' will be solved.

[axel_knutt]

The base on the valve stem moulded into the tube is slightly smaller than the hole in the rim, so what they've effectively done is to turn the valve and rim into a punch and die that slices the valve clean out of the tube. The solution is simple:

1) Deburr the holes.
2) Put a leather washer on the base of the stem (inner tube rubber is just more of the same material that's already failing).
3) Avoid using pumps. with no hose so that you aren't yanking the stem to and fro, and aggravating the wear.

I've only ever had one of these punctures, because the measures above completely fixed the problem.

[Sum]

...
2. The failures have not been straightforward punctures - no thorns, no nails, etc - they have all been ruptures of the rubber of the inner tube immediately adjacent to the base of the valve stem. In some cases the stem has started to pull out of the tube.
3. There are no snags or protrusions or sharp edges on the wheel rim anywhere near the valve entry point.
4. In all cases, the failures have occurred in new - or nearly new - tubes. Also in all cases the valve stem nut has been tightened onto the rim with gentle help from a pair of pliers.
...

I had a cycling colleague who experienced the same. It eventually turned out that they were fully tightening the valve stem nut before inflating. I assume it's not the same here, noting the OP's years of experience, but ever since then I've always ruled that possibility out first when others had the same issue.

[peterb]

Some years ago I had a similar problem - eventually turned out the tubes (all Continental purchased at the same time) were faulty, and the fault not unknown. I now use tubes with a smooth barrel (eg. Michelin, Decathlon). There is no real need for the valve nut (or dust cap, but that's another potential can of worms )

[Nearholmer]

Tightening the stem nut, but ….

I wonder if the tyre, and with it the tube, is being dragged round the wheel-rim to an abnormal degree. I can’t quite rationalise how if the motor is driving the front wheel, but if the motor is driving the rear wheel, through the chain, maybe it’s causing far more torque than was the case with human-only power. So, maybe it needs a new back tyre that is tighter on the rim.

[Paulkentuk]

It sounds as though your Swych kit may potentially
be applying more torque through to your back wheel, than the tyre / inner tube setup you have, is potentially designed for ?

I'd go back to Swych's Customer Service with the issue, (and details of that tyre / inner tube setup) and ask them what combination they would recommend under the circumstances ?

[Cowsham]

Don't do 4. Finger tight is all that is needed if you need to fit to use the vale stem nut.

Yes. The nut can be useful when fitting and inflating, but I run it slightly away from the rim once that's done.

Jonathan

PS: Welcome.

+1
The over tight nut may be twisting the tube inside or pinching the base of the valve stem -- finger tight is plenty

[Paulatic]

When ATV's came into my life 40 years ago experience soon told us not to even put the valve retaining nut on. Low pressure tyres seem to drag on the tube and pull at the valve. If the the valve stem got dragged inside so be it. At least it didn’t tear the tube.

[Cyclothesist]

It sounds as though your Swych kit may potentially
be applying more torque through to your back wheel, than the tyre / inner tube setup you have, is potentially designed for ?

I'd go back to Swych's Customer Service with the issue, (and details of that tyre / inner tube setup) and ask them what combination they would recommend under the circumstances ?

The OP indicates the motor is on the front wheel making that unlikely.
As other posters have already said, over tightening the valve lockring and running under inflated tubes is the likely culprit.

[peetee]

......... Inflation pressure - My habit and preference over my 60 years of cycling has been to inflate bike tyres to a pressure level rather lower than is recommended by the manufacturer, and I have done the same with the tyre in this case. I have not noticed any harmful effects of this practice in the past....

the syptoms are 100% consistent with the tyre being ridden on when it is too soft. This is much more likely with an e-bike, because the rider is less likely to feel the attendant extra drag of the underinflated tyre. I suspect that if you simply inflate the tyres to the recommended pressure, the 'mystery' will be solved.

My thoughts exactly. I suspect the tyre is ‘creeping’ round the rim. If you want to be sure (and potentially risk another tube) the easy way to check is to deflate the tyre, rotate it so the manufacturer name is directly above the valve then re-inflate to your preferred pressure, ride and watch to see if it wanders.

[Brucey]

when looking at anything broken, it is often difficult to tell what happened first; however, the only way the tube can get itself rucked up like that is if the tyre was ridden on when it was underinflated, so that is your 'smoking gun'. On one side of the valve, the tube gets all rucked up, and may show clear signs of having been folded up inside the tyre. On the other side of the valve, the tube will see very high tension forces; so high in fact, that the tube can be permanently stretched, which probably explains the 'tube too long' observation. Of course the valve can't move around like everything else can, so sooner or later the tube starts to fail adjacent to the valve, and all the air comes out....

[gregoryoftours]

I'd suspect under inflated rear tyre. Is it also possible that there is a slight delay in between applying the brakes and the motor cutting out? This would certainly be the case if the optional brake sensors have not been fitted and if the rider only stops pedaling at the moment of braking, not a bit before. This would cause there to be a lot of opposing force between the rear rim and the tyre, contributing to the tube getting damaged at the valve base. This could be especially the case if the rear brake is favored.