Small wheel blowouts - options?

[Brucey]

FWIW many new tubes are coated with something which allows them to slide more easily inside the cover. So are many tyres. Were it not the case there would be many more problems than there are for non-talc users.

[Edit; In many cases simple friction is a key ingredient in clincher tyre retention. If there is something present (like wax/release agent for example) that can melt at lower temperatures the friction won't be able to help you in the same way and a tyre can come off the rim; without friction present loose-fitting tyres are inherently unstable, i.e. if one part starts to climb off the rim it will just carry on climbing at that point. For my sins I have sat and watched a new tyre (on a non-hook bead rim, getting hot in the sunshine) do exactly that. Once the tyre and rim were cleaned properly the friction was high enough that there wasn't a recurrence. I wouldn't be taking those rims and tyres down a mountain pass though....]

In terms of noises when the tube fails

a) the tube bursts in air (tyre comes off, tube bulges out from beneath a tyre bead or through a hole in the tyre). This can be as loud as a shotgun report if the pressure is high and the hole is large.
b) tube bursts into the hollow cavity in a twinwall rim, eg because the rim tape has failed. This makes quite a loud pop or bang but you wouldn't mistake it for a) if you had heard both
c) inner tube splits or valve comes away or patch lifts etc whilst the tube is well constrained by the rim and tyre. How loud this sounds is dependant on the size of the hole and how easily the air can then escape.

So if the tyre has rubberised sidewalls, it fits well, the rim tape is full-width plastic and the valve stem seals reasonably well against the rim tape, it doesn't matter how big the hole/split in the tube is, the air can't escape at any great speed and nor will there be a noticeable 'bang' of any kind, probably just a pffft sound. On the other hand if the sidewalls are not rubberised, and the rim tape is porous, or the air can escape from the rim/tyre some other way once the tube is breached, this will make more noise and the tyre will deflate more quickly.

cheers

[Mick F]

On the two occasions it happened to me I heard a loud bang then the tyre came off the rim. In that order. Bang, instant loss of pressure, tyre off, sliding along on ally rim.

............ and BANG!

Mine never went BANG! or came off the rim on either occasion.
They went "ssssssssssssss tut" within about ten feet and then the tyre was totally flat.
No damage to the tyre, just a wrecked tube and a blisteringly hot rim.

[Samuel D]

Mine never went BANG! or came off the rim on either occasion.
They went "ssssssssssssss tut" within about ten feet and then the tyre was totally flat.
No damage to the tyre, just a wrecked tube and a blisteringly hot rim.

Where was the hole relative to tyre and rim features, and what did it look like?

[Mick F]

It was a long split, an inch or two long.
On the lower side, probably next to and touching the the tyre bead. Steel bead, not folding. Continental Grand Prix 406/28.

The tyre was fine as was the rim tape on both occasions.
The rear rim wasn't blisteringly hot, but it was quite warm.

Since changing brakes and rims, no issues of overheating.

[francovendee]

Now I was thinking that heat induced blowouts happened because the inner tube got too hot and the material failed. Is that not the case?

On the two occasions it happened to me I heard a loud bang then the tyre came off the rim. In that order. Bang, instant loss of pressure, tyre off, sliding along on ally rim.

I thought exactly the same. Whilst the peripheral speed past the brake is the same, small or large wheels, the period between the pad touching the same place on the rim is different. I assumed that the longer the rim had to lose the heat before adding more would be the downside against small wheels.

[LittleGreyCat]

One thing not touched on so far.

I am assuming that very little heat is dissipated down the spokes, because they are both very light and slim, and also not integral with the rim but fitted into a hole.

I recall (although I haven't been able to find a picture yet) that at one point there were cast alloy wheels much like modern motor bike wheels where the rim and spokes were all one piece.

I was wondering if this design would give much better heat dissipation down the spokes and also much better cooling because of the flat surface of the large alloy spokes.

I also assume that there were significant downsides to the design because (unlike motorbikes) they are not in common use.

[Brucey]

for heat to get into spokes it has to travel a long way from where it is generated, and it is unlikely to do that without also being able to heat the inner tube (which is rather close to the braking surface) too. Cast wheels have pluses and minuses; more of the latter for bicycle use. Heat conduction down the spokes is unlikely to be a significant advantage and if it was useful, it would be unlikely to cool the rim uniformly.

if you want to cool the braking surface and not heat the tube, the inner wall of the rim needs to be made as thin as possible (so doesn't conduct the heat well) and the conduction of the heat into more exposed parts of the rim (which are well cooled) needs to be as good as possible. Square-shaped rims are probably the best for cooling; they have thick walls at the base of the section, and the sharp-ish corners (which are as close as they possibly can be to where the heat is generated) are very well cooled by the air rushing past.

By contrast aero rims are likely to be a bit crap; the heat can't easily run into the base of the rim, (because the section thickness in the sides is usually minimal) and the smooth airflow doesn't necessarily provide a strong cooling effect. These things probably outweigh the 'increased surface area'.

Note that with caliper brakes (with arms that pivot above the braking surface), the bottom of the brake block needs to wear faster than the top in order for uniform pressure to be maintained. It is therefore likely that with such brakes, there is more heat generated in the lower half of the braking surface than in the top. Cantis and Vs are likely to work the other way. This effect may not be large but if you had your choice you would probably choose to have more heat being generated in the lower half of the braking surface, I think.

cheers

[pwa]

I have pondered this issue before and wondered if it might be feasible to line the inside of the rim up to but not including the hook with some sort of insulating material / coat. So that the tube is not directly in contact with metal. But I haven't thought of anything suitable and I suspect if there were something, it would be on the market already.

[Brucey]

I have often wondered how much insulation is provided by various different types of rim tape.

cheers

[reohn2]

I have pondered this issue before and wondered if it might be feasible to line the inside of the rim up to but not including the hook with some sort of insulating material / coat. So that the tube is not directly in contact with metal. But I haven't thought of anything suitable and I suspect if there were something, it would be on the market already.

The only real answer to such a potentially catastrophic problem is to remove the cause away from the rim to where it can't cause any such problems.
On a large wheel bike the problem is reduced though can be confounded by unnecessarily high pressure tyres ie; 23mm 100+psi +heavy rider,long descent,bad braking practice,etc.
On a small wheel loaded bike,especially as in the OP's case,the small wheels and high pressure small section tyres accentuate the problem because the wheel can't dissipate the heat fast enough.
Drums,(cable operated) discs or a third drum dragbrake solve any potential problems.
There's one other solution,don't ride small wheel rim braked bikes in big hills or if you do be aware of the problem and stop frequently on long descents to allow the rims to cool.

[Mick F]

I have pondered this issue before and wondered if it might be feasible to line the inside of the rim up to but not including the hook with some sort of insulating material / coat. So that the tube is not directly in contact with metal ................

I've been thinking ..............

The rim tape goes right across and up the return of the side of the rim.
The tyre bead almost touches the tape, and the feathered edge of the tyre probably goes over the tape edge.
Therefore the tube isn't in contact with the metal of the rim ..................... perhaps?

No doubt all rims aren't the same, and no doubt all rim tape isn't the same. It all comes down to the relative widths and the tyre.
Not got the details of the Moulton MT20 rims, and I can't remember the tapes.

For the record, this is the profile of my present CR18 rims, and a link to the rim tapes I have .......... 22mm

https://www.sjscycles.co.uk/rims-tape/s ... 30e1db89cd

[pwa]

That combination looks like it will leave a few mm of exposed metal at each edge of the tape. The trouble is, you don't want to lose depth from the hook so you need to be accurate with any "solution" to the gap. The tyre still needs to sit well.

[speedsixdave]

Gents,
OP here. Thanks very much for the continuing contemplation on this - some on topic and some not so much, but all interesting!
I think I will investigate the front hub brake further and consider how to make it work on a leading link fork. The advantages of the front brake for me are: easy to swap out for a normal wheel when not touring; will be more effective than a rear brake for stopping and slowing; and much easier to deal with the cable run to an additional third lever without affecting separability. I've looked briefly at the Sturmey X-RDC rear cassette hub with hub brake too: the reviews are mixed, it is an 8/9 speed cassette and I am on Shimano 11 speed, and it's 135mm OLD and I'm on 130. I suspect none of these are insurmountable issues.

Thinking about other heat sinks, am I right in thinking Scott-Matthauser or someone used to make brake pad holders with cooling fins?

I didn't really want to get into the rights and wrongs of the Moulton concept in general - rather a topic for a different day. Suffice to say I think that the cultured ride is an absolute joy and persuades me and some other folk (MickF amongst them) that at least some of the compromises are worth it for the ride.

[gazza_d]

I do remember when out with clubs people having issues with punctures on descents which always seemed to be down to braking constantly.
All of them seemed to be due to patches lifting rather than blowouts though

I've never experienced a flat like this either on big wheelers, or Moultons (over last 20 years or so).
I wonder what rim tapes and tubes. I always use thick cotton Velox tapes, and schwalbe tubes
I also wonder if thicker tape could insulate tube slightly and whether different makes of tubes are more heat resistant due to better joins etc.
Kinetics in Glasgow do disc brake mods for Moultons, but that may be a bit drastic

[Brucey]

I've looked briefly at the Sturmey X-RDC rear cassette hub with hub brake too: the reviews are mixed, it is an 8/9 speed cassette and I am on Shimano 11 speed, and it's 135mm OLD and I'm on 130. I suspect none of these are insurmountable issues.

the other option is a brake that screws onto a hub that is threaded on the left side too; this can be removed as necessary. You can buy/modify 11s cassettes which fix to a 10s freehub body.

Your main objective is presumably to take the heat input out of the main brakes; a drag brake on the rear wheel will do that even if it won't stop you.

Thinking about other heat sinks, am I right in thinking Scott-Matthauser or someone used to make brake pad holders with cooling fins?

yes, but they were a waste of time; the heat doesn't conduct through the pad material, so the fins can't possibly do any good.

cheers