Small wheel blowouts - options?

[reohn2]

So if MickF goes out on his Moulton and finds the longest, twistiest steep descent he can, one where letting go of the brakes is not an option, he should be alright if he descends at 3mph? A tedious way to descend, I know, but in extreme circumstances, to avoid a blowout.....

Needing to be aware of a blowout to such an extent to me indicates a design flaw in the machine.

It puts small wheel bikes alongside tandems, which have similar problems with braking heat.

We solved that potential problem with one tandem with discs for local and winter use and one tandem with rim brakes and an Arai drag for touring and big hill use.

[Carlton green]

Deleted content, duplicate post.

[mattheus]

Sturdy, heavy, simple "dutch"-style roadsters (with either 1 or 3 speeds) are an APPALLING design for climbing the Tourmalet (dunno about the descent).

That doesn't make them a bad design - owners just need to be aware of their strengths and weaknesses.

[Carlton green]

The heat a rim gets too depends on the balance of heat in vs heat out.

The total amount of power is simply the rate of descent - how fast you are gaining energy through gravity.

This heat either goes into heating the rim, or heating the air.

Heating the air can come from air resistance, or heat transfer from the hot rim.

Now, consider a very slow descent.

There is very little heat entering the rim. That heat is transferrred to the air through heat transfer, which easily keeps up without the rim getting very hot.

Next, consider a very fast descent, with no braking at all.

No heat enters the rim. All the heat is dissipated in air resistance.

Finally, consider a medium speed descent. Perhaps half of the energy from gravity enters the rim. Heat transfer can't keep up without the rim gaining a lot of temperature.

So there is some rate of descent for any given hill, cyclist and bike which will produce a maximum temperature in the rim. Any descent either slower or faster will result in cooler rims. At the limit, either a very, very slow descent or a very fast descent without braking, will result in rims at ambient temperature.

That’s my view of things too and I find that slow descents are best, a cyclist should be in control at all times and if descending fast there is a real question about a cyclists ability to respond to changing circumstances. I would just add a couple of additional comments which I hope might help someone.

Based on distant memories of science I would suggest that wider rims and wider tyres are a better option (than narrow) in terms of tolerance of heat energy input. Wider rims have a larger area from which to loose heat by radiation and conduction/convection to the surrounding air and they have a greater thermal mass too. Wider tyres have a higher internal volume so energy entering them increases the air pressure at a lower (diluted) rate.

[Mick F]

.........but you have experienced blowouts on your Moulton on the short but steep hills of Cornwall and Devon,which IIRC you put down to inefficient brakes and black coloured rims.

Since changing the brakes and the rims, they never even get more than gently warm now.

I have felt them often to make sure what I'm saying is true.

When the others got hot, they were blisteringly hot. Fry and egg sort of hot. It was on two separate occasions that the tube blew. Melted more like!

1st was 11th April 2016 - bike was brand new 2nd March that year.
I was descending locally not far from home.
2nd was 22nd May 2017 descending the back road into Boscastle on my way to meet Chat Noir on his ride round the SW Peninsula.

[Carlton green]

.........but you have experienced blowouts on your Moulton on the short but steep hills of Cornwall and Devon,which IIRC you put down to inefficient brakes and black coloured rims.

Since changing the brakes and the rims, they never even get more than gently warm now.

I have felt them often to make sure what I'm saying is true.

When the others got hot, they were blisteringly hot. Fry and egg sort of hot. It was on two separate occasions that the tube blew. Melted more like!

1st was 11th April 2016 - bike was brand new 2nd March that year.
I was descending locally not far from home.
2nd was 22nd May 2017 descending the back road into Boscastle on my way to meet Chat Noir on his ride round the SW Peninsula.

IIRC the earlier rims were painted black and your new ones are plain polished alloy. Whilst black is notionally a good colour for radiating heat the paint might have been acting as an insulant too and have been stopping heat loss by conduction/convection to the surrounding air.

From some basic laws of Physics the energy into the rims would be the same regardless of (effectively working) rims and blocks but the temperature build up and energy out rate is a function of the rims properties.

[mattheus]

The only blowout I have witnessed on a descent was on the Galibier - one of the leaders in a Sportive braked (quite) late for a hairpin and BANG!
This was on "race" tyres, and probably rim-brakes given the year. It was a hot day, but still mid-morning, so nowhere near scorchio.

[I was crawling *up* the hill at the time.]

[pwa]

Does tubeless technology provide any hope?

[roubaixtuesday]

.........but you have experienced blowouts on your Moulton on the short but steep hills of Cornwall and Devon,which IIRC you put down to inefficient brakes and black coloured rims.

Since changing the brakes and the rims, they never even get more than gently warm now.

I have felt them often to make sure what I'm saying is true.

When the others got hot, they were blisteringly hot. Fry and egg sort of hot. It was on two separate occasions that the tube blew. Melted more like!

1st was 11th April 2016 - bike was brand new 2nd March that year.
I was descending locally not far from home.
2nd was 22nd May 2017 descending the back road into Boscastle on my way to meet Chat Noir on his ride round the SW Peninsula.

IIRC the earlier rims were painted black and your new ones are plain polished alloy. Whilst black is notionally a good colour for radiating heat the paint might have been acting as an insulant too and have been stopping heat loss by conduction/convection to the surrounding air.

In previous threads on this, back of the fag packet calculations show that neither radiation changes nor insulation to be significant for the blackness of rims affecting temperature.

(1) Radiation is insignificant compared to convective heat transfer for a rapidly moving bike.
(2) Paint isn't thick enough to be a sufficiently good insulator.

You can probably find the thread with a search.

[reohn2]

Sturdy, heavy, simple "dutch"-style roadsters (with either 1 or 3 speeds) are an APPALLING design for climbing the Tourmalet (dunno about the descent).

That doesn't make them a bad design - owners just need to be aware of their strengths and weaknesses.

Quite!
But heavy Dutch style roadsters have evolved out of a need for a certain criteria ie; utility riding in flat or mildly undulating terrain.
In the case of the OP he apparently bought a bike specifically for touring it,it fell short of that remit due to design flaw that wasn't apparent until this alarming incident.
It could be argued that he should've been aware of the bike's short comings but he wasn't having been AFAIA a long term Moulton owner who had never experienced such an incident previously.
OTOH MickF has had the same problem with his Moulton which,until it happened,had no perception of the machines' shortcomings.

The problem IMO is one of safety that Moulton could have designed out but chose not to nor AFAIA give any warning of in the accompanying literature issued with the bike at point of sale.

I own a Tern folding bike which the handbook states categorically that the bike is intended for tarmac roads only(or words to that effect)so when I take to a gravel track I'm aware of it's short coming on such terrain and accept that it's my own risk.

[reohn2]

.........but you have experienced blowouts on your Moulton on the short but steep hills of Cornwall and Devon,which IIRC you put down to inefficient brakes and black coloured rims.

Since changing the brakes and the rims, they never even get more than gently warm now.

I have felt them often to make sure what I'm saying is true.

When the others got hot, they were blisteringly hot. Fry and egg sort of hot. It was on two separate occasions that the tube blew. Melted more like!

1st was 11th April 2016 - bike was brand new 2nd March that year.
I was descending locally not far from home.
2nd was 22nd May 2017 descending the back road into Boscastle on my way to meet Chat Noir on his ride round the SW Peninsula.

The point I'm making is that the bike sold to you was defective in that under not very extreme conditions it proved unsafe,and you had to modify it to make it safe that's an inherent design flaw by Moulton IMO.

[Samuel D]

The effect of brake efficiency on blowout risk is negligible within the reasonable range of brakes. Other things, especially rim weight, matter far more. Small rims are light and that is the main problem here.

Blow-outs that lift the tyre over the rim are not caused by a rise in air pressure from temperature as is sometimes thought. That effect is small and results in pressures that other, heavier riders routinely use without blowouts on flat roads.

Jobst Brandt and accomplices studied this years ago and reported their findings on Google Groups where you can still read them. They found that clincher tyres stay on the rim mainly by the clinch, i.e. the bead held under the hook by pressure from the inner tube. You can even cut the bead in several places and the rim will retain the tyre at high pressure.

It follows that if the bead goes over the rim the clinch failed. Why it did is debatable but Brandt thought it had to do with the bead becoming soft and more flexible (i.e. subject to a localised excursion over the rim from some random provocation) and the friction of the rubber on the rim becoming lower with temperature rise, allowing the bead to slide up and over.

From this I conclude that, first of all, heavy rims help because their temperature goes up less for a given brake application. Heavier rims often also have more surface area for cooling. Aerodynamic rims would help for the same reason.

Tubeless tyres should be better in this regard if you use them with a tube (without a tube they still suffer from semi-random blowouts at ambient temperature, never mind braking temperatures). Their beads are stiffer circumferentially, since they rely on the bead stretching less to keep them on a hookless rim or any rim without an inner tube.

[pwa]

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.

[Samuel D]

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?

Brandt certainly didn’t think so.

Such a failure would hiss down relatively slowly (seconds) and not make the “loud bang” you heard. A loud, explosive noise is evidence the tube burst outside the tyre.

If you look at the geometry of a deep rim well compared to a circular tube up in the tyre at very low pressure, you can see that if only the unconstrained bottom quadrant or so of the tube stretched into the rim corners we’d have failures with butyl tubes merely from inflation to high pressure unless the tube, as it is being inflated, slid to take in material beyond the quadrant. It must do this even without talc, because I never powder my tubes and they don’t burst on inflation from excessive stretch into the rim well. With latex tubes I can see some minimal evidence of stretching into the corners of the bead-rim interface but that has never been a problem either. Latex tubes, having low hysteresis, are impressively slippery even without any form of lubrication.

Even if occasionally problematic stretching occurs on inflation, no additional stretching occurs with temperature rise from braking, since the tube is constrained by tyre and rim.

Studies and energy calculations show that rims usually only get a little over 100°C with hard braking (carbon rims get hotter at the surface because they conduct heat badly – another reason not to use them). That isn’t hot enough to give an inner tube trouble by itself even if the tube weren’t insulated by tyre and rim tape and only briefly exposed to the rim at that temperature anyway, besides having its own heat capacity, all of which combine to make it unlikely the tube reaches even 100 degrees.

[Brucey]

in theory there are other processes which might retard the bike without creating so much heat; for example if there are chemical bonds severed in one or both materials, this may provide a retarding force without (only) generating heat. However I have no idea if this can make a meaningful contribution to braking effort or not; my suspicion is 'not'.

Another point that was discussed previously was the shape of the rim. I think a square shaped rim will be more effectively cooled near the corners and the corners are also close to the part of the rim where the heat is generated.

There may be a whole load of factors which by themselves seem inconsequential (making only a few percent different) but when added together might make a difference.

I would also comment that we don't know if heat alone was responsible for the failures reported; it may be just one factor. 20" wheels have about 2/3rds the weight, heat sink, cooling potential vs similar section ~27" wheels. My guess is that the fit of the tyre and the fit/type of inner tube are at least as important.

FWIW you can usually hear when the rims are getting too hot; the brake starts making a different noise. I think the difference in noise comes because the brake block surface starts to melt, and that this happens at different temperatures with different brake blocks. When you hear this noise, I think you have a few choices.

1) Alternate to another brake and let the noisy one cool down
2) Brakes off for a while.
3) Come to a halt using both brakes
4) deliberately running off the road (last resort; semi-controlled crash)

Obviously the situation may deprive you of one or more choices. A lot of alpine climbs and main roads have gradients below 10% and it is often possible to moderate your speed using one brake at a time, and keep brake temperatures under control that way. But if you are more heavily laden, punch a smaller hole in the air, on a steeper gradient or can't ride quickly because of tight bends etc, then you can end up in a situation where you start to need both brakes to even hold the speed down acceptably. At this point option 3 is usually the best one. The rims however will get very hot indeed during the stop and this may cause an inner tube to fail. Hopefully this will be at relatively low speed and you won't have a bad accident if it does happen.

When alternating brakes to moderate speed. my method is to change brakes when you hear the sound change. When the brakes are cold you might get five seconds before this occurs. When the brakes are already very hot and not being cooled enough between uses, the interval goes down to less that two seconds and if you cannot let the bike run it is probably time to think about option 3. If you end up in the situation where both brakes are making the 'too hot' noise and have been for a few seconds, and you can't let them off, you are in the mire and you will be lucky to be able to stop without there being problems.

FWIW I have a suspicion that 'better' brake blocks (i.e. that have a more linear friction coefficient during a hard stop) may be that way because they don't start melting so easily. This being the case it might be that you are able to push the rims and tyres to higher temperatures using such brake blocks, without knowing, and therefore be at more risk than normal of a tube failure.

Other brakes will do different thing when they get hot. Obviously it would be best if brakes were not at all vulnerable to getting too hot but (all things being equal) that would mean much heavier brakes/bikes. So the best you can realistically hope for are clear signs of distress (before the brake stops working) and a 'graceful failure mode'. Arguably rim brakes that get noisy before there are other problems are one such.

Some disc brakes just 'go away' (eg because the pads get too hot) before anything worse happens; this is fairly graceful too. Others will get a lot hotter before they work any less well, but when there are problems it can happen very abruptly with no warning; this is not a graceful failure. Drum brakes (of the SA, Arai etc persuasion) tend to 'go away' and then show their discontent more emphatically by smoking. The smoke looks dramatic but the brake usually suffers no permanent ill-effects as a consequence; I'd describe this as another graceful failure.

Re tubeless tyres; not that I fancy them much anyway but I wouldn't fancy them with rim brakes; I have a feeling that many sealants (like the glues that hold old patches on tubes) melt at low-ish temperatures so might allow the tyre to come off the rim more easily. The other thing is that if the sealant is water-based you may get uncontrolled excursions in tyre pressure; most of the liquid sealant will be thrown against the inside of the tyre tread (where it is coolest) but if any is trapped near the rim it may start to boil and this may turn you tyre into a 'steam bomb' which is not terribly appealing. Simply because there not crevices in the same way, I don't think there is such a high risk if you use similar sealant in tubes.

Adding a third (hub) brake to a loaded Moulton is a pragmatic approach that will increase the tolerable brake heat capacity (if that is really the problem) by at least ~+50%

cheers