Small Wheel Blow-outs

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Mick F
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Small Wheel Blow-outs

Postby Mick F » 23 May 2017, 9:54am

It's happened twice to me and my Moulton.
20" wheels.

Descending a steep and restrictive-view hill, braking hard out of necessity due to gravity, and the front tyre blows. Big split, utterly irreparable so the tube is scrap.

Happened only yesterday coming down a VERY steep hill into Boscastle. I took the minor road rather than the main road ......... like I should have done which is a much gentler clear hill that you can whizz down. I couldn't touch the rim to remove the tube it was so hot, and had to wait a few minutes to change the tube. I was off and away agin within fifteen minutes as I always carry spare tubes etc.
Screen Shot 2017-05-23 at 09.50.33.png


Question:
Is there anything that can be done to stop the front brake overheating the rim so much that it wrecks the inner tube?
Mick F. Cornwall

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531colin
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Re: Small Wheel Blow-outs

Postby 531colin » 23 May 2017, 10:01am

Sounds like the tyre bead has lifted out of the rim well? (ie. no split in tyre sidewall?)
If the bead lifts out of the rim, the old bodge was to wrap some tape around the rim bead seat to increase the effective diameter.
Small rims will overheat more than a bigger rim for the same braking.

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Re: Small Wheel Blow-outs

Postby pwa » 23 May 2017, 10:15am

In the 1990s I had two blow outs on 700c rims due to heat, both on long descents in the Alps. That made me think that small rims would not be for me, since the friction and heat build up are shared by a smaller alloy surface area and must be more intense. Is a front drum brake possible?

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Mick F
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Re: Small Wheel Blow-outs

Postby Mick F » 23 May 2017, 10:25am

531colin wrote:Sounds like the tyre bead has lifted out of the rim well? (ie. no split in tyre sidewall?)
Absolutely no defects or issues.
Just heat. So hot, I couldn't touch the rim. I could feel the heat radiating from it a few inches away.

pwa wrote:In the 1990s I had two blow outs on 700c rims due to heat, both on long descents in the Alps. That made me think that small rims would not be for me, since the friction and heat build up are shared by a smaller alloy surface area and must be more intense. Is a front drum brake possible?
Spot on.

Not sure about a drum or disc on a Moulton.
Maybe it's cheaper and easier just to accept the tube loss, and carry more spares?
................ or keep away from hills. :lol:

I looked at the tube earlier. The split is about 10mm long on the inside. I've been attempting to repair it, and I now have two patches on it next to each other, and it's still going down. Having a cuppa, then I may try a third patch.

Can you buy heatproof inner tubes?
Mick F. Cornwall

reohn2
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Re: Small Wheel Blow-outs

Postby reohn2 » 23 May 2017, 10:31am

Heat due to the small wheel circumference with heavy constant braking.
Answer = alternating front and rear pulse braking technique or discs :)
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Re: Small Wheel Blow-outs

Postby pwa » 23 May 2017, 10:34am

If it is possible to use a front drum brake you could have two front brakes with two levers. I have two levers for the back of my tandem and it works fine on the bars. A front drum brake (if it is possible) would allow you to put some of the heat into a non-lethal area on long steep descents. Just like rear drums on traditional tandems.

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Re: Small Wheel Blow-outs

Postby Brucey » 23 May 2017, 11:47am

Of course there is a fundamental problem with the heat build up in smaller wheels as discussed above but I suspect that (if correctly fitted i.e. not pinched) the tube is bursting 'in the corner' i.e. where it is most stretched when fitted. Rubber loses strength pretty quickly when it gets hot, so the more the tube is stretched locally, the easier it will split. Any tiny imperfection in the rim tape (eg even an edge) will help the splitting.

Patching tubes in this location is frequently ineffective again because the tube is so stretched.

I know of a few things that might help with this;

1) use lots of talc when installing the tube
2) use wider tubes (which will be more difficult to fit, less stretched in use)
3) use thicker/ heavier tubes.

It is possible that some tubes are tougher or more heat resistant than others, but if so I don't know about it.

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Re: Small Wheel Blow-outs

Postby AM7 » 23 May 2017, 12:40pm

Mick F wrote:Not sure about a drum or disc on a Moulton.


Moulton don't recommend the use of hub brakes for the front forks (though some models have them at the back).

Would a tubeless setup suffer from the same problems?

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Mick F
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Re: Small Wheel Blow-outs

Postby Mick F » 23 May 2017, 2:49pm

Good info and thoughts there.
I've tried repairing the tube and although it is sealed and fine when out of the tyre, it goes down PDQ when inflated to only as low as 50psi.
The tube is split in the well of the rim, and the rim tape is fine.

Brucey seems right to me. Rubber will obviously loose its strength and I reckon that the internal shape of the Moulton rim gives a sharp stretch to the tube. The rim well is narrow and angled so as I've been fiddling this morning, I wrapped two layers of PVC tape on top of the Velox rim tape in an effort to smooth out the shape. May work, may not, but it's got a brand new tube in there to test it out. :lol:

As for getting a wider tube, I've already thought of that, and the next size up in Erto406 is too wide .............. though I suppose it could be stuffed in. I've been using Schwalbe 28 - 40 406 SV6 tubes (not the lightweight SV6A). In fact, I've just ordered three from SJS as I'm down to only one spare. ie one on each wheel of course, but only one in my saddle bag.

R2 suggests pulse braking.
That's what I do naturally, but if you go down a narrow rough lane and you can't see far due to tree overhang, parked cars, corners etc, it's difficult to "let go" between the pulses because you speed up alarmingly. Going down a half-mile long 25% hills takes no prisoners. :wink:
Rear only braking was skidding without seemingly slowing down. :shock:

I wonder if braking harder and going slowly - walking pace? - would produce less heat than a 10 or 15 mph sort of speed?
Is continuous braking heat production speed dependent or time dependent?
Does going slower and braking harder produce less heat than braking less hard and going faster over the same distance?
Mick F. Cornwall

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Re: Small Wheel Blow-outs

Postby andrew_s » 23 May 2017, 3:06pm

Mick F wrote:Does going slower and braking harder produce less heat than braking less hard and going faster over the same distance?

In general, going slower would produce more heat.
The energy to be disposed of is defined by the height of the hill and the weight of the bike. If you go faster, more of that energy goes into stirring the air about - i.e. more of the braking is done by wind resistance.

If it's steep enough, bendy enough, and with a poor enough surface that you've got to go slowly, your options are#
a) alternate front/rear brakes
b) go slowly enough that a bit of rim cools a bit before it gets back round to the brake. This would be walking pace.
c) stop and rest part way down whilst the rims cool off

Disc brakes have their own heat problems, and I wouldn't expect them to be much better, if any.
There's basically just the disc and the caliper to soak up the heat, and if the caliper gets hot enough you could have boiling fluid (= no brake any more) in hydraulic, or melting plastic bits on mechanical.

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Re: Small Wheel Blow-outs

Postby pwa » 23 May 2017, 3:12pm

Mick F wrote:Good info and thoughts there.
I've tried repairing the tube and although it is sealed and fine when out of the tyre, it goes down PDQ when inflated to only as low as 50psi.
The tube is split in the well of the rim, and the rim tape is fine.

Brucey seems right to me. Rubber will obviously loose its strength and I reckon that the internal shape of the Moulton rim gives a sharp stretch to the tube. The rim well is narrow and angled so as I've been fiddling this morning, I wrapped two layers of PVC tape on top of the Velox rim tape in an effort to smooth out the shape. May work, may not, but it's got a brand new tube in there to test it out. :lol:

As for getting a wider tube, I've already thought of that, and the next size up in Erto406 is too wide .............. though I suppose it could be stuffed in. I've been using Schwalbe 28 - 40 406 SV6 tubes (not the lightweight SV6A). In fact, I've just ordered three from SJS as I'm down to only one spare. ie one on each wheel of course, but only one in my saddle bag.

R2 suggests pulse braking.
That's what I do naturally, but if you go down a narrow rough lane and you can't see far due to tree overhang, parked cars, corners etc, it's difficult to "let go" between the pulses because you speed up alarmingly. Going down a half-mile long 25% hills takes no prisoners. :wink:
Rear only braking was skidding without seemingly slowing down. :shock:

I wonder if braking harder and going slowly - walking pace? - would produce less heat than a 10 or 15 mph sort of speed?
Is continuous braking heat production speed dependent or time dependent?
Does going slower and braking harder produce less heat than braking less hard and going faster over the same distance?


I took some interest in the problem of rim and disc brakes overheating when I was buying a Thorn tandem, and Thorn had a lot of information on tests that were done to determine the relative merits of different braking tactics. Techniques that work (as we all know) include just letting the bike go fast, which is fine when you have a nice straight road, but no help at all on a steep twisty lane. And alternating between the different brakes, two on most bikes but three on some tandems. A third option was going very slowly, at walking speed, which seemed to result in lower rim and brake temperatures than steady braking at a moderate speed.

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Mick F
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Re: Small Wheel Blow-outs

Postby Mick F » 24 May 2017, 7:05am

pwa wrote: ................ A third option was going very slowly, at walking speed, which seemed to result in lower rim and brake temperatures than steady braking at a moderate speed.
This is what seems correct to me. If I'd initially braked harder to slow down, I might have been ok.

Perhaps heat build-up is produced by a combination of wheel rotation speed PLUS braking force. Reduce the speed, and the heat doesn't build up so fast even if it takes longer on the brakes.

I wonder if anyone has some figures on this.
Sounds like another experiment. :D
Mick F. Cornwall

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Re: Small Wheel Blow-outs

Postby pwa » 24 May 2017, 8:26am

Mick F wrote:
pwa wrote: ................ A third option was going very slowly, at walking speed, which seemed to result in lower rim and brake temperatures than steady braking at a moderate speed.
This is what seems correct to me. If I'd initially braked harder to slow down, I might have been ok.

Perhaps heat build-up is produced by a combination of wheel rotation speed PLUS braking force. Reduce the speed, and the heat doesn't build up so fast even if it takes longer on the brakes.

I wonder if anyone has some figures on this.
Sounds like another experiment. :D


You might try exploring the mass of stuff Thorn have to say about this, including links. Somewhere among the mass of stuff they had on their site (or linked sites) when I bought a tandem was data from experiments carried out on a steep hill. On hills which do not have the option of letting it go the speed with the lowest temperature (other than zero, of course) was walking speed. Modest speeds above that resulted in hotter rims. They also succeeded on boiling the fluid in disc brakes, making them useless.

But they were not using small rims, and I wonder if the lower surface area of a small rim means the heat will be even more of a problem. Data for 26" rims may not be very helpful.

One tried and tested tactic is, of course, periodic stopping to let the rims cool.

If it were me I would be looking at a secondary brake, to use as a drag brake when I'm worried about excess heat. I know it is a PITA for you, but that could mean a rear drum brake hub.

(Edit. Just checked the Thorn site and if you look at Links you will find stuff on tandem braking that goes into the physics a bit.)
Last edited by pwa on 24 May 2017, 8:43am, edited 1 time in total.

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Re: Small Wheel Blow-outs

Postby reohn2 » 24 May 2017, 8:39am

Mick F wrote:
pwa wrote: ................ A third option was going very slowly, at walking speed, which seemed to result in lower rim and brake temperatures than steady braking at a moderate speed.
This is what seems correct to me. If I'd initially braked harder to slow down, I might have been ok.

Perhaps heat build-up is produced by a combination of wheel rotation speed PLUS braking force. Reduce the speed, and the heat doesn't build up so fast even if it takes longer on the brakes


I think you're right.

One point,could you double up the inner tube by using an old one slit on the outer circumference and the vale removed then slipped over the active tube,perhaps only on the front it would provide some insulation against heat build up from the rim and add support to the active tube.

I know you can't use discs but when we bought the Circe tandem converting to discs was the first thing I did,and the extra heat build up from rims wasn't something I was happy contemplating.As we run 50mm Big Apples,a front blowout due to the bigger tyre would almost certainly mean an off,not desirable.
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Re: Small Wheel Blow-outs

Postby Brucey » 24 May 2017, 10:09am

at 'normal speeds' dragging the brakes all the way down usually results in higher brake temperatures. This happens for two reasons;

1) the net heat input to the brakes is higher (less potential energy is spent working against air resistance) and
2) the cooling rate of the heated parts goes approximately as the square of the speed, but the heat input goes approximately as the speed.

However this isn't the full story; the cooling rate isn't quite zero even at zero airspeed, so at some (very low) speed the brakes will start to run cooler again. Quite where this will happen speed-wise will vary with the bike, rider and the steepness of the hill etc. But to a first approximation this is likely to vary with the surface area of the hot parts, so again small wheels will suffer worst.

It wouldn't surprise me to find that this 'ride slow=cool brakes' thing needs speeds around walking pace in some cases, i.e. impractially slow.

cheers
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