Small Wheel Blow-outs

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pwa
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Re: Small Wheel Blow-outs

Postby pwa » 24 May 2017, 10:14am

Brucey wrote: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


Looking at what Thorn say, I think that for small wheels that is probably right. Have a look at their Links. Graphs and everything.

Motorcyclists can get their exhausts plasma sprayed with heat shield stuff. Now if we could do that with the inside of wheel rims...

cycle tramp
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Re: Small Wheel Blow-outs

Postby cycle tramp » 24 May 2017, 11:18am

Bit of a silly question but what happens if you get off and walk the steep bit? It sounds like tyre blow outs don't happen a lot and even my memory reminds me that is a very steep hill.... From the point of view of economics a new tube is about 4 pounds...Apologies for being indelicate at this point, but if you're paid at 20 quid per hour, that's about about 33 pence per minute, which means for a 4 pound inner tube you could walk for 12 minutes and break even... (and that's before time to change the tube is taken into consideration) :-)

pwa
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Re: Small Wheel Blow-outs

Postby pwa » 24 May 2017, 11:53am

cycle tramp wrote:Bit of a silly question but what happens if you get off and walk the steep bit? It sounds like tyre blow outs don't happen a lot and even my memory reminds me that is a very steep hill.... From the point of view of economics a new tube is about 4 pounds...Apologies for being indelicate at this point, but if you're paid at 20 quid per hour, that's about about 33 pence per minute, which means for a 4 pound inner tube you could walk for 12 minutes and break even... (and that's before time to change the tube is taken into consideration) :-)


£20 an hour! You must live in London! But yes, getting off and walking is better than having a blowout. And not because of the money. When you have a blowout you lose all tyre pressure in a split second. When it first happened to me in the French Alps I was heading for a corner at about 40mph with a big drop over the barrier. The cost of the inner tube was low on my list of worries. I was lucky in that it was my rear wheel. The tyre came off the rim and wrapped itself around the mech and chain, jamming the rear wheel so that I was sliding along on the alloy, When I came to a halt the rim had a flat on it.

The answer in the Alps, with the sorts of gradients there, was to let the bike build up speed for longer and confine braking to short intense periods just before corners. My own experience is that ceramic coated rims get less hot. I've been told that is not correct, but touching rims with my fingers has told me otherwise. And yes, of course, if in doubt it is better to pull over and give the rims a rest and maybe take a stroll. Better than a crash.

cycle tramp
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Re: Small Wheel Blow-outs

Postby cycle tramp » 24 May 2017, 2:46pm

That's a great anecdote - made better as you survived the situation.. Apologies, if the 20 pound an hour caused any offence.. Mick F...sounds like a hardworking, intelligent sort and i've thought it unfair if that type of person earns less than that... Personally speaking I'm on half that, so a new tube for me us worth a thirty minute stroll.. which is about 2 to 3 miles... Where I live in Somerset, we don't have any hill that high..
..I guess that's the danger of being very good at bicycle mechanics, when you find there's a hill that your bike won't go up or down or a bit or road or track that your bike won't go across.... You end up tearing your bike apart, and spend more time and money rebuilding it to cope with these new conditions even if you end up cycling that bit of road maybe two or four times a year... Then when you work out the cost and then the time involved to make those changes, it works out that it would have been more time efficient to simply get off and walk that bit of road even if you did it 6 times a year for the next 20 years :-)

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Mick F
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Re: Small Wheel Blow-outs

Postby Mick F » 24 May 2017, 3:25pm

£20 an hour eh?

When I was in the RN, we were always told that we were paid 24hrs a day because we were employed 24hrs a day and on call 24hrs a day - especially at sea. Last paypacket I had before I left the RN in 1996 was in the region of £80 a day (before tax) = £29,200 pa = £3.33 per hour. :lol:
I take your point though.
Walk down the hill.

Thanks all, for the info and the thoughts.

I've hammered down hills, sometimes at 40mph+ on the Moulton, and braked very hard for corners and hazards and never had an issue. It's just that I've had a blowout twice now and both times on long very steep restrictive view and/or poor surface. Maybe having a small-wheeled bike, I should keep off them, or very much take my time.
Mick F. Cornwall

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Mick F
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Re: Small Wheel Blow-outs

Postby Mick F » 24 May 2017, 3:28pm

PS
Another question ............

Do the number of spokes on any given rim help with heat dissipation?
I have a 28h on the front.
If I changed to a 36h, would it make any difference?
Mick F. Cornwall

mig
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Re: Small Wheel Blow-outs

Postby mig » 24 May 2017, 4:03pm

does the rain make any difference?

gloomyandy
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Re: Small Wheel Blow-outs

Postby gloomyandy » 24 May 2017, 4:17pm

Mick although this sounds perfect for one of your wonderful experiments, please don't! The thought of you throwing yourself down a steep hill with a wheel with a varying number of spokes braking until the inner tube explodes seems like a step too far! Maybe you need to build some sort of crash test dummy to take your place?

On a slightly more serious note, would a latex tube be better or worse at handling the high temps? I know they are often used in tubs (which are supposedly better for this sort of thing), but maybe (one of, I know that they are supposed to be better for other reasons) the advantage of a tub is the extra rubber between the rim and the tube rather than the tube itself?

Brucey
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Re: Small Wheel Blow-outs

Postby Brucey » 24 May 2017, 5:39pm

Mick F wrote: Do the number of spokes on any given rim help with heat dissipation?
I have a 28h on the front.
If I changed to a 36h, would it make any difference?


not much difference IMHO

mig wrote:does the rain make any difference?


yes I would imagine that it does, but I don't know of any corroborative measurements.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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John1054
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Re: Small Wheel Blow-outs

Postby John1054 » 24 May 2017, 6:55pm

Reminds me of the 1995 CTC tandem testing week in mid Wales. Many tandems fitted with a drag brake, some didn't. one front wheel blow out on a non equipped tandem, luckily well handled by that crew.

I've had my Brompton fitted with disc brakes - very hot when descending long hills, but no tyre problems.

Try and stay "cool" Mick and good luck however you deal with this.

P.S. had Air B inner tubes in my last tandem - no punctures, but regular inflating to account for the loss of pressure, much quicker than with normal tubes.

manybikes
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Re: Small Wheel Blow-outs

Postby manybikes » 24 May 2017, 7:54pm

I've found the quality and size of a smaller size tube more impactive on durability than on larger sizes.
With my Bike Friday I found some slightly over sized which resulted in creasing and ultimately failure. I think it was the Continental that proved to be consistent but they were hard to get.
Counter intuitively, despite Brucey's usually invaluable advice I also found that a 18 tube slightly stretched to be a better bet than a lesser quality 20 inch

Brucey
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Re: Small Wheel Blow-outs

Postby Brucey » 24 May 2017, 9:32pm

manybikes wrote:Counter intuitively....... I also found that a 18 tube slightly stretched to be a better bet than a lesser quality 20 inch


that quality might trump size seems perfectly reasonable to me; I think tubes can vary. The thing is that I don't know which brands/models might be worse than others....

I have noticed that some tubes can be torn lengthwise rather easily these days; I'm sure they didn't used to be like that. For my sins I quite often cut up old inner tubes and make them into all kinds of other things (eg rubber bands, ideal for securing sets of spokes etc...). Occasionally a tube will just split easily when stretched; maybe they got the mix wrong or it degraded prematurely or something. I keep thinking that it ought to be possible to devise a simple test so that the quality of the material can be assessed.

BTW because of the way inner tubes are made (it is a miracle they cost as little as they do IMHO) there are very often thin/weak spots in them. If you pump them up when they are not in a tyre the thin spots will bulge; thin spots are usually where the tube will fail if there is the slightest problem with the rim tape etc.

cheers
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mig
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Re: Small Wheel Blow-outs

Postby mig » 24 May 2017, 10:02pm

if i'm reading this correctly then MickF needs to fit a quality inner tube & walk the bike downhill in the rain to ensure no front blow outs.

:D

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RickH
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Re: Small Wheel Blow-outs

Postby RickH » 24 May 2017, 10:03pm

My first choice of tubes is Schwalbe
Schwalbe wrote:Which special features does a Schwalbe tube offer?

All tubes are inflated and stored for 24 hours to test for air retention. Afterwards, every tube is carefully checked individually by visual inspection. (link)

2nd choice is Conti & anything else is a distant 3rd place.

On braking
If you can try to mimic the alpine descent on a small scale. Rather than trying to maintain a steady speed, use your brakes to slow the bike right down in between short spells of letting it run & try to only use one brake alternately for each slowing. You may need to adjust the timings but, say, 5 second run - slow to walking pace on front brake - 5 seconds run - slow to walking pace on rear brake - repeat. There may be times when you have to abandon this & use both brakes or modify it (extending or reducing the timings due to corners, etc.) but where it is possible it will maximise the time each rim gets to cool between brakings & minimise the time the rim is being heated by the brakes.

Alternatively, modify your route so you only go up these sort of hills which will eliminate the problem entirely! :wink:

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Mick F
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Re: Small Wheel Blow-outs

Postby Mick F » 25 May 2017, 8:26am

RickH wrote:Alternatively, modify your route so you only go up these sort of hills which will eliminate the problem entirely! :wink:
This is the only cure. :lol:
Mick F. Cornwall