Continental brakes - Wrong way round

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gxaustin
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Re: Continental brakes - Wrong way round

Postby gxaustin » 6 Oct 2018, 9:30pm

It's not simple physics for a child or even inexperienced adults, when you have professional riders still locking their wheels up due to panic braking then trying to apply physics to kids just cycling is somewhat futile.


Surely physics is fundamental and irrespective of the age or experience of the observer? I was describing the phenomena not the level of understanding of a child.

Cyril Haearn
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Re: Continental brakes - Wrong way round

Postby Cyril Haearn » 7 Oct 2018, 8:28pm

pwa wrote:A couple of decades ago I was regularly going to the Alps on cycling holidays and I had a couple of inner tube explosions due to overheated rims. So I did some reading to find out what I was doing wrong. It turned out that I was making the mistake, on long fast descents, of retarding the speed more or less constantly, feathering the brakes. That is a recipe for very hot rims and disaster. What I should have been doing, and have done ever since, is have spells with no braking, letting the speed rise,
..
..

Seems scary and "dangerous" :?
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pwa
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Re: Continental brakes - Wrong way round

Postby pwa » 7 Oct 2018, 8:33pm

Cyril Haearn wrote:
pwa wrote:A couple of decades ago I was regularly going to the Alps on cycling holidays and I had a couple of inner tube explosions due to overheated rims. So I did some reading to find out what I was doing wrong. It turned out that I was making the mistake, on long fast descents, of retarding the speed more or less constantly, feathering the brakes. That is a recipe for very hot rims and disaster. What I should have been doing, and have done ever since, is have spells with no braking, letting the speed rise,
..
..

Seems scary and "dangerous" :?

Counter-intuitive, but it is safer to go very fast for a while with cool brakes then brake hard. The alternative, which I survived, is extreme rim overheating and a tyre losing all its air in an instant and wrapping itself around the derailleur. Timid feathering of the brakes can kill you.

Vorpal
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Re: Continental brakes - Wrong way round

Postby Vorpal » 7 Oct 2018, 10:07pm

On a long descent, I alternate between brakes, front - rear - front - etc.

If I am not familiar with the descent, I am cautious with my speed.
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Brucey
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Re: Continental brakes - Wrong way round

Postby Brucey » 7 Oct 2018, 10:34pm

Vorpal wrote:On a long descent, I alternate between brakes, front - rear - front - etc.

If I am not familiar with the descent, I am cautious with my speed.


I do that too. If you have a load on, and the hill is above a certain gradient, it is the only (safe) way of moderating your speed.

On some alpine passes the estimated terminal (ha!) velocity of a loaded touring bike with no brakes is something like 70mph. Unladen it is bit less but 50-60mph is easily possible. In my impetuous youth I have (unladen) gone past cars at about that kind of speed.

One thing I will say for old (rubber) brake blocks is that they give an early warning of the rim getting too hot. What usually happens is that when the rim starts to get too hot, the brake block starts to soften/melt at the surface. This causes several things to happen; the wear rate of the brake block goes up, the brake modulates differently (because the friction is no longer the same) and most importantly the noise of the brake blocks is changed.

The last of these is most important; it tells you when you need to change to the other brake when alternating brakes as suggested above. The point at which this happens varies with the brake block type. On steeper hills you may find that you have to alternate brakes with increasing frequency. When it gets to less than 2s bursts on each brake you are pushing it.

If the hill is very steep then you can get both brakes making that noise. At that point you have two choices; neither are very good ones, but you must decide quickly what you are going to do because it will only get worse if you carry on dragging the brakes. If you carry on using the brakes much longer you may get a blowout (which I think is most usually initiated by the most stretched part of the inner tube, which is pressed against the hottest part of the rim, tending to split when it gets hot). [FWIW if you fit tubes that are not undersized , using lots of talc, they are less stretched in the rim well and less likely to fail in this way.]

The two remaining choices you have are

a) to let the bike run so that both rims cool off, (even 5s may be enough if you are doing a reasonable speed and the air can cool the rims effectively) or
b) to use both brakes to stop the bike, and hope that you don't get a blowout before you stop, or that it will happen when you are going slowly enough for it to be manageable.

I've stopped before now on a steep descent and taken the opportunity to admire the view, (holding the brake on to stop the bike from rolling away) only to find that the brake blocks have melted and welded themselves to the rim; the bike wasn't about to roll away.... IIRC this happened with kool stop brake blocks. I didn't like the way they felt when they got too hot, either, (it seemed like they started to melt too suddenly) so I stopped using them for touring in mountainous areas after that.

Note that as beneficial cooling airflow goes, so does aero drag, more or less. This means that aero rims are (weight for weight, surface area for surface area) liable to be less well cooled than boring old square section ones.

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Chris Jeggo
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Re: Continental brakes - Wrong way round

Postby Chris Jeggo » 8 Oct 2018, 1:31am

David Gordon Wilson in 'Bicycling Science' (MIT Press, 2004) has a section 'Rim temperatures reached during downhill braking'. He aims to estimate the steady state rim temperature achieved during descent of a long, steady gradient at a speed that is held constant by braking.

At very low speeds air resistance is negligible so all retardation is obtained from the brakes, with the brake force essentially constant, so the power dissipation is simply proportional to speed. At such speeds it seems reasonable that Newton's law of cooling applies, so the heat power removed is proportional to the rim temperature's excess over ambient. This is shown in Wilson's Figure 7.14 'Temperature rise of rims during long braking on hills', with excess temperature rising linearly from zero at zero speed to quite high values at a few m/s.

At sufficiently high speeds all retardation is obtained by air resistance. Wilson's Figure 7.12 shows terminal velocity versus gradient for a touring bicycle, a tandem and a HPV, using typical parameters for those cycle types.

With rim excess temperature rising with speed at low speeds, and falling with speed at high speeds, somewhere in between there must be a maximum, the opposite of a sweet spot, a place to be avoided. Wilson estimates this might be somewhere around 10 m/s (22 mph).

Here are a few quotations from Wilson:

"... temperatures reached are likely to be dangerously high for standard road bicycles that rely on rim brakes alone, and even higher for bicycles with smaller wheels and for tandems.... Wheels with deep-section, streamlined rims will run cooler than those with narrow, unstreamlined rims that produce separated air flow, which has little cooling effect."

"It is also important in downhill cycling that braking be applied to both wheels fairly evenly, but with a bias in favour of the rear wheel, because of the extreme danger of a front tyre blowout at speed."

"... the faster the bicycle is allowed to travel on a downhill, the lower will be the rim temperature, but if one has to brake suddenly from high speed ... one will produce a very high transient temperature, and the danger of tyre failure will increase sharply."

So, in this situation, air resistance is your best friend. Go as fast as is safe with the brakes off for as long as is safe, sitting up, sticking your knees, elbows and ears out, deploying your drogue 'chute.... You maximise your use of air resistance by braking firmly, not too early, for bends. If you're worried about transient overheating, there's often somewhere you can stop safely near a hairpin, where you're going slowly anyway, and test your rims. If they burn your fingers then wait there until they have cooled.

What others have said above about using the brakes in bursts and alternating them sounds good to me too - generating transient heat and then dissipating it at speeds high enough for rapid cooling.

Also, it is beneficial if the tyre and rim tape, between them, prevent the inner tube from touching the metal rim.

Brucey
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Re: Continental brakes - Wrong way round

Postby Brucey » 8 Oct 2018, 12:42pm

Chris Jeggo wrote: .... Wheels with deep-section, streamlined rims will run cooler than those with narrow, unstreamlined rims that produce separated air flow, which has little cooling effect....


I'm not at all sure that is entirely correct. The way air cooling works within an airstream essentially favours an object with sharp corners within the airflow; the rate of heat transfer through these regions is very much faster. It is a good part of the reason why heat transfer surfaces have pins, ribs and so forth. If the sharp corners are also where the heat is generated, so much the better.

An ideal rim could arguably have ribbed braking surfaces, for example. The cooling rates are going to be influenced by apparently small changes in rim design; for example there is likely to be a significant difference between (say) two rims of similar profile, one of which has a braking surface that is blended into the rim profile, and the other of which has a small step transitioning into it, for example.

The argument that a separated airflow doesn't cool so effectively (for any given amount of drag) around a non aero rim is flawed in several ways. For one thing the flow isn't separated in the same way around the hottest parts of the rim. For another the flow is still separated around a supposedly 'aero rim' for more of the time than you might expect. In any significant crosswind, the effects of flow separation are considerably worse in a deep section rim.

The rear rim sees a lot of messy air and (on a touring bike) the rear wheel especially is in the lee of the mudguard. This is likely to greatly lower the rate at which heat is extracted from the rim into the air. This means that in practice the rear rim is often not so effectively cooled (you can tell this because when you are alternating the brakes the duration of the rear cycle (before it gets too hot) is usually a bit shorter than the front) and the advice to favour the rear brake isn't such a good idea.

Note also that aero section rims (of any given weight) are invariably built of very thin gauge material in the curved part. Thicknesses of 0.5mm are not uncommon. This very simply means that this part of the rim is of very little value as a heat transfer surface, because the heat cannot conduct easily into that part of the rim. This (in essence) forces the remaining parts of the rim (including those that are in contact with the inner tube to run hotter, exactly what you don't want.

One fatal exception is if you have patches on the inside of an inner tube, where they are pressed against the rim well. This always locally increases the stress in the inner tube anyway (making it more likely to split) and in addition the rubber solution in the joint has a lower melting/softening point than the rest of the tube, meaning that the patch may just come off when the tube gets too hot. The only saving grace is that if a patch does lift, the tyre will usually deflate fairly gracefully, giving you a few seconds to slow down.

Note also that the boring old single wall rim has a major advantage here; the well of the rim is cooled directly by airflow from the outside, making it less likely that the inner tube will get as hot. Also the most strained part of the inner tube (the corners of the rim well) is where the rim is single-walled and well cooled too, again making it much less likely that the tube will fail. Probably if you wanted a twinwall rim to run at a lower temperature, it ought to be perforated so that there is some airflow within the hollow sections.

At the least it is a good deal more complex than DGW suggests; at worst the outcome is the exact reverse of that he suggests, for the reasons given above.

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Chris Jeggo
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Re: Continental brakes - Wrong way round

Postby Chris Jeggo » 8 Oct 2018, 2:34pm

Thanks for that, Brucey. Much food for thought.

I thought you might disagree with my DG Wilson quotations, and I would not disagree with any of your most recent post.

Chris

mercalia
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Re: Continental brakes - Wrong way round

Postby mercalia » 8 Oct 2018, 3:48pm

right hand turns need to stick your right arm out so best brake on the left? ( really no need to put your left hand out, well polite only, but right you might get squashed by an overtaking car?)

pwa
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Re: Continental brakes - Wrong way round

Postby pwa » 8 Oct 2018, 3:56pm

mercalia wrote:right hand turns need to stick your right arm out so best brake on the left? ( really no need to put your left hand out, well polite only, but right you might get squashed by an overtaking car?)

My thoughts exactly, and as my front brake is the best, that's on the left.