Techniques for braking on steep descents

General cycling advice ( NOT technical ! )
rareposter
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Re: Techniques for braking on steep descents

Post by rareposter »

Pebble wrote: 20 Apr 2024, 4:09pm In the mean time - any one else got any thoughts on how to work it out
I think it gets much more complicated because you're not slowing to zero each time - you're scrubbing some speed off, maybe for a corner say, then letting the bike run again.

I've just looked at a stat off a descent in Spain last week - Strava breaks it down into max and average speeds throughout the segment all overlaid onto the segment map so I can see the higher speed parts, where I've braked (lining up for a corner) then how quickly I'm back up to speed but at no point on the descent was I using the brakes for more than a couple of seconds at a time so trying to calculate the amount of energy being put into the brakes would involve dozens of calculations of "well here you slowed x mass from 40mph to 32mph over y seconds" repeated for every braking point on the descent.

The brakes never once faded or pumped up. 160mm rotors F&R with finned pads.

As a simple answer to the original question of:
Techniques for braking on steep descents

When the speed goes from comfortable to edge-of-seat to oh "%$%, I pull the little lever things on the bars a bit until the speed is back down at comfortable

There we go. :-)
cycle tramp
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Re: Techniques for braking on steep descents

Post by cycle tramp »

Jdsk wrote: 20 Apr 2024, 6:59pm
Pebble wrote: 20 Apr 2024, 4:09pm
Jdsk wrote: 20 Apr 2024, 10:07am
Thanks again.

Drag: about 250 W at 10 m/s.

Rolling resistance: about 80 W.

All corrections welcome.
Many thanks for that, does it gave the riders position for that? sat up or some aero position
...
Unspecified. But presumably a normal riding position rather than one that maximises drag... I'll see if I can find how much that increases it.

Jonathan
Don't forget to factor any strong headwind :-D
Obtaing a more comfortable riding position https://www.rivbike.com/blogs/news/how- ... p-bar-bike
Pebble
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Re: Techniques for braking on steep descents

Post by Pebble »

cycle tramp wrote: 20 Apr 2024, 10:15pm
Don't forget to factor any strong headwind :-D
there was a windy day recently (which doesn't really narrow it down much) there was a downhill section I had to peddle to get to the bottom
Marc
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Re: Techniques for braking on steep descents

Post by Marc »

rareposter wrote: 19 Apr 2024, 8:41pm
Marc wrote: 19 Apr 2024, 8:10pm
Pebble wrote: 19 Apr 2024, 7:59pm...they use a highly conductive type of cast iron...
Sorry, but a "highly conductive type of cast iron" doesn't exist.
Thermal conductivity of cast irons can range quite widely depending on the exact alloy.

It's still lower than aluminium and steel though and why on earth would you be making bicycle components from cast iron?! I mean, it's been decades since I've actually had to do anything involving a drum brake but surely they're not made of cast iron are they?!
As far as I can tell, its some cast steel drum in the Sturmey Archer drum brake hubs. The drum "inserts" have a smooth surface and are definitely not as brittle as cast iron. (did my share of machining steel, cast steel and cast iron in the past decades)

rareposter wrote: 19 Apr 2024, 8:41pm
Marc wrote: 19 Apr 2024, 6:46pm Ahem... velomobiles? You hardly get more performance orientated "bikes"
We're going back to the silly play on words detail.
Normal bikes - the ones that everyone thinks of when you say "bicycle".
Ok... :mrgreen:
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Bmblbzzz
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Re: Techniques for braking on steep descents

Post by Bmblbzzz »

cycle tramp wrote: 20 Apr 2024, 12:33pm Personally, I stop half way down to admire the view and let the tension out of my hands... or if there wasn't a view to admire you could always save this thread to your phone. By the time you've got to the end, I strongly suspect your brakes would have cooled considerably :-)
Taking the whole title "techniques for braking on steep descents" rather than just thinking about heat, the one way to improve this would be not to get that tension into your hands in the first place. Which I expect is not just dependent on braking technique and brake calipers/drums/discs/etc, but also how well the levers and bars are matched to your individual hands. Many factors to consider.
cycle tramp
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Re: Techniques for braking on steep descents

Post by cycle tramp »

If it helps I descend in the same slow manner than Thora Hird use to on her stair lift... I suspect that the tension is more mental than physical.... I just can't seem to do big hills with the same joy as I used too..

...a good few years ago I hit a small bank of gravel coming down Cheddar gorge, which had me sprawled out in the road.. on the plus side nothing else was using it at the time...
Obtaing a more comfortable riding position https://www.rivbike.com/blogs/news/how- ... p-bar-bike
Biospace
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Re: Techniques for braking on steep descents

Post by Biospace »

Pebble wrote: 19 Apr 2024, 7:59pm
rareposter wrote: 19 Apr 2024, 6:22pm
Biospace wrote: 19 Apr 2024, 6:01pm My apols, I mentioned the point made in the article under discussion that a drum brake can dissipate safely a lot more heat than a disc, which sent the topic off on a (downhill) tangent. I had hoped there might be some discussion around why the author believed this to be the case.
I'd be interested too cos I've always heard that drum brakes are LESS good than discs at dissipating heat
I would have thought the very design of them make them less good at cooling down. I doubt there is much cooling takes place on the inside of the drum where the friction is taking place - the heat has to make it through to the other side of the iron to get near cold air
(I understand they use a highly conductive type of cast iron, but still only the far side is getting cooled)

Where as on a disc, both friction sides are out in the open
One way of a drum dissipating "a lot more heat than a disc" would be to be able to run at consistently higher temperatures than a disc, in which case, the explanation would appear to be nothing more than the greater (exposed to the airflow) surface area of a drum brake.

Convection can be assumed to be responsible for the great majority of heat loss from a bicycle brake under use, it's possible the convective cooling of a finned drum may be greater per unit area than for a bicycle disc brake because airflow around it is more turbulent than for a disc. I would expect an Arai tandem drag to have a greater surface area than a 180mm disc.

It would seem that discs are better at coping with temporary extreme braking since the entire friction surface is exposed to the air, whereas much of the heat generated by drum shoes has to be conducted through the drum. But, in a situation such as a heavily loaded tandem and long descent where a third brake is advisable, a well designed drum of a slightly smaller overall diameter than a similar disc is very likely to have a larger surface area with more turbulent airflow, allowing it to do more work than the disc without overheating.

Disc brakes presently also likely attract more profit, both at point of bicycle sale but particularly in after-market sales, which clearly has some effect on what people are persuaded to see as desirable.
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531colin
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Re: Techniques for braking on steep descents

Post by 531colin »

If you arrange for a disc brake and a drum brake to produce the same amount of retardation, they will necessarily generate the same amount of heat.
Under those circumstances it will become obvious which will dissipate more heat, or fade sooner.
Biospace
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Re: Techniques for braking on steep descents

Post by Biospace »

531colin wrote: 23 Apr 2024, 6:11pm If you arrange for a disc brake and a drum brake to produce the same amount of retardation, they will necessarily generate the same amount of heat.
Under those circumstances it will become obvious which will dissipate more heat, or fade sooner.
Whichever brake is least well specified for the purpose?
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531colin
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Re: Techniques for braking on steep descents

Post by 531colin »

Sorry, perhaps I should have been more specific
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Cowsham
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Re: Techniques for braking on steep descents

Post by Cowsham »

Katie Kookaburra was hitting some speed going round the Yorkshire Dales in her latest video. About 8 min in.

https://youtu.be/MY93BXf3P4o?feature=shared
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rareposter
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Re: Techniques for braking on steep descents

Post by rareposter »

Cowsham wrote: 24 Apr 2024, 1:31am Katie Kookaburra was hitting some speed going round the Yorkshire Dales in her latest video. About 8 min in.

https://youtu.be/MY93BXf3P4o?feature=shared
Great Dun Fell is one of those descents where it's easy for your speed to run away with you! The wind on there can make it quite treacherous too.

However it looks/feels quicker than it is because cos the road is so narrow.
Pebble
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Re: Techniques for braking on steep descents

Post by Pebble »

531colin wrote: 18 Apr 2024, 6:23pm Tandem drag brake; usually a rear hub ( or drum or internal expanding) brake. Several ways to work it, maybe a friction/ ratchet gear lever worked by pilot or stoker, or a lever worked with the pilots heel, or a a normal handlebar lever worked by pilot or stoker.
Pretty simple mechanical brake, just to “ hold it back a bit” adding in other brakes as required to adjust speed to road conditions, bends etc. I can’t think of a way you might be able to control a brake to hold speed steady?
Possibly a non friction brake, such as in automotive, Exhaust Brake, Jake, Brake, even just leaving it in a low gear.

If some sort of compressor was built into the hub, possibly feeding a chamber in a section of the bike frame. then that could work like an exhaust brake. different settings would determine a chosen max speed on any given slope

Unlike a friction brake the faster you go the greater the resistance, I guess it will follow the usual squared proportion. so at 30mph it would be offering 9x as much resistance than 10mph

Nice concept, but probably expensive and prohibitively heavy.
Jdsk
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Re: Techniques for braking on steep descents

Post by Jdsk »

Pebble wrote: 24 Apr 2024, 9:50am Nice concept, but probably expensive and prohibitively heavy.
I'm with you on the second and third!

: - )

Jonathan
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Re: Techniques for braking on steep descents

Post by Nearholmer »

How about steep inclines used regularly by cyclists be provided with a long rope and a sheave, so that a descending cyclists hooks on at the top, and transfers their potential energy to an ascending cyclist who hooks on at the bottom. Given the likely losses in such a system, both cyclists would probably need to pedal a bit, thereby satisfying the need for exercise and giving a sense of achievement?

Or, a strap-on motor/generator and battery pack, a bit like those old strap-on “auto wheel” things that drove on the top of the front tyre? This would be charged by a descending cyclist, then passed at the bottom to an ascender. It might even take the form of a small trailer-tug, rather than actually be strapped to the bike.
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