Scary big hills, what to expect

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Claireysmurf
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Scary big hills, what to expect

Postby Claireysmurf » 30 Jul 2012, 6:43pm

I normally find the brakes (Tektro caliper) on my Dawes Clubman ok but not brilliant but on saturday I was descending a very steep hill (estimate 800 feet in less than a mile) and found them very lacking; I estimate I was doing 25-30mph with brakes full on. A riding companion who has a good spec Giant MTB with hydraulic disc brakes found that his discs turned blue from the heat.
Friends riding behind me complained about the awful rubbery smell.
Should I expect any different in such difficult conditions?

(Apologies if this is another daft question)

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gentlegreen
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Re: Scary big hills, what to expect

Postby gentlegreen » 30 Jul 2012, 6:55pm

Are you sure your blocks aren't touching your tyres ?

You could try koolstop blocks - they make such good contact with your rims, they'll often squeal.

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Re: Scary big hills, what to expect

Postby Vorpal » 30 Jul 2012, 7:12pm

Claireysmurf wrote:I normally find the brakes (Tektro caliper) on my Dawes Clubman ok but not brilliant but on saturday I was descending a very steep hill (estimate 800 feet in less than a mile) and found them very lacking; I estimate I was doing 25-30mph with brakes full on. A riding companion who has a good spec Giant MTB with hydraulic disc brakes found that his discs turned blue from the heat.
Friends riding behind me complained about the awful rubbery smell.
Should I expect any different in such difficult conditions?

(Apologies if this is another daft question)


Have you checked and adjusted them recently? Has the surface of the brakes pads become embedded with stuff from your wheels/the road? Do you clean the rims regularly?
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Brucey
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Re: Scary big hills, what to expect

Postby Brucey » 30 Jul 2012, 7:18pm

it could be the brakes touching the tyres but it could be the brake blocks overheating; they are rubber too.

On long hills it is often better to 'let yourself go' for a bit and then brake hard, rather than drag the brakes all the way down. If you must drag your brakes, drag them one at a time, then change to the other one after ten seconds or so. This will often prevent either from overheating so badly.

Obviously you need to maintain control, safe speed etc but even so one can often put less work into the brakes this way.

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Claireysmurf
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Re: Scary big hills, what to expect

Postby Claireysmurf » 30 Jul 2012, 7:27pm

The brakes have been adjusted fairly recently (the length of 'pull' to the levers certainly feels ok) and I wiped the blocks and rims over the day before (with clean damp paper towels) when I changed an inner tube.
I certainly didn't have the confidence for about a third of a mile or more to back off the brakes at all as the road surface was slightly poor, the speed was relatively high and my 25mm tyres don't necessarily inspire confidence when I was unsure what was ahead

mig
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Re: Scary big hills, what to expect

Postby mig » 30 Jul 2012, 7:47pm

25-30mph with brakes full on?! :shock:

what do you estimate your top speed to be before the brakes were applied?

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Si
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Re: Scary big hills, what to expect

Postby Si » 30 Jul 2012, 7:51pm

Yep, what Brucey said. Although, initially, it does take a bit of nerve to stay off the brakes, but you'll find that they are more effective if used like this as dragging them all the way down can lead to over heating.

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Mick F
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Re: Scary big hills, what to expect

Postby Mick F » 30 Jul 2012, 8:08pm

I climbed over to Applecross some years ago. Loads of braking on the way up to hold the bike as I got off to wheeze - and take photographs!
Pass.jpg
However, on the way down, I was behind a car taking its time. No problem with that, but I wanted to go a little faster in places, then brake hard - as per Brucey's comments.

I had to stop. Let the car get ahead, then allow my bike to go faster, then brake hard for the corners. To stay behind the car at the speed the car driver wanted to go at, meant my brakes were overheating. Quite frightening to say the least. :shock: 2,000ft+ down to sea level in a couple of miles or so.

So stop, allow them to cool, or whiz as fast as you dare - to cool the wheels down - then brake hard. Repeat as necessary.
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Re: Scary big hills, what to expect

Postby fatboy » 31 Jul 2012, 12:04pm

The blocks make a huge difference to downhill braking. I replaced my pads for Clarks red inserts and found that I got huge brake fade in the Yorshire Dales that I'd not experienced before. When I clean the bike they are going in the bin!

From my experience Tektro make OK brakes by rubbish pads!
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xpc316e
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Re: Scary big hills, what to expect

Postby xpc316e » 31 Jul 2012, 2:35pm

Surely the amount of kinetic energy converted to heat that the brakes have to cope with is the same whether one pulses the brakes on and off, or holds them on at a constant pressure? Is there any concrete experimental evidence to confirm that the on and off method really makes a difference, or is it just anecdotal?
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Re: Re: Scary big hills, what to expect

Postby wirral_cyclist » 31 Jul 2012, 2:45pm

xpc316e wrote:Surely the amount of kinetic energy converted to heat that the brakes have to cope with is the same whether one pulses the brakes on and off, or holds them on at a constant pressure? Is there any concrete experimental evidence to confirm that the on and off method really makes a difference, or is it just anecdotal?


If 'cold' pads and rims work better than 'hot' then pulsing will work better!

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Brucey
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Re: Scary big hills, what to expect

Postby Brucey » 31 Jul 2012, 2:45pm

the total energy dissipated by the brakes is reduced whenever you descend more quickly; it 'costs' about 500W (mostly in air resistance) to do ~30mph and the power goes up with the cube of the speed. This is all work that the brakes would otherwise be doing if you are going more slowly.

Obviously you can't always descend at speed... but very often you can.

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[XAP]Bob
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Re: Scary big hills, what to expect

Postby [XAP]Bob » 31 Jul 2012, 2:57pm

Don't forget the good old air brake.

Open any jackets, leaving the zip joined at the bottom, to make an effective parachute.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
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Re: Scary big hills, what to expect

Postby BigG » 31 Jul 2012, 5:27pm

Brucey wrote:the total energy dissipated by the brakes is reduced whenever you descend more quickly; it 'costs' about 500W (mostly in air resistance) to do ~30mph and the power goes up with the cube of the speed. This is all work that the brakes would otherwise be doing if you are going more slowly.

Obviously you can't always descend at speed... but very often you can.


The amont of work needed to allow you to keep the same speed at the bottom of a descent as at the top does not depend either on how you brake or on how fast you go. It is simply the vertical drop multiplied by the weight of the bike plus rider. Of course Brucey is right that at higher speeds more of this work is done by air resistance and to a very much lesser extent by rolling resistance. If you don't brake at all, all of the work will be done in this way! (If it isn't done by a hedge on a bend at the bottom.) The heating of the brakes is very much dependant on speed with the rate of heating proportional to sspeed. If you come down slowly, the heat will have more time to dissipate resulting in lower twemperatures. Incidentally, I have never had problems with the brakes themselves overheating although they have sometimes become very hot. I have had trouble with the rims overheating causing puncture patches to lift.

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Re: Scary big hills, what to expect

Postby uphillbothways » 31 Jul 2012, 7:19pm

[XAP]Bob wrote:Don't forget the good old air brake.

Open any jackets, leaving the zip joined at the bottom, to make an effective parachute.


Absolutely - watch Moto GP and you'll see riders puffing themselves out under braking to increase their frontal area, then crouching back down into an aero tuck on the other side of the corner. Long descents can be a real problem on recumbents for similar reasons - lower aerodynamic drag means your brakes have to do much more work.