Disc brakes on long descents

[DaveBath]

Am looking for a new commuting machine

Marin Lombard about the right price bracket
http://www.winstanleysbikes.co.uk/produ ... _Bike_2010

I like the look of disc brakes - no rim wear - Does anyone have any thoughts on disc brakes for long descents?

I am 12.5 stone and my daily commute takes me down about 150m

A disc looks like a v small piece of metal to dissipate this sort of energy

[iandriver]

Given the disc brakes bias towards mountain biking and big descents, I wouldn't worry too much. Like anything, there are good and bad components, I'd look for reviews on the particular model of brake.

[frank9755]

Not personal experience but I have heard that cheaper/less good ones can fail suddenly as the disc warps through heating, if the brakes are applied continuously for a long period. There is a fair bit of discussion of this in tandem circles. I believe that some of the newer and larger discs have solved this problem.

I think that Santana, the tandem manufacturer, has something about this on their site. Also try the Tandem Club site.

The problems are associated with coming down from an Alpine pass. I think your little hill should be no problem!

[Tigerbiten]

I warped a rotor and damaged both front calipers on my trike dropping down hills in Scotland-Northern England.
But then the weight of me + trike + trailer + gear was prob ~150kg, around 75kg per brake.
On some hills I was dropping ~1,000' in 2.5 miles at ~25-30 mph, thats an average of only ~7.5% but with some parts above 20%.
The trouble was this was on twisty single track roads so I couldn't just let the bike run free, so I had to ride the brakes down it most of the way.
With dual track roads where I could just let the trike run, I was easily hitting speeds of over 40 mph on similar hills.

YMMV.

Luck ........

[willem jongman]

Discs are great in the mud. On a road bike like this I am not so convinced. They are relatively expensive, and at this price bracket that means you pay the price elsewhere in the bike's equipment. That is obviously true with this bike as well. Also, they imply a very strong an stiff fork, which is not at all comfortable. I think this bike is an altogether crazy mix. For a commuter bike I would like mudguards and a hub generator instead.
Willem

[Deckie]

I am 12.5 stone and my daily commute takes me down about 150m

A disc looks like a v small piece of metal to dissipate this sort of energy

150m over what distance?

To be honest 150m isn't that much. People have problems if they are using them as drag brakes over Alpine descents - 1,000's of feet over a number of miles. For short descents they are not a problem - just use them hard for short periods and remember not to touch the disc at the end of the ride!

As an example I held the disc brake on most of the descent off Shap in to Kendal on our tandem with loaded trailer, this was on top of the rest of the JoGLE trip. No problems at all.

[al_yrpal]

Look at any modern motor vehicle, you will see disc brakes! Why, because they are better!

You won't be trying to disipate the braking energy through a thin aluminium rim, but on a perforated stainless steel rotor with proper braking pads not rubber blocks. Neither will you be stressing the spokes with braking forces. On long descents just be sensible and apply your disc brakes gently and intermittently allowing some respite for cooling just as you would with rim brakes. You will find that disc brakes are far superior in braking performance especially in the wet. I have them on my MTB (hydraulic) and tourer (cable operated) and wouldnt consider going back to any form of rim brakes.

Al

[snibgo]

Look at any modern motor vehicle, you will see disc brakes! Why, because they are better!

The alternative for motorists is, of course, drum brakes. Bicycle rim brakes are closer in the way they operate to disc than drum.

Neither will you be stressing the spokes with braking forces.

Instead, the fork takes more stress. As far as I know, spokes don't need strenthening for the braking forces, but forks do.

As disc brakes operate at a smaller radius than rim brakes, they need a greater force for the same braking capacity.

I'm not dissing disc brakes, just pointing out a couple of points.

[CJ]

Look at any modern motor vehicle, you will see disc brakes!

Irrelevant comparison: motor vehicles use engine braking on long descents and suffer the consequences of damaged brakes if they don't!

Given the disc brakes bias towards mountain biking and big descents, I wouldn't worry too much.

Also irrelevant. The rolling resistance of rough and soft terrain is many times greater than that of a tarmac road and substantially reduces the total amount of energy that has to be absorbed by a brake.

I have hydraulic discs on my mountain-bike and I'm very glad of them most of the time, but on the few occasions I've had to descend more than 300m on tarmac there have been problems, mainly due to overheating of the callipers and adjacent hose. So I'm just as glad to have rim brakes on my tourer - especially when I hear of experiences like those of Tigerbiten!

Cable operation may actually be better when discs are mainly used on road. Or maybe steel-braid jacketed hoses help, and in the hope that they might I recently upgraded the mountain-bike thus. Larger diameter discs would obviously also help. But the "discs" on my tourer are already 622mm

[patricktaylor]

...

Neither will you be stressing the spokes with braking forces.

Instead, the fork takes more stress. As far as I know, spokes don't need strenthening for the braking forces, but forks do ...

With disc brakes, the braking force is applied to the hub, then via the spokes to the rim and road. I'm no engineer, but it seems there should be more load on the spokes than with rim brakes. When the brake is applied, the hub wants to stop but the rim wants to keep turning, and they are connected only by the spokes.

[frank9755]

...

Neither will you be stressing the spokes with braking forces.

Instead, the fork takes more stress. As far as I know, spokes don't need strenthening for the braking forces, but forks do ...

With disc brakes, the braking force is applied to the hub, then via the spokes to the rim and road. I'm no engineer, but it seems there should be more load on the spokes than with rim brakes. When the brake is applied, the hub wants to stop but the rim wants to keep turning, and they are connected only by the spokes.

Patrick, I remember coming to a similar view when I first thought about it, but it is not right because the thing that you are trying to stop is not the rim of the wheel, which is light, but yourself, which is attached to the hub, not the rim!

Therefore, with rim brakes, the spokes do have to deal with braking forces - which they do cope with very well. However the forks are spared the braking forces, hence those lovely, slender forks that you used to get on steel bikes could work fine with rim brakes but not discs.

[aek]

I have a Carrera Gryphon disc spec with Tektro brakes and i use it on a 22 mile daily commute on a mixed route with some long and steep hills.
I must say that i never had any issues with my brakes and i've been doing this commute daily in the past few months.

The only thing that i do need to adjust from time to time is the cable because it does stretch after a while, but i'm happy with them.

The bike you are looking at has more superior Tektro brakes than the ones i've got and i don't think you will have any issues whatsoever.

[[XAP]Bob]

I have a Carrera Gryphon disc spec with Tektro brakes and i use it on a 22 mile daily commute on a mixed route with some long and steep hills.
I must say that i never had any issues with my brakes and i've been doing this commute daily in the past few months.

The only thing that i do need to adjust from time to time is the cable because it does stretch after a while, but i'm happy with them.

The bike you are looking at has more superior Tektro brakes than the ones i've got and i don't think you will have any issues whatsoever.

They're not that long and steep - but it's significantly more than 150m...
Just a shame I'm so slow on the old upright

[rualexander]

However the forks are spared the braking forces, hence those lovely, slender forks that you used to get on steel bikes could work fine with rim brakes but not discs.

The forks still have to cope with the braking forces whether rim or disc brakes, its just that with discs the part of the forks that the forces operate through are down near the dropouts whereas with rim brakes the forces are higher up on the cantilever/v-brake bosses where the forks are thicker, or for caliper brakes on the fork crown, therefore the bending moments on the forks are much lower for rim brakes. I think!

[patricktaylor]

Hmmm... @frank - we've had this discussion before on here. There must be a force through to the road - to resist the momentum of the bike and rider and slow them down (which is why you'd slide over ice), and that has to be via the spokes from a disc brake, surely? With the braking force on the rim, the road resistance is direct, although I can see there's still a force acting through the spokes, but not a rotational force. I think there must be a difference, depending on where on the bike the initial action of braking occurs.