Cycling UK Forum

I'm giving some thought to fitting cable disc brakes on a touring frame. I've never had a cycle with discs so consequently don't know much about them. I want to utilise my current 10 speed Shimano 105 shifters. When I get the frame resprayed, I plan to have the caliper mounts added to the frame at the same time. I would get the wheels rebuilt with the appropriate hubs to accomodate the rotors. Any pitfalls, advice or recommendations please? In particular, which calipers would work with my Shimano 105 shifters? Are the frame mounting points standard for any make of calipers? Which disc hubs would work with my 10 speed cassette? Thanks in advance.

It's all possible and most of the pitfalls can be addressed. First off, there are just three models of brake that'll work with the very short cable pull of a drop bar lever: the "Road" versions of the Avid BB5 and BB7, plus Tektro Lyra. Beware that all three are easily confused with "normal" cable disc brakes from both manufacturers, which need twice as much cable pull, especially the Avid BB5 and BB7 (yup: the exact same model numbers without the "Road" suffix!). And whilst it may seem possible to get those normal cable discs to work - at the top of the hill at least - there's so little travel that the pad wear of one long descent can leave you with no brakes at all by the bottom! So you really do need the special road type. (A fact that some retailers I'd rather not mention unfortunately overlook!)

For hubs: any Shimano 8/9-speed freehub will also accept a 10-speed cassette, so any mountain/trekking hub will be just fine. Your bike will need a 135mm rear droput spacing. It's probably 130 at present (if equipped with roadie stuff) but that's easily sorted by the builder cold-setting the frame before the brake fittings are brazed on.

I suggest the rear brake is located inside the rear triangle, out of the way of the luggage carrier. Cable disc brakes are very bulky and get in the way of a rear carrier when fitted in the usual location behind the seatstay. It's possible to get special carriers or to mount a normal one in a special way, but none of these work-arounds are particularly attractive and most of them compromise the security of this heavily loaded fixing point, which as we know to our cost, tends to rattle loose even in the perfect situation of the carrier leg clamped snug against the frame. The one snag with the inside rear triangle brake location is that the cable route involves an upward facing casing end, thereby vulnerable to the ingress of water. So you have to re-grease the cable more often to avoid it seizing up, especially in freezing conditions.

Trouble is, these cable discs are designed primarily for cyclo-cross, not touring, so the designers aren't that bothered about the things that bother us, such as rear carriers and long-term maintenance. I'm afraid it also means there's a bit of a question mark over whether the relatively small discs they come with are up to the job of holding back a heavy bloke on a loaded tourer, all the way down a two thousand metre (or more) descent. That's a heck of a lot of potential energy to put into two small rings of metal and nobody ever has to do anything like that much braking in a cyclo-cross race. Sure they need powerful brakes, but not for long at a time. However, there are people who use discs for touring and seem to get on okay.

Meanwhile at the front we have another small snag (apart from interference with front panniers, which you possibly do not want anyway). Touring forks are a bit flexible, it makes for a comfortable ride, but there may not be enough strength and stiffness for the concentration of brake torque on the most flexible lower part of one taper-gauge fork blade - rather than the whole length of both. Disc brakes are usually mounted on plain gauge, non-tapered forks, for that good reason. Even if an existing (not made for discs) touring fork is strong enough, it is likely to flex sufficiently that the disc will rub on one pad then the other when you rock the bike from side to side, i.e. when climbing out of the saddle. But maybe you'll not be doing much of that, or not find it annoying.

Cyclenut wrote:First off, there are just three models of brake that'll work with the very short cable pull of a drop bar lever: the "Road" versions of the Avid BB5 and BB7, plus Tektro Lyra.

... and you'll probably have trouble actually finding anything other than the BB7 Roads. Fortunately these are good brakes, powerful, easy to adjust, easy to get pads for.

I have a BB7 Road on the front of my tourer and it works very well so far. I'm much happier with it than with V-brakes and drops or cantilevers. However I haven't yet toured with camping gear on mountains though, so must reserve judgement somewhat. It definitely feels like the future, though. I will put one on the back too once I can afford a rohloff-drilled disc!

CJ's point about the front fork is very valid though. My Fort/Intec CrMo frame has quite a beefy but tapered fork which seems to resist the twisting of the brake quite well. However it is pretty uncomfortable! Now my last touring bike was a Moulton, so everything feels uncomfortable after such a cultured ride, but even with 50mm Marathon Supremes it does bash your hands up. I think this is probably the primary issue with discs, and probably the last one anyone thinks about for a touring bike. On something like a Cannondale Headshock - ie well-thought-out and executed front suspension - a disc seems like a definite step forward. But without suspension it may prove more of a hindrance than a help.

The other point to remember is that disc braking adds loading to the spokes and requires a dished front wheel. Neither of these is good for the longer-term reliability of the wheel.

If you are worried about having powerful enough braking, have you tried Magura hydraulic rim brakes? I've run these for over 10 years without any maintenance other than snap-fitting replacement blocks.

Second vote for the Magura rim brakes. CRC seem to have them in stock at the moment at a reasonable price. Only any good with straight bars though.

If you tour with heavy loads on big hills, be aware that discs can get so hot as to warp, needing a new disc to get you back on the road. In normal use this is not an issue but it could be in extreme conditions.

I wouldn't fit a disc brake on a conventional front fork - fork twist under heavy braking is the last thing you want, and it's likely to become a problem just when you need the handling to be predictable, i.e. when you have to brake hard from speed, on a descent, with a heavy load.
The only way to avoid it would be to have twin discs, one on each side, but the weight penalty would be huge, and I'm not sure if any hubs are available which would allow this.
On suspension forks it's unlikely to be a problem, as they can be much stronger in torsion than forks which have to flex to provide comfort.

you can get some forks that are made for discs and 700c wheels, both in steel and carbon (or atleast you could last time I looked). But if my P2 is anything to go by, they will be a little less comfortable than a proper touring fork.

I wouldn't worry about over heating on big hills anymore than you would for a rim brake: if it gets hot enough to warp a disc it would have probably got hot enough to blow the tyre on a rim braked bike!

Have you tried some good old frog leg cantis yet? These might improve yur braking a fair bit without the need for all the hassle with discs!

Si wrote:I wouldn't worry about over heating on big hills anymore than you would for a rim brake: if it gets hot enough to warp a disc it would have probably got hot enough to blow the tyre on a rim braked bike!

In my touring carrer I've ridden hundreds, yes hundreds, of high passes in the mountain ranges of Europe, involving descents of 500, 1000, sometimes even 2000 metres altitude, on rim braked touring bikes without ever blowing a tyre. I did once get some odd punctures (slow ones) on a tandem in the Pyrenees which I attributed to contact with hot aluminium, but no more of those after simply applying a layer of PVC tape.

I've made only a few comparable descents on my disc braked mountain-bike. It's for off-road after all, where the rolling drag is higher so less energy goes into the brakes. And not much of Britain gets that high. But whenever I have descended something like 500m, on tarmac (that's an important distinction), the discs have got smoking hot, occasionally hot enough to stick on, so I've had to wait quite a while for them to cool and release. I don't like to think what it would be like with another 500m descent, easier rolling tyres, luggage on the bike ...

Sure I've heard of blow outs, but I've also heard of warped discs.

I know that some people do tour with discs, so I'm open to persuasion that it might be okay. But as yet there are not many of them, and even fewer still who tour amongst really big mountains, so it worries me that I have even heard of warped discs. We need more data, but so long as I'm not prepared to risk my own money and my own neck, over the high passes where I like to tour, I think it would be unfair to recommend that course to other people.

I've made only a few comparable descents on my disc braked mountain-bike. It's for off-road after all, where the rolling drag is higher so less energy goes into the brakes. And not much of Britain gets that high. But whenever I have descended something like 500m, on tarmac (that's an important distinction), the discs have got smoking hot, occasionally hot enough to stick on, so I've had to wait quite a while for them to cool and release. I don't like to think what it would be like with another 500m descent, easier rolling tyres, luggage on the bike ...

Were you using Hope's perchance

TBH, I've not got up to those sort of drops with discs - 300-400m is more the norm (MTB on road, moderate load). And yes, they have got very hot, but never had a jam or warp. But I have witnessed rim break tyre explosion - pretty scarey.

Of course, whatever you go or, technique is an important factor. If you ride down hills with your brakes dragging on all the way then you are asking for trouble. If alternatively, you try to whack 'em on before the corners and then release until the next corner, they'll be in a lot better shape to stop you. Although it does take a little more nerve

Should also note that this all applies to solos. Tandems are a different kettle of fish and using many types of disc with them can lead to problems - but then a drag brake would be the best solution here.

Just googled warped disc and found this on Singletrackworld.com:

skidartist wrote:
I warped a rear rotor and turned both rotors black coming down Mont Ventoux, but that was a 7000ft road descent with 16 stone of me and a trailer too. A celebratory squirt of water on the disks and the resultant big cloud of steam probably didn't help. And then for the rest of the holiday 'squik squik squik squik'

Maybe okay without the water

I seem to remember in an old issue of a MTB magazine (possibly Singletrack?) a disc brake test whereby the discs where attached to a static machine and spun (I guess using a motor and flywheel arrangement), before the brakes were applied. I don't remember any of the tests causing warped rotors or sticking pads, but then these were hydraulic and the limiting factor seemed to be boiling brake fluid.

It was interesting to note in the pictures of the tests that many of the rotors did get to red-hot (literally) temperatures!

CJ wrote:

skidartist wrote:
I warped a rear rotor and turned both rotors black coming down Mont Ventoux, but that was a 7000ft road descent with 16 stone of me and a trailer too. A celebratory squirt of water on the disks and the resultant big cloud of steam probably didn't help. And then for the rest of the holiday 'squik squik squik squik'

Maybe okay without the water

Possibly, not something I would've done!

Interesting how it was the rear brake rotor that warped. I would've thought more energy would be scrubbed on the front brake. Though skidartist may've been using different sized rotors and of course braking technique will probably have influenced it.

Oh yes, the old trick of squirting water on to the disks at the bottom of a long 45mph down hill - twas like being in the bicycle wing of the Red Arrows

That's one of the most satisfying bits of being caught in a downpour. If you have a slight tail wind, get up to about 25 mph then stop reasonably quickly you get a long cloud of steam blowing off your disc!

I guess this is obvious but I'll say it any way! Careful choice of rotor can help reduce warping. My cheap and heavy base level shimano discs (M525 Hydraulic set) had great reviews for not warping.