Rear Brake Not Opening (specialized dolce)

avoidingmyphd

Can anyone suggest a solution to the problem I am having with a rear brake. Once the lever is pulled, the caliper won't spring back open properly again, so it rubs badly and half of my lever pull becomes useless. It has done this since new, but replacing the cable yesterday brought it back to the front of my mind! It's still doing it with brand new cables and all the routing nicely cleaned out.

Now, I can solve it by using an insanely long piece of housing between the barrel adjuster on the caliper and the lug on the frame. By insanely I mean ~ 30cm, installed in a big loop around the seatpost! Any shorter though, and the problem returns.

I am a bit worried that it's a geometry problem: the gap for that bit of housing is very small, much smaller than on our other bikes [this is a 51 spec dolce frame. the brake has done this since new in 2009], and leaves no room for movement in the housing, which just pushes the brake closed constantly. The reason it got worse when i replaced the cables seems to be that the new housings (jagwire racer) are stiffer than the old ones (stock housing).

Any suggestions?
Brakes with short arms and therefore bigger gap to caliper?
Very flexible housing?

Weejie54

The brakes shouldn't have to rely on the flexibility of longer housings. It is the gap between the end of the barrel adjuster to the clamp bolt that provides the relative action of the mechanism. The calipers should return back on release.
If you haven't tried dismantling the caliper itself, and lubricating and adjusting pivot points in reassembly, then perhaps this should be done. If the problem persists (and you are sure there is no binding in the cable) then perhaps you should consider changing the 'Ultra Light' brakes for a more reputable model - Ultegra or something.

avoidingmyphd

Weejie54 wrote:

The brakes shouldn't have to rely on the flexibility of longer housings. It is the gap between the end of the barrel adjuster to the clamp bolt that provides the relative action of the mechanism. The calipers should return back on release.
If you haven't tried dismantling the caliper itself, and lubricating and adjusting pivot points in reassembly, then perhaps this should be done. If the problem persists (and you are sure there is no binding in the cable) then perhaps you should consider changing the 'Ultra Light' brakes for a more reputable model - Ultegra or something.

Thanks Weejie54. I totally agree that they shouldn't be depending on the housing flexibility. They are already upgraded to some tektro ones which are fine on our other bikes.
I haven't dismantled and relubed the caliper - mainly because that's what I'm trying to avoid (!) but also because it functions perfectly in hand and on the bike with the silly long housing section installed, so I can't help suspecting that the problem lies in the housing rather than with the caliper itself... Two brake models have now performed the same on this bike, with two sets of cables and housing.
I might put the brake on my bike to see if it works there...

topdude

Sounds like the frame size / geometry is causing the problem. The calliper can be very close to the top tube on a small compact frame causing binding in that last section of brake cable.
How about fitting a single length of cable outer from the brake lever to the calliper using frame clips or even cable ties instead of the usual housings welded to the top tube.
This would allow you to position the cable anywhere on the top tube and reduce the sharp bend to the calliper.

Weejie54

This would allow you to position the cable anywhere on the top tube and reduce the sharp bend to the calliper.

That's the strange thing - the bend doesn't seem to be particularly sharp - less so, in fact, than most of my road frames.
The single length of housing might work, however.

keef66

Not particularly elegant, but you could try a band-on barrel adjuster, further forwards on the top tube than the current boss and pointing backwards??

Or Nokon outers are supposed to allow sharper bends??

avoidingmyphd

Thank you all for your help.
It is not just the actual angle that matters, it's how short a length of housing the cable has to make the angle in. I'm pretty sure that the problem is the geometry of the bike which makes this length stupidly short.

I'm going to try to fashion a cable stop, maybe using Alloy Stick-On Cable Guide and Cable Guide Stopper for 5mm Housings (Converts Open Loops to a Frame Stop) from here: http://www.jagwireusa.com/index.php/pro ... ssories/if I can find stock, or similar products from elsewhere. Thanks keef66 for that plan. I'll see how elegantly it works...

If that fails, I'll go for topdude's suggestion of fully housing the cable and routing it as tidily as possible.

I'll post what happens, because if it is a geometry problem that means I'm not the only sufferer...

Weejie54

The angle is quite relevant - the angle between the top tube and seat stays is relatively shallow. I would even suggest that you shorten the housing - it may seem like a barmy suggestion but I reckon if you have just a tiny fraction more housing than is needed to stretch from the cable guide to the barrel adjuster, you would have less of a bend. If you take the ideal of a straight line between the guide and the brake adjuster, then using too long a housing would introduce a bend in the cable. If you had a right angle between points, then increasing the length of housing would make the bend less acute. What we have here is a shallow angle. Much as it seems illogical, I would try it with a shorter housing.

because if it is a geometry problem that means I'm not the only sufferer..

I would expect there to be more recorded experiences like yours as a result.

avoidingmyphd

Weejie54 wrote:

The angle is quite relevant - the angle between the top tube and seat stays is relatively shallow. I would even suggest that you shorten the housing

Oh, I know it's totally relevant - but for any given angle, the cable routing gets easier the bigger the gap the housing has to cross. A 90 degree 10mm gap is impossible because housing won't flex into right angles. A 90 degree 1metre gap would be no problem (well, for housing. The bike would be a weird shape!).

I'm at my wits end with the housing length. We've tried everything from so short it's clearly going to pop out of the cable stop, to 30cm looped around the seatpost. The longer the better but unfortunately it doesn't start working happily until it's impractically long.

I would expect there to be more recorded experiences like yours as a result.

Yeah, me too, admittedly. This is still one reason I'm sypathetic with your suspicion isn't actually a geometry problem at all. Although there aren't that many Dolce's riding around. I'd certainly be blaming myself if the first set up wasn't done at the bike shop...

Weejie54

Oh, I know it's totally relevant - but for any given angle, the cable routing gets easier the bigger the gap the housing has to cross. A 90 degree 10mm gap is impossible because housing won't flex into right angles.

A 90 degree would be a problem but a few degrees would be much less of a problem.

Nevertheless, you have tried a short run. Bang goes my theory!

This is still one reason I'm sypathetic with your suspicion isn't actually a geometry problem at all.

I'm not saying that the geometry isn't a problem - just that it is not necessarily the problem. I find it difficult to accept that Specialized hadn't sorted this out if it had been an inherent design fault - setting the cable guide in a different position, for example.

msw

Does it work OK without any housing at all?

I ask because I had a similar problem on a front brake caused by too long a housing, as Weejie54 suggests. When the lever was pulled the housing would deform past the vertical, bending out and then in again; this would cause the end to sit at an angle in the barrel adjuster rather than slotting neatly back into line. Because it was the front I could see what was happening and jog it back into position manually until I had a chance to cut the housing so it only had to make one curve.

I see that yours has to make 2 curves anyway because of where it is, but if it works without any housing then wouldn't that suggest that at some point the housing becomes long enough to interfere with the curve? Could you set it up like that and cut the housing to the length and curve of the naked cable?

avoidingmyphd

UPDATE - we have now solved this problem using a Nokon Extension Kit. The rear brake is now working properly for the first time since the bike was new a couple of years ago, having previously gone through lots of attempts to fix it, a change of caliper and cable changes without succeeding.

I'm pretty sure it's a geometry problem on the Dolce. Thanks to everyone who came onto the thread to propose solutions!