Serious problems with V-brakes on a 29er

[gkmac]

I have spent the past several hours trying to fine-tune some V brakes for my Schlumpf 29er with absolutely no success whatsoever.

The symptom was that I could stand the unicycle upside down, spin the wheel and check that the brakes are clear of the rim. They stop the wheel when the lever is pulled, and return to an equal clearance distance from the rim when released.

Sounds good right? Trouble is I would then sit on the unicycle and try to idle it with either foot down, and then one of the brakes would somehow make contact with the rim and try to prevent me from moving the wheel. (Not that idling a 29er makes much sense, but similar foot-forces must come from trying to get the unicycle moving surely).

I looked carefully at the brake pads and somehow they go off centre, so if my right foot is pushing down then the right brake pad somehow moves towards the rim and rubs it. And vice versa with the left foot.

It seems that my foot power is making something flex, most probably the frame.

I have adjusted the cable so the brakes are as wide apart as possible, but the right pad is still rubbing the rim. They are so far apart that only one of the pads seems to touch when I apply the brake when not sitting on the uni and hence seemingly have braking power at all. In other words, useless.

I've also made countless other adjustments on everything else, from the height of the moveable brake bosses, the springs that make the brake pads retract equally and even the curved washers that allow the pads to stay at an angle. Nothing works.

I have tried three different sets of V brakes (albeit with the same cable and brake lever) with no improvement. I have spun the wheel round and looking at the rim, it doesn't appear to meander from side to side so the wheel must be true.

And I have never had this problem with the secondhand Magura HS-33 brakes fitted to my KH 24" XC Muni. They seem to rest closer to the rim, but even when idling on either foot this problem does not occur.

Has anyone else ever come across this? Is it solvable, or am I just going to have to ride a virtual 45" wheel at speed without a brake???

[Mikefule]

If you've tried three sets of brakes, and if you are sure they were all set up correctly when the uni was upside down and under no stress, and if you are sure that the wheel is not buckled, then the problem must be flexing of either the frame or the wheel.

You have no control over flexing of the frame. Getting the wheel hand-tensioned at a bike shop could reduce the flexibility. A wheel is like a circular suspension bridge, designed to have some flexibility as the load changes. Too much flexibility can produce the symptoms you've described.

Good luck.

[rob.northcott]

Possibly an obvious question, but are you sure your leg isn't touching the brake arm and pushing it into the rim?
I think that's why Magura hydraulic brakes are so popular with unicyclists - nothing to get pushed by your leg (the slave cylinders stay rigidly fixed to the frame, only the pad moves). Plenty of people ride with cable brakes of some sort though, so it must be possible to avoid leg/brake contact (if that's what the problem is).

Rob

EDIT: woohoo, I made it into a sig!

[forget_your_life]

what you have is a very flexy frame..

it doesnt happen on your KH because thats a very stiff frame.

[Into the blue]

Maybe a brake brace would solve the problem.
http://www.skijor.com/hr_brake_brace_241x300.jpg

[joemarshall]

Are the bearing holders and the torque arm connection tight enough? The only time I've seen such a wobbly wheel is when the bearing holders were super-loose. Even the stock frame that comes with the Schlumpf isn't so flexy you can't fit a v-brake. See how much you can push the wheel to the side when it's in the frame upside down.

Similarly, check the frame brace bolts, if the braces are loose, then it'll be more flexy, or if you're running the two frame braces very close together. Be *very* careful when tightening these as you can snap the braces by messing with them. It might be worth looking at them carefully to check neither of them have snapped.

The cable - is it very short, and do you hit it with your leg when riding? That might do something funny.

The brakes themselves, are they nicely on the bosses, so they can't slide forwards and backwards on them?

Whilst v-brakes can take some messing around to make work perfectly (see below), they should just fit and be okay even on flexy frames, so most likely something on the frame is loose.

Also, while you're talking about v-brakes, to make them work well, make sure the pads are toed in - which means the front of them is closer to the wheel than the back.

To do this, get a square of card (like a cereal packet), pop it under the back of the brake pad and pull the lever hard, undo the bolt on the brake pad, whilst still pulling the lever hard, pull it a bit more, then do up the pad before releasing the brake lever.

This makes them work way way better than if they're setup without toe-in.

Joe

[gkmac]

Originally Posted by rob.northcott

Possibly an obvious question, but are you sure your leg isn't touching the brake arm and pushing it into the rim?

No. I did ponder that possibility when trying to adjust the retraction springs or whatever they're called, as when the brake pads didn't retract equally I reached down and pushed the brake arms to centre them manually.

Originally Posted by joemarshall

Are the bearing holders and the torque arm connection tight enough? The only time I've seen such a wobbly wheel is when the bearing holders were super-loose. Even the stock frame that comes with the Schlumpf isn't so flexy you can't fit a v-brake. See how much you can push the wheel to the side when it's in the frame upside down.

Well I did push the frame slightly in various places, and even my weak arms managed to make the right hand side of the frame wobble significanty. The cause of this was that one of the bearing holder nuts was quite loose.

I've tightened this and everything else on the frame and also toed-in the brake pads as Joe suggested, and by tightening the retraction springs on both sides a lot... I think I've hit the jackpot. I can now idle on either foot without a brake pad interfering.

I've been able to bring the brake pads a bit closer together now; when the brake lever is fully on, the wheel does stop but not as brutally as my Magura muni setup does, which I think is good since locking the wheel means the brake lever is your ejector seat button. When looking for posts with similar problems I found that Roger sets up his brakes to be non-locking so his brake pads are still clear in high-torque situations.

Of course the real test will come when I can ride it proper. My injury instigated riding quarantine period is nearly over...

Originally Posted by rob.northcott

EDIT: woohoo, I made it into a sig!

Robert, that was one of the best on-topic birthday greetings I've ever had. I sincerely hope you were talking about my muni tyre pressure and not my freestyle.