Coker wheel rubbing on brakes

[TonyMelton]

Howdy

I've just installed a pair of Magura HS-11 brakes with a brake booster arch on my Coker. They work great on the downhills, but on the uphills when I'm pouring on the torque there's a distinct rubbing sound coming from the brake area on the right hand side on the uni. Its kind of annoying and I'd like to fix it before I try to do an 80km ride on it [ www.cyclechallenge.org.nz ]. I'm not entirely certain that the noise is in fact coming from the brakes, as I can't see the brakes while I'm riding. But most likely that's what its from as that's the only thing I've changed recently. Any help from experienced Coker brake setter-uppers would be appreciated!

As far as I can see the Airfoil rim isn't wildly out of true, and there's a good 3-4mm clearence between the pads and rim on both sides the whole way round. It could possibly be the join of the rim hitting the pad, but again it seems to be in the wrong place in the pedal stroke and almost the join is pretty smooth. I can't replicate the noise by lifting the uni off the ground and spinning the wheel - seems that large pedalling forces are involved. I have noticed that there is a lot of lateral movement in the wheel - I can flex the wheel by hand so that it touches the brake pads on either side. This plus the torque would suggest to me that the noise comes from the wheel moving to the side when I pedal hard. In which case I wonder whether there's anything I can do about it at all?

I've already tried moving the brake on the right hand side out as far away from the rim as possible, and its in the furthest away possible position now and it still rubs.

Aarrrg! What to do?@!?

Tony

[Rowan]

You have two options to choose from. [list=a]You could:[*]Send it to U-turn and get him to tension the spokes until it is equal to the Strongest Coker wheel in the world, hopefully preventing the wheel from flexing while you torque. Check out the update too while you are at it.[*]You could send it to me for Christmas, and it would solve all your braking worries. No-brakes, no-squeeks. I'm sure I could get used to it or just chop the damn brakes off.[/list=a]

[U-Turn]

Thanks, Rowan. Yes Tony, its essentially all about the tension, although if you have a narrow hub that will contribute to the problem. In addition, a stiffer frame couldn't help.

[TonyMelton]

OK, looks like I could try increasing the spoke tension. (The frame is a custom made one with ovalised tubing so it should be reasonably stiff. The hub is the standard Suzue, though.)

How did you get such massive tension on your Super Coker spokes, David? Could I do it by hand with my spoke wrench or would I need special tools? Could a bike shop apply sufficient tension to stop the flex?


Tony


No sig

[nathan]

Yeah, Tony, as U-Turn says, the problem is spoke tension. Once I upgraded to the airfoil rim and a KH36 frame (stiffer than the stock Coker), I still experienced the exact same thing you are. Now, I no longer have a hint of rubbing and although the KH is replaced with a Hunter, the big change is to U-Turn's "The Strongest Coker Wheel in the World". It's bombproof.

You can probably tighten your stock spokes quite a bit - maybe enough to completely eliminate the problem. Try 1/4 or 1/2 turn at a time on each spoke. Tight is right. Just make sure your spoke wrench fits right so you don't damage the nipples.

---Nathan

[U-Turn]

The Suzue hub is a fantastic hub, but is really quite narrow for that size wheel. As a result, the spoke angle means that most of the spoke tension is perceived radially by the wheel. Little of the tension is seen laterally, which is required for lateral stiffness. On my 20", the Suzue is a beautiful width and results in a nice angle and a super strong wheel. The wide hub was designed (by Chris Reeder and George Barnes) to be a nearly linear scale-up of width to match the increase in diameter from 20" to 36", to achieve basically the same spoke angle as the Suzue on a 20".

What I'm trying to say is that increased tension will help, but I'm not sure that it will solve your problem because the geometry of your wheel is inherently flawed. Even if you achieve the same tension I do (which is painfully difficult, I'm sorry to say), there may not be enough of a lateral component to stop the lateral flex. But this is one of the reasons the Super wheel is actually super. It corrects a host of problems, including the geometry and tension ones. There are many, many steps to building a Super wheel, it takes a lot of time, and the components are expensive. But the results are worth it.

[nathan]

Couldn't've said it better. What he said!

---Nathan

[GizmoDuck]

Slightly off topic, but is it possible to get proper stainless steel spokes rather than the cheap galvanised ones that come on the Deluxe Coker?

[cyberbellum]

Originally posted by U-Turn

Even if you achieve the same tension I do (which is painfully difficult, I'm sorry to say), there may not be enough of a lateral component to stop the lateral flex.

On a cautionary note, don't even try unless your wheel has a geometry similar to U-Turns wheel.

When the rim of a tensioned spoke wheel is deflected from true the spokes on one side get tighter and the spokes on the other side get slacker. If the angle of the spokes is large then these force changes are large and the wheel is stiff laterally. If the left and right spokes are nearly vertical then there is very little change in tension and the wheel is laterally soft.

These changes in tension due to lateral rim deflection are virtually independent of the static tension in the spokes. When the rim moves to the side one set of spokes gets longer and the other shorter, which is what causes the changes in tensions. If the rim moves the same amount the spokes lengthen and shorten the same amount regardless of the base tension, right? It's just geometry. Since the elasticity of the spokes is pretty much a constant (until they begin to yeild or permanently stretch), then the changes in tension are pretty constant too. (Elasticity is the ratio of length change to force change. Highly elastic materials lengthen more under stress than inelastic materials, but they all lengthen under stress)

So what happens when the base tension is high? The rim is tremendously compressed by the spoke tension. If the spokes are nearly vertical the rim isn't held laterally, and once it moves a little too much to the side then it can dump the spoke tension and expand by folding itself into a nice potato-chip shape. Which it will most certainly will do if you tension up to U-turn's ultimate wheel tension without having the lateral stiffness that the wider hub provides.

Just out of curiosity, where do your brakes rub? Have you tried marking a pencil line on the braking surface and climbing a hill to see where it gets scuffed off? (obviously don't deliberately apply the brakes when you do this experiment.)

Good luck,

Tim

[U-Turn]

Originally posted by GizmoDuck
Slightly off topic, but is it possible to get proper stainless steel spokes rather than the cheap galvanised ones that come on the Deluxe Coker?

Yes it is. Tom Miller can provide you with them, or my wheels come with them automatically.

[GizmoDuck]

Thanks Dave,

How do I get in touch with Tom Miller? Can those spokes be made available through unicycle.com? Are they butted spokes?

That would be a huge improvement. My spokes are corroding after all the Coker MUni rides.

I was really worried last week that I had made some big flat spots on my Coker rim, but it was actually the tyre which was not sitting properly on the rim. But stronger spokes would definitely make me more at ease with using the Coker off-road.

Cheers,

Ken

[U-Turn]

Originally posted by GizmoDuck
Thanks Dave,

How do I get in touch with Tom Miller? Can those spokes be made available through unicycle.com? Are they butted spokes?

That would be a huge improvement. My spokes are corroding after all the Coker MUni rides.

I was really worried last week that I had made some big flat spots on my Coker rim, but it was actually the tyre which was not sitting properly on the rim. But stronger spokes would definitely make me more at ease with using the Coker off-road.

Cheers,

Ken

The Unicycle Factory. It's best to order from Tom direct. If the international aspect of things makes it difficult or if you don't want to make a phone call PM me and I can handle it. I will be placing another order with Tom soon. The spokes are straight 14 gauge spokes and you will need the special nipples; I could handle that too if you'd like.

[unisk8r]

Dave, what kind of hubs are you presently using in your "super wheels"?

[U-Turn]

Originally posted by unisk8r
Dave, what kind of hubs are you presently using in your "super wheels"?

I'm currently using a Suzue hub widened by Tom Miller in the same manner as George Barnes did his wide hubs (George no longer does them).

[GizmoDuck]

Originally posted by U-Turn
The Unicycle Factory. It's best to order from Tom direct. If the international aspect of things makes it difficult or if you don't want to make a phone call PM me and I can handle it. I will be placing another order with Tom soon. The spokes are straight 14 gauge spokes and you will need the special nipples; I could handle that too if you'd like.

Thanks Dave,

I might check to see if the other NZ Coker riders (there are only four Coker's in the country as far as I am aware) want some too and we'll get a stockpile to rebuild our wheels with.

Cheers!

Ken