dbs

After a lenghly discussion on this forum, I changed my 5-speed freewheel to a 7-speed freewheel (albeit at the risk of bending my axle). Part of the process was to re-dish the wheel.

Instead of me investing in the $150 worth of tools to do this (and do it badly in that it would be my first time) I elected to give it to my LBS. (ok may $150 is too much but tools of any quality will still be much more than the $20 the LBS charged to do it)

I just got a call from the LBS. I was told they couldn't totally align the wheel in the center because the spoke tension on one side would be too loose and the spoke tension on the other side would exceed the recommended maximum. Does this sound right?

I would think the process would be that you'd loosen one side and tighten the other side (which would add tension to the side you just loosened) and in the end, they'd be the same tension plus the wheel center would be shifted to one side.

What am I missing?

DannoXYZ 

What kind of bike? Going from 5-speed to 7-spd requires increasing the axle/dropout width from 120 to 126mm. This reduces the amount of dishing needed. If you try to squeeze a 7-spd freewheel onto a 120mm wheel and fix the off-centre hub by dishing the wheel, you may end up with such a great difference in spoke tension between drive & non-drive side that the wheel will be extremely weak.

The problem is not too much spoke-tension, but rather the tension-difference between the two sides.

Wil Davis

If the original wheel was in good shape and there were no problems with the axle length and spacers, I would have thought that re-dishing would be fairly simple. If I couldn't do the simple tweak (as you describe; tighten one side and loosen the other), I think I'd slacken all the nipples so that there was one thread exposed, and start again.

- Wil

Bill Kapaun

Could it be that the spoke ends would protrude through the nipple on one side???

dbs

Correct. I had to increase the distance between the dropouts from 120mm. I actually went to 128mm. This was a simple thing to do. But still I don't understand the imbalance in spoke tension. If you loosen one side it will be tightened when you tighten the other side. In the end they should balance. If they weren't balanced then the wheel should continue to move until it was balanced. It would reach equilibrium or a force balance between spokes pulling in one direction versus the spokes pulling in the opposite direction.

DannoXYZ 

Hmmm, did you add all of that extra 8mm to the right side? Or 2mm on the left and 6mm on the right? If you did the 1st, it would cause quite a lot of dishing needed. If you did the 2nd option, that would require less dishing. However, both would still require more dishing than the original.

Now the forces from the spokes pulling on the rim doesn't balance out. That's because they aren't pulling in exactly opposite directions on each side of the rim. You'd need a hub with flanges as large in diameter as the rim itself. Instead, most of the spoke-tension goes to pulling the rim towards the hub radially. The lateral axial pull on the rim is then based upon how far from centre the hub-flange is. The tension is then the sine-function of the angle between the spoke and centre.

What dishing does is that the spokes with higher tension then moves the rim laterally to that side. Kinda like this photo (ignore rim on back part of wheel):



The more you need to move the rim laterally, the greater the differences in spoke-tension between the left & right sides. Maybe the shop did run into a problem with that differnece, the right side spokes may have bottomed out on the nipple perhaps?

dbs

DannoXYZ

I moved each side 4mm. I actually had a local frame builder do it using his surface plate and dial indicators.

I understand the primary force vector from the spoke is radial direction and not parallel with the axle but the simple fact is at some point the forces need to balance on there will be movement. That is simple physics. It will move until all the pulling and pushing is equal.

I'll run to the LBS and get a first hand look and explanation on what is going on.

Wil Davis

I think your demo pic might be more meaningful if you used a rear wheel rather than a front wheel…

- Wil

dbs

I just thought of it. The reason is due to the asymetry of the hub/freewheel combination relative to the frame. On the non-freewheel side, the lateral force vector will be greater for a given spoke tension because the angle of the spoke is less vertical than on the freewheel side. And because the lateral vector generated from the spokes on the freewheel side must exactly balance (equal but opposite) lateral force generated from the non-freewheel side, the tension in the spokes on the freewheel side must be much greater than their opposite counterparts. Hence to reach a force vector balance you may get into a situation where the one spoke is very loose and one is very tight. Again, this is caused by the asymetry between the hub location and the drop outs. Simple.

vpiuva

I think what happened is they ran out of spoke tightening length (too long and nipple bottomed out) on the drive side, and if they tensioned the non-drive side to an appropriate level, the wheel is not dished enough. If the drive side spokes were shorter, they could acheive an acceptable tension on both sides of the wheel and remain dished. The flanges are the same width apart on a 120 and a 126 hub, at least for Campy both are 57mm.

Retro Grouch 

Sounds right to me. Dishing a typical rear wheel results in the drive side spokes having greater tension than the non-drive spokes. The greater the amount of dish, the greater the tension differential.

cohophysh

I am still somewhat confused...if you open the rear triangle equally (cold set method) and you centered the wheel between the two drop outs using spacers, is there still a need to redish the wheel?

vpiuva

Yes, because the 7 speed freewheel is wider than the 5 (or a standard 6) speed. If you bent only the drive side chainstay to accomodate the new width, then no redishing would be necessary, but you don't and can't do that.

dbs

Yes the flange to flange length is the same but when you add the freewheel, the flange on the freewheel side is closer to the center of the dropout than the flange on the non-freewheel side. As a result, if you are trying to center the wheel centerline on the centerline of the hub-freewheel combination the spokes from the freewheel flange will be more vertical than the spokes on the non-freewheel side. As a result of this asymetical geometry, for a given spoke tension the amount of force generated parallel to the axle is less for the spokes on the freewheel side than on the non-freewheel side. Because of this and the need that these two forces must be equal, the tension in the spokes on the freewheel side must be greater than the tension in the spokes on the non-freewheel side. This is independent of spokes bottomout, length, etc. It a simple need to balance forces.

I reviewed my wheel with the person and the LBS and this asymetry is very obvious. I don't expect this is any different on any other bike. The drive side spokes carry the higher tension.

vpiuva

My comment on the flange width being the same for a 120 and a 126 hub was to say that wheels were built like this for years, so yours can be, too. Your existing drive side spokes are just too long and bottoming out. Don't let the LBS convince you this can't be done, cause it was and is.

cohophysh

So I assume redishing is needed for both MTB and road bike? and the worst that could happen if it isn't done is rear wheel failure?

DannoXYZ 

Ok, another related idea is the offset of the flanges from the centreline of the axle between the dropouts. Strangely, if you added 4mm to each side, the hub-flanges should still be offset an identical amount as before. They will also each be 4mm further away from the dropouts. You shouldn't have needed any dishing of the rim if you had added 4mm to each side... odd...