Spoke Tension on Rear Wheel
Recently I completed building a rear wheel. The components used were a new Velocity Dyad 40-hole rim, new Wheelsmith double-butted spokes (2.0/1.7/2.0mm), and I reused the original Suntour hub (the bike is a mid-80s Miyata used for fully-loaded touring).
To get the wheel dished properly a large difference in tension exists between the drive-side and the left-side spokes. Per my Park Tools spoke tension meter the average tension on the drive-side spokes is 135kg, and the average tension on the left-side spokes is 75kg [note: all spokes came within 5% of these average values]. Velocity recommends a spoke tension of 100kg to 110kg for this rim.
In his article "Wheelbuilding" <http://www.sheldonbrown.com/wheelbuild.html> Sheldon Brown wrote:
Average freewheel-side tension should be up to shop standards for the type of spokes and rim being used. More important is that it be even. Don't worry about the left side tension on rear wheels. If the freewheel side is correctly tensioned, and the wheel is correctly dished, the left side will be quite a bit looser. You should still check the left side for uniformity of tension.
I'm wondering if the tension I have on these spokes is reasonable -- or if they are too tight. I pretty much split the difference, making half the spokes too tight and the remaining spokes too loose compared to the range Velocity recommends. If I understand what Sheldon wrote it seems that I should reduce the tension on all the spokes -- but I'm worried that this would make my left-side spokes too loose. Should I stay with the wheel as it currently is or does it need to be changed?
P.S. When I used "spocalc" to determine the spoke lengths for this build the difference between the drive- and left-side spokes the software gave was 1.5mm. The source that I purchased the spokes from didn't think that it made much difference if the spokes I used varied by 1mm or 2mm, and ended up giving me spokes that differed by 1mm. In retrospect it appears as though I should have asked for a difference of 2mm. Agreed?
The 9-speed and 10-speed rear wheels that I've built have the non-driveside spokes at about 65% of the tension of the driveside spokes (driveside at 158 kgf). Yours are 55%, so that does seems extreme to me, especially on an "'80's" style hub. I would expect pre-8-speed rear wheels to have less tension differential. I reuilt a 1980 Shimano 600 6-speed hub and it did not have such a large differential but I no longer have the wheel or the numbers.
I upgraded the freewheel from a 5- to a 7-speed. This necessitated moving all of the spacers on the rear axle to the drive-side. As a result this rear wheel has more dish than normal.
I'm wondering if it is necessary to space the hub over that far. Could you use a different spacer combination that puts the cogs a bit closer to the driveside stays?
Originally Posted by Manzanita
I wouldn't worry about the 135 kgf on the driveside spokes. I'd be more concerned about not having enough tension on the non-driveside spokes.
With all of the spacers on the drive-side of the rear axle there is about a 1mm gap between the smallest cog (on the 7-speed freewheel) and the drive-side dropout. When I did the 5- to 7-speed freewheel conversion I tried various spacer combinations but found that only by putting the spacers all on one side could I get the 7-speed freewheel to fit.
I see exactly what you are getting at. Moving the freewheel to the right would mean that I wouldn't need as much dish (which means that I wouldn't require as large a difference in tension between the left- and drive-side spokes).
At this point I'm thinking that I'll just run the wheel as is and see what happens. If spokes begin to fail then I'll probably change the spoke tensions to address the failure mode. Another possible solution would be to get some new spokes (so that I have more than a 1mm difference between those on the left- and the drive-side). However I'd like to make this my last resort so to speak.
Thanks again for your comments and suggestions.
me have long head tube
7-speed freehweel on a 126mm-spaced hub and rear dropouts gets you more extreme dishing than an 8/9/10-speed gearing on a 130mm-spaced hub.
Generally it's best to max out the allowable tension of the drive-side spokes (to 100 or 110 in this case) and then the non-drive-side will be whatever it is.
In your case, using this for fully-loaded touring, I think you're facing two problems.
1) unless the hub design has a reinforced axle, you've got a fair chance of bending the drive-side axle, since so much of it is exposed to torque and shear forces between the bearings and the dropout. This is why 8-speed freewheels were rarely viable, and 7-speed weren't for bigger guys or loads. I just built up a rear wheel wtih 7-speed cassette for my commuting bike. I'd say just go with a 6-speed freewheel.
2) the dish sounds excessive for touring application. You could just space out the non-drive-side a few more mm, and then flex the dropouts apart when installing the rear wheel.
Making a kilometer blurry
If you get new non-drive spokes, you might consider thinner spokes for the non-drive side (1.8-1.5-1.8). They will stretch more to reach tension, so they will be more likely to remain taught during a big bump (they'll still have tension when they are shorter).
With a 40h Dyad, you're not likely to see any problems for a long time either way.
me have long head tube
Excellent points, both. But if you use spokes that are 1.8mm at the ends, you should also use spoke washers so that you're sure they seat properly in the hub flange. You might just want to use DT Revolution or similar spokes (2.0/1.5/2.0mm).
Originally Posted by waterrockets
Kevin, changing spoke length is not going to alleviate the tension differential. And new spokes can break almost as fast as old spokes if there is not enough tension. Low tension spoke failure on the non-driveside rear occurs in the "J" bend of the spoke at the hub flange due to excessive flexture.
Originally Posted by Manzanita
If I were you I'd ride the wheel and watch it closely. It may become necessary to rebuild the wheel with a modern 8, 9, 10-speed hub, but that means new cassette and chain, and shifters if you want indexed.
Good luck, let us know how it goes.