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Houston, we have a (small) problem

by Carl Brewer last modified 2010-07-26 22:46

Our track powertaps are not quite right

I'll cut to the chase (I'm pretty busy working on sprintTracker, my little python program to track sprinters times etc), I'm responsible for some 10 track modified Powertap hubs, two are mine, the rest belong to the VIS, the NTID and Hilton Clarke.

There's a small problem with them involving the chainline.  We never noticed it on mine because it's only about 3.5mm out and I'm no great torque machine and both my and Emily's bikes have reasonably long chainstays so the chainline problem doesn't really show up. However, under some of the NTID and VIS boys who have real motors we hear noises at high power outputs, so we investigated the chainline of the hubs.

Best illustrated with a couple of (poor quality!) photos :

pt track hub chainline  That's what they look like

campy track hub chainline  That's what it should look like

 As you can see, even with my crappy mobile phone photography and quickly cobbled up bit of cardboard measuring device, the PT hub puts the sprocket about 3.5mm (the width of the lockring) too far towards the middle of the bike.  I think the guys at Wheelbuilder made a mistake reading the width of the hub and assumed that the sprocket was where the lockring is,  which it isn't. Most people would never notice, the 3.5mm deviation is small and under enduro riders would not show up at all, but put them under a big sprinter putting out a lot of torque and it makes noises and runs rough.

The fix is pretty easy, the hubs have a steel axle end cap that you can see in the top picture (with the flat side to allow you to do it up), that needs to be 3.5mm shorter and the other side needs to be 3.5mm longer.  Then, all the wheels need to be re-dished.  Bugger, most of them were put together by Daryl Perkins and he tied and soldered them, which is a PITA to re-do.

Anyway, these things happen and I'm sure the guys at Wheelbuilder will send us corrected end caps ASAP.  They're smart people and proud of the work they do, they'll want to take responsibility for this and fix it.  In the mean time we can machine down the existing drive-side end caps and put washers under the off-side ones.  It's fiddly and shouldn't have to happen but this is prototype and first generation stuff, we expect a few teething issues.  It's the price of being on the bleeding edge.

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