|The 7 speed cassette only left about 3mm of thread for the lockring, not enough! You can see a few marks on some of the threads in this pic from the top sprocket slipping off.|
In the last post I documented how I used a Dura-Ace track sprocket as the top sprocket/lockring on the rear hub, however it wasn't completely engaged with all the threads on the freehub. I was riding along and shifted into the top gear and felt it slip and then heard a clack. The top sprocket had tightened down then popped over a thread - a sign that there wasn't enough thread engagement for it to handle the torque of pedalling. The normal uniglide lockring threading is somewhat loose anyway which isn't a suprise as it's segmented with the freehub splines.
The root of the problem is that I am running 7 speeds on a freehub only meant for 6 sprockets. According to http://www.sheldonbrown.com/cribsheet-spacing.html a 7 speed cassette with spacers has a stack height of 31.9mm. My setup is actually an 8 speed cassette with the top two sprockets removed and the dura-ace track sprocket put at the top.
|8 speed cassette with top two sprockets removed and my dura-ace track sprocket comes in at 31.4mm|
|Freehub width measures at about 29mm|
This meant that my cassette was 2-3mm too tall for the freehub, 8 speed spacers are 3mm so if I used the thinner 2.5mm 9 speed spacers I could reduce 5 x 0.5mm which would get the stack height down to 29mm or so which is bang on correct. The only problem was that I may have to use a narrower 9 speed chain to suit.
I was a bit nervous about tightening the lockring because I was afraid of overloading the threads and damaging them. However since it was now engaging all of the freehub threads this didn't happen and I got it down nice and tight. I took it for a quick test ride out the back and it worked fine, although the friction shifting was now quite tetchy, trimming was required on most shifts. This is most likely due to using the wider 8 speed chain (7.2mm pin width) on a cassette with 9 speed spacing. I've got a 9 speed chain on order so that should fix that, the pin width of it is 6.8mm which is less than half a millimetre, however it should make a big difference.
In hindsight, I should have seen that the lockring engaging only half of the threads would not hold, particularly in such a highly stressed part. It needs to be strong enough to withhold the cranking of a heavy-set cyclist going up a steep hill fully loaded. Anyway, now I'm quite confident that it's up to the job because it's fitting like how it was intended to. That's all for now.