Saturday, January 23, 2010

Crank Swap -- Part 1

This weekend I started the process of swapping cranks and bottom brackets. So far it's gone pretty cleanly and easily, and thus far, I haven't had to order any parts, though I doubt that's going to hold true. Here's what's up:

I started the project late Saturday with a small pile of tools and a transparent straightedge with metric and English rules. I have a set of calipers, but the ruler is close enough for what I needed. What I was interested in is what's called Q-factor. Q-factor is essentially the distance between the outer faces of the crank arms, at the pedals. This is important because it determines how far apart your feet will be as you ride your bike.

Q factor is really easy to measure, but not with the cranks in their normal, opposed position on the bottom bracket. The easy way to measure is to pop off the left-side crank arm, and put it back onto the bottom bracket 180-degrees out of phase from its normal orientation, so it's mounted the same way as the right side crank arm. Then you snug it back down (don't overtighten or you'll trash the crank's mating surfaces), measure outer face to outer face, and voila! That's your Q.

In the past I've found that riding a bike with a relatively wide Q-factor hurts my knees. I can't really remember whether that was with or without clipless pedals, but I do remember the achy knees, and it was worrisome because I've already had one knee scoped, and would rather not go back again for that. The ache set in after only a couple of rides, too, as I recall -- not good. The ache resulted from installing a bottom bracket ordered online that turned out to be 10mm too wide. It was intended as a replacement for the BB on my Paramount, which was in touring/commuting guise at that point, but as it happens, the axle was a 128 or so, not the 118 I'd ordered. That's a big change, and it was really uncomfortable, so it came off, was replaced by a locally sourced and narrower part, and it went off to my parts box. Anyway, from that experience, I've tried to stick with cranks spaced somewhere in the neighborhood of the bike I have the most experience with, which is my Kestrel.

The Kestrel has 8-speed 105SC crank arms mounted on a 107 or 108mm Phil Wood bottom bracket, and it has a Q I measured the other day at about 152mm. However, I've only had that crankset on that bike for three seasons, and when I measured the Schwinn (which at the time wore the Kestrel's original 105SC 7-spd cranks and the same size BB spindle), it came in at 149mm. Finally, with its old-fashioned SR Apex crankset, the Motobecane measured out at 146mm. So 150 or so is the magic number for me, and I'm thinking a little under is better than a little over.

All of this measuring in mind, on Sunday, I pulled the bottom brackets out of both the Motobecane (Swiss-threaded cup and cone, 122mm) and Schwinn (English-threaded cartridge bearing, again at 107 or 108mm), cleaned out the bottom bracket shells on both bikes, sat down to work on the Motobecane.Months ago, I'd picked up new old-stock Shimano UN-72 bottom bracket (113), which is prized in the tinkerer community because the pressed-on cup can be tapped off pretty easily, and with that cup removed, the bottom bracket can be used with Phil Wood bottom bracket rings in a completely unsupported configuration. In addition to jewelry-quality bottom brackets and hubs, Phil Wood makes rings threaded for French and Swiss threading, so the Phil/Shimano combination is great for older bikes for which a $110 bottom bracket may be a little hard to justify.

As the scuttlebutt said they would, the Swiss rings mated to the Shimano bottom bracket like they were made for each other, and the bottom bracket installed quickly and painlessly. With the 105SC 7-speed cranks bolted up and the front derailleur travel adjusted to fit the rings, the Q measures 155mm. Which is on the wrong side of 150mm, but still narrow relative to many contemporary cranksets -- hmm. Let's hold onto that thought for a moment.

The Schwinn is still sitting there in the attic without a bottom bracket installed, and no crankset. I have upstairs that old 128mm bottom bracket, which simple math leads me to believe will result in a Q with the Apex cranks of around 152mm. Smack-dab on top of the Kestrel.

So what are my options? Well, I could just leave the Motobecane with a 155 spacing and see how it feels. 3mm isn't much, after all. Or, I could mix it up again. The Kestrel's Phil Wood bottom bracket would work just fine with the 105 7-speed cranks and Phil Swiss cups currently on the Motobecane, and should get the Q down to 150mm or just under. That would leave the bottom bracket shell of the Kestrel empty, but I have the 108mm Shimano UN-73 just taken from the Schwinn that would bolt in easily enough, and keep me at 152 or so on that bike. I'd rather have the expensive BB on the Kestrel, honestly, but there's nothing that says the Moto isn't deserving. If I did that (and I plan to), I'd have three bikes very close to 150, without having to spend any money.

I've been on the rollers pretty regularly since the holidays, and my left knee is starting to ache, which isn't good, considering the relatively low number of miles. Since my last cleat fitting, I've swapped the cranks and replaced the saddle a couple of times, so there are probably some variances at work, there, that can be rectified. I've got an appointment scheduled at Belmont WheelWorks next week for that, and I'm hopeful that'll help.

In the mean time I think I'll keep the swap going, getting the Kestrel apart and then back together, undoing and redoing the Motobecane, and finishing up the Schwinn.

All for now,


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