Auto Ads by Adsense

Monday, June 29, 2009

Cranks, Bottom Brackets, and Upgrades

Pardo was on my case about my cranks for about a year or so, ever since he built up my custom frame. His reason was that the pedal eyes of the Mavic 631 cranks had a reputation for cracking eventually, and that I didn't inspect the cranks frequently enough to catch any cracks. I resisted it for a year or so, since I was in Germany, and I figured that since those cranks had survived all of my bike trips since 1998, it would be fine.

Well, after I came back from Germany, I discovered a creak in the Phil Wood BB, and there was also a bit of chain rub when I pushed hard while climbing. I had The Bike Doctor look at it, but neither he nor I could get rid of the creak. Furthermore, while he had the cranks off, I felt the Phil Wood BB, and it felt rough to my hands.

After much consultation with Pardo, I settled on the Ultegra SL triple. Pardo thought that the bearings would hold up better than the Phil Woods, and he didn't think the design sucked. In fact, the Shimano Hollowtech cranks are a copy of an old Bullseye design, with an integrated spider and spindle, and a pinch-bolted left crank arm. Shimano had simply adopted it after both Octalink and Octalink II had demonstrable failures in the field and the Bullseye patent had expired. It seemed strange that Shimano bothered to wait for the patent to expire, when skipping the Octalink/Octalink II system might have saved them from those fiascoes, but such is life.

Original Bullseye Crank with Pinch Bolt (See entire gallery)

The most expensive bike shop in the world likes numbers, so we started by putting together a spreadsheet with all the components weighed. We noted a few things: first of all, Shimano's chainrings are now heavier than the same equivalent chainrings from 1992, even though the ones from 1992 were uncut, and had no ramps and pins while the latest and greatest stuff is pre-worn out from all those ramps cut into it! Maybe the steel pins pushed into it added some weight, but still, one would expect such small pins to make little difference to overall weight. Secondly, Shimano's assembly is shoddy. First of all, none of the bolts were installed with any grease in the threads! This is a big no-no. Secondly, for the middle and big chainrings, 4 of the bolts were aluminum and one was steel, indicating that the assembler had reached into the wrong bin for the bolts when assembling the crank.

The old Bullseye design had an axial loading bolt that protruded from the spindle. Shimano improved this by using a recessed bolt so your heel won't strike it. They did, however, make you use a special tool to loosen and tighten it. We guessed that they did so to avoid having folks over-tighten it, which could add too much preload onto the spindle and cause early bearing failure. However, the tool they provided had no way to apply a torque wrench to it, which left us scratching our heads. Pardo's solution was simple and elegant --- he drilled a hole in the middle of the recessed bolt, stuck an allen head onto it and a bolt behind it, and now I can tighten and loosen the pre-load screw, and even apply a torque wrench on it with a hex head. On top of that, I won't have to carry a special tool, since I carry a full complement of Allen keys anyway when I ride!

Modified Ultegra SL center knob (See entire gallery)

The installation went smoothly, and Pardo and I borrowed a torque wrench from Roberto and we proceeded to tune our fingers to how tight the pinch bolts should be. It turns out that while the axial bolt only needs to be finger tight, the pinch bolts needed to be pretty tight, as in, I had to use my entire wrist strength, but it would have been wrong to use body weight on it. Needless to say, we installed the lighter chainrings on the crankset before installing it, with my 24t steel chainring (lighter than Shimano's 30t aluminum chainring), my old 39t and 49t aluminum rings from circa 1993.

Incidentally, when we pulled the Phil Wood BB, once it was out of the retaining rings without any preload on the bearings, the spindle spun smooth as butter. So despite treating the bike as a submersible in the Tour across France last year, and numerous rain rides, the Phil Wood is still good for at least another 15000 miles. So if your Phil Wood feels rough in the retaining rings, maybe it's just the preload. You won't be able to tell without pulling the entire BB. Pardo thinks that the roughness when preloaded could also be due to grit in the seals (entirely possible, given that I treated the bike to some pretty wet weather in France last year)

Enough with the geeky stuff, how does it ride? The answer: it is nothing short of amazing. It was obvious from the design that the result would be stiffer. If you were Miguel Indurain or Mike Samuel (200 pounds) or Roberto Peon (also 200 pounds), you would feel it, but skinny me? (149 pounds) I'm sure I don't put out more than 250 watts on a good day.

My first impression when getting on it was, WOW, this feels STIFF. And not in a bad way. There's absolutely no budging at all, no matter how hard I push. But most importantly, all drivetrain noise went away! No creak, and more importantly, no rubbing of the chain against the front dérailleur! Pardo examined the Mavic 631 crank, and found that the spider, while it covered a huge surface area, was really thin --- this makes a difference because beam stiffness is the square of the cross section, so that explains the increased flexibility. Pardo didn't think I was in danger of cracking the spider before the pedal eye gave way, however. So I was flexing the entire spider, causing the chainrings to move towards and away from the frame on every pedal stroke, and if I was riding hard, that would cause my crank to twist so hard that I could not provide sufficient trim on the front dérailleur to keep the chain from touching the dérailleur!

Mavic Starfish 631 Crank (note the thinness of the spider). Full Gallery

Is there a performance difference? No, not that I can tell. (I rode a familiar road at my standard hard pace, and the speedometer showed no difference) But the entire assembly is about 230 grams lighter, and the noise has gone away, and the bearings will last longer (and when they fail, they'll be easily serviced, unlike the Phil Woods, which you'd have to get a special tool to press the bearings out of). Clearly I shouldn't have resisted this upgrade for so long. Highly recommended!

Anyone want to buy a used, Phil Wood BB + Retaining Rings? Or a Mavic 631 crankset?

P.S. The 2010 Shimano Ultegra cranks have been restyled so that non-Shimano chainrings don't look right when mounted on them, so if you want to avoid that "feature", you better buy up the 2009 Ultegra crankset!
P.P.S. I found out years later that the creak was because of a frame that was cracking!

No comments: