Monday, November 26, 2007

Review - Cannondale SI SRM cranks - Mechanicals

The Cannondale I bought had one feature in particular in which I was interested - the Cannondale SI version of the SRM crankset.

Three things differentiate this crankset from other non-Cannondale cranks.

First, it has an oversize BB30 bottom bracket. The BB refers to the fact that it's a bottom bracket. The 30 refers to the internal diameter of the bearings. A previously proprietary design, Cannondale has released specs for use by other companies - an open source design if you will. As someone that works with open source software (linux and putty among other things), it's refreshing to see a good design shared with others.

The whole oversized-bearing concept isn't new - the standard 68mm shell will accept outboard bearings with much larger than current outboard mounted 24mm I.D. (internal diameter) bearings. Placing the bearings outside the bottom bracket shell has its disadvantages, the primary one being the fact that the bearings are so far apart from left to right. The crankarms have to be further apart too, or designed carefully to keep them relatively narrow. One thing that kept me away from Shimano, FSA, and related cranksets are their relatively wide Q-factors, the distance between the pedals left-to-right. Campy has consistently kept their Q-factor relatively small and Cannondale's SI cranksets follow a similar philosophy.

Instead of pushing the bearings outside the shell, Cannondale incorporates oversized bearings by stretching the bottom bracket shell itself. This is nothing new - Cannondale was the first to use a huge headset bearing, they use the widest (and single tube) suspension fork leg, and they first appeared on the map by mass producing oversized aluminum framesets. At any rate, 30mm I.D. bearings sit inside a much bigger than normal bottom bracket shell. With the outside-to-outside measurement the standard 68mm (instead of 90mm), it is a narrower design, more efficient due to the shorter bottom bracket axle, and gives the crankarm designer leeway to stiffen up the cranks laterally.

So my new Cannondale has such a bottom bracket shell and a corresponding oversized-bearing bottom bracket. The latter is nice because it uses a large diameter hollow aluminum axle - light and stiff. The short axle focuses leg power closer to the center plane of the bike. And the simple design (pressed in bearings) means less parts and a cleaner install. Sort of like integrated headsets versus the cup and cone ones - it's much easier now with the integrated headsets (no complex tools necessary). The integrated bottom bracket is similar, except that to replace the bearings (when was the last time I wore out a bottom bracket?) you need to press the new ones back into place. Cannondale has conveniently included the tools for doing this, so no worries on my part.

The second difference is the Cannondale specific Hollowgram SI (System Integrated) crankarms. Although the bottom bracket shell is an open standard, Cannondale has their own thoughts on what cranks should connect to the bottom bracket. Currently Specialized has a BB30 crank/BB combo, and a few other companies too, but nothing too widespread. Cannondale's cranks are hollow aluminum, extremely stiff, and very light. They are a modern version of the Pong cranks from way back when. They are among the lightest and stiffest cranks out there. With a standard width bottom bracket, the crank can be a little wider to make for a very laterally stiff arm. In addition the crank can follow the chainstay profile to give maximum clearance for the rider's shoe (and booties - but more on that later).

The final significant difference in the crank contains the proprietary SRM chainring spider, the force measuring setup which first widely appeared under Greg Lemond. The original power measuring system, the SRM measures the twisting forces between the bottom bracket axle and the chainrings. Because both left and right arms exert force on the axle, the SRM measures power from both legs. (I mention this because the Ergometer only measures the difference between the right and left leg.) The spider normally adds some weight, but the SI crankset allows the crankset to come out at something like 657 grams whereas the other "light" SRMs (Dura Ace, FSA) are more like 800 grams, with the standard one weighing in at about the same weight. Remember my way of measuring weight? Every 45 grams is 0.1 pounds - and this means the SI SRMs are about 150 grams, or 0.3 pounds, lighter than other SRMs. Not bad for just the cranks, right?

The SI SRM has a nice stiff bottom bracket, a narrow Q factor, and works in the efficiently stiff SystemSix frameset I've wanted to use. Great! So how are the cranks?

Ah, well. Hm. Let me go over what I found in my limited testing so far.

The shifting seems fine, but new chainrings always seem fine for the first few thousand miles. So that's sort of a wash. Appearance wise I'm not a fan of the black rings - I prefer the chiseled look of the Campy Record rings I have on my Primary Giant. Perhaps at some point I'll swap out the rings. Till then though, I'll suffer along. I'm also keeping the original KMC chain. It's quiet enough but my peace of mind is asking me to install a Campy 10s chain. I'll wait till the sand is off the roads and do the chain, perhaps the chainrings. Of course if I'm my normal self, the chain won't happen till spring of 2009 and the chainrings only when they wear out.

The cranks seem very stiff but it's hard to quantify since the cranks sit on a much stiffer frame. Regardless, I did notice that while sitting on the trainer (same rear skewer, same wheels, same tires), the bottom bracket shell barely moves on the Cannondale but sways noticeably with the Giant TCR carbon (I'd estimate the visible movement as Giant = 10-15 mm and Cannondale = 5 mm) . The Giant's Record chainrings therefore move a bit too, but there is a chainring "sway" (a left right movement as you pedal) which is noticeably separate from the BB sway. The sway is about 2-5 mm relative to the bottom bracket shell. The SI has almost neither - perhaps a 1-2 mm chainring sway to accompany that 5 mm or less bottom bracket sway. However, since chainring sways also develop over a couple thousand miles of riding, I'll have to reserve judgement on that.

I should point out that the cranks are shorter than my Records - I've returned to 170s after about five or so years on 175 mm cranks. I've been rethinking the long crank theory - I think that it's a good thing for someone who is short on fitness (relatively speaking) but, just like a long stroke engine loses ability to rev as highly as a short stroke engine, I am starting to think that short crankarms allow a rider to reach higher top speeds.

This upcoming season will be an experiment of sorts - I feel comfy on both 175s and 170s and I want to see if a shorter crank will help when I'm fit. As part of my "controlled" experiment, I want to measure my output differences between the carbon Giant TCR with 175s and the SystemSix with the 170mm SI cranks. I'll be doing this (hopefully soon) on a friend's Computrainer, a device which will allow me to swap bikes quickly without too much fuss, all while keeping the cranks, pedals, and bike intact.

A significant difference with the shorter cranks is that I found that my comfy "loafing along" cadence immediately shot up about 10-15 rpms, from about 85-90 rpms to about 100-105 rpms. I also found that my legs remembered the 170mm radius pedal circle - I felt immediately comfortable spinning the cranks around. This remained especially true when cresting short hills - I experienced the "rev-up" feeling as my cadence increased over the top of the hill. With long cranks my preference was to shift into a higher gear, but with short cranks, revving up a bit was fine. It's almost as if long cranks encourage me to be lazy (i.e. shift instead of pedal faster) whereas the shorter cranks encourage me to pedal.

The short cranks means that my legs make smaller circles. This means, significantly, that my legs don't come up as high. Therefore my legs don't intrude into my torso territory quite as noticeably as they do with the 175s. This in turn means that I can hold a lower, flatter position on the bike. In addition the SystemSix (52 cm size) has a one centimeter shorter headtube than the S sized Giant TCR, allowing me to drop the bars by about the same amount. With more drop, a higher seat height, and less thigh intrusion into my torso, my position is much flatter. So much so that my back has noticed the radical change - and I've had to start doing some exercises to accommodate its complaining. With more hours on the bike I hope to be able to turn this aerodynamic improvement into better overall performance. The flatter back also recruits more glutes and hamstrings - so I hope that I'll develop more power too. More aero, more power. Good stuff.

My sprint remains a mystery as I have some winter bars on the bike (wide with an ergo drop position) and I can't sprint to save my life with ergo bars. I'll have to wait and see how it goes when I swap back to my non-ergo crit bars. I hope that the shorter cranks will allow me to turn the pedals over faster in a sprint. Although my leverage has suffered (about 3% less), I'm hoping that my 10-15% increase in cadence will help compensate for that. In other words, if I apply 3% less power but I can pedal much faster (up to 120 rpms for a sprint), I hope that the higher revs will allow me to exert as much or more power relative to a lower rpm, high leverage sprint. My body is not great with sustained power (we're talking sustained over 1/3 of a circle with a 175 mm radius), but quick, short bursts work perfectly for me. I hope the shorter cranks, with their smaller 170 mm pedaling radius, will allow me to find that top speed power, power that I can't even contemplate reaching with the longer cranks turning their wider, lazier circles.

As far as the nitty gritty goes, the cranks did require a few re-torques after I put the bike together. I'd strongly recommend carrying an 8mm allen wrench the first few times you ride these things, or, preferably, ride the trainer for an hour or two with your toolkit nearby.

On a negative note - the nice silver finish is actually painted on the crankarm, and even only 6 hours of booties rubbing the arm slightly has already damaged the finish. I don't have big feet - size 41s - and my shoes, although close in to the cranks, normally don't cause such damage to a Campy crank for a year or two. Nevertheless the Cannondale SIs are irrevocably scarred in less than half a dozen hours of riding. Unfortunate.

This ends my mechanical review of the Cannondale SI SRM cranks. When I have enough experience with the SRM and its software, that review will follow. And when I have more of a feel for the SystemSix frame, fork, and how that rides, I'll post a more comprehensive review of that as well. The components, in case you didn't know, are the ubiquitous Record stuff. I have virtually identical components on my other two bikes and, well, there isn't much to say there. It simply works.

With such an investment in my bike, additional investments in related gear (some of which I'll review later), and some (obsessive?) thinking about factors which affect my racing, it's obvious that I'm looking to regain some of that "incredible but lost" 1992 form. The first step was getting some mechanical help (it has a psychological effect too) in the form of lighter and stiffer equipment. This is done. The next steps are as follows (and in no particular order): increase overall strength and conditioning; optimize balance of aerodynamics and weight in wheelset/s; optimize position on bike.

I hope to utilize the SRM a little more than the PowerTap, not because it's "better" or "different" but simply because I've never done any steady training with the PowerTap. My January and February training accounts for a huge portion of my annual hours on the saddle, and with the SRM, I'll be able to measure progress a little more accurately.

I plan to review some other equipment for my "strength and conditioning" phase, and I'll be commenting on my wheelset and rider position thoughts, explorations, and findings.

2008 awaits :)

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