Brad wrote:
> ---- I believe this can be explained by gearing.
> Take a 170-pound cat 2 rider & 30-pound bike (200 lb together), charging
up an 8% grade at 10.5mph. Then the rider is expending about 354 Watts. A
42/26 gear gives a cadence of 82rpm, an average force on the pedals of about
52lb, tension on the chain is 108lb, the radius of the rear cog is about 1.5
inch, and the rider is exerting a torque of about 19 ft-lb on the rear
wheel.
> Now take the same rider expending 354 Watts up a hill, but increase the
total weight from 200 lb to 400lb to represent a heavy touring load. The
rider can now only go 5.7mph, and for a cadence of 80rpm a gear of 32/36 (!)
is appropriate. Now the average pedal force is still 53lb, but the chain
tension has increased to 145lb, the cog radius has increased to 2.86 inch,
and the torque on the rear wheel is now 34.5 ft-lb.
> So: when a cat-2 racer went touring with a heavy load the torque applied
to the rear wheel doubled, the force on the chainring increased by 50% and
the applied force on the pedal eye was the same.
> But let's go back and check on Mr. Cipollini: rolling at 40mph on the
flat in a 53/11 gear with a cadence of 104 and a power output of 1292 Watts,
the pedal force is now 152lb, the chain tension is 248lb, the cog radius is
only .87inch and the torque on the wheel is 'only' 18.1 ft-lb. (this is
just an example, I hear he maxes out at 2000Watts)
> So in a finishing sprint in a high gear, Mario is much less likely to pull
the wheel out of the dropout, but substantially more likely to break a crank
or chain.
Some more technobabble for the masses. How can anybody possibly go from 200lbs racing weight to 400lbs touring weight? Even on an 8 month tour with tent and all, my total gear weighed in at less than 50lbs, so a correct ration would be from 200lbs to perhaps 260lbs to account for the added weight of gear and touring bike equipment and accessories. You also neglected to divide the forces over the number of teeth engaged (the radius is a red herring because it is not important, the number of teeth engaged is!), both front and rear. Jan never mentioned what type of Campagnolo hub he had that slipped, but the torque that can be applied by the Campagnolo cam is far superior to the 35 ft-lb that you mention in touring mode, so it is more than likely wear in the cam, defect, or incorrect tightening the reason of slippage. In a sprint there is also an extremely high side-to-side force on the bicycle and therefore the wheels in particular that is not as common on a touring bike. Sprinters use the frame/wheels as potent levers to counteract the pedals to give added speed, whereas this is not the case in loaded touring.
Personally, including the many years in various shops, I have never seen any crank break, except for those that had first undergone extreme stress, such as that witnessed in a pedal hitting in a corner or crash... If you think of it, how many race-employed Campagnolo pedals have you ever seen that didn't have cornering scrapes? Now on touring bikes, how many cornering scrapes? In touring these sudden stresses basically do not occur. Touring likely does reach the same ultimate stress levels of racing, but it is never as constant, nor as fraught with these and similar 'incidents'. I have ridden large mileages with Magistroni, Campagnolo, Ofmega, Miche, TA, Stronglight and SR cranks; without ever breaking any. I have, unfortunately broken pedal spindles (one lyotard and one gipiemme), stems (one Cinelli and another 3TTT), a couple hub flanges (one Normandy and one radially-spoked Suzue BMX hub) and an alloy saddle rail. While perhaps not as harsh as some others on my components, I do believe I can categorize my serious riding of years past as relatively strenuous.
Steven Maasland Moorestown, NJ
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