PC: Initial Impression and First Question

Triathlon Forum
1

When I say “initial impression” that’s just what I mean, my impression when first seeing them. They won’t go on the bike until Monday because bragging rights are on the line this Sunday. So…first thought was…Did Frank take a set of connecting rods out of a big block, long stroke, mountain motor and call them “cranks”?

Looking through the included literature I noticed there was talk about clutches breaking beyond a certain load. It says “100lbs”, so, if my cranks are basically 7" long, does this mean it would take 700 lb-in to break them? This worries me a little, as I ride on desolate mountain roads that are often closed to traffic. Maybe I should get an even larger bag for bike so I can fit a tent in it. Any experiences with clutches breaking under peak loads?

2

When I say “initial impression” that’s just what I mean, my impression when first seeing them. They won’t go on the bike until Monday because bragging rights are on the line this Sunday. So…first thought was…Did Frank take a set of connecting rods out of a big block, long stroke, mountain motor and call them “cranks”?

Looking through the included literature I noticed there was talk about clutches breaking beyond a certain load. It says “100lbs”, so, if my cranks are basically 7" long, does this mean it would take 700 lb-in to break them? This worries me a little, as I ride on desolate mountain roads that are often closed to traffic. Maybe I should get an even larger bag for bike so I can fit a tent in it. Any experiences with clutches breaking under peak loads?

The “rated” load for the clutch is 847 in-lbs. For a 7 inch crank that is about 100 lbs tangential force. Since pedal forces are hardly ever tangential this load is hardly ever exceeded, especially when riding at a reasonable cadence when the pedal is moving away from you. The most common maneuver these clutches break is when powering away from a stop. The most common time is in the 6 week to 3 month time frame when people are starting to feel comfortable with them and start to ride them “hard”. The pros rarely break them so we think it is more a smoothness issue than actual power per se, except for these powerful starts.

We recently had a customer who has broken two with a PowerTap so there is data. He says his torque when they broke was something like 400 ft-lbs and both were with powerful starts.

Anyhow, failure is rare but it does occur which is why we warn people. I wouldn’t head out into really desolate areas until you have developed confidence in them and in your abilities and smoothness with them. When in very desolate areas I would stay in “smaller” gears to avoid the high torque situation to minimize risk of failure there (that is a better workout anyhow). Even if it breaks on a ride you will always be able to get back (once you are adapted) by one-legging it or, if necessary, walking the ups and coasting the downs. Also, take a cell phone with you if there is reception where you are.

Frank

3

Frank, I think the larger concern would be riding out to a desolate area and have both your hip flexors “break” when you are 30 miles away from home. This is likely a greater issue than applying the >840 in-lbs of torque to the cranks :-).

Seriously though, even a smaller 140 lb rider such as myself, might be able to get to this load from a standing start hammering away from a stop light on the downstroke. Is 847 in-lbs where the cranks have been known to fail (ie minimum) and is the average much higher ?

4

Frank, I think the larger concern would be riding out to a desolate area and have both your hip flexors “break” when you are 30 miles away from home. This is likely a greater issue than applying the >840 in-lbs of torque to the cranks :-).

Seriously though, even a smaller 140 lb rider such as myself, might be able to get to this load from a standing start hammering away from a stop light on the downstroke. Is 847 in-lbs where the cranks have been known to fail (ie minimum) and is the average much higher ?

Most of the time our users don’t have torque data when they fail so we don’t know what the torques are. However, in view of how people are riding them (hard, including hammering up hills, etc.) without failure I susppect that that rated load is conservative and the clutches can and often do take more than that. As we have had more experience with these we have modified our manufacturing techniques to provide better radial support for the clutches and the failure rate has dropped substantially in the last year or so. I would like to get it to zero but, of course, that is impossible.

Unfortunately, sometimes (very rarely) the failures occur at light loads when some internal parts (springs) in the clutches fail on their own. This is not an age thing as most of our original stuff is still out there and these spring failures have occurred, once, on the first ride or we have seen them in clutches we get from the supplier. It is one reason we give a 2 year mechanical warranty on these things, just in case.

Frank

5

I never worried about it. Apparently, maybe I should. I’ll crank pretty hard on them on some hills, and, besides the grinding of the derailleur and the visible bending going on in the whole bottom bracket area, I haven’t had any problems. I think I’ll be a little more conservative on them, and at least maybe not sprint on them…especially going uphill.

6

I never worried about it. Apparently, maybe I should. I’ll crank pretty hard on them on some hills, and, besides the grinding of the derailleur and the visible bending going on in the whole bottom bracket area, I haven’t had any problems. I think I’ll be a little more conservative on them, and at least maybe not sprint on them…especially going uphill.

You are not alone on how you ride them and haven’t had a problem. I think most people who have put the time in on them to become fully adapted “forget” and just ride them, doing what comes natural, and never have a problem. It is what I hope people will do but I warn them of the possibilities, just in case.

7

Well, much of the time will be out of the saddle uphill low rpm (50-75) work, so that’s why I was wondering. However, I just looked at some data on my CP software and at 760 watts I’m only putting out 220 in-lbs of torque, so it doesn’t sound like it should be a problem.

8

Well, much of the time will be out of the saddle uphill low rpm (50-75) work, so that’s why I was wondering. However, I just looked at some data on my CP software and at 760 watts I’m only putting out 220 in-lbs of torque, so it doesn’t sound like it should be a problem.

I believe that is torque at the wheel hub, which is reduced some by the gearing (if you are in a 48/12 your torque at the front hub is times 4, Francois, correct me if I am wrong). The harder you ride them the closer you are to failure but it is usually not a problem for most people except when starting from a dead stop.

9

A friend of mine blew his clutch out starting from a red light. It wasn’t like it was a super power start - just a normal, let’s get going start.

10

I may be the customer that Frank is referring to who has broken two clutches with a PowerTap. First off, I want to make it clear that the powercranks have without a doubt improved my spin and turned me into a much faster cyclist. I started with the cranks in Jan. of 04 because I had a pretty severe leg strength differences; a problem that is close to being completely solved. The cranks have increased my overall max power, and my sustainable power.

I have broken the clutch twice, but I would not call the situations in which I broke them “powerful starts.” The first time I broke the clutch I was doing a workout that involved standing and hammering coming out of a 15-17 mph pace. My cadence was high, around 90 rpms. I am not the most powerful rider, if a race comes down to a sprint I’m probably going to lose. I’m not sure what my torque was when the first clutch broke, but I know my power was right around 400. Anyone one who is decent sprinter can produce that kind of power in a couple of seconds. Frank was very quick and considerate in getting the crank fixed and sent back to me. His explanation was that I shouldn’t be hammering on the cranks, and that my spin probably wasn’t smoothed out enough. At this time, I had been on the cranks for about 7 months, using them for about 80% of my rides. From my point of view, I had been on the cranks long enough to have a pretty smooth spin. This clutch breaking resulted in a fairly hard crash to my right side.

The second time I broke the clutch was about 2 months ago, so I had been on the cranks for 10-11 months. Thankfully, I was sitting when I broke the clutch this time so I didn’t crash. I was going up a hill, around 70 rpm’s putting out about 350 watts, when it broke. When it happened there was a loud snapping noise. I was about 15 miles away from home; I tried to pedal with one foot for a while, but I surrendered and called a friend to come pick me up.

I know all bike parts have a chance at breaking. That is a calculated risk that anyone takes when they get on a bike. However, I would consider these two incidents of the clutch breaking in normal riding conditions. After the second clutch broke, Frank was very quick in getting the crank fixed. He had no good reason why it had happened . “I’m not sure how to explain this” was what he told me. He explained it to just be a freak accident. All this being said, I still ride the powercranks. If I was smarter, I probably would not be using the cranks. I try to never get out of the saddle in them, and never sprint on them. Whenever I ride on the trainer I try to use them. I have close to no confidence in the product’s safety.

11

I may be the customer that Frank is referring to who has broken two clutches with a PowerTap. First off, I want to make it clear that the powercranks have without a doubt improved my spin and turned me into a much faster cyclist. I started with the cranks in Jan. of 04 because I had a pretty severe leg strength differences; a problem that is close to being completely solved. The cranks have increased my overall max power, and my sustainable power.

I have broken the clutch twice, but I would not call the situations in which I broke them “powerful starts.” The first time I broke the clutch I was doing a workout that involved standing and hammering coming out of a 15-17 mph pace. My cadence was high, around 90 rpms. I am not the most powerful rider, if a race comes down to a sprint I’m probably going to lose. I’m not sure what my torque was when the first clutch broke, but I know my power was right around 400. Anyone one who is decent sprinter can produce that kind of power in a couple of seconds. Frank was very quick and considerate in getting the crank fixed and sent back to me. His explanation was that I shouldn’t be hammering on the cranks, and that my spin probably wasn’t smoothed out enough. At this time, I had been on the cranks for about 7 months, using them for about 80% of my rides. From my point of view, I had been on the cranks long enough to have a pretty smooth spin. This clutch breaking resulted in a fairly hard crash to my right side.

The second time I broke the clutch was about 2 months ago, so I had been on the cranks for 10-11 months. Thankfully, I was sitting when I broke the clutch this time so I didn’t crash. I was going up a hill, around 70 rpm’s putting out about 350 watts, when it broke. When it happened there was a loud snapping noise. I was about 15 miles away from home; I tried to pedal with one foot for a while, but I surrendered and called a friend to come pick me up.

I know all bike parts have a chance at breaking. That is a calculated risk that anyone takes when they get on a bike. However, I would consider these two incidents of the clutch breaking in normal riding conditions. After the second clutch broke, Frank was very quick in getting the crank fixed. He had no good reason why it had happened . “I’m not sure how to explain this” was what he told me. He explained it to just be a freak accident. All this being said, I still ride the powercranks. If I was smarter, I probably would not be using the cranks. I try to never get out of the saddle in them, and never sprint on them. Whenever I ride on the trainer I try to use them. I have close to no confidence in the product’s safety.

All I can say is we can’t explain every break. When the breaks seem to occur at less than extra ordinary torques we will also replace the crank arm to one that has been manufactured to what we consider to be “better” support. We think that is the explanation for some of these failures and since we have been shipping the model 4 cranks with this new crank support the failures have been less. Like I said, it doesn’t seem to be a power issue per se because we know people can ride them at 800-1000 watts, or more, without problem and the pros hardly ever break them, at least they break them at a lesser incidence that they are a fraction of our users. That is why we think it seems more of a smoothness issue or a “murphy’s law, that clutch’s time was up” issue.

All we can do is tell people about it ahead of time for their potential safety and to help them improve the reliability as much as possible, try to improve the product as best we can, and repair them if and when it occurs. It is primarily intended as a training device although it has proven itself to be reliable enough that people have and do race on them.