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Technical FAQ with Lennard Zinn: Crank length

  • By Lennard Zinn
  • Published Jan. 14, 2003
  • Updated Feb. 24, 2011 at 7:47 PM EDT

Dear readers;

I get a lot of questions about cranks – crank lengths, non-standard cranks, compatibility, etc., so I thought I’d just combine a number of them here.

–Lennard

Dear Lennard;
I would like to read your comments or opinions on a product called PowerCranks (see at Powercranks.com). I would like to buy a pair to improve my technique. Thanks for your help. –Helton

Dear Helton;
PowerCranks are cranks with a clutch bearing at the bottom-bracket connection in order to only engage the bottom bracket in the forward direction. You can only keep them turning if you pull up and around the entire stroke, since they spin freely backward. You can find articles about PowerCranks in back issues of VeloNews, namely May 31, 1999 and July 12, 1999, as well as Inside Triathlon in February 2002 and July 1999. I rode many months and many miles on PowerCranks. I certainly believe that they improve one’s technique and strength on the backstroke, since they will not come back up if you don’t pull ‘em up. That said, I am not sure that they do not also encourage you to soft-pedal the downstroke to avoid having to pull up as hard and fast.

I think that if you are gettingPowerCranks that you need two bikes, one with standard cranks and the other with PowerCranks. Do at least some power workouts on the standard cranks to not lose quad power, and, of course, race on the standard cranks, since those heavy, hard-to-pedal PowerCranks are much slower.

–Lennard

Dear Lennard;
About two months ago I had a set of PowerCranks installed on my road bike. Knowing there was an adjustment period I started out slowly and increased my training time gradually.  However, after about three weeks of riding I began to experience pain to my right knee.  It got to such a point that I couldn’t bend it and swelling set in.

I had the shop take off the PowerCranks and now my knee is finally getting back to some semblance of normalcy, albeit a bit tender.  The bike was set up exactly the same, especially the crank lengths.  I suspect it was the Q-factor that played a role with my knee condition. Do you know of anyone else who might have had a similar experience and do you have any information regarding the Q-factor?  I’ve been racing for 12 years and this is the first time my knee has been this sore.

–Darryl

Response from Dr. Frank Day, inventor, founder and President of PowerCranks:
Knee pain on using the PowerCranks occurs occasionally. This is the first instance that I have heard of it being associated with swelling. Typically the complaint is pain at the back of the knee where the hamstring inserts on the tibia and is probably associated with an “overuse” injury.

Typically, when users allow a little rest to occur and they go back  to the PowerCranks this problem does not recur. Your problem seems a little different. You say you started out slowly and have a strong base so overuse injury seems unlikely. The Q of the basic PowerCranks is essentially the same as regular cranks, so if this is your model the problem is probably not related to Q factor. If your model is the adjustable, the Q is about an inch wider, which bothers some people but I would be surprised if it could cause swelling.

In a previous life I was a physician. If this recurs when you go back to the PowerCranks it suggests to me that your knee may be unstable and the pulling the PowerCranks are forcing you to do may be causing abnormal knee movement leading to the pain and swelling and I would suggest a trip to the orthopod to have it checked out.

–Frank

Dear Lennard;
I’ve always enjoyed your insights on cycling, and I have a question for you. I’ve been talking to the importer regarding the Rotor crank. He mentioned you have been using it. I am thinking about investing but would like to hear a real-world opinion rather than a guy trying to sell these.

I am a serious 30-34 Masters Expert XC racer, and I pedaled this crank at the trade show and it felt very smooth! Please tell me what you think. I was also wondering if you were able to get a weight on the crank/bbset? I’m a 30-34 Expert Master MTB racer. I do a lot of training. My other question is, if I was to get the Rotor crank, would you think it would be detrimental to not have this on each one of my 3 bikes that I ride. I have a road bike, hardtail, and a FS and I ride each one pretty much each week. If I only had one of these cranksets on one of my training bikes would it screw me up?

–Dave

Dear Dave;
The Rotor crank is made in Spain and features a cam system so that the dead spot is largely eliminated. When one arm is pointed straight down, the other one is already 15 degrees or so (adjustable) ahead of vertically up. As this arm travels to the bottom, the other arm comes up faster and is again 15 degrees ahead when the other arm is at the bottom. We mention it in our Interbike 2002 coverage in VeloNews.

I think that Rotor cranks are a real advantage under some conditions. I have a pair on one of my mountain bikes and another pair on one of my road bikes. I think the Rotor is particularly good for climbing and sprinting. Your foot is always ready to push down – very efficient when out of the saddle especially. On a mountain bike when sprinting over short, steep hills out of the saddle, one normally loses traction because of the varying speed of the tire – fast when you are stomping down and slow when the feet are at the top and bottom. With the Rotor, the wheel speed is more constant and traction is far better.

Time trialing should also be good with it. Relative to standard cranks, there is certainly more weight and more resistance from all of the cams and bearings moving. You can particularly notice this resistance on cold days when all of the grease thickens. With a steel BB, the MTB 180mm triple I have weighs 1376 grams. Witha titanium BB spindle, the custom road 200mm (yes, you are reading this number right!) double I have weighs 1272 grams.

Astute readers will see that this is about a pound more than a Dura-Ace 180mm crank and BB. In the 2002 Vuelta, 5 of the 9 Relax-Fuenlabrada riders used Rotors throughout, and some figured prominently in long breaks. I switch back and forth between four road bikes, three mountain bikes, a cyclocross bike and a tandem all of the time with standard and Rotorcranks (as well as vastly different crank lengths from 175mm to 203mm).

It takes only a few strokes to get used to the Rotor each time, since the pedaling action is so natural. If you think of swimming or cross-country skiing, the arms move slowly when pushing the body along and fast on the return stroke. It is similar with the legs when cross-country skiing and I think with running as well – certainly it is with snow-shoeing. It takes a lot longer (10 minutes?) to get used to the standard cranks after using the Rotors. They feel very weird and unnatural for a while. I can’t say if or under what conditions Rotors are faster or not.

That’s the best I’ve got at this point. Down the road (or up the road), I will do quantified tests, but the results will not be available for sometime yet.

–Lennard
Dear Lennard;
I am a middle of the pack triathlete who at age 48 is losing speed and strength quicker than I care to admit.  I am riding a KestrelKM 40, which I have found to be the most comfortable bike yet.  My question is how much is gained by going to a longer crank arm? I presently have 175 but the local bike shop suggested 180. He stated that for time trials, a longer arm is better.  If you have poor spin technique there is no efficiency lost, but extra power gained. If this is so, why isn’t everyone on 180′s?
Any advice is appreciated.  (I am 5ft 11 in with an inseam of 29 inches.)

–Bob

Dear Bob;
This is too huge of a question to tackle here. A few years back, we did several crank-length tests with an ergometer I built and using Boone and Sweet Wings cranks varying from 100mm to 220mm with riders varyingfrom 5’2” to 6’6”. With no time to get used to them, the results were inconclusive. Short cranks were more efficient at low power outputs and long ones were more efficient at high power outputs for all riders.

Some shorter riders complained of knee strain with the super-long ones. In VeloNews back issues, these articles are in the April 10, 1995, April 29,1996, July 1,1996, March 1, 1999 issues. My own sense of it is that I go much faster on long climbs with longcranks (I am 6’6” and have bikes with cranks varying from 175mm to 203mm). I build custom bikes predominantly for tall people and equip a high percentage of them with custom cranks in the 195mm to 210mm range (for riders ranging from 6’5” to 6’10”). I am planning some on-road crank length tests soon.

–Lennard

Dear Lennard;
I’ve got an XTR 952 BB that only creaks when fairly heavy pressureis being applied to the pedals, mostly uphill but a hard gear on the flatcan cause it also. If I’m on the flat and change to easier gears, the creaking will usually disappear. The mechanic at the local shop removed and greased the bb, cranks and pedals, but it didn’t make any difference. –Fred

Dear Fred;
The only success I have had with this problem is greasing the BB cup threads and the inside of the shell and the insides of the cups where they fit over the cartridge. Use heavy grease. It usually works
If that does not work, try plumbers tape on the threads.

–Lennard

Answer from Shimano:
As far as the BB goes, The creak could be coming from a number ofplaces;

1 Make sure that the bottom bracket shell has been properlyfaced and chased.

2 Lubricate the threads and face of bottom bracket shell.

3 Lubricate cartridge shell, threads, non-drive cup threads,inside of the cup and flange.

4 Lubricate splines on both spindle and crankarms.

5 Remove and lubricate all chainring bolts. Replace and tightento torque

6 Periodically remove and lubricate interface of spider/chainringand lockring  on drive side crankarm. Replace and tighten to torque.

7 Inspect and lubricate internal pedal assemblies. If this doesn’t work, I’d start looking at the internal weld joints inside the frame. It’s very common for these points to creak in titanium and aluminum frames. If so, shoot some spray lube into the weld vents of the frame towards the joint that is creaking
–Jason W. Leith
Technical Representative/Cycling Components Division
SHIMANO AMERICAN CORPORATIONDear Lennard;
My question is in regards to mtb cranks for big guys.  I’m 6’5″and 230 lbs.  Over the past year I have ruined two XTR cranksets. The problem is that they need constant attention or they come loose and strip off on the drive side.  Shimano says it is the user’s responsibility to maintain the equipment at the proper torque level.  I would expect some durability from such an expensive product.

My shop mechanic thinks that as long as my legs are combined with my weight that I’m overpowering the set-up.  Do you think this is a credible response and if so wha tare your recommendations?  Especially since there are few 180 mm cranks around for MTB’s. As a bit of background information the cranks were installed by a shop and not by me.

–James

Dear James;
I have certainly seen some people with the loosening problem you mention. I have 180mm XTRs and have had no problems like that (6’6″, 174lbs). I think it is critical to get the tightening torque high enough initially, after making sure the splines are engaged.

–Lennard

Answer from Shimano:
During installation of the crankarms to the spindle it is critical that the entire interface is properly lubricated. Make sure the splines are correctly aligned. You will feel them engage with each other when installed by hand. Use a rubber mallet, and lightly tap the crankarms a couple of times onto the spindle interface to get them started. Once you have done this, begin installing the washer and crankbolt. Use a torque wrench to insure the crankarms are tightened to specified torque range (35-50 Nm/305-435in. lbs.)

It is important to check the bolts on a regular basis to make sure the torque is maintained. Note: Many times during the installation process, the splines of the crankarm can become permanently damaged due to mis-installation or mis-alignment. This can cause the cranks to come loose on a regular basis. Also, make sure you are using the correct bottom bracket. Failure to use the correct bottom bracket will result in permanent damage to the crank. You must use either the BB-M950 or BB-M952.
–Jason W. Leith
Technical Representative/Cycling Components Division
SHIMANO AMERICAN CORPORATION

James’s response back:
It helps, but I find it interesting that even Shimano notes in their response that “many times during the installation process, the splines of the crankarm can become permanently damaged due to mis-installation or mis-alignment.”

It puts the consumer in a difficult position between the shop and the manufacturer.  It indicates to me by their own admission that maybe it’s improperly engineered.  I would guess that between the shop, Shimano and me, we are all to blame to some extent.

In the future as a consumer I will look for a company that backs their products better. This leads back to my one of my original questions:  What do you think would be a good replacement crankset? And last but not least I tried that Progold lube you recommended a few months ago in VeloNews and that stuff is great.

–James

Dear James;
I don’t know off hand of any non-custom mountain cranks that come in 180mm (besides the Rotors mentioned above). I use XTR for that reason.Please inform me, y’all, if you know of others.

–Lennard

Dear Lennard;
I will be upgrading my crankset soon and am considering going 10-speedeventually.  Unfortunately I cannot afford to upgrade the entire groupset at this time.  I currently run a 9-speed Veloce triple drive train. Am I able to use a 10-speed Chorus double crankset with 10-speed front derailleur with my 9-speed chain and rear setup for now?  I’ve read where this should be compatible, but should I remove the resin insert?

–Shannon

Dear Shannon;
Yes, it is compatible. It is probably not necessary to remove the resin insert. At least with a Shimano or Connex or SRAM 9-speed chain, I know from personal experience that there is no reason to remove it.

–Lennard


VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books including the pair of successful maintenance guides “Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance,” and “Zinn & the Art of Triathalon Bikes.” Readers can send brief technical questions directly to Zinn.

FILED UNDER: Bikes and Tech / Technical FAQ TAGS: / /

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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