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Discussion Starter · #1 ·
I do realize a recent post was just made on crank lengths, so my apologies to the poster if this distracts attention away from their concerns.

As a shorter rider, 5', I have been reading articles on crank lengths based on inseams due to some knee problems that have arisen on my old bike with 170 cranks. To quicken things I'll try posing my questions in point form.

1)Based on calculations I have seen, my inseam of 69cm, would dictate a 150mm crank. I'm wondering if such a drastic change will be beneficial. I know people suggest trying them and seeing what my personal preference is, but I'm curious as to opinions of others who have tried.

2)How can this be built? I realized there are custom cranks, but as you can probably guess I'm not looking to drop that kind of money for cranks just yet. Right now I'm looking at a Campy Centaur setup with a compact crank. From what I have read the Centaur compact crank has a bolt center diameter of 110mm (BCD). Does anyone know, or have they tried using crank arms from a BMX bike or Mountain with a 110 BCD. Can they just be interchanged with the Campy 170's or is there a problem with the bottom bracket.

I realize I could also drill and tap the holes myself but I'm not completely confident that I would be able to do this with my tools and with good precision.

Now for the fun part

3)Any suggestions as to how to Frankenstein a crank are welcome, be creative, and keep in mind I have not built the bike up as of yet, Centaur is just my initial leaning. So nothing is set in stone, and no options are off the table.

-cc
 

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commutercraig said:
2)How can this be built? I realized there are custom cranks, but as you can probably guess I'm not looking to drop that kind of money for cranks just yet. Right now I'm looking at a Campy Centaur setup with a compact crank. From what I have read the Centaur compact crank has a bolt center diameter of 110mm (BCD). Does anyone know, or have they tried using crank arms from a BMX bike or Mountain with a 110 BCD. Can they just be interchanged with the Campy 170's or is there a problem with the bottom bracket.
J&B Importers sells an MTB crank under thier house Pyramid label that is available in a 152mm version. It is similar to this: http://74.8.32.132/nondealer/product.phtml?p=1793&large=1. The part# for the 152mm version is 20402. They are sure to be cheaply constructed & heavy, it appears that the 48-38-28 chainring set is not removeable, and they likely won't work with a Campy BB. On the plus side, you'd be paying too much if someone charged you more than $30, and for that at least you can see what shorter cranks feel like and decide whether it is worth pursuing.
 

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Doing the math

commutercraig said:
1)Based on calculations I have seen, my inseam of 69cm, would dictate a 150mm crank. I'm wondering if such a drastic change will be beneficial.
Be VERY careful about these calculations. The assumptions going into them are NOT supported by any valid studies. They are simply the mathematical expressions of the author's theory that "crank length must be proportional to some body dimension." No quality study has ever shown this. That doesn't mean that you might not like shorter cranks, just that you can't calculate the right length. Humans are highly adaptable, and this confuses attempts to study the effect of crank length. Very short cranks tends to mean very high cadence, and this may not work for you. 165s are readily available, so you might try some of these to see if you like shorter cranks better. BMX cranks are available in short lengths.
 

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suggestions-

1- a 1" wood plank is about 2cm thick- try cutting out the world's thickest Lemond Wedge.. you know, just a big thick spacer... and use bolts that are 2cm longer to attach your cleats to your shoes, through this spacer. If you know what "tapping" means, you are surely handy enough to try this out. Heck, you could probably just duct tape a board to an old pair of pedals just to see how it feels. So cheap, and maybe you decide it's not worth the hassle.

2- Harris Cyclery can get you Bulletproof cranks in 140 mm, 150 mm, 155 mm, 160 mm, 165 mm, 170 mm for $35... 110 bcd crank arms only. Lots of nicer (more expensive) shorter cranks available for track and fixed gear bikes. http://sheldonbrown.com/harris/fixed.html#cranks

Any decent high end shop should be able to put together some custom cranks for you for less than the cost of Campy- not as nice, but whatever length you want. I bet the guys at Harris would do it for a reasonable price, if you need to go with on-line service.

You said "Frankenstein"- so I mention the products for Tandems called "crank shorteners"- I googled "tandem crank shortener" and found lots of hits including http://www.tandemseast.com/parts/cranks.html (so you get the idea.)

Finally, the Nashbar compact crankset looks like it would be easy to clamp in to a milling machine, and put another set of holes above the existing ones, then thread the new holes and cut off the old ones. This is a fun project because you get to order odd sized taps in right and left hand versions! HAH! Makes those bulletproof cranks look free!

My "stoker" is 4 feet tall, and she hammers on 170mm cranks. Yeah, they are too big- but by the time I get around to doing anything about it, we will have put thousands of miles on the bike, and she'll be up to 5 feet tall.

If I had to guess I would think your best bet (considering you are thinking Campy) is to build a compact 110 bcd crankset on 165 track cranks, and if that's not enough, put another 5mm or so of flat shims under your cleats.

DogMeat
( no clydsdale myself)
 

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Discussion Starter · #7 ·
thanks

Thanks for the ideas. Armed with the knowledge that I'm not teetering on the brink of madness by comtemplating such ideas is encouraging. When I finally get my bike I'll take it to the lab and begin experimenting with myself.
 
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