Sabospins

Ok, this started out as a question for a tall man (6'7"-225lb-38"inseam) on what he might work on to take advatage of the obvious dissadvantage of being so tall in this sport.

Now I am recieving some rather compelling arguments about using a MUCH longer crank, namely a 225mm.

The following is a fellow making the argument FOR them, and I am looking for other opinions.

>>>You've got long enough legs to use them. If a 5'11 rider used them, he would probably develop some knee trouble, he would feel like his seat was too low at the top of the stroke, even though he would be extending properly at the bottom.

175's are designed for an inseam up to about 33in or so. (There are varying formulas out there to figure crank lengths.)

They say that everyone is usually faster on longer cranks, up to about 180mm once the get used to them, but the difference is very slight if you are only going up 5mm.

You on the other hand will have 45mm more leverage, almost another 2 inches!

Lets look at the math: You weigh 225lbs. when you are standing on the pedals you are probably putting about 100% of that weight on the downstroke?? (not sure) maybe more if you are pulling on the bars.

the differnce between 175mm and 220mm cranks in regards to torque, is huge!

T=LLxF

LL is lever length
F is Force 225lbs.


LL = 175mm = 6.889 inches or .574 feet x 225lbs = 129 foot lbs of torque with 175mm cranks


LL= 220mm = .721 feet x 225lbs = 162 foot lbs of torque!

It will be the like having another 57 lbs pushing on your current cranks.

(you would need another 57lbs of force to get the same torque from your current set up)


Ahhhhhhhhh.... The magic of the lever arm. Who said we'd never use physics 101 after we graduated<<<<

lotek 

sabospins,

I've never heard of anything longer than a 190mm crank,
and from what I know they are pretty scarce. 180mm should be fairly easy to find.
One compelling argument for the shorter crank is that you can generally spin at a higher cadence, and save
your knees.
Just my .02 worth
Marty

superchivo

You sound like a man who needs to talk to Leonard Zinn. Besides being a mechanical genius and a very tall man himself, Leonard specializes in making frames and cranks for tall riders. He has some very well developed (and tested) ideas on long cranks. 225 might be too long. The longest Zinn makes are 220. His rule of thumb is to multiply your inseam (in mm) by 0.21 or 0.216 to get a range of appropriate crank lengths.

https://www.zinncycles.com/cranks.aspx

Dave Stohler

Another thing to think about: grounding out the crank arm on the pavement. Not fun, especially at speed.

fore

162 - 129 = 33.

Phatman

maybe you should try a stock 175 for a bit and see how that feels. It is easy to get injured with the wrong crank length, and 220 sounds a bit extreme to me.

Xtrmyorick

Yeah, but .574*(225+57)=162
Which = .721*225

57 extra pounds would increase torque by 33 foot-pounds, same as extending the cranks from 175 to 220, which was the original point.

ParamountScapin 

Check out the latest Velonews.com tech column by Lennard Zinn, who specializes in building bikes and special long cranksets for tall folks. Lennard is something like 6'6", himself, so knows the problems first-hand. You can also look at his website, www.zinncycles.com, for additional info. And, he is good at responding to e-mails if you have specific questions. I know he has built many bikes for very tall riders, some with double top-tubes they are so large. He also does such things as raising the BB on his frames to allow for much longer crankarms. And, has had many sets of longer crankarms built and sources for such items. 190mm may about the longest stock crankarm, but there are longer ones available. Talk to Zinn.

Smoothie104

sabospins,

As long as you ease in to your new cranks, you're not going to get injured. Plus you can lower your saddle and get a little more aero.

Remember, the bicycle is a machine, powered by your legs, working a set of levers. Not everyone has the same length legs, why should everyone have the same length levers? Is everyones femur within 5mm difference in length?

Should we all ride the same size frame too?

You have longer than average legs, you will benefit from longer than average levers. What may seem extreme to people with shorter legs shouldn't matter, they have shorter legs. If everyone we knew rode 130mm cranks, and I suggested 175mm, someone would say "are you crazy....175?

And all this talk of not being able to keep as fast a cadence? thats Horseshlt.....gearing is gearing, a 39x14 @ 90 rpm = 21.51 mph etc.....regardless of what crank length is.

With 220mm cranks, you just move your feet in a 3.54 inch bigger circle. Considering you legs are at LEAST 3.54 inches longer than the shortest legged guy who is using 175's Where is the problem?

Bottom line: If you use a crank thats fit to YOUR leg, YOU will be much more efficient. And there's no way its the same crank that comes on a bike built for people who are 5'10"

Face the wall, take a book, jam the spine up your crotch like a bike seat, mark the top level on the wall, measure in mm and multiply by .21 225mm is probably too long, but I can promise you, 175 is too short.

That is all.

Michel Gagnon

One one hand, the longer your cranks, the more leverage you get and the more torque you can provide. But on the other hand, the range at which our own legs are the most effective is very limited, so the shorter the cranks, the longer one stays in the best range. Crank length is a compromise between both functions.

There are many formulae around. Generally, mountain bikers prefer longer cranks because they prefer to have more torque in difficult passages whereas road and touring bikers prefer shorter ones to favour smoothness. Right now, most people are "fitted" with too long cranks -- a result of the "one size fits all" philosophy of bike manufacturers. 50-cm bikes, for instance, should generally be fitted with 160 or 165 mm cranks.

As for the best formula for cycling on pavement, I don't remember, but I ride 175 mm with 35" legs. Mathematical proportions for 38" legs would give you 190 mm. Spécialités TA offers the widest range of cranks and I think they go from 150 mm to 185 or 190 mm. You could see them at https://www.peterwhitecycles.com

Regards,