Mountain Bike Reviews Forum banner

1 - 10 of 10 Posts

·
Avid Fun-Junkie
Joined
·
56 Posts
Discussion Starter · #1 ·
This is an honest question about adaptive tech. I've got a freind who knows I've ridden and built for years. He has asked about using 2 differnt size crank arms to cover one of his legs now being an 1" or so shorter than other due to injury.
I've never done anything like that. Is it feasible to go 170 on one side and 175 other or would the result be to unsuable?
ideas? Anyone done this? Thanx.
 

·
Professional Crastinator
Joined
·
6,178 Posts
I don't see where it would cause any mechanical issue.

They even have kids cranks with 2 holes in each arm for when the kids' legs get longer.

You would probably have to buy 2 full crank sets, though, unless you get on down to your local bicycle co-op and haggle for scraps. Heck, they'll prob'ly give 'em to you in that case.

-F
 

·
Registered
Joined
·
804 Posts
I don't see why he couldn't try it out. I know people with shoe soles that are thicker to cover that kind of issue, but it is doubtful that would work well on a bike shoe. Unless he has flats and 5.10s.
A 165mm and 175mm arm set up would give him about 3/8ths of an inch, it would be some compensation for his difference.
Get a used set that matches what he has in the length he is looking for, swap out the arm. Sell it off if it ends up not feeling right to him.
 

·
EAT MORE GRIME
(ノಠ益ಠ)ノ彡┻━┻
Joined
·
6,516 Posts
it can and has been done. it's same as making a pedal shim, only neater

EDIT: nope. not same as shim my bad. but I have seen setups like this. thinking more about it... a shim is probably better
 

·
Registered
Joined
·
1,054 Posts
One of my bikes came factory assembled with a 170 & 175 crank arms. I don't know how long a rode like that and never noticed! I didn't find out until I bought new Bullseye cranks to replace them.
 

·
Registered
Joined
·
184 Posts
I'm sure it would work functionally but it seems it would be very odd to ride. I understand one leg being shorter than the other but a shorter crank on that side would mean that not only would it not travel down as far, but it would also not come up to the top of the stroke as high. Also, because it is covering a shorter circular distance in the same amount of time as the other side, I believe it is going to have a slower angular velocity, just like cassette gears.

IMO different thickness pedals would be a better solution and give more than a 5mm change without the drawbacks, be cheaper, and save the hassle if it doesn't work great. If you aren't worried about pedal strike or weight, just buy 2 sets of pedals with threaded pins and make a "pedal sandwich" for the shorter leg.
 

·
Carbon & Ti rule
Joined
·
5,410 Posts
I think it would help a lot, after all with 1 leg 1 inch shorter, It must make him rock on the seat a bit.

If you have ever ridden with rotor cranks, You will know how quick the body can adapt to what feels wrong at 1st.
 

·
Registered
Joined
·
2,701 Posts
I'm sure it would work functionally but it seems it would be very odd to ride. I understand one leg being shorter than the other but a shorter crank on that side would mean that not only would it not travel down as far, but it would also not come up to the top of the stroke as high. Also, because it is covering a shorter circular distance in the same amount of time as the other side, I believe it is going to have a slower angular velocity, just like cassette gears.

IMO different thickness pedals would be a better solution and give more than a 5mm change without the drawbacks, be cheaper, and save the hassle if it doesn't work great. If you aren't worried about pedal strike or weight, just buy 2 sets of pedals with threaded pins and make a "pedal sandwich" for the shorter leg.
I vote for this answer. Though he would be ok as long as he fit the bike to the shorter leg. The longer leg would just feel as if the bike was slightly misfit. Pedal seems to be the best way to get an efficient fit on the bike and still compensate for your friends situation.

Posted via mobile
 

·
Bicyclochondriac.
Joined
·
14,589 Posts
This is an honest question about adaptive tech. I've got a freind who knows I've ridden and built for years. He has asked about using 2 differnt size crank arms to cover one of his legs now being an 1" or so shorter than other due to injury.
I've never done anything like that. Is it feasible to go 170 on one side and 175 other or would the result be to unsuable?
ideas? Anyone done this? Thanx.
I can speak to this question with loads of first hand experience. I have one leg 50mm shorter than the other. The answer is a little complicated, and it depends on where the leg length discrepancy comes from: Below the knee or above the knee.

If the discrepancy is below the knee, then using a shim (or a lifted shoe) is the answer. It will make your knees even, and your two legs will have a very similar bend at the top and bottom of the stroke.

Introducing a different crank length is not going to help, and in some ways is counterproductive. The problem is that while the shorter crank might even out the discrepancy at the bottom of the stroke (the pedal will be higher), the longer crank pedal will be higher at the top of the stoke, which is the opposite of what you would actually want. In addition, since the lower part of the leg is longer on the longer leg, the resulting bend at the top of the stroke is deeper. Thus, it makes it hard to set up a saddle height that is optimal for both legs.

If the discrepancy is above the knee: This is more complicated. Adding a lift of shim or lift can help at the bottom of the stroke, but now you have a much longer effective leg length below the knee on the shorter leg than the longer leg, and at the top of the stroke the shorter leg is now bent more deeply. Different crank lengths can help this. The problem is that as the two cranks get to be very different lengths, you are essentially in two different gearings.

In my case, my left leg is 50mm shorter than my right. 15mm is below the knee, 35mm is above. I normally wear a lift that is 50mm at the heel of most shoes. Of course, with bike shoes, it mostly matters how much the ball of your foot (where the cleat is) is lifted.

About 12 years ago, I bought a mess of cheap square taper cranks ranging from 160 to 175, and tried loads of combinations for about two years until I settled on a 5mm difference. Running the same lengths felt a little weird because my left leg felt too bent at the top of the stroke. at 10mm difference, the difference in the gearing and the size of the circle I was spinning was just noticeable. 5mm was just right.

Also, from using shoes with different lift heights under the ball of the foot, 40mm seems to be the sweet spot.

SO in the end it is a combo of lift/shim and different crank lengths that worked for me, but it is never going to be truly "optimal".


Here is my most recent shoe:
0204131356.jpg 0204131425.jpg

BTW, does your friend wear a lifted shoe? If he does not, he better start doing so ASAP. An inch is enough to cause him some really awful back and/or hip problems years down the road.

One last thing: If you want current mid-to-high end crank buying different lengths can be expensive, and a real pain in the ass. Long story that is another thread.
 
1 - 10 of 10 Posts
Top