Would it be silly to use a 172.5 on the drive side and a 170 on the other? Would i notice the difference? I used to think that I had to have 175 cranks, but eventually realized that I couldn't tell the difference between 170 and 175. What do you all think, bad idea?

Depends on how bad your limp is? Ya, it's a bad idea unless one leg is 2.5mm shorter than the other

Funny - I just bought a '72 Alan that had a 175 NDS and a 170 drive side crank on it. I emailed the original owner and she said it came that way, and she never noticed.

My first thought was bad idea but the more I thought, I decided that you should try it and let us know how it turns out. I can't think of a reason you would want to though. Why? Also, the downside might be knee issues. If you try it, pay attention to your knees. They will tell you if they don't like it. Don't ignore them.

I think I will try it out just because of velonomes sage advice.

I tried it, unknowingly, when I put together a dirt rider from part sets I had on hand. The chainring side was 165 and the other was a 170. Both looked identical as they were from the same manuf- Sakae.

I got 1 lap( 7 miles ) around around Salem Lake when the knees started to tell me something was not right. Started moving the seat,raising the post, just nothing would make it right. Turned around and went home. 2 weeks later I decided to go forward on another project and decided to rob the dirt rider's crank. Looked at them as I cleaned them up and realized the difference.

Knees are better and I'm left with 2 orphan crank arms.

Let me say it now...."told ya so"

Most people have one leg a bit shorter than the other, just like feet are rarely the same size. The problem as described above was probably due to the exaggeration of the mismatch with the non matching crank arms.

Thanks for real advice.

I was thinking the same thing before I read repechage's post. I have one leg shorter than the other. And I sometimes wonder if I get sciatic pain from cranking on the longer leg.

Some words of caution. Sometimes the longer leg is the result of the femeral joint not being fully seated in the socket. Chiropractors are often licensed to manipulate only the spinal column, but here in Japan they have more freedom to treat patients. Sometimes I get them to check to see if my femurs are aligned in their sockets. In addition there are people called "bone-setters and yet others called "Seitai" practitioners who perform a rather radical kind of chiropractic. I had a Seitai doc completely resettle my right femeral joint. Sciatic pain and back ailments vanished after 3-4 treatments. That was about four years ago — I may need to go again even though I regularly see a chiropractor.

As to who is able to examine you in your area, I have no idea. So before you ride with mismatched cranks — IMHO — I'd be inclined to check out your skeleton. If you are even on both sides, I'd ride a matched pair. If you have a naturally longer femur on one side, you could try out your current mismatch perhaps with advice from an expert.

Assuming you thought I was jesting, I was not, I was/ am serious. A smaller crank makes smaller circles, a 2.5mm difference in radius is a 5mm in diameter that's about an 1 3/4" difference. That would cause a noticable limp if your leg was that much shorter.

In additon the shorter side will be wanting a different gear inch since it's making it's orbit slower than the longer side. What starts as crank size ends up in the drive train. so ya....size matters.

Draw it out on paper and you'll see the difference in diameter is significant, if not damaging to your kness as 3speedslow noticed, it would drive me nuts! ......see now I've gotten all wordy

According to my ruler and conversion charts 5mm is just a hair over 3/16. Come to find out i dont have the proper chainring for this crank. I believe i have some in storage. I didn't know if this was a good idea or not, but now I am curious. I will let you all know when i try It out! thanks for all answers.

i'm curious too. i have different length cranks on different bikes. in theory would the problem be caused by pedaling a crank that isnt the right size for your legs, or from the mismatch?

It strikes me as a bad idea. I could possibly see it being okay if one leg was in fact noticeably longer, but IMHO the vast majority of people are better off with matching crank arms. If it's a bike you plan to ride a lot, this may lead to joint and/or back problems in the long term. No I'm not a doctor, I only play one on the internet. It also begs the question: What's the proper saddle height for mismatched cranks? Either 2.5mm too high for one leg, 2.5mm too low for the other, or +/-1.25mm off for both legs. If someone raised my saddle 2.5mm, I truly believe I would notice and would want it lowered before the end of a long ride. I know it's not much, but why not try to get as close to the right fit as possible? I think there is an unnecessary risk of developing problems from the cumulative effects of added pelvis rotation caused by using different crank lengths.

Btw, what type of crank? Maybe someone has an extra arm to make a match.

Great, if anybody has a spare, lone 165 mm Sakae SX crank laying around i'm available to adopt !

This one is the 165, I need the left crank arm.

Opps. Your right....but still a difference in circum. and still a wrong thing to do, odd sized cranks on the same bike.

No, He is actually not right, see my above post.

My knee jerk reaction was NO! Don't do it! But the problem is really my OCD. If I knew I was riding with two different length crank arms, that would be all I could think about while riding. OTOH, if you try it and don't notice anything different (and it doesn't cause you any pain), then why not ride the crank that way? If indeed you have one leg a little longer (like most people supposedly), hopefully the longer leg gets the longer crank arm or you can make the problem worse!!

I agree that this is a bad idea unless there is an orthopedic. Two years ago, I rode with a very fast one legged man who only had one crank arm and strapped his prosthetic leg to the rack.

BTW, I CAN tell the difference between 170 and 175. The most pronounced difference is when riding singlespeed. I can climb better with 175 and spin better with 170. I once tried 165 on a small road bike. I could spin like hell in the flats, but climbing was another story.

Oooooooo........your more right

Yup...175 = pedal strikes, 170's are fine.

It does depend on the person- some people can feel small differences, some are obliviously to it.

My right leg it 1/2" shorter than my left, and my pedal stroke is best described as "wonky". I don't have any kind of shims or similar to even out the discrepancy. I get by with a high cadence so I'm not stressing my knees, but I definitely feel it when I start adding miles. My left ITB starts complaining, mostly.

One of my friends built a bike for ultra-marathon rides (the Seattle-to-Spokane "Cannonball", STP, etc.). When I looked at it at the end of a one-day STP, he had the splined steel cranks off by one spline, that is, they were set at 175°/185° from each other. He'd never noticed.