Fat Louie

I'm curious as to how much of a difference shorter cranks would help with cadence on a road bike. I'm trying to focus more on cadence these days rather than mashing. I would think that a shorter crank would be faster to complete a revolution... I currently use 175mm cranks because that's usually what comes with a 61cm bike... I'm thinking of going to a little shorter crank so I can get my cadence up a little. I'm just curious if anyone here has tried that and if shorter cranks caused any discomfort or anything else that I didn't think of. It doesn't seem like. 2.5mm or 5mm would make a huge difference, but I know if my seat drops a few mm, it definitely makes a difference.

ModelT

I used to ride a bike with 180's and felt like I got better leverage than my current 170's.

Bill Kapaun

It worked great for me, although YMMV.
See my post here-
https://www.bikeforums.net/fifty-plus-50/361045-your-cranks-too-long.html

Note- I've got bad knees, which the shorter cranks definitely helped.
Screw the "leverage"! That's why they make lower gears!

deraltekluge

If you use the same gear, a shorter crank requires more force. Torque is force times radius. Decrease the radius, and you have to exert more force to produce the same torque.

rideorglide

I have shorter legs in relation to my torso. My change was for comfort and fit, with no real care about being possibly slower. I figured I would spin faster to compensate, if anything.

I experimented by shifting to 165mm cranks from 170/175 to try a different fit for my legs. The change revolved around comfort. It also allows me to lower the saddle a few millimeters (an insignificant amount for most, but a little more comfortable for me.).

I'm happy with the new bike and cranks and don't have any complaints, but I realize I may have sacrificed some speed/leverage. Knees haven't been complaining, but are they any better, I can't really say.

I think you have to be prepared to lose peed if you go shorter, and maybe have a compelling fit/comfort reason to make the shift.

Velo Dog

I'm 6'4", and most of my bikes (63-65cm) have 175mm cranks. I'm used to that length and spin at 90-100 comfortably. A couple of years ago, though, somebody gave me a 62cm Allez with 170mm cranks. I made the frame fit all right with a taller (quill) stem and long seatpost, but I never did get used to the cranks. I could spin a little faster, but that's not really a problem for me, and I felt like I had less leverage when I was climbing

ericthered

Wouldn't you need to raise the saddle by 5mm/10mm to keep the same pedal to seat distance? I guess you could slide the seat back a little as well.
Eric

TurboTurtle

I would agree. Would seem the real cure for the knees was shortening the saddle to bottom-of-pedeal-stroke distance which could have been done by merely lowering the saddle. - TF

TurboTurtle

Leonard Zinn, who is about 6' 9", totally believes that crank length should be somehow proportional to the body. Unfortunately, all his testing has shown that there is no difference. He even had different lengths on each side once by mistake and coudn't show any difference.

Your upside cadence limitation is determined mostly by how fast your nerves can fire your muscles at the top of the pedal stroke and stop the firing at the bottom. I cannot see how crank length would affect this.

TF

Bill Kapaun

The problem with most? bad knees is at the top of the stroke where the knee is bent the most. If the knee doesn't have the flexibility, you want to bend it LESS!
Read the link I posted above.

Bill Kapaun

That might be believable if your cadence was over 1000!

Antelope 70cm

I was going to try Zinn's 220mm cranks but they won't clear my chain stays. I use Dura Ace 180mm.

TurboTurtle

What happens when you try to pedal fast? You start bouncing, right? Why? Because you cannot stop pushing down at the bottom of the pedal stroke. Your foot cannot go down any further, so your butt comes up off the saddle = bounce.

Muscles don't act in isolation. Your nervous system is intricately involved in pedaling a bike. It is also part of fatigue.

TF

Fat Louie

Thanks all for the replies- Bill, reading your post, it looks like your situation is similar to what I'm trying to accomplish. One of the reasons that I think shorter cranks may help is that when I do my one legged drills on the trainer, I'm not smooth. While I have been able to get my cadence average up to around 90, I still don't feel like I have a good circular pedaling motion. I also have knee pain and hip pain as well, but it seems to be getting better these days (yeah for glucosamine and SamE!).

Others have mentioned the extra leverage and torque with longer cranks- while I enjoy the feel of a quick sprint every now and then as well, in reality losing a few seconds here or there won't make that much of a difference since I cycle for fitness- not being able to put so much torque will probably save me breaking spokes as well!

Bill Kapaun

You spin 90! I'm jealous!

Forget the leverage/torque aspect. You accomplish the same thing with lower gears!
"Torque-wise", a 48/16 with 175's is = to a 48/17 with 165's! You'll probably accelerate better, because your knees will allow you to spin up faster.

I used to ride the 46T big ring with a 13-28 cassette. I'm now running 38T with a 12-23 for my flat land riding. My "take off" gearing is virtually identical and I can use all the gears.
I have bought/made the tools to make "custom gearings" on my cassette, so I've been able to experiment a lot there and get combo that works GREAT for ME.

I thought I had a line on some 165MM cranks, but that fell through. It seems I pretty much would have to go to "road" cranks, but that entails buying different rings (130mm vs 110mm). There seem to be some possibility of using a BMX type crank by giving up my "granny" ring.
Budget limitations prevent me from taking a chance on buying anything that won't work. I really can't afford it IF it does work!
Go for the shorter cranks and just change the cassette to match your riding.

What bike do you have? Rings/cassette sizes?

Bill Kapaun

I can only speak for myself, but I didn't have ANY problem on the down stroke!
My feet were coming off the pedals at 68 RPM on the TOP.
I'd exceeded the free movement of my knee and I was lifting the entire leg. All that inertia wanted to keep going up!
I was also wasting a lot of energy, because as the other leg was finishing the down stroke, energy had to be used to carry the bad leg over the top!
I think the Bio Pace rings also helped too, since the crank speed is supposed to be the slowest at 6-12 o'clock.

How long have you had bad knee(s)? Or are you just talking out your hat?
You might as well diagnose riding position for my bad back! You don't have a clue about that either!

Fat Louie

It took me two years to work up to that, but now I'm between 85 and 90 most of the time. I started around 65rpm and 90rpm seemed impossible for any extended period of time... I think part of the problem was I wasn't shifting enough. Now I'm constantly shifting and my average speed is slowly getting faster.

I'm riding my 2007 specialized roubaix most of the time. I swapped the 12/27 for a 12/25 10 speed cassette and have the standard 50-39-30 triple up front. My other bike is a 2005 Schwinn Fastback Pro with 11/23 in the rear and a 50-36 compact up front.

jens5

I have a 61cm frame and tried 175 crank arms. Did a metric and almost ruined my knee's. The Dr. explained that although 1/2 cm. doesn't seem like a lot. the rotational difference is huge. Went back to 170's and all is well. BTW, I'm over 50 and a Clyde.

Richard

asabike

I have 185 cranks and spin at90 or so rpm,when I used 220mm I could spin at 90 or so, 175 the same.It seems to me that it is a matter of practice to gt to 90-100 rpm.the crank length is just for power.

ronjon10

I've got cranks with 3 lengths. 175, 172.5 & 170. The 175 was the size recommended to me by a good bike fitter and it works great for me. The 172.5 is fine and also works great for me. The 170 is annoying as heck to me. I spin fine with it, but I just don't feel like I'm getting anywhere.

TurboTurtle

Unless you are on a single speed, crank length does not change power. - TF

asabike

Why would the power only by affected on fixed, and only fixed? I noticed a huge power difference between 170mm and 220mm cranks.

ericthered

Wouldn't a better way to say it rather than power is that it effects gearing. Just as a change in the rear cassete,front rings, or wheel diameter, a change in crank link changes the moment arm and hence the sum of the power need to turn a crank revolution. With a geared bike, it would shift the gearing range, with a fixed it would change the the fixed gear ratio.
Eric