Bicycle Mechanics - Compact crank question

I've got a Shimano FC-R700 with 172.5 mm cranks and am interested in trying a shorter crank. Do I have to buy a whole new crank set or can I get just the sorter arms--and from where and is it easier (and/or cheaper) than getting a new crank set??

You can buy the crank without the rings but it is seldom cheaper than just finding the crank with rings.

Measure out 2.5 millimeters and see if you think you would notice the difference. It is about 1/10 of an inch

I'm curious as to why you want to try shorter cranks. There are lots of good reasons. What is motivating you here?

Measure out 2.5 millimeters and see if you think you would notice the difference. It is about 1/10 of an inch

You wouldn't think 2.5 mm would make a difference but it can. A longer crank arm requires more force to turn the crank and if the arm is too long or you're not used to it, the result can be stress and femoral-patellar pain in the quadriceps tendon at the patellar insertion. I know this from experience, having ridden a hilly century on a bike I'd been riding for only a couple of weeks. The geometry was almost identical to the bike on which I'd ridden other centuries except for the crank arms which were 175 mm vs 172.5 on my old bike. My knees hurt like crazy every time I rode that bike after the century until I switched to a 172.5 mm crank then everything was fine.

" Measure out 2.5 millimeters and see if you think you would notice the difference. It is about 1/10 of an inch"
Well it actually works out to twice that if you are trying to alleviate knee pain.
1/10" shorter means the seat goes up a like amount.
The "high pedal is now 2/10" lower than the seat. Not that much on a 2.5mm shorter crank, but make it 5mm different and it starts getting very noticeable. If you have sore knees, sometimes an inch is like a foot!
I'm looking to go to 165's on my MB/commuter for that very reason. I've got way more low gears than I need, so I'm want to try shorter cranks with a faster cadence/lower gear.

What is your inseam? Crank length is usually related to that. If you are having knee pain it could be related more to positioning. Have you looked at that? Do you have another bike where you don't experience pain? If so have you compared your position on the two? Are you cleats worn? Do you have a slightly bent pedal spindle or crank arm?

These are free to check out and really, you need to eliminate these possibilities as well otherwise even the correct crank length won't solve the problem. I'd look at these issues first before shelling out money on an experiment. Also, some knee pain is a sign off long term damage and it may not go away quickly or completely even after you have corrected the root cause.

Knee pain sucks. Good luck.

What is your inseam? Crank length is usually related to that. If you are having knee pain it could be related more to positioning. Have you looked at that? Do you have another bike where you don't experience pain? If so have you compared your position on the two? Are you cleats worn? Do you have a slightly bent pedal spindle or crank arm?



My inseam is 33" and the crank length that works for me is 172.5 mm. The bike I bought had 175 mm cranks but was otherwise set up the same as the bike I had been riding including saddle height and position. Since installing a new crank with 172.5 mm arms, I've had no knee pain on the new bike over about 700 miles. I realize this a N = 1 and the p value is worthless but for me it's significant. The crank I installed, by the way, was a Shimano R700, replacing an FSA compact carbon crank. I was constantly having to adjust derailleur position and tension to try to prevent the chain from being dropped with the FSA. With the Shimano, it has been "set it and forget it," with very smooth shifting and no drops. I wouldn't have thought there could be a difference in two cranks with the same size rings but there is.

A longer crank arm requires more force to turn the crank and if the arm is too long or you're not used to it, the result can be stress and femoral-patellar pain in the quadriceps tendon at the patellar insertion.
Actually, just the opposite. A longer crank requires less force to turn under the same conditions. A crank arm is a lever and a longer lever requires less force.

What a longer crank does require is that your feet travel further per rotation and the flexion of your knee is greater at the top of the stroke if you adjust the seat height to keep the extension at the bottom the same.

BTW, your problem with the FSA crank isn't unique. Apparently FSA had a run of badly made cranks that caused the shifting difficulties you experienced. A local bike shop had a number of new bikes that came with the FSA cranks and had the manufacturer replace them with Shimanos after a raft of customer complaints.

Has the OP solved his problem?

For the record, 2.5mm does make a difference in terms of bio-mechanics and neuro-muscular memory - ask anyone who has ever changed cleats or saddle position, mid-season, and then developed hip, knee or neck issues a several 1000km later. The problem is, because of the length of time usually needed to cause these kinds of injuries, most people never connect the injury to the change they made several months earlier.

[QUOTE=HillRider;4899033]Actually, just the opposite. A longer crank requires less force to turn under the same conditions. A crank arm is a lever and a longer lever requires less force.

What a longer crank does require is that your feet travel further per rotation and the flexion of your knee is greater at the top of the stroke if you adjust the seat height to keep the extension at the bottom the same.[QUOTE]
You're right, of course, yet the pain I experienced using longer crank arms was no different than the pain I experienced from increasing weight on squats. Maybe the biomechanics of loading forces on the knee and range of motion are similar.

Interesting to learn about the FSA cranks.

What is your inseam? Crank length is usually related to that.
I thought it was more important to base it on femur (thigh) length?

That's a good point but a bit beyond what we were talking about in general.

There is a relationship (ratio) between upper and lower legs. Depending on that ratio, riders tend either toward spinning and a higher, more forward saddle position or turning a bigger gear, at a lower cadence with a lower and more reward saddle position. Crank length falls in there based on which way a riders body is more naturally inclined towards and will be loosely related to inseam (it is a good starting point).

For the OP I was looking to see if a smaller rider was turn a big crank (which happens a lot in Asia) or if he was in the ballpark (which he was). Then it was all about any changes he had made in the recent past. I usually start here when looking at knee pain. Then it's things like bent parts, cleats that have moved and rider habits.