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Yep, the "common knowledge" of short cranks is true...(12 posts)

Yep, the "common knowledge" of short cranks is true...Eug
May 30, 2003 8:33 PM
Finally had a few short rides on my 165 mm Ultegras. (Bike came with 170 mm 105s - 54 cm Trek.)
Caveat: I'm just a recreational rider, kinda out of shape too (15 mph range).

1) The 5 mm difference is EASILY noticeable, even for me, a novice rider.
2) Easier to spin. It helps me use my hamstrings, and I now notice more often how weak they are. :(
3) Initially harder on the hills. Fortunately, it's not a big deal since I have the luxury of a triple and I can just learn to spin better on a lower gear.
4) Less stress on the knees. I haven't done long rides yet, but even on short rides, I had slight twinges after 10 km with my longer cranks. The twinges basically disappeared with my last few rides (12-13 km each) on my 165 mm. I will withhold final judgement until some 30-50 km rides though. I suspect the difference is not just the less extreme knee flexion, but also the fact that one is "forced" to spin.

I wonder if I would have noticed the same thing with 167.5 mm.
re: Yep, the "common knowledge" of short cranks is true...Jeff Rage
May 30, 2003 9:59 PM
This is an interesting topic, I never even thought about the length of my cranks. HOw tall are you? I wonder is being a shorter person if I would benefit from smaller cranks. Of course, I'm not that good of a "spinner." I like the feel of higher gears.

Oh yeah, I also wonder if this would carry over into Mtn Biking?
Argh! Putting the crank length debate to rest...Sprint-Nick
May 31, 2003 4:00 AM
Okay this is just my 2 cents... but my opinion is Sheldon Brown and other people who have formulas for why to buy different crank lengths are out to lunch. I'm sure they have their theories but I have mine as well. So here is my deductive reasoning why it will most likely help line their pockets more than it will help most riders except say a pro racer who needs that extra couple % power output. So let me make a few assumptions and do some math:

1) 6'0 foot tall person vs 5'4 tall person... its fair to say a person who is 5'4 probably has a 30" inseam whereas the 6'0 tall has about a 36" (probably not quite but just to make the math simpler).

2) Say the ideal crank length is 172.5 mm for someone who is 6'0.

3) Now figure out what percentage 30" is of 36"... so 30/36=0.83*100=83%

4) If a 6'0 tall person needs 172.5 mm cranks a person who is 5'4 obviously needs cranks that are 83% of that right?

5) So 83% of 172.5 mm is 172.5*0.83=143.75

Deduction #1***Anyone have any 143.75 mm cranks???

1) Now lets just calculate the difference between 172.5 and 167.5. 167.5/172.5=97%

Deduction #2***A 3% difference for a 17% difference in inseam... Hmmm...

Then lets just take a look at the distance covered by your feet by a 172.5 mm crank vs 167.5 mm... say averaging 100 rpm for 60 mins.

1) 172.5 cranks - 172.5*pi (circumfrance of circle)=541.65 mm per pedal stroke 541.65*100rpm*60mins=3,249,900mm=3.2499 km
2) 167.5 cranks - 167.5*pi=525.95*60mins=3.1577 km

So theres 0.0922 km differce... 92.2 m. That'll let you spin 2.98 rpm faster - back to that 3% difference figure... is that worth a couple hundred dollars for a new set of cranks? I think on the track when you need the extra clearance + are spinning that much faster its necessary but for most people to buy new cranks it isn't.

Now I don't disagree it can't be noticeable like the person who posted this orginally said or it may be better on some peoples knees, recruit different muscles, etc. But alas thats a different topic. :)

My 2 cents,
It's about feel...merckx56
May 31, 2003 5:09 AM
Nick your assumptions about inseam are way off. I'm 6'1" and have a 32" inseam and ride 175s on the road. I can tell a difference between the 172.5 cranks on my rain bike and the 175s on the race rig. More leverage=more power.
It's highly personal, and there really is no formula for determining what length to ride. My wattage figures went up with the longer cranks and bigger inside chainring. Some people spin, some people mash.
Princess and the Peafiltersweep
May 31, 2003 5:54 AM
That is interesting. When my bike was built up, I ordered a 175 like on my previous bike. After several months of riding the new bike, I noticed that it had a 172.5 crank. At first I was a bit miffed, but realized if I hadn't noticed by then, what difference did it make?
Placebo effect?Continental
May 31, 2003 6:30 AM
The gear ratios, weights, wheels, tires, and frame stiffness of the two bikes certainly make more difference than 2.5 mm in crank length. I'll bet that if you put two identical cranksets on two identical bikes, but one crankset was engraved with "172.5 mm" and the other was engraved with "175 mm" people would swear that the 172.5 mm crank was easier to spin and the 175 mm crank gave better leverage, when in fact they were identical.
Dunno about 2.5 mm, but 5 mm is quite noticeable...Eug
May 31, 2003 6:39 AM
I don't have two road bikes to compare different cranks, but I tried my cranks on the same bikes.

MTB: 170-->175 - Instant knee pain - completely unusable for me. Spinning felt awkward.

Road: 170-->165 - Spinning easier. Knee pain less, but the jury is still out on that one.

I don't know if I'd notice the same thing with only a 2.5 mm difference, but I suspect that a more advanced rider might.
2.5 mm is noticeable...satanas
Jun 1, 2003 11:06 AM
When I started riding I used 170mm as this was standard.

Some years later first MTB came with 180mm(!) cranks - all the rage in 1985. I hated them, could never get comfortable, find a decent seat height, spin, etc.

Next MTB, back to 170mm , but sold bike due to handling problems. Then 175mm, since this was the only available length in XT. These were okay after I got used to them (about 2 months into a four month tour), but could never really spin comfortably.

So, decided to split the difference and try 172.5mm on the road bike. These felt right from day one. Better acceleration from rest and climbing, no real problem with spin. Tried 175mm for comparison - could climb in a higher gear (lower rpm) but not go faster, plus couldn't spin comfortably. Back to 172.5mm...

Then tried fixed, first 170mm, since had them lying around. No problems. Next 165mm, since "everybody" used them on the track, but couldn't climb or accelerate very well - hated them. Tried 172.5mm, but legs were almost torn off on descents - felt that way anyhow.

Currently: use 172.5mm both on and offroad, except for 170mm on the fixed. I wish I could get along with 175mm offroad, but unfortunately I just don't like them.

It has been suggested several times that the most efficient length is the one that you are used to but the only way you'll find out is to try for yourself...
I bought this BECAUSE of my knees... Also, some other points.Eug
May 31, 2003 6:23 AM
I would have preferred to have saved my $110, since Ultegra cranks are not much lighter than 105, and at my weight and level I don't notice any difference in stiffness.

However, it was a last ditch effort to save my knees. Years of cleat and seat adjustment on my MTB (170 mm) didn't solve my unfortunate problem with knee pain with every ride. The pain with my road bike (170 mm also) is much less, but I still get it, esp. with longer rides.

I remember when I tried 175 mm cranks on my MTB. (Everybody told me longer cranks are better for cranking over ultra-steep mini-hills and obstacles.) Instant pain, no matter how much adjustment I tried. Went back to 170 and the pain went back to my "normal" amount. So if I'm using 170 mm on my MTB I figured I'd try something even shorter on my road bike. Ironically, the 175s didn't help me that much on those minihills and obstacles anyway, and I enjoy the added clearance the 170s give me on the MTB vs. 175.

As for spinning. The strange part is that I'm NOT really spinning that much FASTER on my 165 mm cranks than I was on my 170 mm. However on the 170 mm I have to think harder to spin at that rate, but with 165 spinning feels more "natural" if that makes any sense. I'm still trying to get used to it though since I'm not used to spinning all the time. (I mentioned my weak hamstrings, etc.)

For the time being I don't know if my performance has really changed significantly overall. However, even if it is slightly worse I wouldn't mind if it means my knees are much more comfortable.

But like I said, I'll wait until I've done some longer rides until I make final judgement. I'm going for a 50 km tomorrow. Here's hoping...
Interesting new pointSprint-Nick
May 31, 2003 5:23 PM

You made a very interesting point when talking about shorter cranks and knee pain.

Now its my 2 cents that Sheldon Brown and other bike fitters go overboart fitting crank lengths to height/insean as can be seen from calculations I did. 3% difference in crank length making up for 10+% in inseam. However, you have brought up an excellent point that for people with knee problems shorter cranks may be a viable option. I hope your ride today goes well. Keep us updated!

54 cm Trek, 5'7", 30.75" inseam, 155#. See other post re: MTB nmEug
May 31, 2003 6:28 AM
I remember a semi-objective test in VeloNewsMR_GRUMPY
May 31, 2003 6:56 PM
It was about 6 or 7 years ago, when they took quite a few riders of all sizes and had them use different size cranks. The results were quite different than everybody thought they would turn out.
Most, but not all of the tall riders did better on the long cranks, but there were a few that put out more power on medium size cranks. Again, most of the medium size riders did better on longer cranks, but like the tall riders, there were a few that did better on short cranks. Most of the short riders did best on short or medium size cranks, but there were a few that did their best on 175's or even 180's. The results seem to show that longer cranks work for most, but not all riders.
Myself, when I switched from 172.5's to 175's, it took 2-3 weeks to get used to them. I now find that I can spin just as fast as any other rider that I know. In the winter training season, I work on low-load spin-ups, where I get up into the 160-170 range. I find that in the real world, anything over 125-130 is a waste of time and/ or effort. Maybe if I rode track, I would work on it more.