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Is it dumb to run different crank lengths? eg. 167.5 + 170.(10 posts)

Is it dumb to run different crank lengths? eg. 167.5 + 170.Eug
Apr 17, 2003 7:08 AM
In my last resort quest to rid myself of knee pain, I've asked my store to look for 167.5 mm Ultegras or 165 mm 105s.

I'm currently running 170, and was wondering if I need to change both cranks, if only one leg is getting the pain.

I've heard of people running different crank lengths with success before specifically for this reason, but somehow it just seems a bit odd to me. I'm thinking I should start first with running both at 167.5 and if OK, maybe trying 167.5 + 170. Or is trying the latter just going to introduce a whole new set of problems?
bad ideaelviento
Apr 17, 2003 7:25 AM
I am 99% sure the crank length is not the cuase of the problem. Suppose the seat height is good, then the only difference that a longer crank makes is that your knee is maybe 5mm higher at the top of the stroke. Unless you are super short or super tall, I just can't believe that extra 5mm will cause knee pain.
Well, 175 mm cranks (mountain bike) killed my knees.Eug
Apr 17, 2003 7:56 AM
You may be right that going to 167.5 mm cranks (from 170 mm won't do anything. Like I said, it's a last resort.

However, I do know that on my mountain bike, going from 170 mm to 175 mm cranks absolutely killed my knees - both of them. Adjusting the fit and saddle height did absolutely nothing.

I went back to 170 mm on the mountain bike and with a few minor adjustments to the fit, within a couple of rides most of the pain went away.

I'm not super short, at 5'7" with 30.5-31" inseam. 170 mm is usually the recommended crank length for me.
re: Is it dumb to run different crank lengths? eg. 167.5 + 170.KEN2
Apr 17, 2003 7:32 AM
Have you looked at your bum knee and how it tracks through the range of motion, i.e. does it sway in and out (towards and away from the top tube), or is it angling out or in for the entire stroke? It's more likely that you need a shim on one side or the other of the cleat/shoe interface, than you need a different length crankarm.
My bum knee is straight as an arrow according to my spin teacherEug
Apr 17, 2003 7:51 AM
The weird thing is that my other knee does sway a bit according to her, but it's not that one that hurts.

My bum knee doesn't sway at all. She says it looks straight. Furthermore, if I use just toe clips, the foot inversion/eversion falls into the exact same stance as my cleated shoe.

My knee is over the pedal spindle as should be when checked.
Funny story... but truejhr
Apr 17, 2003 9:45 AM
When I was in college (approx. 100years ago)my roomate and I both raced. He had a medium level Puegot (sp) that he raced at the time. In a local crit he was involved in a crash damaging the front fork and one of his crank arms. He had the bike repaired and continued to ride it. Because of money issues the shop replaced only the bent crankarm. He rode the bike after this crash for about a year. When he got a new nicer bike (Batavus with Record) the new bike pedaled oddly for a while. Despite numerous attempts to check the new bike, nothing wrong could be found. Eventually, the funny sensation disappeared, only to reappear on the old Puegot that he now used as a rainy day bike. Finally, he discovered that the Puegot had two different length cranks on it (170mm and 175mm). He had ridden the bike that way for at least a year following the accident that required replacing a crank arm. He suffered no ill effects during this time and in fact won a number of races with this bike. Thereafter whenever he got in a slump he would threaten dig out the mismatched cranks again.

Moral of the story is it probably wouldn't hurt you to try mismatched cranks, but it may not help either. I think for most leg length discrepancies, they put a lift in the shoe, or a shim under the cleat to equalize leg length. I have never heard of anyone intentionally riding two different length cranks.

my 2 cents...sievers11
Apr 17, 2003 9:50 AM
1) I wouldn't run different length crank arms, but I would try the smaller 167.5 on both. In certain instances the difference of 2.5 on crank length equaled 3 degrees in the knee at the top of the stroke. Running different lenghts could cause your other knee too much stress, and cause you to wabble on the saddle.

2) Make sure your saddle is on the marginally high side, this will give you less angle in your knee at the top of your stroke.

3) Get some peddles that are free floating, like speedplays. They will allows your knees to move as they want, this is a big deal even if you don't notice the movement.

4) Learn to circular peddle at 90 rpm if you are not already. Unclip one foot on the trainer or on a flat empty road, and try pedaling with one foot...that is the motion you should be pedaling with both. It is easyer to work on this in the off season on a trainer than durring the season, to off season is a great time to increase your cadence and work on your stroke. If you learn to circular peddle it will add more power to your stroke and lessen the need to mash. Mashing at a low cadence will kill your knees.

5) Last suggestion, are you training to hard to fast...this is a common problem early in the season.

The goal here is to get less stress on your knees, spinning faster will help that out, as well as makeing the angle greater at the top of your stroke. Getting a smaller crank will do both, but don't mix crank arm lenghts.
Hmmm... My spin class bikes are 175 mm...Eug
Apr 17, 2003 10:25 AM
I was taking a spin class with these Schwinn "pro" bikes, which have mountain SPD pedals. However, I just found out they're 175 mm.

That would probably explain why my spin class usually gives me even more knee pain than the road bike does.
Spin bikes used to be sure they changed? (nm)ColnagoFE
Apr 18, 2003 9:50 AM
What pedals are you running?El Kabong
Apr 18, 2003 6:12 AM
Like the others, I find it hard to believe that a 2.5 mm difference in crank length is going to solve your problem. Are you running pedals that use a spring to try and center your foot? I find that my knees are very sensitive to that. Try some Speedplays and see if they help.

El K.