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Effect of Crank Length on Tolerable Seat Tube Angle
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This January, I switched from 165mm to 150mm cranks on my tri bike. I did this in an attempt to reduce hamstring strain (I have very tight hamstrings), flatten out my back with lower effective position, and also reduce knee tracking out to the side. Previously, I had to keep the front end fairly high to avoid hamstring strain during races, and I was hoping to be able to lower it by going to shorter cranks.

With the 165mm cranks, I had been riding with an effective seat angle of about 80 degrees and it was comfortable (except for hamstring strain during races if I had the front end too low). After switching to 150 mm cranks (and raising my seat 15 mm), this winter, I found that I wasn't able to tolerate the 80 degree seat tube angle with the same amount of seat to armrest drop as I previously had, it felt like I had way too much weight on my arms and shoulders. I had to move the seat back to about 78 degrees effective seat tube angle to be able to tolerate the same amount of seat to armrest drop as I had before with the 165 mm cranks.

As a result of the change to shorter cranks, I don't seem to be able to sustain a flatter position as a result of the shorter cranks, however, they did stop my knees from going out to the side at the top of stroke and so far I haven't had strained hamstrings. My 20 min FTP (on the trainer) seems to be about the same with the 150mm cranks as it was with 165mm. I probably won't definitively know about the hamstring strain until a race, since I've only rode on the trainer so far since January.

Has anyone else switched to very short cranks and found they needed to move their seat back a fair bit to be comfortable?

As a sidenote: After the switch to 155 mm cranks and with the the original 80 degree seat angle, I did try moving my cleats fairly far back (with speedplay cleat extender plate) to see if this would take some of the weight off my arms and shoulders (attempt to mimic the position in space at 90 deg crank angle (downstroke) that my feet would have been with the 165 mm cranks), but it didn't seem to make a noticeable difference. That being said, I did keep the near midfoot cleat position because it seemed okay otherwise and I think it keeps my calves a bit fresher.
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Re: Effect of Crank Length on Tolerable Seat Tube Angle [Brad79] [ In reply to ]
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Brad79 wrote:
This January, I switched from 165mm to 150mm cranks on my tri bike. I did this in an attempt to reduce hamstring strain (I have very tight hamstrings), flatten out my back with lower effective position, and also reduce knee tracking out to the side.

Brad,

Since your seat should be adjusted just as high from bottom of stroke to seat, I don't see how going to shorter cranks actually improves the situation for tight hamstrings. It's when the leg is extended that one has problems with tight hamstrings. That said, it one's gut in interfering with their quads as they reach the top of the stroke, then shorter cranks with really help out. The whole "improving hip angle" argument is a bit of a red herring in my opinion. IIRC Slowman measures the hip angle through the extended leg.

YMMV,

Hugh





Genetics load the gun, lifestyle pulls the trigger.
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Re: Effect of Crank Length on Tolerable Seat Tube Angle [sciguy] [ In reply to ]
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Hugh,

The pain I was experiencing was at the inside of my thigh just in front of the sit bones where the saddle contacts. My bike fitter had suggested to me that the pain was hamstring related and was mainly caused when my legs were at TDC with a closed hip angle, not when my legs were fully extended at the bottom of stroke.

As an interim measure, the fitter raised my front end (left seat height the same) to open up my closed hip angle and that seemed to mostly eliminate the pain (I only experienced it afterwards for a brief time during a couple of very hilly races)

The fitter suggested that shorter cranks would also be helpful to open up my closed hip angle and allow me to lower the front end without experiencing pain. I can't say for certain yet whether it has helped for that.

The one thing I've noticed is I have to move the seat back quite a bit to be comfortable with the same drop as I had before (this is accounting for a raise in seat position to keep the same leg extension as I had before).
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Re: Effect of Crank Length on Tolerable Seat Tube Angle [Brad79] [ In reply to ]
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Brad79 wrote:
Hugh,

The one thing I've noticed is I have to move the seat back quite a bit to be comfortable with the same drop as I had before (this is accounting for a raise in seat position to keep the same leg extension as I had before).

Brad,

You do realize that moving the saddle back closes down the hip angle and acts to decrease the effective seat tube angle?

.


Hugh





Genetics load the gun, lifestyle pulls the trigger.
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Re: Effect of Crank Length on Tolerable Seat Tube Angle [sciguy] [ In reply to ]
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to both of you (and everybody):

the hip angle opens at the TOP of the pedal stroke with shorter cranks, OR it gives you a flatter back, depending on whether you change your handlebar height. if you keep your armrest elevation drop unchanged, you must move your bars up the same amount you shorten your cranks, because you are raising the saddle. if you changed from 165mm to 150mm, and you wanted the same back angle, you raise your saddle 15mm, you raise your bars 15mm.

because the pedal sits lower to the ground at top dead center, you actually have an extra 30mm of distance between a point on your knee and a point on your torso. this is going to reduce the stress on your hamstring.

if you don't change your handlebars at all you have half that amount of stress reduction on your hamstring, and you have a slightly more open hip angle.

but, you are producing less torque, because the lever arm is shorter. not less power, but less torque. so you have to pedal a slightly higher cadence to make up for the reduction in torque (power = torque x rpm).

to your point. when you shorten the crank, you also move the pedal back, rearward, at 3 o'clock on the drive side (when the pedal is pointing forward). this kind of artificially steepens the seat angle. that's why you're feeling more weight on the front of your bike. when you shorten the crank, you have to move the saddle back that same amount to keep the same "effective" seat angle, and really you ought to move your pads and extensions back the same amount to keep all things equal.

Dan Empfield
aka Slowman
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