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-   -   Bike fit question after crank length change (https://www.bikeforums.net/general-cycling-discussion/798780-bike-fit-question-after-crank-length-change.html)

 xfimpg 02-14-12 07:33 PM

Bike fit question after crank length change

Hi
If you are fitted on a 170mm crank length, but then change to a 175mm, do you lower your seat by 5mm to compensate for the difference? How's my math?

Quote:
 Originally Posted by xfimpg (Post 13852675) Hi If you are fitted on a 170mm crank length, but then change to a 175mm, do you lower your seat by 5mm to compensate for the difference? How's my math?
The difference is +2.5 mm at TDC and at BDC, lower the saddle 2.5 mm.

 StephenH 02-14-12 11:03 PM

No, the difference is +5 mm at TDC and BDC. Anyway, lowering 5 mm sounds good, or just adjust till it feels right.

 bored117 02-14-12 11:44 PM

5mm sounds good. Just making sure to keep proper extension of leg.

 DX-MAN 02-14-12 11:59 PM

The pedal, at full bottom, is 5mm lower. To lower your saddle 2.5 will risk some strain in your legs (most likely knees); small diffs make big issues, I have found.

How did you 'get fitted' to the bike before the crank change? The heel-on-pedal method tells you to drop 5mm. The mathematical methods would likely come in closer to 4mm.

 MichaelW 02-15-12 03:53 AM

The pedal is also 5mm further forward at 3:00. Move your saddle forward 4.32mm to compensate.

 xfimpg 02-15-12 07:13 AM

I agree with the general consensus of lowering 5mm and forward by 2-5 to 5mm, depending on feel. Thanks for your input guys.

 bud16415 02-15-12 09:55 AM

There is always the thicker peddle method also or shoes with thicker soles. Winter socks might also compensate.

Have you noticed much difference with the longer crank length while riding?

5mm = .2 inches

 xfimpg 02-15-12 09:59 AM

Quote:
 Originally Posted by bud16415 (Post 13854662) There is always the thicker peddle method also or shoes with thicker soles. Winter socks might also compensate. Have you noticed much difference with the longer crank length while riding? 5mm = .2 inches
Quite negligable to be honest. I've had 170, 172.5 and 175 in the past, I found that it allows me to spin a bit more going to a 170. This is helpful because I am a masher.

 bud16415 02-15-12 11:23 AM

Quote:
 Originally Posted by xfimpg (Post 13854679) Quite negligable to be honest. I've had 170, 172.5 and 175 in the past, I found that it allows me to spin a bit more going to a 170. This is helpful because I am a masher.
I’ve rode a lot of different crank lengths and like you I didn’t notice much between 170 and 175. About 3/8 of an inch total in diameter is well within my comfort zone and dropping one gear seemed to spin me up more than crank length did.

 fietsbob 02-15-12 11:48 AM

thicker soles on your shoes will do it.

 IK_biker 02-15-12 06:25 PM

Done this a few years back.
Crank length 170 -> 165
I think I have had a specific thread on this - use the search function.
Fore/aft is optional, depends on cockpit aim and changes in body flexibility.

 LarDasse74 02-15-12 06:44 PM

Quote:
 Originally Posted by xfimpg (Post 13854108) I agree with the general consensus of lowering 5mm and forward by 2-5 to 5mm, depending on feel. Thanks for your input guys.
Don't bother adjusting the fore/aft of the seat - the pedals do go more forward by 5mm, but also more rearward by 5mm. The seat height is the only adjustment that is really affected, and you should just set it up so you get your proper leg extension at BDC.