Saddle height and crank length question

spasypaddy

Had a bike fit back in april on my roadie and my saddle to BB is 712mm and this is on a 172.5mm crank. Now i bought a CX bike which has 175mm cranks so what height should my saddle be?

its quite important as i had the fit done to sort a knee complaint out

Bobbinogs

Not sure what you mean by this:

my crank to BB is 712mm

If you mean that the saddle to BB length is recommended to be 712mm then that would have been calculated with your feet on the pedals of the 172.5 cranks. So, in my opinion, if you now have 175mm cranks then just take 2.5mm off the 712. 2.5mm shouldn't make that much difference though.

EDIT: Might be worth getting in touch with the bloke who did your bikefit. They should have kept the pictures and data, etc., so will know how much tolerance you have.

spasypaddy

yeah sorry, fixed the error.

i will email him as hes very helpful, just thought id also ask here as it could take a few days to get a reply.

Monty Dog

Try 2.5mm lower than the height with a 172.5mm crank

spasypaddy

but then i feel cramped monty as thats what im currently doing.

nickwill

I actually went the other way which might sound counter intuitive. I felt cramped and took the seatpost up by 2.5mm. I found this to give me my best ever position on the bike. If in doubt, I would leave the saddle in its original position as 2.5mm is a very small measurement, and I suspect well within the margin of error for any bike fitting.

gwillis

I'm probably going to get corrected but there are a number of factors that could change how a bike feels such as a saddle wearing in and sagging slightly, thicker pad in a pair of shorts or even swapping to different shoes. When I picked up a campag crank recently which was 2.5mm diff to my usual my bike fitter said the difference would be minimal and I think if I recall was told to decrease the saddle height .

alex16zx

if you take it down by 2.5mm obviously you will feel a bit cramped because the distance from the top of your pedal stroke to your saddle will be 5mm shorter! just play around...you won't be able to get them feeling the same with different crank lengths.

ireland57

If you were well fitted to the bike with 172.5 mm cranks then you should be LOWER by 2.5mm with the longer cranks (so 709.5mm).

Otherwise you are now stretching at the bottom of the stroke.

If the knee issue is caused in part by the "folding" of the knee at the top of the pedal stroke a shorter crank may be of more help.

I found two things when I changed from 172.5mm cranks to 165's on my road bike.

1. The relief to my knees was felt the minute I road out my driveway; my legs liked pedalling in smaller circles. I can do rides now that I couldn't before because my knees would stop me.

2. But..........the loss in leverage is significant. Even though it only measures a few percent I do miss the leverage on the hills and when I'm pushing out front or solo.

I've also had a bike fit done. The difference in crank length and strength required may not be noticed by experienced riders but it's a big issue for someone like me.....new to road riding 7 mths ago and never done endurance sports of any type (all sprint sports).

spasypaddy

ive just had an email from my fitter and he recommends dropping it by 2.5mm

so thats what i'll do unless it feels incredibly uncomfortable at which point it'll go back to the normal height.