Changes to fit when changing crank length

Joe Exotic

Hi all

Im changing my crank length to 165mm Ultegra (Reccomendation from Bike fit)

Im going to install myself and try to fit the bike myself also, my riding for the next few months will be primarily on Zwift.

HAs anyone any advice in what to watch for when i do the fit E.G. shorter cranks means a higher saddle etc.

Any advice appreciated ?

lennymc

probably best to contact the person who did the bike fit and see what they say....

Joe Exotic

lennymc wrote: »

probably best to contact the person who did the bike fit and see what they say....

Not in business anymore as far as i know, think he moved to Austrailia

Effects

What was it at previously?

Joe Exotic

Effects wrote: »

What was it at previously?

Crank length ? 172.5 .mm

magicbastarder

interesting question i've wondered about before too. the bottom of the pedal stroke will be higher, but the top will be lower. usually fit is based around your leg length while fully extended, which would imply saddle height *should* be raised.

joey100

Probabky in theory you shud raise the saddle to take into account the 7.5mm. This will mean an increase in saddle to bar drop too so you may need to change the bar height too.

If you raise the saddle and leave the bars, your really moving bars awar from you too, reach will be longer. Can measure from saddle tip to back of hood to get an idea of full reach.

Joe Exotic

joey100 wrote: »

Probabky in theory you shud raise the saddle to take into account the 7.5mm. This will mean an increase in saddle to bar drop too so you may need to change the bar height too.

If you raise the saddle and leave the bars, your really moving bars awar from you too, reach will be longer. Can measure from saddle tip to back of hood to get an idea of full reach.

So would a general way of approaching this be

1. get saddle height right
2. Check reach /Adjust bar
3. Saddle position
4. Review saddle height in lieu of other changes
5. Review/set Cleat position on shoes


AM i missing anything ?

magicbastarder

joey100 wrote: »

If you raise the saddle and leave the bars, your really moving bars awar from you too

probably an utterly minimal change though really, on the order of 2mm maybe?

joey100

magicbastarder wrote: »

probably an utterly minimal change though really, on the order of 2mm maybe?

Probably alright, and even an argument that the 7.5mm woundnt be huge either. Some people need it bang on, others don't mind a bit of difference.

I'd start and get saddle height and set back right. No point adjusting height then bars then moving the saddle back, will change your position in relation to the bars. Get saddle sorted first in relation to bb, and then work bars from that.

Effects

joey100 wrote: »

Some people need it bang on, others don't mind a bit of difference

You really think most people will notice 2mm difference?

CramCycle

Having changed to 165mm myself, I wouldn't do anything off the bat, the raise is minimal. Give it a fortnight and if needed, raise by .5cm.

Joe Exotic

CramCycle wrote: »

Having changed to 165mm myself, I wouldn't do anything off the bat, the raise is minimal. Give it a fortnight and if needed, raise by .5cm.

Ya i might do this, did you notice much benefit of the change ?

joey100

Effects wrote: »

You really think most people will notice 2mm difference?

Personally nope, not at all. But some people are convinced they can. I use 172.5 and 170mm cranks and have same saddle height on all my bikes, even put the wrong power meter on the bike one time and had a 170mm left crank and 172.5mm right crank, nó idea until I removed them.

Was more a general thing that if yiu raise your saddle or drop it you should probably look at how that will impact the saddle to handlebar drop.

Daroxtar

Shortening the crank puts your knee slightly further back in relation to the pedal on your down stroke so saddle setback is just as important to keep an eye on as saddle height.

gaffmaster

In theory, you can raise the saddle the same amount as the difference in crank lengths.
In practice, it's better to have a saddle too low than too high.

If you are swapping crank length to open your hips up, you stand to benefit from a 15mm opening of the hip at the top of the pedal stroke, if you raise the saddle the same amount as the difference in crank lengths: 7.5mm. This is because at the top of your pedal stroke, the pedal axle is now 7.5mm lower than before, added to the 7.5mm rise you can now in theory accommodate.

I'd swap the cranks and try the bike out with that single change, then raise the saddle up incrementally if I felt I needed to. The setback will probably be automatically catered for by the angle of your seat tube.

If you swap the cranks back, don't forget to drop the saddle - don't want ouchy hoop syndrome.

EDIT: Did the bike fitter set your saddle height for your current cranks, or for 165mm ones?

Joe Exotic

gaffmaster wrote: »

In theory, you can raise the saddle the same amount as the difference in crank lengths.
In practice, it's better to have a saddle too low than too high.

If you are swapping crank length to open your hips up, you stand to benefit from a 15mm opening of the hip at the top of the pedal stroke, if you raise the saddle the same amount as the difference in crank lengths: 7.5mm. This is because at the top of your pedal stroke, the pedal axle is now 7.5mm lower than before, added to the 7.5mm rise you can now in theory accommodate.

I'd swap the cranks and try the bike out with that single change, then raise the saddle up incrementally if I felt I needed to. The setback will probably be automatically catered for by the angle of your seat tube.

If you swap the cranks back, don't forget to drop the saddle - don't want ouchy hoop syndrome.

EDIT: Did the bike fitter set your saddle height for your current cranks, or for 165mm ones?

Ha ha Ouchy Hoop syndrome ill remember that one!!!

THat sounds like a good approach ill try that

He set it for 172.5 as it was a good while back and wasnt getting the 165 immeadiatly

Joe Exotic

for Saddle height is the 109% method the one to go for.
I have the bike fit measurement which i will adjust for the 7.5mm channge in crank, but I want to Learn/Understand this better so that i can gradually get confident adjustng all areas of my fit using the original bike fit as a baseline.

What other methods do ye prefer ?

gaffmaster

I'd tend to trust the fitter, if you're comfortable on the bike. Consider what they determined to be your saddle height (measured from centre of Bottom Bracket and taking crank length into consideration).

Most good bike fitters won't have a set method as you described and will look at your overall physiology, then measure your leg angle at full extension - while riding the bike. You need a camera with a high frame rate and some software for measuring the angle.

Methods like 109% won't work for everyone, unfortunately. I've tried a few methods myself over the years and thought I was in a good position. When I got my first bike fit, I was dropped about 15mm! I didn't mind, as it meant I could slam my stem haha.

CramCycle

The 109% method apparently only works for 1/3 of riders, whereas the heel on the pedal works for almost everyone getting that nice 25degree knee angle apparently. There is a guy on youtube who actually goes into scientific literature on it, points out flaws etc. Sports Science is a pain in the ass as a science as you simply never get enough volunteers, and too many drop outs so every study needs to be taken as it stands but he covers it quite well. Dylan Johnson is the name. I don't agree with everything he says but I'll be damned if he doesn't put up a fair and reasoned argument. His attempts at humour with himself as a know it all club rider are a bit challenging.

joey100

Like others have said there's no real one size fits all way to set saddle height. Difficulty with bike fitting is that you adapt, your body adapts to the position and even though it might not be the best or most efficient it will adapt, until it can't. Usually by the time you can't get comfy or it's too much reach/saddle height, your probably way above what you should be.

The other thing I have seen a fair bit online is to not try and copy the pro's positions. Makes sense, they are pro's, they are the top 1%, by their nature they are not normal people on bikes, they might train around 20+ hours a week on the bike and have done since they were kids and when they aren't they are recovering. So because a pro uses a 140mm stem and a 15cm saddle to bar drop with a slammed stem doesn't mean you should.