Saddle Height & Crank Length

ibr17xvii
ibr17xvii Posts: 1,065
in Road general
I've just got a new bike that has 172.5mm cranks whereas on my others they are 175mm. My saddle height is 765mm from the top of the saddle to the centre of the BB & I'm 183cm tall.

I presumed that on my new bike I would need to increase the saddle height by 25mm to 790mm or thereabouts to compensate for the shorter cranks but when I did it was miles too high.

I'm still tinkering around with trying to get it right but I'm in the ballpark of 765mm - 770mm which is basically the same as my others.

That's fine but for my own curiosity I don't really understand why? Was just adding 25mm to what I'm used to too simplistic?

Comments

  • whyamihere
    whyamihere Posts: 7,715
    You're trying to accommodate a 2.5mm change by making a 25mm change. You're out by a factor of 10.
  • ibr17xvii
    ibr17xvii Posts: 1,065
    edited May 2020
    whyamihere said:

    You're trying to accommodate a 2.5mm change by making a 25mm change. You're out by a factor of 10.


    Of course! Why didn't I think of that?!?! How thick am! :#

    You know when you look at something & it doesn't make sense but you can't work out why?!?

    Off now for a lie down in a darkened room...........
  • imposter2.0
    imposter2.0 Posts: 12,028
    ibr17xvii said:



    Off now for a lie down in a darkened room...........

    It’s 8.30 in the morning., you’ve had all night to do that.. ;)

  • ibr17xvii
    ibr17xvii Posts: 1,065
    imposter2.0 said:

    ibr17xvii said:



    Off now for a lie down in a darkened room...........

    It’s 8.30 in the morning., you’ve had all night to do that.. ;)


    I've been up half the night measuring my saddle height. I think I would've been more successful in the dark to be fair :D
  • mugensi
    mugensi Posts: 559
    I wouldn't even bother adjusting the saddle height and see how you get on with the new length cranks. 2.5mm is so small that its the same as wearing thicker socks in winter or changing the saddle to something with slightly more/softer foam padding.
  • imposter2.0
    imposter2.0 Posts: 12,028
    mugensi said:

    I wouldn't even bother adjusting the saddle height and see how you get on with the new length cranks. 2.5mm is so small that its the same as wearing thicker socks in winter or changing the saddle to something with slightly more/softer foam padding.

    Not sure I agree with that. If you know that your reach has changed, it makes sense to alter your saddle to suit.
  • fenix
    fenix Posts: 5,437
    2.5mm is bugger all. I'd probably keep it the same and see if it makes any difference. But I'm not very sensitive.

    I think it was one of the sky riders who kept taking his bike back to the mechanic complaining the height was wrong. It was measured again and again and eventually they twigged that the difference was old saddle Vs new saddle. The old one was worn in and had dropped by a mm or two.
  • ibr17xvii
    ibr17xvii Posts: 1,065
    I've currently got it set to bang on 765mm but not had a chance to get out & test it yet. Not sure I'll notice an odd 2.5mm here & there as much as I did 25mm :#

    This bike is a lot more aggressive than I'm used to in terms of saddle to bar drop & how much seatpost is showing which is giving me some issues with how I'm sat particularly on my right hand side which feels a wee bit uncomfortable at present but I guess I'll get used to it.
  • StillGoing
    StillGoing Posts: 5,211
    It's not 2.5mm difference. Yes 2.5mm to raise your saddle height, but it's 5mm at the top of the pedal stroke, giving you more room to get over the top of the stroke on to the power. In simple terms, if you've raised the saddle height by 2.5mm to compensate for the shorter crank length with the leg extended, the increased distance from the saddle to the top of the crank arm when at the 12 0clock position will be 5mm.
    I ride a bike. Doesn't make me green or a tree hugger. I drive a car too.
  • ibr17xvii
    ibr17xvii Posts: 1,065
    StillGoing said:

    It's not 2.5mm difference. Yes 2.5mm to raise your saddle height, but it's 5mm at the top of the pedal stroke, giving you more room to get over the top of the stroke on to the power. In simple terms, if you've raised the saddle height by 2.5mm to compensate for the shorter crank length with the leg extended, the increased distance from the saddle to the top of the crank arm when at the 12 0clock position will be 5mm.


    Blimey that’s confusing! :#
  • me-109
    me-109 Posts: 1,915
    StillGoing said:

    It's not 2.5mm difference. Yes 2.5mm to raise your saddle height, but it's 5mm at the top of the pedal stroke, giving you more room to get over the top of the stroke on to the power. In simple terms, if you've raised the saddle height by 2.5mm to compensate for the shorter crank length with the leg extended, the increased distance from the saddle to the top of the crank arm when at the 12 0clock position will be 5mm.

    What a load of twaddle!
  • shortfall
    shortfall Posts: 3,288
    Is milemuncher back?
  • ibr17xvii
    ibr17xvii Posts: 1,065
    edited May 2020
    me-109 said:

    StillGoing said:

    It's not 2.5mm difference. Yes 2.5mm to raise your saddle height, but it's 5mm at the top of the pedal stroke, giving you more room to get over the top of the stroke on to the power. In simple terms, if you've raised the saddle height by 2.5mm to compensate for the shorter crank length with the leg extended, the increased distance from the saddle to the top of the crank arm when at the 12 0clock position will be 5mm.

    What a load of twaddle!

    As I was being thick in the original OP I didn't want to appear any more stupid than I already made myself look but I didn't understand a word of StillGoing's post!
  • kevin_stephens
    kevin_stephens Posts: 184
    It depends on why you may want shorter cranks. I have very short legs so changed from 170 to 165 to reduce discomfort from my too acute knee angle at the top of the pedal stroke. It may be that such an acute knee angle may also impede pedalling efficiency for some. With 165 cranks I raised my saddle my 5mm to maintain the same knee and ankle angle at the bottom of the stroke. Therefore there is a combined 10mm reduction in knee height at the top of the stroke. This has a significant impact on reducing my knee angle (considering the trigonometric effect of my short legs) and hence reduced knee discomfort, and for me improved my pedalling efficiency
    I want to climb hills so badly;
    and I climb hills so badly
  • ibr17xvii
    ibr17xvii Posts: 1,065
    kevin_stephens said:

    It depends on why you may want shorter cranks. I have very short legs so changed from 170 to 165 to reduce discomfort from my too acute knee angle at the top of the pedal stroke. It may be that such an acute knee angle may also impede pedalling efficiency for some. With 165 cranks I raised my saddle my 5mm to maintain the same knee and ankle angle at the bottom of the stroke. Therefore there is a combined 10mm reduction in knee height at the top of the stroke. This has a significant impact on reducing my knee angle (considering the trigonometric effect of my short legs) and hence reduced knee discomfort, and for me improved my pedalling efficiency


    The 172.5mm cranks were just what came with the bike. I've not gone shorter for any performance gains or anything along those lines.
  • kevin_stephens
    kevin_stephens Posts: 184
    Sure, in that case raising the saddle by 2.5 mm may or may not have a small impact on comfort
    I want to climb hills so badly;
    and I climb hills so badly
  • ibr17xvii
    ibr17xvii Posts: 1,065
    kevin_stephens said:

    Sure, in that case raising the saddle by 2.5 mm may or may not have a small impact on comfort


    As above 2.5mm is next to nothing, not sure I'd notice. I currently have it ever so slightly higher than my normal height so just need to try it out.
  • N0bodyOfTheGoat
    N0bodyOfTheGoat Posts: 6,057
    edited May 2020
    For many riders, a 25-35 degree bend between their hip joint, just under the knee cap and their ankle joint at bottom dead centre of the crank stroke is optimal. Each 1mm of saddle height changes that bend by ~1 degree.
    ================
    2020 Voodoo Marasa
    2017 Cube Attain GTC Pro Disc 2016
    2016 Voodoo Wazoo
  • StillGoing
    StillGoing Posts: 5,211
    Christ who knew cyclists were thick. If you can't figure out that by measuring your saddle height from the pedal in the 6 0clock position and then shortening the crank arm by 2.5mm and raising the saddle by the same amount doesn't make a 5mm difference between the saddle and pedal when at the 12 o'clock position, lets hope you don't do anything important work wise.
    I ride a bike. Doesn't make me green or a tree hugger. I drive a car too.
  • me-109
    me-109 Posts: 1,915
    I'm not disputing it, but it's completely irrelevant to the OP setting his saddle height.
  • fenix
    fenix Posts: 5,437
    StillGoing said:

    Christ who knew cyclists were thick. If you can't figure out that by measuring your saddle height from the pedal in the 6 0clock position and then shortening the crank arm by 2.5mm and raising the saddle by the same amount doesn't make a 5mm difference between the saddle and pedal when at the 12 o'clock position, lets hope you don't do anything important work wise.

    You ok Hun ?
  • ibr17xvii
    ibr17xvii Posts: 1,065
    StillGoing said:

    Christ who knew cyclists were thick. If you can't figure out that by measuring your saddle height from the pedal in the 6 0clock position and then shortening the crank arm by 2.5mm and raising the saddle by the same amount doesn't make a 5mm difference between the saddle and pedal when at the 12 o'clock position, lets hope you don't do anything important work wise.


    I'm guessing you've never made a mistake in your life & know everything? Your 5146 posts would suggest otherwise unless you are just a super helpful person?

    Thanks for your input anyway.

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