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Measuring seatube angle?

Discussion in 'Bicycle Mechanics and Repairs' started by Joe, 2 Jun 2008.

  1. Joe

    Joe Über Member

    I need a steeper seatube but I have no idea what angle mine is. It's an old frame so I can't find a geometry chart online. Is there a simple way of doing this? My maths skills are pretty rusty and I can't find a way!
    If not....being a traditional 55cm c-c (tt and st) frame, would there be a likely angle?
     
  2. yenrod

    yenrod Guest

    Any chance of posting a pic ? - out of a sheer guess it'll be either a 72' to 75' but as you say its a tad slack - its prob closer to the 72 angle ???
     
  3. OP
    OP
    Joe

    Joe Über Member

    It's not necesarily slack, it's just that to get my knee over the peddle spindle (I've tried being further back and I feel so weak) I need the saddle right forward and it leaves me very unbalanced on the bike. When I had my cyclefit they said ideally I needed a 76 degree st!
    I'll try and get a picture up at some point. Can you tell from a photo? I've got another bike with a 73 and I can't tell if this one is steeper or not:blush:
     
  4. pieinthesky

    pieinthesky New Member

    You might be able to move your seat further foward by fitting an in-line seatpost. The saddle clamp is then directly on top of the seatpost tube instead of being set back about an inch. You can therefore gain another inch which is roughly 2 degrees but depends on your seat height.

    There are seatposts with setback which can be reversed to move the saddle even further fowards but I dont know which ones can do this.

    If you need to measure the seat angle there is a special protractor tool with a spirit level for the purpose. I dont know where you can get them and its probably not worth it for one use, does your LBS have one?

    I measured my seat angle recently in the following way, it is a bit fiddly and requires a bit of maths but is fairly accurate if done carefully.

    1 Find a room with a hard level floor and bang a piece of cotton with a weight on the end from a drawing pin in the ceiling. Make sure the weight is below bottom bracket height.

    2 Put 2 dots on the side of your seat tube, dead in the centre and as near the bottom and as near the top as possible.

    3 Measure the distance between the dots, this is dimension X

    Edit Sorry, posted incomplete - see next post
     
  5. pieinthesky

    pieinthesky New Member

    You might be able to move your seat further foward by fitting an in-line seatpost. The saddle clamp is then directly on top of the seatpost tube instead of being set back about an inch. You can therefore gain another inch which is roughly 2 degrees but depends on your seat height.

    There are seatposts with setback which can be reversed to move the saddle even further fowards but I dont know which ones can do this.

    If you need to measure the seat angle there is a special protractor tool with a spirit level for the purpose. I dont know where you can get them and its probably not worth it for one use, does your LBS have one?

    I measured my seat angle recently in the following way, it is a bit fiddly and requires a bit of maths but is fairly accurate if done carefully.

    1 Find a room with a hard level floor and bang a piece of cotton thread with a weight on the end from a drawing pin in the ceiling. Make sure the weight is below bottom bracket height.

    2 Put 2 dots on the side of your seat tube, dead in the centre and as near the bottom and as near the top as possible.

    3 Measure the distance between the dots, this is dimension X

    4 Get an assistant to hold your bike vertical and rock steady (chock the wheels to stop it moving) so that the front wheel is pointing at the thread.

    5 Measure horizontaly from the thread to the upper dot (dimension Y) and horizontly from the thread to the lower dot (dimensionZ)

    6 Calculate angle as follows

    Seat Angle = 90 - (sin-1 (Y-Z)/X)

    If you cant do that send me X Y and Z and I can work it out.


    An easier but less accurate way is to use a vertical wall instead of the thread and push the front wheel up to the wall and measure as before.

    Sounds complicated but will take less time than it took me to write this post
     
  6. OP
    OP
    Joe

    Joe Über Member

    Thanks Pie!
    I'm already running an inline seatpost. With the saddle almost right forward. Any further back and it both sets off my itbs and feels alot less powerful.
    I'm thinking with a steeper seatube I could get my saddle in the same position over the BB but be a lot more balanced on the bike. As right now my arms are taking a lot of weight and my shoulders are really tense.
     
  7. OP
    OP
    Joe

    Joe Über Member

    Well I've been busy with the plumb lines and tape measures. I've tried twice and got 73.4 and 73.8 but I'm not at all confident about my accuracy. Going by those figures and this picture would you say it's likely that it's a 73.5? It feels steeper than my Surly which is defintely 73 but that I cannot be sure, would I notice 1/2 a degree?
    [​IMG]
     
  8. fossyant

    fossyant Ride It Like You Stole It!

    Location:
    South Manchester
    How you got short thighs ?

    That's about a 74 - I have a 73 and 74, but have the saddle rammed right back.

    Have you run a plumb line from your knee through the axel when the cranks are parallel to the ground.

    If one bike feels right, compare the differences.....if you drop a plumb line from the nose of the saddle, and measure how far behind the BB it is - are both similar (taking into account slight differences in saddle). Also check saddle height - may be too high/low
     
  9. OP
    OP
    Joe

    Joe Über Member

    Neither bike feels right! But the one with the steeper seat angle feels better. Yeh, I've run a plumb line. With the saddle right forward on an inline seatpost I get my knee over the pedal spindle...just about. But it means my arms are taking alot more of my weight than is ideal and especially over long distances I really feel it. I must have freakishly short thighs:sad:
     
  10. fossyant

    fossyant Ride It Like You Stole It!

    Location:
    South Manchester
    What's the bar stem length ?

    I'd say I was fairly average - 5'9" - my saddle nose is set back a good 6-8cm from the BB but I also have a long 13cm stem.

    If you've had a bike fit then it should be about right, especially if you are getting the knee position correct. A longer stem may move some of the weight off your hands as more will be supported by your back - you could have a play with handle bar height as well - first as this is a no-cost option.