TT Bike Positioning

Discussion in 'Cycling Training' started by Roadie_scum, Sep 8, 2007.

  1. Roadie_scum

    Roadie_scum New Member

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    I just did a bit of an experiment to see what size TT frame I might buy were I to buy one... I want to get some feedback and see if this sounds insane. I comfortably ride a 58+cm road bike typically, and I was a little surprised, as my results yielded a 54cm TT bike, were I to get a Cervelo (which was the easiest to assess against as they give stack and reach for their framesets).

    So:

    I took, from seat tip, seat height, vertical seat to BB and seat setback from BB, to form a triangle. I checked for errors by doing pythag on this right angled triangle and re-measuring until (i) my measurements were consistent and (ii) the triangle was pythagorian.

    I measured: Length from greater trochanter to acromion process, length from acromion process to bottom of elbow with 90 degree bend between forearm and upper arm, vertical height of greater troch off seat and distance behind seat tip of greater troch. The greater troch measurements formed another known triangle off the seat tip. I then ran a line parallel with the ground from greater troch to acromion process. I dropped off this line vertically the length from acromion process to elbow. I also calculated the slight difference that 10 degrees raise from troch to process made. Not much. So, long story short, using these triangles I ended up with maximum stack and reach measurements for a size frame, assuming negligible rise from bars to pads. Basically, I ended up with the reach and stack of a 54cm frame. This kind of makes sense - I have always thought that TT frames should be a little smaller. I guess I have short-ish legs, so maybe that is why I need a small frame to achieve an appropriate drop. But it seems like a lot. Obviously my method has lots of assumptions:

    (i) I am flexible enough to achieve the position
    (ii) Seat angle change (fore-aft) is fairly negligible
    (iii) I can get low rise pads
    (iv) The vertical upper arm and 90 degree angle with forearm is a good idea
    (v) I sit similarly on a TT bike to my road bike

    All these seem reasonable, and if not, I can always fiddle with headset spacers and stem length until I actually achieve sufficient flexibility. Many of the assumptions concern only small margins of error.

    One number seems big: ~24-26cm vertical drop seat to elbow pads.

    So what do people think?
     
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  2. 9202

    9202 New Member

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    My road bike is a 54cm frame. My TT bike is a Cervelo P3c, it is a 56cm frame. My LBS who did the fitting prior to ordering the frame told me that the Cervelo P3c frames tend to run this way due to the geometry.

    BTW, I absolutely love the Cervelo!
     
  3. Jono L

    Jono L Well-Known Member

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    Sounds right.


    My roadie= 58cm Top tube + 125mm stem

    But TT bike= 58cm Tob tube+ 60mm stem

    The only reason I've still got a larger frame for my TT is because of my praying mantis style (aka Vervart) legs need the high seat...
     
  4. Roadie_scum

    Roadie_scum New Member

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    Thanks for the feedback.

    Interesting, regarding your bikes. Someone like you maybe needs a custom, so you can be high enough with seat height, yet have short enough top tube and low enough headtube/stack to get your front end low without being stretched.

    I wanna do the ATTA stuff next year... be cool if you came along, except for me being relegated one place further down the finishing list. :)
     
  5. Alex Simmons

    Alex Simmons Member

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    Are you flexible enough? I suppose you have relative youth on your side;) How does this position go in terms of knee - elbow touching?

    Interested to know the verticle drop from saddle to handle bar drops on your road bike.
     
  6. Jono L

    Jono L Well-Known Member

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    The thing for me is I need a relatively high stack height so that I'm not too stretched and also so that I don't smash my knees on the handlebars...

    Somehow I scored my P3SL off Ebay and it's a perfect fit!
     
  7. Roadie_scum

    Roadie_scum New Member

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    Maybe?? :) Empirical method may come into play soon...

    Again, gonna have to take the experimental approach. It's a good question though.

    My roadie atm is a bit too long for me, with a bit too much head tube. I have it as low as it will go, and it is only about 10cm. When I had a bike that was a little better geometry for me, I had either 15cm or 18cm of drop. Can't remember which, it was a couple of years ago. Whatever the case, it was a reasonable amount and it felt really comfortable and good.
     
  8. AndROOb

    AndROOb New Member

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    I use a 54cm road bike and a 51cm Cervelo, with the seatpost in the forward position, with an 80mm adjustable stem set at just below level position. I use this set-up for anything from 10-50 mile events, and it feels great.

    I would err on a smaller size TT frame as it is easier to extend a tight position with seat-post/stem length adjustment, but you cannot shrink a top-tube that is too long to allow you to achieve a good, comfortable position.
     
  9. Bailsibub

    Bailsibub New Member

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    Roadie Scum,

    Yeah, most people ride a smaller TT frame.

    I'm 5'11 and I use 57 cm top tube length with a 120 cm stem on a road bike. (This is a little longer than most guys who are my height because my arms are freakishly long)

    Anyway, on my TT bike, which is also a Cervelo, I'm riding 54 cm frame with a 100 cm stem. This might sound short, but my position is as good as it can get.
     
  10. daveryanwyoming

    daveryanwyoming Well-Known Member

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    That number is big, but not too far from what some fast guys are using. Have you seen this positioning table from BTR: http://www.biketechreview.com/performance/position_table.htm

    24-26 cm drop is on the high side, but not way out of line with some of these folks.

    Personally my limit to aerobar drop is how much my groin is pressed into the saddle nose, at least for longer events like a 40 km TT. I ride a 56 cm road bike with about 10 cm of handlebar drop but ride a 54 cm TT frame with an adjustable stem that gives me 10 to 20 cm of aerobar pad drop. My knees don't hit my chest at the low end of the range but my groin can't handle that for very long. I'm playing with TT saddles hoping to find something that helps and plan to work on lowering my position over the winter for next season's races but can't yet go as low as I'd like.

    Have you tried testing out a similar position on your existing road bike. For instance you could put your bike on a trainer and place a padded rod across the bottom of the bar drops or something like that to get an ultra low drop like your 24-26cm plan. Then ride the trainer for a bit by placing your forearms on that very low horizontal bar. Do your knees clear your chest, can you still see up the road out of the tops of your eyes, can your taint handle the saddle pressure? Seems there are some home fitting trials you can do before throwing down for a new frame. FWIW I really like the adjustable stem on the TT frame. It's really heavy, but weight isn't too important to me for flat or mostly flat TTs.

    -Dave
     
  11. Catabolic_Jones

    Catabolic_Jones New Member

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    I ride a Norco CRR Extra Large which I believe is a 58 cm TT. I recently purchased a 56cm Cervelo P2C.

    I made a mistake, however. In order to get the proper 90 degree angle between the upper and lower arm, a 54cm would have been a better choice.

    Unfortunately, TT'ing is all about these tiny nuances that can save considerable chunks of time. I love the actual activity of pushing to my physiological limit and holding it there; but I'm not a real lover of tinkering with my position; you really need both to become a great TT'er.

    Maybe I'll focus on hill climbs next season. [​IMG]
     
  12. Roadie_scum

    Roadie_scum New Member

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    Thanks for the link! That is quite interesting. The 24-26cm estimate is a maximum, but I could get major adjustment out of a frame using different rise stems, headset spacers and various bars with pads coming off at different heights. Pretty sure a frame that accommodated that kind of drop could be tuned to only have 15cm (it might look funny but whatever). However, a frame that yielded 15cm of drop to slammed on the head tube could never be tuned down more than about 20mm with a negative drop stem.


    Yeah, I think adjustable stem is a good idea for me too, at least until I feel I'm close to optimal. The idea of trying it on a road bike is good, but it couldn't be my one, since it is too big for me and has a whole bunch of head tube. I have a friend with a small-ish TT bike and I am going to play with it on the trainer for a while at some stage this week. Will report back if people are interested.
     
  13. Roadie_scum

    Roadie_scum New Member

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    I thought about this some more and measured from my greater troch to the top of my knee (call this X) then looked at where the circle drawn from my greater troch with X as radius should go, in terms of the theoretical position I have drawn up.. Answer: it doesn't look like it will touch. By my calculations, I should have a good amount of clearance. I think I have a long-ish upper body for a cyclist.
     
  14. Alex Simmons

    Alex Simmons Member

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    Did someone do these measurements for you? :p When reading your post I was visualising all those triangles.:D
     
  15. Roadie_scum

    Roadie_scum New Member

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    A couple of them - I am aware that it might not be perfectly (or even imperfectly within a decent amount of error) accurate. But it's just a sketch-up to get some ideas about what I should look at. I'm not going to buy anything without sitting on bikes and playing with things.

    Hmmm. Yeah. It was convoluted as hell. I think I was a little over-tired when I wrote it. I can barely understand what I was on about. If you saw my diagram it might have been 5% clearer, rendering it still opaque. Nonetheless, I seem to have got some useful thoughts out of the exercise. Thanks guys! :)
     
  16. rmur17

    rmur17 New Member

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    1. What does a pythagorian triangle have to do with it? (pythagorean?)?? Or just a right triangle??
    2. 24-26cm of drop is an awful lot unless you have super long upper arms or can ride tighter than Chris Boardman!
    3. FWIW Cancellara rides a 58cm road bike and 54cm P3C so it certainly can be done.
    4. FWIW my road bike has 25cm of drop from saddle top to the drops and the TT bike currently has around 20cm (hand support vs. elbow support).
     
  17. Roadie_scum

    Roadie_scum New Member

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    1. I measured horizontal seat setback from BB, seat height from BB an vertical drop to BB from seat. These things form a right angle triangle. To check my measurements, I confirmed that the numbers were (like all right triangles), a pythagorean triad. It was instructive, because I wrote one of the numbers down wrong, but when I re-measured, the relationship was pretty good. This suggested to me my measurements were also at least reasonable. Does that make sense?

    2, 3 and 4: Thankyou! I think you know a lot more about this stuff than me... :)
     
  18. rmur17

    rmur17 New Member

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    all the best ...
     
  19. Roadie_scum

    Roadie_scum New Member

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    This is probably strictly correct... what I meant was I checked that I had measured a right angle triangle correctly by checking the consistency of the measurements with pythagoras' formula. That's what I should have said.
     
  20. rmur17

    rmur17 New Member

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    ah i'm just getting crotchety in my old age :)
     
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