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  1. #1
    IndyFab girl jp_nyc's Avatar
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    My current 48cm custom road bike has these dimensions:

    Standover height: 74.1cm
    Top tube: 50.5cm
    Wheelbase: 96.1cm
    Trail: 56.9mm
    Fork offset: 50mm

    I'm looking into buying a used cross bike with these dimensions:

    Standover height: 74.4cm
    Top tube: 51.5cm
    Wheelbase: 97.8cm
    Trail: 64.7mm
    Fork offset: 45mm

    My current road bike takes 700c wheels, while the cross bike is built to take MTB 26" wheels. I'm planning on building the cross as a fixed gear bike. My concern is with the wheelbase and trail differences. Are they significant, and if so, how will I notice them? I'm not able to try to the cross out, so I'll have to go on numbers.

    Thanks for your input.
    JP

  2. #2
    Senior Member AnthonyG's Avatar
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    The differences in trail are the most significant. Trail usualy ranges from 50mm to 70mm with 50mm being a very fast steerer to 70mm being a slow steerer. Both bikes have trail figures that are normal for those type of bikes with the road bike being fairly quick and the cross bike being quite stable. The extra wheelbase in the cross bike will also make it more stable but not by a huge degree. The cross bike will still get around corners but you will use noticeably more effort to do so.

    Something that could be even more important than the figures you have mentioned is the seat tube angle although I suspect that the cross bike should be more relaxed in this dimension which in my view is a good thing.

    Regards, Anthony

  3. #3
    IndyFab girl jp_nyc's Avatar
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    Forgot to mention these numbers:

    Road bike seat tube angle is 74.5, head tube angle is 72.
    'Cross bike seat tube angle is 72, head tube angle is 71.

    Not sure how these angles are measured, so I don't know whether larger angle means more aggressive or more relaxed.

  4. #4
    Senior Member AnthonyG's Avatar
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    Quote Originally Posted by jp_nyc
    Forgot to mention these numbers:

    Road bike seat tube angle is 74.5, head tube angle is 72.
    'Cross bike seat tube angle is 72, head tube angle is 71.

    Not sure how these angles are measured, so I don't know whether larger angle means more aggressive or more relaxed.
    The cross bike is significantly more relaxed in the seat tube angle but that could be a very good thing. It will take the weight off your hands/shoulders and put it on your backside. One of my bugbears with small framed bikes is that the seat tube angles are too steep putting the rider too much infront of KOPS (knee over pedal spindle). See http://sheldonbrown.com/ and go to frames/revisionist frame sizing.

    Given those figures the CROSS bike is actualy smaller and even though on paper the top tube is longer the effective top tube length will be shorter. The cross bikes head tube angle is slightly more relaxed but that's just typical for those style of bikes.

    Regards, Anthony

  5. #5
    IndyFab girl jp_nyc's Avatar
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    Thanks for the info, that's really helpful. The 'cross frame has a slightly compact geometry (whereas the road frame doesn't), so the effective top tube measurement will be shorter than 51.5cm (actual length from seatpost joint to headtube joint). If the seat tube angle is more relaxed than the road, that means the saddle is further behind the BB, correct? That might be something to think about. Since I want to build a fixed gear with it, I'll want to have a more forward position.

    How much of a difference in position (relative to the bb) is there between a 72 and a 74.5 degree seat tube angle?

  6. #6
    Senior Member AnthonyG's Avatar
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    Quote Originally Posted by jp_nyc
    Thanks for the info, that's really helpful. The 'cross frame has a slightly compact geometry (whereas the road frame doesn't), so the effective top tube measurement will be shorter than 51.5cm (actual length from seatpost joint to headtube joint). If the seat tube angle is more relaxed than the road, that means the saddle is further behind the BB, correct? That might be something to think about. Since I want to build a fixed gear with it, I'll want to have a more forward position.

    How much of a difference in position (relative to the bb) is there between a 72 and a 74.5 degree seat tube angle?

    You've got the concept. I'm using the term "effective top tube length" pretty loosly. If you set your seat up to have exactly the same seat setback and height on both frames then effectively the top tube wouldn't be so long on the cross as the seat would be set much further back on the road bike to acheive the same seat setback which therefore EXTENDS its effective top tube length. From what i've read 1 of seat tube angle is about 1 cm of top tube length difference. Before you think that you NEED a more forward riding position than the cross bike provides maybe you should measure your KOPS position on your current bike. I suspect that you are infront of KOPS and the more rearward seating position on the cross bike may be just what you need. You could always use a zero setback seatpost if required anyway.

    Regards, Anthony

  7. #7
    IndyFab girl jp_nyc's Avatar
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    I went back and looked at the 'cross bike specs. The 51.5cm top tube measurement is effective, not actual, so it's only 1cm longer than my road bike.

    You might be right about my being forward relative to KOPS, although I did take that into account when I initially set up. I'm just comfortable where I am right now. I can go back and check. The seat looks pretty well centered on a straight seatpost.

  8. #8
    Senior Member AnthonyG's Avatar
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    Quote Originally Posted by jp_nyc
    I went back and looked at the 'cross bike specs. The 51.5cm top tube measurement is effective, not actual, so it's only 1cm longer than my road bike.

    You might be right about my being forward relative to KOPS, although I did take that into account when I initially set up. I'm just comfortable where I am right now. I can go back and check. The seat looks pretty well centered on a straight seatpost.
    Yeah sorry I'm misusing the term "effective top tube length". Due to the differences in seat tube angle the road bike has a top tube thats longer than the specs suggest if you were comparing it to a bike with a slacker seat tube angle.

    A "normal" seatpost will have the clamps just behind the centreline of the seat tube. A zero setback post will have the clamps centred right on the centreline of the post and rearset will of course have the clamps significantly behind the centreline of the seatpost.

    I'm also short and ride a custom built 48 cm frame but my seat tube angle is only 71 and when you take into account seat setback I've effectively got a 69 seat tube angle! Best thing I ever did honestly so I acknowledge your comfortable with your current setup but honestly from experience it is worth trying a more rearset position (back to KOPS) and see how you feel then. The cross bike with its more relaxed seat tube angle could be supprisingly good.

    Regards, Anthony

  9. #9
    IndyFab girl jp_nyc's Avatar
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    Quote Originally Posted by AnthonyG
    I'm also short and ride a custom built 48 cm frame but my seat tube angle is only 71 and when you take into account seat setback I've effectively got a 69 seat tube angle!

    Regards, Anthony
    OK, now I'm confused. If a greater seat tube angle means a slacker geometry with the seat further back from the bb, why is it that when you push your seat back, you get a smaller angle?

    Maybe it'll be easier for my brain to wrap itself around this if you can tell me how seat tube and head tube angles are measured (which angle, exactly, are we talking about?). Sorry for the basic q, but I guess it's about time I know this.

    Thanks.

  10. #10
    Senior Member AnthonyG's Avatar
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    Quote Originally Posted by jp_nyc
    OK, now I'm confused. If a greater seat tube angle means a slacker geometry with the seat further back from the bb, why is it that when you push your seat back, you get a smaller angle?

    Maybe it'll be easier for my brain to wrap itself around this if you can tell me how seat tube and head tube angles are measured (which angle, exactly, are we talking about?). Sorry for the basic q, but I guess it's about time I know this.

    Thanks.
    No I think you were right the first time. My custom bikes phisical seat tube angle is 71, if it was exactly vertical it would be 90 and as it tilts BACK the numbers get smaller and the angle slacker in cycle speak. Anyway I have some setback AS WELL as the slack seat tube angle to begin with so the EFFECTIVE angle is even slacker at 69.

    Actualy it gets more complicated. Just about EVERYONE out there has an effective seat tube angle thats slacker than the frame geometry suggests. If you want to be precise you measure the angle from the centre of the bottom bracket to the midway point on the seat rails but even this can be a bit airy fairy due to differences in seat tube rails and seats in gereral. Even the concept of seat setback (nose of saddle to vertical line from bottombracket) can be upset by variances in seat design. IE how long the seat is and whereabouts on the saddle your weight centres.

    Does that make any sense or is it getting more complicated.

    Regards, Anthony

  11. #11
    IndyFab girl jp_nyc's Avatar
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    Quote Originally Posted by AnthonyG
    If you want to be precise you measure the angle from the centre of the bottom bracket to the midway point on the seat rails
    I don't get what you're trying to say here. If I've deduced correctly, seat tube angle is measured on the rear side of the seat tube line bisecting the ground, correct? Where does the midway point of the seat rails come in?

    Quote Originally Posted by AnthonyG
    Does that make any sense or is it getting more complicated.
    I guess that depends on your answer...

    In any case, things are starting to come into focus...

  12. #12
    Senior Member AnthonyG's Avatar
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    Quote Originally Posted by jp_nyc
    I don't get what you're trying to say here. If I've deduced correctly, seat tube angle is measured on the rear side of the seat tube line bisecting the ground, correct? Where does the midway point of the seat rails come in?
    You've got it right regarding where the seat tube angle is measured. Thats the simple version but reality is more complicated. Say you have two bikes with identical seat tube angles according to the manufacturers yet one has its seat moved as far forward on the clamp as it will go and one is set as far back on the seat clamp as it will go. The seat that's set forward has a seat tube angle that's effectively steeper than the one thats set back on the rails which is effectively slacker. Thats where measuring to the middle of the seat rails comes in if you want to be accurate/pedantic.

    Look this is a bit advanced anyway but what I've found over the years is that I have to be up on these advanced concepts myself and if I tried to rely on others to be experts then I just ended up getting bad advice. Now to be fair this is partialy because of the fact that sometimes "good" advice is just so far from the norm that people don't accept it anyway.

    Have I mentioned the concept of proportional fit?!!

    Since your riding small frames my bet is that your in the same boat as me and standard cranks are too long for you wheather you know it or not. See http://www.cranklength.info As a reference I'm using 140mm cranks although I'm not suggesting that you need cranks that short. I'm a special case.

    Anyway the concept is that short riders realy should be able to get a riding position where there hip and knee's are traveling through the same range of angles as larger riders and our upper body angles should be something simmilar but the fact is they aren't because small bikes are made to be cost effective rather than fit properly and this unfortunately leads to poor fit and a poorer cycling experience for small riders. Ahh I'm ranting now!

    Here's a picture of my "special"



    EDIT: OK why did I go off on this rant? Thats the problem with rants, you forget why you made them. The point was that steep seat tube angles is one of the dodgy things bike builder do to make a bigger bike seem like it will fit a smaller person but it doesn't realy so this is one of the reasons why I think you may be plesantly supprised as to how the relaxed geometry of the cross bike will fit you.

    Regards, Anthony
    Last edited by AnthonyG; 02-26-06 at 10:37 PM.

  13. #13
    IndyFab girl jp_nyc's Avatar
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    I see what you mean now. For some reason, I read "middle of the seat tube" instead of middle of the seat rail.

    No, the seat on my road bike is centered on the seatpost. Maybe a little forward, but definitely not all the way. The 'cross bike was posted on ebay and I was very tempted to get it since the geometry seemed pretty close to what I need. I was outbid, though.

    But I've learned a few things here, which will come in handy as I keep looking for that new frame. Thanks for taking the time.

    jp

  14. #14
    Senior Member AnthonyG's Avatar
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    Quote Originally Posted by jp_nyc

    But I've learned a few things here, which will come in handy as I keep looking for that new frame. Thanks for taking the time.

    jp
    Your welcome. I'm always happy to "rant" about bike fit.

    Regards, Anthony

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