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  1. #1
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    talk to me about fit

    I'm reading through some of early WWT threads and it's got me thinking about fit. I've currently designed my model based around the geometry of a bike I have really liked riding. But I'm not sure how other people define fit. Do you have a specific 'fit' for you, and then design your frame and it's geometry from there (rhetorical question really - I think that you do this)? So, if I had a fit, would it potentially be the same if I was designing an XC bike or an enduro bike? Would you define your contact points early on, and then design in the front/rear centre and head angle depending on the riding style? I've previously been under the impression that your body position on the bike will be different depending on what the intention of the bike is... so contact points suit the genre of riding. You could have n bikes with n variations of contact points.

    If you are going to tell me that fit comes first, then you design the rest of the bike around the riding style... how do I know if previous bikes I've enjoyed have 'fitted' me well, or have had great geometry?

  2. #2
    pvd
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    Quote Originally Posted by bluechair84 View Post
    how do I know if previous bikes I've enjoyed have 'fitted' me well, or have had great geometry?
    They surely didn't. Arriving at a good fit takes a lot of work. You get the crank length about right, get your cleats sorted (generally, almost to the back of the slots), then put the handlebars in the right place (as low as you can stand and forward enough to provide power climbing). The seat comes last. Set it to about 150 degrees of bend and about 90 degree of angle for your arms from your torso. This is very rough but it will help for now. It's probably going to be the opposite of what many will say, but then, they're wrong.

    It takes a huge amount of work and the swapping of lots of stems and posts before you get close. Lot's of trips out there. You want it so that you can climb and decend super fast. Simple enough. Bracket things to not loose track.

    That's just for body fit. A good geometry is a whole different thing. basically, you want the BB as low as possible, the front wheel as far in front of you (front center) as you can handle in the tight stuff, short chainstays, and as slack a head angle as you can get given stem choice.

    Most folks focus on stupid things like trail and super-short chainstays and loose track of the whole bike. Forget about trail. Make sure that you have enough room for a front derailure between the ST and tire.

  3. #3
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    Quote Originally Posted by bluechair84 View Post
    I've previously been under the impression that your body position on the bike will be different depending on what the intention of the bike is... so contact points suit the genre of riding. You could have n bikes with n variations of contact points.
    Fit is rider and style of riding. The body position for a downhill mountain bike and a road racing time trial bike need to be different. Fit is about getting the rider in the best position the riding they wish to do. Fit has a ton of aspects like the size of the rider but also their flexibility and even personality.

    The frame creates a baseline position but in any given size you can adjust a good bit with components. In general, I think the frame is more about riding style provided the size is correct. The frame sets the headtube angle, bottom bracket, and chainstay length. Front center and seat tube angle put the rider close to where they need to be and provided it is adjusted based on a ballpark fit the rider can get setup the rest of their position using components.

    The best way to arrive at perfect fit is to keep trying different bars, stems, seat heights, cleat positions, etc. Making small changes is the key.

    Quote Originally Posted by bluechair84 View Post
    If you are going to tell me that fit comes first, then you design the rest of the bike around the riding style... how do I know if previous bikes I've enjoyed have 'fitted' me well, or have had great geometry?
    It's pretty simple, if you enjoyed the bike the fit was good. Depending on how much you enjoyed it it may even be great.
    Mark Farnsworth, Raleigh, NC
    http://farnsworthbikes.com

  4. #4
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    Thanks for the replies. I discovered that there are such things as online fit calculators. I didn't know about such things until researching. I've put my details in to the competitive cyclist calc after seeing a recommendation, and have built the results into my own design. Compared to the geo of the Cube Hanzz that I'd originally followed, the top tube 'needed' to be shorter. This reduced the wheelbase too much so I steepened the SA to push the front wheel back out again. It's been interesting seeing a recommendation (albeit, an autonomous one) compared to what I thought would be good.
    So, now I need to focus on that SA thing and see if I've got a roughly correct seat compared to the BB.
    Quote Originally Posted by PVD
    The seat comes last. Set it to about 150 degrees of bend and about 90 degree of angle for your arms from your torso.
    Could you clear this up for me - set the seat 150deg? I'm not sure what you're referring to here. Cheers

  5. #5
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    Quote Originally Posted by Keith Bontrager - 1998
    In my experiences so far, I've found that most riders are suitably positioned with a seat angle that falls between 72 and 74 1/2 degrees. What is important is that I have not noticed any specific correlation between seat angle and my customers' femur bone length. I am reasonably certain that none exists. Still, it is easy to see how the KOPS method can get by. It usually puts the rider in the range of correct fit, although in my experience, the more anatomical proportions vary from the norm, the more off the mark the KOPS method is.
    I'd like to see what PVDs thoughts are, but this will likely do (though it's rather old!)

  6. #6
    pvd
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    I agree with KB in most cases.
    Referring to just seat angle of a frame with regard to fit is basically retarded.

    You are looking to get your knee to bend about 150 degrees at it's maximum given your pedaling shoe position.

  7. #7
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    Quote Originally Posted by pvd View Post
    i agree with kb in most cases.
    Referring to just seat angle of a frame with regard to fit is basically retarded.

    You are looking to get your knee to bend about 150 degrees at it's maximum given your pedaling shoe position.
    ok.

  8. #8
    650b me
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    Fit....isn't that one (of many) good reasons why custom-built frames exist?

    I am but a recent student of bicycle frame design, but my current belief is that there is no right/wrong, no formulas....it's completely dependent on the individual's proportions and preferences. Take me, for example. I have short legs and a long torso. I'd love to see a table of body proportion percentiles, but I have come to my assessment of myself through simple experience. I'm sure I have shorter than average femurs, but my two mtb's have seat angles of 71.5 and 72 degrees. Anything steeper feels weird to me. On top of that, I run setback seatposts and the saddle, at best, in a neutral position, but often it too is slid back. On paper, this doesn't seem right, but it works for me.

    I guess what I'm saying is that fit is as much art as science. It is facets of frame design like this that draw me in and keep me constantly interested.

  9. #9
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    Welby's Law: As an online discussion of bike fit grows longer, the probability of mentioning Sean Kelly approaches 1.

  10. #10
    A_A
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    talk to me about fit-sean_kelly.png

  11. #11
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    I'd say his KOPS looks spot on

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    I'll throw out my opinion on the matter...

    Unless you've got some issues (aches, pains, etc), I won't mess with the placement of your contact points. But I will move pieces around underneath you to achieve weight balance on the wheels and appropriate response (how you want the bike to feel).

    But if you're looking to re-tool your contact points on the bike, that's another matter and starts at the BB, moves to the saddle, and then to the stem/bars. Unless you're looking for a tri fit, and then it completely reverses, starting at the extensions, to the base bar, to the saddle, and lastly to the BB.

  13. #13
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    Quote Originally Posted by A_A View Post
    Click image for larger version. 

Name:	sean_kelly.png 
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ID:	846109
    I have the impression that the original question is not being answered by this picture.
    This rider is old school 1970's and the bike is typical of the period. The rider is either Sprinting or Chasing a break or Time Trialing.

    I'll suggest Time Trialing. Note the placement of the pelvis in relation to the saddle, arms tucked under the whole head/body, the over-lap of elbows/knees.

    Today I would set this rider up for the TT by giving him a more comfortable saddle position = steep Seat tube angle to get saddle placed under his sit bones better.
    The top tube would be considerably longer to stretch those arms out so he can pull on the bars better.

    But none of this theory is relevant to MTB ride styles, because balance on the bike and sit positions are different, so yes, different strokes for different folks on this topic.

    Eric
    If I don't make an attempt, how will I know if it will work?

  14. #14
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    Quote Originally Posted by Eric Malcolm View Post
    I have the impression that the original question is not being answered by this picture.
    I got the impression the image was posted tongue-in-cheek?
    The original question was whether a designed positioned the contact points to create a comfortable riding position, and then created the feel of the bike through geo changes, or started with geo, then found appropriate places for the rider to be positioned. I've come around to the idea that it is fit first.
    Interestingly, I'm looking forward to my first prototype to try this fit first method, to see if I enjoy a frame designed with my dimensions in mind and less so the concept of the frame.

  15. #15
    pvd
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    You should be doing both. Rider fit and a geometry for handling. They are not mutually exclusive. In fact, you need the rider fit to be worked out to make the bike handle correctly or well.
    Most folks really don't do the work needed to arrive at a good fit. They haven't tested the variety of positions from pinner XC hammer to lallygag knobber. There's a lot of room for high power output, aggressive attack position, and comfort in there but you really have to dig for it. Do this long before you design your own bike.

  16. #16
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    Quote Originally Posted by pvd View Post
    You should be doing both. Rider fit and a geometry for handling. They are not mutually exclusive. In fact, you need the rider fit to be worked out to make the bike handle correctly or well.
    Most folks really don't do the work needed to arrive at a good fit. They haven't tested the variety of positions from pinner XC hammer to lallygag knobber. There's a lot of room for high power output, aggressive attack position, and comfort in there but you really have to dig for it. Do this long before you design your own bike.
    Totally Agree!!!!

    Eric
    If I don't make an attempt, how will I know if it will work?

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