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On bikefitting... (really should be working... Ah well)(5 posts)

On bikefitting... (really should be working... Ah well)Jofa
Apr 19, 2001 8:24 AM
I was doing some proper work in Adobe Illustrator, and suddenly found myself to be drawing a bike... damn. Anyway, I've been thinking about bike fit, a subject that crops up a lot on these boards, and I've always been suspicious about the importance of many dimensions/ angles etc in fitting a bike. So here's my 2pence suggestion. (There's a diagram at the bottom of the page)

You'll notice that I've made no reference to either seattube length, or angle. Well, the seattube is telescopic, by nature; and the effective angle is largely modified by sliding the saddle in its rails. It seems to me that the centre of saddle-related measurements is best located at the depression in the saddle (SLR users excepted) made by the bottom of the pelvis- the 'sit- bones'. All other options, to my knowledge, have too many variables: the seat- cluster of the frame can be more or less anywhere, horizontally or vertically, and moreover it may be measured to the top or the centre. The tip, rear or centre of the saddle, though preferable, don't take into account the saddle design. The location of the pelvis on the saddle is easy to identify, and all modifications can subsequently be taken care of with the seatpost.

Neither have I referenced the toptube length. Again, there are too many variables at play: the toptube may be angled or horizontal; it is affected by the seattube angle (the seat cluster again); and, of course, stem length, of which more in a minute. Using 'effective' or 'imaginary' top tube measurements, can be more of a hindrance than a help, as no-one has any idea who's measuring what.

What does seem important, is the relative positions of the pelvis and the handlebar (which may be best taken to be the lever hoods, or the drops; I propose the tops, for simplicity and accuracy). This takes into account the toptube, but without the variables. Any adjustments here, on an existing frame, require that the stem be swapped out for one with varying angle and length.

So as far as basic fit goes, a frame of ANY size may be made to fit properly (as long as it isn't way too big, for standover clearance), just by modifications of the seatpost and stem. Where the frame tubes occur, within this crucial triangle of BB, pelvis, and handlebar, is irrelevant- you could draw a frame wherever you wanted, within reason, on my diagram without affecting the rider position.

However... we all want bikes to handle well. There is a lot of talk of weight distribution etc, but again, I think that variations here are minimal. Most manufacturers build bikes with chainstays between 40 and 41 cm these days, and BB heights around 27 cm; if you know the relative position of the saddle that is correct for you, then there's nothing more to be done around the rear triangle: it's already fixed. And that will be the same whatever bike you get on, irrespective of seattube angle, or whatever. If you already know where the handlebar should be, then the only thing left is the position of the front wheel, which is largely defined by the location of the head tube. This is the one part of the frame, IMO, that actually matters- because it is the centre about which the whole mechanism (bike + rider) bends (bad word, I know, can't think of the correct one). The location and angle of the head tube are impossible to treat seperately, because they both affect two important (to handling) variables: the length of the control lever (stem), and the position of the front wheel (which, to add to the equation, is also affected by the fork rake). In buying a new bike, however, it is simpler: you usually have no choice about head angle or fork rake, because they're decided by the manufacturer or builder, and I think that's correct. You do, however, have a choice about the bike length (different size frame) and can therefore choose one which gives your preferred stem length, which, in turn, affects the wheelbase of the bike. So 'weight distribution' is reall
CONTINUED... (the post got cut up)Jofa
Apr 19, 2001 8:27 AM
... So 'weight distribution' is really about the location of the front wheel.

I suspect that there is a 'ideal' ratio between the stem length ('5' on the diagram) and the pelvis to bar distance ('3'). I've no way of testing this out, but perhaps some people could post the dimensions from their bikes to see if they show a pattern.

This is all, clearly, retrospective; it assumes that a given rider has already decided on the relative locations of the 'three red blobs'- BB, Pelvis, and handlebar- by various means- experience, a mathematical system, divine intervention, whatever. And it is certainly no 'system' for determining fit- it's just my attempt to clarify the picture, and simplify the variables (a complex diagram and 800 words, yeah right, Jofa). But my contention is that many aspects of the bike frame have no effect on the fit, or behaviour, of the bike: in fact, that the first five dimensions on the diagram are the ONLY dimensions that need be considered, in fitting a new bike.

I've not taken into account crank length- this is crucial, but can be changed; nor the effect of differing flexion of differently shaped frames (wobbly long seatposts, flexy BB's etc), as that's a whole other issue.

I don't mean to step on any toes here- shoot me down if you think it's nonsense (or so obvious that it didn't need saying)- I'm fairly new to this board, and this may have all been covered before. In any case, I've just managed to put off important work another hour...

I'd be interested to hear any responses. (begin flaming now...!)

Jofa
well, great picture!ET
Apr 19, 2001 8:57 AM
First, welcome! And don't be too deterred from posting even if some may not agree with you. You may learn from us and we may get input from you as well. Free exchange of ideas is good.

Before you get too carried away, some of your criteria are just proxies for others, e.g. if you know where your saddle rests on its rails for a given measurable setback and seat tube angle, it is easy to do the math and translate it to another bike with different measurements. I've do it myself all the time in deciding fit issues. And this may be easier than figuring out exactly where your pelvis is resting right down to the tenth of a cm. So you may be overemphasizing this pelvic stuff a bit. I don't think anyone wants to be looking primarily at head tube angles and wheelbase measurements and the like, certainly unless they've already ridden many bikes and claim to want a certain exact measurement. Finally, "pelvis to handlebar, horizontal" and "pelvis to handlebar vertical" are not independent components of reach, e.g. one may find a shorter horizontal but longer vertical reach to the handlebars just as good.

I don't agree with you either in your claim that all those other measurements don't matter. As two examples, I can't go higher than 73 degrees on my seat tube angle or I may very well run out of seat rail for any normal seatpost, certainly on a 74. Also, a very small frame (with high standover clearance) will necessitate for many that the stem be built up way higher than recommended, to say the least. I'm not going to even discuss here what effect, say, being way forward or back on the rails w.r.t. the triangle would do to ride; this is less clear but not necessarily trivial.

I don't mind the current system of measurements, but I wish they would standardize their c-t sizing to always mean c-t of top tube; that would clear up a lot of confusion.
well, great picture!Jofa
Apr 19, 2001 9:35 AM
Quick response, ET! Thanks for your comments, and for your kind welcome. You're absolutely right- the criteria I propose are proxies- but in this case, I think the proxies are easier to deal with. I've always had difficulty in working out set-back and seat angles, and instead resort to a plumb-line and ruler to translate a seat position. Positing the key locations- pelvis (I still support that...!) and handlebar, simply with two measurements (x and y, if you like), seems easier, and saves juggling many different numbers around. With this in mind, the 'horizontal/ vertical' distances are not meant to be used independently, rather to precisely describe a preferred location.
To an extent, this is a theoretical proposition: clearly, if the seattube angle forces the seatpost out of it's adjustment range, then it matters. I mean to dismiss it only within that ~3 degree range (unless you run a seat with 20cm rails). The stem issue on a small frame is easily solved with a stem that has a high rise (which probably isn't made- but I did say it's theoretical). I certainly don't recommend running a frame that is much to small, for all sorts of issues relating to mechanical stresses, but I only mentioned it to stress the point that to dwell over the frame tubes themselves, in fitting a bike, only clouds the issue. Hopefully it may be of some use to inexperienced riders who are completely confused by all the differing information being thrown at them... (or confuse them even more?)
Top tube lengthtommyb
Apr 20, 2001 9:43 PM
First off, great analysis.

However, I still feel that top tube length matters. One of the critical dimensions for handling is the horizontal distance between the rider's contact on the handlebars and the front wheel axle. You don't have that marked as a discreet dimension. It is affected by the head tube angle and fork rake, but is also dependent on the ratio of the top tube length to stem length. A bike with a short top tube and long stem would put the handlebars further out toward the front hub (or even in front of it in the case of extreme 'Superman' positions). That definitely affects handling. If you treat the eight dimensions you specify as fixed, but alter the top tube to stem ratio, you will change the bike's wheelbase. I think in order to define a bike's fit, this needs to be considered.

I guess just adding wheelbase to your list of critical dimensions would take care of it. In reality, I don't think there is that much variation in this dimension on race bikes, but it makes a big difference between a touring bike and a racer.