Exact Size Bike Fitting myths



jasong

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Nov 24, 2003
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Looking at several bike geometry details / fit guides, it would seem to be that one could really err on the side of choosing the right bike frame size without serious consequences. It seems that one bike size out of a set offered by xyz mfctr follows fit guide recommendations. If the surrounding frame sizes differ by 2cm, it would seem true that both a size above and a size below could be altered to fit this person just fine.

The one mistake that can't be made with the fit is that the seat tube must be allowed to get the .883 x inseam length recommendation or whatever allows that 20-30 angle at bottom dead center. This seems most easily made impossible by a true square geometry with a level top tube (for the shorter people, as the largest of 3 possible frame fits) or a compact frame (for the larger people, as the smallest of 3 possible frame fits). For example, as a 6'00 person with 34.5" inseam, my 2003 Litespeed Capella at full seat post extension puts me at the right leg angle. Unless I go with a 350mm post (currently 330), I cannot experiment with smaller angles unless buying a seat that gives a slightly higher rise. I probably have too small a frame since it's virtual top tube length is 56.3 cm. Calculations would suggest a 58.7cm frame; yet I fit the Litespeed fairly well with some mods. Thank the LBS for that idea and having the seat post 2in too low on their "custom fit" they charge $100 for.

However, frames that aren't quite as compact or square won't ever offer this screw up possibility (of not giving low upper-lower leg angle) since full seat post extension (assuming availability of a 330mm post isn't a big deal) would give possibility for more than full leg extension.

Now for top tube concerns. There are stems that range from 70mm to 140mm of varying angles. Assuming that the fork steerer tube hasn't been trimmed completely, and with the use of spacers above or below the steerer tube, a large combination of (x,y) coordinates of the handlerbar and stem junction is possible. One can see 7cm difference for a given angle, which if placing the ideal frame in the middle, gives an easy 2-3cm difference on either side for error. That accounts for the +/- one size frame difference in addition to the frame that hits the recommendation. There me be a strange angle rise resulting from putting a big person on a small bike, but the (x,y) points (z in the direction parallel to the top tube) are going to be the same between the "incorrect" and "correct" frame in the wireframe intersection points of the rider.

Yeah, cranks come in their recommended sizes differently but can be easily replaced and often cover two-three sizes. Light stems can easily be had for $10-30. I guess turning radius can be adversely affected on a unnecessarily small frame that fits within these guidelines, but I don't really see how the arguments for importance on bike fit gain credence.

One thing these ideas don't take into account is where the relative body inertial moment & center of gravity sits relative to the frame size and under different conditions. But heavier people probably don't have their weight calculated into their bike fit, and the same for light people, so those arguments could weigh against that in the same manner. Stronger riders less interested in comfortable frames might choose something smaller; low mileage riders interested in comfort might choose something large letting the frame take the abuse and suck up some energy. Preference isn't calculated into the "exacting measurement fit" either.

But as for where the relative points in space of elbows, shoulders, head, butt, knees, and feet, these points seem far more flexible than what the literature would suggest.

Changing a frame rarely seems like it would ever be an issue if one measures their own inseam and buys a frame based on the .67 formula. Erring on the + or - side would easily make the recommend points achievable.

How can one justify the $$$'s one would spend for an custom frame build? Or even for wanting the 1cm increments touted as so important by some companies.
 

boudreaux

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Oct 16, 2003
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Originally posted by jasong
Looking at several bike geometry details / fit guides, it would seem to be that one could really err on the side of choosing the right bike frame size without serious consequences. It seems that one bike size out of a set offered by xyz mfctr follows fit guide recommendations. If the surrounding frame sizes differ by 2cm, it would seem true that both a size above and a size below could be altered to fit this person just fine.

The one mistake that can't be made with the fit is that the seat tube must be allowed to get the .883 x inseam length recommendation or whatever allows that 20-30 angle at bottom dead center. This seems most easily made impossible by a true square geometry with a level top tube (for the shorter people, as the largest of 3 possible frame fits) or a compact frame (for the larger people, as the smallest of 3 possible frame fits). For example, as a 6'00 person with 34.5" inseam, my 2003 Litespeed Capella at full seat post extension puts me at the right leg angle. Unless I go with a 350mm post (currently 330), I cannot experiment with smaller angles unless buying a seat that gives a slightly higher rise. I probably have too small a frame since it's virtual top tube length is 56.3 cm. Calculations would suggest a 58.7cm frame; yet I fit the Litespeed fairly well with some mods. Thank the LBS for that idea and having the seat post 2in too low on their "custom fit" they charge $100 for.

However, frames that aren't quite as compact or square won't ever offer this screw up possibility (of not giving low upper-lower leg angle) since full seat post extension (assuming availability of a 330mm post isn't a big deal) would give possibility for more than full leg extension.

Now for top tube concerns. There are stems that range from 70mm to 140mm of varying angles. Assuming that the fork steerer tube hasn't been trimmed completely, and with the use of spacers above or below the steerer tube, a large combination of (x,y) coordinates of the handlerbar and stem junction is possible. One can see 7cm difference for a given angle, which if placing the ideal frame in the middle, gives an easy 2-3cm difference on either side for error. That accounts for the +/- one size frame difference in addition to the frame that hits the recommendation. There me be a strange angle rise resulting from putting a big person on a small bike, but the (x,y) points (z in the direction parallel to the top tube) are going to be the same between the "incorrect" and "correct" frame in the wireframe intersection points of the rider.

Yeah, cranks come in their recommended sizes differently but can be easily replaced and often cover two-three sizes. Light stems can easily be had for $10-30. I guess turning radius can be adversely affected on a unnecessarily small frame that fits within these guidelines, but I don't really see how the arguments for importance on bike fit gain credence.

One thing these ideas don't take into account is where the relative body inertial moment & center of gravity sits relative to the frame size and under different conditions. But heavier people probably don't have their weight calculated into their bike fit, and the same for light people, so those arguments could weigh against that in the same manner. Stronger riders less interested in comfortable frames might choose something smaller; low mileage riders interested in comfort might choose something large letting the frame take the abuse and suck up some energy. Preference isn't calculated into the "exacting measurement fit" either.

But as for where the relative points in space of elbows, shoulders, head, butt, knees, and feet, these points seem far more flexible than what the literature would suggest.

Changing a frame rarely seems like it would ever be an issue if one measures their own inseam and buys a frame based on the .67 formula. Erring on the + or - side would easily make the recommend points achievable.

How can one justify the $$$'s one would spend for an custom frame build? Or even for wanting the 1cm increments touted as so important by some companies.
Sounds like alot of speculation from someone who has not ridden alot of different sized bikes and tried to make them fit. Sure, you can 'force fit' one, and get by.Been there done that several times,both too big and too small. The 'right' size with correct stem, bars,and crank is just so much better.The formula puts me on a 60. I find a 57 in both a cdale Cdale and Merckx to be much better than a 58 in either,and a 56 in both would be too small. And that's all I'm gonna say about 1cm increments. Y'all already know what I thinks about hooey and generalizations.
 

jasong

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Nov 24, 2003
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The point is that the critical points definitely can be put at the same points in space more or less on 3 frames (not considering extremes). Would you agree with that?

What doesn't fit for you on a 56cm?
 

buffedupboy

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Feb 28, 2004
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I don't actually understand what you are trying to say with your post.

1) Are you saying it is not worth it to go custom and can't justify the difference in price?

2) Are you posting to knock your LBS for charging you money for a faux fitting?

3) Are you trying to tell us what we already know about bike fitting?

4) Are you trying to tell us that there is no difference between a compact frame and a standard geometry frame except for the standover height?

5) Are you trying to say something that I didn't get in your posting?

Whatever it is, the whole compact vs traditional is just a load of s***. The only difference people should look at when comparing both frames is the front centre and wheelbase. The reason why compacts appear to ride livelier is because normally people are riding bikes which have a smaller wheelbase compared to the standard geometry bikes that they are used to. It has nothing to do with centre of gravity, or whatever else people are touting.

I personally think they can make any frame with a 3cm increment and get away with it. However, we are spoilt for choice and have different preferences for riding characteristics. Hence the frames come in 1-2cm increments to allow for longer wheelbases, longer front centres, shorter standovers, preference for longer stems and hence handling characteristics.

Customs are just what they are, one-off wonders. You get what you pay for, a bike that no one else has. And if you have freakish body measurements you finally have a chance to own a freakish bike! (kidding). Anyways, I would pay for a custom bike not for the fit, but for the knowledge that I am the only one with this bike.

regards,
Sean
 

jasong

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Nov 24, 2003
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I think the points are varied, but summarized in the response to boudreaux. Derivations from that generalization follow in your 1,2, and maybe 5. Not 4. I don't think 3, and would like counterexamples because if 3 is true, then it means a lot.

Not knocking specifically my LBS or I would have mentioned their name, but knocking the popular mentality that they are unanimously accurate and won't sell frames that don't fit people properly. Also contributing to that idea that individuals can make reasonable bike buying decisions without the fear of screwing up too much (you know, like spending $1500 for something that's the wrong size). And especially with mail order advice or advice obtained from reading forums like these or other fit guides. It seems unfortunate a lot of people can't get in this sport because they can't cough up the at least $1k and that's too bad.

Aside from that, what do you see as being the exceptions to what I've written? That's what I was interested in. Putting out those ideas and having counterexamples listed or explanations given from those that have a lot of knowledge in this area.
 

boudreaux

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Oct 16, 2003
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Originally posted by jasong
The point is that the critical points definitely can be put at the same points in space more or less on 3 frames (not considering extremes). Would you agree with that?

What doesn't fit for you on a 56cm?
Toptubes too short and the fix is not 140 stems. And yeah, I've heard that 3 point babble before. Did you cut and paste it from another source?There is more to it than that.
 

jstraw

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Jan 16, 2004
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I don't pretend to know this stuff so help me out here...the three point stuff matters...we all have an optimum position for our own body to be in and that's defined by saddle, bars and pedals...but isn't it equally true that this has nothing to do with bike handling, that that position can be almost anywhere in terms of center of gravity, releative to almost any kind of frame geometry and with almost any kind of wheelbase and with any sort of bottom bracket height...yes?
 

meehs

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Nov 7, 2003
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I don't get the point of the post at all... Am I missing something? Why would you want to shoe-horn your ass onto a small frame with a 400mm seatpost and a 140mm stem when you can get a bike that actually fits??? What's your point? A overly long stem messes-up the handling characteristics of the bike and with 300mm of seatpost sticking out of the seat tube your ride-feel is going to be based 90% on the flex of all that seat post and 10% on your frame. That's just ridiculous!
 

manilacyclist

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Oct 21, 2003
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Just to help you on this issue of fitting.

There's an article on bike fit in this month's Bicycling. The article is a great read if you're looking for that custom fit.
 

Aztec

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Jul 8, 2003
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I disagree strongly with the original post.

Boudreaux nails it re: long stems. Ever ride a 140 stem? Probably not as safe either.

What about a low front end from too short of a seat tube? You mention spacers and angled stems. There are bigtime limits there. 2-3cm spacer plus a goofy +13 degree or whatever stem isn't aesthetic at the very least.

What about standover height? Unless you have a decent amount of seatpost above the collar, you are likely to zero standover. That makes unclipping at stoplights, etc., wobbly.

I'd say there's room to err by 1-2cm in any direction from the ideal. No more for a high performance ride.
 

serenaslu

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Feb 3, 2004
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Originally posted by boudreaux
Toptubes too short and the fix is not 140 stems. And yeah, I've heard that 3 point babble before. Did you cut and paste it from another source?There is more to it than that.

Absolutely correct!! As an example, in current Cdale frames I would take a 55cm. (stubby legs and longer torso for my height, just the way I'm made) I constantly have had LBS's try to stick me on a 54cm with a longer stem. Would it work, sure. Would it be nearly as comfortable. Nope, not for me; maybe for someone else. Having the bars turn on a longer lever can feel and react dramatically different. But as Cdale on only markets complete bikes in even sizes, most of my LBS's would prefer the former. I also know a dealer who is 100 miles away but shrugged his shoulders and said, "no big deal" he can get a 55 frameset in 3 days and build it just as easy. Who'd you think I'd rather deal with?
 

tcklyde

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Dec 17, 2003
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I strongly agree with bourdreaux, Aztec, serenaslu, et. al., with the added point that different riding also demands different sizing. I commute on a bike with a shorter top tube and higher bars so I have a more up right position to navigate heavy traffic. On weekend and after work fitness rides, I prefer a more prone position.

A 140 mm stem is craziness, man!
 

Tonto

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Nov 3, 2003
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I changed my 110mm stem to a 50cm stem and this made a load of difference in terms of comfort. I had been getting pain in the upper arms and the gonads :eek:
I certainly would be wary of a 140mm stem though.
 

boudreaux

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Oct 16, 2003
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Originally posted by Tonto
I changed my 110mm stem to a 50cm stem and this made a load of difference in terms of comfort. I had been getting pain in the upper arms and the gonads :eek:
I certainly would be wary of a 140mm stem though.
I'd have got a smaller frame.