Building small frames
Thu, November 18, 2010 
Frame design, in many ways, was much simpler when I was building back in the 1980s.
Top tubes were always level; it was not acceptable, to me or my customers, to build a frame with a sloping top tube.
Once a rider had established his correct frame size, he set the handlebar stem about 4 or 5 centimeters above the head bearings.
Then he set his saddle to the correct height and he was for the most part, good to go.
The handlebar stem could be adjusted a centimeter up or down as the rider wished.
The other point that made everything simpler was the fact that a person could go buy a frame of any make, in the same size, and the seat to handlebar height ratio would be the same.
If the top tube length varied slightly it could be corrected with a longer or shorter stem. Handlebar drop never even entered into the equation because it was automatic once you had the frame size right. The level top tube was in fact a point of reference.
Many years before I started riding, the wheel size for a racing bicycle was set at 27 inches diameter, or 700c as it is known today. This means that there is a fixed distance from the ground to the bottom of the head tube. This is always the same for any size frame with 700c wheels.
On a level top tube frame, it doesn’t matter that the builder changes the bottom bracket height. If he raises it, and the seat tube length remains the same, then he also raises the top tube and the head tube becomes longer. This is because the bottom of the head tube remains in a fixed position.
The rider’s saddle height is measured from the pedals (Or BB center.) to the top of the saddle. So although the rider is sitting higher because of the high bottom bracket, because the head tube has become longer by an equal amount, the seat to handlebar height difference always remains the same for any given size.
Each frame size will have its own saddle to handlebar height difference, which increases as frames become larger, decreases for the smaller sizes. On a modern sloping top tube frame, raising or lowering the bottom braket height will not necesarily alter the head tube length.
With today’s design, the bottom of the head tube is still in a fixed position, but the top of the head tube can be anywhere; it is not governed by a level top tube as it used to be. There seems to be no standard point of reference between the different manufacturers.
Where today’s design has an advantage it is in building very small frames. The bike pictured top left, is my personal bike; it is a 51 centimeter. (Center to top.) You can see in the picture that if the position of the bottom head lug is fixed, a framebuilder can only lower the top tube another 2 cm. and the lugs merge. To all practical purposes a 49 cm. is the smallest level top tube frame he can build.
The only way to go smaller is to shorten the seat tube by raising the bottom bracket. This really goes against the requirements of the rider, because the last thing a person with short legs needs is to be higher from the ground.
I was asked just this week how would I go about designing frames for women. If the woman was 5’ 4” or taller it was no problem; I would just build according to the customers measurements as I would for a man.
If the woman was less than 5’ 4” then it was not so much a case of building a frame to fit, but one of how small can I build this frame? There is not only a limit to how short can I make the seat tube, but there is a limit the how short one can make a top tube.
The whole problem is fitting two large wheels into a frame that has reached the limit for those size wheels; it restricts what you can do. Smaller wheels are available but rims and tires are limited to a much narrower choice than for the standard 700c.
Frames are now sized like tee-shirts; extra-small, small, medium, and large. My advice to female under 5’ 4” would be to buy a frame in the smallest size possible. There will be no problem with the seat tube length, and there is now a far wider range of handlebar stem lengths and angles than were ever available in my day.
With this wider range of handlebar stems, it should be possible for most riders to dial in a near perfect position. The only problem as I see it is that another dimension has been added to the equation, and that is handlebar drop.
In the old days one only had to concern themselves with saddle height and reach, drop took care of itself with the correct size frame.
I always maintained that a rider’s arms (On the drops.) should be in direct opposition to the legs. Choose a combination of drop and reach that will achieve this.
Back in 2007 I wrote an article which included a chart that gave a drop measurement and was based on my old fit philosophy from the days when top tubes were level. Some people have found it useful.
Your comments and input as always are appreciated.
Reader Comments (16)
Interesting article. Georgena Terry posted some videos on YouTube which explained her design philosophy for small frames; essentially, having reached the same conclusions as you did, she decided that the only way to build a smaller frame would be to use a smaller front wheel, as this would shorten the fork and lengthen the headtube, so the seat tube could be shortened. She explains it better than I can.
As you note, with sloping top-tubes, it's now pointless to measure seat-tube length as a frame size indicator.
Even more, with a choice of set-back, set-forward or clamp-on-top posts, combined with normal saddle rail adjustment, I can get the same "effective" seat tube angle (BB to saddle) on bikes with a range of actual seat tube angles (or amorphous carbon structures with only a passing resemblance to a seat tube), each of which will have a different (effective) top tube length for a given riding position, depending on how far forward or back and up or down the nominal measurement point is.
It makes more sense now, I think, to measure bike frames in terms of "reach" and "stack". I'm not sure where this methodology originates, but it is described here:
http://forums.roadbikereview.com/showpost.php?p=2907645&postcount=421
It takes out all the adjustable and swappable dimensions, and only measures the relationship between the two frame fit hard points: BB and top headset race.
This allows bikes with any tubing shape - even monocoque designs with no tubes - to be compared directly with meaningful measurements that don't rely on "effective" tube positions and imaginary constructs.
When people ask "what frame size do I need?", it is almost impossible to answer them without knowing how the frame is built. If a person doesn't know what size top tube they need, they are pretty well screwed, unless they buy the frame from a good fitter. My three road bikes range in "frame" size from 55, to 59, to 60cm. They all have a 57-1/2 or 58cm top tube. One is a "classic" style, one has a deep slope to the top tube, and the third has a slight "upwards" slope to the top tube, so that the head tube can be 1cm longer.
The main reason for the sloping TT phenomenon is $$$. Less frame sized to make and stock.
Very interesting article, Dave. As a short woman (not much over 5' tall), I had to make do with 19-20" frames all of my adult life. Until I discovered the Surly Long Haul Trucker. It's not a racing bike, but it does everything I've asked of it, and my 42cm fits like a glove. Being built around 26" wheels no doubt makes this possible, as does the sloping top tube. And as far as finding suitable tires, I've not felt handicapped by the offerings.
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I'm genuinely confused by the illustration. It seems to show that the ground to handlebar distance is the same for all frame sizes (that use a particular wheel size). But if that's true, how can "the seat to handlebar height difference always remains the same"?
Eric,
I changed the picture to clarify. The witness line points to the bottom of the head tube; that is the point that remains constant. It also states that in the text. (Second paragraph below the picture.)
That the handlebars happen to be at the same height on the particular bike is irrelivant and unfortunate.
Also, I was pointing out that the saddle to bar height remans constant for any given size, regardless of the BB height. On a modern sloping top tube frame, raising or lowering the bottom braket height will not necesarily alter the head tube length.
Each frame size will have its own saddle to handlebar height difference, which increases as frames become larger, decreases for the smaller sizes. (I have added these points to the text.)
I hope this clears up the confusion.
Dave
Hi! I'm a 5'2" woman with very long legs and small feet. I have been roadbiking for about 10 years, am a mathematician and love to compute angles :-) so it's rare I read something new and the shoe size thing is just so logical and never occurred to me. Wow, what a clever guy.
I have a Terry with a small front wheel. This is not to make the top tube horizontal but to shorten the top tube. If not there is toe overlap for some people. I also have a mid 90s cannondale that is extremely small and hence has a sloping top tube. It has a 48 top tube and I have an 11cm stem on it. It is a great fit, but with a slight toe overlap, that was never a real problem for me. Lastly I have a litespeed bella in size S, so not the smallest one, with 700c wheels. It's great, even though the top tube is over 51 cm. With today's stems available, you can go to as little 60cm (I use an 80) and the salsa poco bars have a very very short reach. All these are more important than the perfect frame geometry, I think. It's good if you can have it all though but I agree that frame size is pretty much determined by height and with today's components you have some room to work.
Dave-
Yes, that makes a lot more sense. Thanks!
-Eric
My daughter is of average-to-short overall height, with very long legs. She rides one of the Terry 'Symmetry' frames with a 700C rear wheel and a 24" front wheel - it works out great for her, although finding tubes and tires for that front wheel is a challenge.....
Nice job, Dave, I think maybe some drawings would help illustrate the bottom bracket differences, it's hard to visualize that. One thing about the Terry Videos was she did pay attention to "trail" also with the changes she makes.
The only thing I don't like about modern stems is that you have to throw down 85.00 to get a decent one with four handlebar bolts instead of two. I get paranoid about bolts snapping. They're probably so expensive because of liablility.
Yes my modern alumunum Trek is ripping fast but definitely more complicated to fit myself on. I notice evey microscopic change on this bike.
Robert,
If the bottom bracket is moved higher, the seat tube is pushed up and the top tube is also higher off the ground. So if the top tube remains level, the head tube becomes longer because the bottom of the head tube stays in a fixed position.
I wrote a piece on bottom bracket height previously; here's the link.
Dave
Aren't bicycles a blast? Anytime we're not racing around with the wind in our face, we can spend endless hours gabbing about the last millimeter of something or another. Or sharing blog time with a Guru like Dave Moulton. Dammit Man! I love this stuff!. Steel!
Been a height impared chap, I have had problems with top tube height, I do wish to point out that a 27X 1 1/4" tyred wheel is MUCH taller that a 700 x 23 tyred wheel, and I found that switching from 27" to 700c makes a diffrence in stand over height.
Interesting post.
I'd agree - modern frames all over the place sizing wise. The "small", "medium", "large" sizing is partly a carry over from mountain bikes - where that's been the norm since day one. Of course, it's also for cost reasons - less frame sizes to manufacture and stock. This is especially true with carbon frames, since molds are expensive.
The modern headsets are also more compact, lowering the bar height, which affects fit. The giant "Stack-O-Spacers" found on some bikes to compensate can look pretty strange. The newer stuff is light and easy to work on - no doubt - but at least the old style quill stems allowed adjustment.
Dave, I really enjoy reading your blog!. This is an interesting topic. Being an old fart, I prefer steel over plastic, horizontal TT over sloping etc.
I find it interestng that so much of the discussion about fitting is related to the top of drop bars. How about an article discussing drop bar shapes in combination with old braks vs. "brifters" from an eronomic perspective.
BTW: I enjoyed using your fitting chart to find out how far out I was with my current steed (83 Colnago). Right at the upper limit with a 58 ctc ST. My previous mount was 61 (72 Motobecane Le Champion).