Dave Moulton

After measuring and studying hundreds if not thousands of customer’s measurements over the years as a custom frame builder. I came to the conclusion that although human bodies are all different; they do follow certain rules of nature.

Tall people are not scaled up models of short people. Most of the height difference is in the legs; body length differs by a lesser proportion.

If you have long legs then you also have long arms. Short legs; short arms. This makes sense since most animals are four legged; why should we be any different?

Leg length is a combination of the inside leg measurement plus the length of the foot. Length of the foot is important because when pedaling the toe is pointed downward at the bottom of the pedal stroke; so the foot becomes an extension of the leg.

People, who have a short inside leg measurement for their height, generally have longer feet. (Bigger shoe size.) It is as if they were designed as a much taller person, but their heel got turned further up their leg; making a short leg, long foot.

Imagine two people both six feet tall standing side by side; one has a 34 inch inside leg, the other a 32 inch leg measurement. Because they are the same height it follows the one with the shorter leg has a body 2 inches longer; he also has longer feet and shorter arms than the other guy. They can both fit on the same size frame, (59 cm. center to top i.e. 57 center to center.)

They will both have close to the same seat height, because the short leg guy has a longer foot so his seat needs to go higher than his inseam would suggest. They can also use the same top tube length and handlebar stem, because one has short body long arms; the other long body, short arms; making their reach the same. Minor adjustments in seat height and stem length may be called for.

It has occurred to me that with these compact frames on the market now and only available in small, medium, and large; customers are only ball parking frame size anyway. It has long been my opinion that frame size is linked to the overall height of the rider more than any other measurement because of the rules of nature I have just spoken of.

So I have just formulated this based on my own frame sizing chart. If you are 5’ 3” to 5’ 5” frame size equals Height divide by 3.3. For people 5’ 6” to 5’ 10” frame size = Height divide by 3.2 and if you are 5’ 11” to 6’ 4” frame size = Height divide by 3.1

A example would be someone 6’ 2” = 74” divide this by 3.1 = 23.87 in. (61 cm. measured center to top. i.e. 59cm. center to center. A person 5’ 7” = 67” divide this by 3.2 = 20.93 in. (53cm. center to top. i.e. 51cm. center to center.) The easiest way to convert from inches to centimeters is to get a tape measure with both on and simply read across.

Reach which is top tube plus stem length; is frame size center to top plus 10cm. If you want something more accurate go to the chart on my website (Link below) go to ‘Bicycles’ page 6 ‘Bike tech.' That chart was derived not by any mathematical formula but by records of custom frames built over many years. Most people find it pretty accurate. The above is an attempt to come up with a simple formula that comes close to this. Please don’t change your position based on this alone because this is new thinking.

What I would like to know from everyone is; is this of any help or does it complicate things further and also does it come close to what you are riding now?

jukt

No, thanks for the great post. And the chart.

Most LBS use standover. I'm 5' 10", 30 " inseam. The Chimp. Don't know what size my bike is. I just ride it.

The most important part may be the rider. My body hurt like *&$@ until I got stretched out. Now, I ride fine.

I don't wear shoes and pedals, like to move my feet around, use differant muscles.

rcams

For me this supports my own experience a lot better than most of the other guidelines I've tried. I'm 5"11" and can't remember my inseam (shoe size 10.5 - 11), but every time I measure inseam and use the latest guide I'm looking at, I come up with a frame size of right around 55 cm - which is what I'm riding. The problem is that I always feel "cramped" no matter how high I set my seatpost (max. now).

When I used your formula I come up with a 58 cm c-t size and intuitively I was planning to just toss the 55 cm I keep coming up with and go to a 57 on my next bike. I think you're onto something. BTW, I have over 23,000 miles on two nearly identical frames that are 55 cm. and have always felt like they were a bit too small....its not like I'm on a new unfamiliar bike, or inexperienced.

halfspeed

I'm 6' 0", 35" inseam and I ride 61cm frames. Your chart says 59. Maybe on some frames a 59 would work, but not anything I ride.

Geometry can a have a huge effect that you do not account for in your measurement. Top tube length and head/seat angles can change fit significantly.

Also, while the reach may be the same on long and short torsoed riders, weight and musculature distribution is not. That also may influence ideal top tube length.

Finally, I'm not convinced that extra foot length is equivalent to extra height, especially as crank lengths change.

chimivee

Interesting. It does seem oversimplified, but using that formula for me, it spits out the same frame size that other forumulas generate and that I ride. Hmmm..


At 5'10", a 30" inseam is a bit short... So, according to Moulton's theory you should have big feet. I'm guessing average for that height is size 10 (?)... What size shoe do you wear?

DXchulo

Very insightful. I'm a 53, and it kind of sucks because most bikes come in 52 or 54.

I wonder what his theory on penis size is.

rybowen

Works for me, almost too well. I'm 5'10 and 1/2, so I fall between 3.2 and 3.1. The first gives me a 56, the second a 57. I fit either pretty well with the right stem. I think I have very average proportions though, so I wouldn't be a good test case.

BikeWNC

Dave, I think your chart is pretty close. It is hard to compare TT lengths without knowing STA though. I am 190cm tall, 90.5 inseam and my bike has a 60cm TT with a 12cm stem and a 72 STA. You would put me on a 57 TT with a 13.5 stem. If your bike had a steeper STA then we would be very close.

My wife rides a bike at her height of 161cm which has a 51 TT and a 9cm stem which is what you suggest. Her STA is 74 degrees (maybe 74.5).

Obviously, years of observation have given you a pretty good handle on what works for most people.

Steelrider

Anecdotally, the formula seems to work pretty damn well for me. My custom is about a 58.75 ST and a 58.5 TT with a 14 stem (which seems a little long now that I'm less concerned about having a flat back and being stretched out). I'm 6'1" with a long torso, long arms (35.5 sleeve), and (I think)shorter-than-average barefoot standover of 33.75" (with a 13 shoe) and the formula spits out a frame size of 59.812885. Coincidentally, I just bought a frame that measures 60cm unconventionally, because it uses the top-of-top tube as "theoretical center" - which means that actual center is almost right at the formula length (top tube has very narrow profile). Gonna have to think about what this is telling me in concrete terms, but I do feel better about the frame size choice I made!

Steelrider

You want the truth? You can't handle the truth!

mirona

Your chart puts me on a 54 which is exactly what I have now. My shoe size is 1 1/2 sizes larger though

Dave Moulton

Traditionally isn’t that linked to shoe size?

txcyn

Im 5'1" was riding a 50C, now a 47C it fits much better. Also works with your theory.

shaq-d

dave,

sounds good to me. how're u measuring reach though? you're not just saying that everyone should get 10cm stems are you? if so, that's a good starting point, and hard to go wrong, but not ideal...

sd

jkimbro

From what I've been reading it seems that you can get a good "fit" from a frame geometry that is compact (and therefore maybe a bit smaller than traditional frames) by adjusting the stem length/height and the saddle position/height. Of course, the frame size still has to be within "the ballpark." I know that is the theory of compact frame manufacturers and although I'm not endorsing this theory, it does seem to make some sense. The exception(s) would be if the frame was either too large (making these adjustments impossible) or grossly too small, placing the center of gravity somewhere it shouldn't be which can have an effect on weight distribution and/or rake and trail which affects handling.

Dave Moulton

No that was not what I meant. If the frame size is 59 then the combined top tube length and stem would be 69cm. It could be a 57 top tube with a 12cm stem or a 58cm top tube with an 11cm. stem. I am not talking of custom frames here I mean stock frames which could be any top tube length.

On the compact frames with the sloping top tube the only way you can estimate where the handlebars should be is to measure horizontally across from the seat post combining an imaginary top tube length plus the stem length.

scrantr

Your sizing nailed it for me. 6 feet even, 32 inseam, big feet, knuckles dragging on the sidewalk when I walk.

I checked the Co-Motion web site to make sure how they measure their frames. I'm riding a 57cm Americano, measured center-to-center. The hybrid I use for a sloppy weather commuter is rather smaller, but has more seat post and stem showing. My previous road frame was a 23-inch measured top-to-center (from 1975, when men were men and stock frames were available in 2-inch increments) tourer.

seely

Well it puts me on roughly a 57.5 c to c frame which is pretty spot on. I ride a 56cm but its too small... most 58's I've tried are too big. I would need to go +10cm and maybe a little more on the reach, since I have arms to my knees but its definately very close.

forum*rider

hmmm, according to that my bike is too small. I'm 5' 3" with a 28.something inseam. I should ride a 19" correct? My current mtb is 18", and yet it feels fine.

Eh, as long as it works it's good enough for me.

Crispin

What about me? I have a 38 inch (=96.7 cm) inseam. My height is 1.89 m. I don't know what that is in foot.

Dave Moulton

You are fine. I was referring only to road bikes; MTBs tend to run an inch or so smaller with more seat post showing. 48cm. which is just under 19inches, is the smallest size conventional road frame you can build. That is one with a level top tube and 27 inch wheels because at that point the top tube and down tubes merge together.


This formula does not work for you because you are 6’ 2 ½” tall but you have exceptionally long legs; 38 inch inside leg. For you I would recommend a 64cm. frame center to top, which is 62cm. measured center to center.

MichaelW

Looking at frame geometry charts, it seems that all frames from the smallest to the largest, are built the same height off the ground (ie bottom bracket height). There is less than1cm difference between a tiny women's frame and a clydesdale and that distance is set to be correct for a medium frame.
Would it make sense to select a crank size proportionate to the leg, then design a BB height to give safe cornering clearance. This would mean that every rider has their pedal at the same height off the ground, but the BB would vary by several cm depending on their size.
If you look at v small frames, you see riders with the saddle and BB way too high off the ground. Imagine scaling up the smallest size to the largest with a proportionate BB height. (ie increase every dimension by 50%), It goes from 26.6 to 39.9cm

seely

I figured it out once, and the actually difference between a 170mm crank and a 175mm crank is something miniscule like .4"... I don't think it really makes as big a difference as some might lead you to believe. As for cornering clearance, if you are going hard into a corner, your inside leg is probably up anyways, so it doesn't really matter if you have 167's or 180's... they won't strike.

Dave Moulton

You have touched on the dilemma all frame builders face. Building a traditional road frame with a level top tube you can only lower it until it merges with the down tube and you are left with a tiny head tube. When you have reached this point the only way you can make the seat tube any shorter is to raise the bottom bracket. But as you so rightly point out the last thing a short person needs is a bike with a high bottom bracket.

However now it is acceptable to build a frame with a sloping top tube, building small frames got easier. As for crank length as ‘seely’ mentions in the post after yours; the difference between the longest and shortest length is not that great. Also the modern clipless pedals offer more ground clearance; so yes bottom brackets could be lowered. But to do so lengthens the down tube and the chainstays so you build a frame that is not quite as stiff. So it all really depends on what you are designing the frame for.

JavaMan

I'm 6'4", 36" inseam, and ride a 62 cm bike. I also have 180 mm cranks.
Tom