Monday, February 27, 2006

A Different Thought on Frame Sizing


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.

Choosing frame size on inseam alone does not work across the board because for example it is common for a 6 foot man to have an inseam as short as 30 inches; and you can’t put a 6 foot man on a 51 cm. frame which is what his inseam suggests.

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.

I have 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 metric and inches on and simply read across. If you don’t have a tape measure the formula is “inches x 2.54”

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 The chart was derived not by any mathematical formula but by records of custom frames built over many years. Most people find it pretty accurate. What I have put forward here in this Blog is an attempt to come up with a simple formula that comes close to this. If you are an experienced rider don’t change your position based on this alone because this is new thinking. It is intended as a place to start for a newcomer to the sport.

Feedback would be apprecated.

10 comments:

Anonymous said...

Dearest Davie-boy,
This is an absolutely brilliant observation on vertical human body scalings! I'm currently scopeing an online auction for a bike, and was faltering somewhat at the whole 'scaling consideration gauging thing', what with different frame sizes in different units, etc!!

Funnily enough I am in fact a "stumpy-legged freak"; but it is a condition I have learnt to live delight in - after all, I'm a ghastly abominstion to nature, and not every body can say that!!!
Though it wasn't always so easy you know, no... as a youth I used to lash my feet together with leathern belts and, with a deft handspring, launch my poor inverted little freak-child self at my wardrobe, upon which the slack in the centre bindings would snag over the top corner, allowing my to dangle to my heart's (dis)content; waiting against hope that my stumpy little legs would stretch. Hey-ho; apparently I am genetically indisposed to being 'normal', but I don't see why that should stop me from riding a bike!!!

Yes yes, did you know that although I stand at a modest 5'10", when seated I tower over fellows measuring 6'7" for heavens sake?! Weren't you aware of this? No, no; I suppose not. Any way, like I say, it's not too bad

Little did I realise
For years interminable my family, friends and colleagues have suffered my somewhat vague ruminations on such concepts as heretowith most erruditely encapsulated

David said...

Thanks for sharing all your thoughts on this. I am 6'8" and have always ridden bikes that were too small. I think I may just seek out a builder.

justlonelyme said...

I know that you wrote this some time ago, but I'm thinking about sizing now and I'm curious about a couple of things. My old bike is by your calculations and by other experts the right size for me, however I always feel like I want to be on a taller bike. So although I'm suppose to be on a 56cm frame I always like riding 58cm frames. Should I just suck it up and ride the right sized bike for me? If I go with what I like, just a bit taller, will I be hurting myself in some way? Thanks for your time and any comments you may have.

Dave Moulton said...

This is only a guideline, not mandatory. It is also the set up for a racing position, if your riding style is more leisurely then by all means a slightly larger frame will mean less drop from the saddle to the handlebars.

Just remember that a lager frame will have a longer top tube and try to avoid a situation where you are stretched out too far as this can cause backache. Other than that there would be no permanent physical harm.

Anonymous said...

Dave,

I'm guessing you probably get a million requests for fit assessments a day. So, on the possibility that you'll help me, a random stranger, let me provide you with my info:

I'm 5'10 and 1/4"
I have a 33 and 1/4" inseam (maybe 33.5 if I push)
size 9 US shoes.

What would be your honest assessment of a frame size based on your previous user data? Because of my shorter feet (and hence shorter arms) I'm curious to see how I stack up. I'm especially concerned with the top tube length suggestion, as my arms are indeed short.

Thanks

Adam

Len said...

Here is what I found after 45 years on a road bike. I am not normal.
Most shops would recommend that I take a 56cm (center to center) frame, but
I have short legs, a huge torso, size 13 feet, and "long arms"
About 20 years ago I finally got sized by Bertrand in Canada, and sure enuf, it said I needed a 56 c-c seat tube and ---a 59 c-c--- top tube, if I used a 12 cm stem.

They finally built it more or less that way, but I have come to realize that I can fit well enuf on a frame that is seat (58 c-c) with top tube at(58 c-c) and a 12 cm stem, but to center me on the frame nicely, I need a 72 degree seat tube.

I once got stuck with the bike shop's recommendation of a 56 cm Gios Torino (with a 55 top tube) That was a royal pain and I could almost clean my front tire with my moustache on that bike.

EdZ said...

Dave, I have to firmly disagree with your thesis.

1) While among children a measure of some limb segments (especially hand) can be linked to potential growth, neither foot, leg or arm length among fully developed adults are well correlated with height other than the simple observation that taller people tend to have longer legs, longer arms and larger feet than shorter ones. People with shorter legs don't have larger feet. If there is a correlation its between the length of underarm and foot-- feet are nearly always a bit shorter but never longer.

2) People don't always ride toe down or ankle. I might suggest that your observation was derived from looking at a lot of people compensating from having their saddles too high (became quite common in the US in the 1980s to a certain extent due to YOUR influence!). The body is very good at adapting and compensating--- save when some form of injury sets in.

3) Saddle height is not a factor of inseam but application, frame geometry, body form/condition and, above all, the proportion of the outseam (not inseam since different people can have the same inseam but very different hip positions) and lower leg lengths.

Does fit to height work? Yes. Fit by arm length, fit by hat size (given that hat size increases by age it even takes in account declining flexibility), fit by !#@. The human body can adapt very well and given the observation that most people don't ride 3 weeks Tour at limit (most professional cyclists don't either) and don't need to change bicycles under stress, it does not really matter. All these fitting tables, sizing bicycles etc. are, after all, more voodoo than biometrics.

Is there a better and less good fit to a bicycle? Yes.. but inseam, height don't provide sufficient information.

Having examined the fit (human interface) of large numbers of elite cyclists (it was a hobby) on their bicycles and spoken with many of their builders and mechanics, its clear just how arbitrary and fad driven most riders are. Its more psychology than much anything else. That's the stuff that makes a cyclist ride faster on horse piss!

Dave Moulton said...

EdZ
I said at the end of the piece, “Feedback appreciated” and it is, but why so much anger? I’m just and old guy who built a few frames, just throwing out some ideas to entertain and make people think, it is not the Gospel According to St. John.
Dave.

EdZ said...

"I said at the end of the piece, “Feedback appreciated” and it is, but why so much anger?"

Whose angry? If there is any anger in cycling its against the observation that cycling has long stopped being about riding bicycles but institutionalized doping, manipulation and money (but not enough, as in soccer, to provide unlimited indulgences and political cover).

"I’m just and old guy who built a few frames,"

And extremely nice ones at that--- even if not always my taste (although some were).

"just throwing out some ideas to entertain and make people think,"

Exactly.

That said.. Can't I disagree with you?

artmonkey said...

Thanks Dave,

I was just looking for an approximate bike size, but I found a lot more. Your insights remind me of the center-point of a revolving wheel. Perhaps that is an apt metaphore as I attempt to deviate from my work demands and write, paint and ride again.

On your website I did come across your comments on the cycle forum 'attackers' - I have found encounters with lack of goodwill and negative projections to be a big challenge since there is a small toxicity pool of this stuff within my workplace. Actually, it also connects to your comments on workfroce attitudes - since I am responsible for a dept. with some employees who have a fairly hostile approach to personal responsibility and work tasks and it just clouds the atmpshere and is like goo in the arteries - don't quite know how to send such negative stuff away - but working on it - moreso - looking for an out- to be around others that better 'get it' - and are more aligned with positive contribution

not sure why I am telling you this - just that I see my circumstance or a familar with the thoughts reflected in your writing

more than good writing - you are good energy, Dave

have enjoyed reading you and learning about you

and you have proven that perhaps it is worth the time to document creative endeavours for other's access

keeping it all inside can, perhaps be limiting

on plans....let it happen vs the plan - you advocate one while admonishing yourself for no long strategy - I guess they go hand in hand - I definitely wouldn't know -letting fear tumble me possibility more than once - your choices seem nicely instict-based

Thanks for the inspiration and good sense.