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« Rear Dropouts | Main | My best 12 hour ride. »
Monday
Feb272006

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.

Reader Comments (30)

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
March 5, 2007 | Unregistered Commenter Anonymous
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.
April 3, 2007 | Unregistered Commenter David
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.
May 19, 2007 | Unregistered Commenter justlonelyme
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.
May 19, 2007 | Unregistered Commenter Dave Moulton
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
September 13, 2007 | Unregistered Commenter Anonymous
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.
November 6, 2007 | Unregistered Commenter Len
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!
January 3, 2008 | Unregistered Commenter EdZ
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.
January 3, 2008 | Unregistered Commenter Dave Moulton
"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?
January 3, 2008 | Unregistered Commenter EdZ
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.
July 19, 2008 | Unregistered Commenter artmonkey

I'm 6ft2, with a rather long inseam, and wear size 10-1/2.
I've have difficulty finding a good frame size... the ones that fit have a top tube much too long, and I have to go hunting for a short stem... which is a bummer, because a shorter wheelbase would afford better handling.

The frame I liked most, my feet would overlap the front wheel, the chainstays were ultra short, and the headtube steeper than normal. Never had anything that fit (and handled) as well ever since.

July 28, 2008 | Unregistered Commentersupertimm

Dave I am 6ft tall and I find that all my bikes which are 23" frames have top tubes that cause me to stretch. I have size 9 feet and I think my inseam is 33". With my seat far forward and an 80mm stem hoiked up high I am still about 20mm too short?
Not sure what is short on me but I think it is my arms?

August 15, 2008 | Unregistered CommenterGary Knight

I think I will have to go custom as every top tube on a traditional lugged steel frame is around 22" to 22"1/2 I feel I need to just sell all my bikes and start again.

August 15, 2008 | Unregistered CommenterGary Knight

About the way I see it. I'm 5'7" and a 53cm top to center is most excellent indeed!

September 9, 2008 | Unregistered CommenterVintage Velo

I am 5'6' and have a 28" inseam. I need advice on a frame size. I am getting different advice from local shops, but most want to special order either a 49-50cm road bike. I don't have the opportunity to test the bike before ordering and can't return before ordering.

Please help.

October 17, 2008 | Unregistered CommenterOmar Meer

I'm 5'11" and have ridden road bikes with top tubes from 56cm cc to 58.5 cc to 58. cc . The 56 cc was too short. Needed 135 mm stem. The 58.5 felt good with a 110 stem. My current 58 has a 125 mm stem. Notice a pattern ?. I like to be stretched out apparently. Also, I noticed that I'm more comfortable behined the cranks about an inch and a half. Does this help anyone ? .

February 17, 2009 | Unregistered CommenterDead Squirrel Dave

I was employed at Jack Sibbit's bike shop in Longsight, Manchester. Most customers stated wheelbase and frame size and left the other details to us. One time this guy came in with a blueprint with every measurement detailed. Jack gazed at the blueprint then said "Don't you ride races with corners in them?" He had spotted that the overlap between his pedals and the front wheel meant he could only ride straight ahead and not turn corners.

April 2, 2009 | Unregistered Commenterbikedunc

Dave, I'm a serious cyclist of 25 years experience, and even made an attempt at framebuilding after attending UBI's school taught by Ron Sutphen - who said many great things about you. One of my reasons to try framebuilding was the idea that as a long-limbed person I always found myself on 58cm frames rather than the 56cm frames that my 178cm height suggested. I was doing that to get my arms some reach, not to reduce my crotch-to-toptube distance. (heh heh)

What I've noticed as I've ungracefully entered middle age is that stem length preferences have a lot to do with top tube length and torso/arm accommodation, but aren't used nearly as often by most who casually discuss frame sizing. It's almost like people assume everyone is on 11 or 12 cm stems. Certainly 12cm stems were the standard when I began riding road bikes seriously in 1985... at least they were where I was shopping for mass-produced road bikes in the USA at that time. But I find that I don't like the steering of a 12cm stem and prefer 10 or 9 cms much better. Why are so many road frames designed around longer stems? Custom and habit is the only explanation I can muster.

Anyway, I love this set of Bike Fit posts you've done. Lots of great observations that my experience proves correct.

May 7, 2009 | Unregistered CommenterSean O

While all long-time road cyclists develop their own preferences and adaptations, there is one very interesting aspect about Dave's fitting advice for which I will forever be an admirer. It's the one about proportionally long-femured riders needing to be higher as opposed to the commonly recommended further back. I have found that this is the single best piece of fitting advice I've ever read, and the one which has the most dramatic effect on my riding.

Decades of familiarity with bike fitting advice, suggestions and rules of thumb had conditioned me to accept without question the common wisdom of saddle further back (and correspondingly lower). This makes someone with long thighs have to bend much more sharply, and at the same time end up with an inefficient pedal stroke due to a thigh angle which is too shallow, and a saddle height which is too low. It might be in the ballpark for a proportionally average rider, but it totally disadvantages the long-femured rider compared to everyone else. It's much harder on the thighs and the knees, and because the sharp bend restricts the ability of the diaphragm to move freely, it greatly restricts breathing in a way which affects all aspects of riding the bike. The solution is quite properly straddling the saddle higher, not further back. This fully restores reasonable leg angles, and it opens up the body for proper breathing when using drop bars.

To be fair, my first encounter with higher versus farther back came a decade before I read this on Dave's site. It came from the small custom builder I consulted more than a decade ago. But even then, I questioned that advice, and I never really gave it a chance, thereby guaranteeing myself another decade of inefficient and uncomfortable riding. Dave's site is the ONLY place I've ever seen this written down. Much time spent in trial and error riding since then has confirmed that at least for me, Dave's way is the best way.

May 20, 2009 | Unregistered CommenterPierre

Dave, et al,
I have been riding bikes that are" too big for me" since my first "real" bike ( a Peugeot PX-10 in 1968.) By all rights, I should be riding something around 53, 54 cm, but I am perfectly comfortable on a 58 (C-T, Moulton.) I can almost straddle the top tube flat footed, but have never experienced any discomfort from the proximity of the top tube to my delicate parts, since I don't generally place my feet on the ground while cycling. I guess I am agreeing with you that we all have different criteria and ideas about what constitutes good fit. My saddle is still a bit higher than the bars and I am comfortable, which, after all is what counts.
It's all about the enjoyment for me: since I'm not racing a more relaxed fit is what I choose.

June 10, 2009 | Unregistered Commenterhal faulkner

Dave, I am 5'11" and a inner leg of 31. My feet are between 8.5 to 9.
i have always ridden a frame of 21.5 (centre bracket to top). Also the top tube is 21.5 (cntre to cntre). I have recently realised that a stem of 112 cm's is very comfortable( I have suffered a bad back over the last ten year's).I also ride 175 mm cranks now finding them very smooth pedaling.

If I go on your guidelines my frame is too small. Also saddle back does suit me, I think if my saddle was put up instead I don't think my weight distributon would be right( I have had experience in the past with saddle being forward/up and I was noticably siting upright, not looking like a cyclist, present saddle height being 27.5 inches with saddle as back as possible).

Would the frame size from your guidlines correct the saddle back preference and the long stem (ps. Wev'e read about fork rake and trail affecting handling, does a longer stem affect handling as I seem more stable and accurate round bends than with a short stem?)

June 29, 2009 | Unregistered CommenterJohn

Sorry that should have been a 125 mm stem not 112 cm's !!!

June 29, 2009 | Unregistered CommenterJohn

I've ridden bikes with a 56 cc top tube, 58 cc and 58.5. The 56 was a hair crunched, even with a 135 mm stem. The other two were great as far as reach. I ran a 110 stem. Funny, that measured the same as the 56 with a 135 stem. The 58 is a compact, the 58.5 was a 61 cc frame. The latter was considered a little big. Best feel on the road I ever had. After all this, I realized ignore "pro sizing" or should I say wannbe pro. Later.

September 11, 2009 | Unregistered CommenterDave

I rather like the formula. The results are similar to formulas that I've used and/or half "invented" which are more complex. Such methods involve other body part & partial meas. and like yours, as a matter of course end up quite close to simpler methods, a consensus. The formula yeilds similar results to on-line calcs. as well as corresponding to actual findings in my own size as well as with other individuals withwhom I've been involved.
It's just plain handy, a good starting point for core size and I've suggested that others use it since first having read your formula some time ago.
Consider this: your formula renders results pertaining to center to top as oppsoed to center to center, fine, as C to T prevails in most off the peg bikes. However, why the 3cm. disparity between your CT vs. CC meas. ?
Bikes have roughly a 1" TT OD. so given the distance 1/2 the diam. of the TT = 1/2". Why then would the resulting difference be a full 2cm rather than closer to 1.27 cm ?
I'm not splitting hairs here just trying to get closer. I've seen this very same discrepency more than once, it bugs me.

December 3, 2009 | Unregistered CommenterDan

Compared that sizing formula with the frame size of touring bike I had (before it got stolen). Out by half an inch - not bad considering I'm 5'0", have disproportionately short legs and long arms.

Interesting exercise as that was, there's little point knowing the frame size needed when good quality women's bikes in that size are hardly ever available. :-(

March 19, 2010 | Unregistered CommenterArachne

I have spent the last 3 or 4 years screwing around with comfort, efficiency, and sizing, and I have zeroed in on a couple of comfortable combinations.

I have three conclusions, I guess:

1) Any theory is a starting point only. Trial and error is a necessary ingredient in bike fitting.

2) Nothing is more important than fit, and thus buying a new bike is an exercise in shooting craps, especially given the often bizarre "expert" advice in bike shops.

3) A slightly smallish bike can be adjusted and fitted with stems to make it work. (Crank arm size is another whole dimension to the discussion. I am 63, and I find shorter arms have an allure about them.) I personally think there is some sense to the "fist" axiom for seat posts. I am 6'1.5", a slight long-torsoed, and If I ordered a new bike tomorrow, would buy a 58 or 59.

August 12, 2010 | Unregistered CommenterTim

Is the chart available somwhere ?
Thanks,
Loz

May 28, 2012 | Unregistered CommenterLoz

I am obviously not the sort of human cyclist Dave has ever come across. I have long arms( wing span 6ft3) short legs(30 inches), small feet( UK 7) and a long torso(5'8tall on my short legs)
Makes bike fitting a bit of a challenge but at least it makes know-it-alls in bike shops think twice when they realise the frame they think I need according to my leg length (inseam) would require a 16cm stem.
That said I ride reasonably comfortably on a number of different size bicycles even when my feet don't always reach the ground
Love the site

July 16, 2012 | Unregistered CommenterBryant

Dave, great blog, but I cannot seem to find the sizing chart anywhere. Wiould you have a link to your website?

thanks

December 10, 2012 | Unregistered Commentersam

I am a 60 year old woman, looking for a road bike with upright handlebars, but am having difficulty finding a good fit. My height is 5'3" and my inseam is 30". I wear a women's 8 in shoes. I have been riding for years on a frame around 52 cm which is what the height charts recommend, but my neck and shoulders kill me after a few hours on the bike. Any suggestions for fitting me.

April 5, 2013 | Unregistered CommenterMaureen
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