Frame sizing?
 

I'm trying to wrap my head around how frame sizes are determined. I see road bike frame sizes sold as xx CM, i.e. 54CM. When looking at frame dimensions, none of them are 54. Is there a formula used to determine the frame size from the dimensions?

If they don't have me confused enough with the xxCM sizing, they throw S, M, L, XL... at me for MTB or Hybrid. Is there any standard for those sizes?

I'm 5'8" with a 30" inseam and have a Trek Verve 2 in M on order. I was able to test a Verve 1 M size before ordering and it fit. I'm just wondering how the whole sizing works if I want to look for a used road bike.

Thanks
Jim

Don't really need to wrap your head around it. There is no standard for bicycle sizing. The manufacturers all sort of do the same thing, but there are minor differences in how they go about figuring everything else about the bike that is built around it.

Essentially if you have no other experience to go by, then go with the suggested manufacturer sizing. Even for the same manufacturer, they might suggest a different size for one model than another model. I'd highly recommend that you try on any bike you think you might want to buy. But the more experience you have the more you might be able to guess and buy the correct size without trying it.

But even with 50 years of riding bikes, I'd only do that for the bikes I have experience with. Which currently is only road bikes. Any other type, and I'd have to try them out first.

I am a similar size (5'8" and 30" inseam) and every time I've ever asked a bike shop fitter what size frame he/she thinks is right for me in a road bike frame, they have all said 54cm.

I was a bike store owner for over 12 years. (Seven of those as a Trek dealership.) I am short, and know a few things about sizing. The sizing on a Verve is A different animal than a road bike. You should be looking at a bike with a 17 - 18" seat tube. (no more) The top tube length is longer than a road bike. The Verve is designed to have quite a bit of seat post out of the frame. There is something called "big bike syndrome" where newbies buy a bike that is too big, I know., I was always trying to get people to get on the right page, in regards to sizing. Go to a bike shop, try 'em out. Most bike shops let you "test ride".

With State and Wabi bikes the frame corresponds to the length of the seat tube

There are at least 2 ways of measuring seat tubes - center of bottom bracket to top of seat tube and center of bottom bracket to intersection of the center of the top tube and center of seat tube. Either works, but you have to know which is used. BITD, I believe the English and French makers sized their bikes to top of STs, and Italians to the center of the TT. And, of course, the Brits usually measured in imperial units, and the Italians in metric.

I'm not sure how people can buy a bike via the 'net, unless they've had an opportunity to try out the model and size they're buying before buying. JMO, of course ... some people obviously are successful at selling bikes via the 'net.

This could/should give you a bench mark
https://www.purecycles.com/pages/pure-fix-sizing-chart

I know what range of frames fits me, 58 to 61, with the ideal about 59. The bikes that fit are in that range so if I know if the ST is measured is either ctt or ctc, I can judge if it will be in my range or not. If it is on the border line of the range and does not ID ctc or ctt, it is not in the running.

If Napoleon said a 57 is extra large, I would say it is a medium. To many adds stating s, m, lg, etc. that doesn't tell me anything.

That won't work for many modern road bikes and few if any mountain bikes. Unless I'm misunderstanding what you are saying.

It also ignores that the top tube length and or frame reach can be significantly less or more and still have the same seat tube length. Which is probably part of the switching to letter sizes by some of the manufacturers.

I suspect they are trying to overcome the weight of people still thinking of sizing from the days of bikes with perfectly horizontal top tubes and a head tube that grew in length as the seat tube grew.

Websites like Bike Insights and Geometry Geeks are your friend, especially if you already have a sense of whether your current bike fits perfectly, too large, or too small. Bike Insights in particular has been a godsend since it's borderline impossible to try out any of the frames I'm interested in picking up right now.

I'm 5'7" with a 30" inseam.
I run 2 different measurements to fit bikes I like to ride, then tweek with stem length.
For bikes where I predominantly sit down, I look for an effective top tube of 54cm for drop bar, or 59cm for flat bars.
For bikes where I predominantly stand, I look for a reach of about 43cm.

When you find measurements that fit your bike style, it becomes way easier to find your bikes because you can ignore all the marketing hype and unique sizing.

It always seemed to me that fitting should be based on:

1. How far do I like my saddle (the part where my sitbones rest) from the BB?
2. How far behind the BB do I like that part of the saddle?
3. How much reach do I like from that part of the saddle to where I grip the bars? (For drop bars, that would be to the base of the hoods.)
4. How far below (or above) that part of the saddle do I want that part of the bars?

You really can't size a bike without knowing those four things.

For me, in inches, those four measurements are: 29, 10, 31, and zero.

For the record. A frame size in cm is nominally the distance from the centre of the bottom bracket to the top of the seat tube.
However. This is from pre compact frame days where all top tubes where horizontal. So, it they reduce the length of the seat tube and angle the top tube, has the frame now been made smaller?
No it hasn't.
So, the nominal size of a frame in cm is from the centre of the BB to where the top of the seat tube, WOULD have been, if it was a horizontal top tube.
That's the theory anyway.
Then we can always debate whether we are measuring to the centre of the top tube or to the top of the seat tube.
There are still more factors at play but that's the basic theory.

[QUOTE=bike eagle;22300147]It always seemed to me that fitting should be based on:

1. How far do I like my saddle (the part where my sitbones rest) from the BB?
2. How far behind the BB do I like that part of the saddle?
3. How much reach do I like from that part of the saddle to where I grip the bars? (For drop bars, that would be to the base of the hoods.)
4. How far below (or above) that part of the saddle do I want that part of the bars?

You really can't size a bike without knowing those four things.

I agree with bike eagle's concept, except my criteria are slightly different. I made a story stick with my four critical dimensions, so I can check whether a bike will fit me very quickly and make adjustments. My four critical dimensions are:
1. The distance from the top of the petal at the bottom of it's circle to my seated sit bone height.
2. The horizontal distance from the center of the pedal at the horizontal, front position to my seated sit bone location.
3. The horizontal distance from my seated sit bone location to my hand location on the bars/brakes.
4. How far below the sit bone level I want my bars.
5. I have to be able to stand over the cross bar, at least with the bike tilted on an angle.
If the bike cannot be adjusted to fit my criteria, I cannot use the bike. I have bikes that work from 58cm to 64cm measured to the top of the seat tube from the center of the BB. I cannot lower the bars any more on the tall bikes. Being an old geezer, I doubt I will be able to do that anyway.
Fred

I like it! My measurements didn't incorporate crank arm length, which yours does. Good system!

I would just add that for my purposes, crank length itself is a critical dimension. I ride 170mm cranks and would not use longer cranks.

When it comes to just the frame and not actual fit for the rider, stack and reach pretty much tell you what you need to know.

Otto

I agree. However most new to cycling and many cyclists that haven't given fit a thought won't have any idea what to do with those numbers.

I recently had surgery which made me taller https://leglengtheningsurgery.com/. I gained about 2 inches and I'm not sure if i should get a custom bike built to suit my needs or if i should get a general bike and go off the suggested manufacturer sizing. Of course, I want a long-term, sturdy ride but I haven't had any experience with custom bikes, yet. Any advice?

Make a new thread for new questions. Especially if you are not the OP.

Welcome to BF.

You need tough skin for the tough love we might give you sometimes.

What he said!!!

I agree that's the primary viewpoint, but there are confounding factors in some of the newer extreme geometries.
Traditionally, road bikes have been pretty 'upright' with seat tube and head tube angles clustered around 74 degrees or so, and generally parallel.
With the creation of various genres (cyclocross, endurance racers, gravel bikes, etc...), there are much larger deviations. For example, in the gravel category it is not unusual to see a 73-74 degree seat tube with a 71-72 degree head tube...a 3 degree difference is not that unusual, and can quickly create a 1 cm offset somewhere. This is a meaningful deviation from the two parameter model. Stack/reach are generally within range for 'normal' people that have inseams that are less than half their height, but the current geometries are less useful when your inseam is half your height (like mine is). The non-parallel head/seat tubes also change the geometry...the reach shrinks as you increase the "effective stack", generally corrected with a longer stem, for example.

As has been mentioned manufacturers have strange ways of sizing their bikes. My CAAD 12 is a 52 but if you actually measure the ST to where it intersects with a virtual TT it is a 54. When I measure a frame I do it with that method. When people tell you that you should use your height to determine frame size they are mistaken. Someone who is 5'10" with a 32" inseam is going to require a frame that is different than someone who is 5'10" with a 36" inseam.