# Bicycle Mechanics - How to measure a bike

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View Full Version : How to measure a bike

Myosmith
07-19-11, 05:10 PM
I was always taught to measure a bike from the center of the BB to the center of the top tube at the angle of the seat tube. But now few bikes have horizontal top tubes and I've seen a number of different ways to denote frame size. Is there one universally accepted method of measuring a frame so you can compare apples to apples?

07-19-11, 05:38 PM
I don't know if there is a "universal" method, but the one I've always used, even today, is the stand-over height and "effective" top tube length. With stocking feet, the stand-over height of the bike should provide 2 - 3 inches of clearance these days for road bikes, and 4 - 6 inches of clearance for off-road. Back in the old days, it was 1 - 2 inches for road, and 2 - 3 inches for off-road.

As for the effective top-tube length, it seems to still follow a rough rule of starting around 20 inches for someone 5ft tall, plus 1/3rd of an inch for each inch of rider height. Fine adjustments can be done with stem height and length later. Most, but not all catalogs will publish "effective" top tube lengths.

HillRider
07-19-11, 05:41 PM
Is there one universally accepted method of measuring a frame so you can compare apples to apples?
No, and there really never has been. For level top-tube frames, the size can be expressed two ways, center-to-center (center of the bottom bracket spindle to the center of the top tube) and center-to-top (center of the bb to the top of the top tube). Even those aren't universal as some are expressed as center of the bb to the top of the seat tube.

For newer sloping top tube frames those dimensions are not applicable at all and the size is often expressed as c-t-c or c-t-t using an imaginary "virtual" level top tube as the measuring point.

ultraman6970
07-19-11, 08:00 PM
No... with old frames in classic steel some manufacturers do center to center, other ones do center to top (top tube), then around 90 something the thing started to get weird because some manufacturers like moser started to manufacture shorter frames and bikes with a 1 to 3 cm neck over the top tube then putting some average size engraved in the BB shell (my last steel found is like that).

With sloping the thing was even worse so basically now with sloping frames what u have to do is look at the virtual or effective length of the frame because the decal or what the manufacturer says doesnt even match sometimes the size of the seat tube.

Somebody mentioned the stand up, well that thing doesnt work that much because not all the bikes have the top tube at the same level, u have road bikes that have higher or lower BB shells too, and I dont even want to start talking about a track bike because the stand over is even higher. So you can have a 53 frame and a 55 frame with the same stand over height, you pick the 55 (if your real size is 53) and you could be picking a frame too big, or what happens with hipsters, a hipster that uses a 56 road frame will pick a 54 or a 53 track frame based in the stand over size, track bikes can have the bb shell like 2 cms higher than a road bike, top tube is higher also, then there u have it, they always pick a smaller track frame because of this weird idea that the stand over is the way to go.

07-19-11, 08:27 PM
Stand over height doesn't work for every maker, but many use the measurement from ground to the of the top of the top tube (including sloped ones) at the mid point of the top tube. So this is like an "average" height. And if you look on sites like Nashbar, and maybe REI, and other manufacturers, etc. they sometimes publish stand-over height based on a recommended wheel size for certain models of bikes. But the definition attempts to over come the lack of consistency in sloping top tubes. This gets you close enough to buy with sufficient confidence.

For example, go to: http://www.nashbar.com/bikes/Product_10053_10052_526956_-1_202906_10000_202337

and then near the frame select drop down combo-box, to the right in red should be "Sizing Chart." Click that (disable Adblock Plus for that page if you use that) and you will get a pop-up window with graphical chart and table. Sierra Trading Post and REI also use a similar Stand Over Height. But you can't forget the other term, which is "effective" top tube length, which is based on an imaginary length if the top tube were horizontal and intersected the theoretical seat tube extended to meet a level top tube.

Myosmith
07-19-11, 09:40 PM
OK. Thanks. I thought I was missing something when two supposedly 21" frames fit me completely differently and one seems markedly smaller than the other.

07-20-11, 12:34 AM
OK. Thanks. I thought I was missing something when two supposedly 21" frames fit me completely differently and one seems markedly smaller than the other.

I know that exact feeling. With sloping top tubes these days, you can't really tell. But heck, great reason to go to the shop and try the ride!

Kimmo
07-20-11, 01:25 AM
Sizing frames by seat tube length has always been stupid; sloping top tubes just highlighted the fact.

Who cares how long the seat tube is, as long as there's enough standover clearance? You put the seat at whatever height you want.

The best way to do it has always been, and is now totally obviously, effective top tube length (maybe with head tube length as an extra variable).

07-20-11, 01:33 AM
...
Who cares how long the seat tube is, as long as there's enough standover clearance? You put the seat at whatever height you want.

The best way to do it has always been, and is now totally obviously, effective top tube length (maybe with head tube length as an extra variable).

I sort of care how long the seat tube is because I've sheared a seat post in the old days. Ouch. And I broke the binder bolt lug - like cracked it due to too long a seat post. But yeah, I don't care that much these days with the replaceable collars on frames and also the possibility of selecting a particular maker or even using two binders or even something simple like a hose clamp in addition to a binder.

But your statement about effective top tube cracks me up. There's a kid just turned 18. Great guy and scout and just aged out. Didn't get his Eagle, but made Life Scout. Up until earlier this year, he was on bike with a massively long seat post, but his knees hit the handlebars. He definitely needed a LONGER top tube. He got a new bike a few months ago. He's 6'4" and has a massively long torso too. Hopefully, he's stopped growing taller. But I swear he still grew about a 1/2 inch this year. He may yet need another bike.

fietsbob
07-20-11, 01:51 AM
Learn what top tube length works for you, and Judge around that ,
the stem length can be a way to make that right..

Myself , a well sized frame in a comfortable riding posture
when I look down, I look straight down thru the fork axis.

Kimmo
07-20-11, 02:24 AM
I sort of care how long the seat tube is because I've sheared a seat post in the old days.

It's just not that relevant to sizing; that's more about the seat post length than anything else... though in the old days, seat posts only came in short and shorter sizes...

LesterOfPuppets
07-20-11, 02:54 AM
One method that some folks use is Stack and Reach. (http://www.slowtwitch.com/mainheadings/techctr/stack.html) Stack is measured from a horizontal line that intersects the center of the bottom bracket to a horizontal line that intersects the center of the top of the head tube. Reach is measured between vertical lines that intersect the two.

Kimmo
07-20-11, 02:57 AM
There we go; a sensible approach.

Won't hold my breath for it to catch on...

On second thought, it's a bit of a pain cause it's hard to measure. Back to advocating effective top tube and head tube length.

LesterOfPuppets
07-20-11, 02:58 AM
Learn what top tube length works for you, and Judge around that ,
the stem length can be a way to make that right..

That's pretty much what I do. 15" Mongoose or 18" Trek work for me:

http://farm6.static.flickr.com/5264/5599860468_338cf583df_b.jpg (http://www.flickr.com/photos/lesterofpuppets/5599860468/)

Kimmo
07-20-11, 03:01 AM
Ha, nice pic.

Much of a mission to place the second bike...?

07-20-11, 04:18 AM
It's just not that relevant to sizing; that's more about the seat post length than anything else... though in the old days, seat posts only came in short and shorter sizes...

I'm a heavy guy. Torque on the seat collar area and fatigue on aluminum alloy seat posts can be a painful experience. Sizing of seat tube is fairly critical if you're a big person. The stresses on a seat post cantilevered more than 250 mms is big. The solution for vendors is to bulk up the wall thickness or move to Chromoly for the post. But that increases weight. You can get a frame that got a longer seat tube that adds just a few more ounces to the frame instead, and this will allow you to use both shorter and longer seat posts, with shorter ones making for far lower seat post binder stresses.

MichaelW
07-20-11, 04:42 AM
If you want to setup identical riding position on a second bike with a different frame geometry or shape, I find that the best approach is to use [X,Y] coordinates, using the bottom bracket as [0,0].
I usually mark the x=0 point on the top tube then measure up, forward and back from this mark to the saddle and bars.

Myosmith
07-20-11, 05:47 AM
I like that stack and reach idea.

wrk101
07-20-11, 09:43 AM
I was always taught to measure a bike from the center of the BB to the center of the top tube at the angle of the seat tube. But now few bikes have horizontal top tubes and I've seen a number of different ways to denote frame size. Is there one universally accepted method of measuring a frame so you can compare apples to apples?

+1 Never was one, and there isn't one now. On the vintage bikes, several mfrs were center to center, others were center to top. Doesn't really matter anyway. Just target your size, and measure the bike that way. I always measure center to center, so I don't care that Schwinn measured C to T, etc.

And with the sloping top tubes, some mfrs have just gone to S, M, L designations.

On my vintage bikes, I tend to ride anything in the 55 to 57 cm range, and go with longer or shorter stems to compensate for reach.

Other than my MTB, I never have a lot of seat post showing. I do see people on road bikes with an insane amount of post showing.