sbuckaroo

Folks,

I see messages where people post a picture of a bike (especially vintage) and others manage to figure out the bike size just from looking at the picture. How do y'all do that? Thx.

cxwrench

Magic 8 ball.

genejockey 

Head tube length and seat stay/seat tube angle.

Broctoon

Lots of experience with bikes helps you develop an eye for it. One thing you can notice is the space between the top tube and down tube on the head tube. With traditional geometry (horizontal top tube) this dimension helps you get a pretty close guess at the frame’s size. It gets trickier with endurance frames that have a sloping top tube. If you know the approximate wheel size, this too can help with approximating the frame size just by looking at a picture.

sbuckaroo

I see. It must be somewhat difficult to eyeball, but I guess it comes with experience. Thanks for your response.

genejockey 

That's it. BUT, you can gain a lot of experience quickly, by, for example, looking at bike frames on Ebay.

70sSanO

Bikes sizes are traditionally measured “from” the center of the bottom bracket. The “where” has differed from center of top tube or top of top tube; with some using top of seat tube.

I use the top and downtube attachment distance at the headtube.

An overly simplistic explanation for a vintage bike that has a horizontal top tube, is, as the size gets smaller, the top tube has to be closer to the bottom bracket and by default that brings it closer to the downtube and closer together at the head tube. The head tube generally gets shorter as sizes get smaller.

It is a bit tricky trying to figure a 54cm and 56cm, or 60cm and 62cm from just a picture, but it becomes more obvious when the sizes are further apart. 50cm vs. 60cm is a slam dunk.

John

Ironfish653

Also, if you have a particular brand or era you're looking for, like "Pre- TREK" era Klein bikes or Cannondale ST's, for example; you can do some research (FB owners' groups are a good place) to find out if they have any sizing idiosyncrasies (run big, run small, etc)
Road bikes usually are fairly close to size, but MTBs, especially early ones can vary wildly from brand to brand

urbanknight

Back in the steel frame traditional geometry era, I was fairly good at guessing size by sight. Compact geometry and wildly varied tubing thickness gives me no clue.

noimagination

There's this really cool invention that will make it completely unnecessary for you to post a picture and ask others to figure out the size of your bike. I'm not sure you've heard of it, it's called: a tape measure.

icemilkcoffee 

I am surprised that in the era of 'there's an app for that', there is no app that will help you eyeball a bike's size from a picture. It can key off of the wheel size, which is typically fixed at 700c, and then extrapolate the frame size from that.

genejockey 

This is very true. Once you get to oversize tubing, sloping top tubes, and dropped seat stays, guestimating size becomes much more difficult. But mostly I've been in the market for traditional steel frames. For example, last year I was looking for a Centurion Ironman in 57 cm, and a lot of the listings in Craigs List or FB Marketplace just say things like "Medium" or "Large", or the seller doesn't know how to measure, or doesn't say whether they measured C-T or C-C, and some measure the top tube instead of the seat tube. I got pretty good at spotting 58s based on the length of the head tube and where the pump peg was on the head tube in relation to the down tube.

seypat

Steel frame era frames also might have a pump peg knob on the back of the head tube. Where it's positioned will tell you the right size or one off. Same with the head tube decal. You can compare different sizes and see where the decal is. Basically, your'e determining how much space is between the TT and DT.

urbanknight

Most of the time it's because the person is looking at an ad for said bike, quite often miles away from them, so they are unable to measure the bike manually.

easyupbug 

Well done Bike Forum only two out of more than a dozen responses to a newbie that are not condescending.

urbanknight

Hmmm, I only see 2 posts that ARE condescending or sarcastic... 3 if you count yours.

CliffordK

Yep... with the horizontal top tube.

Measurements vary somewhat by things like brake reach and fork design. So a bike with fenders and a LOT of space for the tires will have a shorter head tube than a pure racing bike.

Nonetheless, if one is really only interested in sizes within a few cm, one can get close enough.

c_m_shooter

Head tube length and top tube height compared to rear tire.

Paul Barnard

Head tube length is most often the easiest reference point. Generally, the greater the distance between the top tube and down tube intersection at the head tube, the larger the bike. With road bikes, the thing that is consistent from bike to bike is wheel size. The size of the frame relative to wheel size is also an indicator. Over time, you just develop an eye for it.

3alarmer

.
...it's an inexact science, even with a tape measure. I once drove two hours one way, from here to Merced, to look at an old Paramount that I had asked the seller to measure, because I couldn't tell from the pictures. He was only off by two inches.

BillyD

Ikr

indyfabz

Yeah. I don't see much condescension or sarcasm here. That comment left me scratching my head.

urbanknight

Yeah, I learned that the seller's tape measure is often less accurate than my guess based on a picture.

bwilli88 

Eyecrometer. having looked at a lot of bikes gives one experience.

mstateglfr 

idk