rraabfaber

Hi all

I've been looking for a replacement bike on Craigslist. I'm a little confused by bike sizes. Help? I know how to fit the bike if it's physically in front of me, but I don't want to drive all over town trying them out if they're just going to be wrong. For instance, I saw a nice looking Gary Fisher Tassajara. It says it's a Size 17, which I'm assuming is Fisher's 17.5" medium. But what does that mean? How is it measured?

I'm 5'7" and my crotch is at 31".

Gracias Amigos

maddmaxx 

The measurement varies from Manufacturer to manufacturer. Nominally, the 17.5 refers to the length of the seatpost from the center of the bottom bracket to the top of the seattube. If measured in inches it probably refers to a mountain bike or possibly a hybrid. Road bikes will be measured in centimeters.

Now, here's where it gets hinky. Some manufacturere measure to the top of the seat tube (C to T), some measure to the top of the top bar and others measure to the center of the top bar (C to C). You need to do your homework and find out how that manufacturer measured in that model year.

To make matters worse, the length of the seat tube may be less important than the length of the top tube (or virtual top tube if the bike is of compact design). In the old days, one could assume that a classic road bike had about the same top tube length as the seat tube height. For MTB's and modern compact design road bikes, this is no longer a given. Some manufacturers make bikes that are "long" (Haro for example makes most of its MTB's with a longer top tube than seat tube). For that brand/model a seemingly smaller bike is a better fit for any given rider.

You have a little homework to do if you want to become knowledgeable in this area. Start by trying to get the specs for bikes that you know fit you (top tube length and seat tube length) and use these to compare to the published (if you can find them) specs on any other bike that you are interested in.

It sounds worse than it is and with a little practice you will get the hang of it.

westsam

I have a Trek that is 17.5 [hybrid] and it is measured from the center of the crank hub to the top of the seat post tube. hope this helps

deraltekluge

Be careful with your terminology...it's the seat TUBE, not the seat POST that's involved in the measurement.

The different frame geometries in use add to the problem of comparing bikes. Bikes could have the same size number but be quite different in actual size, or have very different numbers but fit the same. See Revisionist Theory of Bicycle Sizing by Sheldon "The Long & Short Of It" Brown

rraabfaber

Well, that's a good start. Thanks for the info, guys!

desederada

Well if it helps I'm 5'3' with a 30" inseam and I ride a 15.5 Gary Fisher.

Jet Travis

Homework helps, as maddmaxx points out. Ultimately, you might think of it as dating. You learn a lot from a test drive.

maddmaxx 

Just for fun (and once more for new members) I will go through some of the math for bike fit starting points. Remember, this math is for a starting point only and may not reflect on the preferences of any one individual rider. The starting point appears to be aimed at sizing for a road bike so make note that when buying a mountain bike for real mountain bike purposes (where one is in more danger of coming off the saddle suddenly and encountering the top tube more violently) a size smaller or at least a shorter frame with possibly a longer top tube is appropriate.

The basic bike size is 2/3 of your inseam height. Do not cheat on that height and use your pants inseam. Take the measurement in shoes. Measure by standing upright, back against a wall. Use a large book or similar item that can be kept square to the wall. Pull the book up untill its uncomfortable and measure (an assistant helps) the height to the floor. This height in CM points at the size of an appropriate road bike...........................but, your body may require more than that. To accomodate people with longer legs than average (many women) or shorter legs than average there is a modifier.

Divide your total height by the measured inseam height. You will get a number slightly greater than 2. This number points to the fact that your trunk may be longer/shorter than your inseam height tells us. If the ratio is greater than 2.2 your are relatively short legged/long trunked and you need to look at bikes that have longer top tubes compared to their stated size or you just need to think about a bike one size larger. (In this case you have to make do with less standover clearance to be comfortable on the bike when riding). If the ratio is less than 2.0, then you have long legs for your size. You need to look for a bike with a shorter top tube (typical of WSD designs for all of you long legged girls) or a bicycle that is one size smaller.

All of this provides a starting point............and only a starting point. Riders will vary in their choices from here and the bicycle is very adjustable. It's good to start somewhere near this point however as that will provide a wider range of adjustments in both directions.

deraltekluge

Don't forget that that'd be quite different for bikes with horizontal top tubes than for bikes with sloping top tubes. Which are you considering? Bikes that are really the same size, and fit the same size rider, can have size numbers that are several inches different.

maddmaxx 

If you stay within the roadbike style you will find that the sizes are about the same depending on where the manufacturer takes the measurement......see post 2 above (the sloping top tube bike will still have the seat tube taken to the same height). MTB's may appear to have shorter seat tubes because their bottom bracket is higher off the ground. (same for most cyclocross frames)

Another significant tube length is the head tube. Shorter head tubes (the TT/TRI frame is the extreme limit of this) places the bars lower in relation to the seat because forks do not vary very much from axle to crown. (road bike forks or basic non-suspension corrected MTB forks)
Very short head tube frames may not be comfortable for most 50+ riders. TT riders ignore that statement please.

oilman_15106

I'm 5'7" and most 17 inch Mt. bike frames are too large for me.

cruzMOKS

Is your total height also with shoes or without?

maddmaxx 

^^^^ I would assume that it's with shoes. Your only looking for and indicator here that says your long/short legged for your height. These answers only represent where to begin looking......ie at standard size and above or at standard and below or a lot below for a mountain bike. You may also find that you don't like the bikes that the math points at. If you want to ride in a more upright position, then a shorter (length, not height) bike is probably in order.

The biggest reason to perform this exercise is to break out of the thinking mold that leads to "standover height" being the best way to size a bike. It isn't, not by a long shot.