Cyclist0383

In sizing a bike one hears a lot the seat tube. A 58cm frame will have a 58cm ST, either center to top or center to center. This seems to have been the standard before the advent of compact geometery.

Correct me if I'm wrong, but now it seems with compact geometery that the top tube is the length to look at when buying a new bike. I've seen some bikes that are listed as "58cm" really being 53cm T-T with a longer TT to compensate.

So, my questions are these; Is the TT the more important feature to look at when buying a new compact frame bike? If so, how does one determin ones TT size?

Cadd

Great question! But I don't have the answer. I'm actually wondering myself as well. I feel that I'm too "stretched out" sometimes. Is there an easy way for someone to determine the correct top tube length?

LordOpie

It's the whole package. Don't forget to look at the head-tube as well, or more specifically, where the handlebars are in relationship to the saddle after you've got the saddle in position.

eubi

The following description applies to a road bike:

Look at where the top tube and down tube centerlines would meet in front of the bike. This point should be directly above the front axle.

If this criteria is met, under a static load the frame will behave like a triangle, even though it's a trapezoid (or a trapezium, if you're English). In other words, there will be no bending in the frame tubes, only tension or compression.

The seat tube needs to be long enough so your legs can pedal correctly, and the seat post won't fall out of the seat tube. Typically how a bike frame is sized.

Head tube and seat tube should be parallel, based on how responsive the bike is designed to be.

I'm not a frame designer, but I think you can see how it all has to come together.

phidauex

In the old days, bikes were only made in one geometry, and as the seat tube got longer, the entire thing got taller, but not necessarily longer, and you'd 'correct' it with stems and bars. The only relevant measurement was seat tube length. But these days bikes change size proportionally, which fits human bodies better. We still use seat tube length as a measure, but its actually a silly thing to use, because seat posts come in all sorts of lengths, but stems that deviate more than a little bit from ideal can seriously affect handling.

I'd argue that the more important measurement is the top tube length, since you have a huge amount of adjustability in seat posts.

Of course, no single measurement will determine if a bike 'fits' you or not. I'd suggest getting a loose idea of what size of bike will fit a person of your inseam, and then try several bikes in that general size range +/- an inch or two, and see which one's top tube fits your arms and torso's best. If you end up with a bike a size up or a size down from what your inseam suggests, don't worry about it! Standover height isn't that important, but if your reach isn't right, you'll never feel comfortable on the bike.

Try a lot of bikes, and if what feels best to you doesn't seem to match what the chart or the 'expert' says, then don't worry! Just make sure you've got the right posture so you are feeling the right thing. Bike fit is a complex and unpredictable thing, which is why master framebuilders are paid so damn well.

peace,
sam

halfspeed

For a road bike...
There are three common wrong ways to fit a bike:
1) Seat tube alone
2) Top tube alone
3) Standover alone

The actual critical measures are cockpit length and saddle/handlebar drop. Cockpit length is the distance from the center of the saddle to the center of the end of the stem/handlebar join. Unfortunately, these are not on a spec sheet because saddle height and stem are adjustable for the rider. They are also affected by the head and and seat tube angle.

The point being, that two bikes with the same top tube length can fit very differently. The same is true with standover and seat tube length.

That's why buying a bike on the internet is a crap shoot unless you really understand geometry.

Cyclist0383

Buying site unseen is the situation I'm in. Unfortunetly all the LBSes here in Riga are focused on alluminum mountain bikes. I've been looking for a steel touring frame for months. There is nothing here. I've found one crappy Chinese alu frame without a fork for $200. No thanks. A friend is comming from the States and I would like her to bring me a frame.

halfspeed

With the American dollar being so weak, there's never been a better time for considering a custom or production bike from somebody like Bruce Gordon or Rivendell depending upon your budget. At least that way, you can talk to the people who design and build the bikes which is a lot better than going to an anonymous web site.

Retro Grouch 

The easiest sizing adjustment that you can make on a bicycle is saddle height. Why, then, would you use seat tube height as the primary frame sizing specification?

my58vw

I agree that you can not look at seat tube and top tube seperatly. Although tope tube is a much more important measure that seat tube IMO. For example I am 6'6" with a redicuosly large (38.5 inch) inseem but a short torso. I ride a 63 cm trek with a 63 cm seat tube and a 60 inch TT. While the top tube could be longer (like 62 cm) and seat tube, I bought this frame because it had just about the longest TT I could find in the bike range that I wanted (1.5 - 2k) at the time (wow things have changed).

Now my TT bike has a 60 cm TT and a 58 cm seat tube. I still have an approprate reach but my seat post is longer. Both bikes fit because of the tope tube although the seat tubes are 5 cm different. If you can not reach the handlebars then you are in trouble while you can get a longer seat post if needed. Now the only time this is trumped is when the seat post is so long that you can not get a short enough seat post but that is rare...

DCCommuter

Here is a good article on the subject: https://www.sheldonbrown.com/frame-sizing.html

Basically, you can't use a single number to quantify a frame size because the geometry differs.

I also like this article by Peter White: https://www.peterwhitecycles.com/fitting.htm

He makes a very good point that you can't fit a bike just by measuring your body. He has a bike that fit him perfectly 30 years ago, and his body hasn't changed size, but it no longer fits because he is less flexible.

On the other hand, this site, https://www.wrenchscience.com allows you to put in your body measurements and they recommend a bike size for you. It's a little simplistic but it gets you started.

One thing to keep in mind in terms of your position on the bike, your body only makes contact with the bike in three points -- the pedals, the seat, and the handlebars. So your posture on the bike can be controlled just by changing the relationship between those three points. The bottom bracket doesn't move, but you can move the seat up and down and back and forth, and you can adjust the handlebars with stems of varying length and angle. For instance, Giant has been very successful with making bikes with fewer frame sizes but more options in seat and handlebar positions. The Wrench Science site gives you two key measurements -- "Overall Reach" which is top tube length plus stem length, and saddle height above the bottom bracket.

However, while posture is important, it's only part of the picture. It doesn't take into account factors such as weight distribution and steering geometry. If you had two bikes, one with a 40 cm top tube and a 28 cm stem, and the other with a 68 cm top tube and a 0 cm stem, both have a 68 cm overall reach, but neither would be very rideable.

Portis

I am no "expert" but just went through the bike buying process. I think the top tube measurement is more important to me. BAsed on my height, all of the online calculators put me on way to big of a bike in terms of seat tube.

I agree that it is critical to have both the right size, but if you can only have one, either top tube or seat tube very close to perfect, than i take the top tube. Granted having both of them perfect is better. Then again, having a body with perfect proportions would be nice to. Not everyone does.