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Small frame sizing question. (48 to 42?)

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Small frame sizing question. (48 to 42?)

Old 08-09-12, 12:20 AM
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lalalisa
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Small frame sizing question. (48 to 42?)

I'm currently riding a 48cm Trek and it seems to fit me quite well. I'm 5'3".

I'm in the market for a touring bike though and I found one which the seller says is a 42cm (CC). However the top tube length measurement is the same as my current bike and the standover height is just a half inch lower (probably better for me). A 42 cm. just seems quite small, but if the other measurements are the same might it be a good fit?
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Old 08-09-12, 12:53 AM
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If it really is a 42 cm and you ride a 48, it's going to be much too small, you'd need to have your seatpost a mile in the air. However, I'm almost certain that the seller isn't measuring along the seattube, but is dropping the tape straight down from the toptube: if the 42cm frame had the same length toptube as your 48, that would produce some really wild geometry, more relaxed than your typical touring bike. And if the seattube really was six cm shorter, I'm pretty sure that would produce more than a half-inch decrease in stand-over height.

That said, usually you can ride a frame that's 1-2 cm smaller than your ideal size and be perfectly happy. I'd have a look at the bike in person before you decide.
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Old 08-09-12, 04:27 AM
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lalalisa, If the seller is incorrectly measuring a bike with a sloping top tube (angles down towards the seat post rather than being horizonal) that could account for the difference. Best to look at the bike in person.

Brad
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Old 08-09-12, 07:22 PM
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I was able to find the measurements from the old Trek Catalog:

Seat tube length 41.7 (old bike: 47.5)
Top tube length 54 (53.6)

He lists a measurement of 28.5 for SO height, and my current bike is just under 29.5.

So, it really does look like the big difference is the seat tube/size ... a bit confused on the sizing still, unfortunately I can't go check it out in person.

(Seat tube angle and head tube angle are 73 and 70.5, compared to 73.5 and 72)
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Old 08-09-12, 07:34 PM
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lalalisa, What is the model of bicycle you're considering?

Brad
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Old 08-09-12, 07:42 PM
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Sorry, its an old Trek 520.

Right now I'm riding a Tri Series '86, back in that year I believe it had the same geometry as the 520.
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Old 08-09-12, 07:49 PM
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Basically, all bikes 50cm and shorter are the same. The top tube can't be any shorter or you'll have the dreaded toe overlap. Toe overlap is not a big deal, but some people fear it so bike manufacturers do all they can to avoid it. The head tube can't be any shorter, you need enough to connect the down tube and the top tube, with enough room for a quill stem if that is what is used.
To shorten the top tube and alleviate the toe overlap, manufacturers will make steep seat tube angles and lax head tube angles. But this will only change the top tube by about 1.5 cm at most. Another option is to raise the bottom bracket, the top tube can be shorter, but the stand over is higher. One other option to shorten the top tube without having toe overlap is to use smaller wheels, but manufacturers don't do this often because they believe shorter people won't like this.

Newer bikes have compact frames, so instead of a level top tube, it angles downward. But again, the head tube will still be at the same height as a traditional frame. This can give you the difference in seat tube angle with just a bit smaller stand over height.

I would have to look at the bikes, but most likely, the 48 and 42 are probably almost identical.
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Old 08-09-12, 07:53 PM
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Originally Posted by lalalisa View Post
I'm currently riding a 48cm Trek and it seems to fit me quite well. I'm 5'3".
For 5'3", your 48 cm frame should be a good fit. I go along with your judgement.

With modern frames that have sloping top tubes, don't even look at seat tube length unless you have trouble straddling the bike. Then it's most likely way too big. Instead, measure the horizontal distance from the center of the head tube to the center of the seat post. That's the "virtual" or "functional" top tube length. Your benchmark is the virtual top tube length of the bike you're comfortable on. At your size I'll bet your handlebar stem is quite short, so you don't have a lot of wiggle room. Look for bikes that come within a centimeter of that benchmark.
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Old 08-10-12, 07:23 AM
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Originally Posted by lalalisa View Post
Sorry, its an old Trek 520.

Right now I'm riding a Tri Series '86, back in that year I believe it had the same geometry as the 520.
In that case I would suggest a pass on that 520 and find one closer to 48 cm.

Brad.
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Old 08-10-12, 08:11 AM
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Lalalisa, you have to concentrate in the length of the frame AKA virtual top tube or effective top tube. Sure the guy saying that is a 42 is measuring a bike with sloping top tube and pretty much the manufacturers can put a 20 cm center to center seat tube in a bike if they wanted to, will look awe full but they can. As for the top tube you wont find racing road bikes built with tubulars or 700c wheels shorter than 48ish cm center to center top tube.


As for the stand over I personally dont use that because if the manufacturer has the idea of making the bike with a taller (or shorter) front tube and higher (or lower) top tube (as a higher or lower BB shell) you might end up with a frame that can be too big or too small depending where the manufacturer put the top tube. There is this rule of thumb thing that can help you as a guide but is not a good measurment to just go and buy a frame based in stand over.

As for your old bike, 47x53, I have not seen the bike but sure has a neck where the seat post clamp is and the top tube is sloping too. IMO what you are riding is not even a 48 at all, they call it like that though. Since are touring bikes they have to make the frame longer or the rider will touch the fenders with the shoes, that makes the bike kind'a slow to handle but work for many people, and very smooth to ride too.

If what you have fits you, then concentrate in the same top tube length (virtual or effective top tube) than the bike you are using.

Hope this helps.
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Old 08-10-12, 09:37 AM
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Originally Posted by oldbobcat View Post
For 5'3", your 48 cm frame should be a good fit. I go along with your judgement.

With modern frames that have sloping top tubes, don't even look at seat tube length unless you have trouble straddling the bike. Then it's most likely way too big. Instead, measure the horizontal distance from the center of the head tube to the center of the seat post. That's the "virtual" or "functional" top tube length. Your benchmark is the virtual top tube length of the bike you're comfortable on. At your size I'll bet your handlebar stem is quite short, so you don't have a lot of wiggle room. Look for bikes that come within a centimeter of that benchmark.
I'll have to disagree on all counts. At 5'3", a 48 cm frame is probably on the large side. My daughter at 5'6" rides a 49 cm touring bike while my 5' tall wife rides a 42 to 44 cm frame. lalalisa probably feels that the 42 is too small because she has been put on a bike that is too large and it's what she is used to. My wife rode a Sears 27" (wheel size) "10 speed". It was really a 23" frame (my size) and a more properly sized bike felt 'small' to her. It took roughly 20 years and many size iterations to reach the proper sized bike because she kept feeling that bikes were just too small.

The top tube length is an important parameter but it's given far too much importance. For small persons, standover height is probably the most important parameter since it can't really be changed. If they can't stand over the bike comfortably, all the other measurements are probably going to be out of whack as well, including the top tube length.

The best way to proceed would be to check standover first, then start looking at top tube lengths and other parameters. This allows the rider to weed out bikes that won't work quickly.
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