Hey there all,
I've picked up a old Raleigh that I thought would be my size. A 56cm CC but it seems that I've under estimated the sizing. The reason I went for this size is because that's what I use for my "new" bike. What I'd like to know is how the sizing differs from the old types of steel frames compared to the newer compact frames. Also to ride on the current bike I'd just have to extend the seat a bit but I've reached the maximum extension although it is not sticking out that much. Is there a problem using longer seat post's on older frames? I heard that it could stress the frame more?
Thanks
Badger

By "underestimated the sizing" please be specific about whether the Raleigh is too large or too small.

I'm going to assume too small.

Yes, vintage sizings are real measurements because the TT is generally level.
On compact frames the TT is not level so they measure to wherever they want - sometimes the TT, sometimes the top of the ST (which is cut at an arbitrary length)

The TT also has a 1" diameter on the Raleigh, a modern frames tubes are generally much larger, so even if the modern TT is level, the effective height measured c-t will be larger than on a vintage bike with the same c-c measurement.

You can run a longer seatpost. Just make sure you have it inserted above the minimum. You may end up with quite a bit of saddle to bar drop. Whether you like that or not is personal preference.

You may also find that the TT is too short on the Raleigh.

How old? How long is the stem? How long is the top tube of your 'new' Raleigh, and how long is the top tube of your other bike?

Old Raleighs often come with the seat mounted to the front of the seat post. Flipping the clamp around to the back of the post has the effect of moving the seat back about 3", which makes the bike feel much bigger. Then raise the seat a bit and maybe put a longer stem on it, and it may fit fine. Depends on... well, depends on a lot of things.

This is how I was taught to estimate (measure) size of vintage bicycles...



I am five foot ten inches and a set tube, measured at 56cm, center to center, is a good fit, allowing me about an inch clearance between my crotch and the top of the top tube.

Hope this is a help.

The subject is often discussed and debated. Fit is individual, sizing is general.
Get in the general vicinity, and go from there.

If you already have a bike that fits, take measurements from it, not the tubes on your bike.
Measure from the center of the BB to your pelvic "weld" on the saddle-1
Measure from the center of the BB to your normal hand position (drops or hoods)-2
Measure from the pelvic weld on the saddle to your normal hand position (drops or hoods)-3



Adjust these as close as possible to the bike that fits, and you'll be pretty much in the ballpark.
It takes some trial and error, and a few rides, and some fine-tuning.

Logic says if your other bike fits, and you get these dimensions pretty close to them, it should also fit pretty well.
You'll have to adjust, most likely, for differences in geometry, ride, etc.
You may want a more aggressive position on one bike vs. another, for example.

It generally takes me about 100 miles, minimum, to fit a bike the way I want it. For the first bunch of rides, I carry the allen wrenches I need to make adjustments. This saves a ton of time in the long run.

If you don't already have a bike that fits, see randyjawa's post and go from there.
You'll need to consult some charts, possible a bike shop or an experienced rider/wrench.
You'd need to get a frame in the ballpark for your size, and this is generally within a 2cm range, maybe 4cm. Then you need to fit it to yourself. This takes more trial and error, but that's pretty much the only way. Even fitting on a trainer will be different than your road fit, but it's close.

I've seen some really good "fit analysis" software, but change bikes, and you start over.
Don't be afraid to spend $50 on a fit, though, as it may save you time and money in the long run.
However, be aware that the $50 fit analysis may lead to $200 in new parts.

Just don't compare modern and classic frames and think you can fit one by mentally comparing it to the other.

Part of the discrepancy may be due to crank arm length. Many bicyles prior to 1975 were spec'd with 165mm/6-1/2" crankarms versus the 170mm/6-3/4" length that later became standard.

A lot of older seat posts were made pretty short. There is nothing wrong with getting a newer seat post and using it on a older frame. It wont cause any significant stress on the frame as long the post extends far enough below the top tube inside the frame. More often than not, a longer post will extend further into the seat tube than the original one did.

Also, the frame is not too small if it can be setup to fit you the same way your newer frame does without drastic changes. My bike, for example:



I had to change my stem, seat post a bar to achieve the fit that I desired for my riding style and body's dimensions.