CaptainPlanet

I see a lot of vintage bikes use the same style frames for road bikes and comfort bikes. They have the same wheel and brake setup, and the differences are just the handle bar types, brake types, and stem angles. Do they share the same geometries?. So what kind of sizing guide should I use when searching for a vintage bike? Road or hybrid or they follow their own convention?

fabiofarelli

I don't know what your definition of road- and comfort bikes is but racing bikes never had the same geometry as a bike for 'normal' use. Let's say bikes with fenders.
And since the sloping frames everything has changed. Now we have maybe four sizes while in the seventies/eighties twenty was not unusual for a racing frame.

sloar 

For me I ride a 56cm vintage frame and a 54 in modern frames.

SJX426 

@CaptainPlanet - if you really want to dive into the subject, Dave Moulton has some interesting view points based on all the bike frames he as built including California Masi.
Dave Moulton's Blog - Dave Moulton's Bike Blog (squarespace.com)

cyccommute 

It depends. “Comfort bikes” aren’t hybrids. Hybrids may, or may not, be the similar to road bikes. Some fast hybrids are road bikes with flat bars and some are more like the more laid back comfort bikes. Sizing really hasn’t changed although the length of the seat tube may not be the same on new bikes as old bikes. A 58cm bike from the 1980s will have similar proportions to a 58cm bike from today. The top tube length is still proportional to that size frame even if the seat tube is several centimeters shorter.

Your example is an outlier. It’s also deceptive. There may be only 1cm between the frame sizes but there is a very limited range of frame sizes.

You assumption of the number of frames sizes currently available is also untrue. Sloping top tubes haven’t resulted in fewer sizes. For example, in 1983, Trek made bikes in 19”, 21”, 22.5”, 24”, and 25.5” or five sizes (same range as your example). In 1995, they made bikes in 43, 50, 52, 54, 56, 58, 60, and 62 cm (not all models came in all sizes). That’s eight sizes. And they have a wider range of sizes.

Trek today offers bikes in the same 8 sizes, although not necessarily in all models. And Trek is the same as most other manufacturers. There aren’t only “four sizes”. There are more sizes then there was when the top tubes were horizontal and the bikes fit a wider range of person sizes. The “good old days” aren’t always better than today.

fabiofarelli

Maybe with twenty-one available sizes in 1983 Alan (the example) had a bit more than usual but the Colnago Master and Arabesque were available in seventeen sizes. I guess Trek made some progress from five to eight. I don't want to argue about 'the good old days', just answering a question. It's not that simple.

repechage

Mod bikes often are dimensioned in stack and reach, can be interpolated to classic machines, just requires some good measures.

cyccommute 

No, it’s not that simple but your statement that “…now we have maybe four sizes…” because of sloping top tubes is demonstrably false even though it is oft repeated. As I pointed out, the Alan example is silly because there don’t need to be 1cm differences in the frames. If, back in 1983, they would have produced a bike smaller than 50 cm which is appropriate for someone 162cm tall (5’3”), it is far too large for someone smaller than that. Would you ride a bike that is built for someone who is 10cm taller than you are?

cyccommute 

Stack and reach are touted as being a better way of measuring bikes but they are just variants of standover and top tube length. Measuring and fitting bicycles really haven’t changed for about 100 years. People are just trying to mystifying it. In the end, no one asks what your “stack and reach” measurement when you walk into a shop to buy a new bike. The very first thing hey ask you do is to stand over the bike and see if you have enough room between the top tube and the sensitive bits. That’s standover.

merziac

repechage

if you want to compare mod to classic - ya start
with the numbers that are reasonably easy to reference. Reach takes out setback, we will assume that the seat on either bike will have the same position in relation to the crank axis.
from there, one would want to know can they get the bars in the same or near same location.

Plenty of ways to achieve it. With mod bikes as things are almost always sloping, was suggesting a way to get to a common measure to point.

not knowing the type of mod bike, the more recent are extending the headtube, that might guide one to a larger level top tube frame, let the numbers tell the tale.

70sSanO

Interesting discussion. I’ve wondered from purely a road bike geometry perspective, how much things have changed. Not materials, aerodynamics, stiffness, etc., just the numbers.

Modern mountain bikes share little with their “golden age” counterparts. You can’t lay one over the other and have any semblance.

But for road bikes are we still around 73/73 or 74/73? Bottom bracket height about the same. Similar chainstay and trail.

However the dots are connected, sloped or horizontal, can I replicate an older road bike setup with a modern road bike. Not necessarily stated size to size.

John

Choke 

Eddy went with 16....

cyccommute 

I agree that the saddle will be in the same position in relation to the crank axis which is why comparing the top tube length is as valid a measurement as reach. It’s also an easier measurement to make since you don’t have to drop a plumb line to the crank from a horizontal line. Even with sloping top tube it’s fairly easy to get a consistent horizontal line to the seat post.

The idea of measuring the “stack” to the top of the head tube makes no sense to me. The top of the head tube can be higher than the top of the top tube which would make putting people on a smaller frame unless that measurement is taken into account. Head tube lengths vary a whole lot (I’ve checked) so the stack will vary a lot from bike model to bike model. The stack is also another measurement that is difficult to make…probably even more difficult than doing reach. You have to project a line from the center of the crank to the head tube then raise a plumb line to the top of the head tube. That’s a complicated measurement to make. Standover accomplishes the same thing with less effort and can be done on the fly.

I would agree. Road bikes haven’t changed all that much, even with the advent of the sloping top tube. For direct comparison consider a 1985 Cannondale touring to a 2010 Cannondale touring. Here’s the geometry chart for the 1985




And the geometry chart for the 2010 (and guide for the letters). The only differences are in fork rake and seat tube length. Even with the sloping top tube, the difference between the old seat tube and the newer one is 2” which ain’t much



Granted there aren’t too many road bikes where the same model was made for 30+ years but road bikes haven’t really changed all that much. Look at the 1985 Trek 760 compared to a 2021 Émonda ALR 5. There are just minor differences.


As you said, that doesn’t hold with mountain bikes. They are radically different from the first mountain bikes. Modern mountain bikes are even radically different from 90s mountain bikes but have come back just a little towards older mountain bikes.

cyccommute 

But, like the Alan, there are lots of increments to a limited range. Trek (and other companies) may not offer as many size choices but they have a much wider range to fit a wider range of customers. Do we really need 1cm differences in size when there aren’t bikes to fit people under a certain size?

fabiofarelli

50x55 from 1981, the lady (1.56) smiles.

70sSanO

cyccommute, thank you for all the info. You put in a lot of effort to respond.

John

Road Fan

There's also the aspect of finding "what could work for me?" Practically, at an LBS you ideally get steered to what the manuf recommends for your body size (measured somehow). There also some detailed guides on calculating the "ideal" geo, and these are mainly for road bikes. and for understanding what to think about in sizing.

Good non-quantitative articles include the one mentioned above, by Dave Moulton. Also there is the one written by Peter White, on his shop's website, and the websites (or are they blogs?) by Steve Hogg in Australia.

cyccommute 

That doesn’t prove what you think it proves. The lady in my life is 1.52 m tall. Her bikes are a Specialized Vita that is a 36x39 (seat tube X top tube) and a Terry Symmetry that is 40x50. 5cm of top tube length is a lot of top tube length. Look at your Alan chart. 5 cm of top tube length is the difference between a 48cm bike and a 60 or, on that chart, 12 sizes larger or 10cm larger. I ride a 58cm bike and wouldn’t want to ride a bike that had a top tube for a 68cm bike.

There’s also the problem of the 50cm seat tube. Would you ride a bike that had that radically sloping top tube…sloping top tubes aren’t new, by the way…with a seat post that is from 10cm to 14 cm taller than it needs to be? For my example, that would have a seat tube that is 68 to 72 cm long. It would look just as wrong as that bike looks.

We’ve gone through a dozen or more bikes over the last 40 years trying to find an off-the-shelf bike (couldn’t afford custom) that fit her. I can walk into any shop in the world, point to a bike in my size and walk out with it without riding while knowing that it will fit and have been able to do that for all of the 40 years I’ve been riding. My wife cannot. We have had to hunt far and wide and make many modifications to get bikes that finally fit.

DSCN1059 by Stuart Black, on Flickr



The lady actually enjoys riding both of these because the front end is actually in her current zip code rather than in the next county over.

fabiofarelli

It seems I pushed a button ..
Sorry, its not possible for me to answer now, I have some very busy days. The fact English is not my first language doesn't help either.
And I recognize the problems for small-sized adults.

Choke 

Well....I'll disagree on the 'wider range of fit'. While the seat tube measurements may have a wider range the top tube doesn't for the most part. And I would think most of us here would agree that the top tube measurement is the more important of the two.

Let's look at the geometry posted in this thread.

Alan
Seat tube - 48-63 = 15cm range
Top tube - 50-61 = 11cm range

Trek 1983
ST - 19 (48.25)-25.5 (64.75) = 16.5cm range
TT - I don't see it on the page that you linked.

Trek 1995
ST 47-62 = 15cm range
TT 51.8-58.5 = 6.7cm range

Eddy Merckx
ST 48-63 = 15cm range
TT 51.8-59.3 = 7.5cm range

Cannondale 1985
ST 19 (48.25)-25 (63.5) = 15.25 cm range
TT 20.3 (51.56)-23.25 (59) = 7.5cm range

Cannondale 2010
ST 43.2-63.5 = 20.3cm range
TT 53.3-59.7 = 6.4cm range
While that 20.3 range looks impressive, it's only over 5 sizes so there are some absolutely huge jumps.

Trek 1985 760
ST 48.9-60.9 = 12cm range
TT 53.5-59 = 5.5cm range

Trek 2021 Emonda ALR 5
ST 44.4-59.3 = 14.9cm range
TT 51.3-59.8 = 8.5cm range

So, while a couple of the seat tube measurements do vary a lot the range itself has stayed around 15cm and the top tube range has mostly stayed around 7.5cm. The one thing item that does vary a lot is the number of jumps between sizes; because the newer bikes tend to have fewer sizes the jump up to the next size is a lot larger than it was in the Alan and Merckx examples where there were a lot of size options. Take the Emonda for example, the TT lengths are: 51.3, 52.1, 53.4, 54.3, 56, 57.3, 58.6 and 59.8. Compare that to the Merckx where the jumps are 0.5cm for the most part....that is where the benefit of having a lot more sizes comes in to play.

iab 

I'm curious to know, what is the benefit? Are you saying 1cm in the seat tube and 0.5 cm in the top tube will make a bike unrideable to any user? Using resources on a frame is more efficient that changing a stem or changing the height of a seat post?

Choke 

That's not what I'm saying.

What I am saying is that when there are more sizes one has an better chance of finding a frame with the desired measurements. Let's take the two that I mentioned last, the Merckx and the Emonda, and let's say that my preferred TT length is 55cm. With the Merckx I have a choice of a 54.8 or 55.5 TT. The first one is very close to my requirement and I'm sure that I'm not sensitive enough to notice a 2mm difference, though I know people who claim to be that sensitive. I could choose the 55.5 and try to find a 0.5cm shorter stem, though that's not easy to do these days - I do remember the time when stems were made in 5mm increments. Or maybe the 5mm difference is also not enough to notice.

With the Emonda my choices are a 54.3 or 56 TT. I may not notice the 7mm difference either but 2mm is the better option if I have to choose. I could pick the 56 with 1cm smaller stem and likely be fine as well, though with level TT frames that could present a possible problem in the way of standover height (obviously not the case with the Emonda).

Think of it like adding more cogs to the rear, the overall range may not change but the jumps between cogs are smaller.

iab 

You didn't answer my second question. What is more efficient. Changing a frame to accomodate your needs or changing a stem?

Efficiencies can be in business or environmental measures. And in both cases, it is a win for the customer. They can lower price and conserve resources. Having EM increments is entirely wasteful and should be discouraged.

cyccommute 

Let’s look at the magnitude of those changes you are saying are important. 5mm is the less than 1/4”. To put it in more real world terms, that’s the thickness of 3 US quarters. Go out and pick up a 5mm allen wrench. That’s the dimensions you are talking about. I’m not sensitive to top tube length so I might not be the best judge but even the most picky of riders probably isn’t going to think that 5 mm is going to make the bike uncomfortable. 2mm is even more ridiculous.

And when I’m talking about frame size range, the older bikes may have had lots of sizes but the range of sizes is very limited. Perhaps they should have considered something for riders on the smaller end of the spectrum rather than just lump them into the “and others” size.

I did make a mistake with the 2021 Trek. Their sizes range from 44cm to 62cm with a total of 8 different sizes. It also should be noted that you’ll see a whole lot more Treks even with their (supposed) limited sizing than you will see of either Alans or Merckx. Trek must have done something right.