Modern sizing vs. classic sizing?
 

I have been perusing info on modern road bike offerings and I am wondering if they use a different method of determining size?

On a classic bike, my ideal size is a seat post tube of 58-60cm with nearly the same top tube (I am casual enough of a rider to be a bit flexible).

It seems so many of the new offerings will have short seat tubes, sloping top tubes, and a seat post that is a mile out of the tube. Short of going in to get sized (not ready to buy), is there a general rule of thumb one can use to determine how a new offering compares to a classic steel bike?

S, M, L, and I think XL,.....
I think most shops will start with a Medium with you....then adjust the the seatpost and stem to suit.... If things still looks too small, they might move on to a Large....
The economy of modern bike production, Less sizes = less cost to the manufacturer.....

The sloped top tube/compact geometry frame sizing normally uses the length of a horizontal line extended from the centerline of the head tube to a point on the centerline of the seatpost as the "effective" top tube length.

Here's Giant's table for comparing traditional geometry horizontal top tube frame sizes with compact geometry frames.

http://i32.photobucket.com/albums/d7...ps699756f6.jpg

Here is a drawing showing the Giant Defy 3 geometry.

http://i32.photobucket.com/albums/d7...psf74d4bb6.jpg

Frame size with traditional (horizontal top tube) geometry is usually the length of the seat tube measured from the center of the crank to either the intersection of the centerlines of the seat tube and top tube (center-to-center), or from the center of the crank to the top of the seat tube (center-to-top).

Thanks guys,

I guess I would gravitate towards a 58cm or large as the super tall seat post doesn't sit we'll with me (pun intended).

I like the old methodology but understand the economies of scale and limiting production to 4 sizes.

I wouldn't worry too much about the taller seatpost because the ride may actually benefit from it. A lot of newer road bikes come with carbon seatpost and the flex will help on eatting up some bumps.

I'm really surprised by the slack angles

Think that's to help the aluminum ride better? ;)

Otherwise, +1 on using the "effective top tube" to help you make sense of the modern frames. It's good that they're providing some kind of continuity there. :thumb:

I don't even try to decipher it.

I know my classic sizing, and modern-wise, they've been the same, if shown as cm.
The S/M/L/XL type of sizing isn't so bad, modern stems help a lot with that.

The stuff about Rider Height is pure BULL PUCKY!

These charts do not take into consideration Inseam Length which equates to Stand Over height of the top tube.

For example, I'm 5'9 but I have a very long torso with very short legs (28 1/2" inseam). There are many people with the opposite situation, long legs and a short torso and so on. I can ride 54cm to 57cm frames with 53cm to 57cm top tubes but I prefer 55cm x 53 or 54cm.

Ideally on a road bike a rider should have 1" to 2" of clearance between their private area and the top tube when standing over the bike. On mountain bikes the clearance is usually more. This helps prevent a painful experience in case of a quick dismount! ;-(

Over the years, top tube lengths on most production bikes has had little to do with rider fit. Many bike makers used the same wheel base length on all of their frame sizes on steel, aluminum and Ti frames. Why? Who knows, but probably based on manufacturing expediency!

On smaller frame sizes they "corrected" the frame by increasing the seat tube angle to 74 or 75 degrees while decreasing the head tube angle to 70 or 71 degrees. This had the effect of pushing the rider forward on the frame allowing a short top tube. It also gave sufficient pedal clearance with the front wheel. The end result was a bike that handled like a wheelbarrow!

Large frames where built with the opposite geometry. Seat tubes were laid back to 72 degrees while head tubes were 74 or 75 degrees which produced a longer top tube. This tended to produce fast handling but "twitchy" riding bikes.

In most instances top tube length can be compensated for by adjusting the seat forward or back and/or changing the stem length (also bar reach).

On "plastic" (CFRP = Carbon Fiber Reinforced Plastic) or custom frames these issues are usually not a problem.


Sloping top tubes allow for a longer head tube which raises the bar and stem. This feature first became popular on MTBs and hybrids. It also allows for a more upright riding position.

For racing bikes, this is compensated for by using a longer stem mounted lower to provide adequate rider aerodynamics (wind resistance reduction).


I got interested in racing back in 1974. I read the Italian C.O.N.I book cover to cover several times. In the end I decided that the sections on frame fit were pure bunk for most bike riders! These were written to teach young Italian competitive cyclists how to force their bodies into the most aerodynamic positions plus put out the most power. The book was intended to prepare young riders for the Olympics!

I was 30 years old at the time and realized that these techniques were like training young ballet dancers. After age 18 or 19 the human body becomes less adaptive.

That's the big issue I have with most frame fit experts! Not everyone want's to become a Cat 1 racer! For most riders, a bike needs to be COMFORTABLE! It should fit them not the opposite!

Happy riding!

Robbie,

These are my "most modern" bikes! :)

my 1992 Paramount OS

http://bikeforums.net/attachment.php...hmentid=313631

And my early 90s Gitane Team bike

http://bikeforums.net/attachment.php...hmentid=313632

They're both 55cm ST and 54cm TT and fit me perfectly!

For me it's all, or mostly, about the span between the sit bones and the ball of my foot. A 58 is responsive, quick to handle and I can throw the bike into turns. But anymore I like the more relaxed and predicable feel of a 60 to 62. More fluid and comfortable somehow.
In others words, there's a range, and fit is about feel.

Stack and reach for the win:

http://fitwerx.com/stack-and-reach

I've posted this before, but here it is again.

Top is from 1933 57cm seat tube, 59cm top tube, middle is from 1950s 58cm seat tube, I think 58cm top tube, bottom is 2009 size L.

http://farm8.staticflickr.com/7053/8...650f8f19_b.jpg




This shows comparing the bikes with the seats in the same position. The bars are all relatively the same. BB moves forwards and backwards. Wheelbase is vastly different.

http://farm8.staticflickr.com/7049/8...3738c326_b.jpg




This has the BB in the same spot. Surprising how the 1950s and 2009 are pretty much the same, only a longer wheelbase on the 1950s.

http://farm8.staticflickr.com/7054/8...d52dbf51_b.jpg

Sloping top tubes still look ugly to me.

The chart is about right for me -- my perfect traditional road bike frame size is 55cm C-T, and I stand 5'8".

My only bike (well, besides the Trek) is a Raleigh 22 1/2" frame, which measures 56cm square. I'm 5'7" (seem to have lost an inch? :) ), and it fits me great. Stand over is grazing. Speaking of which, I disagree with the poster above who stated that you should have 1" to 2" standover. I've ridden this frame for nearly 40 years and it has never been an issue. I think that standover height is meaningless for the most part.

Also, I could be wrong, but modern shops seem to fit people to smaller frames than used to be typical?

While I'm loath to disagree with a guy who is much more knowledgable than I am, I agree with you. A bike with a 1 to 2 inch standover height is way too small for me. IMO. But then, I grew up in a different era( though the same as Chas, roughly) and got used to a sort of "French fit".

BTW. I've lost an inch too. You and I must be about the same era. ;)

Way back when, It seems that a smaller than "average" frame was recommended for Criterium racing, as I remember. I was a hill climber so maybe that's why I like a taller frame. Dunno.

I just don't think that you can boil fit down to 1or 2 tube measurements. Back in the 70's and 80's , fit was all about seat tube length with little regard to top tube length. Then with Giant's introduction of "compact geometry" and sloping top tubes, we started caring about top tube length with little regard for effective seat tube length. Now, it sounds like some people are concentrating on head tube length with no regard for any other part of the front triangle. Then you have to consider handling and where the rider's center of gravity sits between the two wheels. As the bike gets taller, the rider moves further back over the rear wheel. The chainstays should be longer on big bikesthan small ones. Giant keeps 420 chainstays on all of the sizes posted above. Longer chainstays aren't fashionable though. I really don't think that proper fit or handling is a big priority for the big bike companies today.

The most important measurement to me is the top tube length, followed closely by the head tube. For compact frames, you should size by the effective or horizontal (or theoretical) top tube distance. Although I prefer the look of traditional frames, I like the generally longer head tubes on compact frames -- and they make a lot of sense with threadless stems.

In a traditional frame, I generally ride a size 56-57 top tube, depending on the seat-tube angle. I own two compact frames and fortunately both of them are sized in cms. From my experience, the compact frames that are sized S-M-L-XL never fit me quite right. They are always a little too long or short across the top.

I agree about disagreeing with posters that are probably much more knowledgeable than I am. The Trek 400 I have is a 54, and while I have the contact points set up very close to the Raleigh, it still feels small to me. And yes, I would guess we are about the same "vintage". :lol:

I find the comparative frame drawings that iab posted to be rather interesting!

I prefer my bikes to "graze" me as well, for something in between the Eddy and French fits, as they're described now. They just look right that way. ;)

Seems like the trend toward smaller frames started in racing (lighter, stiffer, more aero) a couple decades ago and got propelled into the mainstream by brifters, where the default hand position was now on the hoods.

You can read all the guides you like on the web but nothing beats riding a lot on a lot of different bikes with a little (mental) notebook and a tape measure ready at hand.

Very true with one caveat. For smaller riders where the range of fit is not as great as normal-sized riders, angles become increasingly important. a 1 degree angle change can be a drastic change and can be the deciding factor between a stem size in the acceptable range or "twitchy". Knowing angles and lengths makes predicting how the bike will handle for a long ride instead of a 5 minute test ride much more predictable.

yes, you're right. I should have mentioned angles too. I heard there are some pretty good levels available in app from, not sure how useful that is when measuring bikes.

Interesting comparison of Eddy's bike vs. A. Schleck's bike: http://viciouscycleblog.com/2010/12/06/the-eddy-fit/