Holzwurst

Just joined here today, motivated by a very unusual and confusing experience. I have been searching carefully the past 3 months for a good used hybrid and thought I had a handle on sizes. I am 5'11" and understood that I needed a Large, or 19/20" bike. That was backed up by trying different models here and there. Well, today I chased a couple of new leads and ended up buying 2 bikes because I couldn't decide which to get. One is a Scott Metrix 40 fairly new, the other a Cannondale C400, early 2000's. Here's the twist. The Scott is a Medium and measures 19". The Cdale is a Large and measures 17". Should I be surprised by this?

katsup

How do you know they are a medium and large? Does it say so on the frame?

Some manufactures refer to their sizes by cm or in and not by S,M,L,XL.

Holzwurst

yes... big clear stickers on each bike. And I measured both myself. I won't be keeping the cannondale since it's too small although being a comfort bike it actually doesn't feel bad at all when i raise the seat. Just very surprised to discover a large bike being 17".....!

hokiefyd 

Welcome!

Unfortunately, sizing isn't an exact science. I'm not even sure it's an art. Frames used to be measured by the length of the seat post (center to top, usually), but this metric became less relevant as top tubes became less horizontal and more angled (angled down towards the back). For example, I have a 20" Trek Verve that is a larger frame than my 21" Trek 750. The 750 has a horizontal top tube, so the head tube is at about the same height at as the seat tube. But the Verve has a sloped top tube, so the head tube area is much higher than the seat tube, and the frame is physically larger than the 750, which has a larger measurement number (21 vs 20)

I think this is one of the reasons some manufacturers began to refer to their frames with more subjective terms, like "large" and "small" and such. Unfortunately, there are no set definitions, and they're probably most relevant when compared with other bikes within the same model from the same manufacturer. That is, a large C400 should be larger than a medium C400, but it may or may not compare well to a large Specialized or large Giant, etc.

It does seem odd that a large C400 would measure only 17" at the seat tube. Are you measuring center to top? Does the top tube have a pretty large slope to it?

DorkDisk

C series featured shorter seat tubes to provide better compatibility with suspension seat posts.

TakingMyTime

Although sizing may follow certain patterns, there is no standardized formula. You did the right thing. You went out and picked 2 bikes that fit you.

DaveSSS 

Sizing is an exact science if you pay attention to the stack and reach numbers. Those are the two most recent additions to the many dimensions listed for frames. The seat tube angle is also relevant, but mostly with regard to choosing the right seatpost offset.

Having owned many frames over the years, I also know what will fit me, based on the TT length, seat tube angle and head tube length, so I never have a problem comparing one brand of frame to another. What the manufacturer calls that size doesn't matter.

BrocLuno

It depends on whether the frame came from the road engineers, or the MTB engineers ... Road bikes are measured in cm, MTB's are measured in inches.

And as the MTB frames get more unisex and made for dropper seatposts, the measurements get further away from what we were used to.

Crank to seat height, seat to bar reach, bar elevation to seat all play a huge role in fit. But many frames can be accessorized to get into a good comfort zone. Stem length and angle, seatpost off-set, even crank arm length can all be tweaked to get you riding comfortably. Don't toss the odd bike just yet. Ride it for a while and see if you can dial it in

prj71

^^^This.

Ignore small medium and large. Depending on bike manufacturer I'm either a medium or large.

prj71

Incorrect. MTB geometry is listed in mm. Every once in a while a MTB manufacture will give you the option on their website to switch to imperial, but most are metric.

BrocLuno

Not any I have seen ... Around here, all the talk is in inches. But, maybe that's just the North Bay and Santa Cruz.

Why do MTB'ers call them 29'er vs 700, or 27.5 vs 650?

My bikes are all listed on the frame in inches (sticker on the seatpost).

Guess we'll have to agree to disagree...

hokiefyd 

The bicycle industry has long been a hodgepodge of different measuring systems. Many of my frames have sizing decals in inches, and yet the actual geometry diagram on the website lists various metrics (trail, fork offset, top tube length, etc) in metric. Some of my frames have the subjective 'M' or 'L' grade, but also give either a metric or standard number translation. And some just have the subjective size grade.

The tire sizing nomenclature is another example of the mixed measuring system. A 29" tire is nominally 737mm tall (29 inches is supposed to indicate the total diameter of the tire). Going by the numbers, that's a 622mm diameter wheel with a 57.5mm tire. 622 + 57.5 + 57.5 = 737. Translated directly to imperial measurements, that'd be a 29 x 2.25 tire. That's a much different size than a 700mm tire, which would strictly be a 622mm wheel with a 39mm tire. 622 + 39 + 39 = 700. This would be a 700x39 tire. The only thing these two nomenclatures have in common (29er and 700c) is the wheel diameter. And the nominal size really describes only a general diameter of a specific size. A 700x40 tire would nominally be 702mm tall, and a 700x25 tire would nominally be 672mm tall. But they're still both called "700" or "700c", due mostly to tradition.

All that really matters is you have a bike that fits well and that you enjoy riding. :-)

hokiefyd 

To follow-up on tire sizes, the ISO/ERTRO is supposed to "fix" the nomenclature problem. Most tires are dual-labeled (or even tri-labeled). A 700x38 tire (traditional size nomenclature) can also be described as a 38-622 tire. That is, 38mm wide/round and fitting on a 622mm wheel. This is theoretically more descriptive of the actual size in practice. A 25-622 and a 45-622 both fit a 622mm wheel, and you know how wide each tire is (so you could figure the actual diameter). Both tires would probably also be labeled 700x25 or 700x45, but we know that neither one of them is actually 700mm in diameter.

Having said that, a 38mm tire is still sometimes not really a 38mm tire. A 38mm tire might inflate up to only 35mm for example, depending on various factors like rim width, inflation pressure, and even designed size by the manufacturer. Some tend to make their tires a little smaller than the stated size -- this helps them advertise a lighter tire for those shopping by weight. Manufacturer A's 38mm tire might actually be 38mm and weigh 500 grams. Manufacturer B's 38mm tire might actually be 35mm and weigh 450 grams. Nobody advertises the "actual" size -- but they do advertise the weight, and that's often what sells.

prj71

I'm referring to the geometry numbers listed on the bike manufacturers site. I see what you are saying though. Small, medium, large might be something like 16", 18' and 20"

BrocLuno

Yeah, my wife's bike is listed on the frame as 17.5 which I assure you is not cm ...

And tires for MTB's are 2.1, or 2.5. or 3.2, etc. All inches. Dumb I know, when teh metric size is in the small print on the sidewall. I don't know why they are not all just listed as both in the big print? Then everyone would be happy

epnnf

yep, bicycle sizing is confusing- just like lots of other things about bicycles (tires, as stated above; metric/inch; saddles; everything is nonstandard). Manufacturers have started to list 'stack' & 'reach' in their specs. So just find what works for you (which may take ...decades), then stick w/it. Ride on!

Darth Lefty 

BrocLuno

Yeah, this hybrid forum ought to be a place where we can work these inconsistencies out ... I'll have to think about that and refer to MFG's sites for my existing bikes to see what they say? I have mostly bikes with 1-1/8" steer tubes, but 2 with 1" quill stem steer tubes. Yes, they can be measured in mm, but the designs obviously started in the "English" system.

And as the frames have gotten more sloped to fit long seat tubes and dropper posts, it changes the way we have to discuss stuff. The old days of horizontal top tubes are going away. Hard to describe K and Y frames w/o newer measurements like stack and reach, slack, etc.