Lombard

If you scroll down to geometry specs, you will see Cannondale and Trek don't agree with you:

https://www.cannondale.com/en-us/bik...napse-carbon-4

https://www.trekbikes.com/us/en_US/b...ode=white_grey

Look carefully how long the seat tube is on both of these size 56 bikes. Hint: It's not 56cm.

smd4

Modern bikes with sloping top tubes would be misleading if they used the actual seat tube measurement. My 58 cm Cinelli (seat tube, c to c) has a 56 cm top tube (c to c). If that was the size of my bike it would be way too small.

If you don’t get that a bike is primarily sized using a rider’s height and inseam to determine a size based on seat tube length, rather than torso/arm length and top tube, then I really have no words.

tomato coupe

I think it's more accurate to say sizing is roughly based on the effective seat tube length, which is measured to where a hypothetical horizontal top tube would intersect the seat tube.

Lombard

It sounds like Cinelli and I'm sure others size their bikes different than Cannondale or Trek do. Look again and you will see the top tube length is very close to the stated sizes, though that number skews quite a bit on the smallest sizes.

Lombard

Possibly. Though interestingly, while you see effective top tube length posted, you never see effective seat tube.

squirtdad

modern bike bikes use the equivalent sizing.... ie a sloping seat tube bike of 58 will fit like a classic bike of 58 cm
see the size chart for cervelo, it links height to size https://www.cervelo.com/en-US/bikes/caledonia

in fact the size chart for trek that you listed does the same thing , size is equivalent to classic frame size and most direct comparison is height

so to repeat, size is based on seat tube length, either actual lengths in classic designs or effective length in sloping tube designs.....

The effective seat tube length is the distance between the bottom bracket and the point at which a virtual horizontal top tube would intersect the seat tube.

squirtdad

agree

rsbob 

“Remember darling, don’t be a snook. it’s not how you feel, but how you look marvelous”. SNL

DaveSSS 

Once you understand stack and reach, frame size numbers become meaningless. A lot of brands use the length of a short seat tube on a sloping TT frame as the size. It doesn't mean a thing. I do pay attention to the seat tube angle on the smaller sizes. A 75 degree STA with a proprietary 15mm setback post we be a no go for me. I'd want 32mm of setback.

Lombard

Well, OK. But a person's height also includes their torso length. Arm length is meaningful too.

Where do you see this on the Trek site?

OK, this makes some sense now. I guess I could verify this by taking a level from the front of the top tube and measuring effective seat tube from BB. What I would like to know is why you never see "effective seat tube" listed in specs?

I would agree with this. As we all know too well, a size 56 bike from one brand can be quite different from a size 56 bike from another brand.

DaveSSS 

Effective seat tube length was common in the early days of sloping top tube frames, before stack and reach. You don't see that much today. Stack and reach makes selecting the right size so much easier.

smd4

Man, glad I don't have to deal with any of that nonsense!

Jrasero

For smaller riders like myself 5'5", it's simply stand over height, granted SOH doesn't dictate overall fit but if I can't clear the bike then usually the bike's reach and other elements are too big for me either way. For normal sized or taller riders reach probably

Calsun

My first consideration is the amount of fork rake with a bike. Chainstay length is also important for climbing. Bikes like the Trek Domane and Specialized Roubaix are excellent in this regard and provide a lot more control at speed on steep down hill roads. After deciding on the bike then I evaluate the size overall. For touring I would go with the larger frame but for general riding the smaller frame if on the fence.

urbanknight

It's my opinion that the most crucial measurements are whichever ones you are close to the limits of. For example, my long femur and religious following of KOPS has me with a saddle slammed almost all the way back on a 25mm setback post and a 73.5 STA, so I simply won't buy a bike with a 74 or greater STA. Others may want a shorter reach to avoid needing a super short stem, or a low stack height for a super aero position. If you are really an in betweener on all those measurements, you're golden and have a lot of wiggle room. At that point you can focus on other aspects, like how the various dimensions affect handling or comfort

DaveSSS 

Fork rake (offset) doesn't mean anything without a head tube angle to go with it and a calculation of the resulting trail. You could have 40 or 50mm of offset and the same trail.

urbanknight

In other words, the sizing is nominal.

rm -rf

Stack and Reach worked for me when I got a new road bike. I wanted a similar fit to my older bike.

Got an older bike without stack and reach measurements? Here's an easy method -- see this 2017 post.

Using simple measurements with the rear tire against a wall:

Kapusta

For fit? Stack and Reach.

smd4

There used to be a time when sizing a bike wasn't a pain in the ass.

Kapusta

If you want to use the old "X number of inches of standover" or "handful of exposed seatpost" you are still welcome to do that.

smd4

Oh. Thanks.

terrymorse 

If you add an imaginary horizontal top tube to the frame, that old "fist full of seatpost" method still works pretty well. My bike (56cm), fit to me (5'11"), with a fat fist full of seatpost above the imaginary top tube:

GhostRider62

will 30mm gp5000 fit with 4mm clearance is key

Kapusta

Well if it works on one person for one bike then it must be reliable