Most manufacturers that produce road going bicycles will measure the top tube length of the bicycle to gather a marked frame size. The exception to this is on very small and very large bike frames.
Most manufacturers that produce off-road going bicycles will measure the seat tube length of the bicycle frame to gather a marked frame size.
Firstly, I argue that the seat tube measurement of frame sizing is a waste of time. You can change your seatpost height dramatically with the turn of an allen key. There is tonnes of adjustment. The effective top tube length is considerably harder to change, and when you do make any changes with a new stem it affects the handling of the bike! Obviously stand-over height is important to consider, but stand-over is generally not an issue when people purchase the right sized bike for them.
Now, I would argue that a touring bike is more of a road going bicycle than an off-road bicycle. Although this might be the case, Surly touring frames are measured by their seat tube just like off-road bicycles. Surly are not the only ones, it seems that there is a mixed bag of frame sizing from all kinds of manufacturers.
Why? Well, there is some logic behind Surly’s decision and that is to better differentiate between sizes, as the top tube length changes less considerably than the seat tube. The logic is definitely lost though, as it is often very confusing when comparing smaller sized Surly bikes to other manufacturers. Let me give you an example: a 50cm Surly and a 50cm Salsa have a 3cm difference in their effective top tube length! The 50cm Salsa is, in fact, shorter than the 42cm Surly!
Let’s compare Surly and Salsa
Surly frame size and effective top tube length (26″ wheel):
42cm is 50.5cm effectively (Same as 50cm Salsa Vaya)
46cm is 51.5cm effectively (Same as 52cm Salsa Vaya)
50cm is 53cm effectively
52cm is 54cm effectively (Same as 54cm Salsa Vaya)
54cm is 55.5cm effectively (Same as 56cm Salsa Vaya)
56cm is 57cm effectively (Same as 57cm Salsa Vaya)
58cm is 58.6cm effectively (Same as 58cm Salsa Vaya)
60cm is 60cm effectively (Same as 60cm Salsa Vaya)
62cm is 61cm effectively
Surly frame size and effective top tube length (700c wheel):
56cm is 57cm effectively (Same as 57cm Salsa Vaya)
58cm is 58.6cm effectively (Same as 58cm Salsa Vaya)
60cm is 60cm effectively (Same as 60cm Salsa Vaya)
62cm is 61cm effectively
64cm is 62cm effectively
The lesson learnt?
I can almost guarantee that your road frame size will be different to your touring frame size. Get sized up at a specialist bicycle shop (get clued up by reading my resource Understanding Bike Fit). Make sure the employee knows about frame sizing. Measure the effective top tube length of every bike you try. Compare the geometry charts between manufacturers to make sure that you’re comparing apples and apples!
Just wonder what is the captain frame size of your tandem ?? … I’m only 6′ 4” but with 99 cm inseam and we have a heavily modified Santana , ( 180mm 35 deg . stem , 400 mm seat post with a very min. insert ) . My gf is 5′ 6” . In fact the cokpit is very tight for me . My toes already hit the front wheel ( very afraid to not able to put my regular 180mm crank arm ) .
Hi Martin. The captain frame size has been optomised for road handlebars, so it has an effective top tube of about 620mm, and a seat tube of about 600mm. That works out to a stack/reach of 643/402mm. Alee
Just about all bikes I’ve ever bought have been sized by their Seat Tube length including the old CCM Formula 10 road bike (23in) I bought in the 70’s and Sekini road bike (60cm) I bought in the 80’s. The mountain bikes I’ve bought recently have all been measured at the seat tube (all in the 19 to 20.5 in range). But even here there are differences. Most measure from the center of the bottom bracket (but there are some exceptions) and some measure to the top of the seat tube, and others measure to the top of the top tube where it meets the seat tube (as Surly does). I recently traded a 60cm Surly Pacer for a 58cm (700c) Disc Trucker as the seat tube measurement gave me a more comfortable standover height but the top tube measurement was shorter by only 1cm, and the Pacer had me stretched out in a more aggressive riding position, that I didn’t want on the touring bike.