Sizing guides: I don't get it

964cup Posts: 1,362
edited June 2013 in Road general
Before you start, I know I can go and have a bike fit, although opinions on here seem to vary quite considerably about both the value and the accuracy thereof. In the meantime, bear with me...

I'm looking for (yet) another bike. I'm going to buy used, to try before you buy is often not an option.

I'm 6'1", 34" inseam.

I have a Mekk Poggio. Parallel 73, 56 ST, 565 TT, 170 head tube, 70 BB height. Mekk say that's the right size for me, and wouldn't put me on their 58 unless I was much taller. I'm very comfortable on it with a 25mm setback post and a 120 stem.

So I look at a Specialized Tarmac. 73.25/73.5 geometry, 53 ST, 565 TT, 170 head tube, 69 BB height. Forget the seat tube; that just means more post exposed 'cos the actual TT curves down at the back; the angular difference is 0.25 degrees over about 80cm, or effectively SFA (3.4mm). Why, then, are Specialized convinced I need a 58, with both a longer TT and a taller head tube?

Canyon think I'm a 56; Cervelo think I'm a 58. Etc ad nauseam.

I'd accept this if there were radical geometry differences in their sizing, but actually (for the type of frames I'm looking at, i.e. not team-specials with tiny head tubes) they are all basically the same; no more than 0.5 degrees of difference in head tube or seat tube angle, head and top tube length (horizontal) within 5mm. I mean there are no differences in the 56 frames that you can't make up with a 10mm change of stem and/or a 5mm steerer spacer.

But the 58 frames are much larger - take Specialized: 35mm taller head tube, 17mm longer top-tube.

Top tube lengths can be adjusted by reducing stem length, of course, although this alters handling and weight distribution. Similarly one can adjust seatpost set-back and saddle fore-aft, subject to knees. But the head tube is a different story; assuming the same compression ring cover height, the Spesh Tarmac 58 has about 1cm more headtube than the maximum height of an uncut steerer excluding the stem on the 56 (about 25mm). In other words, even if I slam the stem on the 58, it'll be at least 10mm above my current position - in reality about 25mm, as I run about 15mm of spacers at the moment.

It's not just manufacturers. My LBS (Giant, mainly) want to put me on an M/L (a 56). The USA's largest Cervelo reseller (according to them) immediately said I was a 58.

Is there simply no science to this at all?


  • jibberjim
    jibberjim Posts: 2,810
    No, there's not a science to it, the best position is highly variable both in terms of riding you do - you want a different position to climb for an hour at 10% to descending different again for riding on the flat, probably a different again for sprinting, then other questions about how long you'll be riding, if speed, or comfort or something else is the priority.

    A "bike fit" is not a science, it's an attempt to compromise all those features into a single bike. Different people will compromise it in different ways - and how you present yourself to a shop or their own biases will change that.

    In any case, if there's ever a doubt pick the smaller, it's a lot easier to make a position "bigger" but pretty hard to make it smaller.
    Jibbering Sports Stuff: