Anyone know about frame geometry. I refer to effective top tube.

At my height (5'9") I am recommended the small in Focus Izalco Max and the medium in Scott Foil, but the effective top tubes are near identical .  Focus 535mm in small, 551mm in medium. Scott 535mm in small, 550mm in medium.

The Scott even has a longer stem (110mm).

Can someone explain this. To me the effective top tube is how far you have to lean over to reach the bars.


Many thanks!!!


CXR94Di2 [2795 posts] 4 months ago

That's a problem with sizing. I would highly recommend you test ride both bikes and sizes to get a good feel, a proper hours ride if you can

If I was to make a decision, I would go for the smaller frame and longer stem. A long stem makes for a planted secure ride. There are some caveats though
1. If the stack is too low and you feel like you're craning your neck to look forward ( larger frame-higher stack-shorter stem)

2. The seat post isnt too far out of the frame on the small size.

The two numbers I mainly look at are reach and stack. The top tube puts me in the guide size and then I check the other numbers to confirm.

alotronic [652 posts] 4 months ago

Can of worms  1

I am a ittle shorter than you but very much between sizes. The figures that make more sense to keep an eye on are reach and stack which tell you how far forward and above the middle of the crankset the top of the head tube is. SOunds obscure but what with sloping top tubes and various lengths of forks those are the most reliable figures you can compare bikes on. Best plan is to look up the reach and stack for a bike you know you fit and get something close to that. For intance I have a relatively longer back and shorter legs so a frame with a longer reach and shorter reach and shorter (ideally sloped) seat tube suits me. 

Of course an endurance bike will be a little shorter and taller (less reach and more stack) than an out and out racer. Also new 'gravel' bikes are often *meant* to be run with shorter stems.

A good example is genesis who changed their model line up between 2015 and 2016 and went from six sizes to five - I have three (!) of them, two post 2016 are small but the one from 2015 is a medium - they are all within 4mm on reach and stack. So it will vary from one make to another.

Having ridden for a while I 'know my numbers' and can pick bikes off geometry charts but ideally you will want to be riding lots of models and sizes and working with a good bike shop. Arguably being sized correctly from a shop who know what they are talking about is more important than what brand you are buying (shoot me down for that!) 

FWIW it's worth IF you are average on leg/back length then you are *probably* an M, I ride bikes with 535 -540 virtual top tubes with 12-10 cm stems; you might be pushing it with a S as you are about 3/4 of an inch taller than me... 


matthewn5 [1442 posts] 4 months ago

+1 for stack and reach. The only measurements that you really need to look at in terms of fit.

You can work out (by extrapolating from the stack and reach of a bike you know) how many spacers you'll need under the stem, and how long a stem you'll need. Works like a charm, so when you get on your new bike, the position is just the same as the position you're already happy with.

TheBillder [89 posts] 4 months ago

Yep, stack and reach, and a rough way to see how racy the position is is to divide the stack by the reach. The lower the number, the more stretched out you'll be, and from that base line you can use stem spacers and seat fore and aft adjustment to fine-tune, or indeed different stem lengths but then you have to add money unless you have access to an impressive spares box.

Whilst you can use this technique to get the same each time as per matthewn5, I have recently used it to get something deliberately different, starting from what I had and knowing which direction I wanted to go in.

It's not perfect - lots of people will tell you it's more complicated than that and I guess they are right, but for comparative purposes it's pretty useful.

One other suggestion: if you find geometry you like in a shop but you don't care for the components, frame material, or the price, stack and reach can help you find something similar to put onto your shortlist.