Frame sizing...going from 54 to 53 or 55?

lef

I'm after some guidance with frame sizing on a Viper Ventoux frame I have my eyes. My two current bikes are 54cm frames - Focus Izalco with a 543 top tube & 548 top tube length for the Spesh Allez.

The Viper 53 size has a top tube length of 538 and the 55 a 556.8. All sizes centre-centre. I'm thinking I need the 53 size but with maybe a 10mm longer stem if necessary. I realise that a bike fit is the best option but if buying a frame online this isn't feasible and of course building the bike and finding its too small would be an expensive mistake.

Any thoughts would be much appreciated!

majormantra

Have a read of this:

http://www.bikeforums.net/showthread.ph ... mine-reach

Especially this point:

The round rule of thumb is that steeper STAs increase the reach by 1cm per degree, but in the smaller sizes, it's only 8-9mm per degree.

You need to take into account seat angle as well when comparing frame. Are there any local shops that will do a fit for you. You can usually get a basic one for around £30 to £50. They aren't as good as a proper pro fit but they will give you some guidance and maybe a little peace of mind.

lef

well I have previously had a fit which was great but its the 53 cm / 55cm thats throwing me. I probably should get back in touch with them and send them the geometry of the Viper and they might be able to give some direction, thanks.

lef

well I have previously had a fit which was great but its the 53 cm / 55cm thats throwing me. I probably should get back in touch with them and send them the geometry of the Viper and they might be able to give some direction, thanks.

lef

just had time to read the link, very interesting stuff. Might actually be worth drawing the geometry up to overlay the different ones (I use CAD for work) to compare reach. thanks for the link