I want to purchase a road bike, it’s going to be second hand off fleabay probably so I need to know what sort of size I need. My MTB frame is 17.5, anyone have any idea what road-bike frame would be?

Thanks in advance..

54cm or 56cm probably depending on arm/leg length and top tube length required.

I’d say 54.

What’s your height and inside leg? Assuming you are about 5’6″ to 5’9 and a 28-30″ inside leg, you’ll be looking for a 54cm frame (sometimes referred to as a “Medium”). Any taller and you’ll need a 56cm.

Cool, I thought so. I just want the option to ride from my door with a bike instead of having to travel 45 mins every time to the Chilterns for some good riding. Thanks.

It depends on the geo but I’d guess at about 52cm with a sloping top tube or 54cm with a horizontal one. I’ll base this on your mtb being a medium. Top tube length seems to me the only thing that really matters on a road bike as standover isn’t an issue at all.

Estimated frame calculator here. A number of others on the web.

Probably not helpful but recommend at least sitting on the bike on a turbo trainer or better still ride the bike.

Ideally you want a bike fit as it’s more critical to have the correct size in a road bike due to longer periods spent in a single seated position.