justin10054

I've been looking into the possibility of picking up an old lugged steel mountain bike, putting on drop bars, and using it as more of a dirt road bike. I'm just curious how the geometry differs from road bikes in terms of selecting a frame that fits.

For instance, I'm looking at a frame that measures 52x55cm which would be perfect for me in a road bike since my legs are a little on the shorter side and my preferred frame size of 54cm tends to involve a little bit of crotch interference. However, every bit of mountain bike sizing info I can find suggests I should be riding a much smaller frame. Does that sizing info go out the window if I plan on using the frame as more of a road bike or are there other factors to frame geometry that I should be considering (maybe bottom bracket height)?

Italuminium

Justin, one thing you have to keep in mind is the relation between top tube length and headtube angle; slacker headtubes (like old lugged MTB's are likely to have) push the handlebars further back - all things being equal - than more acute angles.

So if you're used to riding a 54 road bike, a 55 or 56 cm top tube on a frame with slacker angles could fit well within your fit range. I ride 56 frames, normally, with 10 cm-ish stems, but my SSMTB based on old frame has a 60 cm TT and a 10 cm stem, but has the same reach as a 56 cm road bike.

I think the frame you're looking at could work pretty well, although the higher bb on older MTB's means that you'll be sitting a little higher up and might need a bit of a taller stem to get the fit right, but that's no problem at all with quill stems, especially not in your size range.

Eric S.

I don't know what the geometry of a Bridgestone MB-1 is, but I had mine set up with drop bars for awhile and it made a fantastic heavy-duty road bike.

Slash5

MTB sizing is usually small with long seatposts to provide more standover room - for those unexpected dismounts when riding off road.
Same reason modern MTB bikes have sloped top tubes.
Assuming you are not mountain biking you can go for a more normal fit - which will help bring the headtube/bars up.

70sSanO

Because classic/vintage mountain bike geometry varies by era (80's/90's) and manufacturer, I think you need to look at some manufacturer's catalogs/tech manuals.

For example, you can google vintage Trek bikes and get a lot of info. You can also use the archive on retrobike.

I have a 1995 Trek 970 that has an effective top tube length of 580mm (58cm) for an 18" frame. It also gives standover height.

For my 1992 Serotta size 44L (17-3/8") the effective top tube is 565mm (56.5cm).

Find a few frame candidates and go do the research. It does take some time, but hopefully you will end up with a bike that meets your expectations.

John