Hi.

I have a 53cm road bike that fits me perfectly running 28mm 700c wheels. That's (give or take) 21 inches. I'm looking at old-school horizontal-tube 80s MTB frames for a drop-bar conversion, and research seems to indicate that an 18" is right for me. I'm fine with that (and there's a mint High Sierra frame for $100, which is good in SoCal), but I'm a bit confused about how MTB frames are measured. I assume MTBs have a higher BB for clearance, which might contribute, but the results are confusing. I'd go stand over a bike at a store, but everything new has a sloping top tube. Any thoughts?

Thanks!

Found this. 18" should be perfect.

MTBs are sized smaller than road bikes to compensate for wider tires and longer top tubes.

My road bike is 54 cm and my 29er is a 17".

Hope this helps.