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!