I’m planning to get an old rigid-frame mountain bike for use mostly as a city commuter, and also for gravel road excursions and the occasional road tour. What size should I look for? I’m 6 feet tall but have a relatively short 30” inseam. I consider 58cm/23” my normal road bike size. The MTB sizing recommendations seem very small and more in line with off-road riding: according to one chart I should get a 19” frame. Shouldn’t I look for something larger for road riding?

For a 26" MTB, 19" frame should be OK.

I'm 5'10 with a 30" inseam.

My road bike is a 21" Trek 520, and my MTB is an 18" mid 90's Nishiki Colorado. These sizes work for me. I have a bit more crotch clearance on the MTB than the 520, but the next size up would probably be too high.

At any rate, providing the standover height is OK, it's not so much the seat tube length that matters as the top tube. Seat height is easy to fix, but there are limits to what you can achive with stems.

This bike sizing calculator worked pretty well for me.

https://www.bikefitting.com/English/FSR/FSR.aspx

I also have a 21" Trek 520! It's a 1992, great bike. It's the smallest frame (as measured by the seat tube) I've ever had. But it's got a long top tube, long stem, and the handlebars extend forward pretty far, so it fits my long torso pretty well. I think it's still a tad short in the seat tube, but with the high BB and big 45mm tires I use, I was afraid the next size up (23") would have been too big. I figured it would be better a bit small than too big. It puts the seat quite a bit higher than the bars, but fortunately I like it that way. The bike is getting to be a classic and I want it to last me a long time. It's held up well through 18 years of daily beatings on the rough streets where I live, but last October I broke the steering tube. I'm thinking of retiring it from the daily grind of commuting and saving it for touring and excursions, and replacing it with an old MTB.

no 19 is fine. it's weird how they measure them though. it's different. like they don't measure the entire tube or something. I recently got rid of an 18" even though I measured it much bigger according to road specs. I now have a 20" MTB which is the correct size for me and my legs and it is way more comfortable. I stayed with the same brand and geometry so it was interesting to see the difference the bigger frame alone made

I have asked myself this same question. I am new to bikes and jumped right into commuting to work with an old body.
Now 8 months later I have 6 working bikes and a couple in process. I just paid $300 for an Italian frame because it had a long seat tube but a shorter than usual top tube.

Anyway I measured all my frames...guess what, every one is a little different. When the snow came I switched from my Gazelle which is an '80's racing framed road bike with 27" wheels to an old Giant Iguana MTB with 26" studded tires. As far as comfort I was not happy because the stem on the MTB was short vertically and long horizonally, the Gazelle has a Nitto Technomic long vertival reach stem and that got my head way up more. With bifocals I saw better. The MTB has me in a crouch so that with hat(s) and hood, bifocals and cold windy weather it's hard to see down the road.

You can bet something is gonna change for next year. The reach to the bars and stem on my MTB just isn't right. I am riding it to work every day but it's not very enjoyable cause i can't look around easily. I like a frame that's bigger. The Gazelle I can just barely stand over. Underway it's really relaxing to ride, high,but quick to turn with a short wheelbase.The mtb by comparison is work in every sense on the road.
bill

I've seen pics of that Giant Iguana, and I want it. Such a beautiful bike

BTW, what size is it?

lose the hood
accident waiting to happen

Hello all,
Consider trying this bike frame sizing:

https://www.ebicycles.com/bicycle-tools/frame-sizer

you can input inches/cm, male/female, road/mtb/bmx/kids. It includes crank arm length recommendations as well. It doesn't go into detail supplying other tube lengths, just seat tube.

THANKS THAT WORKS GREAT! (and matches the bikes sizes I own and ride comfortably and matches the size I've come up with with other methods)

Yeah, but they even say that the MTB's are for off-road roding. I'd figure that for roads and streets you'd want something more along the lines of road bike sizing, but the MTB geometry seems very different and not really comparable.

Its because MTBs aren't intended for road riding. While they certainly can be used, its not their intention.

I would stick pretty close to recommendations. MTB frames are supposed to be smaller. I normally ride a 57cm road bike, but I rode a 19.5" Trek 820 for a while and it was plenty comfortable. Expect to have a lot of seat post showing.

I had a early 90's Giant Rincon that was a size 21" if memory serves me correctly. I think it was considered a mountain/hybrid bike. At 5'9 I thought the bike fit me fine if not just slightly big. I only commuted on it and road riding was just fine. It did not have a sloping top tube like a lot of newer type mountain bikes do.

Your best bet is go out a ride it. When I was younger I felt like I could ride that bike forever. If I took better care of it I probably wouldn't have it just sitting in my moms basement.

Early 90's Mountain bikes can be very different from the later ones. The Rincon had attachement points for two watter bottles and brazeons for both racks and fenders both front and rear. My understanding is that some of them were simply touring frames with 26" wheel sizes. I'd say the road bike frame size would apply.