I'm thinking about buying an old rigid MTB to fit with slicks and fenders for commuting. It was always my understanding that you go with way more standover on a MTB than a roadie to give your crotch more space when bouncing around offroad. This bike will never see dirt.

I ride a 16" MTB and a 49cm road bike. Do I go with a 16" for the commuter or would I want to go bigger?

Go with the reach. Stay with a small or a 15 or a 16 or whatever it is for you. MTB's do have more standover, it's true. That's still convenient with a commuter bike, it gives you something like stepthrough when you have panniers or a kid seat.

MTB sizing by seat tube length is extremely variable, as the top tube can slope down by so many different angles. Reach will be more important for you, and you'll just have to test-ride. But yes, for a MTB that you plan to never take off road, I would tend to size it up in the seattube some, as long as it doesn't put the bars out of reach.

FWIW & this might be useless info, I found my 1st MTB (red). thought it was a blast. did some mods & got a really fast commute time out of it. one day at my LBS was told it was too small 18" & told to find a 20" which I did (same brand & model) (grey). I don't think I remember noticing much difference in "fit"

I ride a 19" modern MTB off road. For a flat bar bike I like a 59-61cm center of seat tube to center of head tube top tube length. On my vintage Mongoose, that means I can ride up to the 23" frame. I also have a 23.5 High Sierra that fits the bill. My 88 or 89 RockHopper is a 20" frame, but apparently has a longer top tube as all 3 measure the same.

I disagree that the seat tube length sizing on mountain bikes is variable. The actual length may vary but a 19” (or 16”) mountain bike is still going to have the same reach as any other 19” mountain bike. That’s where. Someone going from a 16” mountain bike to a 19” mountain bike is going to have trouble with reaching the bars.

A 19” road bike is 49cm, by the way but the proportions are very different.

I have two mtb at home that are labeled as 20" (with a sticker on the seat tube), but different top tube angles and lengths make them vastly different size bikes.

Sounds like the most important thing is to look for something with an effective TT length of 20.8" like my current road commuter (and ride before buying if possible).

Thanks!

Yes, TT length is most important, effective seat tube length is easily modified by the extendible seat post.

The 0.8" part of that measurement is probably unimportant. Look for a top tube that is "around" 21". Varying by 1/2" to even 3/4" either way probably won't be all that noticeable.

I would doubt that they are "vastly different" enough that someone who rides a 16" mountain bike could use either one of them.

On the other hand, I have five 20" mountain bikes that aren't "vastly" different in the way that I fit on them. That's from a cruiser type mountain bike to two dual suspension bikes.

I'm not suggesting OP could ride them, I'm saying one of them is my bike, and the other is significantly too small to be my bike.

I would say that one of them is marked in error then. I've owned 22 mountain bikes (including the 5 I have now). I haven't ridden any mountain bike is has a 20" frame that is too small nor, for that matter, is too big. Granted I don't have any with wheels larger than 26". 29er could mess up the sizing but for someone who rides a 16" frame, a 29er probably isn't a good idea to begin with.