Mountain Bike sizing

[sam.g]

I've always been confused by mountain bike frame sizes stated in inches. How does road bike sizing relate to mountain bike sizes? If I ride a 56 road frame with an optimal top tube of 55, what would be the approximate equivalent mountain bike frame size?

Sam in Cincy

[Mud]

I ride a 56cm TT and a 23" TT on my Bianchi WUSS. Virtual TT length is the key since more and more frames have a sloping TT which throws off tradition MB measurements.

[Peter P.]

18-19 inches c-c, or virtual seat tube size, whatever.

Here's how I calculated it: I took my road frame size and converted to inches. Then I subtracted the size of my mountain bike frame. The difference is 4.25 inches.

I converted your frame size to inches, then subtracted 4.25 inches. One rule of thumb is to subtract 3-5" from your road bike size to find your mountain bike size.

For your information, I have 8" of seatpost exposed, from the rails to the seat collar. That's plenty for good crotch clearance. Of course, some will argue that top tube length is more important, but on a mountain bike you move around a lot so top tube is less important.

Now that I've found what fits me then yes; when I look at mountain bike frames I first seek a top tube comparable to what feels right, then look to see if the seat tube is short enough for the 3-5" rule, for crotch safety.

[shanerpvt]

I'm guessing your about 5'10" or 6'? If thats the case, you could ride a medium or large in most brands. It depends more on the type of riding you would be doing. A smaller bike will excell in techenical situations. A larger bike will be more XCish, allowing you to stretch out & pedal. Once you know what kind of riding you'll be doing - go ride both sizes if you are in acceptable height spectrum for both a large & medium.

good luck,
shaner

[zap]

I ride xc so my mtb position is very close to my road bikes. Virtual TT are usually longer on xc bikes than road bikes so stems will be that much shorter.

I just multiply important mtb " dims by 2.54.