What's the difference between the actual and virtual frame size on a Trek mtb? I'm new to mountain bikes and going to be buying used on CL or eBay (or bikeforums.net), but need some clarification on the sizing before I move forward.

Thanks in advance.

assuming you are talking about top tube length, the actual is a measurement from the center of the head tube to the center of, say the seat tube coller.(A lot of bikes have sloping top tubes for added standover height) The virtual top tube lenght would be a level imaginary line from the center of the head tube to the center of the top of the seatpost, at ride height. Frame sizing is usually determined by a mesurement fron the center of the bottom bracket to the top of the seatpost. If there is a virtual frame size, I'm not aware of it. But then........ There's alot that I'm not aware of.

Actually what I'm talking about specifically is the sticker found on the Trek bikes I've seen. There are two numbers on this sticker and I have no idea what the difference is, this is why I'm confused now.

It lists both "actual" size and "virtual" size. I know I need a 17.5" bike (LBS helped me with that), but which 17.5" do I need??? actual or virtual?

here's an example of what I'm talking about:

I believe that you should go with the virtual. Basically it's saying the design of the frame makes it like an 18.5 inch tube even though the actual length is 17.5.

yeh as freefallman said, the actual length is 17.5 so go for the virtual one.

Honestly, I don't think an inch difference in sizing will matter that much beyond being a personal choice. Mountain bike sizing is often less than an exact science and if you feel comfortable on an 18.5 go with it.

Actual = length of Seattube from center of bottom bracket to center of top tube's intersection with seattube's centerline.

Virtual = Length of seattube from center of bottom bracket to center of an imaginary (virtual) horizontal top tube where it would intersect with the Seattube.

Freefall's first post is a good summarization.

If you get a really old MTB, it'll have a horizontal top tube, so only one number to deal with.

Do you have proof of this?

The measurement is supposed to be from the center of the BB to the top of the "seat" tube. TT height is irrelevant with that number. TT height would be over in the "standover height" section.

I'll see if I can find some proof that applies to treks.

For the vintage of the frame in the pic shown, I'm quite sure the measurement relates to BB to toptube's intersection with seattube. It could very well be the top of the top tube, as opposed to the center, as those were the two common measuring points.

The decals helped you pick the right frame size when you replaced your horizontal top tube MTB with a sloping top tube frame.

As typical MTB design migrates further from the diamond frame, top of the seattube could be becoming a common measurement point for some manufacturers.

EDIT: It looks like their actual may very well be the top of the seat collar. Virtual looks to be the top of the imaginary horizontal top tube's intersection with the centerline of the seattube.

I wouldn't call the height of the top tube's intersection with the seattube to be anymore irrelevant than seat collar height. Some manus back in that day would have seat collars that rose well above the top of the top tube (GT, KHS and Iron Horse come to mind). So if you compared bikes by Trek and KHS going by that measurement, you'd come out with a significantly smaller feeling KHS.

It also seems like standover should be measured right around the center of the top tube, not at the seat tube intersection.

Even more Trek MTB sizing tidbits:

It looks like Bottom Bracket to Top Tube Center-To-Center up until 1990, possibly 1991.

Bottom Bracket to seat collar in 1992, with the BB to top tube dimension still shown in catalogs.

And Holy Cow! Trek is still doing that Virtual Size bit. I dunno what to make of that.

I think I'm too old to figure out MTB sizing past the mid 90s, road sizing beyond 2002 or so.

I knew everyone here would have a better explanation of the actual/virtual sizing thing than even the LBS's do. nice work!

Looks like I need the 17.5" virtual / 16.5" actual variety.

Thanks much for the help.

now I'm off to figure out what gear I'll need to ride a mountain bike (I've only ridden road bikes before). fun fun!

OK, isn't Virtual TT length just a Trek-ism for ETT? Why do they feel the need to come up with proprietary terminology?