Mountain + Bike + ???? = ????

May 13, 2004 12:59 PM

Mountain bike tips:

Being awarded a Trek 4900™ Mountain Bike, why I ask here as I'm looking for a more personal help about it. How do I measure my inseam? Should I be exact or will having shorter or taller specs be helpful? If so, why please.

posted by thomcatspike to Sports, Hobbies, & Recreation (9 answers total)

Response by poster: 5'7'-8' is my height, more lanky than short built.
posted by thomcatspike at 1:11 PM on May 13, 2004

The inseam measurement is taken from the crotch to the floor in stockinged feet. It is not your trouser size, which is typically a couple of inches shorter than your actual inseam. The best way to measure your inseam is to place the spine of a book between your legs and measure from the spine of the book to the floor.

looks like a really cool bike, thomcat -- have fun!
posted by matteo at 1:12 PM on May 13, 2004

Response by poster: 7"-8"
posted by thomcatspike at 1:12 PM on May 13, 2004

Is it possible for you to try out different frame sizes in person? That's the best way to make sure you're getting a bike that feels right. You could go to a Trek dealer and do some test rides...
posted by COBRA! at 1:20 PM on May 13, 2004

The 4900 comes in 6 sizes. I'm 5'10, and just got a mountain bike in size 23 inches (note, these measurements aren't exact manufacturer to manufacturer), which is almost too large for me. The 22 will almost definitely be too large for you, and the 21's probably too close to call. I'd go for the 19.5 if you have to order by mail.

Regarding frame size, this use to be much more critical than it is now. Folks are making oversized seat posts and stems to allow you to fit yourself on a very small frame.

All other things equal, a smaller frame will be a slightly rougher, tighter handling (twitchier) ride than a larger one.

But, don't take my word for it. The inestimable Sheldon breaks it down for you here
posted by daver at 1:40 PM on May 13, 2004

I'm going to assume this will be for trails and not a commute bike or something. If you really will only be using it on pavement, then clearance doesn't matter as much. For trails, however, read on.

Generally for riding on uneven surfaces, you want good clearance over the top tube - the conventional wisdom is around 4 inches. If you expect very technical riding or lots of dismounts, you might err on the side of more clearance. You won't have any trouble finding bike guys who go on and on about the superior stiffness of the smaller frames, either, although if they're racers they may tell you that good clearance is overrated.

But whatever you do for clearance, be sure you're comfortable across the top of the bike, too, since there's only so much stem/bar switching you can do to tune the fit.

Stiffness (eg, in small frames or with aluminum) increases transmission of vibration through the frame, which is less of an issue on dirt than on the road. Twitchiness usually refers to the responsiveness of the front end and has more to do with front-end bike geometry than frame size. In other words, don't worry about these things now; concentrate on clearance and distance across the top of the bike. Plus, you're getting a suspension fork; you're looking at mushiness more than twitch.

The 4900 frames are not terribly idiosyncratic as size designations go, but front suspension lifts the front of a bike up a little. So unless your crotch height is at least 34", and/or you are a very expert trail rider with cat-like reflexes, you might find the 19.5" frame a bit close for comfort on the trail, if you know what I mean. And I think you do.
posted by caitlinb at 2:53 PM on May 13, 2004

There are some fairly exact methods of setting your seat height based on inseam length, which will help prevent/avoid nasty knee stress injuries.

I don't recall the formula at all, unfortunately.

You should google around, see if you can find it, and base your decision on that. The size of frame is irrelevent if it won't let you set the seat height correctly.
posted by five fresh fish at 6:32 PM on May 13, 2004

With a mountain bike, especially a traditional design like the 4900, you won't have any trouble (a) having plenty of room to adjust seat height and (b) getting an extra-long seatpost if necessary. And saddle height and standover ranges are related, anyway, so unless you want a teeny tiny bicycle, the kind once available only to professional circus clowns, and which have safety concerns all their own, you will be able to get the saddle height you need on a bike with the right clearance.

Saddle height is more than just leg length, too. It depends on positioning, pedals/shoes, and where you like to have your heels as you ride. It's an important, fine-tune adjustment that should be done with the bike on a stand so you can pedal normally to test it. Like poorly adjusted clipless pedals, a saddle that's too low can cause knee injury, but you don't want it too high on the trail, either, because you need some extra clearance when you're out of the saddle over rough terrain.
posted by caitlinb at 9:11 PM on May 13, 2004

But more importantly, are you getting the silver or the blue?
posted by milovoo at 2:42 PM on May 14, 2004