I just bought a Trek 7200fx after riding a Mongoose 26" for years. I've been surprised at the comparatively rough and vibration-filled ride of the aluminum-framed Trek versus my old bike.

I assume its a combination of the smaller wheels and tires, and the aluminum frame.

I plan to do 80% road and 20% bike trails.. but now I'm wondering how rough those bike trails are going to be on my bad lower back!?

Wearing bike shorts? Maybe try the tires inflated on the low side of their range? Not sure whether or not the seat post pre-load is adjustable or not, if so, maybe play around with that. If you are bottoming it out, make it a little stiffer. Remover the seat post and see if there is an adjustment screw on the bottom.

The aluminum frames are more harsh, have you considered a 7200 instead? The front suspension might help some, and it's only about $30 more.

Were you riding that Mongoose recently, and now making a transistion, or have you been off a bike for a while?

Mongoose is about six years old.. still has a great ride, but the rear brake spring is missing, and LBS guy thought I should "upgrade" to a lighter, more modern bike instead of paying to have the brake fixed.

I've fiddled with the Trek's seat suspension.. maybe that will help.. but geez, that old Mongoose sure is comfy and seems just as fast.

Most of my rides are 20 miles or less..

Thanks..

it may take you a few weeks to adjust to the new bike, you may even want to consider taking the saddle off your mongoose, I'm sure your butt is quite used to it.

Maybe try swapping the seat from your Mongoose to the Trek? Just the saddle, not the post. It could be that you are so used to the saddle, and of course it's well broken in to fit you, that it will make a huge difference. If so, and the old saddle really could use replacing, ask the shop for something similar and if they will give you any trade in for the original from the Trek, since it should have virtually no wear.

Twahl...you're a good man

As Twahl points out, aluminum frames are notoriously stiff and unforgiving.

You might consider a suspension seatpost, and/or larger-diameter tires. If you're going to be doing even some off-pavement riding, a bigger "footprint" might help, and you can often run regular MTB tires at lower pressures for a little more cushion.