I'm planning a ride from Birmingham to London on the Grand Union Canal (UK), which will be quite bumpy for most of the 145 mile distance. I have a front suspension bike and can't afford a new full suspension bike. Do suspension seat posts actually work? If so, I'll get one of them. Can anyone recommend a good model?
You might find a suspension seatpost to be comfortable, but a suspension seatpost is not a substitute for a full suspension bike. The purpose of a suspension seatpost is purely comfort (though as mikes mentions, not everyone agrees that they achieve this goal), where as the purpose of a full suspension bike is first and foremost control. Suspension on the wheels helps them stay on the ground over rough terrain, which a suspension seatpost will not do. Any extra "comfort" is simply a side benefit.
If you're looking to smooth out your ride, the #1 thing you can do is lower your tire pressure. You will lose some efficiency when the ground is smooth, but if you're already riding 145 on a bike with a front suspension, I suspect efficiency isn't your top concern to begin with.
It seems everyone I know has a love/hate relationship with them.In other words liked it or didn't.My main complaint was while trying to maintain a consistant cadence I could feel the seat height changing.It seemed to interfere with my pedal stroke.Mine was an inexpensive telescope type,with no adjustments.My reccomendation would be stay away from the lowend models.Also I would try it in similar conditions prior to your trip.
I've heard very good things about the cane creek thudbuster suspension seatposts. I'm probably going to invest in one for some longer gravel road rides I'm planning for the summer. They come in both the LT (long travel) and ST (short travel).
I think the ST model would be a good addition to a touring or cross bike used for a longer bumpy ride to take some of the harshness out of the bumps. I'm basing my opinion mostly on the review at http://www.thecyclistsite.com/reviews/2009/11/27/cane-creek-thudbuster-st-seat-post-out-of-the-box.html
I've had to use a cheap spring-based suspension seatpost for a road bike, because it was the only 27.2mm post I had in my pile.
The main problem with the lower end ones is that they have spring action only in one direction. They have no damping other than a hard stop.
As you sit on the saddle the spring holds it up against the stop, and then it takes an extra hit from below to compress the spring.
So these seatposts spread out the initial up-thrust, but don't really minimise road buzz or small bumps.
If you sit down a bit heavily they can compress, but then they push you back up as well.
If you do go this way, tweak the adjuster which is up the main post (remove post from frame to get to it) and find a position which works for your body-weight. Ideally, be hard enough to be firm but soft enough to compress under just a bit more than your normal riding body weight.
Plus they add some small additional weight to your bike which is a negative for any climbing.
Answer possibly better or worse than nothing, and no substitute for a real suspension bike.