These have become a lot more common and are now frequently fitted to new hybrids. They used to be a bit of a gimmick to comfort people who thought they were missing out on the whole suspension revolution. In the meantime, they’ve quietly got better and are actually quite a good idea. They work best if you have a fairly upright riding position, which puts most of your weight onto your saddle. The suspension takes the edge off the constant jolting in and out of potholes.
Suspension seat posts often help if you get a sore back and shoulders through cycling. They are even more effective in combination with a good-quality saddle.
Getting the seat height right can take a little bit of getting used to. When you sit on the saddle it squashes the post a little bit. This is called ‘sag’ and is supposed to happen. It has a side effect, however, since when you get off the saddle, it pops upwards a little bit, making it seem like it’s set too high. You just have to get used to lifting yourself up a little higher to get up onto the saddle. The alternative is to set it to your normal height, so that it’s easier to get on to but, once you’re aboard, the slight sag means you’re sitting too low down so your legs never get to have a proper stretch.
The standard pattern seat post – there is remarkably little variation in design – works by trapping a spring between two telescoping parts of post. The bottom part of the post that fits into the frame looks normal. The top section of the post is narrower and slides into the bottom part. This top part may be covered with a flexible rubber boot to keep the dirt out. The spring lives inside between the two parts and may either be a long metal coil spring or an elastomer rod.
Before you ride, the sag in the seat post has to be set up so that it settles into place the correct amount under your weight. The post is supposed to give a little bit so that it’s got room to spring upwards, supporting you if the bike drops into a dip. It has to be able to compress as well, so that if you hit a bump the bike can ride upwards, without kicking you upwards with it. The ideal is to set the preload on the spring inside the post, so that, when you sit on the bike, the natural resting point of the saddle is some way between the two extremes.
The total travel on seat posts isn’t a great deal of distance, usually around 40mm (11/2 in), so you’re looking for the seat to sink about a quarter of that when you sit on it – about 10mm (3/8 in). You’ll need assistance for this bit as you have to sit on the bike and then let someone else measure how much difference you’ve made.
Step 1: Lean the bike against a wall and measure from the top of the seat post – where the post meets the clamp – to the bottom of the knurled nut. Take a note of the measurement. Leave the bike against the wall and climb on. Sit still in your normal riding position. Get a friend to measure again between the same two points as before. The difference between the two measurements should be around 10mm (3/8 in).
Step 2: If the sag isn’t right, undo and remove the clamp holding the seat post into the frame. Turn the post upside down. You’ll be able to fit an Allen key in the cap in the bottom. If you’ve got more than 10mm (3/8 in) sag, the spring is too soggy – turn the Allen key clockwise, adding preload. If you’ve got less, the spring is too firm – turn the Allen key anticlockwise, reducing preload. It’s important not to undo the cap so far it protrudes out of the end of the post.
Step 3: You’ll have to refit the post in the frame and repeat the measurements to check that the amount of sag is correct. It may take several goes to find the right place. Once you’re confident about the sag, you’ll need to reset your seat height. Use an Allen key (4mm, 5mm or 6mm) or a quick-release lever, which you loosen until you can move the post freely.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Seatposts”