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22. Seats and Seatposts #4015 . Top

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  1. Seat Post Diameter
     When buying a seat post, you need to get one that matches the inner diameter of your frame tube. This is different for different frames. The seat post diameter is usually stamped near the bottom of the seatpost. If not, you have to measure it. On my Rocky Mountain, the new alloy post I bought is stamped 26.8 mm, but I had to measure the old post. Sheldon Brown says 27.2 mm has become more of a standard. However when you measure a seat post with a caliper, you discover they can vary slightly, and are not really exactly round and uniform. I initially measured mine to be 26.8 mm, and in places is not completely round.

    At website, you can see a typical seat post called MEC 350mm Alloy Seat Post. 350mm is the length. That model comes with the following diameters to fit different tube sizes: 25.4, 26.8, 27.0, 27.2, 30.0, 30.9

  2. Seat Post length
     On all my old road bikes the seat post was quite short, usually 160 mm. For safety, you want at least 100 mm of the post to be inside the frame tube, so on my old bike, the highest the seat could be adjusted was 60 mm above the seat tube. For a typical mountain bike, the lengths are much longer, often 350 mm or 400 mm. On my Rocky Mountain bike, I have a relatively small frame and I use a long 400 mm seat post (since I am 6 foot tall).

  3. Offset
     The simplest seatpost does not have any "offset" or "layback". But if you want to get a bit more length out of a given frame, you can consider a seatpost that is offset back from the centerline of the seat tube.

  4. Seat Attachment Clamp
     First, lets talk about the seat itself, and then the clamps. The seat itself has two parallel rails. The post has one of three types of clamp which tighten onto these rails. The seat rails allow you to adjust the seat forward or back. The clamp allows adjustment of the angle of the seat. There are three main types of clamp as listed below. Three types of clamp are:

     1. Two bolt system (2 vertical bolts)
     1. Rocker clamp (one vertical bolt)
     3. Single cross bolt with toothed washer

    It's difficult to explain the different types of clamp if you haven't seen them. So I refer you to a couple of high quality videos. In the videos, pay attention to the method of adjusting the seat angle.

  5. Video
     The woman in this video shows the three most common systems of seat attachment. She starts with the single bolt, rocker type seat post. She shows how to slide it forward and back with the rails and how to adjust the angle.
     0:00 She starts with what I call the "rocker Clamp" system
     2:06 She briefly shows what I call the single cross bolt system
     2:20 Introduces what I call the "Two Vertical Bolt" system

  6. Clint Gibbs Video
     This video shows how to swap out or install a seat. His main emphasis is the "two bolt" system, which is what I prefer. He also quickly shows a single cross bolt system. Good explanation of "bottom plate" and "top plate". He emphasizes keeping the bolts in place when changing the seat. Has a lot of irrelevant detail regarding grease on the bolts, torque wrench, etc. At 3:40 he talks about why he prefers the 2 bolt system.

  7. Adjusting Seat Angle
     Controlling the angle of the seat is much easier with the two bolt seat posts than with the old single bolt / rocker clamp posts. As was shown in the Clint Gibbs video above, the method of adjustment is to loosen one bolt and tighten the other. This gives you exactly the angle you want. By contrast, the old rocker clamp system is harder to control the angle. You think you've got the seat at the angle you want with the bolts loose, but when you tighten it, it changes. So you had to iterate away to get the angle you want. And any adjustment, you had to start all over.

  8. Problems with Rocker clamp system
     All my bikes came with the old single bolt rocker clamps, and on all my bikes I was always at one extreme end of the rocker post adjustment. I had a particular problem with my Rocky Mountain because even with the rocker clamp at one extreme end, the nose of the seat was still too high. And also the little rocker teeth were worn away such that the seat angle could slip. The situation was made worse by having a soft seat, because then my pelvis would sink into the soft back of the seat, making the nose relatively higher. A ball breaker. So the first thing I did was replace the seat with a standard firm seat. This improved things greatly, but I still had the curved clamp at the extreme end of the adjustment. But then I discovered the two bolt seat posts, which give you total control over the angle of the seat. So I bought a new post for 24.00 and easily put it onto the bike. The new seat setup is much more comfortable.

  9. Seat Post clamps
     The height of the seat post is adjusted by sliding the post up and down in the frame, then tightening a bolt or clamp which compressed the seat tube of the frame. The seat tube has a slot about 5 or 6 cm long which allows the clamp to tighten on the tube. Traditional road and touring bikes simply had a built in bolt which pulled the tube together. With mountain bikes, the clamp became a separate item. The clamp could be either a quick release, or via an allen key. You can easily change clamps. Some people prefer bolt clamps to quick release because it makes it slightly harder to steal the seat.

  10. Seized Seat Post
     A common problem is seized seat posts. They can become bonded to the frame tube by corrosion, and can be very difficult to remove. Don't let this happen, because there are lots of times you want to be able to remove the seat. And check any used bike you buy. I recommend you quickly loosen the seat post clamp once every few months and make sure it will twist and slide easily. Last week I discovered one of our seat posts had seized solidly in the frame. Yet a year ago we could easily remove it when putting the bike on top of the car. The normal recommended way to twist the post is by using the seat itself as a wrench. However that didn't give me enough leverage in this case. Eventually I had to use a huge 15" crescent wrench on the flat section at the top of the seat post, and with all my strength I could just barely twist it. But it was still tight, no matter how many times I exhausted myself twisting it back and forth. I sprayed WD40 into it for several days, but it didn't get looser. Eventually I took it to a bike store, and at first the mechanics had no more success than myself. But eventually I told the mechanic to get the biggest adjustable wrench and I would twist the saddle while he pulled up. So I twisted back and forth and he pulled up, and the post gradually came up. The amazing thing was that once it was out, the WD 40 oil covered the surface and it would slip right back in with no problem.

  11. One piece or two
     On all the traditional one piece seat posts I have, there is a flat section at the top of the post just under the seat. That flat section is 22mm wide. To twist a stiff seat post, I inserted a giant adjustable wrench onto this section and twisted. However the bike mechanic was initially concerned it might twist off the top of the post, since the top is usually a separate piece. However, twisting with a wrench is no different than twisting with the seat, just a bit more leverage. My new two bolt seat post does not have this section, and may in fact have the top clamp attached in some manner.