Adjusting an Integrated Seat Mast
This is an excerpt from Essential Bicycle Maintenance & Repair by Daimeon Shanks.
Adjusting an Integrated Seat Mast
Some new carbon road frames do away with the traditional seatpost setup altogether and instead have an integrated seat mast. Basically, the seatpost becomes an integrated part of the frame, and the saddle mounts directly to the masthead. The benefits of an integrated seat mast are lower weight, increased stiffness, the possibility of aerodynamically shaped seat masts, and a striking aesthetic appeal. The downsides are difficult to travel with (fitting the frame and seat mast in a bike bag or box), loss of resale value if seat mast is cut, and increased cost.
When working with a bike with an integrated seat mast, it's important not to clamp the seat mast in a work stand unless an adaptor is provided by the frame's manufacturer. You must use a euro style repair stand that holds the bike by the fork dropouts and the bottom bracket shell.
Of the two styles, the mast cap is more common. Ridley, Trek, Scott, and most of the other major brands that offer frames with integrated seat masts all use a mast cap. The cap will have one or two pinch bolts and may or may not have elastomers (rubber bumpers) or shims that are inserted between the top of the mast and the clamp. As is true whenever you are clamping something to a frame, pay extra attention to the torque specifications provided with the frame.
Frames that have an integrated seat mast must be cut down to achieve proper saddle height. It's extremely important to be sure of your measurements before you cut the seat mast because, once it is cut, more mast cannot be added. Most mast caps have a centimeter or two in adjustment built in, but this is not much of a safety net. Some frames are designed in such a way that if you did mess up your measurements or wanted to resell your frame, you can cut the seat mast off near the junction of the seat tube and top tube and install a standard seatpost clamp. Then you can simply install and use a standard seatpost. Before you take a hacksaw to your new frame, pull out your old bike and double-check that your saddle is in the correct position and that you're happy with those measurements.
1) Measure Saddle Height
On your existing bike, measure your saddle height from the center of the bottom bracket to the top of your saddle. Measure along the line of the seat tube. If you're using different size cranks than what is currently on your bike, measure from the pedal axle, with the crank arm in the down position and aligned with the seat tube.
2) Install Saddle and Clamp
Install the saddle and clamp on your new frame, leaving the seat mast uncut. Fully bottom out the clamp on the mast.
3) Measure New Saddle Height
Using the same method described in step 1, measure the saddle height of your new frame with the seat mast uncut. Subtract the measurement from your old bike from your new. This is the amount of seat mast that is to be cut off.
4) Mark Seat Mast
Carefully measure from the top of the seat mast the amount to be cut, and mark with a pen or notch with a flat-blade screwdriver or other utensil.
5) Install Cutting Guide
If your frame came with a cutting guide, install on the seat mast where it is marked. Park makes an excellent seat mast cutting guide that will work for almost all frames if one isn't provided. If the seat mast is round, you can use a standard steerer tube cutting guide.
6) Cut the Seat Mast
Use a hacksaw equipped with either a fine-toothed blade (24 teeth or more) or a carbon-specific blade. Lightly smooth the cut with a fine file or sandpaper.
7) Install Seat Clamp and Saddle
Install the seat clamp, bottomingout on the seat mast. Some frames require the use of elastomers between the seat mast and seat clamp. Measure the saddle height and adjust if necessary. If the saddle height is a little too low, you may be able to raise the clamp a small amount either with shims or by simply raising the clamp (within the minimum insertion mark). If the saddle is too high, you must cut the seat mast again.
Read more from Essential Bicycle Maintenance & Repair by Daimeon Shanks.More Excerpts From Essential Bicycle Maintenance & Repair
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