Ask R.C: What’s up with integrated seat masts?

What’s up with integrated seat masts? Are there any advantages to a one piece seat mast as compared to a regular seatpost, or is is a fashion statement?
-Reginald Votuer: Oklahoma City

Integrated seat masts can offer measureable benefits, both an increase in rigidiy, as well as a reduction in weight. This assumes that the concpt is executed correctly. The stiffness of a tube increases by the power of two as its diameter is enlarged. This means that even the slight reduction in the width of seatpost necessary to allow it to slide into the frame makes it more flexible than the frame, unless substantially more material is added to beef it up.

Standard frames must be reinforced around the seat tube/top tube junction because seatposts don’t fit the inside of the frame perfectly (if they did, the post would permanently jam in the frame), so the weight of the rider is constantly rocking the post inside the seat tube. The integrated seat mast can be a lighter, thinner tube and still exceed the stiffness of a standard frame and seatpost-and it needs no extra reinforcement at the top tube junction.

Another advantage of an integrated seat mast is that it is not restricted to a specific shape (round, in the case of a standard seatpost) or wall thickness, so the frame designer can incorporate an aero-profile tube, or any profile for that matter, into the seat tube. This is a benefit because most riders use a rear-offset saddle position which when added to the seat-tube angle, puts a bending load into the seat tube and frame that the additional cross-section of an ovalized or aero-profile tube neatly compensates for.

There are downsides to every aspect of frame design and the integrated seat mast has a few. The first has little to do with the customer. Because the seat mast must be left long to adapt for tall riders, the delicate mast is subject to damage in shipping and requires a taller, more-expensive-to ship carton. The second potential negative is that the mast must be cut to fit one person’s anatomy, and thus may pose a potential problem should the owner decide to resell it to a taller customer. Bike makers that offer integrated seat mast designs are now offering longer top-clamps to eliminate this problem. The third, and most often voiced concern is that somehow you or your bike shop will cut the seat mast too short and turn your $10,000 carbon fiber dream bike into a garage ornament. Any shop that sells elite level racing bicycles, however, should be equipped with the tools and the talent to cut a seatmast to the correct height. It’s not rocket science.

Tip: Measure the distance from the center of the bottom bracket axle, to the top of the saddle, through the centerline of the seat tube and jot it down. This important measurement will help you fit your new bike whether you choose an integrated seat mast, or standard frame.

Integrated seat masts are not for everybody, and not for every bike brand either. It’s an all-or-nothing design feature that only provides a performance benefit when it is executed without compromise. BH, Look, Giant and Colnago are good examples. Trek, however, has struggled with the concept. Its pricey Madone has a massive external sheath that fits over the sleek aero seat mast which gives almost 80 millimeters of vertical adjustment. This compromise, no doubt a handshake between Trek and its more reluctant dealers, is simply an unsightly (as I view it) external seatpost. If you are an experienced rider with a history of well-fitted bicycles, you will have no problem taking a few measurements from an existing bike to cut and fit your one-piece seat tube. All top bike makers incorporate a seat clamp that has 15 to 25 millimeters of vertical adjustment to allow fine tuning, and that is all a seasoned rider should ever need. Novice riders, however, should stay clear. You will be adjusting your saddle quite often in order to establish your optimum position and getting it wrong would be a costly error.

Tip: Most riders will need to raise and lower their saddle about one centimeter to compensate for early season muscle stiffness and peak-season flexibility, so take this into consideration when you measure and cut your seat mast.



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