Tuesday, June 9, 2009

TechTalk: The Seatpost: Under-Appreciated Hero

The seatpost appears at first glance to be one of the most un-glamorous components on a bike. It just serves to connect your saddle to the frame. But it plays an important part in the comfort and performance of your bicycle in many ways.

When fitting a particular model and size of bicycle to a certain rider, the first, and often only, adjustment is made by changing the seat height. This is done by loosening the clamp that holds the seat post in place in the frame, raising or lowering the position of the seatpost within the seat tube of the frame. The goal is find the right height to provide you with a full extention of your legs (although not 100% straight) while pedaling, and a safe reach to the ground while sitting on the saddle.

The clamp used to hold the seatpost in place in the frame can be either a quick-release clamp, or a bolt-on clamp. The quick-release clamp is convenient when multiple family members are sharing a bike, or in bike rental fleets, when seat height adjustments need to be made on a regular basis. However, the quick-release lever mechanism can wear out with repeated use over time, causing the seat to slip down as you ride. For this reason, most serious bike enthusiasts prefer a bolt-on seatpost clamp. Adjusting a bolt-on seatpost clamp is still fairly easy using a hex wrench found on most multi-tools that most cyclists carry with them while riding.

The exception is for some downhill and freeride mountain bikers who want to have the ability to adjust their seat height often depending on riding conditions. For example, when riding uphill, it's best to have the seat height set to give you the optimum full leg extension. But, when you head downhill over very rough terrain, it's nice to be able to drop the seat height a bit to allow you to more easily shift your weight around as needed as you maneuver the bike through obstacles and steep drops. To allow these types of riders to make this adjustment even more quickly and conveniently, some manufacturers have designed seatposts with a remote control lever system that lets you change the seat height with the flick of a switch.

In addition to saddle height, the seatpost provides other ways to adjust the fit of the bike. Situated on the top of the seatpost is the saddle clamp. The saddle clamp holds on tightly to the part of the saddle called the rails. Usually, the rails are long enough to provide some forward-backward adjustment to the position of the saddle, allowing you to make fine adjustments to the horizontal position of your body in relation to the pedals and handlebars.

The saddle clamp also lets you adjust the tilt of your saddle. Most people prefer a perfectly level saddle, but some are more comfortable with a slight "nose up" or "nose down" tilt to their saddle. Some saddle clamps provide a smooth, continuous range of tilt adjustment; these are called micro-adjust clamps. Conversely, some saddle clamps have a series of ridges that give discreet "clicks" for each incremental adjustment. The incremental clamps are not able to fine-tune the position as well as micro-adjust clamps, but typically they are less prone to slippage under heavy loads. The best micro-adjust saddle clamps use a two-bolt mechanism, with the bolts pulling in opposite directions from the front and rear of the seatpost (see the photo of the Thomson seatpost, above left). Two-bolt saddle clamps won't slip; their only downside is slightly more complicated installation.

On some lower-end bicycles, the seatpost and saddle clamp are separate pieces. These seatposts are referred to as pillar, or simply "straight" seatposts. The saddle clamp is usually a steel assembly held together with two nuts on the left and right sides (see photo at right).
A suspension seatpost provides its own internal shock absorbers, usually a spring, air, or a squishy grade of rubber or plastic, to help smooth out rough streets and trails. These seatposts come stock on many hybrid and comfort bikes, but are available as an upgrade option for most other bikes. You do pay somewhat of a penalty with added weight of a suspension seatpost, however.
There are two measurements required when selecting a seatpost for a specific bicycle. The most critical is the seatpost diameter. This is usually something in the range of 24mm through 35mm. There is no "standard" diameter, although manufacturers have settled on a couple of most common sizes. 27.2mm has become kind of the "unofficial" standard for many bikes, although some road bikes and many heavy-duty mountian bikes today use a 31.6mm diameter. Your seatpost diameter must match the inside diameter of your bike frame's seat tube exactly; even a 0.1mm difference will mean that the seatpost won't fit, or will not stay in place. All Surly Bikes frames use a 27.2mm seatpost, except for the Instigator, which uses 29.4mm.
You must select the seatpost clamp size to match the outer diameter of the bike frame's seat tube. Even if two different frames both take a 27.2mm seatpost, the size of seat post clamp needed may be different based on differences in the thickness of the material in the frame's seat tube.
The other seatpost measurement is overall length. Most road bikes can get by with a seatpost length of about 250mm. However, mountain bikes usually have more standover clearance over the frame's top tube, so a longer seatpost is required. Usually 350mm is sufficient, but 410mm is another common size that is often needed. You want your seatpost to be long enough so that there is enough of the seatpost extending down inside the frame's seat tube to provide plenty of grip on the seat post. If too much seatpost is extending outside the frame without enough inside the frame, the leverage generated by your weight on the seatpost could snap it in half or rip it right out of the frame. I'll leave the gruesome consequences of that scenario up to your imagination...
After wheels, derailleurs, and shifters, the seatpost is one of the components that cycling enthusiasts often upgrade on their bikes when they want to shave a little weight and/or provide their bike with some more "bling." Higher-end, lower-weight grades of aluminum seatposts are available, and of course, carbon fiber is always another option. You should always consider the intended use when selecting a seatpost; carbon fiber works well for most road bikes and some non-extreme cross-country mountain biking, but is not recommended for freeride or downhill mountain bikes, or other extreme-conditions riding.

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