From the pages of Velo: Getting the most from your post
Editor’s Note: Over the final days of 2012, we’ll be featuring our favorite 12 stories of the year, as voted by the Velo edit staff. First up is Lennard Zinn’s VeloLab seatpost test, which first appeared in the June 2012 issue.
Getting the Most from your Post
Being in the saddle for hours, particularly on bumpy pavement or dirt roads, can be a real pain in the ass. Literally.
Altering the saddle, body position, tire pressure, frame material and design can all reduce this, but what role does the seatpost play in your comfort, or lack thereof? We set out to answer this question by testing 14 seatposts of different materials, setback and design philosophy. At Boulder’s Microbac accredited test lab, we measured vibration damping over small bumps with an accelerometer, and we measured linear deflection (flex) under body weight in two dimensions.
The Test Subjects
We tested carbon, aluminum and titanium seatposts. Some were straight (having zero offset), and some had setback (measured in millimeters relative to the axis of the seatpost). All of the straight models were paired with a setback version of the same make and model to compare the effect of setback on bump-damping performance. Additionally, two of the setback seatposts (Ritchey WCS) were of the same design, one being carbon and one being aluminum, to compare the effect of construction material on vibration and bump performance. Two of the carbon setback seatposts had design features specifically intended to smooth the ride (Specialized FACT with Zertz elastomer inserts and Cannondale Flash with SAVE flattened section), and one of the aluminum setback seatposts (Cane Creek Thudbuster/ST) had a parallelogram suspension design with an elastomer spring in it. All of the seatposts measured 27.2mm in diameter.
We tested the seatposts with each saddle mounted in exactly the same position relative to the cranks and handlebar on the bike. We used three new Fi’zi:k Antares saddles with tubular steel k’ium rails in order to speed setup of successive tests while ensuring accuracy.
Deflection and Damping: A Quick Lesson
Deflection, or flex, and damping, or vibration absorption, are both aspects of suspension. Flex is a static quantity based on load. With a rider’s weight applied, a seatpost flexes so that the saddle moves in an arc, which we measured on both X and Y (vertical and horizontal) axes. The bike’s fork was mounted to a steel test table and the rear tire, pumped to 120psi, rested on top of the table. We used dial indicators to measure the postion of the saddle with and without a rider; no additional force was applied; the difference between the two measurements is the amount of flex.
Damping is related to the speed of movement, or how quickly the post flexes. When riding, the amount the seatpost and saddle are flexed has little relevance over the small bumps on a rough road, but vibration damping insulates the rider from the harshness of the road. On a completely smooth surface, the stiffest seatpost would be most efficient; it would move the least in response to pedaling motion. When hitting big bumps, though, the total amount the seatpost and saddle can flex will protect the rider from a potentially damaging jolt, while the damping decreases how much it flexes back and forth after the hit.
Damping was measured while riding a set of rollers with 1/4-inch tall ridges, welded longitudinally along the front roller and the forward of the two rear rollers. This produced a high-frequency vibration similar to riding fast on a bumpy road. An accelerometer mounted to the saddle rail took 1,000 readings per second (1,000 Hz) and measured the magnitude of the up-and-down acceleration of the saddle rail, which was then converted into a G-force measurement; 1 G is equal to the acceleration due to gravity. We measured for 30 seconds, garnering a total of 30,000 data points per vibration test for each seatpost. A lower number of Gs for any given seatpost would indicate more vibration damping by the seatpost and saddle, and therefore less transfer to the rider.
The winners in the vibration damping test, and the only ones under 0.6 G of acceleration, were the FSA K-Force Light SB25 carbon seatpost and the Specialized S-Works FACT Carbon post. It is unclear why the FSA was so effective at damping vibration, but perhaps the blocky carbon structure at the clamp is able to effectively soak up vibrations. The Specialized seatpost, originally made for the Roubaix road bike, is specifically designed to damp vibration. The active ingredient is an elastomer bumper pressed into the top of the post; based on this test, it does its job well.
Coming in a close third, at 0.616 G, was the Cane Creek Thudbuster/ST. We included this mountain-bike suspension post to see how it stacked up against more traditional designs. While it does perform well in the vibration test, one could get superior performance (especially over the kind of small bumps encountered regularly on the road) with a considerable weight savings by using the FSA or the Specialized post.
The poorest performer in this test, with more than twice the amount of Gs as the top three performers, was the Ritchey WCS carbon straight post. Second to last, and the only other post measuring over a full G of acceleration on these small, rapid impacts was the Thomson Masterpiece straight post.
How Seatpost Material Affects Vibration
The Ritchey WCS setback seatposts in carbon and aluminum share the same design and are both very lightweight relative to others in their class. When it comes to vibration damping, the carbon version outshines its aluminum cousin, albeit not by a huge margin—less than 0.1 G. Bottom Line: Carbon seatposts absorb road vibration better. Five of the top six posts in this test were carbon; the only aluminum post in the top six is built with pivots and an elastomer.
Damping: Setback vs. Straight
We set the saddle in the identical position (height, fore-aft, and tilt) with every seatpost. So that means that on straight seatposts, the saddle was shoved most of the way back on its rails, and on setback seatposts, the saddle was shoved most of the way forward on its rails. And we were measuring both variables (damping and flex) at the saddle, so the saddle plays a part in all of our tests, just as it does for you when you’re out riding. Our test showed that having the saddle pushed back on a straight seatpost resulted in the rider being bounced around more than on a setback seatpost with the saddle pushed forward.
With the Moots Cinch titanium posts and the Zipp Service Course SL aluminum posts, the performance difference between the straight seatpost and the setback version of the same model was minimal. The difference in vibration damping performance of the Ritchey WCS carbon seatposts was huge, however. The vibration passed on to the rider by the zero-setback Ritchey WCS carbon seatpost was nearly double that of its 25mm setback sibling. Bottom Line: Setback posts offer more vibration damping than straight posts; how much more will vary by manufacturer.
Damping: Seatposts with Suspension Features
The Specialized S-Works FACT Carbon seatpost almost won the overall test, with the Cane Creek Thudbuster/ST and Cannondale SAVE both coming in the top 1/3 overall. Bottom Line: Suspension can save your ass. Of the top five seatposts for vibration damping, three of them incorporate suspension designs.
As you might expect, the Cane Creek Thudbuster/ST far exceeded the flex, both horizontally and vertically, of any of the other seatposts. The model tested was the ST (short travel) version with 33mm of travel. Just by sitting on it, the saddle moved down over half an inch and back almost 0.8 inches. This probably explains the Thudbuster/ST’s widespread usage as a stoker seatpost on road tandems. While it offers little advantage over much lighter posts on high-frequency vibrations, it takes out the big bumps that the stoker cannot see.
Interestingly, the seatpost with the worst high-frequency vibration performance, the Ritchey WCS Carbon straight post, had the second-highest vertical flex reading, behind only the Thudbuster/ST. Apparently, since it let the saddle rails move so far, the rails were less inhibited from passing on the vibration to the rider. Strangely, its sister post, the Ritchey WCS Carbon with 25mm of setback, had the next highest flex — clearly due in large part to the seatpost and not just the seat rails, because the saddle was pushed most of the way forward. Even stranger, both Ritchey carbon posts exhibited more saddle flex than the Cannondale SAVE post with the flattened section specifically designed to flex.
Due to the fact that we measured flex at the tail of the saddle, the seatposts with the lowest flex numbers were all setback seatposts: the Thomson Masterpiece (16mm setback), the Zipp Service Course SL aluminum (20mm setback), and the Moots Cinch titanium (19mm setback). Breaking up the run of setback posts, the Moots Cinch titanium straight post just barely took the next spot ahead of the FSA K-Force Light SB25 carbon (25mm setback). The stiffest seatpost, and the only one to register under 0.15 inch flex in either direction (and it did it in both) was the Thomson Masterpiece setback, while the Zipp Service Course SL aluminum setback was the next stiffest and the only other one under 0.16 inch flex in either direction (and it did it in both).
How Seatpost Material Affects Saddle Flex
We tested only one model in both aluminum and carbon, the Ritchey WCS. Comparing the two — both with 25mm setback — we find that the flex of the carbon model is greater in both the horizontal and vertical directions than that of the aluminum one. Bottom Line: Carbon posts deflect more than similar aluminum posts, which is good for big hits.
Flex: Setback vs. Straight
In all cases except one, both the vertical and horizontal flex on the setback posts were less than on the straight posts of the same make and model. That one exception was the Ritchey WCS Carbon: The vertical flex with the straight WCS post was indeed greater than that of the setback version, but the horizontal flex of the straight WCS was slightly less than that of the setback WCS, breaking an otherwise straight flush of straight over setback. Perhaps that is due to the superlight Ritchey WCS Carbon setback post flexing just below the head, allowing it to move back 0.01 inch more under the rider’s weight. Bottom Line: If you’re concerned about maximizing seated pedaling efficiency, consider a setback post.
Flex: Seatposts with Suspension Features
As noted and expected, the flex of the Cane Creek Thudbuster/ST greatly exceeded that of all of the other seatposts. It is the only one in this test with pivots; it is designed to move a long way while others can only flex along their length. The movement of the Specialized post with the Zertz elastomer plug is quite modest; it finished in fifth in horizontal movement and seventh in vertical flex. The Cannondale Flash post finished sixth in horizontal and fourth in vertical flex. Both Ritchey carbon posts still had more flex than the Cannondale SAVE. Bottom Line: The Specialized and Cannondale seatposts excel at damping vibration, however, the designs do not compromise pedaling efficiency.
Ease of Saddle Installation and Adjustment
This was not intended to be a test of the ease of setup of the various seatposts, but given that we had to swap and quickly set up saddle positions on 14 seatposts many times over two days of testing, we learned some things about that as well. The Ritchey single-bolt design, once you figured out how to avoid dropping all of the little separate pieces, was by far the easiest and fastest to install and adjust. The other cross-bolt designs — the Moots and the Specialized — were also easy to install and, with more effort to overcome friction in the head, could be rotated to the proper position quickly. The two-bolt design of the rest of the posts, with one ahead and one behind the saddle clamp, allowed precise angular adjustment, but finalizing the riding position took longer. Finally, the pair of two-bolt posts (Cannondale and Cane Creek), in which one of the bolts was a thumb screw, were a significant pain to finalize the saddle tilt adjustment because the Allen socket bolt had to be loosened completely every time you wanted to turn the thumb screw.
In general, when choosing between a straight or setback post (if your frame seat angle doesn’t already dictate which one you must use to achieve your desired position), a setback post will give you greater pedaling efficiency and more high-frequency vibration damping, while a straight post will give you more flex on big bumps, lower weight, and a more jarring ride on high-frequency small bumps.
When choosing between aluminum and carbon seatposts, our tests indicate that the carbon seatpost will offer more vibration damping on high-frequency bumpy surfaces and more flex for big bumps, while also being lighter; but they are more expensive.
All in all, if you don’t want to get pummeled on high-frequency vibrations as much, avoid zero-setback seatposts and gravitate either toward FSA’s K-Force Light SB25 carbon setback post or toward one with some suspension features built into it. If you’re going to be hitting really big bumps, get a Thudbuster. If you can’t accept the weight or the looks of the Thudbuster and still want some suspension on big bumps, go for a Ritchey WCS carbon post or a Cannondale SAVE Carbon. If you want a light, stiff post, get the Thomson Masterpiece setback.
For smooth roads, pick the Thomson Masterpiece setback seatpost. For bigger hits, like on rough cyclocross or on potholes or big pavé stones, we recommend a Ritchey carbon WCS post. For a long road ride, the FSA K-Force Light SB25 post is our pick, thanks to its excellent vibration damping — it is noticeably more comfortable than an oversized aluminum post, especially on long rides, dirt roads, or concrete or asphalt with lots of small cracks and expansion joints.