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Tech: Researching how we sit on bicycles

Lennard Zinn /
Blind saddle identification test: I’m riding with basketball dribble goggles to prevent being able to look down and getting a hint about which saddle I’m riding on (the saddle was covered by a box until I swung my leg over it to prevent seeing it). Photo: Lennard Zinn | VeloNews.com

“Basic research is what I am doing when I don’t know what I am doing.” — Wernher von Braun

Fizik approached University of Colorado associate professor of integrative physiology Rodger Kram, Ph.D. about finding a foolproof way to predict which saddle will provide maximum comfort to a given rider, thinking that the project might take a few months. Almost three years down the road, Fizik and Kram’s laboratory are instead deeply engaged in researching much more fundamental aspects of the connection between bicycle and rider. And they have discovered that many experienced riders cannot tell the difference between different saddles in a blind test (fortunately, BikeRadar’s Ben Delaney and I were able to).

To predict a given rider’s comfort on a certain saddle, one of the first questions is, “How much of a rider’s weight is on the saddle?” It’s clear that this varies with how hard the rider pushes on the pedals, but a search of the literature came up with no studies answering this question. So, that became the subject of the first study that Kram’s lab and Fizik undertook. Thus far, the collaboration has resulted in six completed studies and three currently in process, all of which are designed to systematically quantify the Bicycle-Rider Interface Forces (BRIF).

The CU Locomotion Lab is generally devoted to the study of the human body in motion and primarily studies running — Kram himself is an avid runner who has not missed a single day of running in almost a decade. While the lab has equipment like the world’s steepest treadmill and a steeplechase hurdle wired with force sensors, it also has lots of bicycle contraptions gathering dust in the adjacent storage hallway. Among Kram’s former graduate students are Todd Carver, founder of ReTul bicycle fitting systems, and Allen Lim, founder of Skratch Labs and coach of many top American pro riders (as well as the man who connected Fizik with Kram); both Carver and Lim conducted bicycle research for their graduate degrees in the Locomotion Lab.

When Fizik product manager Luca Viano and marketing manager Alberto Fonte began discussions with him, any initial reluctance on Dr. Kram’s part fell away when it became clear that studying how the weight on the saddle varies with power output would be breaking new ground; it would become a great way for him and his graduate students to answer a fundamental question.

“I get lots of requests from companies to test their products,” says Kram, “but we won’t do that. We are interested in much broader questions.”

And even though Viano and Fonte may have intended to get some answers specific to Fizik, the study of fundamental relationships between the bicycle and its rider piqued their curiosity, too. Fizik, which makes shoes, pedals, handlebars, stems, seatposts, grips, and handlebar tape along with its sister brands Crank Brothers and Brooks (all subsidiaries of Selle Royal, one of the world’s largest saddle makers), has the tagline that it “owns the surfaces where the human body connects with the machine.”

Power Study: How does power output affect the bicycle-rider interface forces (BRIF) at the saddle, stem, and cranks during cycling with a fixed cadence?

In order to measure the forces at the three points of contact with the rider at specific power outputs, lab students retooled a ReTul Muve stationary adjustable-fit bike with multi-channel force transducers under the saddle, handlebar, and bottom bracket, and they installed a PowerTap rear wheel. The vertical forces, of course, have to add up to 100 percent of the rider’s weight, while the horizontal forces have to add to zero (if not, the rider would make net forward, backward, or lateral movement).

Indeed, as power went up from 1 watt per kilogram body weight of the rider to 4 W/kg, the weight on the saddle dropped, as did the weight on the stem; the weight on the bottom bracket went up accordingly. At 2 W/kg, the BRIF are approximately 40/15/45 percent of body weight on the saddle, stem, and bottom bracket respectively; did you expect that more weight would be on the feet than on the butt or hands? And with each increase of 1 W/kg, weight on the saddle drops by approximately 3 percent, weight on the stem drops by approximately 1 percent, and weight on the bottom bracket increases by approximately 5 percent. Horizontal forces are: 20 percent of the rider’s weight pushing back on the saddle, 12 percent pushing forward on the bar, and 8 percent pushing forward on the bottom bracket.

Cadence Study: How does cadence affect the BRIF at the saddle, stem, and bottom bracket at a moderate, fixed average power output of 2 watts per kilogram of body weight?

One would expect the opposite effect from cadence as with power output, and, indeed, at a fixed power output of 2 W/kg with cadence increasing from 60 to 110 RPM, saddle vertical forces weakly increased, bottom bracket vertical forces weakly decreased, and stem vertical forces stayed nearly constant.

Hand Position Study: What effects do the three standard hand positions (tops, hoods, drops) have on the BRIF?

One would expect that as the rider shifts his hands from the tops of the bars to the hoods to the drops, vertical forces on the bars would increase, offset by a decrease in vertical forces on the saddle. However, the surprising result was that the lowest amount of weight was carried on the bars when the rider’s hands were on the hoods, and the higher weight carried on the bars when in the drops was close to the same as the weight on the bars when on the tops! Who’d a thunk it?

Road vs. Triathlon Position and Saddle Study: How do saddle types and rider position (road vs. triathlon) affect the bicycle-rider interface forces (BRIF) at standardized power and cadence (2 W/kg, 90 RPM).

Comparing the BRIF on the saddle, bottom bracket, and stem in a road position with a standard saddle and with a noseless triathlon saddle showed minimal difference in vertical forces (no difference at the saddle, a tiny bit more on the stem and a bit less on the bottom bracket with the noseless triathlon saddle than with the standard saddle).

– Comparing the BRIF in the road position with a road saddle with that using a triathlon position and a noseless triathlon saddle showed much more weight on the stem with the triathlon setup and much less weight on the saddle, with also a bit less weight on the bottom bracket.

Handlebar Stack and Reach Study: What effects will changing the handlebar stack height and reach by +/- 2cm and 4cm have on the BRIF at the saddle, stem and bottom bracket?

One would think that raising the stem from -4cm to -2cm to zero to +2cm and +4cm would result in a steady decrease in weight on the handlebar matched by the same increase in weight on the saddle. However, it didn’t come out that way; indeed, the weight on the stem dropped steadily, but it was met by an increase in weight on the bottom bracket, not on the saddle; the weight on the saddle stayed approximately constant.

Contrary to what one might think, Kram’s students found that as reach to the handlebar increased from -2cm to zero to +2cm and +4cm, saddle vertical forces increased, and stem vertical forces decreased. This clearly shows that using force transducers (rather than using visual means, plumb bobs, goniometers, etc.) would not be a good way to determine bike fit.

Front-Rear Wheel Rider Weight Distribution: How does the weight distribution compare between amateur riders using various fit protocols and riders on two professional cycling teams?

Amateur riders in this study were in one of three categories: self-fit, fit using Specialized Body Geometry (BG) method, and fit using the ReTul system. These were compared with the fore-aft distribution on the two wheels of the riders on BMC Racing and Garmin-Sharp during the 2014 USA Pro Challenge.

Rather than having the most aggressive fits (i.e., the most weight on the front wheel) of the riders in this test, the pros were in the middle.

Interestingly, the 14 pro riders were tightly clustered around 60 percent of the weight on the rear wheel and 40 percent on the front (average: 59.6% rear, 40.4% front). This was within a couple of percent of that of the self-fit amateur riders (61.5% rear/38.5% front). Interestingly, the ReTul fits were more aggressive (55.3% rear/44.7% front), while the BG fits were the least aggressive (67.1% rear/32.9% front) in terms of weight distribution over the wheels.

Center of Pressure (CoP) Location on Saddle Study (ongoing): As the saddle fore-aft position is varied, how will CoP Location change in relation to the saddle?

The interesting preliminary results of this test are that, as the saddle is moved back and forth by very significant amounts while leaving the handlebar position unchanged, the center of pressure on the saddle remains unchanged. This is a surprise, as one would instead expect that the rider’s butt might stay in the same place, dictated by reach to the bar, as the saddle moves back and forth, so the pressure would move more onto the nose as the saddle is shoved back, and vice versa. Instead, it shows that the rider moves his butt back and forth with the saddle to always stay in the same place on the saddle.

For this test, the saddle transducer is measuring the torque on the saddle front-to-back and side-to-side as the rider pedals at a constant 2W/kg (determined in study #1 as the ideal power output at which to perform future tests). The center of pressure is the point on the saddle where the location of pressure averages out to over one pedal stroke from all of the various torques on the saddle in three dimensions. This is very distinct for the pressure-mapping systems that actually map the pressure gradient on the top of the saddle by means of a pressure-sensing pad.

Saddle Discrimination Ability Study (ongoing): Are individual riders able to discern differences between saddles during stationary bicycle riding?

Riders left the room while lab students either changed or did not change between a pair of saddles. They covered it with a box before the rider came back into the room; the box stayed on until the rider was clipped in and about to sit down on the saddle. The rider wore glasses that prevented looking down and pedaled for five minutes at 2W/kg on each saddle. Three tests were done, and the saddle was switched either once or twice. Even though all riders (all males) in this test and in all of these tests rode at least 100 miles/week all year, many riders could not distinguish between saddles (one saddle even had a cutout and one didn’t) and even couldn’t tell if the saddle had been switched out or not. Former VeloNews editor-in-chief Ben Delaney and I were both able to distinguish between a Specialized Romin and a Fizik Antares. Whew!

Subjective Rider Comfort Study (Ongoing): Are there individual anatomical measures (e.g. leg length, pelvic width, spine flexibility) that can predict saddle comfort?

The students believe they will be able to identify anatomical variables that correlate with the perceived saddle discomfort (a saddle discomfort scale will be used to rate them, rather than a saddle comfort scale).

It seems quite amazing that no studies have been conducted in the past on these topics. This basic research funded by Fizik and undertaken by CU graduate students promises to continue to throw off interesting results as well as useful information for manufacturers working to improve their products.

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