![]() ![]() Aluminum: – Carbon fiber stems don’t offer the weight savings that, say, carbon fiber rims do. The tested stems vary from 5 to 10 degrees which should result in deflection variations of up to about 3%, so keep that in mind when comparing two stems of different angles.Ĭarbon Fiber vs. We tested a couple of stems that are available in both 7 and 17 degree angles and found that 17 degree stems deflected an average of 7% less. One other item we wanted to test was the difference in like stems with different angles. While we were able to measure a difference, less than 0.1 mm at 100 pounds, it was not enough to say that there is a meaningful difference, and was likely within margin of error of the testing. Mounting Direction: We also wanted to see if mounting a stem with a rise vs a drop made a difference in the deflection. We selected a few random stems and tested them both with titanium and steel bolts and found no meaningful difference in deflection in stems with one material over the other. defl) / weight) * 1000īolt Material: One question we wanted to answer was if there was a noticeable difference between stems with titanium and steel bolts. Stiffness to Weight Ratio: This was calculated using =((1/avg. The stems were similar enough in length that we did not normalize any stems in this test, but in the future we may test some stems that need to be normalized for a fair comparison. If you’re interested in relative values–for example, if you want to know whether stem A is stiffer than stem B–then these results should be quite useful regardless of the actual length you intend to use.įor this test we used 11cm stems in every case except 2 (Pro Vibe Sprint Carbon and Syntace F119) which are both 10.5cm. Our first pass at a stem test setup measures combined bending and torsional displacement, so it would not be valid to extrapolate to longer or shorter stems, even within the same model, at least if you’re looking for absolute values. While torsional displacement is directly proportional to stem length, bending displacement is proportional to the cube of the length. A composite stems is a different beast, because layups (AKA laminate schedules) can vary significantly with length. Still, normalizing to length within a single stem make and model is valid to a first approximation, at least with metal stems. It’s true that a good engineer will use thinner walls on shorter stems (which see lower stresses) and thicker walls on longer stems (which see higher stresses). In technical terms, this is “normalizing for stem length.” We can do this because torsional displacement is directly proportional to length for a tube of a given cross section. Normalizing for Stem Length: In theory, it’s possible to measure a particular stem of one length and extrapolate its results to longer or shorter versions of the same stem model. This is the same as applying 75 lbf up on the one side of the bar and 75 lbf down on the other the moment at the stem is the same. The pedal is about half as far from the frame centerline as the handlebar is, so it would take a reaction force of 150 lbs at the right-hand bar to counteract the pedaling force. For a sprinting rider, let’s say he is pushing on the left pedal with 300 lbf (in other words, he can squat 600 lbs). ![]() Jason selected a load of 100 pounds because it’s a nice round number, and pretty realistic to what some riders may experience. Loads Tested: While the actual load doesn’t matter because the response of the structure is linear. The measurement was taken at the point of load, again 10 inches from the stem centerline. Each stem was mounted with it’s recommend torque specs and preloaded with 20 pounds of force applied 10 inches from the stem (about half way between a road bar and mtb bar) Once preloaded the equipment was zeroed and another 100 pounds of force was added and a measurement recorded. All stems tested were 11cm with the exception of a few which are not available in those lengths. Testing Method: Each stem was mounted in the fixture and the testing performed 3 times and then averaged. ![]() He adds real value to all of the tests we perform.ĭisclaimer: A lot of typing and numbers have gone into this article and we apologize in advance for any typos, but would warn that the possibility of mistakes is present. Jason has worked for several companies in the bicycle industry and never fails to amaze me with the depth of his cycling related knowledge. Jason’s a mechanical engineer whose graduate work focused on the intersection of composite materials and finite element analysis. We’ve once again brought back our favorite engineer, Jason Krantz.
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