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tmoritz
06-11-11, 09:57 PM
I was thinking about building a variation on the Bilenky Viewpoint hybrid tandem. This would be a tandem touring bike. Since the stoker rides up front over the 20" front wheel I want to make sure headset bearings can handle the loads and that I have sufficient distance between the bearings. Anybody have guidelines for shock loads on a small diameter bicycle wheel? Are there industry practices for estimating the design maximum bending moment generated by the front wheel/fork assembly when under shock loads?

ftwelder
06-15-11, 05:13 AM
Most people use a vast existing knowledge base for frame engineering. I have looked at some testing standards and they fall way short of what I would consider reasonable strength.

Mark Kelly
06-15-11, 06:15 AM
Yves Champoux has some interesting measurements of vertical acceleration from bumps in his 2007 Paper "Bicycle Structural Dynamic" (http://www.sandv.com/downloads/0707cham.pdf) He measured vertical shock forces of ~800 and ~1100 N for front and rear wheels traversing a bump. I can't find any reference to the size of the bump but it appeared to be a piece of 12mm dowel. These loads would be of the order of twice the static load in the tests and since the accelerometers show the static load as a zero baseline you'd need to add them for total load eg 1200 N and 1600 N respectively.

This would probably make a reasonable base for estimation of repetitive load for the bearing. The maximal load could be calculated by considering the usual 15% rule for tyre compression: turning that on its head, the largest load before rim damage will be around 7 times the static load.

The maximal bending moment could be calculated by considering the shock load as vertical and mutliplying be the moment arm which will be the horizontal disatance between the hub centre and the headset centre and thus equal to axle to crown distance x cos (headtube angle) + rake x sin (headtube angle).

unterhausen
06-15-11, 08:00 AM
All else being equal, the moment on the headset bearings is smaller with a smaller wheel. It doesn't seem like a good idea to have a very slack head angle, because the wheel flop would be a pain. Of course, all things aren't equal because the front wheel being under the front rider will increase the load a little over a standard tandem. You'd have to come up with a design to calculate how much.