Darryl.Richman's picture

I thought I would just make the comparison between the JD and the R52/R62 (R62 shown) a little stronger:

I tried to get these two images correct to scale, since the bikes have nearly identical wheelbase numbers (55.6" for the JD, 1400mm/55.1" for the R62). The JD is a 1928 model, the first with a front brake.

At every point, the design choices by the two companies are interesting. For example, let's look at the front forks. Harley has a leading link design, with the heavily valenced fender moving with the suspension. Its larger tires add comfort to the ride as well as probably making the bike work better on the unpaved roads of the era. BMW's trailing link design mounts the fender on the fork, so it doesn't move with the suspension, and sufficient clearance must be provided under the fender for the full travel of the wheel. OTOH, mounting the fender on the fork reduces unsprung weight and the mass that the front spring has to control.

Or, look at the 3 speed transmissions. The BMW unit is huge relative to the Harley box (which is directly behind the starter pedal and under the battery box in the image above). Even accounting for the fact that much of the visible size of the BMW transmission is in fact a toolbox (the JD's toolbox is mounted on the forks under the lamp and the horn), the long shafts that must run longitudinally make the unit take up a lot of space. All of that length runs across the frame on the chain driven Harley.

In spite of the visually large castings on the BMW, it's 70+ lb. weight advantage is due in part to the extensive use of aluminum. BMW's double-decker heads not only transfer away a lot more heat than the unit cast Harley cylinders, but they are much lighter.


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