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Repair, Restoration and Modification of BMW 2V Boxer a.k.a. Airheads

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	Front Fork Spring Rates

Front Fork Spring Rate Comparison

So, here I was: my 1982 G/S was feeling very soft in the front end, especially with that 9gal tank full of gas. I was hoping that a set of progressive fork springs would take care of it, so I bought a Works Performance Fork Tuning Kit, part no. 250X. Installed it using the medium size pre-load spacer and found it very stiff. Shortened the PVC tube by 1/2 in and included the shortest pre-load spacer: the front end still felt very stiff and was not well balanced with the rear stock shock. Changing the pre-load spacer did not change much in the feel. I ran about 5000km before just recently, I attacked the problem again.

First, I researched the subject and found two simple formulas that I could use to make a qualitative assessment where I was with my front suspension in comparison. Mind you, I am not a professional, just an "average Joe with an non-calibrated buttmeter" but I was up for a little science experiment. For this, I used data from

a 1983 BMW R80RT stock front fork spring and
a 1982 BMW R80G/S stock front fork spring and
the Works Performance Tuning Kit 250X

The BMW fork springs have two springrates, but are very soft and bottomed out easily on my bike. I developed the idea of combining parts of the BMW stock spring with the Works spring and did some measurements and calculations:

Please note that the only uncertainty in those calculations is the material of the spring, which is a constant in the calculation. Also, since none of the springs are progressively wound, the deflection vs force function is a linear function in first approximation. So I consider those above values only correct in their relation to each other, not absolutely.

Those numbers really did not tell me anything. According to my "calculations"

the 1982 BMW R80G/S front springs bottom out at applied 285lbs force @ 9.33in deflection
the 1983 BMW R80RT front springs bottom out at applied 277.5lbs force @ 9.59in deflection
the Works Performance Tuning Kit 250X front spring bottoms out at applied 337.5lbs force @ 7.46in deflection and
"my solution" using a combination of three springs bottoms out at applied 337.5lbs force @ 9.23in deflection

My solution by the way was cutting the G/S fork springs and utilise both spring rates together with the Works soft spring. With the calculations in mind, it should be softer from the start, but stiffen up nicely, but not as "bad" as the pure Works solution.

It actually worked out very nicely! The balance between front and rear suspension is much better now, the front fork action on gravel is much more defined. The bike is unfortunately diving again but without bottoming out. I take that any day over the stiff Works front end.

I will try this combination in Labrador and on the TAT NM/OK later this year and go from there. The next (ultimate) step should that not prove successful, is adding two RaceTech cartridge emulators. But I am not quite there yet.

Front Fork Spring Rates

Front Fork Spring Rate Comparison