The sidecar was driven carefully and cautiously on local journeys to get the combination checked for road worthiness.
Next it had to be tested at various speeds and under different conditions to check that it had no unpredictable characteristics.
An industrial park on a Sunday provided some quiet roads for testing.
The motorcycle and sidecar were tested through left and right hand turns.
Stops and starts were also performed including emergency stops.
Right hand turns were solid and predictable with the weight of the motorcycle being supported by the sidecar. If pushed hard the combination would drift rather than tip especially on loose surfaces. This was reassuring; but of course braking when turning could change this situation as could road camber and gradient.
Left hand turns felt more unnatural with the motorcycle unable to lean into the bend and nothing on the outside to stop it tipping over.
Only the weight of the sidecar would stop it tipping in this situation hence the importance of it being 1/3rd the weight of the motorcycle.
Too much speed on left hand bends caused the sidecar to lift and it was useful to find this limit.
It was also discovered that the sidecar wheel lifting was not a imminent crash situation, but that it could be balanced with careful control of the steering.
Braking and accelerating were manageable provided they were done progressively. Emergency stops required steering input to keep the sidecar from slewing too much.
Shortest stopping distances for various speeds were measured to get a feel for braking capability.
The BMW was equip with drum brakes front and rear with twin leading shoes at the front. Not as progressive at discs but you could lock both wheels under heavy braking.
The test was performed on a dry road on new tyres and without a passenger.
From this testing the only concern was the change in "poise" of the combination when switching between left and right turns (and visa versa). This felt unsettling; and although everything worked fine, it felt like the nose was going to dig in during the right hand turns.
The conclusion was that the suspension was a little soft and that stiffer springs at the back of the bike and on the sidecar wheel would reduce this shift.
The sidecar spring was removed from the shock absorber unit and placed on a basic spring tester.
The tester was loaded with weights to find the deflection for a given force.
Each disc brake weighed 8.3Kg and the distance from the fulcrum to the added weight was twice the distance of the spring to the pivot.
Calculations were as follows.
Force applied : 4 x 8.3kg x 9.81 =325N
Multiplied by 2 for the mechanical advantage of the lever = 650N
Original Spring length = 170mm
Compressed length = 135mm
Difference = 35mm or 0.035m
Hooke's law states F=kx
F = Force, k = Spring constant and x = spring displacement
650N = k x 0.035
so
k = 18500 or a spring rate or 1.88kg/mm
To upgrade the motorcycle, some "Sidecar Ready" Hagon shock absorbers were ordered from Motorworks.
The new Hagon shocks were also put on the spring tester to be compared to the standard shocks. This would indicate what the increase in spring rate was and give a feel for what might be needed on the sidecar.
Curve 1 = lowest preload
Curve 3 = maximum preload.
The graph indicated that even with the highest pre-load, the original springs were still softer than the new Hagon units.
On the road the new shocks felt immediately better. It was noticeable from the outset that there was less movement at the back under accelerating and braking. Likewise cornering was significantly more sure footed, even with the existing (soft) sidecar shock absorber.
To look at options for the sidecar spring contact was make with Mike at Shock Factory.
Mike explained that spring rate could probably be increased without the need to also increase damping, but that there was a limit to how far this could go before the damper became ineffective against the spring.
The first step was to measure "wheel spring rate", which referred to the performance of the shock absorber when actually mounted on the sidecar. This would take into account leverage of the swinging arm and other subtleties of the shock application that would determine the performance of the spring - a more accurate measurement than just measuring the spring on it's own.
Measurement on the sidecar showed that a load of 5.5kgs gave a spring deflection of 23mm.
This equated to a spring rate of 2.53kg/mm of spring compression
For a quick comparison, a spring from one of the original BMW rear shock was cut and fitted to the sidecar shock absorber using some adapter plates. This gave a slightly springy ride because the damping was not matched to the spring but certainly improved the "feel" of the sidecar in the corners.
Comparing the original spring and the new cut down spring the results were as follows.
Original shock spring rate = 1.55kg/mm
BMW sprung shock rate = 2.62kg/mm
So the new "hybrid" sidecar shock absorber was ~1.5 times stiffer than the old one. As this was felt to be a little stiff for the damper, a rate of 2.3Kg/mm was specified.
Mike at the Shock Factory put me in touch with Tony at South Yorkshire Springs LTD.
Tony advised that a spring the same size as the original sidecar spring but with a rate of 2.3Kg/mm would not have much travel - only about 50mm.
Better to go with a larger ID of 43mm and get 88mm of travel.
So the final specification (in imperial for manufacturing purposes) was :
Length = 6.7"
ID = 1.7"
Rate 130lb/in.
The new larger spring (shown at the bottom here) was made to fit with some aluminium adaptor discs.
The result was an improved ride all round, with less risk of bottoming out with a heavy passenger, and less nose diving on right hand bends.
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