Saturday, September 12, 2015

Armstrong Lever Dampers - an explanation

I recently found an article about the Armstrong Lever Action Shock Absorber as fitted to the Triumph Vitesse and other Triumph cars. The article was on the North American Singer Owners Club website.

The Armstrong Lever Action Shock Absorber

The Armstrong New Super Double-Acting Self-Regulating Hydraulic Shock Absorber is of the Vertical Cylinder Type. All working parts are submerged in oil.


CONSTRUCTION

The body A is a zinc alloy die casting and bolts directly on to the frame of the car, the two cylinders B and C being connected by passages E and F. The double crank G and arm H are a force fit on serrated portions of spindle I, which rotates in the body A on generous double bearings. Connecting rods J connect the crank G to pistons K to which non-return recuperating valves N are fitted. The arm H is connected to the axle of the car link L.

HOW IT WORKS

As the axle moves to and from the car frame, so the pistons move in and out of their respective cylinders pumping oil from one to the other. The interior of the body is filled with oil to within 3/8" from top of cover, any shortage of oil beneath the pistons is instantly made good though the recuperating valves N.

CAM VALVE (SELF REGULATING)

As the car axle moves to and from the car frame the oil pumped from the compression cylinder B to rebound cylinder C or cylinders C to B, and has to pass between the taper ended valve P and scew Q, which is adjustable to offer any desired resistance to the action of the axle.

As will be seen from the drawing the upper end of the valve F is held in contact with the cam on spindle I, which means that on fairly good roads the valve P is in its lightest setting and shock absorber gives a normal resistance--just enough--yet not too much to produce harshness, but when bad roads are encountered the spindle I oscillates and the contour of the cam depresses the tapered end of valve P further into the central port of screw Q and thereby increases the resistance of the shock absorber.

The automatic, variable resistance obtained from the cam movement makes this shock absorber entirely self-regulating, the resistance automatically increasing as required thus enabling the car to pass over bad with comfort equal to good roads.

SCREW TAPER VALVE

As the axle moves towards the frame the oil is pumped from the cylinder B to cylinder C, but as it has to pass the spring loaded valve R a resistance governed by the tension on the spring is offered to the movement of the axle.

On the return or rebound stroke the oil is pumped from cylinder C to cylinder B, and as the ball valve only opens in one direction the oil must now find its way to cylinder B past the taper screw S, which is adjustable to offer any desired resistance to the rebound of the car spring.

ADJUSTMENT OF CAM VALVE

Both rebound and compression are controlled by screw Q, to tighten rebound and compression slack nut T, taking care that the screw Q does not turn. Turn screw Q half a turn inwards and lock up the nut, taking care the the screw Q does not turn.

To reduce the resistance follow the above instructions, but turn screw outwards.

SCREW TAPER VALVE ADJUSTMENT

Both rebound and compression are controlled by screw S.
The same instructions apply as for cam valve, but refer to screw S and nut U.

Note.--Shock absorbers adjusted too lightly make your car feel harsh and hard; we recommend slacking off first in case of doubt.
These shock absorbers will only work satisfactorily on Armstrong Super Shock Absorber Oil. Examine your shock absorber oil lever regularly. The correct level is 3/8" from top of cover.

Double Note: I personally have found these shocks to be quite reliable and I have been able to resurect several of these by just general cleaning and fresh oil. You should try that first prior to sending it out for rebuilding.
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Adjustable dampers

I had heard these existed and have now found a picture. These can be adjusted on the car without taking them apart.
MG works adjustable Armstrong lever arm dampers
MG works adjustable Armstrong lever dampers
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Non adjustable dampers

Adjusting the units: There is a large hex head 'nut' at the bottom of the units, this can be unscrewed to remove the damper assembly, take care of the 'O' ring. As below:

The damper assembly looks as below when assembled:
Armstrong lever arm damper assembly
Armstrong lever arm damper assembly
The little washers between the large diameter coil spring and the valve body set the preload on the BUMP damper valve. So add more washers to get more BUMP damping, try one or two washers at a time, you can easily find washers of similar id and od, if not file some.The little nut that compresses the smaller inner coil spring controls the REBOUND damping. This nut is soldered on its thread to stop it moving, easy to unsolder and resolder. Screw this nut in increases the REBOUND damping. Try one turn at a time, it is quite sensitive.
Do both together, clamp them together in a vice, then by hand pressure on the end of the levers you can compare their resistances to get them both similar.
Refit to car and test. Hours of fun there then!

Rebound valve adjustment starting point ~ compress back of the car and release, eg kneel on bumper and get off the suspension should return the car to static ride height and no more. It should not bounce beyond static ride height.

Bump damping adjustment ~ although it is ideal to set this low to get minimum vertical upward acceleration that you feel through your seat/spine. Ideally set bump damping as low as you can to prevent bouncing after a series of bumps or wallowing after quick steering inputs.

Borrowed from a Morgan website.
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Lever Arm Oil

Armstrong sold their own special grade of oil specially for lever arm dampers. What should be used now?

Motor bike shock absorber oil seems to be the recommended substitute. It is available in several different viscosities. 20 as used in older bikes is what can be used. Modern lighter grades will not be stiff enough. Stiffer oil such as 30 may damage the seals.

Some people have successfully used trolley jack oil.  However it has been said such oil may suffer from foaming when dampers work hard and quickly - just when you want the damping action.

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Simplied later design


Original Armstrong lever arm damper assembly with o-ring
Later simplified Armstrong lever arm damper assembly without o-ring

According to Alex Pringle on the TR Forum: The old Armstrong Company merged with James Lyon in the early 1970s, and considerable change resulted.

Lever arm shock absorbers, as we think of them, had numerous and varied commercial and industrial applications . . . . automatic door closers being the most often noticed. The old Armstrong company thought of all of these items as appropriate for reconditioning, whereas the new company preferred to simplify (and cheapen) designs - and to abandon reconditioning in favour of straight replacement.

Lever arm shocks manufactured post 1974 or thereabouts were of the simplified variety, until the eventual cessation of mainstream production, towards the end of the decade.
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