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Suspension

Front suspension

Front suspension
 
Front Strut

Front Suspension

The front suspension of a Scorpion is made up of separate struts on each side.

These struts are sealed and did not originally take modern canister shock absorber inserts.

Front Strut

Rear Suspension

Rear suspension
 
Rear Strut
Rear Suspension
The rear suspension also had independent stuts on each side. These are mounted onto a wishbone arrangement.
Rear Strut

To meet USA laws, the Scorpion suspension was set about one inch higher than that of the Montecarlo. This was acomplished with longer springs (with different load rates), but otherwise the setup is very similar to the series 1 Montecarlo.

Rebuilding Scorpion Struts

The majority of Scorpions will have had or will require work on the suspension struts. Sadly these original pieces are not available new. Short of re-engineering the whole suspension or purchasing a complete kit from the Monte Hospital; one option is to try and rebuild the struts themselves with new seals.

Bernie Benz is quite an expert on struts and has made many modifications to the Lancias and Audis he owns. He wrote this following piece for Lanciana.


One can very successfully rebuild either the original equipment Beta struts (those with removable heads, late units have a rolled-in-place head closure which I have not attempted to disassemble) or the Koni after-market, full strut replacements, for minimal cost and with both life and performance better than new!

In background, lets review basic shock strut principals, design, and failure modes. Apart from being a major component of the suspension geometry, the primary function of the strut is to absorb and dissipate unwanted energy. Bump energy is stored in the spring as it is compressed and this energy is then dissipated in the strut in a controlled rebound release to eliminate wheel oscillation. Thus, the strut applies the damping necessary to control the resonant oscillations of the spring and mass of the suspension; a highly tuned dynamic mechanical mechanism.

Valving: First, the foot valve system, a bi-directional valving system located in the closed bottom of the cylinder, allowing fluid to freely enter the lower cylinder volume from the concentric outer reservoir as the piston moves up (wheel moving down in a "rebound") and prevents reverse flow (below a preset relief pressure) returning from the cylinder to the reservoir upon piston down stroke (wheel moving up in "bounce"). Second, piston internal valving is also a bi-directional valving system, each controlling the differential pressure required for fluid flow from the high to the low pressure side of the moving piston; one valve system for each direction.

That's all there is to valving, but in addition to these pressure operated valving systems which control fluid flow, there are wear-related leakage paths that short circuit the valving systems and degrade damping performance. In MacPherson struts both the piston and the rod bushing, are subject to considerable wear-causing lateral forces, i.e. the reaction forces of acceleration, breaking and cornering. Thus, these are the primary internal wear related failure areas, and are further described as follows:

  • First, there is the piston to cylinder dynamic seal; leakage here affects both bounce and rebound. This seal is a Nylon piston ring (called a wear ring) in all but OEM Scorpion struts, which use a steel piston ring. Second, the top end rod bushing not only guides rod movement and is subject to the fore mentioned lateral acceleration wear forces, but it also must act as a fluid seal for the upper cylinder volume. Wear induced leakage at this point further degrades rebound performance, which is the most important shock parameter. Thus, these two wear-prone areas are the principal cause of strut failure, i.e. when their combined leakage becomes a significant portion of the rebound fluid flow.

    In operation, the combination of the valving and the differential fluid volume displaced by piston motion because of solid rod volume on the upper side, the fluid flow external to the cylinder is always into the cylinder at the bottom. Through the foot valve, and out the top, through the bushing leakage path. Excessive rod bushing leakage also causes another problem. In rebound, excessive fluid flow from the cylinder through the rod bushing clearance and into the seal area will overload the return passages to the reservoir, which, in turn, over pressurizes the rod seal volume and blows the seal, causing the dreaded and obvious external fluid leakage. Believe it or not, the struts were ineffective, "worn out", before the external leakage became evident!

    The Beta OEM strut design differs from the Koni, in several areas. The Beta bushing leakage rod seal area is returned to the reservoir through a tube to a point near the bottom of the reservoir, whereas the Koni design vents the seal area directly through holes into the air space on the top of the reservoir. This difference may allow air entrapment in the Koni working fluid under severe action and also accounts for Koni "leak down" when standing idle, which requires several long "squeaky" strokes to refill, a characteristic of even new Koni's, but not evident in Beta struts. Advantage Beta!

  • A second difference is that the Koni head design allows for a 20% to 40% longer rod bushing, with proportional longer bushing life. Advantage Koni! Koni's adjustable feature alters two piston fluid flow characteristics:

    1. It adjusts a bi-directional piston bypass orifice, thus, when restricted it compensates for increasing piston ring seal leakage due to wear
    2. it also adjusts the cracking pressure of the piston rebound valving to partially compensate for rod bushing wear.

Rebuilding: First, before disassembling the strut, inspect the piston rod. If the rod has been running dry long enough to have worn through the chrome plated finish, replace, don't attempt to rebuild.

Next disassemble the strut, dump the fluid, and clean the parts in solvent. Do not disassemble the foot or piston valving, as these components are not subject to degrading wear.

For rebuilding the Beta struts Lancia has top end rebuild kits available which contain the complete head assembly (head with bushing and drain tube, seal housing with rod seal and scraper, o-ring and nut) for about $30 each. I have rather elected to replace individual worn components to incorporate design improvements for less than $10 each. By rebuilding at the component level one can increase the bushing length by 1/8" or 1/4" (Beta struts), thus increasing bearing area comparable to Koni, maybe doubling wear and leakage life. The bushings are of the Teflon/powdered bronze lined, split steel shell type in both struts, Garlock DU being the American made brand. Although I have identified a replacement seal for the Beta struts, I have not found it necessary to replace the later, spring loaded lip type. Early Beta seals were not spring loaded, and thus should be replaced. Koni uses a special combination seal/scraper for which I have no replacement but then. Garter spring loaded seals normally do not need replacing unless they have been running dry for a long time. As the new bushings and adequate drainage minimize the seal pressure.

Nylon piston ring wear is corrected by placing a paper shim behind the old nylon wear ring in the bottom of the piston ring groove. A.0025 to .005 thick, as required, bond paper ring reduces the piston clearance in the cylinder to a light push fit. I have replaced the Scorpion steel piston ring with a tight sealing Quad ring (a quad section O-ring that is not subject to spiral failure) which improves performance under short stroke conditions by eliminating leakage at low piston velocities.

As for replacement fluid, automatic transmission fluid (Dextron ATF) has ideal characteristics for this high shear rate application, and it has a viscosity (180 ssu @ 100F)just slightly higher than the stock fluids.

Filling: With fluid in the reservoir submerge the foot valve end of the assembled insert and prime the cylinder by stroking the rod, When the cylinder is completely full ( fluid weeping through the bushing clearance) and with the piston at the bottom of the cylinder, lower the insert to the bottom of the reservoir. In this assembled position the reservoir fluid level should be about an inch below the head; i.e. minimum air space in reservoir of about one inch.

Last, button up the head using a new O-ring static seal. With this rebuild and improvements, the strut will be better than new. Piston ring and rod bushing wear being the likely cause for the next rebuild in about 30 to 50 thousand miles.

Technical Data:

Cylinder bore diameter 27mm 1.062"
Piston rod diameter 20mm 0.787"
Head static seal O-ring 2-128 Nitrile
Rod dynamic seal 20 x 34 x 7mm
Lip - none
Use Parker U-Cup M950B1N842020 Nitrile
Rod bushing Garlock du (TFE/Steel) MB2020DU
Rod scraper Original
Piston ring .078w x .045th Steel
Shim behind ring .002/5 Paper
Quad ring replacement 04-020 Nitrile
Fluid Dextron ATF - amount unknown
 
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Last updated: 2002-09-01