Jacking & Tuck Under

The dreaded jacking & tuck under on the fixed spring car is caused by the sideways force on the outside rear wheel relative to the position of the pivot (the uj) during cornering. This sideways force, generated by the grip of the tyre, is proportional to the weight on the wheel which is greater on the outside wheel.
One way to reduce the likelyhood of tuck under would be to lower the car which would take the line of action of the force further away from the pivot. Another way would be to reduce the force on the outside wheel by sharing more of the force with the inside wheel, this is what the swing spring does. By allowing the spring to pivot, more of the weight of the car is taken by the inside rear wheel.
Obviously the weight on the outside front wheel increases to compensate, thats why you need to fit a stronger front anti-roll bar at the same time
Charles Bushell, 14-May-2007, 10:10pm on TSSC

On the early Spitfires the road spring was attached firmly to the differential using 6 studs and nuts. During hard cornering, body roll would cause the inside rear wheel to be pulled up and lose adhesion to with the road surface. When this happened the swing axle would have a tendency to tuck under resulting in what is called "wheel tuck". This would produce a severe positive camber position of the inside wheel so if the direction is changed rapidly (as in slalom or right-left turn combinations) you have immediate and gross oversteer.
Triumph modified the rear suspension with the introduction of the MkIV model using a different way of attaching the road spring. Instead of a fixed mount, there was now a metal box with a pivot pin that allowed the spring to rock when the body rolled in turns.
Previously you had POSITIVE camber. With a swing spring you should have a couple of degrees of NEGATIVE camber.
You need a round file to enlarge the holes in the anti-roll bar mounting plate on the chassis into slots to take the longer distance between the studs on the larger bar's U-bolt. Enlarge the front slot "forwards" and the rear slot rearwards by approx equal amounts until the new U-bolt will fit.
You could drill the holes out larger to do the same thing, but that is removing more metal than is necessary and I would advise "slotting" the holes instead.
Chris Taylor, 29-Dec-2006, 05:30pm on TSSC

….the idea of the swing spring1 is to allow some compliance of the spring relative to the roll of the chassis under a cornering situation.
basically what used to happen on the original fixed spring was as the car went into a corner the inside wheel would drop down at an alarming angle of positive camber so inducing the rear of the car to "jack up"creating the infamous axle hop which would throw the back end of the car out sometimes resulting in a spin.the swing spring was designed to prevent this situation by allowing the main leaf of the spring to pivot on a fulcrum arrangement on top of the diff casing so allowing the road wheels to maintain a more even
camber attitude relative to the road surface by keeping this main leaf more or less parallel to the road surface resulting in better road holding.it was a simple and cheap modification made by triumph during production and is quite effective.hope this enlightens you..
Paul Carter, 14-May-2007, 09:55pm on TSSC

The original swing-axle independent rear suspension of the Triumph Herald and Spitfire had its limitations. Setting of "static camber" was critical. There was a tendency of the rear wheels to tuck under during certain cornering conditions, such as quick transitions. This was due largely to the inherent limitations of a swing-axle suspension as well as the high effective roll center created by a solidly mounted transverse leaf spring atop the differential.
Standard-Triumph USA's Competition Manager "Kas" Kastner developed a beautifully simple solution to the problem: the camber compensator. This single leaf spring, bolted to each axle's vertical link at the radius arm mount and fastened by a single bolt to a mount on the bottom of the differential, was an easy, bolt-on modification that greatly reduced the tendency of the rear wheels to tuck under.
De-arching and stiffening the "main" transverse rear spring was also a common and recommended modification, one used much by Triumph itself on the "Works" Spitfires, by Sir Jack Brabham on his Coventry-Climax-engined Heralds and by many other tuners and owners of Heralds, Spitfires and GT6's.
For the 1971 Spitfire Mk.IV, Triumph redesigned that transverse swing and mount so that all but the main leaf would rock or "swing" rather than being solidly clamped to the top of the differential. This lowered the effective roll center and greatly reduced not only the roll stiffness in the rear but also the amount of camber change during cornering.* This reduced roll stiffness, and the potential of increased oversteer that would result otherwise, was tamed by increasing the diameter of the front anti-sway bar.
The updated rear spring and front sway bar can be bolted onto earlier Spitifres. However, the camber compensator remains a simple and effective means of "taming" the rear suspension of a Herald, Vitesse/Sports 6 or Vitesse 2L, pre-Mk.IV Spitfire or "Mk.1" GT6.
*I have seen figures quoted for the total camber change of a given rear wheel of as much as 21 degrees for the original suspension, and about 7 degrees for the swing-spring suspension.
From Triumph Al Hoag's Corners USA

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