Mounting a tramp bar

 In Brakes, Wheels, Suspension And Steering, MGA-MGB, Midget

When full power is applied to the driving wheels, the axle experiences a reaction torque which tries to rotate it in the opposite direction from the wheels.   A tramp bar is a device which prevents this from happening, and can be thought of in simple terms as a bar running parallel to the front half of the spring, attached to the axle at its rear end and the chassis at the front, roughly as a parallelogram.  As it is pivoted at both ends, the axle can move up and down in the vertical plane, but as soon as it tries to rotate in reaction to power being applied the bar restrains it from doing so.  However …. ! ….

Although it’s a parallelogram in principle, in practice it isn’t. This is because the effective length of the spring changes as the spring flattens and curves when the axle moves up and down, the centre of the arc through which the axle moves is not the front eye bolt. This means that one side of the “parallelogram” is changing length, while the other side is fixed in length, so it’s not a simple parallelogram situation.

The way I have always approached it is to jack the axle right through its range and measure the movement of the tramp bar mounting point (tom the car to the ceiling, or have a few heavy helpers sit on it to allow the axle to go almost right to the bump stop).

Every inch of axle movement, plot the position of the tramp bar eye where it mounts to the axle bracket using a reference grid based on the chassis. These points can be plotted on a full scale drawing and from that the arc drawn and the true geometric centre of the arc found. This centre is where the tramp bar mount should be in order to prevent it restraining the axle movement and confusing the spring action.

In our K, we had the axle mount 3.0″ below the bottom of the shock plate, directly under the axle centre. The centre of arc through which the tramp bar eye moved plotted at 2.52″ below the bottom of the plate into which the front spring mounts, and 2.3″ behind the back edge of the spring mount plate.

Once the chassis bracket is made & fitted, attach the tramp bar at the axle end and then as a check you should be able to repeat the jacking exercise. Offer the front eye of the tramp bar up to its mount but don’t put the bolt in it. Instead, observe the alignment between the front eye of the tramp bar and the hole in the chassis bracket. As the axle moves throughout its range, these should stay aligned with each other. This was the case in the tramp bar mount on the K.

If there is more than 1mm relative movement between the two, then the tramp bar will be restraining the axle and adding effective stiffness to the suspension. If this is the case, generally the stiffness increases with vertical axle displacement.

Less than 1mm is acceptable if there is compliant bushing in the tramp bar; if it is rose jointed then the tolerance needs to be virtually zero or the tramp bar and spring will forever fight each other.

Apart from the suspension stiffness, this also causes fatigue and sometimes failure at the chassis bracket mounts.

Getting the right length is important, but so is getting the right height of the tramp bar mount, and it is even more sensitive to height error than length error. 1 mm difference in height affects the trueness of the geometric centre about 4-5 times more than 1 mm out in length.

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