Last updated 13 July 2008
Rear Suspension Design
With the assistance of Allan
Stainforth's excellent "Competition Car Suspension" the design of the
rear suspension has been completed,
though the spring and damper have not been detailed. That will have to
be done on the car as I can't justify the time trying to draw in 3D!
The spring rates, etc will also need to be calculated when more
information on corner weights are known.
A hardboard 2D model was used to optimise the arm lengths.
Using the model generated the following data, which I thought was acceptable.
Body Roll (Deg)
Roll Center (Inch)
Camber (Deg)
Bump (+)
Droop (-)
Scrub (mm)
Swing Arm Length (Inch)
Lower arm 265mm, O/B pivot 23mm higher than I/B
to ground. Upper arm 178mm 15deg rising from inboard pivot to outboard.
0
1.2
0.0
0
0
47
1.2
-1.7
1
0
35
1.8
0.5
-1
0
66
0.6
2.2
0
2
1.0
0.2
0
0
46
0.9
-1.4
1
0
34
1.6
1.2
-1
-1
60
0.7
2.6
1
4
1.0
0.8
0
0
40
1.0
-1.2
1
0
30
1.2
1.8
-1
-1
58
0.2
3.0
1
Droop (-)
Basic
CAD software was used to finalise the design, and confirm with greater
accuracy the camber and scrub changes. I have incorporated wider wheel
arches to allow a 4inch increase (2 inches per side) in track, and
allow more space behind the wheel.
Scrub is 3.0mm at full droop (40mm) and 0.8mm at full bump (60mm). This varies as the body rolls, the tendency being to reduce the change. Roll center lies between 22 and 39mm above ground. Camber is 3.8deg negative at full bump, 1.2deg positive at full droop, but again is effected by body-roll.
Rear Suspension Modifications
While designing the layout I wanted to minimise modifications to the body tub as much as practicable. I want to improve the car, but still retain it's original looks. It's taken a long time to achieve but I am happy with the results so far.To mount the lower wishbone the same method as used at the front has been employed, using the same mounting points. The chassis was drilled and thick wall tubes welded in place to prevent crushing the chassis rails. A simple jig was used to ensure accuracy and consistency.
Having purchased a posh new Mig welder, which made a massive improvement in the consistency of my welding, I was eager to weld anything and everything. This revealed some areas that looked in good condition, until met with a few amps. . .
Surgery was required, and new metal let in . . .
The original cross brace between the rear shock absorber towers was completely cut out to clear the diff. A new, stronger brace was installed, but this required a notch to clear the diff, and allow clearance for the body tub above.
For some reason there were two holes drilled in the rear towers. These were plated in, and the area at the top reinforced, with new metal.
Finally the front of the tower was boxed in, making the whole tower much stronger than original, and should keep debris and water out, making it more resistant to corrosion.
The completed assembly is below. The mounting brackets from the front set-up can be seen. The ends of the wishbones still need to be sealed. The vertical link is temporary, and the MDF is to give the correct spacing for the wheel, to make sure clearances were okay.
From the rear the main diff mounting points can be seen.
From above the space for the coil-over is obvious. This will be finalised once the body is in position, as the top will connect to the body. I couldn't find a way to get a sufficiently robust structure where I needed it. The body will need strengthening, but it was always intended to incorporate a full roll cage into the car, partly for safety, and partly to stiffen the structure.
The adjusters can also be seen. These are not yet sitting correctly. When welded in some distortion took place, which is expected. I reamed them out put my reamer won't get the last 2-3mm. Need to get a parallel reamer, or put a chamfer on the bottom of the adjuster.
And an overview of the, more or less, complete setup . . .
Rear Vertical Links
Having
finally decided to go to the Ford 108mm PCD, to give a better choice of
wheels, and using the original Sierra discs, 253.5OD, but only a 33mm
offset.The vertical links are custom made is all 3mm mild steel, with a 6mm thick pad that the bearing housing bolts to. Three track rod ends allow fine control over camber and toe. The bottom wishbone mounts to the chassis can be shimmed to give further camber adjustment if necessary.
There
is plenty of clearance around the body. Modifications will be required
to the body-tub to accommodate the shock absorber with coil-over
springs. An cross brace will be fitted across the wishbone to strenghten it. This will be done once the best position for the shock absorber is confirmed. The intention is to tie the roll cage into this new mounting
point to maximise stiffness and strength.
Still
got the hydraulic lines and handbrake cable to run, but they'll be done
once body tub is back on and runs optimum can be identified.