Custom Ford Sierra Upright – steering arm

Getting the dimensions correct for this custom Ford Sierra Upright hasn’t been easy. The reason for making a new upright was the steering arm on the standard upright being wrong for my particular application. Therefore, getting the next stage right, is very important.

The next job on the custom front upright was to double check the CAD measurements against the real car. The goal being to position the ball joint of the track rod end to avoid bump steer and get correct Ackermann geometry.

I’d swapped from a front mounted Ford Escort MKII rack to a rear mounted Citroen BX rack. The wheelbase is also different to a standard 4×4 Sierra (mine – 108″, Sierra 102.7″)

Bump Steer

steering arm height ratio = a÷b = c÷d
steering arm height percentage(%) = 100 x (distance between lower inner ball joint and track rod pivot ÷ distance between inner top & bottom ball joint)

The distance between the top, inner A arm ball joint and the lower, inner A arm ball joint is 200mm. The inner steering arm ball joint is 29mm above the lower, inner ball joint.

steering arm height ratio(c÷d) = 171÷29 = 5.9
steering arm height percentage(%) = 100 x (29÷200) = 14.5%

The outer upper & lower ball joints now sit 214mm apart, therefore the track rod end needs to be roughly 31mm above the lower ball joint.

(a÷b = 183÷31 = 5.9) or (214×14.5% = 31)

It’s a little more complicated than that when you add in the Ackermann geometry. Plotting all the intersecting lines in 3D showed the final result moves less than 1mm. Before I made these custom uprights, the track rod end was 70mm out in one axis alone!

It’s hard to see in the photo below, but there is actually a red laser line just about visible on the floor and upright. It passes straight over the end of the track rod.
Custom Ford Sierra Upright


This represents the Ackermann geometry and is centred over the top of the rear differential.

The Citroen track rod end has been propped up at the right height and the joint threaded in as far as it goes, in order to get it in the correct position. The plan is to cut 8mm off each side to the rack to get some adjustment.

With the upright in place I also wanted to double check the size of a couple spacers. I’m using Mercedes SLK front calipers and 300mm Jaguar X type front discs (re-drilled). It’s one thing measuring everything and drawing it up in CAD but nothing settles the mind more than bolting things up and having a quick ‘double check’.

Unfortunately the thread of the lower Austin Ambassador ball joint touched the drive shaft at full suspension travel. Luckily, there is about 8mm surplus thread and  a quick lick of the sander will sort things.

The discs in the shot below are Cosworth 4×4 Scorpio (278mm) discs. You can see the gap between the caliper body and the edge of the disc. I did this to emphasis how much bigger the Jag discs are.

Custom Ford Sierra Upright

There are a lot of things to plot out when making uprights and for each line there has been a large amount of reading up.

Supension Geometry

Once everything was assembled, the measurements, when checked against my calculations, thankfully checked out fine. I just hope Kangaloosh is as good as it seems to be.

I’ve owned and driven Hot Rods and Kit Cars and they drove pretty good (however that was 20 years ago). The suspension geometry of these cars was never checked and was probably way off given the wheelbase difference of target and donor vehicles. These days, most hot hatch backs handle pretty good. If I was to get out of a modern car into a poorly set up 80’s Rod, would I still think they handle well? – maybe not…..that’s why I’m trying to get things as close as possible.

The shot below is when I tack welded the steering arm in place. It took 3 hours to get the ball joint in the correct position. Until the rest of the removable arm is complete it can still move and I’ll be back virtually to square one.

Custom Front Sierra Uprights - Steering arm

The red laser line passes straight through the centre of both track rod end and the SAI line (roughly the lower ball joint in this case).

Custom Ford Sierra Upright CAD

Steel Ford Sierra UprightWith all the measures, so far looking as intended, I can make an additional couple mounts on my upright jig for the steering arm.
The Jaguar Discs need a tiny bit of machining, so that the pads perfectly match the surface area of the disc and the stud pattern is 5×112.

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