Ackermann Angle and Sierra Uprights

I’d swapped from front mounted Ford Escort MK II rack to a rear mounted Citroen BX rack. I needed to get my Ackerman Angle correct.

see here:

To do this I just reversed the uprights and made some new rack extensions.

The next night I looked at what I’d done and thought ‘Oh Balls!’.

For some reason I’d forgotten all about ‘Ackermann Angle– most likely brain damage

I’d reamed out the steering arms so that the track rod ends could mount to the underside of the steering arms. This was to get the ball joint at the correct height to avoid Bump Steer.

Haynes Roadster Rear Mounted Rack
Ford Sierra Upright - reversed
The above shot shows the ‘pink’ reversed upright superimposed over the ‘correct’ grey one.

I knew the steering rack was now far too wide. You can’t just reverse uprights as it screws up the Ackermann Geometry.

As soon as I saw my work, with fresh eyes I knew my mistake.

The front drive shafts on the Sierra Front 4×4 Differential has 23 splines. The Early MK5 Escorts also have 23 splines. When a pair of Escort Uprights (one BNIB) came up for pennies on ebay, I decided to take the chance that my super rare Granada 5 stud 4×4 front hubs would also fit the upright. – They did.

There are 2 types of uprights fitted to the MK5 Escort, 23 and 26 Spline. If I simple wanted 4 stud flanges, then I could use the Escort items. However, I want to use 5 Stud Granada Flanges and the Sierra/Granada flanges, which are a larger diameter and are the same size as the later 26 Spline Escort items. Therefore, I would actaully need 26 spline uprights.

The height of the steering arm also looked pretty spot on. Was I getting lucky again?

Ford Escort MK5 Upright

The next task was to measure them and draw them up in CAD. I wanted to check the Ackermann Angle as accurately possible.

Ford Escort Upright MK5 - CAD

As you can see, although the splines fit, they are useless to me.

To connect them up I would have to shorten the top A arm and extend the bottom. The Steering Axis Inclination would be miles off.

Ford Escort MK5 - upright
Ford Escort MK5 - upright CAD

Finally, although a waste of time, I thought I’d see how close the Ackermann Geometry would be to ‘true’.

Ackerman Angle Geometry

True Ackermann AngleLines drawn from both SAI pivot axis, should pass through the track rod end ball joint, to a central point on the rear axle line.

Ford Escort MK5 - upright CAD

This is how the Ackermann Angle looked.
Ford Escort MK5 - upright CAD

In the above picture, steering pivot on the red super-imposed Escort upright should lie on the white line and not the red one. The red line also has a ‘faked’ bend so it could pass through the steering pivot. Basically, Escort uprights are as bad, if not worse, than the already useless Sierra items.

It looks like I’ll be making my own uprights on my new Milling Machine. 🙂

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