Escort / Capri uprights or Sierra?

I have just responded to a posting on Sylva chat regarding castor, tail and the Seirra uprights.

Many SVA's have been failed on lack of self-centering. Even though cars apprear to have and adequate castor angle, they still lack self-centering. Why?

There seems to be two culprits. The first is friction. New steering racks can be very tight. The second is the Sierra upright geometry.

If you were to look from the side of a car and draw a line from the centre of upper ball joint to the centre of the lower balljoint and extend it, it should strike the ground in front of the tyre contact patch. This line is called the steering axis. The distance from this immaginary line to the centre of the contact patch is the amount of trail. It is trail that gives castoring effect.

On the sierra upright, the stub axle is offset to the front of the car. This reduces the relative distance between the steering axis and the contact patch, reducing the amount of trail. So 8 to 10 degrees of castor angle may yeild enough trail with an Escort upright where the stub axle passes through the steering axis. However this amount of castor angle will yeild little or no trail with a Sierra upright.

Why did Ford do this? Firstly the upright casting was not optimised for use with double wishbones on a 500Kg 2 seat sports car! Secondly, castor angle has other effects on steering, not just self-cenetering. Castor angle also yeilds negative camber gain on the outside loaded wheel as it is turned. It also adds a damping factor to the self-centering caused by jacking. When the wheel is turned, the castor angle tries to jack the fornt of the car up. Have a look at the picture above. The Sainsbury's trolly wheel has lots of trail but no castor angle. It centres but has a tendency to wobble. The next picture shows a typical car setup with trail genterated by castor. This gives trail, damping and camber gain. Camber angle and trail can be individually controlled by offsetting the stub axle with respect to the steering axis.

Conculusion. The effects of castor angle and tail must be investigated individually. Trial gives self-centering, castor angle, negative camber gain and damping. It is the lack of trail on Seirra based kit cars that causes a lack of self-centering. There are 3 remidies. Raceleda makes a Sierra upright replacement that has no offset. Dial in more castor. This will increase trail but will also affect steering feel and induce bump-steer as the position of the steering arms will have been altered, you have to add a lot to make a difference. The last option is to offset the top ball joint position in order to minimise the offset. The next kit I build will not have Sierra uprights!

Comments above are not necessarily those of Sylva Autokits.