Front Wheel Castor Angles

[duncana]

Has anyone got any ideas on what the shimming arrangement should be on the lower wishbone brackets to get an approximate setting of the castor angle?

 

I thought there should be equal shims on both brackets to set the correct camber angle but what about the castor ?

 

At present I have about 4 shims on both back lower wishbone brackets, taking the wishbone out about 10mm. There are no shims on both front wishbone brackets.

[jean]

As a general rule on our cars the castor angle is defined by the trunnion. Late TR4s up to TR6 are fitted with 3 deg trunnion what will give a 3 deg positive caster angle setting, am I right.

This is a good average setting for road use, light steering at low speed and good straight line stability.The more you will increase the caster the more force will be required to steer the car, wonder why modern cars have power steering.

For proper camber setting you will have to take the car to an alignement rigg. Check if they have chims available, what in most cases they have, and ask them to set the front wheel geometry.

It is quite normal that for a 30 year old car the amount of shims might differ from one side to another, once you hit a curbstone very hard you might be requested to add or remove a shim.

Be aware too many shims could point towards prior accident damage      ???

Jean

[marvmul]

Duncana - I think you have a lot of caster with the distribution of the shims you described. In Kas Kastner's book you can find that adding shims on the rear lower bracket increases the caster : the projection of the upright to the ground will be further ahead of the contact area of the tyre, so more caster.

To measure the caster you have to get the distance between the projection points of the upright and the vertical axis (through the stub axle centre) to the ground, with the suspension in neutral position and the steering straight ahead. I think you have to remove the wheel and hub and put a jack under the trunnion to measure this.

[jean]

I think we should be very carefull with forcing the caster above a value that is not designed for the car concerned. Please be aware that Kas Kastner is giving tips to modify TRs for racing, and this is mods are not necessarily recommended for normal use.

Racing cars are often taken apart after each race to replace parts that have suffered stress input above their design criterias, not to speek about tyre wear. Excessive caster can be detrimental on tyres with concern to wear.

The upper fulcrums/wishbones are designed and set to accept 3 deg caster angle, any more deg. will produce an additional load on the upper fulcrums and bushes. This extra torsion effect will probably not lead to a failure during a race season, and Kas did probably not mention in his book that they will be thrown away anyhow at regular intervals.

Believe me or not, I was sometimes horrified by seeing modification made to suspensions with the goal to counter a bent frames  and compensate accident damage    

Jean

[duncana]

Thanks for the advice. I think I'll try and find somewhere that can measure and set the front wheel geometry. All the suspension/steering has just been renewed so this'll be the time to do it.