Negative Caster Question

[SFjames]

I have asked this question and received conflicting answers.

I have a car made in the 1940's. A Desoto. The car specifications state that the Castor should be set to between 1 and 3 degrees of NEGATIVE caster. It is a king pin independent coil spring front suspension.

The thing is wandering a lot. It is restored with everything new and tight.

The question asked is, if one wants more "caster effect" in a car that is designed for negative castor...does one adjust toward zero or away from zero?

Several people I have asked have said you go toward zero. But, one person I asked is emphatic that you go away from zero to 4 or 5 degrees negative. His comment comment has to do with the fact that in a negative caster design the king pin is slanted the opposite way...

I get what the one person is telling me, however, the King Pin Inclination angle is positive 4.75 to 6 degrees on this car...so I am leaning (pun intended) on heading toward zero to get more Caster Effect.

Can anyone shed any light on this?

Thanks, James.

 

[BrianPetersen]

Add a positive number (more "caster effect" i.e. more mechanical trail) to the negative number (the original specifications).

I'd try about 2 degrees positive - the other side of zero! - and see what happens.

It can be hard from today's perspective to see what the original designers were thinking back then.

If the car does not have power steering (likely) they may have been aiming for as little steering effort as possible. Using today's steering geometry may introduce today's almost-universal reliance upon power steering ... but you won't really know until you try it.

Ruff numbers a 4 degree change in caster will (taking a ruff guess at your wheel OD) change mechanical trail by about 1 inch, and if you apply some cornering load it will increase the tie-rod loads and therefore the steering effort, but by my estimation it should be manageable even without power steering. You will notice it.

 

[cibachrome]

Make sure you have the REAR suspension buttoned up tight, too.

If you have put radial tires on the car instead of the bias tires it came with, the wander is a result of this because radial have MUCH less self-aligning torque and camber stiffness. You would add more caster (That would be more positive caster) to help this situation out. Possibly some complications from this change, depending on the suspension type.

 

[sierra4000]

Check the steering system friction,
extremely tightened the steering gear create strong wandering feel.

 

[LionelHutz]

It'd be better to ask if you want to move in the positive or negative direction rather than towards or away from 0. Moving towards zero can be going in either direction depending on where you start. The answer could also be to move away from zero, but in the positive direction. Overall, it's a very clumsy way to look at it.

Positive caster gives mechanical trail, which typically helps. If you are already positive, I doubt moving in the negative direction will help.