I explained how lowering a car can affect roll centre and what to do to resolve it in a lot of detail in this post Roll Centre Correction
Several people have struggled to understand why balljoint spacers or mounting the balljoint on top of the wishbone doesn`t actually do anything. I will try and explain the reasons in this post.
Driving a lowered car without changing anything is possible, I did it for 7 years. What it does do is mean you need more steering lock when cornering, the car is sluggish to respond and doesn`t feel as communicative. Given the choice I would always address the roll centre now I have driven the car with and without it.
Double wishbone suspension is a different issue and in this explanation I`m specifically talking about single wishbone strut type suspension.
This is a typical car at stock ride height and running McPherson strut suspension. Original image created by Dave on ClubGTi
At standard ride height, the centre of the balljoint is lower than the centre of the wishbone pivot.
This is what you are trying to maintain when lowering the car.
Lowering the car has the undesirable effect of moving the chassis lower than the balljoint and excessive lowering results in the situation on this drawing where the wishbone is pointing upwards at the wheel end.
This is where all the confusion arises. The wishbone angle is a simple visual guide to see if the car is lowered too much.
To address the issue, people have fitted balljoint spacers or mounted the balljoint on top of the wishbone in the mistaken view this will address the problem.
I do understand that some spacers also move the balljoint outboard and forwards to modify the caster and camber. What is does NOT do is affect the roll centre.
The spacer is fitted between the balljoint and wishbone.
The wishbone is now lower at the wheel end and that is what people look at, thinking that the wishbone is now pointing down so assume the roll centre issue has been addressed.
This is incorrect.
The Intersection line is completely unchanged.
The only options are to raise the wishbone pivot at the chassis end, which is difficult and often impossible.
The solution usually employed is to fit balljoint extenders.
To make it easier to understand I will show longer balljoints being used instead. This is not something I`m comfortable in actually using but they are shown to explain the theory. Balljoint extenders are a far better solution, but the effect on roll centre correction is identical.
The critical point is that the centre of the balljoint has been lowered.