First I want to say yes I am happy with the performance of BMR rear control arms. I had wheel hop on the street and the first thing I did was go with BMR relocation brackets, using 1 hole lower than stock when my car was not lowered yet, and went with poly poly bushings on LCA's. The wheel hop disappeared and my traction increased.
The following is from an email with Kelly at BMR. If you want the full email with the diagrams he provided I would need your email address. But below is the text of his response to me. AS = Antisquat and IC = Instant Center if I remember correctly.
Paul, explaining AS/IC is very complex. I will attach some pictures at the end of this email, that I made up years ago, to help you understand.
I have never changed my mind about suspension geometry. If I recommend something to someone, there is definitely a reason for it. IE: If someone is stock height…and they tell me they “will” be lowering the car….then I tell them to install the UCA into the upper position. But they “should” use LCA Relos, if that is the case (when they lower the car)
The upper most hole I speak of, is the highest hole from the ground, when the parts are installed on the car.
A stock ride height car, without relocation brackets, works best with the UCA in the lowest position. Once the car gets relocation brackets, then the UCA needs to be moved up. On a lowered car, I always recommend LCA Relos, and the UCA in the Upper position. If someone does not want LCA Relos, or does not have them, and they have a lowered car….the lowest position of the UCA is going to work better than the upper – but I don’t prefer this.
You read what I posted correctly. The upper positions are designed for a lowered car, that utilizes our LCA Relos. The lower hole, is designed for a stock height car, and no relos. BUT – a lowered car (with no relos) can work well in the lowest position, as well.
In other words, if someone never uses our LCA relos, they will likely have better luck with the lower position on our mount. If someone does use our relos, I always recommend the upper hole on the UCA Mount.
The goal is to have the angles (imaginary) intersect at a point that provides the best overall performance. When you drop a car, by lowering it, you are already dropping the angle down, of the UCA – because the UCA is attached the body of the car that gets lowered.
When you have a lowered car, THEN lower the UCA down….and also drop the LCA down in our brackets – the angles are WAY to aggressive/steep….and you “hit” the tires way too hard…and you also run into instability issues in corners.