Do you measure the 7° lay back the batwing or on the kingpin. I did 7° on the batwing for my straight axle and that puts the kingpin at roughly 15°. Want to make sure this is right before I do my final welds. Running a spring behind setup. TIA!!
I assume you are referring to setting caster angle when you say layback angle. In that case, you need to measure on a surface that is parallel to the kingpin axis.
The kingpin boss is the true place to get the measurement, the batwings can be welded on at any unknown angle.
I put up to 10 degrees in my cars. As it makes it a little harder to steer in a parking lot it goes straight as an arrow down the highway
Do you have the car mocked up enough to account for tire rake? That should be done first due to increasing rake reduces caster. If not at least set the frame up with 2 or 3 degrees of rake then measure your caster. And if you are following the math you can see with an aftermarket front crossmember that has 10 degrees of built in caster gets installed with the frame flat in a jig, you give up 2 or 3 degrees to frame rake, and you have backed into 7 or 8 degrees of caster. And as others have said, measure off the kingpin top. Of course if you have adjustable spring perches it's easier to get the caster.
What ever you do make sure you take your measurements with the car perfectly level, with each tire at the exact same spot or you will be chasing your tail. When I had my new driveway put in I had the guy make this one spot in the back as level as possible.
7* is what they are supposed to have. My sedan is 10* and the other car 7*. I would be comfortable with anything from 7* - 10*, but it should be 7*