Quote:
Originally posted by techinspector1
bulletnose, it's interesting to note that you designed the suspension to preclude an instant center by angling the upper and lower arms at the same angle. I did the same thing on mine, except that both upper and lower arms are parallel with the ground, meaning that the actual dive in the car will be determined by the front spring rate. I didn't like the idea of a constantly-changing caster, which is what you get with arms that are laid in at different angles. With a heavy car such as a production bodied car, anti-dive is helpful, but if you have very little weight on the front as in my car (115" wheelbase, motor set back 17" as measured from the front spindle centerline to the nearest spark plug hole), it's not such a big deal.
For those of you who don't understand anti-dive, it is calculated by dividing the distance from the ground of the instant center of the control arms by the distance from the ground of the center of gravity of the car. Let's do a for-instance. The center of gravity height of the car is roughly at the camshaft height of the motor. My cam is 20" off the ground. Now, let's say that you lay the lower arm in level with the ground and it is 8" off the ground. Now you lay the upper arm in at whatever angle you think you want, lets say 2 degrees. Looking at the side of the car, run an imaginary line through the lower control arm bushings toward the back of the car. Now run an imaginary line through the upper control arm bushings toward the back of the car. In this case, the instant center is going to be at 8" above the ground regardless of the angle of the upper arm, because the lower arm is parallel with the ground and the upper arm line will intersect it at some point behind the center of gravity. Divide 8" by 20" and you get .40 or 40% anti-dive. You can angle the arms to get any anti-dive figure you want, even over 100% by angling the upper and lower arms down in the front, thus projecting the instant center above the center of gravity. I suspect you'd have a fairly evil-handling car if you did, but it can be done. Heck, by having over 100%, you could make the nose of the car RISE on braking. I suspect this 5 degree angle that everyone is talking about on the upper arms came directly from the Mustang II car that someone laid an angle finder on and said, hey, if it works for Ford, it'll work for me.
Dave, my setup has absolutely zero bumpsteer through 9 inches of travel (4 1/2" bump, 4 1/2" droop). I've set up fixtures and checked it with a dial indicator several times and even had a fabricator buddy come by the house to verify it. Of course, I had to give up some things to get it, 100% Ackerman being the most important. I have SOME Ackerman in the car as a natural function of the way a front suspension works. This car is an experiment on wheels. Normally, to gain Ackerman, you would draw a line from the center of the rear axle, through the lower balljoint centerline and extend that line out to the centerline of the tie rod hole in the steering arm. This will give 100% positive Ackerman. As you move your steering arm tie rod hole further inboard, you reduce Ackerman. Some race cars have even experimented with negative Ackerman, having the imaginary line drawn through the steering arm tie rod hole and the centerline of the lower balljoint laying somewhere outside the car!!! I capitulated to neutral Ackerman for the sake of zero bumpsteer. With the centerline of the inner tie rod joint (where it connects to the rack) exactly in line with the inner control arm bushings on the lower arm, I fabricated my steering arm hole to be at the same position as the centerline of the lower balljoint. This means that the tie rod and the lower control arm are exactly the same length and operate on the exact same pivot centers. To make this work, I fabricated the lower arm bushings 17" wide on centers, so that the tie rods run behind the front lower control arm bushings. In other works, starting at the front of the frame, the front lower control arm bushing is first, then the tie rod, then the rear lower control arm bushing. This is the only way I could figure to have absolutely zero bumpsteer. The car isn't on the road yet, so I have no idea of the transient handling characteristics. I'm just hoping for the best. That's the kind of guy I am, a hot rodder at heart, try it and see how it works. Am I babbling here?? I hope some of this makes sense to you guys. :LOL:
