IMCA Wheel Rate/Pull Bars
Bob,I have a couple of tech questions pertaining to a four-bar floated IMCA modified rear suspension.
1. What formula would you use to calculate the wheel rate on the rear axle? Most formulas that I've seen are for independent suspension on the front.
2. How do pull bar angles come into play with antisquat? Or is anti-squat not a factor with the four-bar suspension? I'm trying to find ways to quantify set ups. Thanks for your help.
In oval track racing, there's no value or gain in knowing the wheel rate for a straight axle suspension system. The rear axle supports the springs that support the car. All the chassis knows is that it's sitting on two springs. Those springs could be mounted on a concrete slab for all the chassis knows. What we need to know is the spring rates and the spring base, or width between the top of the springs. This forms the spring base.
The spring rates, especially the spring split if they're different, as well as the width of the spring base along with the rear roll center height and the center of gravity height of the sprung mass together make up the dynamic components we need to monitor and consider for chassis setup.
In early automotive design and engineering, there was a need to know the affect on the suspension of sudden movement due to roadway irregularities such as bumps (speed bumps, and so on) and pot holes. These are phenomena that serve to impart forces to the chassis components from outside the vehicle. In racing, we don't usually have those influences apart from a rough dirt track.
Our focus in setting up a race car is always concerned with the forces acting on the car in the longitudinal and lateral directions at the center of gravity. Those forces are concentrated centrally inside the car and impart effects and influences that move from the CG to the suspension to the tire and finally to the track, not the reverse.
This area of dynamics has been a passion for me for the past 15 years and my early research was at the Virginia Tech engineering library where I found a 1940-something book on automotive engineering. In one chapter was an explanation, much like I described above, and a sketch showing a chassis resting on two springs like I described. I had finally found the true explanation of straight axle dynamics and I've personally proven that methodology many times since then.
As for antisquat, yes this effect is a part of the four-bar system. The lift arm or pull bar, depending on which you use, acts as the link that promotes antisquat. The lift arm can actually lift the entire rear of the car, in some cases upon acceleration. Be sure not to put too much angle in the third link or it will cause the car to be upset and loose on entry.
Push Vs. Pull Slave Cylinders
What's the difference in a push-style and a pull-style clutch slave cylinder? Is there a functional performance difference, and what clutch setup would be best for either one?
Right now I have a race car with a 350ci, Saginaw four-speed and a ram clutch setup. I see a lot of pull-styles in racing magazines rather than the push-style, like I have on my racer I'm building. The car isn't running yet so I can't say if I like the push style yet or not.
The real difference is in the mounting. One will be adaptable to your type of car better than the other. The difference is in the location of the fluid inlet. The push-style is behind the piston and only needs a seal at the piston, while the rest of the chamber is closed.
The pull-type has the inlet in front of the piston and it must seal at the piston as well as at the "push" rod. So, with the two seals, theoretically, there's more to go wrong. Other than that, both should work fine.