Q: What are the advantages & disadvantages of a 3 link or 4 link rear suspension and what do you thing about floater brakes for a dirt track modified?
A: Regarding the 4 link suspension, I assume you are talking about the parallel links that require a J bar or panhard bar for lateral location of the rear end and not the factory style where the top links converge at the rear end.
For a dirt track modified, the 3 link or four link will win races. You will have to do things a little differently between the two suspensions, but either one will do the job.
A 3 link suspension is nice because it is very simple and very adjustable. There is however a lot of misconceptions (and/or voodoo) with the introduction of spring (or rubber biscuit) links. The best recommendation is to stay away from the spring links. The problem with these links is that they are compressible (obviously because of the spring) and therefore the suspension geometry is free to change. This is a problem for repeatability on the race track and directional stability. With solid links, th e three link suspension is reliable and will achieve the same handling response as the four link
The 4 link suspension is very good on short (90" and less) wheelbase cars when you use it as a Z link (one forward and one running to the back). On an IMCA type modified, running all four links to the front is fine. Using the 4 link does require a "Bird Cage" or some type of floater for the links otherwise you will bind the suspension and forever be bending and cracking things. These floaters, however, require more hardware and more maintenance and possibly another point for failures during a race.
Geometrically, you can achieve anti-squat just as well with either suspension. Rear axle steer is typically not a big influence on dirt track handling, but both systems will respond the same so long as the 4 link is not a Z link.
Biasing the left or right wheel's weight transfer during acceleration can be done with the top link of the 3 link and the torque arm of the 4 link. You will get the same results but from a different piece of hardware.
The 4 link will tend to be more rugged due to the additional link sharing the loads. On a dirt track this may help due to the roughness of some dirt tracks and the resulting body contact with the competitors.
Both suspensions are about equal as far as response and controllability. There is the hardware aspect of the 4 link which adds initial cost and more costs during a big crash, but maybe a bit more rugged otherwise. There are other small differences but t hey are negligible on dirt tracks.
Regarding the brake floaters, you really are adding cost at questionable benefit. How much braking are you doing on a dirt track, anyway? Most of the braking on a dirt track car is done at the front wheels. The engine provides a good portion of braking at the rear wheels and the actual rear brakes are only a small contribution to the total braking. Honestly, I don't know that rear brake floaters are much of a benefit to anybody except to the guy selling the extra hardware.