Described in its simplest form, a birdcage is the working foundation of a 4-bar (4 link) chassis suspension system in that it stabilizes and regulates the movement of the rear part of the car. It also provides a platform to control the moving relationship between the chassis’ shifting weight and the car’s rear traction. Sounds easy enough right? but the first time you saw one you probably thought (I know I did) ‘hmmm… Looks complicated.’ And the more you learn about them, the more you realize that they really can be. Still, the things about your car that they control are crucial to your car’s performance and you will never outrun everyone else until it’s at least close to right. The more right your birdcage setup is, the better your chances.
First things first. Let’s all get on the same page of our relevant terminology.
Indexing: when a birdcage rotates on the axle housing this movement is called “indexing.”
Side Bite:when the outside edge of your sliding tires (the right in particular) assists the bottom of the tire in gaining and holding traction. And look, we’re not going to debate the philosophy behind why or why not side-bite does or does not exist. We’re just going to take note of 35 years of dirt racing experience and know that it does.
Roll Steer: when the rear end itself rotates horizontally under the chassis, from the driveshaft’s center, to some degree steering the rear end of the car. This is a direct result of the chassis’ movement, interpreted by the birdcages for the rear end.
Zero Indexing: OK, let’s be sure and get this one straight. There are a lot of teams that will index their car’s birdcage a few degrees from center so that when the chassis movement is at its apex, the cage will be at 0°. No offense to anyone, but that’s not ‘Zero Indexed,’ that’s ‘Indexed TO Zero.’ True zero indexing is when the cages are at zero vertical degrees from center when the car is stopped and level. True zero indexing is what PPM recommends because of how indexing from that position loads the rear end. (We’ll get to that in a minute.)
OK, to get our brains in the right spot; this story is much like our story on droop limiters in that race car suspension (particularly on dirt) is all about managing centrifugal and centripetal force. So get your mind there and imagine the up and down push and pull on the chassis as the car circles the track.
Now, let’s start from our standard position: 4-up and 4-down and remember, it’s not the bars’ job to support the weight of the chassis, that duty belongs to your shocks and springs, and that’s what we mean when we say “load the rear end.” We’re talking about directing the centrifugal and centripetal forces of a moving chassis onto its shocks and springs. So, on the most basic level, the cages should never be set up in such a way that allows for lock-up. Instead, the movement of the chassis should direct the birdcages to increasingly put more of the load produced by the shifting weight onto the shocks, and the more effectively it does that, the more traction you will have. Just keep in mind, you don’t always want all the traction that it’s possible to have. You want the right amount of traction for the track conditions and the driver’s style of handling the car.
When you go to adjust your cages from there, there are a couple of things to keep in mind. Firstly, remember too our visualization of the chassis twisting (or rolling) as it goes into the corners and realize the forces acting on the opposing sides of the rear end will be exactly equal and opposite to one another, i.e. what one side does, the other will do the opposite. Also, our true zero indexing is faster than ‘Indexed TO Zero’, and that’s what we meant a moment ago when we said “HOW (true zero indexing) loads the rear end.”
There are a couple of reasons that true zero indexing works better. For one, it simply doesn’t have as far to go in order to best engage the shocks. Now considering that side bite creates forward momentum, the faster the cages can load the shocks, the sooner your side bite starts to dig in. Moreover, it effectively changes the pivot point of the birdcage’s action from the shock mount to the correct point: the center of the axle housing.
Lastly, before I show you a quick reference table to illustrate the points of this article, let’s think about the relationship all this has to roll steer. To be sure, roll steer can be a wildly effective tool to have at your disposal if you can master it and use it correctly and, like most things in this life it is a delicate balance that works best. In some general terms, the less roll steer you have in a set up, the tighter the handling will feel and vica-versa. The more roll steer will have a looser feel. The trick is to tune it to the right spot (once again) for track conditions and your driver’s style. But everyone must be ready to compromise if the most effective possible set-up is to be achieved. Driver, you may have to deal with a little slop on the straight-aways in order you get you through that corner fast enough if the track is really slick. Crew chief, you may have to just let him fight with it a little more coming out of the corners if he thinks he can beat his rival into the corner with it… delicate balance. The point is to win the race, not to be the one that is right if there’s a fuss.