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Old 09-28-2008, 05:45 PM   #1
Twisted Minis
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Default Max Fish's 4-link Article

I have posted this in the past, its helpful, so I figure it should be here as well. Could it possibly become sticky? It answers a lot of commonly asked questions.
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Why so much talk about rear suspension? It seems these days no self-respecting minitruck is show worthy without an attention commanding “Goldberg device” (look it up) hanging out of a cavernous hole in the bed. Most rear suspension designs are limited to certain confines that don’t create many severe affects, but there are major design considerations that should be taken in account. Regardless of how you lift the frame off of the rear end, the links that connect the rear end to the chassis are what affect the dynamic relationship of the vehicle to the road. Before we go into the actual suspension designs I think it would benefit us to define the affects.
Every suspension design has its benefits, even the tried-and-true 4-link has infinite design options to be an advantage in one area more than the other. The dynamics that are affected by the design, that are important to us here are…

- Roll-center
- Roll-steer
- Anti-squat
- Pinion angle

Also, before we even discuss these we need to define instantaneous center or instant center. What instant center is is a theoretical point in space that is used to calculate suspension affect. The way to find instant center is easy once you “get it.” When you have two or more, bars, arms or what ever you are working with, each part is only as long as a straight line drawn from pivot to pivot. If you were to extend the line drawn through those points into theoretical “space” and do the same through the pivots of the other related parts, hopefully, at some point, they will meet. The point at which they meet is the instant center. If this doesn’t make any sense keep thinking about it until it does. To continue reading without understanding instant center is not going to be as fruitful as it should be.

Now that you know what instant center is let’s talk about roll-center. Every suspension design sets a point at which the vehicle “rolls” from side-to-side. Most of the time the point moves around depending on where the suspension is in it’s travel, but it is still important. On a straight axle type rear suspension the roll-center is “set” by whatever is used to eliminate lateral movement (i.e. panhard bar or even a triangulated type link system.) What happens, is that the center of gravity acts upon the vehicle through its roll-center. The higher the roll-center the lower the vehicle feels in cornering. Don’t get greedy though, too high of a roll-center and the car will act odd. The chances of having a high roll-center at the front suspension is not good and the vehicle will act unbalanced. Any decent rear suspension design will have a roll center around the center of the rear end, which is fine.
Roll-steer is a little more important and could cause some really poor dynamic affects. Roll-steer also comes in flavors, roll-understeer and roll-oversteer. Every straight-axle rear suspension design, that actually affects performance, has roll-steer in some amount. Roll-steer is always measured when the car is leaning toward the outside of a corner. For example, if you were to turn left the vehicle would lean to the right, so with the right rear wheel being compressed and the left wheel in droop and geometry being what it is, the rear end would try to steer the vehicle into or out of the corner. The issue is caused when the rear end steers out of the corner causing roll-oversteer. What happens at the controls of the vehicle is once the corner is entered and the vehicle takes its “set” into the corner, the rear end suddenly steers the car further than initially anticipated. Now under normal driving conditions this is at best annoying, but really driving into a corner and having the rear end oversteer can put the car into a spin. In really severe conditions the vehicle would be extremely difficult to drive over 30mph, a small bump in the road would really upset the chassis and almost can’t be controlled. Again the confines of the typical minitruck doesn’t really leave much room for severe oversteer and severe understeer is hard to achieve.
Anti-squat is really not a major concern, except that it should be at least taken into consideration while building a link system that is limited to short bars. Anti-squat also affects roll-steer and vise-versa (roll-center can also affect roll-steer, but isn’t a concern with a properly designed system.) What anti-squat does is fight the affect of weight transfer trying to “squat” the rear end while under acceleration. To create or eliminate anti-squat, you would design the link system to have the instant-center a certain height, related to the center of gravity. A higher instant-center has more anti-squat while a lower instant-center has less. At some point too high of an instant-center will actually lift the rear up as the vehicle accelerates. And too low will push the rear down (though a little tougher to achieve.)
Pinion angle is probably the one thing that most builders are completely aware of, but many are not sure how to set up properly. The pinion angle should be between 1 and 3 degrees pointed down. Most of us have already heard that, but the thing is that the major detail left out is that it should be 1 to 3 degrees pointed down from parallel to the transmission (or the front driveshaft on a 2 piece drive line.) What that means is the pinion could, under certain circumstances, point up. We don’t have near enough space here for me to explain how to design the suspension that you may desire, but give us some time we will get there.

Now that the boring part is over lets get to the subject at hand… Suspension design. I am going to describe the major benefits of each design. (None of this is my opinion (unless noted.) So don’t go and think that I am just bad-mouthing every body else’s ideas.) For a suspension design to be un-drivable, it would have to be really bad. Also if you just can’t quite fit the perfect design into your confines you will have to compromise. That’s part of what makes a good fabricator, the ability to adapt to their limitations. So here we go…

The ladder-bar and 2-link are both very similar. In fact a ladder bar is a type of 2-link, but not all 2-links could be considered ladder-bars. The ladder-bar design is great for high horse power, high traction cars that the owner doesn’t want to spend the hours or days it would take to tune a good 4-link for the track. The first major downfall of the 2-link is that it doesn’t allow much articulation (one wheel up and one wheel down.) The only way that the design can articulate is to flex a little bit of everything. NASCAR uses a 2-link rear suspension that is specially engineered to allow an acceptable amount flex in order to articulate. The second major issue is pinion angle. The 2-link by nature cannot keep the pinion parallel to the transmission so useable travel is limited to the acceptable amount of pinion change. On the other hand the 2-link is fairly easy to tune, to install, and to purchase. Anti-squat is easy to figure because the instant-center is the forward pivot. Roll-center and roll-steer aren’t really a concern because the suspension won’t articulate so on the same coin the need for a swaybar is just not there either. The 2-link will require a panhard bar, track locator or other similar lateral limiter.
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