Rear Suspension

This whole website is pretty cool. By watching the stats I can see that the most popular tags for searches which end up on my site is rear suspension setup, in particular four link setups.

I will, in the next couple weeks, publish a very in depth post on adjusting your four link correctly. It will be a long post to be sure I cover it properly. But to begin with, let’s cover some basics.

As I am sure you have deduced, I am a drag racer, so this discussion refers to drag racing setups. If you want info for a street setup, or something to go around corners, send me an email and I can help you individually.

Let’s start with some basic high school physics. Newton’s law of motion states that for every action there is an equal and opposite reaction. This applies to almost everything we are doing while trying to accelerate a car down the quarter mile, and is very handy to remember when designing or modifying anything on the car.

Pistons are pushing down on the connecting rods, and the crankshaft, most of us understand that, but what are they pushing against? They push up against the cylinder heads, and, here it comes, they push up the same amount that they are pushing down, Newton got it right. That is an example for you, back to the rear suspension.

Your driveshaft is rotating, clockwise when viewed from the front. This turns the pinion gear in the front of your rear axle, the pinion gear is meshed with the ring gear, turning the rotation ninety degrees to drive the rear wheels. Try to picture the following, it can be tricky, but once you get it you will have a better understanding of what is happening in the rear suspension. As the pinion gear is rotating, meshed with the ring gear, if the slicks are stuck hard to the track, in effect preventing the ring gear from turning, or at least impeding the ring gear from turning, the pinion gear is literally trying to “climb up” the ring gear. Picture the ring gear as stationary, the pinion then climbs up the ring. Back to Newton, even when the tires start to rotate as the car accelerates forward, the pinion gear is still trying to climb up the ring gear, by an equal amount of force. Equal to, and opposite the amount of force trying to drive the car forward, (ok, there are some minute losses, friction and such, but for this discussion we can forget about them). And it is a huge amount of force, (torque). Think about that 750 ft lbs of torque your engine makes, multiplied through the torque converter, before hitting your first gearset which multiplies it even more, it is a huge amount of torque, (I am saving the calculations for another post). Now if the pinion is trying to climb up the ring gear, what is stopping it? The axle housing stops it, and in turn, whatever is holding the axle housing in the car. . . four links, ladder bar, leaf spring, etc.

Now, rather than visualizing the pinion climbing the ring gear, picture the whole axle housing trying to rotate up and around the rear axle, in the opposite direction to tire rotation, (and with an equal amount of force, Newton again). The housing is being driven around the rear axle by the pinion gear. It is this rotational force that we are harnessing with a four link or ladder bar or any other rear suspension setup.

A four link has two bars per side, a top bar and bottom bar. Pictured from the side, and under acceleration, (people often forget that under braking, or deceleration, everything happens the other way, ever see a car try to turn when backing off at the top end?), the bottom bar is being pushed forward, and being compressed by the rotational force of the axle housing, while the top bar is being pulled backwards and upwards in tension, trying to lift the chassis of the car. Adjusting the angle of the bars, controls the direction of the push and the pull, pushing the car forward, pulling up on the chassis, lifting the chassis, in turn transferring weight to the rear tires.

This all refers to the basic load directions in a four link rear suspension, so that you can picture what is happening back there when you release the trans brake, or mash the throttle. I have tried to make it basic enough to follow, yet assume the reader has a bit of an understanding of the terms I am using. These loads are all similar in a ladder bar setup, or leaf springs. Stock coil spring rear end setups, like some Mustangs and Camaros are in one form or another a four link, but with different geometry for anti-squat and anti-dive, as well as general passenger comfort. This is the first time I have ever really written a technical article, and hope I am making the concepts clear and understandable. I will work out how to add drawings in future articles to help clarify some of the concepts. If you have any questions, don’t quite understand what it is I am trying to get across, or want some clarification, please send me an email, I would love to hear. If you have any ideas for future posts, I would love to hear them too.

I will continue this discussion on rear suspensions soon, so check back often.