Thread: Rearend links- 4 link, 3 link, parellel, angled???

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When I was 10 years old, my mom bought a set of World Book Encyclopedia. She couldn't afford them, bless her heart, but she somehow bought them anyway. By the time I got to high school, I just about had them worn out from leafing back and forth and reading. So, I am familiar with what you're saying.

OK, I have spent quite a bit of time studying the whole issue of the 4 link system. I still dont quite understand all of it but I'm gaining on it.

When using bushings like these (picture below) from moog, does it matter which way it goes in?
I would assume whatever I do on the right side of the car, its opposite on the left.

Also, I understand that the upper and lower bars need to be the same length (or close) to not change the pinion angle during travel.

I can only get about 13", eye to eye, on the top bars.
That is also a comfortable length for the lower bars.

Is that too short to work properly?

Thanks again to all who have participated in this thread and my education on rear suspension. I really apreciate it. Couldnt have done it without you.

And while I'm on the subject of rear suspensions-

Techinspector, In your drawing of the different instant centers, (Thanks) , How does one decide what to use.
I have read much about % of anti-squat.
All of your instant centers are at 100% anti squat (on the neutral line)
Let me know what you think about this. Its the part I dont understand yet.

The site below talks about this (see diagram below)

http://www.baselinesuspensions.com/i...A_Drag_Car.htm

This is taken from that page-

Normal/Neutral Line:
1. Draw a line up through the center of your front wheel.
2. Draw a Horizontal line through the CG until it crosses the line drawn through your front wheel and mark this intersection point. The Normal Line (or Neutral Line) is the line drawn from the Rear Tire contact patch through this intersection. Neutral Line

The location of the IC relative to the CG and the Normal Line (or Neutral line) is what determines what the chassis is going to do. One term used to describe the chassis movement for a 4 link suspension is Anti Squat. An IC that is located ABOVE the Normal Line is said to have more than 100% Anti-Squat. An IC located BELOW the Normal Line is said to have less than 100% Anti Squat.

~4-link settings of 100% Anti Squat should accelerate the car w/o any raising or squatting of the rear of the car.
~4-link settings with MORE than 100% Anti Squat will Raise the rear end and hit the tires HARDER.
~4-link settings with LESS than 100% Anti Squat will cause the rear end to Squat and hit the tires SOFTER.

~IC's that are located IN FRONT of the Center of Gravity (CG) will tend to lift the NOSE of the car.
~IC's that are located BEHIND the Center of Gravity (CG) will tend to lift the REAR of the car.

I think equal length is desirable but many factory rear coil setups use unequal. I also like the idea of the top bars being triangulated.

I do too, Willowbilly. But I dont have the room to get much angle on them.

Originally Posted by willowbilly3

I think equal length is desirable but many factory rear coil setups use unequal. I also like the idea of the top bars being triangulated.

I've thought about this unequal rear length (shorter on top) and have come to the conclusion that this is to maintain pinion angle closer to zero on acceleration/bump. If you look at the geometry on a front suspension, you'll see that the top arm is always shorter than the bottom arm. What this does is allow camber gain in bump, so that the tread of the tire remains more parallel to the road surface on bump, instead of rolling under and losing traction. If you look at the rear suspension from the side as we're doing here and think of it as a front suspension, a shorter top link would do the same thing. On acceleration, the pinion climbs the ring gear (or tries to) and results in a less than desireable pinion angle relative to the front/rear u-joint relationship. As the diff rises (or body drops depending on how you look at it), the shorter top bar will introduce the same "camber gain" into the differential that the shorter upper control arm on the front suspension uses and counteracts the tendency of the pinion to change the u-joint angle.

Now, again, I see it this way because of the way the factory sets up a rear suspension for the diff to rise (body drop) in the car on acceleration. I have never seen diff drop (body rise) on a factory car. I've seen it on Mopars that were set up for drag racing, but not on factory installs for grandma going to bingo. Now, if a guy were to set up the instant center at a point that would generate body LIFT (diff drop), then I would think that the shorter top bars would be a detriment and maybe even going the other way with longer top bars would be advantageous. Then again, I could be wrong.

Instant Center Length: Somewhere in Billy's explanations, there is the statement that the instant center should not be less than 60" from the rear pivot points. He said the center could be out ahead of the car in thin air, but not shorter than 60".

Link Lengths: Here's a thought. You have the links connecting ahead of the diff tube, which limits the length of them with the available space ahead of the diff. What if you were to make your diff mounts behind the diff tube and lengthen the links. Angles on bump and rebound wouldn't be quite as acute and as a result, the instant center wouldn't change so drastically in bump and rebound. I could see re-inforced triangular plates mounting the links maybe 11 inches behind the diff tube and giving a link length of 24". This should also give you a little freedom to angle the top links and keep from using the dreaded Panhard bar to control lateral movement. You would have the added benefit of not twisting the rubber bushings as far circumferentially. Think about it and tell me if I'm all wet.

It won't matter which way you install the rubber bushings, but you must make the ID of the receptacle steel bushings compatible with the step on the rubber bushings. They are a press fit. I would have one of the OEM control arms in my hands to carefully measure them to get it right before turning the steel bushings for fit.

Instant Center: Like I said in an earlier post, with a drag race 4-link, there are somewhere just short of 100 different combinations of placement of the IC. If all cars and all conditions were equal, then there wouldn't be but one set of holes in the forward brackets. But that obviously is not the case. I think the best you can do is to make the IC at least 60" long and put it on the squat line as your center set of holes. Make as many adjustment holes up and down from there as you wish to make so that you can experiment with the placement of the IC for your particular combination of weight, wheelbase, CG and road/track conditions.

In my opinion, here it is in a nutshell.....what you posted above....
"~4-link settings of 100% Anti Squat should accelerate the car w/o any raising or squatting of the rear of the car.
~4-link settings with MORE than 100% Anti Squat will Raise the rear end and hit the tires HARDER.
~4-link settings with LESS than 100% Anti Squat will cause the rear end to Squat and hit the tires SOFTER."
The Mopars I have seen that raised the body obviously had the IC set ABOVE the squat line in order to hit the tires harder. If you're raising the body, that means you're pushing the diff down.

The current generation of very fast cars I see at Firebird neither rise nor squat. On launch, the body remains where it was in relation to the diff when the car was staged. That tells me that they must have the 4-links set somewhere close to 100%.

Or you could run the links rearward toward the bumper

Here's my idea of extending the link mount back behind the diff....

And here's (I think) Jerry's idea of extending the links to the rear for attachment.....

Just playin' around here because I love to do this sort of thing. I'm working on my desk with a ruler at 1/8 scale. If any of you other guys like doing drafting/drawing, a dandy board can be had for cheap. Go to one of the home improvement outlets and buy a door. Cover it with a smooth, thin sheet of linoleum and make a down and dirty stand for it from some inexpensive 1 1/2 X 1 1/2 lumber. Makes a dandy board and you can easily draft out 1/2 scale automotive projects. T-squares and such can be bought pretty cheaply from drafting supply outlets. Drafting 1/2 scale and dividing the lines with the sharp points on your 6" dial caliper can get you pretty darned close. When done with your drawing, multiply times 2 and you have the dimensions for the actual parts.

Anyway, here's another idea for limiting lateral movement of the diff without having to use a Panhard bar. In my opinion, those things are an engineering abomination and that Panhard guy should have had his head pinched off as a baby.

Just wondering Techinspector, what do you have against panhard bars. What have they ever done to you!?

As the diff goes through bump and droop, it follows an arc created by the Panhard bar as the bar swings up and down. The bar will be at its longest at rest and will become shorter in both bump and droop, pulling the diff laterally in opposition to the main suspension components, which operate on a completely different arc. It is not in synch with anything else on the car geometrically. Sort of reminds me of an inbred child. Just not right.

As awful as it is, the characteristics of tugging and pushing of the bar can be minimized by intalling it parallel with the diff and making the bar as long as is humanly possible. That involves making brackets to attach it to the diff as far to one side as possible, all the way over to the backing plate. The bracket on the other side should mount the bar onto the frame all the way over by the backing plate on the other side of the car. It's still an engineering abomination, but at least in this manner it's a minimized abomination.

Agreed..... A Watts linkage takes a bit more planning and requires even more room then a panhard bar...but they do work a whole lot better....

Originally Posted by Dave Severson

Agreed..... A Watts linkage takes a bit more planning and requires even more room then a panhard bar...but they do work a whole lot better....

Watts that?

Sorry I just had to say it.....

Go to your room.