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Welcome to the next installment of the

Topic of the Week


We did the three link, now it's time to do the four link suspension.

You know...

link placement
joint construction
squat & anti-squat (and I don't know diddly-squat)
etc.
 

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How about beginning a list of design elements that usually always work and should be suggested in most all 4 link setups?
Maybe starting with arm lengths?
Are the bottom arms usually always longer than the tops?
Should you usually always try have close to level top arms?
etc.,
 

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Also I am interestd in leraning more how to desgin the front 4 link...the rear seams pretty simple, but in the front you have to deal with steering issues. Another thing I would like to learn about is spring rates.
 

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Antisquat

I tried to draw up a decent picture to illustrate how to calculate antisquat. You don't need CAD to do it, a pad and paper work fine.

One absolutely necessary piece of info is the Center of Gravity of the vehicle, and that is not hard to figure out with the COG spreadsheet.

To figure it out:
1) Draw out a side view with the basics of your setup. Draw the tires, axles, and control arms. If find it easiest to draw a horizontal line through the COG.
2) figure out your instant center by drawing imaginary lines until they connect, that is your IC. That is the blue point in the pic.
3) Draw a line from the point on the ground below the centerline
of your rear axle through the instant center, it should look like the blue line.
4) Draw a line vertically through the front axle centerline. (not shown)
5) The point that your line drawn in step 3 and 4 intersect is your Point of Convergence (POC). This is the purple dot on the drawing.

The height of your POC divided by the height of your COG, multiplied by 100 is your Antisquat percentage. The green line on the diagram shows where 100% antisquat is; if your POC is above this line then your rear end will rise under acceleration, below this line and your rear will squat.

What's the perfect number? Who knows.
 

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Roll Axis

Here's a couple pics from the Thomas Gillespie book Fundamentals of Vehicle Dynamics that show how to calculate the roll axis of a 4-link.





The flatter the roll axis is, the less roll steer (aka rear steer) that will be present.

Hopefully, this will get the topic going now that the basics are covered.

[edit: attached pics rather than linking]
 

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Re: Antisquat

JohnnyJ said:
The green line on the diagram shows where 100% antisquat is; if your POC is above this line then your rear end will rise under acceleration, below this line and your rear will squat.
I agree with all the tech you posted, sumed it up nicely how to figure your anti-squat and COG but the comment above does not seem to be accurate in the real world.

I have been researching and designing my 4 link rear for about 6 months and from what I gather from the people that are more experienced than me say that you will begin to have squat chararacteristics at around 50% anti squat in a rockcrawler. Is this true I am not sure but since we are all searching for the perfect number I highly recommend building adjustability into your design. Below is a CAD picture of my setup, I have anti-squat adjustability between 50% and 102% and up front I can adjust between 83% and 174%. These figures can be adjusted even further up or down by changing my ride height. I believe that in doing this I will be able to find the "perfect" anti-squat for MY rig!

Notice in the drawing the white circles on the brackets those are adjustment holes along with 2 holes in the frame for the upper control ARM up front.. My roll axis sits about level with the bottom of the frame! :D

 

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I love Mechanical Desktop for modeling. It is truly an amazing piece of software. Excellent topic that I know nothing about, but have always wondered. Keep up the good work guys..

TPIJeep, what's your wheelbase? What reference did you use while investigating this suspension type?

Chris
 

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Re: Re: Antisquat

TPIJeep said:


I agree with all the tech you posted, sumed it up nicely how to figure your anti-squat and COG but the comment above does not seem to be accurate in the real world.

I kinda winced as I typed in that comment, and figured of all things it would bring out some sort of disagreement. I even intentionally left out any sort of comment about "acceptable" antisquat values for fear of throwing this thread off the deep end and turning it into a low squat vs high squat with strap holy war type debate.

I was over at friends today and I was joking around about needing a squat-o-meter, similar to those junky clinometers they sell through jc whipme that will help show you how close to tipping you are. (As if your ass-o-meter can't tell you that by your current pucker factor.)

It seems that real world and book theory most collide on this subject more than any other. The problem is once you take the rig from level ground it seems that weight transfer throws the whole static calcuation away, and the lovely AS values calculated during design are thrown out the window. I still believe that it should be taken into consideration, but I don't believe it's the holy grail of suspension design, but that's just my opinion. I think that your suggestion of making the level of AS adjustable is excellent, since different people like different amounts depending on the rig and environment they are in.

One thing that intrigues me about your statement is talk about AS on the front end. I, too, do not claim to be the god of suspension, but I've not heard or read of doing AS calcs on the front end, but instead doing anti-dive with the brake proportion being taken into account.

edit: TPIjeep - I am jealous of your autocad capabilities.. your 3D full renderings make my drawings look like I was a kid using crayons.
 

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Re: Re: Re: Antisquat

JohnnyJ said:

One thing that intrigues me about your statement is talk about AS on the front end. I, too, do not claim to be the god of suspension, but I've not heard or read of doing AS calcs on the front end, but instead doing anti-dive with the brake proportion being taken into account.

I said anti-squat for the front end but that is really anti-lift the flip of that on braking would be anti dive. I made a little adjustability up there incase it wanted to stand on the front bumper when braking. I would like to see is sorta plant the front tires on acceleration. There has been VERY LITTLE discussion on front link design Anti-Dive values most people say your limited to space so just build what you can and be happy with it.. Well if I am unhappy with it i would like a few options!


On with the topic at hand, Now that we have the Anti-Squat portion done with the conclusion that get it somewhere between 50% and 150% with adjustablitly we move on to another factor which is roll axis.

The diagrams really do make sense if you look at them long enough! You want your roll axis to be as high as possible and and flat as possible, This will make your more stable when off camber and help control rear steer. Link placement here is important, you can tweak your roll axis by moving the links inward or outward while keeping the height the same. Doing so will not affect your Anti-Squat values. If you cannot figure out the diagrams I am sure we can help to explain them.

Here is a pic of my roll axis(green line)
 

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dang - pretty good thread so far.

I'm curious to know how people are calculating their loads and determining tube dia/thk, joint size and bracket design. This is the area where a lack of knowledge could kill you (or someone else) - no pun intended.

Brian
 

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NVR FNSH said:

I'm curious to know how people are calculating their loads and determining tube dia/thk, joint size and bracket design. Brian
Personally I think anything less then 3/4" heims on a link system is crazy. As far as calculating its a matter of choice, how much you want to spend, what size tires, rig weight and horsepower. I think the RE bearings will work well for most link systems as long as you have enough vertical link seperation on the axle. As far as materail size most heims and RE bearings can be made to go inside 1 and 1.5" ID tubing with an adapter or straight tapping the tube.

As far as the brackets go make sure you have lots of weld surface and get good penetration when welding.. in a double shear setup 1/4" thick should be more than enough... although I am building all my brackets out of 3/8" !!

I am running the 1" 1-1/4" shank heim joints with 2" .250" wall tubing, but my rig weighs 5k, has 40" boggers and 450 hp.

:D
 

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Arm length...
Uppers 37" 1.5" .25wall
Lowers 40" 1.25" .25 wall sleeved with 1.75 .25 wall
7/8" heims
Cherokee coils
I believe my AS to be at 103% but have not had it on the road yet. <2 months remaining. :bounce2:

Shorter uppers doesn't necessarily mean that the pinion rotates the wrong direction. Mine doesn't. It still depends on placement and the rotation through the arcs.

Here's a few pics...
 

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Re: Re: Re: Re: Antisquat

JohnnyJ said:

... seems that real world and book theory most collide on this subject more than any other. The problem is once you take the rig from level ground it seems that weight transfer throws the whole static calcuation away, and the lovely AS values calculated during design are thrown out the wAS on the front end. I, too, do not claim to be the god of suspension, but I've not heard or read of doing AS calcs on the front end, but instead doing anti-dive with the brake proportion being taken into account.
TPIJeep said:

I said anti-squat for the front end but that is really anti-lift the flip of that on braking would be anti dive. I made a little adjustability up there incase it wanted to stand on the front bumper when braking. I would like to see is sorta plant the front tires on acceleration. There has been VERY LITTLE discussion on front link design Anti-Dive values most people say your limited to space so just build what you can and be happy with it.. Well if I am unhappy with it i would like a few options!
On with the topic at hand, Now that we have the Anti-Squat portion done with the conclusion that get it somewhere between 50% and 150% with adjustablitly we move on to another factor which is roll axis. [/Bold]

I hardly think you have it all wrapped up and done fellas ...
How about
In tall rocks I'd like to see my back end rise and my front end squat when climbing up ... so I won't need to fill my front tires with water :flipoff2:
Then when braking going down hill I'd like my rig to squat in the rear and rise in the front so it won't want to endo as soon as it hits bottom.
 

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Re: Re: Re: Re: Re: Antisquat

Jaffer said:
JohnnyJ said:




I hardly think you have it all wrapped up and done fellas ...
How about
In tall rocks I'd like to see my back end rise and my front end squat when climbing up ... so I won't need to fill my front tires with water :flipoff2:
Then when braking going down hill I'd like my rig to squat in the rear and rise in the front so it won't want to endo as soon as it hits bottom.
Sounds like you want a 64 with hydro:p :p :p word up player! peace & chicken grease ,I'm outa here gota bounce hommie, keep it on the down low, Ieeet:D
 

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Tpi what is the advantage of using 2 seperate links on the front drivers side couldn't the same thing be be done with say a Y on the drivers side i was just wondering if it may be done to help with adjustment or control. I was going to ask you how you plan on ataching your panhard bar to the axle when the pig is in the way or are you mounting it to the top of the pig by a rock ring support plate. :beer: :beer:
 

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Jaffer.... Thanks for the input, and how you offered so much tech on how to do that.... Hence why I built adjustability cause I don't know, nor think I have it all wrapped up..

Seabass.. :rolleyes: :rolleyes: :rolleyes: :flipoff2:

Mad Mac,

I built seperate arms to allow some adjustability and so I can tweak my pinion angle if need be..

As far as the panhard goes take a look at this pic.. the mount IS above the pig.. :D


 

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Tpi is there any reason for the upper panhard mount to be so long what is the distance from the top of the axle to bottom of the frame. I also wanted to know if keeping the panhard as flat as posible at rest had somthing to do with your design. Looks good so far though man keep it up looks to be you got learning autocad licked.

:beer:
 

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MAD MAC said:
Tpi is there any reason for the upper panhard mount to be so long what is the distance from the top of the axle to bottom of the frame. I also wanted to know if keeping the panhard as flat as posible at rest had somthing to do with your design.
You want your panhard bar to run at the same angle as your drag link to minimize bumpsteer, that is the reason for the long mount... that and to get it away from the upper control arm!
 
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