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Suspension gurus: rear axle and adjustable roll center

cwdodson88

cwdodson88

Well-known member
Joined
Oct 27, 2011
Location
The Dalles, Oregon
So I'm working on the installation of a Ford 8.8 into a 122 wagon.Based off my stock chassis mounts and a parallel 4 link. After running through the setup of brackets for that I've got a few things going. I have between 41% and 198% antisquat assuming I use camshaft height as my center of gravity. I have ample room to mount the shocks in front of the axle, links are similar to stock leaving a good deal of room for locating devices.

Question: would I be better shooting for my rear roll center to be chassis mount and adjustable or axle mount and barely adjustable?

Option A: use the factory cross brace (aft of the axle) and build an adjustable mount for a watts link bellcrank and mount arms that hit the axle centerline about 14" from the vertical centerline of the diff.

Option b: mount an adjustable panhard making an axle bracket that brings the rod across the diff housing at just about or above the diff housing centerline.

Looking for a versatile rear suspension that can go from nearly a dd to track day in a matter of a Friday night with a good beer and a few friends.

Edit: my rear coilovers have 5.5 inches of total adjustability between the mounts and the spring sleeve.
 
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The only thing I can offer by way of an answer is...it depends. Watts linkages have advantages in RC location and location adjustment as the RC is fixed at the pivot. A panhard rod is simple and lighter but the RC migrates with movement. Axle mount vs chassis mount is whether you want the RC relative to the tire (axle mount) or CG. Axle mount normally equals more oversteer while chassis mount is more prone to understeer.

I'm not seeing option A - can you take a picture. I don't recall the rear axle locating differences between the wagon and sedan.

Option B - just try to get the adjustment so that the panhard rod is level in each position. This is a good compromise.

When designing this stuff, try to engineer in as much adjustment as possible (I know this doesn't need saying), then you can fix on the fly. That said - make sure your front RC isn't 4" below ground. Drop the upper control arms up front...try out some of the modelling software on line. I use Racing Aspirations (http://www.racingaspirations.com/) to get an idea what's going to happen.
 
The CoG at camshaft height sounds very high. It's usually at about the crankshaft or the drivers arse height.

Plan B sounds the best to me. If I understand you correctly, you want to adjust the height of the suspension (for different uses) using the spring collars then adjust the panhard rod (presumably by moving it to one of a number of holes arranged vertically) to match?
A watts linkage is probably technically better, but adds a lot more fabrication and spanner work to change.
 
Canuck - I was hoping that you would drop some knowledge on me. Basically there is a cross member between the gas tank/spare tire hole, and the axle. Dropping a plumb line from foremost part of the 2x3" x-member and its about 1/4" to 1/2" away (aft) from the diff cover at ride height. Idea being to drop a 12" long stinger with holes for the bell crank mount 1/2" mount every 3/4", keeping it about 2" from the diff cover. From the bell crank, two rods about 14" long running to mounts on the horizontal center line of the axle.

Crogthomas - the cam height for these pushrod motors is only about 14" at ride height, given that the rear CG will be higher due to less mass, more unsprung weight, the CG should be somewhere between 17-19" so I used 18" rear and 14" front for my roll axis. Also, my axle center line front and rear is 12.5".

Problem I'm seeing with a panhard is the length of the brackets possibly interfering with the ground or the floor if I'm going to have any adjustability of the roll center to match the shock/axle adjustability.

I'll get pics tonight of what I'm thinking.
 
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cwdodson88 said:
Crogthomas - the cam height for these pushrod motors is only about 14" at ride height, given that the rear CG will be higher due to less mass, more unsprung weight, the CG should be somewhere between 17-19" so I used 18" rear and 14" front for my roll axis. Also, my axle center line front and rear is 12.5".

I'll get pics tonight of what I'm thinking.
Click to expand...

Your front roll centre can't be 14" off the ground...physically. Also it's very unlikely that your rear roll centre is more than about 12.5". Just because your front wheel is 12.5" to the middle doesn't mean it's locating your front roll centre. To work this out you need to map where your front suspension pick-up points are. You need ball joint centres and bushing centres (for a 2D model - it's not that hard to work out - for a 3D model it's a bit of work). Then you draw a line that goes from the ball joints through the mount pivot points and see where they intersect. That's your instant centre. Then draw a line from the centre of the contact patch to the IC ... that's your RC. As your upper control arms are basically level or pointing up (depends on if you're lowered or not) - that puts your RC at ground level.

The line between the RC and CG is your roll couple - a lever through which the CG acts on the suspension. It's very long in a 122 and that's what makes them roll so much. Lowered cars are worse - so everyone goes with the giant ARB to counteract this force.
 
opps sorry typing too fast, I meant that to be the front end estimated center of gravity.... not enough coffee this morning. I'll take pics of my drawings. My boss is going to plug the mounting points into some software he has over the weekend using the drawings I have of my front and rear suspension, chassis dimensions, and whatnots.
 
snapped some pics... left my front suspension drawings at work with my boss last night, so heres a couple rough sketches of the rear, and the space constraints.

<iframe src="https://drive.google.com/file/d/0B9M6uoq2GqnQT21pWnRvVzMwQk0/preview" width="640" height="480"></iframe>
<iframe src="https://drive.google.com/file/d/0B9M6uoq2GqnQR1cxdmh4dkxRbFk/preview" width="640" height="480"></iframe>
<iframe src="https://drive.google.com/file/d/0B9M6uoq2GqnQTTUtUUFfWm96U3M/preview" width="640" height="480"></iframe>
<iframe src="https://drive.google.com/file/d/0B9M6uoq2GqnQWER1SEJyY09TRVU/preview" width="640" height="480"></iframe>
<iframe src="https://drive.google.com/file/d/0B9M6uoq2GqnQWTRSVUV2V2FzT0E/preview" width="640" height="480"></iframe>
 
With a watts linkage the suspension will move up and down in a path perpendicular to the linkages when they are both parallel to each other. In other words, in your diagram the axle will move to the side, just like a panhard rod setup. Thus giving you no advantages over the panhard rod. This is the reason the mountings for a watts linkage are at different heights.

You can map it out yourself on paper, or make up a model with cardboard or Lego to experiment.
 
Parallel is parallel, the rods are parallel to each other, thats all that matters, my pivot angle may be a little off, but it shouldn't matter.
 
On the SCCA FP Sprite that I worked on many years ago,I put the center mount UNDER the diff,and ran a diagonal link to the side of the chassis, with an adjustable height control.The rear was a 3 link setup, with adj. trailing arms to adj. rear steer. We took 1 second off our best lap times at Lime Rock on a bumpy track. The car was well mannered and much smoother on R/L turns. The car is a multi time Div champ and a National Championship winner.FOOD FOR THOUGHT!
 
That's why I'm wondering how much benefit a highly adjustable rear roll center with the watts link will add compared to a not very adjustable panhard setup
 
Ok, crog- I owe you an apology. With my axl mounts at the same height like in my drawing the axle would be on a straight plane, but the travel would be at an unattractive angle. So I need to drop one mount and raise the other so the difference is approximately the same as the length of the bell crank.

https://youtu.be/3fBZgoXhrgE
 
Are you sure you have enough room for the Watts link? Looks pretty tight. The diff cover is over 3" from the mount surface to where the bracket for the pivot would attach.
 
Since my diff almost hits that cross bar, plan is to move the cross bar back, and take the wow out of it. Going back to the tape line, and having the pivot mount threaded and as tight to the new spare tire box as possible. It'll probably be 1/4" plate threaded for 1/2-20 and a nut weleded on each hole. All that and I should have about 1.5-2"
 
cwdodson88 said:
Ok, crog- I owe you an apology. With my axl mounts at the same height like in my drawing the axle would be on a straight plane, but the travel would be at an unattractive angle. So I need to drop one mount and raise the other so the difference is approximately the same as the length of the bell crank.

https://youtu.be/3fBZgoXhrgE
Click to expand...

Exactly. Apology accepted. The horizontal links will always be at a equal but opposite angle to each other. At only one point in their travel they will be parallel. It is that point which determines the path the axle (or body I suppose) will take, perpendicular to the parallel bars. Follow convention and it will work properly. James Watt knew what he was doing.

That said I'd seriously consider using a Panhard rod for your application. It will simplify mounting greatly and I don't think it is any disadvantage. Yes, the Watts linkage is technically superior, but do some quick trig and you'll see that a Panhard bar has very little sideways movement when compared to other things like mounting bush flex and tyre distortion.
Take for example a 1200mm long Panhard rod on a vehicle with say 160mm of suspension travel. With the vehicle on the bump stops one side and the other side topped out (the most extreme case) the total sideways movement will be about 10mm. In normal use, much less.

With long travel suspension and/or a limited width for the linkage, like on a 4x4 or narrowed dragster axle, I'd consider a Watts linkage, but not in your situation.
 
I need some more fab practice, and engineering experience, and I have some material so I'm going to give the watts link a shot, I'll post pics. Thanks for the input.
 
A good enough reason to give it a go. What sort of bearing are you going to use in the centre?
 
Simple solution is a bronze flanged bushing, the other option is a sealed ball bearing. Either way it should be doable. I may see if I can work up something in solid works and have it run in the spare 5axis at work
 
Ok, so Ive got some drawings for the center pivot, but we had a machine crash so my thoughts of using the spare 5 axis have been shot down for a while.

But, I have another question:
Should I box the back of these in?
<iframe src="https://drive.google.com/file/d/0B9M6uoq2GqnQaHQzTGpsRE1ydjQ/preview" width="640" height="480"></iframe>
I was thinking that I could cut a piece of box tube at an angle and put a axle curve on it and weld it up the sides.

What say the gurus of suspension?
 
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