The system on sdetweil's truck is a variation of the trailing arm suspension used on Chevy truck rears and NASCAR cup cars. In theory the lower trailing arms should intersect and pivot at the front U joint of the driveshaft. In this case they are truncated but look like that would still be the case if the centerline of the bars were extended until they meet. On the cup cars side to side movement is resisted by the panhard bar (a bar running across the chassis connected to the axle on one end and the chassis on the other), rotation is stopped by solidly mounting the rear of the trailing arms to the axle. A panhard bar's downside is that the rear axle move differently at each end due to the asymetric mounting arrangement. This isn't a disadvantage with the cup cars since they only turn left, instead the bar's length and mounting angle can be used to their advantage in controlling traction and handling. When you see a pitcreww member stick a long wrench into a hole in the rear window and turn it, he is adjusting the height of the chassis end of the trackbar mount. On the cup cars the rear coil springs sit on top the trailing arms in front of the axle rather than over the axle. Since SW's trailing arms have a flexible mounting to the axle at the rear, A panhard bar would not work since there would not be any resistance to axle rotation. Instead two upper bars are mounted at a significant angle to provide resistance to both side to side movement and rotation, and provide symetrical movement of the rear axle. Like two unequal length A arms in an IFS the difference in length and angle between the upper and lower bars is designed to give a certain amount of pinion angle change as the suspension drops coresponding to the chassis/ground angle change.

To take it one step forwards, a 3 point or three arm suspension is just a similar suspension geometry with the two upper arms extended and mounted with a pivot where they meet. In some cases the position of the upper and lower arms are reversed, with the longer straighter arms at the top and the short angled arms at the bottom to give more depth space for trunk floor or spare tire placement.

 

This is the trailing arm setup. A buddy of mine is building this '84 Bronco. The arms are based on the Nascar cup car design and these were built by a professional race chassis builder. While they look like they will bind during up/down motion, they are designed to twist, due to the C-channel construction and short welds.
The panhard bar is made as long as possible to limit the amount of side travel in the arc path.

 

Exactly, except I can only assume it is not completed yet. There should be a flange welded to the axle and a matching flange welded to each track bar and then bolted together. The first launch with the rear as set up (there doesn't appear to be a flange or perch welded to the axle?)will result in the axle spinning in the U bolts and turning the drive shaft into a balloon animal. It also looks like the pinion angle is a bit steep, but that may be because it is not sitting on the suspension? Note that if you extend the traling arms forwards they look like they would intersect at the front U joint. If they had been that long, there would be less twisting force on the front end of the arms to deal with.
Very clean nice work! Do the shocks have enough travel to not bottom out when the bags are deflated? Bottoming out the shocks even just supporting the weight of the vehicle can permanently damage them. Will he be adding a sway bar?

 

Ya, it's no where near finished. The frame stops at the firewall, waiting for the frontend parts. The truck is sitting at ride height and won't be a frame dragger, so the shocks look to be a good length. It's been a while since I last seen it and he wants me to come down to pick my brains on some ideas. It's running a Lincoln 4.6 DOHC, 4R70W, modified Mustang II frontend, Baer brakes and 20" Torq Thrust II. It's mainly a show truck, but should be good on the street.

 

Ax,

One more question, what the maximum angle at which rear shocks should be mounted. What I'm talking about is the angle in toward the center of the axle when looking from the rear of the truck. I called Bilstein and talked with a tech and they suggested I talk with Steve Duck from Race Car Dynamics. When talking with Steve, he was concerned that my ride height from eylet to eyelet was only 10 3/4" and this really limited the Bilstein shocks that could be used (because of the limited travel). He suggested moving the shock mounts so I could get a shock that has a greater travel. One option would be to change the upper mount and possibly moving the mounts toward the center of the truck. I just want to know how many degrees I could go in before having a negative effect on the ride. 30 degrees.....35 degrees?

 

Most shock manufacturers say no more than 30* from vertical. The greater the angle the less effective the shock's damping will be. At ~45* the shock just start rotating instead of compressing. Easiest solution would be to extend the lower mount downwards. As long as you stay within the rim diameter there souldn't be any issue. A more complex solution often used on very low nosed sports cars etc. would be a bellcrank system, which will allow any mounting angle up to horizontal.

 

good/bad/or indifferent, my shocks are at 35~ degree angle (suspension at ride height). Its 45 degrees all the way down. AirRide didn't think this was a problem

the lower eye is 8 inches from the ground.
the eye to eye distance at ride height is 21-8 = 13inches
all the way down is 17.5 -8 = 9.5 inches.
all the way up is 24.5 -8 = 16.5 inches.
so I have a 7 inch vertical travel.

Sam

 

At least you are installing shocks. There is so many guys running bags that say they don't need shocks, it rides like a Cadillac. Shocks are need for all applications. The fact is, you built a nice driver and you aren't road racing it or hauling loads, so you will probably be fine with that setup. If you find it bottoming out too much or a really spongy ride, you can look at up grading the shocks later.

 

No shocks?!.. are they nuts? it'll be all over the road!

Sam

 

Jeepers... looks exactly like this 104-yr-old design of Henry Ford's...

 

Well, sort of. The early Fords trailing arms, driveshaft and torque tube are fixed solid to the axle and the pivot point is the u-joint.
One thing I noticed about the arms on the Chevys of the '50s and '60s is, on the short box trucks, the trailing arms intersect close to the forward u-joint while the longbox uses the same suspension so the forward pivot points are way back from the u-joint. I would imagine there is enough play in the system that it isn't a factor.
When guys use a 2 link setup, where the arms are parallel, fixed solid to the axle and are short, there is insuffictiant articulation for movement in the axle and it moves as on solid unit. Left side goes up, so does the right. It's fine for going straight, but cornering must be a b**ch. These are typically the same guys that don't need shocks.

 

Shawn, Right on!
Sam, I assume and hope that your setup was engineered and tested, but IMHO there is a great deal of difference between "no problem" from the people selling the bag setup and "this is the way our shocks work" from the shock manufacturers. Each has their own point of view. There is the physics of angular displacement that can't be argued with.
If there is enough interest I can elaborate more on shock theory and practice, or can discuss it off line on an individual basis.

 

Ax,

Anything you share is appreciated....and I'm sure a few other people feel the same way. I decided to follow your advice and I custom made some lower shock mounts. I think it will bring my angle in from 30 degrees to about 20 degrees or so and it also increased my ride height between shock mounts.....more choices for shock types now.

 

Ax, please carry on. I have a good grasp on the designs and concepts, but I only have experiance with my truck. I did a homemade flip kit at first, then installed the IRS. On the front, I have done custom mods to the drop I-beam setup. I'm very happy with the rear, but the front has a very firm ride. It handles like a Mustang, but rides like a 1 ton. I don't know if it's the Slam bags or the QA1 shocks that are making it so firm. I don't want to lay out the $400 a piece for the double adjustable shocks and find out they are just as firm. Open discusions like these, help me to research new possibilities.
I've concidered fabing a custom SALA setup. I modeled the '97-'04 F150 suspension in Solidworks to see if it can fit my frame by making a new crossmember.
Springs and shocks are a major variable and every vehicle is different. You could go through 40 shocks before you find the right one. Understanding every aspect of the design helps to pinpoint a better choice.

 

Shawn,
Fortunately IIRC the QA1 shocks are rebuildable, which means they can be revalved if they are too stiff. Are yours the single adjustable? If so the adjustment changes the rebound (extension) only, and adjusting thru the range from stiff to soft would have very little affect on the ride. Rebound controls the transference of weight from corner to corner (weight transfer always occurs on the diagonally opposite wheels) The down and dirty way to determine if the compression of the shocks is too stiff for your application would be to temporarily disconnect or remove the shocks and take a ride over a familiar road where you find the ride too harsh to see how much better it gets (ignore the after bounce or porpoising effect of no shocks). If it improves a lot then the shocks likely need to be revalved softer on compression, or possibly are binding durring the suspension travel. If it is still harsh, then you may need to go for larger bags (I don't know a lot about bag use, I can only assume there are different sizes for more or less weight support?) that require less air pressure to reach road height, but I find it hard to conceive of bags ever getting too stiff under the front of the truck to cause a harsh ride. I'd check for suspension binding by first applying bread wrapper twist ties snugly around the shock shaft and pushing them down against the shock body. Once again take that test drive and when you return measure how far up the shaft the tie was pushed. This gives you an indication of how much shock movement you are getting. Put the front end up on runup blocks and block the rear wheels up a similar amount. Measure the front end height from the floor to the frame at a convenient place near as possible to the axle. Place a floor jack under one front corner of your truck and deflate the bag (or remove the spring if using springs rather than bags). Now slowly raise and lower the jack about 1" more than the measured travel in both directions while watching and LISTENING to the shock as it moves thru it's travel. Does it hang and release, make noises, or bottom out at any point thru the entire range? If so, then disconnect one end of the shock and repeat the height sweep in the suspected bind area while holding the loosened end near the mounting point with your hands. If you can't hold it there, find out where the bind is occuring. Body of the shock contacting the mount or other part of the suspension or the mounting stud twisting in such a manner that the eye bushings are compressed to the point the stud hits the metal part of the eye are common contact points. Do whatever is required to alieviate whatever is causing the binding condition.
What exactly is a SALA setup, I'm not familiar with what type of front suspension Ford used in their trucks during those years. Are you talking about the twin I beam setup? That was never one of Ford's better ideas IMHO.
There is a lot to take into account in designing your own suspension and front end besides just fitting it in place. I'd suggest you do some significant research before investing any time or money. I'd start with an excellent yet easy to understand inexpensive soft cover book: Tune to Win by Carroll Smith. It's a good reference book for any gearhead to have in their library (or bathroom).