weird ass rear suspension problem

well, i finally got my truck to the point where it can move under its own power. I had a major tranny leak so i drove it a 1/4 mile down the road to my uncles garage to gain access to his pit. Im way too fat to fit under this thing. It rides real nice going down the road at 10mph w/ no shocks hooked up.

after i fixed the leak i decided to let my father drive it back to the shop. The second he hit the paved road he tried to spin the tires. Which we all knew he was gonna do. The strangest thing happened. The tires never broke traction, and the rear of the truck went straight up causing the rear coils to fall out. Then the whole damn thing came back down resting the bed rails on the tires.

I copied the rear suspension from a '62 chevy 1/2ton pickup. So ladder bars, coil overs, and panhard bar. Im planning on using the original lever action shocks my truck came with, but they werent hooked up when this happened. Rear tires are 7.50-16s, i cant believe they didnt break free.

The engine is a 454 going into a ford 9", not posi.

Im thinking limiting straps would be a quick fix. I've never seen the rear end of a car lift like that. All of my experiences with cars tells me if anything lifts it should be the front.

What the hells going on?

 

Are the ladder bars too short?

 

how short is too short? They measure roughly about 30", i cant be sure right now the trucks in my shop.

 

some drag cars the rear actually lifts. it forces the rear tires down giving more traction. Lifting the front is cool but slower, I was told by achassis guy if you can dial the suspension in to get the rear of the car to lift you will get max traction, which would explain why the tires didn't spin. Brandon

 

Pics of your setup? whats the wheelbase? wieght distribution? Wheres your instant center?

 

May be hard on the rear shocks, but they will act as limit straps when installed...just like front shocks keep the A-arms from coming to full droop in front...JMO

 

Once you put shocks on it it wont move as far to lose the springs.You might need to buy different shocks too.You can try them,but the shocks hooked should be the cure as the truck is lifting up and there isnt anything to control how far it is moving,shocks will do that.AlsoChevy trucks have there springs bolted in....

Steve

 

Im just thinking if the truck is light and the bars are short the leverage could lift it.

 

Post pics to see what you have going on. Short arms and your instant center is out. What angle is your lower arms at?

 

Hillbilly,,,you did the right thing !! You just did not finish the job....
The suspension should LIFT the rear of the frame under acceleration...
Think about what is happening ! If the frame lifts up, it is trying to push the tire DOWN into the pavement = traction !
We work very hard to get this lift effect on our dirt track race cars.
Take a look at pics of a modern dirt late model or imca modified. (under power) You will see the rear end rotated way forward and the left front wheel @ full droop ( if not off the ground completly ) Roughly 80% of the cars weight is on the rear tires at this point ! How is that for traction ? LOL !
The shocks are the travel limiter on most stock type suspension. You would not have lost the springs (1) if the proper length shocks were installed and (2) on stock type coil suspension there is always a strap or cup bolting one end of the coil to the vehicle..
This "truck arm" type of rear suspension is really just a super long torgue arm, or from another perspective ..long traction bars.
What this gives you is a lever that is using the torque of the engine to lift the weight of the vehicle.
Nextel , sprint, or winston cup (how old am I ?? LOL ) cars have used this for decades.
If you have the room under your car I highly reccomend this suspension to anyone. Use with conventional coils, or coilovers, or air bags .. It is easily adaptable.

So fasten the coils down , put some shocks on it and try it again..
Let us know how it works.
Dave

 

i need to re-work my rear mounts they bent when the frame came back down, but hey thats why we test stuff right.

 

WHOA!!!!! get out your cutting tools and change those mounts!!

First off those are not ladder bars, and they are pointed up, they should be level! Remount them level with the ground or your going to have all kinds of handling issues. Thaats probably the majority of your problems. Also do a search on split rear bones. Not a good design at all.

When the chassis is lifting those wishbones are going to cause the wheelbase to shorten, check you trans tailshaft for signs of the yoke bottoming out.

 

Hill, looking at the pics I am guessing cowl steering ?

Also. it looks like you did not totally "copy " the chevy pickup..
The short arms @ the steep angle amplify the lift...
This effect is one of the reasons why the p/u arms are so long, to soften the "jacking" effect..
I wish I could have witnessed you dad getting on the gas ! I bet it spit the springs out real quick and real hard...LOL !
Dave

 

my truck, 70 chevy, same as yours . the sping is bolted on the top &bottom.

 

Tman, I wasnt going to tell him to rebuild the thing..But since you brought it up !
I would level the arms @ ride height, extend them to the 2x3 crossmember, and angle them towards the center so that the driveshaft will just fit between the mounts. Also, unless those lever shocks are modern hyd. units I would not use them. Coils have a much higher frequency rate than the leaf spring those were designed for.
Just my 2 cents.
Dave

 

um dats Da Tinman, Tman wouldnt have been so nice! And for whatis its worth, I'd cut that chassis in half and throw the back away and start over.

 

yeah, the coils came out pretty fast. I'll relocate the arms parrelell to the ground. Im thinking to the lower part of the Z. Once again i ask, How short is too short when it comes to these arms?

 

for street use, a lot longer, you wan the mounting point as close to the trans output as you can get, solves the lift issues, and make it react in a better arc.

You should really angle those bars in at the front or your going to break them when they try to twist during cornering

 

See picture for one possible explanation.

 

T- IN- man , Sorry about that !
(my shorthand inadvertantly called you by another Hamb'ers name)..
My first choice would be to extend them all the way to the trans mount.
Since hill (shorthand ,again ) has that big 2x3 crossmember right there..
I think that it would be an ok compromise solution. Yes , a lot longer is better, but, I think that it will better than double the current length to do as suggested in my earlier post.
Dave

 

that was my next question. So your saying i would have the same problem with 4-link then? I've done alot of jeeps before and have had to put limiting straps on them. but that wasnt because of power/torque it was because of too much articulation.

 

Yeah, as mentioned, those arms should be angled in toward the center of the vehicle. That lets the rear suspension articulate more freely and allows one wheel to lift without the other following. Also, in the Chevy pickups, the springs are bolted to the arm and the frame. Another thing to keep in mind is the original application arms were two LONG pieces of C-channel welded back to back. This and being mounted with a BIG rubber bushing up front allowed them to twist a bit to prevent binding and the ensuing breakage that will eventually occur if the axle can't articulate freely. I have a truck arm setup on the rear of my '34 built from drilled I beam using a transverse spring. It works very well. There's lots of info on the hamb that shows how to build one of these setups correctly. Check it out before you hurt yerself or worse.

 

lets say i made something like these ladder bars. and weld a batwing style mount to the axle and another mount to the horizontal part on the frame. would that work?
http://www.rockymountainsusp.com/tractionbars.htm

 

You said you had coilovers. Those are just coils. Ladder bars or trailing arms, whatever you use, as long as they are fixed solid to the rear, need to be LONG and angled towards and as close to the driveshaft as possible, otherwise you just have a big ass hinge and any articulation comes from components bending and twisting. Your frame is inviting mucho flex with that tall vertical section, should be angled back, like at a 45° angle or there about.

 

Yep.

that would be the hardest workin Panhard bar in Hot rodding.

step one-
bones paralell to the frame, as long as You can make them, and as close to one another as possible on the front end- triangulate, Yo.

step 2. tube shocks. the knee action shocks will not compensate for pinion twist the same way as a good 'ol tube shock. anywhere as cool lookin? no.
performance gain? You betcha.

step 3- spring mounts. You need to re-think those. alot.

 

Too much going on in the caboose for me with that setup!
I'd simplify the whole works.

Parallel leaf springs.

(EG only)You can get a long set from a pre 2001 Caravan/Voyager (Multi-leaf springs as opposed to later mono-leafs) along with the bolted on spring mounts.
Regardless...A trip to the scrapyard and you have a full, strong suspension from something.
Get some measurements of what you need for overall length and length to the axle center and take a tape to the scrappers. You'll find something.
You can adjust spring rate by adding or subtracting leaves usually and height by using lowering blocks.
If you need "traction control" LoL, make top links to the existing upper arm mounts to make it a leaf/link setup like the old AMX Rambler.
Lever shocks would be good if the leaf springs have several leafs to make up the pack but tube shocks would be super easy with your frame.

If you do go leaf springs, remember to keep your front bracket about the same height as the center of the axle and let the rear of the spring come up closer to the frame. Gives better control by lessening the potential for roll steering of the rear axle.
Look under a 70's/80's rear leaf spring pickup as a good example.
Note how the springs are positioned with the rear bushing attached very close to the bed floor on upsidedown shackles?
Thats why...



Now...that front suspension appears to have some issues worth addressing as well....
F1 axle?
You bolted radius arm adapters to the spring seats? Thats not a bad way to do it.

But the arms themselves look a bit weak and the frame brackets look really iffy!
We can't really judge thickness and strength from those pics but it's definately worth rechecking and maybe a closeup of what you have so people can offer suggestions etc.
The front doesn't look like it would be nearly the extra work that the rear will be if coils and arms are kept back there.

 

Every thing else will work fine if you change your arm mounting location on the frame. Remount them so they are even with the ground at ride hight or a tad lower in the front for the short arm. Bolt in your coils.

 

LOL, you have now experienced what is known as "anti-squat".. brightened my morning with the humor of it all.

Billy Shope has put together a really neat web page dedicated to rear suspensions. It's not all that complicated and I suggest you read it.

http://www.racetec.cc/shope/

 

The "instant center" referred to earlier is where your suspension applied force to the chassis. YOUR (not correct I.C.) instant center from the side is a couple feet in front of the rear axle center but several feet behind your center of gravity.
To find the longitudinal center of gravity take a guess....or put the complete assembled car on a scale, get a front and rear axle weight and use math to find the equilibrium.
There is still a vertical CG component I can't remember the name for which is the mass balance point from top to bottom represented from the side as a line drawn front to back (longitudinal) through mass of the car.

So there's a single line drawn through the mass that is a center and a point on that line which is the center of gravity.
I'm pretty sure "Roll center" is the where the suspension tries to rotate the car when it leans to the side like when cornering.

If the roll center is higher than the center of mass then the car will be tippy.
The closer the rollcenter is to the surface of the road the harder it will be to tip it and a line drawn longitudinally through the rear and front suspension's roll center indicates the axis the car will move on in a corner and "puts some english" on weight transfer in cornering..
This is the advantage of a fully independent parallel arm suspension like Jaguar or Corvette and illustrates the necessity of fully engineering front and rear suspension geometry, not just building what looks good

If the front is 60% heavier than the rear then the center of gravity will be 60% of the wheelbase from the rear axle forward.
If your instant center is far behind the CG then like a wheelbarrow it's going to lift the rear with severe anti squat (leverage) throwing more vehicle weight to the front.
If the instant center is far in front of the CG it's going to lift the front more making it wheelie prone.
There isn't any difference if your running radius arms or parallel arms.
With radius arms the I.C. is the point where a line drawn through the axle/spindle centerline intersects not the mounting point to the the chassis but the vertical CG.
With parallel arms the I.C. is where lines drawn through the mounting points of the top and bottom intersect.
The shape of the arms is irrelevant, it only matters where they mount.

I don't remember if you need the I.C. in front of or behind the CG but it should be close and then adjusted to work with engine power so that under launch the rear does not rise more than the front and so that the front does rise a little more than the rear but at a minimum.
Weight transfer by the front lifting a bit is good but wheels off the ground is dangerous and wastes momentum. The fastest cars on launch rise almost level, just a small bit more in the front

 

Ok heres my 2c, front spring mount, 454 + anti squat = alot of pressure on the mount, I'd be looking at something more like a '32-on style of mount rather than relying a "suicide" style with nothing to stop (with faliure of the bracket) the whole front end coming out as well.
At least the front brakes won't twist the axle out.