Technical Rear end dances

Yesterday I was driving my 50 Ford Custom and noticed when I goose the throttle it dances one way and when you let off it comes back the other way. I did some exhaust work also and removed some lowering blocks and buttoned it up tight. It is a 3 speed no overdrive. Definitely got my attention.

 

Try installing a set of heavy duty shocks. Chances are your old ones are shot even if they are not leaking and seem ok. If they have over 25000 miles they are definitely shot. While you are at it check the spring bushings, and make sure the axle is solidly located.

 

Are the springs indexed to the axle with the spring center bolt correctly, and are the U bolts good and tight?

 

I am guessing that when you removed the blocks the rear end did not sit back down square on the springs. Just lossen the U bolts up and do some measuring. square it up and retighten.

 

It has air shocks on it and yes they are old and just took a look and one of the air line o-rings blew out.
It had a 4" drop and I removed a 1" block. Everything is tight. I just need to figure out what shock will work with a 3" drop.

 

I'm going to re-check everything, Thank you.

 

I can see why it was dancing. HRP

 

I replaced the rear shocks this morning.
It still wants to wash out when you stab it or back off. Whoa nelly, hold on!
Checked everything, it's all tight and looks like nothing has shifted.

 

I'd say you're getting some rear steer, because as the rear end pushes the car upon acceleration, one side is getting ahead of the other, hence steering the car from the rear. Just my two cents, but why don't you do it right and get rid of those enormous lowering blocks and re-arch the springs. I'm fairly certain the "dancing' will be back under control.

 

Look at the length of the lever arm from the center of the axle tube to the bottom of the spring pad. Now take the lowering block out and see that length. Than figure out the math. It will scare you and you will find another more stable way to lower the car. Spring warp or wrap is happening and eventually will break a leaf. And if its the main leaf it will get very exciting for a short time.

 

I don't particularly care if the car is lowered. I don't want a Yahoo Flume Ride.
I'm going to remove the blocks and see how she rides.

 

I agree 100%

not to change the subject, but a guy wanted me to find out why his IRS car was fine until you get to 45, and suddenly, a violent hard turn to oncoming lane, from the rear "steering". It had mega toe out on both rears. It would feel excellent, then cross lanes "hard" without notice.

I believe the blocks are adding way too much leverage and causing one side spring to wrap up, and get shorter, so it steers the rear end

 

I was wondering why your shoebox has so much travel between axle and bump stop, at first I thought it was on stands or a lift but can't see any.
The travel looks like 4wd stuff.

We have a 1" block welded to our axle and a 2" lowering block . It handles very well and tracks straight as, gas on gas off. We did initially have a slight rear end steer as we got on the gas. Discovered that one axel pad had the locating hole in a slightly different position to the other after a wheel alignment.
Perhaps the lowering block are not an exact pair in hole location.

 

Maybe one spring is much weaker than the other.

 

It was jacked up in those pics so I could get under to take the pictures.
Sorry for the confusion. I will get my son to crawl under and take some pic's sitting level.

 

Sitting level on the ground.

 

In my Pontiac, you could "steer" the car with the gas pedal going down the highway and it would lean a whole lot into curves. Changed the leaf spring bushings, the frame bushings, along with new shackles, new leaf springs, and new shocks. Wow what a difference. It was like driving a new car. I'd say to check and replace the bushings (not a fun job on the frame ones in my car by the way).

 

Ditch the blocks and have lowered springs made.

That, or install a torque arm.

When you get on the gas the pinion climbs the ring gear, and causes the rear end housing to rotate in two directions.

Torque rotates the housing on the plane of the tubes. Since the front of the leaf spring has a fixed anchor, the only movement that can happen is to flex the leaf spring. This effectively reduces the arch, or straightens the front half of the spring under load. When this happens the axle housing is pushed back, effectively lengthening the wheelbase.

To make matters worse, you don't just have spring deflection working here. The blocks you are running amplify that axle movement to the rear under load, as the are a long arm.

You may actually be experiencing an extension of wheelbase, and a subsequent contraction on deceleration, of several inches.

To compound matters, the second direction your axle housing is moving is in relation to the driveshaft. This makes the fore-aft deflection uneven from side-to-side.

If you have a parking brake installed, have a friend you trust put it on.

Have that person put the car in gear and try to drive off, both forward and reverse. with the brake on, while you are watching the position of the rear wheel in the wheel arch. Check both sides.

I would wager that you will see the wheel move fore-aft a noticeable amount. It only takes a little to steer a car.

 

Thanks for the reply's.
This problem just came on recently. About a month ago I replaced the steering box, idler arm bushings, coil springs and sway bar bushings. Had the alignment checked and basically, he just set the toe. I'm just curious if their is any connection to the issue.
My next move is to remove the blocks as mentioned above, replace the spring bushings and see if that fixes it. If not I will get a set of 3" lowered springs. Thanks guys.

 

Rear End Dances: A list.

1. the Hula
2. Shake a Tail feather
3. the Macarina
4. the Twist
5. Urban Bootie dance

 

Yeah, Mike. I was expecting some totally inappropriate videos by now.

 

Let's get some opinions from the guys here that worked in repair shops...besides me

I find it impossible that the car still was in spec on caster/camber, after you did that front end work.

If the car is spooky(AKA dangerous), I certainly would be checking both ends of car, and finding a competent shop to do the alignment. It can't be in spec, IMO. It takes so very little change in front spring ride height, to make a big change in at least camber... so how could it not need anything besides toe? ..and if they did not do that right, could it be a bit toe-out? that would be twitchy feel

 

No offense intended, but looking at the condition of everything back there, this is where I would start, and in the middle of it all, as suggested by many, have the rear springs de-arched so the lowering blocks can be canned.

 

I got my 4" lowered springs from St. Louis Spring (3" de-arch with a reverse eye good for another inch). They know their stuff and the springs were affordable and perfect.

 

I had a similar problem with all stock parts. One of the leafs was broken. Look for a crack in the leafs.

 

New parts do not always equal good parts. Inspect everything.

 

Since the problem did not happen with the even taller blocks, I don't think this is a simple spring wrap issue. Something else happened, or is happening. Either eye bushings are shot, spring could be broken, rearend not sitting square, or ???

While springs with less arch and no blocks would be better, there are plenty of cars running 3 inch blocks that don't have the problem to extent OP is describing. I am sure there is something to be found that is allowing the rearend to shift fore and aft, thus steering the car.

 

Checked everything I could underneath. I did have to re-torque the u-bolts. Going to pull springs out and de-arch and replace the bushings. I'm also going to take it to a buddy of mine that owns a frame shop to check front alignment again.


I'm sure this didn't help matters.

 

I don't see a clear picture of the shackles but they may have flipped. Flip them back and you can probably sell those blocks for scrap aluminum. Harley

 

I've fixed three cars with these symptoms and all were a combination of incorrect front end alignment and the rear axle not being parallel to the front axle. As several people have already said, the big lowering blocks will compromise the rear axle location under heavy acceleration when the springs wind up. You can easily check the front to rear axle alignment with string, or fishing line, which is surprisingly accurate if you are careful.