Clunking when getting on and off the gas

[68mustang]

My 73 has a 3.5" suspension lift and 2" body lift. It has a pretty decent clunk with getting on and off the throttle. There rear drive shaft has a steep angle to it. I have checked u joints and tight bolts. Could this be a problem with the shaft angle? Do people have to do some kind of drop kit?

 

[73azbronco]

engine, tranny?

Engine tranny mounts?

 

[okie4570]

Axle wrap?

 

[Speedrdr]

Is your rear suspension lift a new set of springs or a lift block? I have seen the locating pin on the block shear off and cause a clunking noise.

Randy

 

[DirtDonk]

A picture of your setup would help. But in lieu of that, how steep is the angle? A 3.5" lift should not be overly steep. I've found that when they are, the rear pinion is pointing too far down and needs corrective measures such as shims under the leaf springs.

Another thing, probably one of the most common reasons actually, is the slip joint in the driveshaft binding up. The only way I could get rid of my clunk was to pull it apart and manually grease the splines.
However, most of the time I think a slip joint is more of a clunk when you start. But not so much when you let off the gas.

So you'll have to be the one to do all the detective work. We can't see it or hear it, so it's all on you. We can only point you in different directions.
One of those directions would be to do with the body lift.
Look at your transfer case shifter to make sure it's not just touching the tunnel where it comes through.
Where is the exhaust run? Any part of it close to the body or frame?
What about the back of the driver's side cylinder head? Close to the firewall?
Steering shaft? Do you feel the clunk through the wheel, or is it all in the seat-of-the-pants coming through the floor?

Paul

 

[Yeller]

Locker?

 

[JSBX]

Also check your spring u bolts to make sure they are tight. That should be part of routine maintenance.

 

[DirtDonk]

You said you “checked the u-joints and tight bolts” but does that mean you checked the big U-bolts around the springs?
As said, that’s a good place to check.
Not only that though, but check to make sure that your spring perches are not rounded over on the ends. That used to be a common thing with Broncos and one of the sources of clunks back in the old days.

 

[68mustang]

351 with a c4 trans. I do have shims in it already. I’m not sure why you men by axle wrap, are you referring to it slapping around causing the clunk? The lift is all new springs from wild horse. The cluck is very solid and harsh. Enough so I feel like something is going to break. I will check a few of these things tomorrow and get some pictures to post. Yes it’s a locker. More feed back to come.
thank you to all. This gives me a solid starting spot.

 

[ep67bro]

Check the yoke on the rear axle and the output of the transfer case. You may find they are loose and will have slop in them, the nut maybe loose causing the yoke to be loose on the shaft this can cause a clunk when getting on and off the gas.

 

[DirtDonk]

351 with a c4 trans.

Just out of curiosity, what gear ratios in the differentials, and what tire size are you running?
This can be related to more or less axle-wrap.

I do have shims in it already.

Good, then the driveshaft should not be at a falsely steep angle, other than from the lift.

I’m not sure why you men by axle wrap, are you referring to it slapping around causing the clunk?

Axle-wrap, or spring-wrap, or "pinion-climb" is the action of the springs twisting from the torquing of the axle. Whether from acceleration torque from the engine and the traction of the tires on the ground, or the deceleration of the compression braking and the traction of the tires on the ground.
Generally speaking (and always on a Bronco rear end) when you accelerate, the rear pinion rises from being stuck in-between the engine and the ground. When you decelerate, it drops. When you're static, you measure the pinion angle as "x" and when you accelerate with leaf springs like on a Bronco, the pinion can rise up by quite a few degrees. Changing u-joint angles. In some cases, drastically.
I'll try to find a discussion we had on NorCalBroncos.com with video awhile back and post up a link.

The lift is all new springs from wild horse.

Good. And thanks by the way!
The reason that was asked, I'm sure, is because a lift using blocks will exaggerate the axle-wrap due to the additional leverage applied by moving the pivot point (the centerline of the axle shafts) further away from the main leaf spring, without adding any additional spring stiffness.
An add-a-leaf on the other hand, might reduce spring/axle wrap severity.
Stock springs were bad enough, but I believe most lift springs would be worse. But I don't have any empirical data.

Yes it’s a locker.

What type?
Lockers have some inherent sloppiness to them in some cases. In others, there might be some extra lash in the gears if they were not assembled correctly.
But a big thing is how lockers act on the street. Just as a double-check to rule this out, you should measure both the air pressure in your rear tires, AND the precise height that they are sitting at on the vehicle.
Variataions in tire height, whether due to manufacturing changes, or air pressure differences, can exacerbate typical locker issues.
Such as clunking.

Did you upgrade to 31-spline axle shafts at the same time? Lockers (real lockers anyway) are known for their hunger for 28-spline axles.
A limited-slip is much less harsh on axle shafts and on making noises on the street. But if you're running an actual locking differential, a lot of funny sounds and feels can come from it.
On my Detroit Locker, I don't have any of those symptoms. Just wanky handling in slippery situations. But none of the expected noises.

Paul

 

[68mustang]

I read all this info this morning before work. When I got home I went to work on the bronco. I have borrowed it down the the gears in the rear end. It is a 9 inch. When I jack it up there is a considerable amount of play When moving the wheels. I’m pretty sure I’ll need to replace the rear end gears. I have not changed the axles. What do you recommend for a gear set?

 

[73azbronco]

unless you know how to do gears, let a shop do them.

tire size, tranny, gears, expected use, gets you gears

 

[DirtDonk]

Don’t forget, depending on which locker you have, some of that play, or maybe even all of it, is coming from the locker itself. Not necessarily your gear mesh.
I’m not really sure how to tell the difference, without pulling it apart, but someone here might be able to.

 

[okie4570]

I read all this info this morning before work. When I got home I went to work on the bronco. I have borrowed it down the the gears in the rear end. It is a 9 inch. When I jack it up there is a considerable amount of play When moving the wheels. I’m pretty sure I’ll need to replace the rear end gears. I have not changed the axles. What do you recommend for a gear set?

Before you dive into gears, have someone drive while you stand next to the bronco as they accelerate and deaccelerate so you can listen to it under load........and jack up the front and back or put it on a lift and do the same while you listen.

Also, you didn't hear this noise whatsoever before the lift was installed? I only ask because this doesn't sound like it would be a gear issue. I can't think of a situation where the gear mesh would only make a clunk upon acceleration/deacceleration and not constantly while turning, as in teeth missing or a bearing cap loose, it would be constant and/or frequent clunk. What is "a considerable amount of play when moving the wheels"? If you're not sure what you have in the rear for a locker or LS and if you're not familiar with rear end play, what you experienced could be perfectly normal. And just confirming you did grease the slip yoke on the rear drive shaft?

 

[Broncobowsher]

Find some flat ground in a quiet place. Leave it in park. Start rocking it back and forth against the parking pawl.
Do you get the same clunk? You are loading the driveline in both directions (on and off the gas).