Thumping sound coming from the rear, what did I break?
 

Hey guys... I'm not sure what I broke but I think I broke something.

I did a burnout then a hard launch. I'm making 331 to the rear wheels and I had drag radials on the car. When I let out of it and cruised away, I realized that there was a strange thumping sound coming from the rear of the car. I've got solid diff mounts so I can hear everything coming fromt he drivetrain.

At first, I thought maybe I got something stuck in the tire because the thumping was proportional to the speed of the tire rotation. I put it in the air and there was nothing wrong with either tire, everything rotated just fine with no noises.

I put it back on the ground and drove it down the street and still heard a thump...thump... thump... thump. I gave it some gas and the car didn't feel like it was missing a beat. Just that same thump....thump....thump and it got faster the faster I went.

My question is... what do you guys think I broke? I was thinking that I may have broken a tooth on my ring gear or something but I'm pretty ignorant on the inner workings of the rear end drivetrain.

Power Plant Frame

Really? what part of it is the most likely place for me to look for cracks? I'm assuming toward the rear where it bolts to the diff. I'll go look right now.

I just went to look at it and I got under the car and examined it pretty close but didn't see any cracks... I looked really good around the bolts.

Is it a super sneaky crack or a major one that I would probably see?

Couldn't be suspension bushings?

No sir... just replaced 24 bushings including everyone of the rear bushings.

I'm thinking it's either the PPF, Diff or axles.

I'm hoping it's the Diff and not the PPF but I'll fix whatever is broken... I've come too far to turn back now.

Tires perhaps? Last time I did a big burnouts, I got the same thing you did... thump thump thump... I thought I blew my diff! But my tyres were so bad, they weren't round! Changed them, and it was smooth again :)

I wish that were the problem... I switched back to my street tires and I'm getting the same thing.

Does anybody know where I should look on the ppf for any cracks...where is the most likely spot?

I was thinking it might be that I bent an axle that got twisted.... could that cause that? How would I diagnose that?

Thanks for your help,
Jeremy

as soon as i read the first post i was thinking an axle. it would be speed dependent and you definitely could have effed one up launching on DR's. drag radials break stuff.

Anybody ever used these before:
http://www.driveshaftshop.com/item135656.ctlg

How do they compare to these?
http://www.rotorsportsracing.com/per...drivetrain.htm

i dont think it would be ur internals on ur diff, because u would hear a lot of crunching most likely along with the thump, i was thinking PPF too.

I can almost gaurantee you broke a tooth off of your gears.....I did the exact same thing doing a burnout. At first is wasn't too bad, but got worse until I got on it and barely made it home.....

Once you pull it apart and see if this is what happened....I would by 4:30 gears instead. It's not any stronger, but you will love your 5th gear for once!

One more thing.....it's a very slight possiblity that the bolts came loose on the driveshaft but that would make a bad vibration.

I seriously doubt its your axles, but to answer you question. I have the driveshaft shop axles in my car. I snapped a stock axle about a year ago on et streets at the strip. I upgraded to those and they have taken every thing i've thrown at it, even low 1.5x 60's

I have axels from the Driveshaft shop as well.....and they seem unbreakable!

So I've got 2 votes for the PPF, 1 for axles and 1 for diff.

I'll start with the most popular. Other than removing the PPF and checking it, where is the most likely spot to check for cracks or damage? I"ve been under the car and didn't see anything obvious.

Can anyone point me in the right direction as to how I could test each component? I don't really have the money to replace all 3 so I'd like to replace only what's broken.

Process of elimination yo..

That's my question... how do I eliminate each piece?

The PPF is easy.... I can just take that off rather easily and really examine it, I was hoping someone that had a broken PPF could tell me where theirs broke.

The other two are a little more complicated. I was hoping someone could give me some pointers.:icon_tup:

Here's a couple threads that people have had similar noises.. They're not 3rd gen's though..

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/weird-hard-thumping-sounds-rear-582386/
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/thumping-sound-rear-during-breaking-61779/

Thanks... I already checked the rotors and I've got solid Diff mounts already. That takes care of both of those threads.

I've seen a few threads with photos showing broken PPFs from way back when, so try searching for that. They typically crack at the "ears", usually at the diff end, but sometimes at the trans end as well. And then some break more towards the center, so just drop the whole thing and look it over.

I'd also try getting under there with the car on stands (and trans out of gear and parking brake off), and rotate the driveshaft by hand and just listen for noises from around the diff area. Sounds like you already tried rotating the wheels, but turning the driveshaft will operate the final drive gears and u-joints.

Thanks for the response. I've got an update:

1st... I can't pull the PPF tonight because I don't have the right size extension... will pick it up tomorrow.

2nd... I did rotate the driveshaft and spent quite a bit of time undernetha tugging and pulling on things with no avail. No noises. :(

3rd... I put the car in gear while it was on stands ( I know I'm not supposed to) and let the engine turn the wheels, I heard absolutely nothing out of the ordinary!!! No noise from the diff, no noise from the bearings no noise from the driveshaft or anything else.

I put the car back on the ground and it's still doing it. I had a friend drive it up and down the street and you can't hear anything from outside the car. You can still hear it really well and feel it too from inside the car.

At this point, I'm leaning toward the PPF being cracked somewhere I can't see or the axles. I'm almost 100% it's not the diff.

If anybody comes up with anything you think I might be able to use to solve the mystery, I'd really appreciate it. Once again DigDug... thanks a lot for giving me some more ideas.

Jeremy

drain the diff. to see if there are more than the usual amount of shaving (fine metallic particle is normal but if you see chunks, you are in trouble) or open up the diff and have a look around.

Once I pull the PPF I will definately have a look at the diff fluid for shavings or chunks. I still don't think that's it but if I'm gonna have to pull the axles, I've gotta pull the diff anyway. I'm really hoping it's the ppf cuz that's the easiest thing to fix.

Update:

Pulled the PPF... it's fine. Drained the Diff... no metal. I'm gonna put it all back together and see what else I can do without pulling the axles.

How can I tell if the axles are bent?