Bought car several months back that has a rear knock sound when going over bump or pothole.
It sounds like a hard banging frame sound, very loud.
If both back wheels go over a bump together it is quiet, it is when mainly left rear wheel hits a bump, and more sensitive to this when cornering to the right at the same time.

Due to some wear, i have replaced both rear upper control arms, also rear right side lower control arm.
Upon lifting car on blocks, the rear wheels have no play with all the usual up/down/side to side pulling/pushing action.

All the other bushes seem ok, and no contact with exhaust.

My attention now turned to the subframe, with the car still on blocks and the subframe in effect hanging, I pushed/pulled the wheels again whilst looking at the bushes, no play found.
I also carefully checked that the subframe was not near any underside, both on blocks, and with the wheels on the ground, all ok.

Also the shock absorbers are also approx 18 months old, and are jag parts.

Any good ideas ?

Thanks,
Paul.

 

My wheel nut spanner used to cause a notorious knocking being so inadequately clipped to the jack. All the bits are now wrapped up in old T shirts and Grace has been restored to the Pace and Space!

 

man on the Hill, have you looked at the rubber bushings for the anti-sway bar? It is possible that those have developed some play and atleast 1 side is loose now and is allowing the sway bar to bang against the frame of the car. You can test this by taking a rubber mallet and striking the sway bar near the mount. You will hear a difference if the mount is bad (ie, good side will more of a high pitched thud where if the bar is striking the frame, it will be more of a lower pitch thud).

 

Astromorg, good tip on wrapping the wheel brace with rags.
I did make sure the jack and brace was properly secured previously, also it did have this problem before with the wheel and parts out.


Thermo, I did check the droplinks and bushes by clamping a wrench on each side and twisting to check for play.
For good measure I have just gone out to the car and used a mallet near each mount, all checks out fine.


The banging noise problem sounds like a metal frame hitting the underside of the floor/boot area, it is loud, and it can bang several times at one go.


Even though the testing of the subframe to body bushes seem ok, could they still be the culprit, as you cannot see the innards without removing the bolt/washer ?


Paul.

 

Man, I know this might sound silly, but what about the exhaust system. Does it have a lot of play and able to move side to side? Hitting both wheels into a common thing would cause the exhaust to go straight up and down normally. But, having a single wheel hit something would cause it to go side to side, potentially striking the side of the spare wheel well.

As for the subframe mounts, I would think that if you are hearing the subframe hit the body of the car, if you support the car by the body and have the rear wheels off of the ground, you should see a gap form on the bad mount.

It may be advisable to jack up the rear end of the car and set the rear tires say on top of some 10" (25cm) tall blocks so you can have the weight of the car on the suspension and then using a rubber mallet, start hitting the underside parts of the car. When you hear your sound, you will be in the area of your problem.

 

After driving the car I had another look underneath and just spotted my new Volvo adjustable upper arms were leaving a clean dent in the hub plate cutout under the outer bush end.
With the rear tires sat on blocks, there was no contact, it got closer when pulling the car down.
I took the arms off and ground off the cast edge to give it more clearance.
Whilst I was at it I decided to replace the left lower control arm, this arm tested as having no play on the car, but when it was removed I could get play in it at some angles, this turned out to be the main culprit !


Now the bad clonk is gone, I am just left with some smaller minor rattles to hunt down.


What I have noticed after replacing both upper and lower control arms is the car drives more accurately, with steering better defined, and changing road surfaces having far less influence so needing far less steering correction.


Paul.

 

Hmmmmmm.....having just replaced components in the rear, that turned out to be hitting might have been a thought up front? In other words, what change recently (if there had been recent changes) might account for a new behavior(s).