Rear knocking sound Log Out | Topics | Search
Moderators | Register | Edit Profile

Australian RR Forums » Silver Shadow Series » Rear knocking sound « Previous Next »

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

Benoit Leus
Frequent User
Username: benoitleus

Post Number: 58
Registered: 6-2009
Posted on Thursday, 02 May, 2013 - 07:16 am:   Edit PostDelete PostView Post/Check IP

I just finished my first classic tour after the body restoration of my '79 Shadow.
The car ran well except for a few problems. One of those however seems to be somewhat of a mistery to me.
When pulling away from standstill or accelerating hard there is a distinct knocking sound coming from the rear left corner. I have to feather (pull away real slowly) the accelerator to drive "around" the problem. Once I do that the knocking dissapears and will only come back after i've stopped and pulled away again or really floored it. It seems to be more prone to knocking when I steer to the left while pulling away.
There are no strange sounds coming from the diff, the car handles well, brakes well and sits level.
The car seems to have developped this problem during the 11 months it was immobile while having its body restored as it didn't suffer from this before.
I removed the left driveshaft for inspection, but there doesn't seem to be any indication of something rubbing or other malfunctions. With the driveshaft removed I can easily turn the rear hub, but that also seems perfectly normal. When looking at the side opening of the diff, where the driveshaft goes in, everything seems to be in order too.

I'd appreciate any suggestion as to what to look for next.

Regards,

Benoit
Top of pagePrevious messageNext messageBottom of page Link to this message

Hubert Kelly
Frequent User
Username: h_kelly

Post Number: 71
Registered: 3-2012
Posted on Thursday, 02 May, 2013 - 07:36 am:   Edit PostDelete PostView Post/Check IP

Hi Benoit,
by any chance could the noise be from a sticking rear caliper....(2) or take note, to see if the hand brake cable mount has fallen(unto drive shaft) thus rubbing on the drive shaft.
hk
Top of pagePrevious messageNext messageBottom of page Link to this message

Stefan Morley
Grand Master
Username: myupctoys

Post Number: 370
Registered: 7-2009
Posted on Thursday, 02 May, 2013 - 07:40 am:   Edit PostDelete PostView Post/Check IP

Hi Benoit,

Just a punt, had a similar irritation with my car. Narrowed it down to the exhaust rubber hangers flexing too much and one having been removed for some unknown reason. Exhaust swung enough to knock on the underside of the body with greater preference when to turning in one direction more than the other.

Other possibility is sub frame mount if none of the other suspension issues seem problematic.

Stefan
Top of pagePrevious messageNext messageBottom of page Link to this message

David Gore
Moderator
Username: david_gore

Post Number: 1270
Registered: 4-2003
Posted on Thursday, 02 May, 2013 - 08:43 am:   Edit PostDelete PostView Post/Check IP

Hi Benoit,

I think Stefan is on the right track - it is easy to get the exhaust system alignment wrong when reassembling if allowances are not made for thermal expansion and drive train movement when the car is mobile.

As suggested by Stefan, missing/sagging exhaust rubber hangers are also contributors.
Top of pagePrevious messageNext messageBottom of page Link to this message

Benoit Leus
Frequent User
Username: benoitleus

Post Number: 59
Registered: 6-2009
Posted on Thursday, 02 May, 2013 - 05:05 pm:   Edit PostDelete PostView Post/Check IP

Hubert,

I've checked the callipers and handbrake, but, altough the handbrake mechanism is a little bit loose, they don't seem to be the cause.

Stefan & David,

as the bodywork was totally restored and they had to do a fair bit of welding at the rear, I can very well imagine they hung the exhaust out of the way and indeed got the alignement wrong when reassembling it.
I'll check this tonight and keep you posted.

Benoit
Top of pagePrevious messageNext messageBottom of page Link to this message

Benoit Leus
Frequent User
Username: benoitleus

Post Number: 62
Registered: 6-2009
Posted on Sunday, 05 May, 2013 - 02:48 am:   Edit PostDelete PostView Post/Check IP

Stefan & david,

you were right, it was the exhaust.It took some time to find where it excactly touched, but using a small piece of wood and wedging the exhaust at different places finally brought result.
It's surprising how a few millimeters out of alignement can cause so much noise.

Benoit
Top of pagePrevious messageNext messageBottom of page Link to this message

David Gore
Moderator
Username: david_gore

Post Number: 1273
Registered: 4-2003
Posted on Sunday, 05 May, 2013 - 09:41 am:   Edit PostDelete PostView Post/Check IP

Hi Benoit,

Stefan gets the credit for suggesting the source of the noise and you get the pleasure of driving a beautifully restored Shadow - life is rarely better than this......
Top of pagePrevious messageNext messageBottom of page Link to this message

Benoit Leus
Frequent User
Username: benoitleus

Post Number: 63
Registered: 6-2009
Posted on Tuesday, 07 May, 2013 - 05:41 am:   Edit PostDelete PostView Post/Check IP

Seems I was a bit quick in believing the problem to be solved, even if after the short test run I heard nothing.
Driving it today the knocking sound is still there.
I rechecked the exhaust but that can't be the problem anymore.
As it only occurs when driving off (unless I pull away extremely slowly) or accelerate hard, I am starting to believe it could be propshaft related.
The car was involved in a collision with a wall 11 months ago and then had it's bodywork restored. Before all this there was no knocking sound. Could it be that the engine has moved slightly backwards on its mounts and that now the propshaft touches the floorpan/crossmember (I assume it could move slightly backwars when pulling away or accelerating)?

I welcome any advice.

Benoit

ps. Stefan, you're suggestion that the exhaust could be at fault was still a good one, as now it certainly doesn't rattle anymore when driving over road imperfections.
Top of pagePrevious messageNext messageBottom of page Link to this message

Laurie Fox
Frequent User
Username: laurie_fox

Post Number: 98
Registered: 6-2004
Posted on Tuesday, 07 May, 2013 - 06:09 am:   Edit PostDelete PostView Post/Check IP

Benoit

The conditions under which you get the knocking are both when there is a high torque in the propshaft. This torque will be trying to twist the gearbox and engine clockwise and the engine and gearbox will move accordingly. Is the rear mounting OK?

Regards

Laurie
Top of pagePrevious messageNext messageBottom of page Link to this message

David Gore
Moderator
Username: david_gore

Post Number: 1274
Registered: 4-2003
Posted on Tuesday, 07 May, 2013 - 08:55 am:   Edit PostDelete PostView Post/Check IP

I think Laurie is on the right track; certainly sounds like a defective engine/transmission mount to me.

Due to the impact suffered by the vehicle, I would have replaced all mountings as a precautionary measure regardless of their condition.
Top of pagePrevious messageNext messageBottom of page Link to this message

Benoit Leus
Frequent User
Username: benoitleus

Post Number: 64
Registered: 6-2009
Posted on Tuesday, 07 May, 2013 - 05:23 pm:   Edit PostDelete PostView Post/Check IP

I found this clip on YouTube and the problem seems to be the same (the knocking sound certainly does).

http://www.youtube.com/watch?v=25B4SFZWDRs

I'll check the engine/transmission mounts, but could a bad universal joint also cause this knocking sound ?

Benoit
Top of pagePrevious messageNext messageBottom of page Link to this message

Benoit Leus
Frequent User
Username: benoitleus

Post Number: 67
Registered: 6-2009
Posted on Friday, 10 May, 2013 - 12:21 am:   Edit PostDelete PostView Post/Check IP

I've found the source of the knocking noise.
The propshaft must be touching the floorpan where it passes through the 2nd crossmember and there is a protruding edge from the floorpan. You can clearly see a shiny groove on the propshaft.



I am now trying to find out why it touches :
- the front propshaft universal joint makes a clicking sound, so that needs replacing (I'll replace the rear one too for good measure). At the same time I'll have the propshaft balanced.
- as Laurie suggested I took a good look at both rear engine mounts but at first sight they don't seem to have moved nor do they look to bad. I could imagine that with new rear engine mounts the whole drivetrain combination sits a little higher, reducing even further the distance between propshaft and floorpan.

Left engine mount



Right engine mount



Still, the impact of the accident must have moved something. Am I missing something else ?

Benoit
Top of pagePrevious messageNext messageBottom of page Link to this message

David Gore
Moderator
Username: david_gore

Post Number: 1275
Registered: 4-2003
Posted on Friday, 10 May, 2013 - 08:16 am:   Edit PostDelete PostView Post/Check IP

Benoit,

Have you checked the rear ride height? If it is too low, the prop shaft is closer to the body structure.

I cannot recall whether the rear engine mounts are adjustable, my intuition says they are but cannot be certain so need advice in this regard. If they are adjustable, the engine/transmission may be set too high at the rear and need resetting further away from the body.
Top of pagePrevious messageNext messageBottom of page Link to this message

Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 1014
Registered: 6-2006
Posted on Friday, 10 May, 2013 - 08:32 am:   Edit PostDelete PostView Post/Check IP

Hi Benoit,

the rear engine mounts are adjustable but I seem to remember your accident was fairly low speed, so I wouldn't expect them to move .

The rear engine mounts look good but is hart to tell with the engine weight on them. jack either side of the bell housing up slightly to test them.

I would also check the torque arm mounts and the diff carrier outer mountings for splits in them .

also check the diff carrier itself . They can split and allow things to move and touch under load.

Facebook search Everythingrollsroyce and there are some photos of k diff carrier with a crack almost completely through it.
Top of pagePrevious messageNext messageBottom of page Link to this message

David Gore
Moderator
Username: david_gore

Post Number: 1276
Registered: 4-2003
Posted on Friday, 10 May, 2013 - 02:27 pm:   Edit PostDelete PostView Post/Check IP

This problem has also been covered in the Forum:

http://au.rrforums.net/forum/messages/17001/1368.html
Top of pagePrevious messageNext messageBottom of page Link to this message

Benoit Leus
Frequent User
Username: benoitleus

Post Number: 68
Registered: 6-2009
Posted on Saturday, 11 May, 2013 - 03:38 am:   Edit PostDelete PostView Post/Check IP

Problem solved !

Laurie,

you were absolutely right by mentioning the rear engine mounts. Altough everything looked OK, I adjusted them a tiny bit and the knocking sound from the propshaft has dissapeared completely, even under heavy load.
Paul's suggestion that I check the torque arm mounts was a good one too, as they seem to be past their prime and feel spongy. Luckily that's an easy fix.

Now I only have to replace the leaky radiator and the propshaft UJ's and all will be well again.

Thank you to everyone for your help and suggestions.

Benoit

Add Your Message Here
Post:
Bold text Italics Underline Create a hyperlink Insert a clipart image

Username: Posting Information:
This is a public posting area. Enter your username and password if you have an account. Otherwise, enter your full name as your username and leave the password blank. Your e-mail address is optional.
Please quote Chassis Numbers for all vehicles mentioned.
Password:
E-mail:
Options: Enable HTML code in message
Automatically activate URLs in message
Action: