Help identifying source of “clonking” under rear of car when going over bumps.

[Rob Y]

Hi Everyone,

I have an annoying “clonking” at the rear underside of the car when going over sharp bumps. I thought it was the exhaust hitting the car as there is no central support bracket on my 73 CR car. However when I got under the car I couldn’t see where the exhaust was making contact, nor could I even simulate this by pushing the exhaust. There just didn’t seem to be any movement in the vertical direction. It would be really hard to fit the central bracket for several reasons, but I really don’t think this is the problem.

Interestingly, with my brother in the car today the problem seemed less, so this is with a bit more weight in the car and I guess the suspension compressed a bit more.

I have been unable to simulate the problem with the car stationary, bouncing the suspension with the car standing on its wheels doesn’t yield any clonking at all.

Do you have any ideas what the problem might be and/or a way I can simulate the problem to check where it might be coming from please?

Thanks in advance for any thoughts.

 Rob 

[John Morrison]

Make sure the s/A bolts arereally tight, renowned for a clonk.

John.

[Mike C]

Have a look for cracks in the shock absorber and diff mounting brackets.

[JohnC]

Perhaps a bump stop missing its rubber bit?

[Waldi]

And also check the bolts of the Armstrong dampers, they need to be torqued but can work loose.

Waldi

 

[Rob Y]

Thanks, looks like I need to do some checks around the shock absorbers/Armstrong dampers. Diff mountings have been fixed and strengthened so shouldn’t be that. I don’t think it’s a drive related problem so shock absorber area sounds like a good area to look at. I’ll also check the bump stop, that’s easy to do while I’m looking.

 I will report back once I get a chance to have a look.

Rob

[Rob Y]

Just thinking on this, if it was the shock absorbers wouldn’t I be able to feel/hear this by “bouncing” the back end when it’s stationary?

 Rob 

[Tim D.]

Not always. Sometimes needs more of a jolt than you can do by hand.  My tube shock bracket came loose at the end of a CLM trip.  Real racket going over potholes. Nothing when bounced by hand.  
tim

[Rob Y]

Thanks Tim. The sound I get is not a single clonk, more of a rattle, but a clunky rattle if that makes sense. Nothing when driving or cornering on smooth or undulating roads, like you said it’s potholes or sharp bumps.

Like I said in my original post it felt like the exhaust bashing against the body work or chassis, but I don’t think it’s that now.

was it obvious that the bracket was loose, could you feel/see it easily enough when you checked with the wheels off the ground?

 Rob 

[john.r.davies]

I''d say, get under the car with a pry bar, and lever it on all the suspension points.    Unwanted movement will soon show up.

John

[roughground]

I had the same problem chased it all over the rear but in the end it was slightly loose shock absorbers. 

[jimcwestie]

I had to tighten up the rear telescopic damper bracket to fix a rattle/knock.

Jim

 

[Z320]

If you find all tight, like I always did every time I checked it:

on my TR 4A the problem was the slack castled nut of the ingoing shaft of the diff.

I thought it cannot become slack, because it is fixed with a pin or wire?

But indeed it was because the massiv washer under the castled nut was punched by the time.

Ciao, Marco

Edited August 2, 2021 by Z320

[Z320]

double 

Edited August 2, 2021 by Z320

[John McCormack]

I have telescopics on my TR6 but with my TR2s I have to tighten the lever arm shocks every 6 months or so. they start giving an occasional metallic clunk/rattle which get progressively worse.

If you don't attend to it the mounting holes will wear and it becomes very difficult to get it tight.

[harrytr5]

Armstrong lever arm shock absorbers

The way I fix this problem is longer setscrews 2" x 7/16" UNF and fit nylocs on the other side.

Works for me.

Regards Harry

[Rob Y]

Thanks for all the feedback, seems pretty clear where I need to look. I’ll check it out tomorrow and hopefully find tightening the shock absorber mounting bolts fixes the problem. I will report back when done.

Thanks.

 Rob 

[stuart]

2 hours ago, harrytr5 said:

Armstrong lever arm shock absorbers

The way I fix this problem is longer setscrews 2" x 7/16" UNF and fit nylocs on the other side.

Works for me.

Regards Harry

This ^^

Stuart.

[Peter Cobbold]

check the rubber joint between the lever arm and drop link, another known failure point.

[Motorsport Mickey]

I use longer cap heads, there isn't enough space for a socket to comfortably fit over the bolt head, it often seizes up between the edge of the bolt head and the leaver arm mount frame escutcheon. You torque it up and when you wiggle the socket out off the bolt head the bolt is still not up to torque.

Mick Richards

[Tim D.]

3 hours ago, harrytr5 said:

Armstrong lever arm shock absorbers

The way I fix this problem is longer setscrews 2" x 7/16" UNF and fit nylocs on the other side.

Works for me.

Regards Harry

Also did this for the mounting for my shock conversion after it came loose repeatedly. Been fine now for 5 years. 

[Rob Y]

Thank you gentlemen!

I was quite confident that this would fix the problem from all your comments. Interestingly though I couldn’t get any movement where the damper mounts were with the car on stands and swinging arm supported but the bolts did tighten up and the problem is fixed! The bolts on the offside needed tightening but the near side were tight.

I love problems like this. Simple to fix, but transform the driving experience.

I have had the car for 6 years and this is the first time I have needed to do this, so I will probably wait and see how it goes before fixing the problem with longer bolts and nyloc nuts. I can live with this frequency of adjustment.

So happy with this, I can’t thank you enough.

Cheers, Rob

[Drewmotty]

Nice tough cap head bolts with thick machined washers and high tensile nylocks are the answer. With proper tough fasteners you can get them properly tight without stripping anything. 

Edited September 2, 2021 by Drewmotty

[Alan B]

I have only just picked this topic up. For what its worth I had the same problem on my 6 and thought it was the exhaust. After having spent a long time checking and rechecking the clearances and moving brackets around to no effect I had a closer look at the chassis and found that the chassis cross member which holds the damper mountings had almost parted company with the main chassis frame and was banging up against the bottom of the boot.  Some skilifull welding and strengthening and problem solved.

[DRD]

On 8/3/2021 at 10:01 AM, harrytr5 said:

Armstrong lever arm shock absorbers

The way I fix this problem is longer setscrews 2" x 7/16" UNF and fit nylocs on the other side.

Works for me.

Regards Harry

Same here!