DeDion to A-Frame Bush

[revilla]

I've had a persisent knocking and clonking from my back suspension since I got the car a couple of years ago. Several threads on here revealed a few little issues, none of which have got rid of the main knock (Watts link arms catching brake calipers, solved with spacers made by Bruce, springs touching turrent walls, solved by reshaping slightly, repalced shocks and springs, checked and double checked handbrake cable routing and security, searched for witness marks and found nothing not mentioned above). Car is going on SORN at the end of this month and I've decided to strip the entire rear suspension off for inspection and overhaul.

 

However ... I jacked it up yesterday for one last look and I did notice as I put the jack under the DeDion to A-Frame bush and lifted, it squelched oil out of the bush. The first thing that is clear is that I've got a small loss of oil from somewhere around the rear of the differential which I'll look at while I've got it stripped down (and no, I haven't done any track days 😬), in fact this was something that Nick Potter spotted when he looked into my knock last year and he thought it was throwing oil out of the breather and fitted a copper pigtail - I though it was sorted but it still looks to be loosing some somewhere.

 

Bush fitted is I believe standard metallastic. I believe these don't do too well when contaminated with oil. So my questions are:

 

i) What are the typical kinds of characteristic noises or other symptoms when this bush fails? Mine is mainly a metallic clonk when one back wheel drops into a dip such as a small pothole or drain cover. To me it sounds like it comes from just behind me, but passengers report it comes from just behind them - so it's probably in the middle! Do people think this might be the source of my noises?

 

ii) I guess all will become clear when I strip it over the winter but in the meantime, any easy ways without stripping it to see if there is anything obviously wrong with the bush? I can't say I can see any excessive movement in it and the nylon washers either side look undamaged.

 

[DJ.]

Mine's covered in oil too.

 

I suspect that it would clonk when lifting off and reapplying the accelerator, well that's what I've been listening for to tell me it needs doing anyway.

 

Duncan

[Stationary M25 Traveller]

Rose bearings in watts linkage crash and bang over bumps when they get the slightest bit of wear.

 

I have come to see them as a consumable spare to be changed regularly.

 

Redline suggest changing to radius arms to avoid this.

 

I have a tame supplier of the Rose bearings if needed.

[revilla]

@SM25T - Thanks but last time I took them off when hunting this problem they were as good as the day they went in, certainly no play to feel at all, so I don't think it's them. I will have another good look when I tear it all down over the winter and I will be bear you and your "tame supplier" in mind, thank you.

[Mavic82]

Rear anti roll bar? That's what rattles on mine.

[revilla]

Yes a few people have told me to check the rear ARB and drop links. Problem is ... I don't have any! It's not so much a rattle as a real hard metal-on-metal clonk. Very loud and clear. You would swear it would leave witness marks but several hours under the car haven't found anything. I could sort of be persuaded that it's the shocks topping out as it definitely happens when they extend as the wheel goes over a dip, but I've tried winding the adjustable platforms up and down by a good inch either way which is enough to throw the geometry right out of kilter just to see what happens, but it has no effect whatsoever on the knock at all - it knocks in the same way, to the same extent, over the same bumps. I only have to drive down our road at 30mph running the near wheels over the slightly sunken drain covers to make it do it every single time. No sign of contact between the DeDion and diff. DeDion not hitting the chassis rails at full droop. I've even removed the wheels and shocks and tried swinging the DeDion through all its extreme positions and angles and nothing makes contact, but still it clonks. It's as loud and clear as someone giving the suspension a good belt with a hammer. Keep thinking I've found it but whatever I find and fix, it's still there. Been there for the last two years and 8000 miles, no better, no worse. Drives me mad

[Stationary M25 Traveller]

The other thing I found recently was my boot floor not secure. The aluminium extrusion across the boot between the ali and wooden floor sections was not securely riveted. The self tapper screws fid not bite into the extrusion or rear chassis rail. Fitted spire clips all round and re-riveted extrusion. Much quieter now.

[revilla]

@SM25T - Mmmmm that's certainly something for me to look into. I did look at the possibility of it being something in the boot as I do tend to carry a load of spares and junk, but the knock was still there just the same when I emptied it completely. I even taped down the straps and buckles for the half hood in case they were swinging and slapping the boot wall. In fact knocking the offside one against the cover over the fuel filler neck produced a similar noise and I was hopeful that was it, but no.

 

I've never had the boot floor up, so not really sure what is down there. Is it all just accessible by lifting the boot carpet? If the boot floor was loose I could imaging it slapping as the car dropped over a dip.

 

After seeing what the original builder managed to do to the engine (hole ground in the block, wrong ECU and loom for the engine ...) I wouldn't be surprised if things weren't 100% as they should be at the other end too!

 

Thanks all for helpful input, always appreciated.

[Bob L]

Tracing the knocks and squeaks in a Caterham is almost a full time occupation. I amazed how some disappear apparently all on there own and its not for the want of searching!

As always, you seem to have covered the main culprits and many more. Doesn’t leave much but thoughts are:

Electrical connection block to the rear lights. These are light in weight but may not be fixed (cable tied) to an adjacent frame. Unlikely to give the sound magnitude you describe but simple to check and rectify.

Rear gearbox mounting. Not sure you would call this a bush, more of a rubber type cradle. If you have a minor oil leak this could be worth a look.

Alternatively, fit a 4” peashooter and try higher revs 😬

Good luck and keep us informed.

 

[John Vine]

Just a thought.... could it be the hoodsticks? I had a similar clonk on my old SS-R, and it turned out to be the lower hoodstick hitting the top of the rear panel/chassis.

 

JV

[revilla]

@Bob L - I will check those things that you mention. Funnily enough I haven't fitted a peashooter exhaust but I have noticed how much nicer the car is to drive with some earplugs in and music on, so the knocking is actually detracting from the enjoyment of driving it.

 

@John Vine - A good thought but unfortunately something else which I don't have! I took them off and fitted a low profile boot cover from SBFS when I went with their half hood.

[masiwn]

I have  had exactly the same problem for the last two or three months, your discription is identical to what I am experiencing. I have not had it looked at yet but was thinking of starting with the A frame bush. It will be interesting to see how you get on. If I have any luck I'll let you know. 

Aled

[EEK]

Had this type of clonk on my car from acquisition. Car bought as a refurbishment from a respected Seven specialist, was really bad and detracted from the driving experience. Traced source to the abscence of the two nylon washers either side of the rear A frame bush. These had never been fitted / replaced. Adding some transformed the driving experience.

[p.mole1]

I have spent ages trying to trace a clonk/cracking noise I was convinced it was coming from the suspension, turns out it was one of the diagonal bolted in chassis braces. The bolts were not tight enough

[revilla]

@EEK - This is going back a bit now! In my case it was the main A-frame bush that had gone. From what I was told, those washers were only usually fitted to race cars and should only come into play when the bush is laterally loaded to extremes. Under normal circumstances it shouldn't be clonking without them. I suspect this probably means you bush has gone soft and is allowing a lot more movement than it should and the washers are now cushioning this movement at the ends. Keep an eye on it, it may well chew through the washers quite quickly.

Replacing the bush is an easy job for somebody with a suitable press. In my case I dropped the A-frame off (just 3 bolts from memory) and took it back to Arch to get it blasted and powder coated and have all the bushes replaced. Wasn't at all expensive and came back like it just left the factory.

[DJ.]

On my car the bush is in the De Dion tube, and the A frame bolts onto the ends.

[revilla]

@DJ - Sorry yes of course you are right! My memory was letting my down.

When I sent the A-frame off for blasting and powder coating at Arch I also returned the de Dion tube for blasting, crack testing and powder coating along with all the Watts linkage arms. The two bushes in the front arms of the A-frame were changed but yes the rear one was pressed into the de Dion tube, which makes it a slightly more involved job to change, but still not too bad.