Noisy Rear End

[Glynh]

I’ve just been fumbling around under the car to try to work out why I get knocks from the rear end on power take up and going over bumps. Looking at the metalastic bushes on the A frame and the diff mounts, the first thing that struck me is there’s a mix of 1/2” and 12mm fixings, this is a factory built 2022 car so a bit surprised at that!
Anyway,  it looks as though standard OD washers have been used either side of the metalastic bushes and as the clearance between bush mounting components reduces depending on which side of the car I’m looking, there’s a lot of potential for washer to frame contact which is not too clever. My plan is to work through these, replacing the washers with smaller OD ones (19mm instead of 27mm). Has anyone else done this to good effect or am I barking up the wrong tree? The first picture below is the front diff mountings and the second is an A frame mounting. 

[StevehS3]

I do a nut and bolt check at least once a year and the only ones that have ever needed nipping up are on the radius arms. Might be worth checking those and also the marked area of the chassis. As an observation I would apply some corrosion protection to the tubes. 
 

Edited February 11 by StevehS3

[krisfl]

Same here. Loose radius arms make it sound like the back end is about to fall off.The only thing I've ever found to be loose too. Easy to check if you don't have tilletts seats...🤪

Edited February 12 by krisfl

[heptagon]

Pad knock can be quite loud, had that on a previous Caterham 

[nic.day]

I had a loud knock last year - turned out to be a broken diff mounting bolt

Do a nut and bolt check!!!!

[Geoff Brown]

Strange having two large washers fitted at the forward A Frame attachments. According to my CC Build Manual there should be only one large, chamfered edged washer either side of the forward A frame mount. The outboard side should then have as required spacer washers interposed between the chassis mount & the large washer. Use of the small spacer washers is not only to get the a Frame centred but allegedly to reduce the possibility of transfer of vibrations to the chassis. Two large washers butted up to the chassis does not help !

Any 'clonking' though would possibly due to loose attachments or bushing wear. So a good look & a spanner check is required of all rear end attachments

Edited February 14 by Geoff Brown

[John Vine]

59 minutes ago, Geoff Brown said:

Strange having two large washers fitted at the forward a Frame attachments. According to my CC Build Manual there should be only one large, chamfered edged washer either side of the forward A frame mount. The outboard side should then have as required spacer washers interposed between the chassis mount & the large washer. Use of the small spacer washers is not only to get the a Frame centred but allegedly to reduce the possibility of transfer of vibrations to the chassis. Two large washers butted up to the chassis does not help !

Any 'clonking' though would possibly due to loose attachments or bushing wear. So a good look & a spanner check is required of all rear end attachments

Interesting!

I've just checked my 2008 Assembly Guide, and Geoff is spot on.  The AG shows this arrangement:

 

The AG text reads:

Pack 30R011A shows:

CC online parts info:  79029 and WPH1/2

JV

[StevehS3]

From my 2016 Metric chassis build manual. 
 

[John Vine]

3 hours ago, StevehS3 said:

From my 2016 Metric chassis build manual. 
 

 

Thanks for that.  So, unlike the earlier text-rich AG, the later (pictorial) AG suggests using HM washers (WPH1/2 chamfered) to space the A-frame:

 

I don't know what year the OP's car is, but it looks like it has this later (correct?) arrangement.

JV

[ScottR400D]

1 hour ago, John Vine said:

 

Thanks for that.  So, unlike the earlier text-rich AG, the later (pictorial) AG suggests using HM washers (WPH1/2 chamfered) to space the A-frame:

 

I don't know what year the OP's car is, but it looks like it has this later (correct?) arrangement.

JV

 

From first post: 
“ ……..this is a factory built 2022 car“

[John Vine]

14 minutes ago, ScottR400D said:

 

From first post: 
“ ……..this is a factory built 2022 car“

Thank you, I missed that!

JV

[Glynh]

Thanks for the helpful comments, I just checked on the Caterham website and the differential spacing washers are 1/2” x 3/4” whereas the  WPH1/2” spacing washers fitted to my car (by the factory) are bog standard 12mm washers with an OD of 27mm. Makes no sense, why would you space a metalastic bush with large diameter washers that could hit the bush housing or frame? 

[Geoff Brown]

Using larger washers to pack out the A Frame is the easy way out as inserting the smaller packing washers is a PIA. Hence the change. but I cannot see the sense in it as the bushes are not entirely isolated from the chassis.

[BigCol]

5 hours ago, Geoff Brown said:

but I cannot see the sense in it as the bushes are not entirely isolated from the chassis.

would adding some rubber washers/spacers between the A frame and chassis provide an appropriate amount of isolation but add unwanted movement/compliance?

or should smaller diameter washers simply be used, as was the case in the past?

(my A frame is currently on the garage floor having just taken my diff out - again!  So am interested in the best means of rebuilding)

The 2014 Assembly Guide includes this photo, showing the suggested arrangement:

 

Edited February 14 by BigCol

[John Vine]

8 hours ago, Glynh said:

Thanks for the helpful comments, I just checked on the Caterham website and the differential spacing washers are 1/2” x 3/4” whereas the  WPH1/2” spacing washers fitted to my car (by the factory) are bog standard 12mm washers with an OD of 27mm. Makes no sense, why would you space a metalastic bush with large diameter washers that could hit the bush housing or frame? 

Under "SECTION 2 - PRE-ASSEMBLY INFORMATION", my 2008 Assembly Guide says:

So, large washers are ok, provided they're chamfered and fitted the right way round.

JV

[StevehS3]

Mine appears to be tightened against the bush. I presume the face of the bush is longer than the A frame mount. I don’t remember a chamfer on the washers but it was some time ago. 

Edited to say that neither of my assembly guides mention chamfered washers and they appear plain in the drawings. 

Edited February 14 by StevehS3

[StevehS3]

Actually, looking closer I think you have a gap too and it is clamped onto the face of the bush. 
 

[John Vine]

16 hours ago, StevehS3 said:

Edited to say that neither of my assembly guides mention chamfered washers and they appear plain in the drawings.

That's odd.  Which particular editions are you referring to?

JV

[Geoff Brown]

18 hours ago, BigCol said:

would adding some rubber washers/spacers between the A frame and chassis provide an appropriate amount of isolation but add unwanted movement/compliance?

or should smaller diameter washers simply be used, as was the case in the past?

(my A frame is currently on the garage floor having just taken my diff out - again!  So am interested in the best means of rebuilding)

The 2014 Assembly Guide includes this photo, showing the suggested arrangement:

 

Up to you which direction you take. Smaller diameter shimming washers were the norm until a certain date. Obviously CC saw the use of large washers as the way forward due to the better chassis tolerances ?  My chassis is a Westbury chassis though a 2012 one so maybe the manufacturing tolerances were tightened up much later on as with earlier Westbury chassis the only way to get an accurate centring of the A Frame was to use the packing washers.

Obviously on later chassis large washers were the norm & filled the space accurately.

 

[Glynh]

Metalastic bushes deflect under radial, axial and torsional load by an amount depending on the stiffness of the rubber. Looking at the clearances between the large washers and the bush housings on the photos it wouldn’t take much axial loading for the washers to hit the housing and given that the washers have very little chamfer (if any) I think there’s also a possibility of metal to metal contact during radial loading where the large washers are fitted. 

Edited February 16 by Glynh

[aerobod - near CYYC]

6 minutes ago, Glynh said:

Metalastic bushes deflect under radial, axial and torsional load by an amount depending on the stiffness of the rubber. Looking at the clearances between the large washers and the bush housings on the photos it wouldn’t take much axial loading for the washers to hit the housing. I’m changing mine to smaller OD washers. 

1mm of axial deflection on the bush size used by Caterham is about the most they can take before tearing, as is often found on the A-arm to deDion bush when under track use with worn limiter washers.