Knocking from BMW carbon plate diff.

[V84A]

I have recently bought my first Caterham, having thought about doing so for at least 10 years! The car is a Supersport ford sigma 1.6. It is only 9 months old. The guy who built it looks to me to have made a really nice job, but he is looking to change his career and become self employed so needed the capital. He tells me it came with a BMW diff. but that it whinnied really badly and Caterham replaced the original unit with the carbon plate diff. from the R500. This diff. doesn't whine, but it does clonk. I understand that it is not unusual to get a clonk form the take up of a little play as power is engaged however this is a different clonk from what I understand may be called transmission slap. This occurs most often when coming off the power, particularly slowing for speed bumps. It can get into quite a repetition of ( almost uncontrollable) on/ off clonks as you slow down and then speed up again. I wonder if anyone else has experienced this kind of diff. problem and whether it should be regarded as normal or acceptable? I have to say it is pretty annoying and marring what is otherwise a brilliant experience.

[V84A]

Thanks for all the information. What I didn't mention earlier (not wanting my first post to turn into a shaggy dog story!) is that I did take the Car to Caterham Midlands last week. That was interesting and informative but left me with more question than answers regarding the clonking, probably leading to my begging for help here!

I took the car for a general check over, 1000 mile service, and a tweak around with the pedals (being 6'5" and needing as much help as I can get, leg room wise). I spent time with a very helpful mechanic. I was particularly reassured in that he talked me out of rushing into spending money unnecessarily and when we were discussing the clonking noise from the back axle (whilst the car was on a ramp) he said that it was most likely to be the take up in the necessary play in the diff. It seemed like a reasonable explanation and certainly with the wheels off the ground it was possible to turn the axle and witness the play. It seemed quite a bit to me but when we tried a few other cars in the workshop they were much the same. He also pointed out that any noise would be accentuated by the bare metal and lack of sound proofing and one's close proximity to the axle. He did, however, want to take the car on the road and when he came back he thought the noise was actually play in the universal joints so he put the car back on the ramp and checked those but they seemed to be Ok. At this point he was a bit puzzled and asked another mechanic for a second opinion. After a further test drive the second mechanic said the noise we were hearing was normal from the BMW diff. I have to say it was apparent from the first mechanic's reaction that he was surprised and I could tell he was a bit uncomfortable reporting back to me that it was perfectly normal.

At this point the things that make sense to me are that Caterhams by nature of the lack of sound deadening and the fact that you are sat inches away from the diff. are likely to be noisy (certainly by comparison to most others cars one might drive). I also understand that a element of play in the diff. may be necessary and this may lead to an element of clonk on take up. What doesn't sit comfortably with me is excessive clonking. I can't help think that if what I am experiencing is normal (the mechanic I mentioned agreed that the excessive clonking is pretty irritating)that CC would not want to resolve it, particularly if this diff. is the one they put on their flagship R500.

I am wondering what to do. Certainly the suggestion of a second opinion from a Lotus 7 club veteran does sound like a good idea but I am conscious that the car will soon be coming up to 12 months old (it was registered late in October but being a self build the bits were received somewhat earlier, September I think) so I am thinking that if anything were to happen under a warranty then time might be short.....

[V84A]

Thanks for the further information. I am beginning to think that clonking is fairly normal given the things I am reading and that perhaps what I am experiencing is not unusual. Many thanks also for the offer to try yours as a point of reference, I live in Yorkshire so not a million miles away. Having been out in it a couple more times I am now thinking that the exhaust note, whilst impressive and head turning initially, is also wearing on more than a short blast and that I might look to get some kind of headset (with intercom) to block out the noise and make it possible to chat more easily to my 12 year old navigator. That might solve both the ringing ears and clonking noise!

On the subject of headphones with intercom, has anyone tried that? It seems there are such things designed for aviation but they are expensive.