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Porsche specialists buying, selling, servicing | 911virgin | Uxbridge

Help me understand my coffin arms...

S

screwy

New member
Joined
15 Jan 2014
Messages
42
I've never been one to do something just because someone tells me to, so...

I'm about to embark on replacing the rear coffin arms, diagonal arms and toe control arms on my C4S. I've been reading my Bentley manual and notice that the instructions make a big point of stressing that when you come to tighten up your new coffin arms it is important to put weight back on the wheels or to use a jack to raise the wheel carrier back towards its normal position. But they don't give any reason for this. And it only says this for the coffin arms not for the diagonal arms or toe control arms.

Anyone got any idea why this might be required for the coffin arms in particular?
 
I think it's because otherwise it would cause the rubber bushing on the inboard end of the coffin arm to be under twisted tension when the car is lowered to the floor if the eccentric bolts were tightened with the hub suspended. This obviously doesn't happen if the hub is raised or the wheel is lowered to the floor before the bolt is tightened.

BTW I have a couple of lightly used Meyle toe arms (2000m/4 months use) that I'm selling on ebay if you are interested and want to save a few quid,.
They were off my C2 but I think they fit other models, best to check beforehand.

https://www.ebay.co.uk/itm/283405708386
 
If you torque them up to spec with the wheel in the air then lower it the twisting/expansion of bushes means the torque will then be too high.
 
Do it for all of the arms. It is so the arms are in their normal position and the weight of the car is fully on the springs when the car is sitting on its wheels. The bushes should not under any tension in this position . Remember, any car spends most of its time just sitting on its own not being driven.

If you tighten them when the suspension and springs are extended, then when you lower the car it will sit higher initially because the bush is gripped in the fitting. The weight won't be fully on the springs. Yes it will drop when driven but there may be always tension in the bush and on the rubber.

I hope that makes sense. I did all of my arms last summer. Initially I tightened with the suspension extended - lowered car, didn't sit right, so jacked it all up and re did it. Sat perfectly then.
 
Thank you for the replies.

I feel a bit daft. It seems blindingly obvious now you've explained it. I was focussed on the ball joint end, but I hadn't thought about the twist of the bushing at the inboard end. Once it is tightened up, the only way the coffin arm can move up and down is through the twist in the bushing.

And yes, this must apply to pretty much all the arms. I think I must have misread the warning in Bentley. They have placed it in the section about replacing the coffin arms, but reading it now, I think they intend it to apply to all subsequent sections that cover replacing the other arms.


Thanks All!
 
Prepare yourself for siezed solid eccentric bolts on the inner pivot points of the coffin and toe arms if its never been apart before? Cutting them off with the end of an hacksaw blade without damaging the rear subframe is an experience I wouldn't want to repeat.
Good luck, give yourself plenty of time and walk away for a brew if things get tough is my advice. I ended up replacing both my eccentrics on the toe arms as both were solid and needed cutting off
 
Surely this is a pointless exercise as once you've fit them you have to get the geo done and the bolts are undone, adjusted and re-tightened whilst the car is sat in its natural position on its tyres. Also, aren't the internal sleeves on the bushes a clearance fit on the bolts so the arms can freely rotate anyway?
 
Once tightened the inner metal ferrule is clamped tight. If it wasnt it would effectively be a bearing.

Tightening suspension components when on the ground is normal practice on all cars with metalastic bushes that experience rotational forces.

Regards
Ian
 
:thumb:
 
Hi Screwy,

If you are going to replace the coffin and diagonals, I recommend you change them all. While the upper links in the rear may not be making any noise, the rubber in the bushes will be expired at this stage. Hard and non compliant.

I changed all links last autumn using Hartech re bushed links. They are as good as new, OEM weight and spec, and cheaper than buying new Porsche ones. Even if you decide you use Meyle or equivalent, it is still work changing them.

Also, if you read back on some of my posts, you will see that I had to quickly change my engine mounts once I had replace the suspension as the smoothness of the driving experience highlighted how much the engine was moving. A very simple job. I used the Rein versions as they are OEM.

Just my 2 pence worth, while acknowledging that your budget determines what you do.

As wazz says, you may need saws. Check either his or some other posts (maybe MCs) where you will see what he is talking about. There are photos of the process here somewhere.

If you have to saw a bolt - mentally prepare and allow yourself an hour. If you do it in less time, it's a bonus. If you have a reciprocating saw, buy decent metal blades and use it, but there are some bolts it isn't practical. In my case it was the RH REAR coffin and toe bolts that were seized. It is mind numbing work. I also repeatedly doused all bolts and nuts in penetrant in the days building up to doing the work. If they come out easily, the world is suddenly a wonderful place and it is a 2 min job.

Best of luck
 
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