On most rear-wheel drive vehicles, a flex disc, sometimes called a giubo (which, you may be interested to know, is properly pronounced JOO-boh), is fitted where the gearbox and propshaft flanges meet. Flex discs are designed to help smooth out the transfer of torque between the gearbox and the rear wheels. You can see it in place on my E39 below, with the gearbox crossmember removed, which you need out of the way in order to access the flex disc.
The photo below shows the old flex disc from my car on the left, and the new one on the right. Now, if you’ve been following this blog for any length of time, you will know that I replaced this last March. So why am I doing it again now, and more importantly, why is the old one so distressed already?
Well, there are a few reasons. In the first place, the flex disc is designed to be installed in a particular way, and as it turns out, I installed it the wrong way last time, with the arrows pointing away from the flanges, rather than at them.
You will notice above that parts of the flex disc are thicker than others. Now, if you are looking at the end of the gearbox, the flange will be spinning counter-clockwise when a forward gear is engaged. That means that when you set off from a stop, the gearbox flange and propshaft flange are sort of compressed together as the torque is transferred. The flex disc is intended to absorb that compression and transfer it smoothly through the driveline. When installed properly, it is the thicker parts of the flex disc that compress, while the thinner parts are designed to stretch. As you can see below, all flex discs are marked with arrows to indicate which way they are to be installed. The arrows point to the corresponding flange that the parts of the flex disc are supposed to be fastened to.
As you can see above, I have installed the new one correctly. There was a lesson I learned here, which is, of course, an obvious one: You can’t always trust the internet. I had come across a forum discussion that threw the very simple instructions for installing the flex disc in the E39 Bentley manual into question. I then came across a YouTube video that similarly made a fairly compelling case for installing it differently to the what the manual advised (which I can no longer find). But the worn flex disc above testifies to the fact that this was simply wrong.
Or does it? I’m still not entirely sure, because oddly, even though I had installed the old one the wrong way with respect to the arrows, the flex disc itself was still set up in the correct way (thicker parts compressing, etc.). So that leaves me with two possible conclusions: Either there is something in the design that absolutely necessitates the arrows pointing the correct way, or it was simply that the OE-quality part I purchased was not good enough (I can’t remember what brand it was, but I’ve opted for a Meyle part this time). Another possible contribution to its premature demise is excessive driveline vibration; I did have to replace the engine mounts a few months ago. However, I don’t think they were far enough gone to cause the kind damage that you can see above.
I realise that this all remains somewhat inconclusive. It has been frustrating too, not least because it required me to spend an extra £30 I didn’t want to spend (but really had to, since a worn flex disc is very noticeable when you drive). I have also become an expert at dropping the E39’s exhaust system, something I had never intended to master. But in the end, I guess it bears reiterating: When the internet contradicts the manual, always go with the manual.