Bad repeated lurching motion when accelerating
 

Hi - for a couple of months my A8L W12 has had quite a bad repeating lurching motion when setting off from standstill and there is a grating noise when going over speed bumps. The garage diagnosed worn suspension bushes (they call them "Front Arms" on the invoice) which were replaced in January. But it still happened.
So they diagnosed further that the "Bottom front arms" needed replacement so they were replaced today. I picked the car up and guess what, still exactly the same.
So I've spent £1,600 so far on suspension components and it's no better. Worse, I'm looking at 3 weeks to get it back in.

Is there anything else I need to get them to look at? Without part numbers I don't know exactly what they've changed. I will email them now and ask if they can tell me.

Thanks.

The following image is for the front suspension. Here's what I think they changed based on the descriptions, and I'll add the relevant part numbers as well:

1. 'Front Arms' - This has to be #10 in the attached image. Part numbers are 4H0407505E [or F] (left) and 4H0407506E [or F] (right).
2. 'Bottom Front Arms' - This has to be #1 in the attached image. Part numbers are 4H0407151C (left) and 4H0407152C (right).

That only leaves #7 and #11 to replace, but throwing parts at it is no guarantee that will resolve the issue. Plus, you've already spent around £800 in the above parts, and numbers #7 & #11 will cost another £1,100. As for what they actually replaced, turn the steering full lock and have a look at the suspension. The new parts should be spotless.

Just about to post this, and came across a thread on another forum about the exact same scenario, creaking when going over speed bumps. Apparently, their issue was caused by the ARB links, #30 in the image. Although they had the official Audi stamp, they were made in China. The part number is 4M0411317J [or L], fits both sides and are around £75 each.

https://forum.a8parts.co.uk/attachme...1&d=1740564331

Thanks. Good idea about having a look, I'll raise the suspension which will make it easier and take photos too which should confirm it.

Although the ARB drop links might be a good shout, I'm not 100% on board with that. Although the noise could be as a result of ARB drop links as the noise happens with sudden vertical movement (I recorded the sound and will post later), but would they be the cause of the bad jerking / lurching? I'm not so sure, because that's a longitudinal force and the drop links don't take that load.

Ps - the diagram you posted is one of a possible two sets. The only difference I can see between the two is the inclusion of a vibration damper. But i cant see what links the appropriate drawing to the vehicle...?

I somehow missed the lurching part. With that said, I just came across a thread using the same term, but unfortunately no resolution in it. Link as follows:

https://www.audiworld.com/forums/a8-...issue-2876581/

I also forgot to include the link to the other thread regarding the grating noise:

https://www.audiworld.com/forums/a8-...links-2881584/

The second set that you're referring to is for the armoured version of the car. The first set is for every other variation, regardless of engine. Stupidly, I provided the image of the armoured set, which I've now changed, but the part numbers are correct for yours.

I had a reply from the garage, the parts they fitted first are 4H0407151C & 4H0407152C (#1 in the diagram above).
Then yesterday they changed 4H0407693G & 4H0407694G (#10.& #11 in the diagram above).

#7 could be a culprit because that looks like it transfers longitudinal load into the car AND it could be making the noise too....
What do you think?

I would be more inclined to think lurching is more related to transmission or engine. Engine mounts, gearbox mounts, rear diff mounts, propshaft middle bearing mount. Or gearbox/ torque converter itself. Curved bottom suspension arms can also affect if the bush is worn but then you also likely to get vibration when braking.

At least that explains the cost, with around £1,200 in parts, so far.

I think this is one where you really need 'bum on seat' to feel it for yourself, as 'repeating lurching motion' is open to interpretation, and leads to more questions - how long does it last, is there a frequency to it, is it speed related i.e. move off slowly/quickly, is it weather related i.e. cold/warm, is it engine/transmission related i.e. cold/warm, how long does it last and is that speed related, how severe is the lurch i.e. you can 'feel' it as a driver or you can notice it as a passenger.

The garage have given me the following part numbers which have been changed;
https://i.ibb.co/pjX9CrBm/A8-Suspension.png

And here's a video - with audio - showing;
1) The noise over speed bumps
2) The jerking / lurching motion
https://youtu.be/-ySEm_HFHRs

That is a lot worse than I was thinking, as I was thinking it might only have been a slight noise and lurch.

With that video, maybe ainarssems can narrow down the likely culprit(s).

Video does not help much, it's not the same as real life, especially on a crappy small cheap phone speaker. Maybe would be better on proper speakers. The only thing it does, is that timing kind of shows that at least one of issues is at the front of car.

I have come up with another possibility however, which could tie both issues together, but it's just a speculation.

I am thinking of front driveshaft inner joint by the gearbox. Not sure what D4 is using, CV joint or tripod, I guess tripod, but in either case it could be dry or worn/damaged and make noise when going over bumps as driveshaft angle and length changes and it needs to slide in and out. And also under acceleration if it has play and shifts off centre or if it's binding due to dried out grease/lack of lubrication, it could be the cause of lurching. Does the lurching effect change depending on ride height?