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racdavies
Newbie


Joined: 26 Sep 2017
Posts: 3



PostPosted: Tue Sep 26, 2017 9:23 pm    Post subject: Violent bumping at high speed. Disappears if I brake. Reply with quote

Hi all, I have a Cayenne 2005 4.5S. Been a great car for many years. Recently I noticed a wobble at high speed and easing off from high speed until about 50mph, then smooth again. Not wheel balance (got that done), not disks (replaced all four).

Here's the odd and important confusing thing.

If I feather the brakes, even slightly, it goes away and smooth. Take foot off brakes and wobble comes back, almost like going over cats eyes or road paint... Like a pulsing bumping. Touch brakes, goes away.

I replaced all front bushes, top and bottom wishbones. Bearings fine as is drive shaft mid bearing.

Two garages seen and none can work out why braking removes wobble / bumping. Please help. Spent a fortune and it's still doing it!
 
  
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MaxA
Hockenheim


Joined: 11 Oct 2015
Posts: 708
Location: Helsinki


PostPosted: Wed Sep 27, 2017 9:05 am    Post subject: Reply with quote

That's an odd one. I can only suggest failing wheel hubs?
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Demort
Paul Ricard


Joined: 21 Mar 2015
Posts: 3343
Location: Sussex


PostPosted: Wed Sep 27, 2017 7:50 pm    Post subject: Reply with quote

Just to confirm .. you do mean the steering wheel shakes at high speed etc ?

Wheel wobble is .. rule of thumb here 62 - 74 mph .. above and below that its fine so can you drive through this ?

Brakeing would have the effect of pulling the wheel central .. locating it if you like and altering the frequency of the vibration .

So .. assumeing this is steering wheel shakeing and so some sort of " wheel wobble " then we have the wheel balence , the disc , the hub and the driveshaft .. cant see anything on the diff that would cause this .

We do have suspension bushes and we do have geometry setup but assuming no tyre scrub and the play has been checked which i would have thought it has been you are left with driveshaft or a hub out of balence .

A little more info on the symptoms please Smile
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racdavies
Newbie


Joined: 26 Sep 2017
Posts: 3



PostPosted: Fri Sep 29, 2017 11:42 am    Post subject: Reply with quote

Thank you for having a think about this guys. MaxA, hub is for sure a possibility.

It's a real head scratcher... I've spoken with Porsche Glasgow now and bearing is a possibility. Rare but possible. The brakes pulling any wobble at higher speed suggest hub being grabbed by callipers is stabilised which suggests hub wobble directly (actually more like a thumping like a flat tyre than general steer wobble like with unbalanced wheel or shot disks)

So specific symptoms from 80mph when easing off (only briefly at that speed to overtake perhaps) the wobble gets violent. If, while it doing it I feather the brakes it goes away completely. Remove foot from pedal and it comes back but at around 50mph disappears, or rather diminishes so I can no longer feel it. It may be there and minimal but road surfaces distort what I feel.

The odd thing is that the good garage I use locally that replaced the front wishbones and all bushes (a previous garage diagnosis thought it was bushes, as there was play in them, so we replaced them) didn't notice track rod ends and they checked drive shaft and said that was ok.

Reason I have not gone to Porsche direct is because I live in Outer Hebrides and it's a 3 day, two nights in hotel and 600 miles and ferry to get to Porsche centre, so am stuck locally).

Really appreciate help. Am thinking Hub / bearing somewhere? Just odd it is fine at lower speeds except psi light and losing power as result.
 
  
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Demort
Paul Ricard


Joined: 21 Mar 2015
Posts: 3343
Location: Sussex


PostPosted: Fri Sep 29, 2017 5:24 pm    Post subject: Reply with quote

Interesting .. im sure you have had out of balence wheels before and you describe it as like a flat tyre so very different to normal wheel wobble .

Light pressure on the brakes will stablise it .. it happens violently on over run and is not noticable below 50 .. hmm ..

Dont worry this is me listing everything and trying to figure it out .

It a cayenne so it might have the same wheels front and rear .. some do some dont so is it possible to swop front and rear ?

Im thinking something wrong with a front tyre for this .

Air suspension or shocks ? suspension damping will bounce the wheel in a sense , that would probably be unnerveing and would after a time show up as wear on the tyres .

The speed indicates a rotational item though .. it sounds like most of the suspension has been replaced so im guessing no play up / down and side to side on the front wheels ? .

A wheel bearing will normally give a noise or will have play in it .

Steering feels normal .. ie the rack is not heavy to turn at normal or low speeds ?

Why on over run though , out of balence hub wheel , tyre will be the same i would have thought .. over run is the front of the car bearing down and a load on the front diff .. transfer box .. hmm

Ok so assumeing the garage has had a good look over then its nothing obvious ..

Im assumeing its air suspension so any fault codes in this system .. i would also have liked to see the readings for the shocks but that needs a tester .

Im also thinking geometry .. any tyre wear on any of the tyres ?

Is it possible for a passenger to do a video with sound of whats happening ?

you can post it here or mail it to me here ..

De.Mort@virginmedia.com

I know your kinda stuck with the OPC so far away so happy to do my best here .
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Johnd52
Spa-Francorchamps


Joined: 06 Oct 2014
Posts: 349
Location: North Yorkshire

2011 Porsche Cayman

PostPosted: Fri Sep 29, 2017 6:27 pm    Post subject: Reply with quote

Driveshaft Cv joint? Does it get worse in high torque situation - accelerate in high gear at low rpm?
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racdavies
Newbie


Joined: 26 Sep 2017
Posts: 3



PostPosted: Sun Oct 01, 2017 10:46 am    Post subject: Reply with quote

Hi guys, John, It doesn't seem to happen in high torque situations, or rather you can't feel it.

Demort, your initial sentences spot on. The bit about over run down to 50 the key thing that makes this different to any other issue I have ever had with any number of cars. Something is clearly "loose" or moving where it should not so joints and drive shafts along with hub bearings are to be thoroughly examined.


Same wheels front and rear. This is just too violent to just be a wheel I think. Steering feels normal. I'm not sure a passenger could video it or even hear it really? It comes through the chase or wheel and just a feeling like a flat tyre at speed but all tyres evenly worn and tracking been done twice in last few months... It is possible it's a bearing. I can't hear a constant noise but the Porsche engineer said you don't always.

What surprises me though is at least four pro mechanics and two separate garages have known about this and I have specifically asked them to check things over and neither garage has found anything. However, this may be because of assuming it was the bushes, being as they needed replacing. So that didn't solve it, so maybe this next visit they'll be more thorough now an obvious thing (to them) has been ruled out.

I will let you know if it's solved but it is an interesting anecdotal behaviour that is rare it seems.

Many thanks for having a think and once tested with diagnostics again and thorough underneath check I hope to have some closure!
 
  
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