981 tyres scrubbing when turning

[bcr5784]

cibble10 said:

So I can send someone or everyone over the edge... or maybe help with a theorem or two...

I have a 981 BGTS with 20 inch wheels, SS and PTV.

After pulling out of my garage I have to make a 90 degree left on tarmac. It appears to me that the inner rear left wheel makes a clunky noise as if it is being braked (and feels as if it is, a bit like ABS). I have assumed to date it was a quirk of the PTV at slow speed and on a hard turn. On returning home and reversing the process nothing happens that is out of the ordinary (no odd noises/behaviour).

My previous Cayman R did not exhibit this trait.

My AM V12V sometimes scrubbed a front tyre on a smooth surface (workshop floor) at slow speed on a tight slowish turn.

I am now about to sit in a trench with a tin hat firmly placed on my head...

My 981 does something very similar (possibly the same). In my case it is definitely PSM - the traction light flashes, and I don't have PTV. That said it SOUNDS exactly like a plate-type LSD in operation (Pretty much like an R300 Caterham makes when turning on full lock at low speed - which is definitely LSD in operation - it has no PSM or ABS). In your case I'd think it might be either PSM or PTV - but the flash of the traction light should tell you I imagine.

[cibble10]

I should reflate the tyres in a controlled lab, check all tyres for equal wear, rebalance, refit, confirm air temperature and moisture levels and used a GATSO for confirmation of speed as I turn the corner... on the newly paved area...

But on 2nd thoughts, it is sunny, so I shall go for a spin and quickly check the PSM indicator as I do my usual 90 degree turn

[iandc]

cibble10 said:

I should reflate the tyres in a controlled lab, check all tyres for equal wear, rebalance, refit, confirm air temperature and moisture levels and used a GATSO for confirmation of speed as I turn the corner... on the newly paved area...

But on 2nd thoughts, it is sunny, so I shall go for a spin and quickly check the PSM indicator as I do my usual 90 degree turn

Enjoy the drive and keep the revs up so you can't hear any nasty clunks!

[cibble10]

bcr5784 said:

My 981 does something very similar (possibly the same). In my case it is definitely PSM - the traction light flashes, and I don't have PTV. That said it SOUNDS exactly like a plate-type LSD in operation (Pretty much like an R300 Caterham makes when turning on full lock at low speed - which is definitely LSD in operation - it has no PSM or ABS). In your case I'd think it might be either PSM or PTV - but the flash of the traction light should tell you I imagine.

No flashing traction light..

Good drive in the sunshine though

[cibble10]

iandc said:

Enjoy the drive and keep the revs up so you can't hear any nasty clunks!

And what a sound with Sports Exhaust!!!

Just need to run it in so I can use the full rev range too!!

Edited by cibble10 on Wednesday 31st December 08:33

[bcr5784]

cibble10 said:

No flashing traction light..

Good drive in the sunshine though

Looks like PTV then - not absolutely clear whether it's the LSD or Torque Vectoring . My money would be on the LSD at such low speed, assuming its a plate type rather than a Torsen/quaife/viscous type (anyone know for sure?) Indeed surely the LSD would cut in before PSM tries to intervene on traction (otherwise the LSD is pointless)

[cibble10]

bcr5784 said:

Looks like PTV then - not absolutely clear whether it's the LSD or Torque Vectoring . My money would be on the LSD at such low speed, assuming its a plate type rather than a Torsen/quaife/viscous type (anyone know for sure?) Indeed surely the LSD would cut in before PSM tries to intervene on traction (otherwise the LSD is pointless)

Though no expert in this matter I have had many cars and this is the first to exhibit such a distinct action on a hard turn, so to date I have not been concerned (and I can be a bit OCD about such matters when it concerns my latest car purchase!) and assumed it was/is the PTV as you suggest braking the wheel.

[bcr5784]

cibble10 said:

Though no expert in this matter I have had many cars and this is the first to exhibit such a distinct action on a hard turn, so to date I have not been concerned (and I can be a bit OCD about such matters when it concerns my latest car purchase!) and assumed it was/is the PTV as you suggest braking the wheel.

It might be PTV braking a wheel - but the LSD acts by preventing the differential working by locking the back wheels to each other, and is a separate function (but part of the whole PTV option) on the Cayman. The most common form of RWD LSD uses a set of clutch plates which engage when the speed difference between the driven wheels exceeds a certain amount. The noise the clutch makes when it engages is distinct clunk which sounds (and feels) just like a wheel brake being applied. So it could be either the single wheel brake action PTV uses to improve turn in OR the LSD function to improve traction.

[nsm3]

And all this is happening when pulling out of the garage?

You fellas need to slow down a bit

[Ozzie Osmond]

Some LSD's can make a sort of grinding noise as the plates move. This is usually suppressed by adding a friction modifier to the lubricant. I don't know what particular type of LSD is used in the Boxster.

But I do know tyre scrub noises happen at the front of the car and differential noises happen at the back!!

[bcr5784]

Ozzie Osmond said:

Some LSD's can make a sort of grinding noise as the plates move. This is usually suppressed by adding a friction modifier to the lubricant. I don't know what particular type of LSD is used in the Boxster.

But I do know tyre scrub noises happen at the front of the car and differential noises happen at the back!!

We are talking about noises that SOUND LIKE operation of the REAR brakes here!

I was curious and had a surf around - the Cayman does use a plate type lsd diff (at least the 987 does) , so I would expect it to clunk a bit when it was working. I did read too that it's a bit modestly sized and has serious durability issues when used for racing (which they do in the states). Another potential concern that surfaced is that Porsche might be a bit picky about warranty claims on the LSD (any track days might well be cited as the reason). http://www.planet-9.com/987-cayman-boxster-competi... makes interesting reading if you are, like me, interested (or boring reading if not!)

[cibble10]

nsm3 said:

And all this is happening when pulling out of the garage?

You fellas need to slow down a bit

If I went much slower the car would not be moving

That is until I leave the garage shared parking area!

[nozydog]

Ackerman effect, steering geometry, PTV, LSD (although the latter would probably make for a far more unusual experience) ....But really!? Surely it's simply because there is virtually no weight over the front wheels. Under an extremely lightweight aluminium hood is what can only be described as a large volume of thin air!! My front wheels skip sideways every time I pull off my drive at full lock, but recently with a 40lb bass cab in the front trunk the phenomenon was virtually non existent!!

Edited by nozydog on Tuesday 15th December 13:53

Edited by nozydog on Tuesday 15th December 13:57

[bcr5784]

nozydog said:

Ackerman effect, steering geometry, PTV, LSD (although the latter would probably make for a far more unusual experience) ....But really!? Surely it's simply because there is virtually no weight over the front wheels. Under an extremely lightweight aluminium hood is what can only be described as a large volume of thin air!! My front wheels skip sideways every time I pull off my drive at full lock, but recently with a 40lb bass cab in the front trunk the phenomenon was virtually non existent!!

Edited by nozydog on Tuesday 15th December 13:53

Edited by nozydog on Tuesday 15th December 13:57

It's (lack of) Ackerman. There are two reasons for not having true Ackerman - reverse Ackerman which is (mainly) used on race cars to get better turn in on tight hairpins or - and probably more likely in my view - to reduce wheel arch intrusion because the inside wheel doesn't have to turn so much. It's an issue on both the 911 and the Lotus Esprit. The common factor, a rearward mounted engine where the driver is pushed further forward between the wheelarches than is the case with a front-engined car. In order to create reasonable footroom (or avoid offset pedals) where you have a central tunnel you either have the choice of limited steering lock or don't have true Ackerman. The 981 has much more lock than most mid-engined cars.

On the 981 the scrub can cause the traction control to kick in - which makes a much louder mechanical noise. The scrub itself tends to sound like crushing freshly fallen snow under foot only louder and the front tends to hop forward, PSM sounds more like a tightly set Salisbury LSD. Mine hasn't got PTV so I'm not sure what the Porsche LSD sounds like.

[b2tus]

"The scrub itself tends to sound like crushing freshly fallen snow under foot only louder and the front tends to hop forward"

Couldn't have put it better myself. I have spoken to my local OPC and asked if "they all do that Sir". They confirmed that they all do that but it in no way damages the tyres or steering. Seems my problem is exacerbated by the short sharp slope from my drive up to our lane and then a 90 degree left or right turn. I have to stop before turning right or left onto our lane as there is a bend both ways and the local mums drive like loonies on the school run around here.

Despite this small niggle, I still really do rate the 981 as a driving machine. I had a new Boxster in 2000 but after 2 years moved on to various TVRs (last one a Tuscan) until this year. So after 13 years of TVR hooning, I wondered if I would still get the same buzz with a 981 Boxster.

The fact is yes I do.

[bcr5784]

b2tus said:

They confirmed that they all do that but it in no way damages the tyres or steering. Seems my problem is exacerbated by the short sharp slope from my drive up to our lane and then a 90 degree left or right turn. I have to stop before turning right or left onto our lane as there is a bend both ways and the local mums drive like loonies on the school run around here.

My drive is similar to yours, though not that steep - I expect you'll get noisy PSM intervention on occasion too when conditions get slippier. It was quite worrying until I had worked out what the hell was going on.

I have bought a set of winter wheels and tyres which I hope will make things a bit better in cold conditions - but the weather has been too mild until now to make it sensible to fit them.

To suggest that "doesn't damage the tyres" is a bit fanciful of the OPC. Course it does! Hopefully it doesn't happen often enough to be significant.

[Spartikus]

"The scrub itself tends to sound like crushing freshly fallen snow under foot only louder and the front tends to hop forward"

This describes it nicely. I have a 981 on 20s. It does sound a bit alarming but it's only on full lock so easy to avoid and anyway, full lock is only ever used when manoeuvring slowly

[nsm3]

Or when crashing!

[S666SBD]

Spartikus said:

"The scrub itself tends to sound like crushing freshly fallen snow under foot only louder and the front tends to hop forward"

This describes it nicely. I have a 981 on 20s. It does sound a bit alarming but it's only on full lock so easy to avoid and anyway, full lock is only ever used when manoeuvring slowly

My 981 does the same as described when I back up my parking slope and turn almost full lock onto the straight which is level .

I am attending my Local OPC Dealer's Xmas Party tonight so will test their knowledge of this .

[Timbo_Mint]

Used to happen on My 987 Boxster.

It happens to all wide tyres - the outside of the tyre is moving further than the inside so the inside is skidding. Listen to the sound of the tyres when an Articulated Lorry turns with a tight lock.