New rear tyres (Kumho) - now odd handling

[zcacogp]

Chaps,

A slightly odd one. I know there are some chaps on here who know a LOT about 944 chassis dynamics, so I'm keen to hear your opinion.

I had some new rear tyres put on my S2 last week. It did have Falken FK452's all 'round, and I replaced the rears with Kumho KU31's (they were on special offer and come well recommended on this forum). The old rears were worn, and borderline legal. I had an advisory on an MOT and they needed to be changed before winter.

Since the tyres were changed, the handling of the car is not as it was. It feels very well planted at the back (where the new tyres are), but the front feels less stable and almost a little wayward. The impression is almost that of understeer, certainly of slight lack of control. It's not that it doesn't turn in as well as before, more that it doesn't inspire confidence in the front-end handling in the way that it did before. The sensation is very similar to that of a broken front ARB mount, but I have had it up in the air and checked carefully around the ARB's and there is no sign of anything amiss.

I suspect that the car wasn't jacked correctly when the tyres were fitted; the jack on the drivers' side wasn't well positioned, and looked like it wasn't on the central jacking point as it should be, but I am not 100% confident about this. (I wasn't allowed into the tyre fitting bay when the work was done - the garage owner was a to55er of the first order, and I won't be going back there again. PM me if you want details.) However I have inspected the jacking site carefully since the work was done and there is no sign of anything amiss there either (no damage to paint or shape), so I could be wrong on this point.

I also know that the lower balljoint on the O/S/F wishbone is worn, and needs replacing. However, this was the case before the tyres were changed, so I don't expect it is a contributing factor.

Has anyone else experienced anything like this? Is is a common (or even known) complaint? The car needs a full geo check, which will be done in the next couple of weeks (after the wishbone is replaced) and I will be able to see if there is anything amiss in terms of alignment then.

All comments welcomed.


Oli.

 

[Melv]

Put matching tyres on front....simples!![]

 

[zcacogp]

New (matching) fronts is an option, but given that the current fronts are only about 50% worn I am reluctant to go this route just for diagnostic purposes.

Is this a known problem?


Oli.

 

[pegasus]

Hi Oli,I would agree with Melv,when mixing brands you are mixing in slight changes in sidewall flex etc always best to change like for like if you want things to stay the same,bad jacking will deform the accelerator mount on the floor but will never twist a 944 chassis,it is one of the only cars I have come across that you can jack up in one corner and the doors open and close as normal,it is so stiff.These guys that insist on h/s rules etc that stop you being next to your car when thay attack it should be avoided like the plague,they tried that when I went to have a four wheel alignment done and quickly found out it was being attempted by two total idiots,If I had just left the car thay would have said the job was done and given me the bill. all the best John.

 

[Eric_Oz_S2]

My car once pulled to the side after getting new tyres fitted. Eventually I figured out that when they changed the tyres they jacked up one side of the car. The result was the RHS camber eccentric on the fron strut somehow moved and went into positive camber (obviously the bolt wasn't tightened properly by the last alignment shop).

 

[Fat Albert]

Olli, re-check the tyre pressures, when I had my rears changed I found my tyre place 'checked' my front tyre pressures and pumped them up by a few psi as a 'favour' which then made it feel like it was on tiptoe

 

[zcacogp]

Fattie, thanks. I have yet to check the tyre pressure; the chap who did it said he blew them up to 36 both sides, which is what I asked him for. I'll check, and see if there is improvement to be had from playing around with them. A good idea - thanks.

Eric, This sort of thing will hopefully come to light when I have it geo'd.

John, Helpful, thanks. You seem very confident that a 944 chassis won't be twisted by such a bad jacking, and I guess that makes sense. Yes, I have had the stuck accelerator problem. (And the odd looks from the neighbours when I sorted it out with a hammer in the drivers' footwell ... ) The guy doing the work did a good job of the fitting and balancing, but clearly had no clue as to how to jack a 944 - I showed him the jacking point on the first side he did, whereupon his (tosser of a) boss came and told me that I wasn't welcome in the workshop and would I leave. Your story of the alignment place sounds even worse - and sadly not surprising.

I'll post on here again when I have checked and played with the tyre pressures.


Oli.

 

[Melv]

Watch my lips....change the tyres............[]

However, thought 944's were 2bar/29psi F, 2.5bar/36psi rear........[8|]

 

[edh]

I'm amazed you find any road in London that's free of traffic and / or speed cameras to get any meaningful feel for handling []

Is it nervous & a bit snappy when turning in? That might be a geo change with one of the fronts toeing out, although I can't see a reason why that should suddenly happen.

The other thought is that because you are turning in as before, but with more grip at rear, the front is understeering & basically not going where you predicted. Maybe you then wind on a bit too much steering? I am surprise that the effects are so marked at low speed and very far from any limits though.

Pressures? - I always favoured less difference front / rear, but I guess that will vary car to car..It seemed to change in the handbook from year to year as well.

 

[pegasus]

Hi Oli,I think you are chasing round the houses here,I still have to go with Melv and say the change is down to the tyres,either go back to what you had or change the front,all the best John.

 

[zcacogp]

Chaps,

Dragging this thread up from the near-grave, for an update.

I did as suggested and checked the tyre pressures. The fronts were down; one side more than the other, and I pumped them up to the usual 32 front, 38 rear (pressures I have found to work well for me.)

This, combined with running in the new rear tyres has solved the problem - thanks for the suggestion Fat Albert.


Oli.

 

[Diver944]

Glad to hear it Oli [] I've run Kumhos for many years and always been very happy with them. Even with a different brand on the back, the natural balance of the 944 is unfazed IMHO

 

[Fat Albert]

Nice one Olli!
Having made the suggestion, I really must check mine as I don't have as much feel at the front end as I should

 

[peanut]

glad you got it sorted Oli. I was going to suggest checking the tyre pressurres and also the rotation direction in case some muppet got them the wong way round.

I've just had 4x Khumo Ecstar sport fitted all round and on the way home coming up to a roundabout at 70mph the brake pedal went straight to the floor ?
Never found out why and its never happened since.?

They are a nice tyre for the price but not too good on ice [8|] but then what is !

 

[zcacogp]

Thanks Big Boy. I think the pressure issue has solved it. I'm however not sure why it wasn't a problem before - the front tyres weren't checked when the rears were changed. Maybe the old rears were down on pressure a little too, and compensated for the slightly less-hard fronts. Maybe ...

What actually happens when directional tyres are fitted such that they rotate the wrong way? The water-clearing in the wet will make things very dangerous, but what does it do for the dry-weather performance?


Oli.

 

[peanut]

ORIGINAL: zcacogp

What actually happens when directional tyres are fitted such that they rotate the wrong way? The water-clearing in the wet will make things very dangerous, but what does it do for the dry-weather performance?


Oli.

not sure ? maybe it would only affect wet weather performance . Bit like having your wellies on the wrong feet maybe [][][][]

 

[zcacogp]

Wellies on the wrong feet, or perhaps a bit more like walking backwards. Not sure.

I do know that tyre dynamics are a very complex science, which is still being perfected - both in terms of understanding and materials available. I bet the answer isn't a simple one.


Oli.

 

[RichardLW]

Aiming to keep tyre type and tread the same front and rear, I had to put ZRs on the back of my OH's Merc when they wore out. The Contisport Contact 2 is no longer made in HR as it has been replaced with the CCS3. The front HRs are fine with almost no wear. The car feels like a very odd now - like a wayward 911 if you will - with the front going more or less where pointed but the back is all over the place and it is simply the tyre mix, as it was well balanced before. I have played about with pressures a bit but you can clearly see the different side walls at work, even with the car stationary.

Mel is right - as is so often the case.

 

[Fat Albert]

I had my wheels balanced the other day and had the tyre pressures done at the same time, and it feels like a new car, only 3 years to go on the P6000s up front....

 

[Diver944]

ORIGINAL: zcacogp

What actually happens when directional tyres are fitted such that they rotate the wrong way?

One of our old track day brethren once fitted his road wheels back to front after a day on track and also mixed them up left to right, such that they were all rotating the wrong way. He drove the 30 miles home and noted the car felt 'funny' but when he inspected the tyres they had worn all the chamfers to excess because they were the wrong way round [&:]