996TT handling - more advice sort

[robinm]

A brief history:

I posted topic 198466 asking for advice on reducing understeer on a standard 996TT. As a result I visited JZ Machtech who made adjustments to camber and toe, front and rear.

I have just returned from a 2000 mile trip through France and Italy - most of the time in the Alps.

In the dry grip is fine, still some understeer but no complaints. However, in the wet I found on several occasions I was loosing front-end grip too easily. Going round a 270 degree motorway slip road on a wet surface I could feel the front end starting to slide when other ordinary traffic in front and behind seemed to be having no problem.

Can anyone shed light on this. Have the suspension adjustments somehow caused this loss of front-end grip in the wet?

The rear is completed planted at all speeds in all weathers.

Help!!
( Pirelli Corsa P Zeros all round. 2.5 bar pressure at front)


>> Edited by robinm on Monday 26th September 15:38

[domster]

They probably fecked around with your camber to improve grip in the dry. This can reduce it in the wet. The wheel is angled so that under large lateral load (dry) the tyre has a better contact patch. But in the wet you never get the load - or the contact patch. A friend damaged a 968CS at the 'ring in the wet the day after having his camber done...

Everything geometry-wise tends to be a compromise... comfort vs roll, wet grip vs dry grip.

Also remember that some sports tyres are better in the dry... they need to be warm and have minimal sipes (a dry tread pattern, really). P Zero Corsas will be far better dry than wet, at a guess. Not sure about normal P Zeroes, but it's a sporty tyre.

Best thing you can do to prevent understeer is probably driver training. I realised I had been creating a lot of understeer problems. There is so much you can do (trail braking/left foot braking) to cancel it out on turn in. I bet a rally driver could get your car to tip in neutrally on bald Colway remoulds at the front So you can drive around the 'problem' anyway.

[robinm]

[quote=domster]They probably fecked around with your camber to improve grip in the dry. This can reduce it in the wet. The wheel is angled so that under large lateral load (dry) the tyre has a better contact patch. But in the wet you never get the load - or the contact patch. quote]

That makes sense.

Maybe I'll try a sack of potatoes in the front before trying to drive like a rally-ace on the public roads.

[domster]

The nice thing about transferring weight to the front axle under braking is the sack of potatoes is only there when you want it, LOL

[robinm]

Also if I lost it in a corner I might spud into the ditch and end up with a tatty front end!!!

[porsche4life]

Corsa's can run lower pressure's try 2bar see if it helps (fronts of course). Also you did not mention whether it understeers on power, neutral power or over run, if its on power (likely) try working the throttle to see if it alters the line, LFB could help but thats a bit aggressive for road use ! - better probably to chuck it into the corner and then correct using steering and throttle.

Good luck

[cyrus1971]

My conclusion is that the issue you describe is a fundamental of the 996 weight distribution. I drew a similar conclusion from a long cross alp trip last year. My 996TT was loaded with light luggage (stored in the front) and 2 persons and I noticed the same thing but probably milder than you describe.

I have always had my car set up to be as good in the wet as possible. Due sacrifice to dry weather performance which is always astonishingly good anyhow so it can be squandered. This makes for more consistent handling across all weather conditions. Less variation and hence safety. Incidentally I have P-Zero Rosso’s N4.

Certainly in my 993 C2 I had the same but far more pronounced issue and also in my Boxster S to a lesser extent. Just not enough mass over the front end :-(

[silver993tt]

My 993 Turbo handled fantastically on the way back from Germany last week, might of had something to do with the 150 litres of beer in the car

There was still room for more!

[aliged]

Definitely worth doing the Potato sack experiment. My Boxster is prone to a little understeer in the wet when pushing hard unless it has a loaded front boot......then it'd just AWESOME! Imagine the 996 to be even more transformed on that basis.

[DanH]

All cars can understeer in the wet if you apply too much right foot. Even my noble did and that had the least understeer of any car I've owned.

[abarber]

domster said:

The nice thing about transferring weight to the front axle under braking is the sack of potatoes is only there when you want it, LOL

Getting some weight transfer onto the front is really essential in a 911. Tada, understeer gone!

[robinm]

I don't want to drag this out too much - but I guess people will stop responding when they get bored.

However, 2 things.

First: what I have tried to describe above is not about understeer (although that is still apparent). What I am experiencing in the wet is loss of front-end traction. The car starts to slide if I am not careful, no matter whether under power or not - just speed related (and only in the wet). I think Domster hit the nail on the head - see thread above.

Second. My previous enquiry regarding understeer relates to road use. I am aiming to corner quickly but safely, not on the limit. Ideally I would like to be able to turn into a corner and, as the balance/weight settles, not have to apply additional lock to maintain the desired line. The adjustment to camber/toe has not achieved this. I wonder whether different front rim/tyre sizes would be feasible. Anyone got any views on this?

Sorry if I have exhausted interest on this subject, but comments past and future are much appreciated.

[Raven Flyer]

The geometry change could have actually made things worse, much worse. Did they ask you what you wanted to improve? There are lots of variables... turn in, tyre wear, stability under heavy braking, overall grip level, to list the main ones.

That said, I wonder if the problem lies elsewhere. It only takes a tiny bit of wheel bearing or bush play to ruin the front end grip on any car. Were these thoroughly checked when the geometry was done?

[robinm]

The geometry was changed to settings for "fast road use"

I am going to have it reset to standard and will ask them to check bearings and bushes at the same time. Thanks for that.

[murcielago_boy]

abarber said:

domster said: The nice thing about transferring weight to the front axle under braking is the sack of potatoes is only there when you want it, LOL Getting some weight transfer onto the front is really essential in a 911. Tada, understeer gone!

I agree....on the way INTO a corner up to the apex... BUT.... my stock GT3 can understeer quite a bit on the way OUT of a long corner especially under power...Suppose the solution is to floor it and get ready to catch the back end. NOT something I'll be doing on the road.

Sorry to Hijack... my advice - get your suspension geometry re-set/retuned quickly...

[porsche4life]

Sorry for repeat.. but dropping the tyre pressure at front will increase the contact patch and counter some of the effects of negative camber. Front is so lightly loaded i cannot imagine you having a problem with fronts getting too hot.

I know there are soles out there that follow the Porsche recommendations for tyre pressures ( i.e. leave them alone) but this is not realistic when you are looking for your own personal set up.

However if its sliding simply turning in.. i would be straight off to a geometry check of toe... some garages really do make a mollock of it.

I have tracked my car on a 4 wheel laser job, found it un-driveable and went somewhere else who set it again and hey presto perfect.

[DanH]

Only other issue could be too much of a trailing throttle round the corner. Need a bit of power to push the car around the corner. Used to get that mid corner understeer in my old car with too little throttle.