Understeer or oversteer

[westcr]

Boxster 2.7 with Primewell PZ900 rear and Michelin X front - tyres as fitted when car purchased.

What are the forum experiences of Boxster handling on the limit in the dry?
When pushed, mine exhibits fairly strong understeer (tried out on a nice motorway exit curve!) when I was expecting oversteer. Is this typical or are my tyres crap?

Any comments welcome

 

[944s2]

What the hell are Primewell tyres?

That may be your problem

http://www.tyrereviews.co.uk/Tyre/Primewell/PZ900.htm

Put a decent quality tyre on the car and you will notice difference

Remember the tyre is what keeps you on the road!

 

[Glyn]

The poster says he has lots of understeer, the Primewell tyre are on the rear - so that cant be the problem, however, Ive never heard of them either so i guess there a budget tyre, and also change those Michelin X tyres - go for something N rated.

The cars do understeer but thats when pushed to the extreem, make sure you get good tyres, have the alignment checked too - and if your feeling flushed have a Geo set up

 

[Buddy]

First, put a matched set of N rated tyres on the car then get some instruction on how to drive it.

There are a number of techniques to handling any car and most drivers these days have started out on FWD which are quite different, if you want to read up on the basics get Vic Elfords Porsche High Performance Driving Handbook there are a number of good tips there to give you the beginnings of how to handle a RWD car such as the Boxster.

Actually a poor tyre on the rear can cause understeer especially if the driving technique is wrong but the Michelin X is not a Porsche approved tyre so the problems are possibly going to be compounded.

The Boxster 987 is a well balanced and good handling car but does need the driver to know how, best to have someone who has the experience in the passenger seat.

 

[GreigM]

I've done a lot of track days in a boxster and that's normal dry weather behaviour. They are set up from the factory to understeer and generally you will need some other input/condition to provoke oversteer such as a slippy surface + throttle, dumping the clutch, or some kind of fairly sudden control inputs (like some sort of scandanavian flick, or brutal lift off while turning).... but if just driving along and expecting the throttle to make the back end break away you'll be disappointed (or pleased considering your point of view).

The newer cars with more power (I've only driven up to a 987 3.4 S) may be better at breaking traction at the back and provoking oversteer, but all of the models up to that point had plenty grip at the back meaning it was hard to start the slide.

Even if you do get the back end to start coming round, the boxster isn't a great car to drive sideways due to the absence of a slipper diff and the mid-engine layout - it tends to be twitchy and if you do break traction at the rear then it doesn't like to stop rotating and often you'll end up pointing the other way.

 

[Buddy]

Actually, I found that the 986 S with only around 250bhp was far easier to provoke into breaking away and have driven several 987's both Boxsters and Caymans from the std S to the Boxster sport and the Cayman S Gen2 which I drove on the skid pan, ice hill etc which even on the doughnut it took some effort to make it break away.

If your braking is held until the turn in point then the weight transfer forwards will aid the turn in, but try it first at lower speeds to see how it feels before trying it at higher speeds! I don't want blaming for anyone over cooking it into a corner!

 

[spyderman]

I track my 986 with Pirelli PZero Rosso all round and a track-biased geo. Agree entirely with the above that it will under steer when pushed hard. When it oversteers it tends to snap and pirouette.

If you want your car to handle better it's time to fit some decent tyres, same all round and the N-rated ones are a good starting point. Also get a specialist to do a proper geo on it (by which I don't mean just wheel alignment like wot QwikFit do) as your settings could be all over the shop.

Chris #88

 

[Motorhead]

Just be aware of trying to exploit over-or-understeer on any public road in your Boxster which, with the correct tyres fitted, has very high limits of adhesion. Your cornering speed may be such that you may not have time to recover when those limits are exceeded...!

Best to get along to a track day to discover what those (and your) limits are.

Jeff

 

[Glyn]

ORIGINAL: Buddy



Actually a poor tyre on the rear can cause understeer especially if the driving technique is wrong but the Michelin X is not a Porsche approved tyre so the problems are possibly going to be compounded.

Care to explain how?

 

[Motorhead]

The overall balance of a car when cornering is dependent upon many parameters: C of G postion; front and rear roll-centre heights and roll-stiffnesses; weight transfer under braking or accleration and variations in camber, toe-in/out and tyre slip-angles, etc.

Manufacturers (including Porsche) like to set up their cars with understeering characteristics because this is perceived as being more benign for the average driver (and more intuitive perhaps?) because more lock is required to correct it rather than the opposite lock correction required for an oversteering car.

Any variation in Porsche's baseline suspension/tyre set-up will invariably result in unpredictable handling characteristics.

Jeff

 

[Glyn]

If you have understeer puting more angle on the wheel will not correct it, the understeer is the tyre loosing traction with the road - the condition changes from static friction - (no slip) to sliding friction, adding more angle just compounds the understeer. - straightening the wheel and reapplying angle is the correct way to overcome understeer.

The type of rear tyre fitted will not contribute to understeer

 

[944s2]

Mismatch of tyres will lead to one or other depend on which end excess grip is

 

[Motorhead]

ORIGINAL: Glyn

If you have understeer puting more angle on the wheel will not correct it, the understeer is the tyre loosing traction with the road - the condition changes from static friction - (no slip) to sliding friction, adding more angle just compounds the understeer. - straightening the wheel and reapplying angle is the correct way to overcome understeer.

The type of rear tyre fitted will not contribute to understeer

Perhaps too general a statement to make Glyn. It depends upon whether we're talking FWD or RWD, the cornering speed, whether or not power is being applied, any roll-induced camber and toe changes, all of which can affect the tyre slip-angle.

I think that the first thing to do, whether FWD or RWD, is back-off on the power progressively and then apply the appropriate steering correction.

Matching front and rear tyres will provide a more consistent handling package.

Jeff

 

[Andy Fagan]

ORIGINAL: Glyn

- straightening the wheel and reapplying angle is the correct way to overcome understeer.

Or a bit of left foot braking, although make sure without PSM as it doesn't like this technique [] In fact PSM is a pain on track days whether on or off, but there is an alternative [8|]

 

[Glyn]

To correct understeer the "sliding friction" must be addressed, of course that means taking away anything that is contributing to loss of friction, so, on a FWD car backing off the throttle does help but lifting completely off will add another force - breaking from the engine - so this will add to the understeer or suddenly induce "tuck in" - and then you spin.
The OP has a Boxster so of course we are talking about RWD so lets concentrate on that.
The rear tyres will not contribute to understeer and (as been suggested) adding a little left foot breaking will only be adding more forces to the already lost traction of the front wheels so thats not going to assist either- unless of course you have a brake bias adjustment and bias is set to the rear, which i assume is not the case.
If the Boxster has PSM then the Yaw sensor will be aware of the understeer and will attempt to induce rear wheel breaking on the inside wheel to try to recover the slide whilst backing off the power input.
ABS will also attempt to brake and release individual wheels to make the car go in the direction you are steering.

Loss of traction could be either the quality of tyre, an issue with the road surface (contaminant) wheel geometary, alignment and pressures or of course power being applied excessively in the corner.

In the event of understeer you dont have time to read a manual, consider all the alternatives so reactions must be instinctive and straightening the wheel to obtain grip and reapplying angle - with a backing off from the throttle is the most likely succesfull course of action.
ABS works by simply doing what drivers used to do years ago - cadence breaking - when a wheel locks it releases the brakes and reapplies, cadence breaking was the same thing where the driver released the footbrake and reapplied rapidly repeating this

Next time we have snow - go out and try it, adding breaking will simply cause the car to go straight ahead - and ABS is pretty useless on snow or ice too because all wheels will lock so easilly and when they are all locked the electronics dont know the car is still moving so it doesnt release and reapply the brakes - the car thinks its stopped, so, cadence breaking still has a usefull function in this modern electronic world.

 

[uknick]

There is a small roundabout at the top of my road I like to use as my "stickability test". If I go round this at 30 mph and the car under-steers I know it is time to replace my front tyres.

 

[Andy Fagan]

ORIGINAL: Glyn

The rear tyres will not contribute to understeer and (as been suggested) adding a little left foot breaking will only be adding more forces to the already lost traction of the front wheels so thats not going to assist either- unless of course you have a brake bias adjustment and bias is set to the rear, which i assume is not the case.

I beg to differ. Left foot braking transfers some weight shift to the front and helps regain the front end grip. Nothing to do with brake bias adjustment.

 

[Buddy]

Rear tyres can cause understeer, in the simple fact that if the rear case is softer and so more flexible than the fronts then it effectively changes the response of the rear springs and then the fore/aft balance is incorrect, weight transfer does not take place in the correct way under braking and understeer is likely. The same can happen with tyres on the front which are old and have hardened, on the face of it four good tyres, matched but the older front tyres are not then balanced to the newer rears, I saw this on a visit to CG this afternoon.

Remember that the whole suspension effect is not only operating through the contact area of the tyres but the spring rate is effected by a difference in the flexibility of the tyres.

Regarding the driving, late braking or for that matter as Andy says left foot braking will help induce weight transfer forwards on entering a corner, transfer to the accelerator too early/too late and you've lost the advantage.

Talking to Chris at CG, increasing the castor angle setting improves the turn in by increasing the camber in the turn reducing the liability to understeer. []

 

[944s2]

Oversteer occurs when the rear tyres reach the limit of adhesion in a corner before the front. This leads to 'the back coming out'
The good thing about oversteer is that you normally go through the hedge backwards, thus preventing expensive repairs to the front of your car. If you manage to performed sustained, controlled oversteer this is know as drifting

Understeer occurs when traction is lost at the front wheels while cornering, forcing you wide on a bend despite applying the correct steering angle When viewed by an observer, this action looks as if the driver has applied insufficient steering lock (or under steered). If you're car is understeering, you're scrubbing off speed and missing the optimum line, so it's not a quick way to take a bend

In simple terms depends on end with more grip what you have

 

[spyderman]

Both my road Boxster and the race Boxster understeer to some degree in some corners (Silverstone Copse, Donington Redgate). I tend to not try any correction, just keep the throttle constant and not change the steering angle. Result - I go wide on the run out of the corner. Fortunately its usually after the gravel trap!