Reducing understeer on 996TT - any ideas?
Discussion
I've raised this subject before and received advice regarding changes to toe and camber. I tried that and didn't really notice any improvement.
Anyone got any experience on other ways of reducing understeer without compromising ride too much. How about fitting GT3 rims to the front (they are a bit wider / more rubber contact area)?
Apologies if this subject already overdone.
Anyone got any experience on other ways of reducing understeer without compromising ride too much. How about fitting GT3 rims to the front (they are a bit wider / more rubber contact area)?
Apologies if this subject already overdone.
I've got no experience at all of Porsche handling, but I can offer some thoughts based on general principles that might help?
Wider rims on the front will increase the steering response by increasing the lateral stiffness of the tyre, and increase the peak lateral grip slightly, but also mean that the grip drops off more quickly once you pass that peak. Don't count on it preventing understeer, and it might mean the once you drop off the peak it is even harder to get out of understeer.
Understeer is often a transient problem which can be caused or cured by driver technique. If you can avoid the initial understeer, it may be that once the car has turned in and is accelerating into the bend that the balance is atually OK. To drive round the understeer you need to be sensitive to the way that the throttle, brakes and weight transfer change the understeer/oversteer balance of the car. A general characteristic of rwd cars is that a trailing throttle promotes severe oversteer, mild acceleration promotes understeer, heavy acceleration promotes mild oversteer. You also have lots of transient effects, the main one being the weight transfer onto the front axle as the body pitches forward under braking, and backwards under acceleration, the next one being that the body has a significant inertia in yaw which will constrain how quickly the body can respond to the yaw input your generate. Yank the steering in too quickly and you may push the front tyres past their peak slip angle before the car has turned in. You can reduce the risk of understeer by coming off the brakes as you turn in so the weight transfer promotes oversteer, and by keeping a trailing throttle in the initial turn-in phase. But you need to get a feeling for how far to go because if you overdo it the understeer will be replaced by oversteer which is even more of a problem - it needs a happy medium.
If you find that you are still suffering from understeer after you have set up in the bend you may be able to overcome it with the throttle, but this will depend on how much power you have available relative to the available traction. With all that weight over the driven wheels you my find that the understeer hump is relatively big and it may take a lot of throttle to drive through it with a consequential risk of going too far and provoking excessive oversteer.
If you are unable to drive round the understeer then you can bias the car's setup away from understeer by reducing the roll stiffness at the front and increasing it at the rear to reduce the steady state yaw stability. This is another case where you can have too much of a good thing, ideally you want the car as stable as you can get away with, and only reduce the stability as far as necessary to stop understeer being a problem.
Wider rims on the front will increase the steering response by increasing the lateral stiffness of the tyre, and increase the peak lateral grip slightly, but also mean that the grip drops off more quickly once you pass that peak. Don't count on it preventing understeer, and it might mean the once you drop off the peak it is even harder to get out of understeer.
Understeer is often a transient problem which can be caused or cured by driver technique. If you can avoid the initial understeer, it may be that once the car has turned in and is accelerating into the bend that the balance is atually OK. To drive round the understeer you need to be sensitive to the way that the throttle, brakes and weight transfer change the understeer/oversteer balance of the car. A general characteristic of rwd cars is that a trailing throttle promotes severe oversteer, mild acceleration promotes understeer, heavy acceleration promotes mild oversteer. You also have lots of transient effects, the main one being the weight transfer onto the front axle as the body pitches forward under braking, and backwards under acceleration, the next one being that the body has a significant inertia in yaw which will constrain how quickly the body can respond to the yaw input your generate. Yank the steering in too quickly and you may push the front tyres past their peak slip angle before the car has turned in. You can reduce the risk of understeer by coming off the brakes as you turn in so the weight transfer promotes oversteer, and by keeping a trailing throttle in the initial turn-in phase. But you need to get a feeling for how far to go because if you overdo it the understeer will be replaced by oversteer which is even more of a problem - it needs a happy medium.
If you find that you are still suffering from understeer after you have set up in the bend you may be able to overcome it with the throttle, but this will depend on how much power you have available relative to the available traction. With all that weight over the driven wheels you my find that the understeer hump is relatively big and it may take a lot of throttle to drive through it with a consequential risk of going too far and provoking excessive oversteer.
If you are unable to drive round the understeer then you can bias the car's setup away from understeer by reducing the roll stiffness at the front and increasing it at the rear to reduce the steady state yaw stability. This is another case where you can have too much of a good thing, ideally you want the car as stable as you can get away with, and only reduce the stability as far as necessary to stop understeer being a problem.
Thanks for that. From my own experience and from other comments here, I know that the 996TT and other 911s have understeer built in by the manufacturer for reasons of safety. I also appreciate that it is possible to adjust your driving style to reduce this effect - as you have described.
What I would like to find, though, is some way to adjust the mechanics so that the car's inherent tendency to understeer is reduced.
What I would like to find, though, is some way to adjust the mechanics so that the car's inherent tendency to understeer is reduced.
Hope this table comes out ok. These are general principles for increasing/decreasing understeer/oversteer.
PARAMETER----------INCREASE UNDERSTEER------INCREASE OVERSTEER
Front Tire Pressure --------------Lower ------------------ Higher
Rear Tire Pressure --------------Higher------------------- Lower
Front Tire Width ------------------Smaller------------------ Larger
Rear Tire Width------------------- Larger -------------------Smaller
Front Camber ---------------------More Positive -----------More Negative
Rear Camber ---------------------More Negative -----------More Positive
Front Springs ----------------------Stiffer ---------------------Softer
Rear Springs -----------------------Softer --------------------Stiffer
Front Sway Bar --------------------Stiffer ---------------------Softer
Rear Sway Bar --------------------Softer ---------------------Stiffer
PARAMETER----------INCREASE UNDERSTEER------INCREASE OVERSTEER
Front Tire Pressure --------------Lower ------------------ Higher
Rear Tire Pressure --------------Higher------------------- Lower
Front Tire Width ------------------Smaller------------------ Larger
Rear Tire Width------------------- Larger -------------------Smaller
Front Camber ---------------------More Positive -----------More Negative
Rear Camber ---------------------More Negative -----------More Positive
Front Springs ----------------------Stiffer ---------------------Softer
Rear Springs -----------------------Softer --------------------Stiffer
Front Sway Bar --------------------Stiffer ---------------------Softer
Rear Sway Bar --------------------Softer ---------------------Stiffer
robinm said:
What I would like to find, though, is some way to adjust the mechanics so that the car's inherent tendency to understeer is reduced.
GreenV8S said:
you can bias the car's setup away from understeer by reducing the roll stiffness at the front and increasing it at the rear
Thanks for the comments so far. Interested in the advice regarding softer front springs/roll stiffness. Not being an expert in this area I was thinking that a stiffer front setup would have been the way forward. I'll play with tyre pressures.
Have heard lots of good things about Don Palmer. It's on my list of must does - once I get this understeer sorted!!
aceparts_com said:
Have you done a day with Don Palmer yet? Adjust the nut behind the wheel....
Have heard lots of good things about Don Palmer. It's on my list of must does - once I get this understeer sorted!!
If you're experiencing understeer then your corner entry speed is too high for the spec of your car, so drive slower! 
obviously not much help if you want to actually go faster though ..
you need to identify at what point the car starts to understeer .. unless you can work out what's causing it you may end up altering a suspension component which adversely affects something else.
for instance if you have a relatively softly sprung road car with corner entry understeer then don't start playing with springs and roll bars until you've addressed the damping .. corner entry understeer (ie that very first initial turn on the steering wheel) can largely be addressed by ramping up the compression on the front dampers .. however dampers tend to address transient body attitude changes and once you reach the steady state phase of the corner you need to be thinking more towards what's then controlling the total roll and load transfer of the vehicle, and how the suspension is changing during roll and it's effect on the tyre's contact patch.
Until you can identify where the problem first begins you'll not know how to treat it, but in the short term you can address driving tricks which can help in increasing the front grip on the turn-in phase, such as trailing throttle or braking towards the apex if you're brave.
obviously not much help if you want to actually go faster though ..
you need to identify at what point the car starts to understeer .. unless you can work out what's causing it you may end up altering a suspension component which adversely affects something else.
for instance if you have a relatively softly sprung road car with corner entry understeer then don't start playing with springs and roll bars until you've addressed the damping .. corner entry understeer (ie that very first initial turn on the steering wheel) can largely be addressed by ramping up the compression on the front dampers .. however dampers tend to address transient body attitude changes and once you reach the steady state phase of the corner you need to be thinking more towards what's then controlling the total roll and load transfer of the vehicle, and how the suspension is changing during roll and it's effect on the tyre's contact patch.
Until you can identify where the problem first begins you'll not know how to treat it, but in the short term you can address driving tricks which can help in increasing the front grip on the turn-in phase, such as trailing throttle or braking towards the apex if you're brave.
I'm not sure I am able to answer Lanan and Trackcar with information that will satisfy their professional expertise.
I know driving styles can be adapted to match the dynamics of the car, but this post is an enquiry into whether it is feesible to adjust the dynamics.
In very simple, laymans' terms when I apply a steering input on entry into a corner the car will invariably run wider than anticipated. I will end up applying more lock in order to achieve the apex. Entering on a trailing throttle or even under braking the understeer is less apparent. As soon as power is applied the front end will start to run wide and with my style of driving is always the limiting factor so far as exit speed is concerned. If I lift off the throttle at any time, mid corner, the car will tighten its line.
Sorry, that's the best I can do.
I know driving styles can be adapted to match the dynamics of the car, but this post is an enquiry into whether it is feesible to adjust the dynamics.
In very simple, laymans' terms when I apply a steering input on entry into a corner the car will invariably run wider than anticipated. I will end up applying more lock in order to achieve the apex. Entering on a trailing throttle or even under braking the understeer is less apparent. As soon as power is applied the front end will start to run wide and with my style of driving is always the limiting factor so far as exit speed is concerned. If I lift off the throttle at any time, mid corner, the car will tighten its line.
Sorry, that's the best I can do.
robinm said:
I'm not sure I am able to answer Lanan and Trackcar with information that will satisfy their professional expertise.
I know driving styles can be adapted to match the dynamics of the car, but this post is an enquiry into whether it is feesible to adjust the dynamics.
In very simple, laymans' terms when I apply a steering input on entry into a corner the car will invariably run wider than anticipated. I will end up applying more lock in order to achieve the apex. Entering on a trailing throttle or even under braking the understeer is less apparent. As soon as power is applied the front end will start to run wide and with my style of driving is always the limiting factor so far as exit speed is concerned. If I lift off the throttle at any time, mid corner, the car will tighten its line.
Sorry, that's the best I can do.
How much throttle are you using in the corner? Another thing that can exacerbate the understeer once turned in is not enough throttle.
In the end 911s are fundamentally light at the front so a little bit prone to turn in understeer. You just learn to work with it although sure you can make some geo changes etc, but they are always a compromise and if you make them you'll find it more darty but also more likely to follow cambers etc.
Not sure if you've done any driver training, so please excuse me if you are an expert, but I learnt a lot about getting to grips with my 911 on the road with just a half day with these guys www.cadence.co.uk/ . I will be going back soon.
The ones i have been in were OK in standard trim set -up.
I know it sounds odd but it may well be the make/model of tyre you are using.
I have moved to Corsa track tyres on a 993TT and 996 GT2 and both moved into much more push after fitting them, which i had to then correct with different geometry.
The reason i mention the make of tyre is because i have on a very different car moved from Silverstone tyres to hankook tyres and also gone wider all round. The inherent understeer has now gone and replaced with a nice balance if treated gently / snaps into wild oversteer if you don't
I know it sounds odd but it may well be the make/model of tyre you are using.
I have moved to Corsa track tyres on a 993TT and 996 GT2 and both moved into much more push after fitting them, which i had to then correct with different geometry.
The reason i mention the make of tyre is because i have on a very different car moved from Silverstone tyres to hankook tyres and also gone wider all round. The inherent understeer has now gone and replaced with a nice balance if treated gently / snaps into wild oversteer if you don't
robinm said:
I'm not sure I am able to answer Lanan and Trackcar with information that will satisfy their professional expertise.
I know driving styles can be adapted to match the dynamics of the car, but this post is an enquiry into whether it is feesible to adjust the dynamics.
In very simple, laymans' terms when I apply a steering input on entry into a corner the car will invariably run wider than anticipated. I will end up applying more lock in order to achieve the apex. Entering on a trailing throttle or even under braking the understeer is less apparent. As soon as power is applied the front end will start to run wide and with my style of driving is always the limiting factor so far as exit speed is concerned. If I lift off the throttle at any time, mid corner, the car will tighten its line.
Sorry, that's the best I can do.
<numpty>
If the front pushes wide shouldn't you be removing lock to get the tyre back to gripping rather than adding more lock and increasing the slip angle of the tyre? </numpty>
Is this a tight corner problem or a long high speed corner problem?
Generally speaking a stabilising understeering balance through high speed corners is what you would aim for so don't be too keen to dial that out .. but if it's compromising your car through lower speed corners it could do with addressing if it's stopping your fun.
You could do with getting some tyre temp readings, or at the very least some good pics showing the attitude of the tyres during the understeer phase.
I take it you want to make the car faster not just better balanced?
Generally speaking a stabilising understeering balance through high speed corners is what you would aim for so don't be too keen to dial that out .. but if it's compromising your car through lower speed corners it could do with addressing if it's stopping your fun.
You could do with getting some tyre temp readings, or at the very least some good pics showing the attitude of the tyres during the understeer phase.
I take it you want to make the car faster not just better balanced?
DanH said:
Not sure if you've done any driver training, so please excuse me if you are an expert, but I learnt a lot about getting to grips with my 911 on the road with just a half day with these guys www.cadence.co.uk/ . I will be going back soon.
Definitley not an expert Dan, and Cadence or Don Palmer a must in due course.
porsche4life said:
I know it sounds odd but it may well be the make/model of tyre you are using.
Using Pirelli Corsa Asymetrico N3s. Havn't tried anything else.
m12_nathan said:
<numpty>
If the front pushes wide shouldn't you be removing lock to get the tyre back to gripping rather than adding more lock and increasing the slip angle of the tyre? </numpty>
Someone with more knowledge than me can give a technical explanation. Surfice to say that understeer isn't total loss of grip, and it is possible to apply more lock to tighten the line.
trackcar said:
Is this a tight corner problem or a long high speed corner problem?
Generally speaking a stabilising understeering balance through high speed corners is what you would aim for so don't be too keen to dial that out .. but if it's compromising your car through lower speed corners it could do with addressing if it's stopping your fun.
You could do with getting some tyre temp readings, or at the very least some good pics showing the attitude of the tyres during the understeer phase.
I take it you want to make the car faster not just better balanced?
It is more of a problem in long, high-speed corners. The rear weight bias plays a much more dominant role the harder and faster the turn-in into tight corners.
Many thanks to everyone for their input. It's appreciated.
Robin. I think that your problem could be mainly driving technique. The massive torque of the turbo engine greatly exagerates the squatting under acceleration the lifts the nose on a 911. No matter what geometry you run, If you are accellerating hard before the apex of a corner, you will almost certainly get understeer in a TT. Is this what is happening?
All i can add is that i went from your tyres to Corsa track tyres and my 993 TT understeered much more. I dialed in more negative front camber and reduced toe to zero. I was also slightly more aggressive on turn in on the wheel, however the faster i went into the corner the less aggressive i needed to be because the car wanted to swap ends so it was a case on getting on the power to calm everything down.
If your are just starting your driving of 911's there is lots and lots you can do to get the car to go where it needs to. Thats half the pleasure. If you do dial out the understeer you could have a very unstable car which only Schumacher can drive quicker than an Astra Estate (not joking)
If your are just starting your driving of 911's there is lots and lots you can do to get the car to go where it needs to. Thats half the pleasure. If you do dial out the understeer you could have a very unstable car which only Schumacher can drive quicker than an Astra Estate (not joking)
If you're happy with teh balance in slower corners but want to reduce the understeer in faster corners (i wouldn't, but then i'm not the driver) .. then try tackling it with aero add-ons, front splitter/lower front ride height etc. A pair of front winglets might be all that's needed.
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