Vicious squirming under heavy (track) braking
Discussion
Tried searching the archives, but there's nothing on this. Would appreciate a diagnosis from the regular trackers and racers here.
Mk 2 GT3 braking very hard from 235 kph down to approx 70 kph on a downhill section squirms horrendously under late and heavy braking, so much so that the car is snaking rather than just squirming. Dry track, ambient temps approx 35-40C. Have a relatively new set of Alcons upfront with Pagid RS29 and similarly recent Porsche OEM discs at the rear with Pagid 421 from JZ Machtech (but fitted by my local OPC). Rear cups are recently scrubbbed in and front ones are a year old and been through a few heat cycles. 2 degrees camber all round, but the last geometry was done a few months ago.
At the next service I will likely get the alignment done anyway but wondered whether the obvious immediate solution is to change the front cups? There are no apparent handling anomolies through the curve and beyond.
What's most likely: something wrong with the fitment of the brakes? Front tyres need changing? Alignment gone wrong? Crap driver? All right, scrub the last one, that's a given, but maybe my technique is wrong (stamp on the pedal so hard, I risk self-emasculation via 6-point harness - seems to have worked OK before).
Mk 2 GT3 braking very hard from 235 kph down to approx 70 kph on a downhill section squirms horrendously under late and heavy braking, so much so that the car is snaking rather than just squirming. Dry track, ambient temps approx 35-40C. Have a relatively new set of Alcons upfront with Pagid RS29 and similarly recent Porsche OEM discs at the rear with Pagid 421 from JZ Machtech (but fitted by my local OPC). Rear cups are recently scrubbbed in and front ones are a year old and been through a few heat cycles. 2 degrees camber all round, but the last geometry was done a few months ago.
At the next service I will likely get the alignment done anyway but wondered whether the obvious immediate solution is to change the front cups? There are no apparent handling anomolies through the curve and beyond.
What's most likely: something wrong with the fitment of the brakes? Front tyres need changing? Alignment gone wrong? Crap driver? All right, scrub the last one, that's a given, but maybe my technique is wrong (stamp on the pedal so hard, I risk self-emasculation via 6-point harness - seems to have worked OK before).
cptsideways said:
Squirming as you describe is often caused by slight or excessive toe out at the rear, have you changed ride height recently by any chance?
Agreed. I had the same symptom on my 968 when I had zero toe on the rear. Adding a degree of toe-in sorted it out perfectly. Any specialist who sets up 911's should be able to sort this out easily. JZ Machtech seem a popular choice.
Interesting - I had assumed the obvious problem was with the front tyres having gone off with age and use. I will get the alignment checked.
Does anyone know how slight toe-out on the rears would manifest itself in terms of tread wear, so I can visually confirm this?
Oh - and to answer cpt's question, no change in ride height, although I do tend to change the wheels regularly between road and track use. Can I assume "dropping" the car from a jack will not affect geometry too much?
Does anyone know how slight toe-out on the rears would manifest itself in terms of tread wear, so I can visually confirm this?
Oh - and to answer cpt's question, no change in ride height, although I do tend to change the wheels regularly between road and track use. Can I assume "dropping" the car from a jack will not affect geometry too much?
Edited by Harris_I on Sunday 5th November 11:10
Harris_I said:
Interesting - I had assumed the obvious problem was with the front tyres having gone off with age and use. I will get the alignment checked.
Does anyone know how slight toe-out on the rears would manifest itself in terms of tread wear, so I can visually confirm this?
Oh - and to answer cpt's question, no change in ride height, although I do tend to change the wheels regularly between road and track use. Can I assume "dropping" the car from a jack will not affect geometry too much?
Does anyone know how slight toe-out on the rears would manifest itself in terms of tread wear, so I can visually confirm this?
Oh - and to answer cpt's question, no change in ride height, although I do tend to change the wheels regularly between road and track use. Can I assume "dropping" the car from a jack will not affect geometry too much?
Edited by Harris_I on Sunday 5th November 11:10
I only really know the 968, but I had zero toe on the rear and had the same symptom, so don't assume it's toeing out necessarily (I'm sure others here can confirm if the 996 GT3 is the same). Mine needs to toe-in slightly to correct it.
And my 1 degree of toe-in doesn't show any measurable extra tyre wear. I believe that under braking, as the weight moves forwards, the rear lifts slightly and so toes out slightly more than when in equilibrium. Hence the toe-in set up becomes neutral under braking, whereas zero toe would become toe-out and so make the rear feel very keen to come around the front.
Can someone more knowledgeable about 911's confirm this is also the conventional wisdom regarding their set up? I presume that their extra weight over the rear wheels might reduce this effect, but I presume the principle is the same.
Now that you mention it, there is one specific corner I find a slight pogo-ing sensation, but I could be splitting hairs here and being overly sensitive.
Anyway, the one thing this proves, I suppose, is that handling and braking is a combination of so many factors that it is impossible to diagnose a problem without a full inspection. I say tyres, cpt and willdew suspect rear toe, and you suspect shock(s). Only one thing for it... get the car booked in...
Anyway, the one thing this proves, I suppose, is that handling and braking is a combination of so many factors that it is impossible to diagnose a problem without a full inspection. I say tyres, cpt and willdew suspect rear toe, and you suspect shock(s). Only one thing for it... get the car booked in...
Might be a geometry problem, or worn bushes, but have you considered that it may be a thermal balance problem? Very heavy high speed braking is going to heat the brakes up considerably, and if the fronts are running a lot hotter than the rears then they may start to fade sooner. This would bias the brakes to the rear, which would obviously make things very unstable.
Harris I, don't panic it will be the geometry out. Once it has been reset properly you'll be a happy bunny again. For example at Le Mans this year I drove a mates 993 tip cab down to the circuit on the Friday four up, very tight squeeze and impossible if it weren't a tip. Anything over 110 (kilometres of course) and the back end started to weave. Nick had noticed this on the trip down but two up it did not manifest itself until over 140. Once home the geometry was checked and it was quite along way out. Problem sorted.
cheers Graeme
cheers Graeme
I think that we must all realise that the geometry when set is only like that while the car is sat still on a flat base. If you raise and/or lower the front or rear (or both) all the geometry changes - so camber and toe all change and the more you lift or depress the front or rear the more it all changes (as it also does as you go around corners).
Now if you have a particular problem downhill with extremely heavy front braking you will have created effectively the same as putting a huge weight on the front of the car and lifting the rear.
So even if all the suspension bushes etc are in good condition you will at that point be running with geometry that the car was not designed to be as stable at under heavy braking - than it would be if it remained level.
Unfortunately our business does not get involved in track racing - so I cannot advise what to change the geometry to - but if others have already solved this then I would first make sure that you follow their advice. If the advice is contradictory for that exact model and problem then I think I would set up the car and then load it as described to find out what changes in the tilted down at the front - and high at the back - position (not always the same on all cars) - and then re-set the standard setting to reduce the biggest changes to toe because it does seem likely that the changes at the rear toe will be the most likely to be extreme (as in your scenario the rear will be extremely light) and that reducing the change would be the right direction to go in.
All such changes are always a compromise as no one setting will be right for all conditions. What you improve in one scenario may be worse in another - so it is always a question of hard work and testing. There are others that may have solved this already and if so please follow their advice.
The difficulties alway come when the problem is a new one and you have to sort it out for yourself.
Although we have not been involved in this particular problem before - the ability to make a plan to follow to find answers and improve performance is a skill that people either have (or develop) or have not. One of our technicians runs a Renault 5 GT Turbo at the third quickest time in the Country (@11.9 for the standing quarter and a terminal speed of 120mph (2nd most powerful ever) - frequently beating both 996 and 993 twin turbos - and still drives it to work) and also one of our Directors used to be involved in the design and development of racing motorcycle engines (one of which finished 3rd in the French GP) and we often help build specials (like one of the first 3 litre 944 turbos etc). I only explain this because - while we are unable to advise what standard settings will overcome this problem (because we have never been involved in solving this problem on a track) our suggestion as how to approach a solution may still be worth following - if no one else has a definite answer and you have to do it yourself.
This is how we would approach the problem if we had to solve it - but sometimes you just get your initial thinking all wrong (whoever you are) and only find our what to do when your initial idea makes things worse - so if there is already a general consensus of what to do I would always just try that first.
Baz Hartech
Now if you have a particular problem downhill with extremely heavy front braking you will have created effectively the same as putting a huge weight on the front of the car and lifting the rear.
So even if all the suspension bushes etc are in good condition you will at that point be running with geometry that the car was not designed to be as stable at under heavy braking - than it would be if it remained level.
Unfortunately our business does not get involved in track racing - so I cannot advise what to change the geometry to - but if others have already solved this then I would first make sure that you follow their advice. If the advice is contradictory for that exact model and problem then I think I would set up the car and then load it as described to find out what changes in the tilted down at the front - and high at the back - position (not always the same on all cars) - and then re-set the standard setting to reduce the biggest changes to toe because it does seem likely that the changes at the rear toe will be the most likely to be extreme (as in your scenario the rear will be extremely light) and that reducing the change would be the right direction to go in.
All such changes are always a compromise as no one setting will be right for all conditions. What you improve in one scenario may be worse in another - so it is always a question of hard work and testing. There are others that may have solved this already and if so please follow their advice.
The difficulties alway come when the problem is a new one and you have to sort it out for yourself.
Although we have not been involved in this particular problem before - the ability to make a plan to follow to find answers and improve performance is a skill that people either have (or develop) or have not. One of our technicians runs a Renault 5 GT Turbo at the third quickest time in the Country (@11.9 for the standing quarter and a terminal speed of 120mph (2nd most powerful ever) - frequently beating both 996 and 993 twin turbos - and still drives it to work) and also one of our Directors used to be involved in the design and development of racing motorcycle engines (one of which finished 3rd in the French GP) and we often help build specials (like one of the first 3 litre 944 turbos etc). I only explain this because - while we are unable to advise what standard settings will overcome this problem (because we have never been involved in solving this problem on a track) our suggestion as how to approach a solution may still be worth following - if no one else has a definite answer and you have to do it yourself.
This is how we would approach the problem if we had to solve it - but sometimes you just get your initial thinking all wrong (whoever you are) and only find our what to do when your initial idea makes things worse - so if there is already a general consensus of what to do I would always just try that first.
Baz Hartech
Hi Harris I - Although I think everyone appreciates that the camber goes the opposite way as the suspension lifts (and more as it falls) - we wanted to check the affect on the rear toe. We expected that it would increase rear toe out (because the tie rod is at the back of the rear suspension) and have just tested it out after setting up a 996 on the 4 wheel laser alignment rig (out of interest really) and sure enough that is what happened.
This means that under your heavy braking down hill - as the front dips and the rear lifts - the rear wheels will hang down and only the outer edge of the tyre will be in proper contact - while it also toes out both sides.
The result is that as the tyre contact area is very reduced (because it is not level with the road but tilted down) and the rear effective weight is lighter - the grip will be reduced under braking and will vary between the rear tyres and which ever one has the most grip will steer the car on an inward curve - the other tyre then taking over and returning the pendulum affect.
To reduce the problem you could consider making the front stiffer (springs and dampers) or try setting the rear suspension with more toe in - so it toes out less under heavy braking (but there may then be - in fact there usually is - a compensating worsening affect under some other conditions somewhere else round the circuit) or perhaps try altering the braking ratio with compensation adjustment between the front and rear brakes.
At the end of the day it is always a compromise and that is why the more dedicated teams excell - hard work and testing always improves the breed. This is often why trial and error and listening to others experiences - may be a quicker route as long as what they advise fits in with the general theory outlined. However - there are a lot of clever people who read a lot and understand theory but have never banged their finger with a hammer - who can get things quite wrong - so always try and work out what is likely to be the right solution (which I have tried to help you with - and hoep I have got it right) and then see if general advice fits in with that scenario. Even if it doesn't there might still be a logical explanation!
Good luck,
Baz Hartech
This means that under your heavy braking down hill - as the front dips and the rear lifts - the rear wheels will hang down and only the outer edge of the tyre will be in proper contact - while it also toes out both sides.
The result is that as the tyre contact area is very reduced (because it is not level with the road but tilted down) and the rear effective weight is lighter - the grip will be reduced under braking and will vary between the rear tyres and which ever one has the most grip will steer the car on an inward curve - the other tyre then taking over and returning the pendulum affect.
To reduce the problem you could consider making the front stiffer (springs and dampers) or try setting the rear suspension with more toe in - so it toes out less under heavy braking (but there may then be - in fact there usually is - a compensating worsening affect under some other conditions somewhere else round the circuit) or perhaps try altering the braking ratio with compensation adjustment between the front and rear brakes.
At the end of the day it is always a compromise and that is why the more dedicated teams excell - hard work and testing always improves the breed. This is often why trial and error and listening to others experiences - may be a quicker route as long as what they advise fits in with the general theory outlined. However - there are a lot of clever people who read a lot and understand theory but have never banged their finger with a hammer - who can get things quite wrong - so always try and work out what is likely to be the right solution (which I have tried to help you with - and hoep I have got it right) and then see if general advice fits in with that scenario. Even if it doesn't there might still be a logical explanation!
Good luck,
Baz Hartech
Interestingly I was back on the same track today (but without having had time to play with the suspension), and just tried a more progressive braking technique at that particular corner so that, as you suggested earlier, I don't dip the front of the car and raise the rear excessively. This seems to partially solve the problem of squirming under braking (confirming your theory), so I guess the next step is play with the toe at the rear, then go back to stamping on the pedal and see how it behaves. Will be interesting to see what effect the extra toe will have on the rest of the circuit...
Harris
Harris
Gassing Station | Porsche General | Top of Page | What's New | My Stuff