rodgling
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posted on 1/10/12 at 08:14 PM |
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lift off oversteer
Experiments in car setup continue. I've sorted out the front-rear grip balance issue by putting stiffer springs on the front, which made the
rear feel a bit wallow-y by comparison, so I put the front springs on the back to stiffen that too. The downside is that the old front springs are too
short so the back has been lowered about an inch, probably meaning that rake is 0 or maybe even the rear is slightly lower than the front (I plan to
fix this ASAP).
The overall effect is generally much improved - the car is much more "tight" feeling and responsive in the corners - but it now suffers
from excessive lift-off oversteer, even if I lift very gently while understeering.
What are some likely causes of this, and what could I try to address it?
I assume it's mainly due to excessive back-to-front weight transfer. Will sorting out the rake (by raising the rear) have a good effect? Or is
it possible that I've over-stiffened the rear and reduced rear grip/increased back-front weight transfer? Any thoughts/suggestions welcome.
[Edited on 1/10/12 by rodgling]
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gottabedone
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posted on 1/10/12 at 08:30 PM |
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could that be caused by the location/height of your rack giving a steering input when your suspension unwinds?
Steve
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britishtrident
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posted on 1/10/12 at 09:15 PM |
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I presume you checked the front and rear toe settings after changing the springs.
An oversteering car will feel "tight" and responsive but the feeling is deceptive, neutral handling is when the front and rear slip
angles are equal this can feel like very mild understeer because the front wheels will follow a slightly wider radius than the rear.
When trying to balance the handling it pays to change the spring rates in fairly small increments and test against the stop watch.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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snapper
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posted on 1/10/12 at 09:15 PM |
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If it wasn't there before then I suggest the rear is to low
However you have changed springs aswell
Only do one thing at a time
Test
Do the next or revert to original and try something else
Keep a written record.
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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rodgling
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posted on 1/10/12 at 09:47 PM |
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I haven't checked rear toe. The toe link mounts onto the wishbone on the in-board end so I would think it should remain unchanged with ride
height alteration but I could be wrong. Can anyone confirm or deny?
I guess the next step is to fix the rear ride height (tomorrow's job is to make some spring spacers), then check toe and camber and try
again.
It's quite possible I've gone too far with spring stiffness at the rear, however it doesn't feel like it so I will persevere with
these spring rates for now. If fixing ride height doesn't help then perhaps a less severe increase in rear spring rate would be the obvious next
step.
I guess what I'm really asking is, is there any particular setup error I might have made (e.g. low rear ride height) that's well known to
cause lift-off oversteer, or is it more subtle than that?
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chillis
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posted on 1/10/12 at 10:19 PM |
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quote: Originally posted by rodgling
Experiments in car setup continue. I've sorted out the front-rear grip balance issue by putting stiffer springs on the front, which made the
rear feel a bit wallow-y by comparison, so I put the front springs on the back to stiffen that too. The downside is that the old front springs are too
short so the back has been lowered about an inch, probably meaning that rake is 0 or maybe even the rear is slightly lower than the front (I plan to
fix this ASAP).
The overall effect is generally much improved - the car is much more "tight" feeling and responsive in the corners - but it now suffers
from excessive lift-off oversteer, even if I lift very gently while understeering.
What are some likely causes of this, and what could I try to address it?
I assume it's mainly due to excessive back-to-front weight transfer. Will sorting out the rake (by raising the rear) have a good effect? Or is
it possible that I've over-stiffened the rear and reduced rear grip/increased back-front weight transfer? Any thoughts/suggestions welcome.
[Edited on 1/10/12 by rodgling]
I seem to remember this one. I think the problem you originally had was too much roll oversteer, to reduce that you need a front front
anti-roll bar. Stiffening the front reduced the oversteer/roll but then the front and rear were unbalanced. You fitted the 'old' fronts
which may have been to stiff as well as to short. Lowering the rear will have thrown out the rear geometry which could also be agrivating things
(induced rear toe out possibly).
The experiment has shown an improvement if the roll is reduced but not neccesarily by the expedient of fitting stiffer front springs. It tends not to
fully understood that you cant just change spring rates significantly at only one end of the car. The spring rate all round has to be changed which
often just makes the whole car stiffer but doesn't address the underlying problem.
Without knowing more about the car, eg corner weights, motion ratio, current spring rates etc I cannot help further as we're getting into full
set up teritory here.
Exactly what car are we talking about here as others may have some 'successful' front/rear spring rates and geometry settings you could
try.
Never under estimate the ingenuity of an idiot!
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Jon Ison
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posted on 1/10/12 at 10:19 PM |
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Can I make a suggestion ?
Get the car set up correctly by someone with both the knowledge and equipment to do it properly.
I was at Blyton Saturday, there was a guy there offering this service and it wasn't expensive, all the chatter in the paddock was "my cars
under steering" "my car feels unstable breaking" "my car as lift off over steer" yet no one except my self took up the
chance to see just what was what regarding planting those 4 little bits of rubber on the tarmac where they should be planted.
My car had been previously set up correctly (ish) but since last time two new rear axle brackets had been fitted and a new front wishbone due to a
little incident at cadwell, I had set the car up to the best of my ability after this and it went well, you saw it at Barkston heath, however my back
axle (live) was 3° out & front tracking 2° out with corner weights all over the place. I drove a totally different car in the afternoon session,
no more locking up under heavy breaking, tyre squeal almost eliminated during cornering.
Whilst it was on the scales been set up a few where watching, one commented/asked "what makes that so bloody quick" I cheekily quickly
answered "me" the truth is I was clever enough to realise maybe £50 spent on set up whilst you cant physically see it would benefit the
car far more than £500 spent on bling.
I got too within a few thousandths of FTD at Barkston, the mere £50 spent at Blyton would/could have been worth at least a second off that time ?
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rodgling
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posted on 2/10/12 at 08:54 AM |
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Jon, I totally hear what you're saying about getting it set up properly - however I want to work through this process myself as I think
I'll learn more that way, plus I find it interesting. I know a pro could get better results faster, but I quite like fiddling around and seeing
what change has what effect. So you will have to put up with my questions on here a while longer :-)
Chillis: the roll was a minor issue, the bigger issue was the front-rear grip balance and the general soft, slightly wallow-y feel of the car. As
advised here, I'm trying to get the spring rates right first, then I'll look at an ARB to sort roll if I still find that a problem.
The car is just over 700 kg, 50/50 front/rear weight distribution, with 425 lb springs at the front and 350 at the rear. This is around what I had on
my old Elise which was a similar weight, so I reckon this is probably a reasonable starting point, certainly feels better than 350/200 which was what
the kit vendor supplied (except for the lift-off oversteer of course).
I'll sort the rear geo and report back... thanks all for the suggestions so far.
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chillis
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posted on 2/10/12 at 09:34 AM |
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quote: Originally posted by rodgling
Jon, I totally hear what you're saying about getting it set up properly - however I want to work through this process myself as I think
I'll learn more that way, plus I find it interesting. I know a pro could get better results faster, but I quite like fiddling around and seeing
what change has what effect. So you will have to put up with my questions on here a while longer :-)
Chillis: the roll was a minor issue, the bigger issue was the front-rear grip balance and the general soft, slightly wallow-y feel of the car. As
advised here, I'm trying to get the spring rates right first, then I'll look at an ARB to sort roll if I still find that a problem.
The car is just over 700 kg, 50/50 front/rear weight distribution, with 425 lb springs at the front and 350 at the rear. This is around what I had on
my old Elise which was a similar weight, so I reckon this is probably a reasonable starting point, certainly feels better than 350/200 which was what
the kit vendor supplied (except for the lift-off oversteer of course).
I'll sort the rear geo and report back... thanks all for the suggestions so far.
Certainly nothing wrong with figuring it out for yourself - you soon learn a lot of other people don't really know when you do it this way!
To work out spring rates (cheaper to calculate a likely spring rate than to buy dozens of sets of springs) you need to know the motion ratio
(sometimes called leverage ratio) and accurate corner weights. I have a set of my weigh 'bathroom' scales which go up to 550lb specially
for doing corner weights at home.
Once your happy with the spring rates, geometry is right you can play with damper settings (start at the softest and go just stiff enough to keep the
springs under control).
If your not sure about geometry these figures have always proved a good starting point for me:
Rear Camber 1 - 1.5 degrees negative
Rear toe in 10 minutes per side
Front Camber half of what you put on the rear ie 0.5 - 0.75 degrees
Front toe again half what you had on the rear ie 5 minutes per side
5-7 degrees of Castor if you can get it or the maximum you can get if less than 5 deg.
If you still get roll over steer after all that then you'll have to look at a front anti roll bar. Also do not ignore the coments by others
regarding steering rack location. Some Locosts have problems here though I would have thought the GKD would be well sorted
Never under estimate the ingenuity of an idiot!
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adithorp
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posted on 2/10/12 at 12:14 PM |
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I'd agree with those ^^^ as a starting point.
You've got to get the alignment, ride height and corner weights set BEFORE you decide there's a problem with anything else.
Once thats all ruled out... What are your dampers set at? You don't mention them in the previous posts.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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rodgling
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posted on 2/10/12 at 12:24 PM |
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OK, I get the message :-) will get geo sorted in line with suggestions below (this is more or less what I had before I started fiddling) before I
spend any more time worrying about lift-off oversteer.
Regarding dampers: I set them to full soft, then dialed them stiffer until I could still bounce a corner of the car by hand quite easily, but so that
the dampers were just stiff enough to stop it bouncing back and forth uncontrolled (this is at the softer end of their range of settings). That
approach feels about right on the road, sound sensible?
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adithorp
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posted on 2/10/12 at 12:35 PM |
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Sound reasonable put worth trying softer to see if it helps.
Don't take this the wrong way but... How used to driving a light, powerfull RWD car are you? I only ask because mine used to understeer and snap
oversteer... then I learn't how to drive it.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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rodgling
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posted on 2/10/12 at 12:53 PM |
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OK, will bear that in mind with the dampers.
Not taken the wrong way, it's a fair question - but I've had this car for a few years, have a boxster also and an Elise for a few years
before that, and have had some limit-handling type driver training in these cars. I'm no driving expert but I would say I do have enough
experience to be able to judge what the car is up to, how to provoke (or not) different kinds of behaviour, and if I like the overall behaviour or
not.
Certainly at the moment it's difficult to lift gently enough when understeering to avoid lift-off oversteer, which wasn't the case at all
with the softer springs, so would like to improve this.
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chillis
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posted on 2/10/12 at 09:15 PM |
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quote: Originally posted by rodgling
OK, will bear that in mind with the dampers.
Not taken the wrong way, it's a fair question - but I've had this car for a few years, have a boxster also and an Elise for a few years
before that, and have had some limit-handling type driver training in these cars. I'm no driving expert but I would say I do have enough
experience to be able to judge what the car is up to, how to provoke (or not) different kinds of behaviour, and if I like the overall behaviour or
not.
Certainly at the moment it's difficult to lift gently enough when understeering to avoid lift-off oversteer, which wasn't the case at all
with the softer springs, so would like to improve this.
Not doubting you ability to pedal, if you couldn't pedal you wouldn't have started this thread! You clearly know the cars not doing what
you want and I would guess your not sure how to sort that.
Its good you want to learn how to set things up. Its very rewarding to get a car set up exactly how YOU want it and its a very personal thing so if
you can do it yourself you get your car to handle your way every time.
As I think you've worked out by now theres an order to follow that makes life easier.
First off get yourself a notebook and pen - all your set up changes should be recored in there as well as your driver appraisals, comments thoughts
etc. It WILL prove invaluable.
1 - measurement - you need to know what you've got to start with. Original springs, dampers etc. Ride heights Corner weights and unsprung
weights plus motion ratios.
2 - baseline - Set the car with the original parts and get the geometry set up on a lazer alignment. The settings I posted earlier will do here. Get
a paint pen and mark everything up adjustment wise so when you swap out parts you can put things back in exactly the right place to preserve the
geometry you have paid to get set. Unless you change ride height the geo. will stay the same.
3 - assesment - drive the car - quite a bit under all likely circumstances if you can. make notes about how it feels, you don't need fancy words
to describe it just notes so you understand how it felt when you read them later.
4 - playtime - This is where yoiu get to change stuff and see what effect it has and whether the calc.s you did have made things better. ie The
measurements from 1 will help you to calculate the springs you'll most likely need. put them on the repeat 3.
5 - repeat 4 for each change - repeat as many times as needed until the car behaves the way you want
I've simplied it I know or I'll be here all night
[Edited on 2/10/12 by chillis]
Never under estimate the ingenuity of an idiot!
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spiderman
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posted on 2/10/12 at 10:22 PM |
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Cheapest and easiest cure, don't lift off, keep the throttle pinned.
Spider
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sdh2903
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posted on 2/10/12 at 10:28 PM |
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quote: Originally posted by spiderman
Cheapest and easiest cure, don't lift off, keep the throttle pinned.
I've been waiting to see how long before someone posted that
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nz_climber
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posted on 3/10/12 at 06:13 AM |
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My car had oversteer on the track (not noticeable on the road). Added 2 clicks on the front shocks - was a massive improvement, but still a bit of
oversteer, 1 more click it became a different car, real neutral feel and gave me a lot more confidence when pushing it..
http://aarons7.wordpress.com
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britishtrident
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posted on 3/10/12 at 06:27 AM |
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quote: Originally posted by sdh2903
quote: Originally posted by spiderman
Cheapest and easiest cure, don't lift off, keep the throttle pinned.
I've been waiting to see how long before someone posted that
Lift off over steer is really to be avoided on a road car, it will rear its head when you have to cope with some unexpected hazardous situation
and bite you.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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chillis
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posted on 3/10/12 at 09:05 AM |
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quote: Originally posted by britishtrident
Lift off over steer is really to be avoided on a road car, it will rear its head when you have to cope with some unexpected hazardous situation
and bite you.
Which is why its best to get the handling sorted in order to minimise unexpected hazardous situations
Never under estimate the ingenuity of an idiot!
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JimSpencer
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posted on 3/10/12 at 10:52 AM |
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Hi
Had to go through all this exercise sorting a Sylva Striker Mk3 - quite a rare beast - so had nobody elses baseline to start from, so have this T
shirt and the following is what I should have done...
1) Fitting stiffer front springs to reduce roll was a bad idea - an Anti Roll Bar is the right tool for the job.. found this out eventually - huge
improvement.
2) Light cars don't need rock hard springs - found this out eventually too..
3) Take the thing straight to Matt at ProComp and get it set up properly - did this bit but after waisting my time and lots of phaffing about
first!
4) Have the shocks dynoed while there and get them in the ball park - did this too, replaced the rubbish ones we had and fitted some 'properly
valved for the weight of the car' ones..
Then and only then:-
5) Take a box of springs to a nice quiet trackday at a circuit with some nice long corners and plenty of run off and have a play..
Made some quite big changes to see what happened, then worked back towards a base set-up that gave a neautral balance, further fine tuned by small
adjustments to damper settings.
Car was balanced really well, could alter the handling at will with small setup changes as by then knew what the changes would do..
But
If you're setting it up for the road then making it a touch understeery on the track's a good plan, perfect for the track will feel very
twitchy on your average road, in my limited experience.
HTH
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barraw
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posted on 9/10/12 at 09:24 PM |
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quote: Originally posted by Jon Ison
Can I make a suggestion ?
Get the car set up correctly by someone with both the knowledge and equipment to do it properly.
I was at Blyton Saturday, there was a guy there offering this service and it wasn't expensive, all the chatter in the paddock was "my cars
under steering" "my car feels unstable breaking" "my car as lift off over steer" yet no one except my self took up the
chance to see just what was what regarding planting those 4 little bits of rubber on the tarmac where they should be planted.
My car had been previously set up correctly (ish) but since last time two new rear axle brackets had been fitted and a new front wishbone due to a
little incident at cadwell, I had set the car up to the best of my ability after this and it went well, you saw it at Barkston heath, however my back
axle (live) was 3° out & front tracking 2° out with corner weights all over the place. I drove a totally different car in the afternoon session,
no more locking up under heavy breaking, tyre squeal almost eliminated during cornering.
Whilst it was on the scales been set up a few where watching, one commented/asked "what makes that so bloody quick" I cheekily quickly
answered "me" the truth is I was clever enough to realise maybe £50 spent on set up whilst you cant physically see it would benefit the
car far more than £500 spent on bling.
I got too within a few thousandths of FTD at Barkston, the mere £50 spent at Blyton would/could have been worth at least a second off that time ?
Thanks for the positive feedback Jon - I'm glad I could help and that you saw the benefit. You were certainly flying!!!!
I've got an advert on this site here: 4-wheel alignment and corner
weighting
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