Tony Posted September 23, 2007 Report Share Posted September 23, 2007 Has anyone experienced the phenomenon? If yes how did it evolve and how did you deal with the situation. Sam@TDi in your experience how would you explain this condition and indeed control/reduce lift-off oversteer. Quote Link to comment Share on other sites More sharing options...
jammy Posted September 23, 2007 Report Share Posted September 23, 2007 use to get a lot of it with my 205 GTI... usually stonkin up to a sharp bend...... but she just pulled straight back in line with no real effort.......now my cossie,,,well you didnt need to lift off to get that braod siding spent most of it's time goin side way's tbh Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted September 23, 2007 Report Share Posted September 23, 2007 Liftoff over steer is a trait common in almost all chassis designs, it's almost impossible to totally eliminate it from a chassis. It happens when the driver turns the chassis (add's yaw) then as the chassis has loaded up and begun to produce lateral acceleration the driver changes the weight balance of the chassis. Often this is done by lifting throttle suddenly hence the name "lift off over steer".... this action throws the weight distribution forward, this shift of ground pressure to the front removes valuable ground pressure from the rear axle and promotes a spin departure Skilled drivers can learn to manipulate this to great effect, a chassis dynamisist can in turn help by making a chassis's lift off over steer habits as progressive and predictable as possible. Many motorsport drivers lift off at corner entry to help a chassis gain an unnaturally large slip angle, you can see it very obviously in the sports of drifting and rallying The bad news for the uninitiated masses is that often their first taste of car control comes whilst they're having an accident, lifting the throttle tends to be an instinctive reaction to panic in a "normal" driver, learning not to automatically lift the throttle is normally the first thing taught in an advance driving school such as police driver training or motor racing schools Quote Link to comment Share on other sites More sharing options...
Tony Posted September 23, 2007 Author Report Share Posted September 23, 2007 Is this similar to the "point of polar inertia" if so would it be wise to index the chassis and manipulate the front/rear coil/brake bias. To be honest Sam where i am struggling here is the inert initiation... Off throttle the inertia should be subliminal until yaw actions are submitted, unless expected down force is reduced... Could chassis calibration control this or is it a global condition inherited by the cars design..... Kind of "that's it" shrug shoulders time. Quote Link to comment Share on other sites More sharing options...
Mat Posted September 23, 2007 Report Share Posted September 23, 2007 ive been known to have it 1 or 2 times Quote Link to comment Share on other sites More sharing options...
Tony Posted September 23, 2007 Author Report Share Posted September 23, 2007 ive been known to have it 1 or 2 times A conservative estimate methinks mat Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted September 23, 2007 Report Share Posted September 23, 2007 It's important to understand that as a driver applies throttle or brakes to the chassis during a turn the chassis will react in an incredibly dynamic way so it's not easy to talk about lift off over steer with sweeping statements or "rules of thumb" as the science of the subject is so massively complex However it's my opinion that having a dynamic index close to 1.00 makes these kinds of control based maneuvers much easier for a driver to initially judge, measure and then control. A good balanced chassis with a DI close to 1.00 is often referred to by drivers as being "very adjustable" at the limit and I think thats a perfect way to describe the effect. Quote Link to comment Share on other sites More sharing options...
Noel Posted September 23, 2007 Report Share Posted September 23, 2007 What about guys who fit 1.5 or 2 way differentials... ? Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted September 23, 2007 Report Share Posted September 23, 2007 That is actually a good point, LSD effect across the axle is extremely desirable in high grip situations but only to a point, if that useful threshold is exceeded then the LSD can quickly become the enemy of grip causing an axle to break away. Violent and extreme 1.5way or 2way LSD settings are often employed in drifting to help deliberately agitate the rear axle, it's important to realize that this was never the original intended use of the LSD though, when set up correctly they increase stability not harm it. Quote Link to comment Share on other sites More sharing options...
Mat Posted September 23, 2007 Report Share Posted September 23, 2007 i agree with a 2 way, as it locks on de-acceleration, should't have much of an adverse effect with a 1.5, as these should only lock on acceleration Quote Link to comment Share on other sites More sharing options...
Tony Posted June 27, 2008 Author Report Share Posted June 27, 2008 We calibrated a MX5 Monday after the owner installed new coilover suspension. Once set the owner was very happy with the handling but the car had developed a knock. It turned out the top mounts had not been modified as per instruction for the shock install so the owner need to remove them all again. Consequently we had to calibrate the chassis again. Once done the car then had a pull ......we measured it again and there was no obvious reason why so we staggered the front camber and castor, but the pull remained. Convinced the pull was pneumatic we moved the rear wheels to the front..... the pull disappeared Next day the owner called to say he now had violent lift off over-steer .. My thoughts now go toward the coil-overs, maybe a fault in the damping ratios. Then the owner called to say he had moved the wheels back and the problem had disappeared .... all the tyres are the same size/ make and country of origin. For the life of me i cannot understand the train of events or identify the real criminal if only to avoid this in the future. Quote Link to comment Share on other sites More sharing options...
Rich Posted June 27, 2008 Report Share Posted June 27, 2008 I remember this, the new tyres were fitted to the rear? Could swapping them to the front have caused it if they haven't scrubbed in yet? Or could the pressure be too high? Quote Link to comment Share on other sites More sharing options...
Tony Posted June 27, 2008 Author Report Share Posted June 27, 2008 I remember this, the new tyres were fitted to the rear? Could swapping them to the front have caused it if they haven't scrubbed in yet? Or could the pressure be too high? The tyres have more or less the same wear +- 2mm.... Pressure spot on. L.O.O.S is normally a trait of inertia born from transition and inherited design.... Never tyres Quote Link to comment Share on other sites More sharing options...
StressedDave Posted June 28, 2008 Report Share Posted June 28, 2008 Just finished twiddling the suspension on that car... aside from the newness of the tyres affecting cornering stiffnesses, the rear dampers needed a different setting. Before adjustment there was a distinct disparity in response times from front and rear. After... Quote Link to comment Share on other sites More sharing options...
Tony Posted June 28, 2008 Author Report Share Posted June 28, 2008 Just finished twiddling the suspension on that car... aside from the newness of the tyres affecting cornering stiffnesses, the rear dampers needed a different setting. Before adjustment there was a distinct disparity in response times from front and rear. After... Is this what Performance 5 found today? Quote Link to comment Share on other sites More sharing options...
StressedDave Posted June 29, 2008 Report Share Posted June 29, 2008 Phil sent him to me as I did the initial design and development and the owner only lives around the corner. I'm not convinced it was true lift-off oversteer, more a function of reduced understeer as part of the design and the slow transition at the rear so a quick steering input could giv elots of response at the front but less at the rear. Quote Link to comment Share on other sites More sharing options...
Tony Posted June 29, 2008 Author Report Share Posted June 29, 2008 Phil sent him to me as I did the initial design and development and the owner only lives around the corner. I'm not convinced it was true lift-off oversteer, more a function of reduced understeer as part of the design and the slow transition at the rear so a quick steering input could giv elots of response at the front but less at the rear. Blimey Dave, i've just realizsed who you are!!! Yes i agree this didn't make sense as "lift-off-oversteer".... It had me puzzled since the calibration was neutral but with rapid migration at yaw. One reason for this is the huge castor gains on the Puredrive suspension, i've done me math and i think it must be due to the top mounts? On this car i couldn't get the castor under 7 degrees ..... I think you will agree this is mighty high for a 5, ever more so since the steering arm is already out of radii with the radii of the lower arm. Maybe some spacers could be included in the kits if the car will not allow reduction of the castor naturally? Quote Link to comment Share on other sites More sharing options...
StressedDave Posted June 29, 2008 Report Share Posted June 29, 2008 Phil sent him to me as I did the initial design and development and the owner only lives around the corner. I'm not convinced it was true lift-off oversteer, more a function of reduced understeer as part of the design and the slow transition at the rear so a quick steering input could giv elots of response at the front but less at the rear. Blimey Dave, i've just realizsed who you are!!! Yes i agree this didn't make sense as "lift-off-oversteer".... It had me puzzled since the calibration was neutral but with rapid migration at yaw. One reason for this is the huge castor gains on the Puredrive suspension, i've done me math and i think it must be due to the top mounts? On this car i couldn't get the castor under 7 degrees ..... I think you will agree this is mighty high for a 5, ever more so since the steering arm is already out of radii with the radii of the lower arm. Maybe some spacers could be included in the kits if the car will not allow reduction of the castor naturally? I'm not sure that computes... the top mounts are standard Mk II mounts AFAIK (Oi Phil...) so there should be no reason why there'd be a limitation in the amount of caster you can get out of the alignment. I'm always happy to hear an explanation though... Quote Link to comment Share on other sites More sharing options...
Tony Posted June 29, 2008 Author Report Share Posted June 29, 2008 It's odd. As we know normally when the sprung is lowered we lose the castor.... Every Puredrive kit i have measured has had a high castor, to the extent now it's to high. I have spoken to Phil about this, not as a complaint more that if we knew why we could sell it since the high castor on the lowered 5 is so valuable after lowering (within reason) Absolutely shocks have been watching developments and contacted me last week to explain the reason is the top mounts? and this condition could be mimicked with new mounts from Performance 5 on all lowering kits. I don't know.... If i had all variants at hand then this could form an explanation regarding the mounts? Quote Link to comment Share on other sites More sharing options...
StressedDave Posted June 30, 2008 Report Share Posted June 30, 2008 It's odd. As we know normally when the sprung is lowered we lose the castor.... Every Puredrive kit i have measured has had a high castor, to the extent now it's to high. I have spoken to Phil about this, not as a complaint more that if we knew why we could sell it since the high castor on the lowered 5 is so valuable after lowering (within reason) Absolutely shocks have been watching developments and contacted me last week to explain the reason is the top mounts? and this condition could be mimicked with new mounts from Performance 5 on all lowering kits. I don't know.... If i had all variants at hand then this could form an explanation regarding the mounts? I think the mounts are a red herring - it's more likely to be a lower ride height at the rear canting the front adjusters clockwise giving more caster. The position of the spring within the coilover should make no difference to caster - that's purely a function of the two suspension arms and the upright pickup positions. Anyway, I don't think there's any such thing as too much caster, especially on power steered cars. Quote Link to comment Share on other sites More sharing options...
Tony Posted June 30, 2008 Author Report Share Posted June 30, 2008 It's odd. As we know normally when the sprung is lowered we lose the castor.... Every Puredrive kit i have measured has had a high castor, to the extent now it's to high. I have spoken to Phil about this, not as a complaint more that if we knew why we could sell it since the high castor on the lowered 5 is so valuable after lowering (within reason) Absolutely shocks have been watching developments and contacted me last week to explain the reason is the top mounts? and this condition could be mimicked with new mounts from Performance 5 on all lowering kits. I don't know.... If i had all variants at hand then this could form an explanation regarding the mounts? I think the mounts are a red herring - it's more likely to be a lower ride height at the rear canting the front adjusters clockwise giving more caster. The position of the spring within the coilover should make no difference to caster - that's purely a function of the two suspension arms and the upright pickup positions. Anyway, I don't think there's any such thing as too much caster, especially on power steered cars. Problem is the castor initiates the toe-out-on-turns and the steering radii so to much can be a problem. I'm good with 6 or 6.5 degrees but 7plus is to much..... i think? Quote Link to comment Share on other sites More sharing options...
StressedDave Posted July 1, 2008 Report Share Posted July 1, 2008 Not according to the kinematics models of the suspension I have. Toe out on turns is a max of 1 degree at 30 degrees of steer, even with 7 degrees of caster. I haven't got the steering arms positioned accurately enough to see what that's doing with bump steer, but any increase at the outlying values is probably immaterial given the significant increase in spring rates over stock. Anyway, back to the next design... Quote Link to comment Share on other sites More sharing options...
Tony Posted July 1, 2008 Author Report Share Posted July 1, 2008 I agree if taken on a 10 degree lock.... The tests i did were at 20 degrees (more accurate) I found> left lock inner wheel Camber loss -50' to +10' Castor sweep 5d 30' to 7d 40' TOOT 23d 30' "Next design?"..... Oh tell Quote Link to comment Share on other sites More sharing options...
StressedDave Posted July 2, 2008 Report Share Posted July 2, 2008 Doing the same for a Mk 3... Quote Link to comment Share on other sites More sharing options...
Tony Posted July 2, 2008 Author Report Share Posted July 2, 2008 Doing the same for a Mk 3... Now that is ggod news Quote Link to comment Share on other sites More sharing options...
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