Oversteer when Limited Slip Diff fitted to FWD
Discussion
Yes I know it's a model, it's actually a FTX Banzai converted to FWD, with 55% weight over the front axle.
I built a limited slip differential that is full of mega viscosity silicone. It pulls away nicely with less wheel spin, and stops smoothly.
Yet, on full lock and sudden full throttle, the back drifts round like it is oversteering.
How is this even possible?
I built a limited slip differential that is full of mega viscosity silicone. It pulls away nicely with less wheel spin, and stops smoothly.
Yet, on full lock and sudden full throttle, the back drifts round like it is oversteering.
How is this even possible?
With extreme weight distributions like that I usually see problems with 'stoppies' and braking related oversteer rather than power oversteer. If I'm visualising your setup correctly, the only reason I can think is that the are essentially pulling in the direction they're pointing and this is producing a lot of yaw moment, and the vehicle is effectively pivoting around the center of mass.
ETA I've seen the opposite problem, where the two front wheels track parallel to each other and have so much grip that the back of the vehicle drags diagonally behind them.
ETA I've seen the opposite problem, where the two front wheels track parallel to each other and have so much grip that the back of the vehicle drags diagonally behind them.
Good advice above, there could also be a bit of effect from the fact that with an LSD, the front grips better highlighting a reduction in rear grip, so could be a bit of both effects.
Not 100% sure, but makes sense to me.
I had a similar effect in (yes I know not the same) a previous Mini R53 track car when I adjusted the front camber for better turn in & went a bit too far & made it very easy to drift into oversteer.
Not 100% sure, but makes sense to me.
I had a similar effect in (yes I know not the same) a previous Mini R53 track car when I adjusted the front camber for better turn in & went a bit too far & made it very easy to drift into oversteer.
Based on an old Autocar article, here's my amateur thoughts on suspension set up:
In a classic understeer-is-safer supermini they make the rear suspension soft and don't fit a rear ARB so that less of the roll forces on the rear body are translated to the rear wheels and therefore there's more weight on the inside rear wheel and so it grips better.
I presume by moving the drive to the front of your car you've moved weight from the rear to the front? Perhaps this now means that the rear suspension and anti-roll bars are too stiff for the remaining weight, meaning that any roll forces unload the inside rear wheel. Also, with the extra weight on the front, the suspension there is now too soft, which makes it grip better.
I'm not a mechanical engineer and haven't done any proper analysis on this, so could well be wrong, but this is what occurs to me.
In a classic understeer-is-safer supermini they make the rear suspension soft and don't fit a rear ARB so that less of the roll forces on the rear body are translated to the rear wheels and therefore there's more weight on the inside rear wheel and so it grips better.
I presume by moving the drive to the front of your car you've moved weight from the rear to the front? Perhaps this now means that the rear suspension and anti-roll bars are too stiff for the remaining weight, meaning that any roll forces unload the inside rear wheel. Also, with the extra weight on the front, the suspension there is now too soft, which makes it grip better.
I'm not a mechanical engineer and haven't done any proper analysis on this, so could well be wrong, but this is what occurs to me.
Also the likes of Lotus and McLaren (perhaps the ex-Lotus people who moved there?) have never liked LSDs. Here's Nick Adam's from Lotus on LSDs on the Elise:
"In this type of competition they do not tend to run high speed (100mph +) corners and therefore the increase in understeer on this type of corner which you get with an LSD is of little negative consequence to them and they therefore are better off with an LSD.
In our experience an Elise or Exige equipped with an LSD is at a disadvantage to one without an LSD on a typical European race track. On top of that the LSD bluntens the steering feel and repsonse of the car which we don't like.
If you want an LSD then by all means fit one, but please understand that there are negative as well as positive effects. In the instance you describe, instead of spinning the inside wheel as you accelerate away at full throttle (which can be easily fixed by modulating the throttle! Very Happy ) a car with a 2:1 LSD like the obne we supply will provide twice the torque to the outside wheel as it does to the inside one. This will increase the slip angle of the outside wheel and the car will tend to oversteer heavily on corner exit, requiring a reduction in throttle if you are not to spin.... It isn't much quicker, honest."
http://sandsmuseum.com/cars/elise/information/misc...
"In this type of competition they do not tend to run high speed (100mph +) corners and therefore the increase in understeer on this type of corner which you get with an LSD is of little negative consequence to them and they therefore are better off with an LSD.
In our experience an Elise or Exige equipped with an LSD is at a disadvantage to one without an LSD on a typical European race track. On top of that the LSD bluntens the steering feel and repsonse of the car which we don't like.
If you want an LSD then by all means fit one, but please understand that there are negative as well as positive effects. In the instance you describe, instead of spinning the inside wheel as you accelerate away at full throttle (which can be easily fixed by modulating the throttle! Very Happy ) a car with a 2:1 LSD like the obne we supply will provide twice the torque to the outside wheel as it does to the inside one. This will increase the slip angle of the outside wheel and the car will tend to oversteer heavily on corner exit, requiring a reduction in throttle if you are not to spin.... It isn't much quicker, honest."
http://sandsmuseum.com/cars/elise/information/misc...
Edited by trevalvole on Friday 17th May 09:50
trevalvole said:
If you want an LSD then by all means fit one, but please understand that there are negative as well as positive effects. In the instance you describe, instead of spinning the inside wheel as you accelerate away at full throttle (which can be easily fixed by modulating the throttle! Very Happy ) a car with a 2:1 LSD like the obne we supply will provide twice the torque to the outside wheel as it does to the inside one. This will increase the slip angle of the outside wheel and the car will tend to oversteer heavily on corner exit, requiring a reduction in throttle if you are not to spin.... It isn't much quicker, honest."
I guess it might depend how much power you have relative to the available grip, but as an enthusiastic RWD driver I can't imagine why you would prefer a car that does a 'one wheel peel' out of slow corners over one that spins both wheels. It is a miserable and frustrating experience to be in a car that can't decide whether it is going to understeer or oversteer through a corner depending on transient weight transfer, and won't put the d*mn power down out of a corner.But I imagine it's preferable for a mass market road car to spin the wheel rather than frighten an inexperienced driver.
GreenV8S said:
trevalvole said:
If you want an LSD then by all means fit one, but please understand that there are negative as well as positive effects. In the instance you describe, instead of spinning the inside wheel as you accelerate away at full throttle (which can be easily fixed by modulating the throttle! Very Happy ) a car with a 2:1 LSD like the obne we supply will provide twice the torque to the outside wheel as it does to the inside one. This will increase the slip angle of the outside wheel and the car will tend to oversteer heavily on corner exit, requiring a reduction in throttle if you are not to spin.... It isn't much quicker, honest."
I guess it might depend how much power you have relative to the available grip, but as an enthusiastic RWD driver I can't imagine why you would prefer a car that does a 'one wheel peel' out of slow corners over one that spins both wheels. It is a miserable and frustrating experience to be in a car that can't decide whether it is going to understeer or oversteer through a corner depending on transient weight transfer, and won't put the d*mn power down out of a corner.But I imagine it's preferable for a mass market road car to spin the wheel rather than frighten an inexperienced driver.
trevalvole said:
Very weird article.It starts with stats saying wheels are 1" wider at the front, which is obviously untrue when the rears were wider. Then it says one is 1.5" wider, when their stats clearly list wheel widths 1" different.
As for LSD comments, when there are so many variety' of LSD's available, their statement seems very generic, and perhaps more aimed at plated type LSD's ?
Certainly less likely for a VC type, and almost certainly not describing a ATB type.
Funny things LSDs, had one on our then new Hilux in 2007, standard fitting at the time on the invincible spec.
In normal 2wd, RWD obviously, it would flick the back end out far too easily on damp corners, replaced the OE tyres with something much better which cured the problem.
Wasn't just us, we had intended to buy the demo but it was written off on a roundabout by a salesman.
In normal 2wd, RWD obviously, it would flick the back end out far too easily on damp corners, replaced the OE tyres with something much better which cured the problem.
Wasn't just us, we had intended to buy the demo but it was written off on a roundabout by a salesman.
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