Contour Enthusiasts Group Archives Forums Modifications Suspension Drop throttle oversteer

Holy smokes!

Now I *know* drop throttle oversteer because of my Lotus-tuned Isuzu Impulse (tail-happy front driver), but today I was taking a corner pretty aggressively (taken it aggressively many times), it was sunny, dry and I had a passenger and some luggage in the trunk.

We ripped around this sweeping corner (I got off the throttle as the corner began..ok, not textbook) and MAN DID THE TAIL EVER COME AROUND! Got back on the throttle and countersteered to pull out but that got me thinking...will the SVT do this as the struts age/sway bar bushings become old???

I thought the E1's were tuned to reduce/eliminate the drop throttle oversteer (which is fun if you're expecting it...I don't expect it from the Contour). I've never been able to induce it before, even on the track. Anyone else have a tail-happy SVT E1?

(I checked tire pressures...they're 36 psi all around. Tires are half worn OEM g-Force KDWs). Stock ride height, no suspension mods of any sort.

It also seems less likely that I should get oversteer with luggage in the trunk and a hitch (weight) hanging off the rear of the car.

I'm gonna have to tear down that road some more and see what's up with that.

Kerry

I believe lift throttle oversteer applies to rear wheel drive cars (correct me if im wrong). On a RWD car, when you lift the throttle when accelerating into a turn, you transfer the weight to the front wheels (losing traction in the rear) causing a loss of control of the car and therefore continuing to veer the direction in which you were turning. On a FWD car, when you lift the throttle the weight is distributed again to the front wheels (which your steering controls) and therefore more control is gained of the car's action.

Not sure what would have caused what you experienced unless my previous explanations are all wrong...

Well, I've had the tail end around many times (without the ebrake of course) but it usually involved trail-braking into the corner.. Deep into the brakes with the front suspension fully loaded you ease off the brakes during turn-in.. weight gets transferred back onto the rear wheels, possibly overwhelming their traction and bringing the back-end around. Combine that with a hint of counter-steering just before turning in to get a "swing" into the rear end and you'll be going around rally style at any intersection.

On my car, dropping the throttle mid-corner simply overloads the front suspension and causes the setup to push even more.. Blech..

Maybe that specific curve has a camber change or a downward slope to it? That could explain a lot of things!

I don't think Antiramie or Pascal have it quite right. Oversteer is a matter of losing lateral traction in the rear tires, while understeer is losing lateral traction in the front tires. Any negative accelaration while turning will transfer weight off the rear tires, which in turn reduces the amount of traction. This will only result in oversteer if the front tires maintain traction. If both sets of tires lose traction, then you get the fun of the 4 wheel drift!

This would be right in a leisurly driving situation, but under "performance" conditions, traction is the limiting factor. With 100% of available traction used up as cornering power under part throttle, transferring weight to the front will overwhelm the rubber's ability to turn the car.. resulting in understeer.

Consequently, taking weight off the rear tires eases their work.. making damn sure they won't slip.

A RWD car will swing the tail under those conditions not because of the weight transfer, but because of the braking force that engine compression is applying to the rear wheels alone.. essentially the same thing as putting in a click or two on the ebrake!

Dips in the road, uneven steering input, camber changes along the corner, uneven throttle input are all things that can upset the theoretical behaviour of a car tough. A conveniently located gravel patch, different asphalt texture or a pool of engine oil also make a huge difference.. turning a plowing behemoth into a tail-wagging demon under high cornering load...

Pascal,

go back and read AndrewR's post, a good bit of what you just posted is wrong.

Rara,

Thanks for pointing it out, I hadn't even seen AndrewR's post in between mine.

I agree on what Andrew says, but I'm not sure where it means I'm wrong?

I was talking about a performance situation, where you can expect the front tires to already be using 100% of available traction. At that point, I agree that negative acceleration transfers more weight on the front tires.. but for sure that would also send them over the edge, inducing that 4 wheel drift. Not a condition I would describe as oversteer!

In a more real-world situation, where there is still some traction left at the front, I agree on having the rear swing out... I guess I simply never experimented that behaviour with a FWD car.. mostly pushing under drop throttle.

I'm just re-running my thinking here, could anyone point out where I messed up in the explanation??

When trail braking into the corner, the lack of weight over the rear tires will cause the oversteer. As you ease off the brake and onto the accelerator you will provide better lateral grip for all four tires.

I'm sorry Pascal, my statement "wrong" came out much harsher than intended.

You are close, but in a few key areas, I believe you are a bit off. Give me a minute, and I will try to explain.

Antiramie's statement here is correct, though maybe not quite complete.

I think something that could help you here, Pascal, is the friction circle concept, which basically reminds us that a tire has grip (traction) in more than one direction, and can be visualised with a circle with a sort of crosshairs through it with each direction representing the tire's grip in acceleration, deceleration, and either way of lateral accel. (I really wish I had a pic handy) Though this really isn't my key point, lol It's the weight transfer overwhelming the tires grip, but I'll get there in a second.

This is the big issue, this statement is backwards. Friction force = the coeff. of friction times the Normal force pressing the two surfaces together. How do you grip better then? Well, you can increase the coeff. of friction by putting on some race tires, or you can increase the normal force pressing the tire to the road. The only way to increase the normal force, is to increase the weight being held up by that wheel, so by transferring away weight from the rear wheels, you decrease thier overall capability to grip, and transfer that extra grip to the front (because that's where the weight transfers to) so when the weight transfers forward, the grip available at the front actually increases.

Gotcha.

Thanks for taking the time to explain guys. I know exactly about that friction circle concept, it completely slipped my mind though..

Got ya on the friction issue improving with weight too. That's exactly what I play with (front/rear damping rates) to affect weight transfer and cornering attitude. My brain's not connecting everything these days.. :rolleyes: thanks again.