Trailing Throttle Oversteer

[TCS4605R]

I'm in my 23rd year of restoration - may last Europa driving experience ended up in the woods at the top of an earthen embankment.  Going at a goodly rate, I backed off in the middle of a curve and the rear end started to come around.  After a couple of useless corrections, I ended up in the woods with a frame shaped like a banana, no chin spoiler and a lot of fiberglass damage.  Classic brain fade and 'trailing throttle over steer'.  I have experienced this before (brain fade and trailing throttle over steer, but no necessarily both at the same time), but not with these consequences.  I have Spax adjustable perch shocks front and rear, don't remember the spring rates I'm using - non-stock springs.  All of the suspension settings were per the Lotus Workshop Manual - also have adjustable rear lower links and adjustable Spyder wishbones on the front with spherical bearing pivots - no rubber in the front suspension except for the sway bar mounts.

I'm familiar with the concept of trailing throttle over steer (obviously), but don't know how to make adjustments (other than slowing down) to minimize it.  My tires were Yokohama 008R's, the weather was warm, tires were warm.  I've often wondered if the rubber mounts at the rear of the gearbox and the two rubber engine mounts deflect enough under rapidly applied deceleration to move the engine/transaxle forward and change the toe-in at the rear to toe-out and cause the rear to steer itself.  The weight bias on the Europa is to the rear which would tend to make it a bit tail-happy and, other than the trailing arm pivot attachment point at the frame, all other rear suspension attachment points are to the transaxle - a transaxle than can move on its rubber mounts.

Anyone have similar experiences and thoughts on the subject - any thoughts on corrective suspension adjustments?

Tom
74 TCS - 4605R

[jbcollier]

Europas tend to steer neutral to mild understeer when on-throttle (S1/2).  Near the limit, if you back off the throttle mid-corner, it switches to oversteer -- as you so ably demonstrated.  You need to get back on the throttle to bring the car back towards neutral.  How successful you'll be depends on how close you are to the limit and how far the over-steering has developed.  I use this very same technique all the time when fast driving on slippery winter roads.

[TCS4605R]

There was to be some adjustments that can be made to minimize this characteristic.  Anyone have any ideas?  The Europa rear suspension is the only car I've seen that uses the half shafts as a suspension component.

Tom

[EuropaTC]

The Europa rear suspension is the only car I've seen that uses the half shafts as a suspension component.

  

Hi Tom.  I remember saying something very similar once on the Yahoo group and getting lots of feedback, most of which highlighted my lack of knowledge at the time... 

But  I think I know where you're coming from on the design concept and I doubt Lotus would do the same now. In fact I know they wouldn't because in the documentary about the S1 Elise there's a specific comparison of the rear suspensions of the Europa & Elise; basically Rackham said that you wouldn't hang the suspension off the engine/gearbox on a modern road car because it was too difficult to control and you'd need to stiffen every mount to a degree that wouldn't be acceptable for road use/comfort. 

And let's face it, there's a lot of rubber in the Europa's rear suspension and all of which will move under load so you could well be on to something with your initial comments.

But I'll come clean and say I don't know the answer to the question of "what to do ?".  As the basic problem is loss of rear downforce/weight transfer then I think you've got to know the spring rates and ride heights before you can get down to serious detail once you move away from the OEM settings. 

All I can suggest is the obvious checking the UJs & rubber mounts with a lever for movement because any significant wear/movement has got to be bad news I guess.   After that it's geometry settings and if you've seen one of my other posts you'll know I've finally got mine set up about 2mm toe in per (rear) wheel and 0.5deg negative camber. The front is 1mm toe in and feels stable on steering & hard braking. 

Brian

[jbcollier]

Half-shaft as a suspension member?  Corvette (60s to the 80s) and Jaguar (61 to late 80s) come to mind first and then Rover (De Dion: albeit not load carrying on the Rover but wheel locating, yes). There are others, I'm sure.  If you include swing-axles then the list grows exponentially (VW, Porsche -- now those two can have some wicked TTO, MB, etc).

[TCS4605R]

Has anyone done any 'bump-steer' measurements on the rear suspension of a Europa?

Tom

[buzzer]

I'm not sure you can significantly change this characteristic of lift off oversteer. It is a fundamental of rear wheel drive cars, especially when you consider the europa's set up..  I am not an expert by any means but basically if you are on throttle in a corner the rear wheels are doing more work. Resisting the lateral cornering force as well as having to cope with driving the car. Assuming everything else equal you get some oversteer. This all to do with slip angles. Now if you lift off the rear wheels stop driving the car forward but start to slow the car down with engine braking. This will continue to give oversteer. Slip angles again.


Dave

[jbcollier]

It has to do with weight transfer as well -- small forces can have a huge effect at the limit -- my FWD Volvo also swings its tail with a trailing throttle.  It's actually great fun.  The Volvo has all sorts of built in safety and dynamic controls but you can bypass them all by driving super smooth.  The electronics get very involved if one end or the other lets go, but, if you approach the limit and have both ends go at the same time, it doesn't sense any imbalance and you can drift all four wheels in a perfect state of bliss.  Then the wife starts in, mind...

[4129R]

 Practice controlling the car on wet flat grass. You can get to find out the characteristics of its handling at quite low speeds, and it is easier and safer than playing in the snow, or on wet or dry roads.

In 1978, my 1975 TCS Europa understeered safely when I was playing with it on a concrete airfield runway, as I never backed off.

[buzzer]

And something else to think about. The way you interact with the car, if you hold the wheel too tight, the car cannot settle and you end up fighting it. This itself can cause a spin! Believe me it really does. I went on a day high speed driving course in my Westfield. I was spinning out at 55mph at first on a high speed bend. Then eventually after a lot of practice I could do 80mph. A huge different and all down to how I handled the car. It was a real eye opener and I woul recommend such a course. Not like a track day where if you spin you get black flagged, it's designed to find you and your car limits so you spin a lot!

Dave