Lift-off understeer

[5star]

What may cause a car to understeer (yes understeer, not oversteer which is the more common result) when lifting off the throttle mid corner?

I'm theorising certain types of LSD may lock on trailing throttle which would push the car forward.

What else could cause this?

[trashbat]

I might be missing the point here, but anyway... You say it's more common to oversteer but surely it's strongly affected by FWD vs RWD?

One of the things I learnt in AD, in my FWD car, is to steer around bends using a balanced throttle. Within reason, more throttle, more steering, so suddenly remove that and you will understeer off, no?

[jcochrane]

My understanding of lift off oversteer mid corner is that this causes a weight shift from rear to front tyres. resulting in a decrease in downward pressure at the rear which in turn causes a decrease in the lateral force they generate so that their lateral acceleration into the corner is also decreased, As a result the car turns in more sharply. This may cause the rear tyres to loose adhesion which will result in an exiting of the road backwards into the shrubbery. I understand that this is also the reason why it is generally advised to put your least worn tyres on the rear. Whether the car is front or rear wheel drive will result in the same outcome.

The expert on this, StressedDave, may be along later to tell me this is a load of b------s and explain what is really going on and I would bow to his expertise.

https://www.driversdomainuk.com/driver- ... road-rage/

[5star]

...You say it's more common to oversteer...

In most cars, if you are balanced in a corner and lift-off the throttle, load transfer to the front will increase grip* at the front, reduce grip* at the rear, and cause the car to tuck-in. This is oversteer. If you're really aggressive with this, on some cars you can lose traction at the rear.

What i'm asking is, in cars that do the opposite and take a wider line when you lift-off the throttle, what could be the cause of this? Whatever is happening has to be having a greater effect than the load transfer that should be causing oversteer rather than understeer.

I'm not asking about how a corner should be taken, etc., but rather the physics behind the above scenario.


*I use the word "grip" for want of a better word. In technical terms it is probably something to do with increased load on the tyre affecting coefficients of friction and slip angle and stuff, rather than "grip" being affected, but I know what I mean .

[5star]

The expert on this, StressedDave, may be along later to tell me this is a load of b------s and explain what is really going on and I would bow to his expertise.

OK, waiting for StressedDave.

[5star]

...

Thanks. I'm working through this.

In the meantime, can you explain what is happening in this scenario? It might be unrelated to the mid-corner understeer question I first had.

I'm travelling at a perfectly constant speed on maintenance throttle, in a straight line. I do a double lane change manoeuvre (without altering throttle) and the front tyres grip without issue.

I then try exactly the same, at the same speed, but this time I'm on trailing throttle, and the car is slowing slightly with more weight over the front tyres as a result. I do a similar double lane change manoeuvre (still on trailing throttle) with same amount and rate of steering input, but this time the car understeers.

What is causing understeer on trailing throttle, vs sharper response on maintenance throttle?

Before I tried this, I would have through additional weight on the front would have provided additional grip on the front and less likelihood of understeer.

[Astraist]

It provides more grip, but less cornering stiffness so you need more steering.

"A sharper response", mind you, is not the same as cornering stiffness because the less cornering stiffness the front has (i.e. the more understeer), the sharper is the transient response.

[5star]

Note, by sharper response I simply mean no understeer.

[TripleS]

Wow; I'm generally happy (justified or not) if I feel the old 406 has sufficient grip to get me round the curly bits reliably, and without showing undue signs of discomfort. Almost invaraibly that's what happens, but being of a nervous disposition I think I'm going to settle for that, even if it is terribly boring.

[5star]

So with force developed at the front only you get more rotation, so effectively you get more broadside. Remember the whole thing about body slip angle in the first post? Yep, more broadside, more body slip angle, more not going round corner.

Am I understanding correctly:

When changing lanes on a motorway, the car doesn't so much change direction (e.g. turn right then turn left), but rather keeps pointing forwards and moves over to the next lane with no perceptible body slip angle. I can see I'm unlikely to "understeer" in this situation.

At the opposite end of the scale, e.g. when turning into a side street, the car changes direction with a very large body slip angle. I can see I'm very likely to experience "understeer" in this situation.

So in the scenario I'm describing, changing from constant speed to trailing throttle is enough additional loading on the tyre to change from a no-body-slip-move-over manoeuvre towards a body-slip-change-direction manoeuvre? And this is why I'm getting "understeer", with all else being constant?

Note, by sharper response I simply mean no understeer.

Ain't no such thing... All cars under steer, no ifs, no buts, no maybes.

From a physics perspective you're correct, so I've put understeer above in quotes. By "understeer" I mean unwanted/unexpected amounts of understeer.

[5star]

While I work through that, I have a third scenario:

Factors:
A. A moderately slow 90 degree corner.
B. Due to the shape of the corner and entry speed, there is no possibility to generate enough rear slip angle to make it a balanced corner.
C. Sporty rear wheel drive. (I never get the problem I'm about to describe in a front wheel drive car or "comfort" RWD car).

When approaching at a constant speed, I find I have very little confidence in the car's grip level - and I end up taking the corner much slower than is possible. I'm not sure exactly where my confidence feedback is coming from, possibly through the steering wheel or seat.

When approaching with a braking zone, I initially plan my brake pressure and braking points visually, but then as I reduce brake pressure to get back on the throttle prior to turn-in, i get the same lack-of-confidence-feeling (all ability to judge available grip just seems to vanish!) and as a result I end up extending the braking towards the corner, or feathering the brake into the corner (not by plan, but because I'm concerned about lack of grip!)

However, if I trail brake using moderate brake pressure (nothing excessive, but definitely braking with intent so there is some forward load transfer) past the turn-in point, the car feels very "planted" and provides significant confidence that there is ample grip.

So to recap, when approaching certain corners in a sporty RWD car on maintenance throttle, trailing throttle or feathering the brakes, my seat-of-the-pants-gripometer has very low confidence and I want to turn-in much slower. But when I'm approaching the turn-in point under moderate braking with the front loaded, I have good grip confidence and am happy to turn-in much faster (because I can feel the grip is there).

What is going on here, and Is it related to my scenarios 1 and 2 in previous posts? Is grip actually lower, or am I just reacting to my inability to ascertain seat-of-the-pants grip levels?

If the latter, is there a better way to deal with this than trail braking?

And to clarify, this is never a problem on corners where I can set the car up to be nicely balanced, e.g. fast flowing corners where I can use the throttle to balance, or corners with enough space to take a line that allows me to balance front/rear cornering force.

[Astraist]

And can you attribute your increased confidence with trailbraking to increased grip at the front, or rather it's simply the loss of speed during trail braking that is putting you at a speed with which you feel confident around the apex?

What if you entered the bend at the speed you would wish to hold at the apex with confidence or, better yet, enter the bend a bit slower so you have the confidence to gently accelerate up to the apex?

With the issue of cornering stiffness at the front and the suspension geometry Dave referred to (Dave, I assume you talk about something of the ilk of compliance steer?), gentle acceleration can require less steering.

If the bend is short enough (which bends fitting your description usually are) you will not gain much speed up to the exit, where you probably wish to accelerate anyway.

[5star]

I'll try and explain scenario 3 a bit more clearly.

Let's say I'm in a procession of cars and I'm maintaing a 2 second gap to the non-descrip car/van/minibus in front. The procession is travelling at an average of 45mph, typical twisty A road speed.

Let's assume the road is open and the limit point isn't a factor.

In certain RWD sports cars, for corners of the type I described, I find I'm needing to slow much more than other non-descrip road users. I then need to accelerate after the bend to catch up with the car in front. I'm sure this must be annoying for cars behind that wonder why I'm taking each corner so slowly. And the reason I do this is I have no confidence that the car has sufficient grip to take the corner at the speed everyone else is driving at. The car just feels vague and inspires no confidence.

In faster bends I have no problem, and will happily keep the rear end loose and steer the car on the throttle. It is just the particular type of corner I describe that is causing issues.

So, to deal with the issue, rather than keep slowing down much more than other cars for these types of corners, I've been trail braking which gives me similar grip feedback and confidence that I get in non-descrip FWD cars and allows me to maintain a similar (safe) pace as others.

[5star]

B. Due to the shape of the corner and entry speed, there is no possibility to generate enough rear slip angle to make it a balanced corner.

Right... Part B. Do you mean that the corner is of short enough duration that the car doesn't reach a steady state? If not then you're talking male primary reproductive glands. If there wasn't sufficient rear slip angle you'd be spinning uncontrollably.

What I mean is, I'm unable to achieve this in the corner:

the ability to properly balance a car so that you can actually change its line by lifting off a bit or applying more power

Where I can achieve balance, I have no issue. Where I can't achieve balance, I struggle with entry speed due to lack of grip feedback (unless I trail brake) to the point that I'm slower than other cars on the road.

Hope this is clearer now!

[5star]

Whereabouts are you based, and what's the car causing these existential problems?

Manchester.

Cars that I have this problem with:
Jaguar F-Type V8S
Nissan GT-R
Porsche Panamera V6 Diesel
Porsche 911 C2S

Cars that don't give me any such problem:
Jaguar F-Type V6S
BMW i8
Porsche Panamera Hybrid
Porsche 911 C4S
Porsche Cayman

If you've ever driven a C2S back to back with a C4S, the difference in steering feel is what I'm referring to - and for some reason I'm translating that to lack of grip.