Mid-corner understeer

[raymondu999]

Sometimes we see on onboard videos where there is a flat out corner, or at least constant speed fast corner, some cars bite well on turn in and can enter the corner wuickly, but even though it was able to carry the speed in, the front washes out through the corner. When I'm on a track day I also feel slightly (though this is just a raw seat of the pants feel) that somehow the front washes out, though only in the slow.

Why though? I don't get it. Why is it there on apex, but not on entry? This is in a fast corner in a situation where braking doesn't come into it. If you're holding constant/being flat on the throttle, wouldn't the use of throttle actuallyinduce slight oversteer, by virtue of the power coming to the wheels and "using up" some of your grip?

[Giblet]

Usually once it starts, it starts to get worse. The temp and grip start to run away as the tires are on the edge anyways, so if on a really quick lap you use that to drift to the outside on exit so you can stay on the power.

I think.

As for your car, is it FWD or RWD?

[marcush.]

I had my proving ground licence renewed the other week and you do quite a bit of driving in those two days.Tell you what the car could over and understeer depending on what you did with it ,ESP,ABS on and off a LOT of this is driver induced .
sure you can blame the track the setup ,a cloud over the track and wrong tyre pressures,but in the end of the day if you fail to read the tyres capability you will either over or understress them and not all of them at the same time to the same degree.Welcome in the arena of variables!
When a car is already at the limit of adhesion at the front feeding more power(increasing speed or maintaining too high speed will lead to more understeer ,only breaking grip at the rear will induce oversteer. Also you don´t want or need oversteer late in the corner as you cannot accelerate (as you lack rear grip already) .So bsically a tiny bit of U/S at the apex is a good thing for early throttle.
The downside is a understeer corner entry is not wanted as you need to slow down too much to be able to turn in .the ideal would be a car that is showing a off throttle corner entry oversteer characteristic allowing for less steering lock and perfect positioning of the car for early acceleration but as soon as you are approaching the area of releasing the power for the next straight it should be on the verge of understeer allowing for maximum acceleration.
But then the quicker the corner the less you ever want to experience oversteer -BUT don´t get me wrong oversteer to me is
exceeding the optimum slip angle at the rear ,understeer is exceeding the optimum slip angle at the front.

[hardingfv32]

Sometimes we see on onboard videos where there is a flat out corner, or at least constant speed fast corner, some cars bite well on turn in and can enter the corner wuickly, but even though it was able to carry the speed in, the front washes out through the corner.

The tires could be overheating as the turn progresses. The car could be setup to put heat into the tires early to improve turn in. The price paid is that the tires continue to gain heat and go down the backside of the performance curve for the rest of the turn.

I have a paper on this if you are interested. PM with email and I will send PDF.

Brian

[marcush.]

tyre shows a characteristic curve of available grip which is not rising all the time but giving a more or less pronounced peak.

the interesting bit is:

you might be either side of the peak -but at exactly the same level of available grip!
But:

staying on the right side off the peak will leave your temps low exploring the other side will net high temps .

So ,in practical terms two cars may have understeer conditions but one is gaining grip as the tyre gets warmer and the other loses ggrip as the tyre gets warmer.
No question that a case where the car is balanced at one point with the front tyres on differnt sides of the maximum available grip this is a case when
:
the car will never ever be able to explore all 4 tyres to the max !
and even worse ,the balöance will degrade very quick on the set of tyres which are running on the hot side.


I think this behaviour is leading teams ,drivers and almost everyone think in terms of temperature when it comes to tyres .The real underlying problem is that tyreperformance cannot be used fully as front and rears are on different sides of the peak.

[Tommy Cookers]

IMO Raymondu's findings are due to any wheel driven car having less weight on the front wheels, more on the back wheels WHEN POWER IS APPLIED, so producing less grip at the front and more at the back, giving mid corner understeer ! (as long as applied power is not too much )

This will happen in fast corners only if he has a lot of power.

[raymondu999]

The temp and grip start to run away as the tires are on the edge anyways, so if on a really quick lap you use that to drift to the outside on exit so you can stay on the power.

The tires could be overheating as the turn progresses. The car could be setup to put heat into the tires early to improve turn in. The price paid is that the tires continue to gain heat and go down the backside of the performance curve for the rest of the turn.

That quick though? I mean; when I turn in, the bite is good - and when I'm turning the wheel it's still fine; but not long (maybe 0.5 to 1 second) after I've stopped *turning* the wheel (the wheel isn't pointing straight; but I'm not moving it) it sort of starts running wide.

As for your car, is it FWD or RWD?

I had it on several cars (not all) - some were 4WD and some were RWD.

Also you don´t want or need oversteer late in the corner as you cannot accelerate (as you lack rear grip already) .So bsically a tiny bit of U/S at the apex is a good thing for early throttle.
[...]
the ideal would be a car that is showing a off throttle corner entry oversteer characteristic allowing for less steering lock and perfect positioning of the car for early acceleration but as soon as you are approaching the area of releasing the power for the next straight it should be on the verge of understeer allowing for maximum acceleration.

I'm not talking of slow corners - again; I'm talking about corners that are flat out. It's a Sepang Turn 5/12 type corner. There is nothing wrong with my turn in - there is no understeer there - it's that when my hands are no longer moving the steering wheel (ie constant lock applied) that it's appearing.

The funny thing is - I tried having a later entry; and cranking more lock on for a weird "late apex" line through the fast corner - and it bit no problem. It's as if it has bite only while the steering wheel is being turned; but not when it's at constant angle (through a fast corner) As such my instinctive thought was that it was something to do with weight transfer.

Take for example Buttons '09 Spain pole - he turns in great to Turns 3 and 9 - but as he's holding constant lock through the apex the nose is washing out. Check out Hamilton's '12 Sepang pole and you'd see the same.

[marcush.]

maybe your washing out is the slip angle? the tyre needs some to generate maximum grip ,so always the trajectory of the tyre will not be the same as the steering angle of the tyre.
admittedly this is a quite shallow angle but still viewed from above it is very visible.

[raymondu999]

Interesting one. Hadn't thought of that. It's possible I guess.

[garygph]

If I remember correctly the shocks will have a big effect on turn in and rate of wheight transfer but once that has happened your roll stiffness will now dictate your wheight transfer. So corner entry is tuned with slow bump/rebound on the shocks and midcorner balance with the roll bars. Just a thought.

[marcush.]

I had many many drivers who interpreted slip angle with oversteer at the rear of the car and only when you took away the uncertainty from them stating don´t worry it´s the slip angle you feel it is necesary to create side force,they were happy and went on with their business.
At the front you can even feel the slip angle in steering force response as the force needed to hold the wheel will diminish when optimum is near .If you don´t know you might interpret this with a washout but that´s only if you still try to push more.
It ´s a fine or very fine line some tyres are offering more feedback then others.I have to admit to feel it but i´m unable to concentrate long enough to keep it exactly there.It´s an art ,at least for my skill level as a driver.

[Jersey Tom]

Before bothering to think about anything overly fancy with temperature... pretty simple,the linear and limit range characteristics of a tire are.two totally different things.

Plus, you can often get power on understeer in power limited corners from the rear load transfer.

[olefud]

Turn in is a transient condition that is rather different from steady state. As has been pointed out, dampers affect turn in but not steady state. You’re right that more force is required for turn in and thus, instinctively, it would seem that steady state should be “easier”. However, during turn in the tires are more evenly loaded which optimizes total grip.

There’s a lot going on. Often the front tires have a bit more steering lock on turn in and may heat a bit. Tires tend to be nonlinear with changing weight and/or slip angles. During steady state turning, RWD throttle increase will often lead to understeer; but may instead lead to oversteer if the additional work asked of the rear tires compromises traction more than the weight transfer enhances it. All wheel drive with 60/40 front torque bias often sees understeer with very modest additional power. Things change even with track surface and small road camber change. There are too many variables to know just what’s happening. It’s better just to make the necessary compromises.

Just my way and oversimplified, but setting the roll stiffness/roll centers to provide the desired steady state performance, particularly optimizing the exit speed is the first priority. Then, if turn in suffers, dampers, Ackerman, even camber curves can be adjusted to gain turn in while maintaining steady state performance.

[Jersey Tom]

Corner entry is transient, but often not that transient. I'd say springs, bars, and nose weight are going to have as much or more an affect than dampers.

I still like my explanation the best Temperature, transients, and all other fanciness aside... what the tire is doing at small steer angles vs large are just two totally different animals.

[Caito]

Corner entry is transient, but often not that transient. I'd say springs, bars, and nose weight are going to have as much or more an affect than dampers.

I still like my explanation the best Temperature, transients, and all other fanciness aside... what the tire is doing at small steer angles vs large are just two totally different animals.

Shouldn't you care more about slip angle rather than steer angle?

In a really, really fast corner you could need little steering input to generate maximum slip angle, whereas in a slow corner you need to turn the steering a lot to reach the maximum slip angle.

Of course steer angle also affects diagonal weight transfer, that's why I ask you.