Cornering...

Topics relating to Advanced Driving in cars
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StressedDave
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Re: Cornering...

Postby StressedDave » Wed Oct 21, 2015 7:20 am

Well, track has nowt to do with turn in. Having massive fronts does. Rear steer is being used to stabilise things - the idea being that putting a lot of steer in gives good turn-in without being a bit dodgy in terms of response. There a lot of second order stuff around handling and sharp inputs. The 'I don't like calculus, me' version is that all cars have understeer. All understeer cars, when you put a sharp input in, give you a higher response than the steady state. In other words, you get more than you need/want before it all settles down (If you think of fishtailing, you're not far out). If that over response takes you over the limit of the tyres then you're potentially Donald Ducked, or at least having to deal with a lot of things you didn't want to within the narrow confines of a road.
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Gareth
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Re: Cornering...

Postby Gareth » Wed Oct 21, 2015 8:34 am

Since everyone else has has a go, I thought I'd poke my nose in as well ...
TheInsanity1234 wrote:If the road curves to the left, I turn the steering wheel in the appropriate direction to get the car to follow the road.

When you turn the steering wheel for a bend, do you ever get to the point where the car is turning more than you need and you have to 'unsteer' a bit?

If so, then you're probably starting to steer later than you need, or too quickly, or both.

Whether or not you are under power when you start to steer also has an effect on the amount you need to turn the steering wheel.
there is only the road, nothing but the road ...

waremark
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Re: Cornering...

Postby waremark » Wed Oct 21, 2015 2:03 pm

IcedKiwi wrote:I just watched the Chris Harris video of GT3RS vs AM GT12 vs 650s and I don't think he mentioned the rear wheel steer (although it does have it) but said the "turn in" was greater than the others due to the sticky massive profile fronts and wider track. Although as he's a journalist I'm sure SD will confirm he's got it all wrong.

I enjoyed that video and posted my own thoughts underneath. Summary, 650s absolutely brilliant, but less performance and general capability may equal more driving enjoyment.

TheInsanity1234
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Re: Cornering...

Postby TheInsanity1234 » Thu Oct 22, 2015 4:12 pm

Strangely Brown wrote:That's exactly right. You say that you turn the wheel. Is that all you do? How hard do you turn it? How far, how quickly? How do you know when to stop?

No. How hard I turn the wheel depends on how much resistance there is to the steering action, however, 99% of the time, the power-assisted steering is good enough that I don't need much more than how much force my fingertips exert.
How far I turn the wheel depends on how sharp the corner is.
How quickly, depends on how fast I'm going. If I'm going rather slowly (think: Supermarket car park), then fairly quick steering movements won't be any issue, but the faster I'm going, the more gradual my steering actions are. Doing 50 and turning the wheel through 1/3rd of a turn suddenly is probably enough to introduce understeer.
How do I know when to stop? Um, well, when the car is going around the bend without deviating from my intended line, that's probably a good clue that you've got the right amount of 'turn' in the wheel.

Strangely Brown wrote:What happens when you turn the wheel? What do you feel?

Well, the car turns in the direction that I move the wheel in. The more I've turned the wheel, the bigger the turning force acting on the car, and the tighter the line is when I'm turning, however, how fast I'm going, and how much power I'm applying also has an effect, i.e, the faster I am, the shallower my line through a bend is for a fixed degree of steering rotation, although you have to be going fairly quickly to see any noticeable difference.
Hopefully, if I've judged the corner well, I feel nothing. The car turns, and I turn along with it, but I shouldn't feel any noticeable force from my seat pushing me sideways.

Strangely Brown wrote:Please excuse me if I speak improperly here, but does your deafness affect your balance? Does that have any affect on what you feel when cornering?

No, my deafness doesn't affect my balance, luckily, just my hearing :P
Well I can't tell you whether my deafness has an effect on what I feel when cornering, because I have no clue what I should be feeling when cornering. All I can say is if I'm going too fast for the bend, I can feel it.

Strangely Brown wrote:What you said was simple in the extreme but if you think about what you actually do and what you feel when doing it, you'll see that it's really not just as simple as, "I turn the wheel". And that brings us back to the point of the thread. Cornering is far more nuanced than arrive, turn, drive.

Hope that is a little clearer?

I'm aware that turning isn't as simple as 2 steps, but I can't be bothered to type out long winded posts analysing every single detail of the turning process :mrgreen:

Your questions have had me thinking, and one or two did stump me, so no loss in that whatsoever. I'll pay a little more attention to what I do when I steer in the future and see what changing things might do :)

Gareth wrote:Since everyone else has has a go, I thought I'd poke my nose in as well ...
TheInsanity1234 wrote:If the road curves to the left, I turn the steering wheel in the appropriate direction to get the car to follow the road.

When you turn the steering wheel for a bend, do you ever get to the point where the car is turning more than you need and you have to 'unsteer' a bit?

I can't say I'm conscious of any occasion where I've done that. I do occasionally under-steer slightly as in, I don't turn the wheel quite enough and I've got to turn it a tad more to keep myself from hitting a car encroaching on my side of the road. Don't recall ever having to reduce my steering in order to not turn in too quickly. Although based on probabilities, I will have done that once or twice without thinking.

Gareth wrote:If so, then you're probably starting to steer later than you need, or too quickly, or both.

Whether or not you are under power when you start to steer also has an effect on the amount you need to turn the steering wheel.

Thanks for that, I'll keep that in mind and try to spot any cases where I steer a bit too much.

Indeed, thanks to my Physics, I'm aware of the basic concepts of how a vehicle behaves when you do different things with them.

Astraist
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Re: Cornering...

Postby Astraist » Sun Oct 25, 2015 1:05 pm

TheInsanity1234 wrote:How quickly, depends on how fast I'm going. If I'm going rather slowly (think: Supermarket car park), then fairly quick steering movements won't be any issue, but the faster I'm going, the more gradual my steering actions are. Doing 50 and turning the wheel through 1/3rd of a turn suddenly is probably enough to introduce understeer.
How do I know when to stop? Um, well, when the car is going around the bend without deviating from my intended line, that's probably a good clue that you've got the right amount of 'turn' in the wheel.
ell, the car turns in the direction that I move the wheel in.


True, but having experimented more with how early and slowly I can turn the wheel, I find that even for U-Turns I average quite a slow rate of turning. I do sometimes choose to steer later than strictly necessary, in order to get a better view, but generally I steer quite slowly all around.

To be pedantic, you understeer to a degree at all times, even at slow-speed maneuvers. You always have the wheel turned slightly more than absolutly necessary because the front slip angle is greater than the rear, so the car doesn't really go where the wheel is pointed. What you call understeer is the point where the desired radius cannot be maintained in the prescribed speed regardless of steering angle.

As for getting on the right line, there isn't one fixed steering angle to get there. It's ideal to use the least steering, and not to have it on for too long as well. Turning the wheel slowly is thus clever not only from a vehicle's dynamics point of view, but simply so you can stop in time. If you steer quickly you will just go over the point of minimum required steering without realising it.

TheInsanity1234 wrote:Hopefully, if I've judged the corner well, I feel nothing. The car turns, and I turn along with it, but I shouldn't feel any noticeable force from my seat pushing me sideways.[...]Well I can't tell you whether my deafness has an effect on what I feel when cornering, because I have no clue what I should be feeling when cornering. All I can say is if I'm going too fast for the bend, I can feel it.


The sensitivity to the actual force pushing sideways is small. We can feel changes in the rate that the force ramps up, and we also feel roll (which also adds, albeit little, to the force pushing sideways), yaw gain and particularly - aligning torque at the steering wheel.

G-force and yaw gain we feel in part in our middle ear, but as far as I know, your deafness shouldn't affect it's function.

IcedKiwi
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Re: Cornering...

Postby IcedKiwi » Wed Oct 28, 2015 8:44 am

On the topic of fancy technology like rear-wheel steering, I'm just reading the October issue of Automotive Engineer which talks about the new AWD system in the Focus RS
GKN wrote:The new gear rations deliver more torque to the rear wheels than the previous system, and when AWD is engaged, the technology drives the rear wheels faster than the front ones. This overspeeding of the rear changes the way the vehicle handles, and on bends makes it turn more sharply as it quickly responds to the inputs provided by the driver

I'm guessing by introducing extra longitudinal slip some helps speed up the rate at which the lateral slip can be generated (sliding tyre has less friction that a rolling one?) to get the rear slip angle and therefore speeding up the reaction time between the driver making an input and the car actually turning.

I wonder if it's a controllable thing or on all the time, I can imagine increased tyre wear rates if the rears are spinning faster. Isn't that why you should always replace 4 tyres on an AWD vehicle because if you replaced just 2, the news ones (with a larger radius) will be constantly slipping and wear down until they're the same diameter of the old ones and the speed differential has gone? Or is that only on fully locked systems and the viscous/clutched AWD/4WD systems take up the speed difference instead of the tyres?

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StressedDave
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Re: Cornering...

Postby StressedDave » Wed Oct 28, 2015 9:46 am

Nice to see that AE can accurately copy and paste a GKN press release... :lol:

Looking at the nice photo and the full description, the system will overspeed the rears relative to the front, but I guess the overspeeding is minimal and in the majority of cases it's zero due to the fact that the rears are disconnected at speeds over 22mph anyway.

The bit that actually makes it handle differently is not the overspeeding but torque vectoring. If you can put a 'turning force' into the equation you can change the cornering forces at the rear - you can use them for stability or de-stability. Takes me back 20+ years ago when we had the same thing working a little more elegantly... If you're bored look up double differentials on t'Net.
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