It's all about mu

[wbradley]

Peameal bacon

[mrceltic]

Rachel McAdams

[Kazuya]

I am proud to have justin bieber.

[Dave-ROR]

Originally Posted by Kazuya

I am proud to have justin bieber.

More than Alanis Morissette?

[DM7]

Driving beyond the limits of a friction circle and generating over/understeer are both slower than driving on the limit of a friction circle.

The manner in which a car transitions from grip to over/understeer (slide) is very important as well. A car that gives early warning to a slide by screeching the tires will let you learn the limits of the friction circle. In terms of feel it's important for a car to transition into a slide progressively and slowly giving you time to correct.

I really see the 86 as a learners car. The grip limit is low enough to allow you to exceed them at low speeds but at the same time the handling is progressive enough to allow you to correct or hold the car in a slide.

Exceeding the limits of the car will teach you to drive up to the limit meaning maximizing the grip available. Having low grip limits lets you explore this feeling at low, less risky speeds.

[Kevstra]

Originally Posted by wbradley

So we now have several expert opinions, including two from engineers refuting Suberman's claim than Mu is constant. I wonder with whom the judge will rule?

Suberman I suggest you refrain from soliciting your professional services on this forum as that would be tantamount to ecoli offering catering services. LOL

Make that 3 engineers.

[stugray]

Originally Posted by DM7

Having low grip limits lets you explore this feeling at low, less risky speeds.

I have actually tried to make this argument for new drivers learning to drive on the track.
I say that for a new driver, just learning to race, on a real track, with a new car, that it would be better to start with street tires rather than going straight to sticky track tires.

My opinion is beat down by an experienced race driver that "you should just go straight to the full race tires".

I still dont understand how it can be a bad idea to start with the less grippy tires to learn how the car reacts and how to recover.

It seems that learning how the car responds with race tires FIRST leaves less room for errors and when it cuts loose you will have much less time to react.

Dont some of the pro racing schools teach the drivers on a wet track just for this reason?

[wparsons]

And that's why not all good racers make good instructors

The last HDPE I was at the instructors said the best upgrade anyone can make for their time attack series is the best tires you can afford, but for learning you're best with something with much lower limits.

One of the instructors even brought his gf in a rental car which worked out great because the limits were so low she could easily play around at the limit without being afraid of the speed.

[Suberman]

So for those of you who actually drew friction circles, two are enough because only half the circle affects each side of the car at any given time, you should now solve for vectors. Anybody can do this because the diagrams don't need units to be understood.

The front tire circle in a steady state corner can have cornering force described by a straight line drawn at right angles from the centre until it intersects with the circumference. The length of this line is the maximum side force available for cornering. Incidentally, the reason differences in actual mu varying with load are not important is because those occur at points within the circle. All that matters is to understand that useable static mu peaks at peak cornering force (or force in any direction as it happens, as Ford proved in the 60's from research on this point).

Then draw the same line in the circle for the rear tire. Same side force (this has to be assumed for practical reasons. In fact, the circles aren't circles and they aren't all the same size but for road cars they might as well be, the changing size of the friction circle with weight transfer isn't actually relevant for these purposes).

Now draw a line at 90 degrees towards the bottom of the circle. It doesn't matter how long it is but the illustration is more meaningful if it either goes right out to the circumference or stops a short distance.

Now "solve for the vector". Connect the two lines with two intersecting lines meeting at 90 degrees to form a box. The diagonal of that box is the resulting vector. The two forces are treated as resolved into one composite force in the direction of the diagonal with a force proportional to the lengths of the lines creating it. That's your g line.

You will note that the diagonal always ends up outside the circle. You can't drive there. Well, not on dry pavement at least.

The only way you can drive that tire forwards (or brake it for that matter) is to reduce the length of the sideways line to end inside the circle. Then the resultant diagonal can remain on the circumference of possible performance.

Mu is what creates slip angle which in turn is what generates that sideways line. But on the drive axle it is also must simultaneously create slip in a different direction, creep is more accurate. The resultant force cannot create a longer line than will run from the centre of the circle to its circumference. That radius line is a constant (well, we know it isn't perfectly but conceptually it is and for road cars it is so close to a circle the concept is useful and generally true).

So here's the thing, because mu x load equals horizontal friction force, mu is independent of direction in the horizontal plane. Less grip is available to create slip angle because some is being used to drive the car forwards, distorting the tire in the direction of travel and not trying to twist the tire. Ergo, this means that always the drive tires run at lower slip angles than they could if they weren't driving. For rwd cars this means understeer. For fwd cars this means understeer. For awd cars this means understeer. Unless you want to change the definition of oversteer which is of course done in the drifting community (the synchronized swimming of motor sports and about as interesting) and the journalists who talk cavalierly about a "dab of oppo" to cure "oversteer" when they actually mean to fix a clumsy slide which may look interesting to some but is the sign of driver error.

Note in some of the videos the driver suggests that a "tidy lap" is called for (the UK videos nearly always say that). That's a euphemism for a lap driven entirely within the friction circles of the tires on that car. It is so so the signal the driver is about to stop horsing around entertaining the ignorant and prove what the car is actually capable of if driven properly. Drives like the Stig on a hot lap then.

QED

Or perhaps as the late David E Davis liked to say:

Cogito ergo zoom

Or if you drive a Mazda cogito ergo zoom zoom.

[stugray]

Suberman - My 17 year old son understands friction circles even better than the above explanation just by: 1-Taking AP physics in HS. and 2 - driving Forza motorsport since he was 10.

In Forza, you can watch the state of the friction circles during playback of your laps.

Anyone that wants to learn about vehicle dynamics should try Forza MotorSport.
It is a true automotive simulation complete with the realistic effects of changing ALL suspension parameters (camber, caster, toe in, tire pressure, shocking dampening, spring rate, anti-sway stiffness, tire pressure, etc.). It is not "just a Game".

[Suberman]

Here is possibly the most talented racing driver of his generation getting it all wrong in his search for the circumference of the friction circle of a Suzuki Liana.

Watching his fast wet lap, and then his class leading dry lap, is educational after you see how he got there.

The links to the top gear shows showing his hot laps are ubiquitous. The outakes are harder to find.

This is one of the most breathtaking series of laps ever recorded and he does them in an 80 hp car.

[wparsons]

Originally Posted by Suberman

So for those of you who actually drew friction circles, two are enough because only half the circle affects each side of the car at any given time, you should now solve for vectors. Anybody can do this because the diagrams don't need units to be understood.

The front tire circle in a steady state corner can have cornering force described by a straight line drawn at right angles from the centre until it intersects with the circumference. The length of this line is the maximum side force available for cornering. Incidentally, the reason differences in actual mu varying with load are not important is because those occur at points within the circle. All that matters is to understand that useable static mu peaks at peak cornering force (or force in any direction as it happens, as Ford proved in the 60's from research on this point).

Then draw the same line in the circle for the rear tire. Same side force (this has to be assumed for practical reasons. In fact, the circles aren't circles and they aren't all the same size but for road cars they might as well be, the changing size of the friction circle with weight transfer isn't actually relevant for these purposes).

Now draw a line at 90 degrees towards the bottom of the circle. It doesn't matter how long it is but the illustration is more meaningful if it either goes right out to the circumference or stops a short distance.

Now "solve for the vector". Connect the two lines with two intersecting lines meeting at 90 degrees to form a box. The diagonal of that box is the resulting vector. The two forces are treated as resolved into one composite force in the direction of the diagonal with a force proportional to the lengths of the lines creating it. That's your g line.

You will note that the diagonal always ends up outside the circle. You can't drive there. Well, not on dry pavement at least.

The only way you can drive that tire forwards (or brake it for that matter) is to reduce the length of the sideways line to end inside the circle. Then the resultant diagonal can remain on the circumference of possible performance.

Mu is what creates slip angle which in turn is what generates that sideways line. But on the drive axle it is also must simultaneously create slip in a different direction, creep is more accurate. The resultant force cannot create a longer line than will run from the centre of the circle to its circumference. That radius line is a constant (well, we know it isn't perfectly but conceptually it is and for road cars it is so close to a circle the concept is useful and generally true).

So here's the thing, because mu x load equals horizontal friction force, mu is independent of direction in the horizontal plane. Less grip is available to create slip angle because some is being used to drive the car forwards, distorting the tire in the direction of travel and not trying to twist the tire. Ergo, this means that always the drive tires run at lower slip angles than they could if they weren't driving. For rwd cars this means understeer. For fwd cars this means understeer. For awd cars this means understeer. Unless you want to change the definition of oversteer which is of course done in the drifting community (the synchronized swimming of motor sports and about as interesting) and the journalists who talk cavalierly about a "dab of oppo" to cure "oversteer" when they actually mean to fix a clumsy slide which may look interesting to some but is the sign of driver error.

All of that blabbering is entirely pointless. Are you still trying to suggest that you don't want to be cornering at maximum g? You failed, because in your diatribe you actually described why you would want to be cornering at maximum g.

If you actually looked at a friction circle of a race driver you would notice that they do in fact hit peak lateral g. A great driver will keep vector length maximized at all times.

You could've simplified your entire post to:

To maximize the usage of grip you need to balance all three inputs, braking, steering and throttle. When approaching a corner you brake hard, at turn in you start to feed in steering while releasing the brakes (which keeps grip within available levels), coast to the apex (or keep very minor "maintenance" throttle) which lets you use all the grip for just cornering, after the apex you start unwinding the steering as you feed in throttle (which also keeps the grip within available levels).

Funnily enough, this has already been stated a few times in your threads.

Originally Posted by Suberman

Note in some of the videos the driver suggests that a "tidy lap" is called for (the UK videos nearly always say that). That's a euphemism for a lap driven entirely within the friction circles of the tires on that car. It is so so the signal the driver is about to stop horsing around entertaining the ignorant and prove what the car is actually capable of if driven properly. Drives like the Stig on a hot lap then.

Watch this lap and try to apply your misguided thinking to it:

[ame="http://www.youtube.com/watch?v=7mg_9GrHQgY"]Caterham R500 Top Gear Lap - YouTube[/ame]

There is lots of places where the car isn't perfectly tidy well behaved, but it sets a blistering fast lap.

Or this video (skip to about 12 minutes for the Stig's lap):

[ame="http://www.youtube.com/watch?v=gD6SzWbPGpE"]TOP GEAR 2013 第一å*£ 第一集 之Pagani Huayra 風之å*� - YouTube[/ame]

In both cases, watch how much the car is squirming around under braking, and under acceleration. If the car was being driving 100% within the friction circle there wouldn't be any squirming under braking or power.

I could go on for days with examples, but at the end of it all you still don't grasp how to actually drive a car to it's very fastest potential.

As for the video of Lewis Hamilton, all that shows is a VERY talented driver trying to coax a terrible handling car into a fast lap. Notice how much he's forcing it to rotate (mildly oversteer) to get around the corners faster? If understeering was actually faster as you suggest you wouldn't see him doing that at all.