I surreptitiously got some learnin' in this thead:
http://www.f1technical.net/forum/viewto ... =6&t=17810
Here's what it was really all about (front wheel drive race cars):
Original post from elsewhere:
I've been busy lately...thinking about things...and the things that intrude on my thoughts about the other things...but so often have so much to do with them...
Have you read this one:
http://jalopnik.com/5930557/why-you-...-to-drive-fast
Alex makes some good points...but some of them, however good, are still debatable. We have to use words to describe what we're doing, or trying to do in any case, to the best of our understanding, which isn't always that good.
Which is where your higher quality nerd comes in. All of the lesser nerds look to them for guidance. You know some of their names: Puhn, Smith, the other Smith, Aird, Dixon, Haney, Bastow, Olley, Rowley, Zapletal, Nowlan, Van Valkenburg, Adams, Doo, Campbell, Norbye, Milliken's, Kasprzak, Roberts, Staniforth, Daniels, Gillespie, Terry, Holmes, Matschinsky, Reimpell, Gillespie, Ortiz, Wright, Mitchell, Pacejka, and probably some more...including guys who share on the internet like Jersey Tom, DaveW, Cibachrome, Dynatune, Z (I know I already covered him, but he's worth a double mention at the very least), Novak, and SRSVW (Who?)...
Nerds, Engineers, Thinkers...What's the difference to the "Just Shut Up and Get In and Drive" crowd eh? I'll tell you - All The Difference in The World when they're the crucial difference that's kicking your ass in the brain!
So what am I gonna rant about this time? Caster. and the Front Wheel Drive Race Car.
I caught myself not thinking again, and in the midst of That I came across some postings by now retired VW race prep guru/crank Dick Shine (SRSVW) that made me stop and think, and some postings by Cibachrome and Jersey Tom that helped me think about how to think about...this.
Have your thoughts about Caster been limited to negative camber gain and weight jacking in turning and hence guiding you in the direction of "more", subject only to the limit of your upper body strength and endurance? That would put you well within the commonly informed consensus. MAYBE you heard that around 11 or more degrees you're too likely to get oscillation overthrusting and a tendency toward the 8th Dimension.
And MAYBE you even know about Mz and Fy and PT and Magic Formula and BMW and sublime steering feel. And if you know that much maybe you know that Danny Nowlan says flat out that your hands and ass are not going to provide you with coherent and synchronized information, and that it's your job if you're worth a --- to get the job done regardless. Others, more of them, advise the use of geometric caster and trail to align your signal inputs.
Just in case you don't know much about all that stuff, a simple and not wrong picture of it looks like this: the tire doesn't want to turn, by an amount of torque typically referred to as Mz, that you feel in the wheel. At Zero Caster that's what you feel, and it varies with steering and slip angle, and it's a characteristic of the Tire. As you add Caster, you feel a percentage of the tires lateral force Fy thru the wheel. Why you would want to do that (besides wanting to avail yourself of negative camber generation and weight jacking) is because the Mz signal peaks Before Fy and is rapidly heading for Zero as Fy is peaking.
Most of us who got our primary education in the physics of driving from Carroll Smith, think in terms of driving the "top" of a tire curve. We only get such a thing when there's enough caster to give us such a curve. How much is that? Depends on tire, determined experimentally, by YOU. There is ultimately no escape from the responsibility described by Nowlan. There is however something to be said for a car that is More rather than Less nice to drive.
But there's more...Dick Shine, from the VW world of Mac Struts, where you'd Think desire for negative camber gain would be maxed out, was famously outspoken on Caster as EVIL.
Say What! Flip thru all those books...dig and dig...and find no unambiguous description of what Shine could possibly be talking about. So is he just a crank? And a collection of championships and wins coincident only and not contributed to by his philosophy with respect to caster?
Jersey Tom related an epiphany he had about Mz...it's an understeering moment input into the car. That it's converging on zero as the tires lateral force Fy is peaking is nice. The torque about the steering axis created by geometric caster (caster trail) and Fy on the other hand "looks" like the Fy curve itself - ie it's maxed when Fy is maxed.
Did you connect those dots? Caster had a COST...charged to you in additional understeering moment...charged against that same "all the Fy you've got"...by crude calculation 5-8% of the work of getting the front end around the corner at some of the numbers that are commonly quoted. On your front grip challenged front wheel drive race car. You wouldn't do it unless you didn't know better. And near as I can tell almost nobody is telling you any better.
You see it right? It's hiding in plain sight. You can think of the chassis and the wheel as ONE solid body, held as such by your hold on the steering wheel, and the Fy X Trail moment is an understeer moment. Stability, wanting to go straight, is the same as not wanting to turn. Not something you typically want your front wheel drive race car to not want to do.
There are some who know...designers for Honda, Alfa, Lancia, Renault, Peugeot, Citroen, Volkswagen...dig up some factory caster specs...note too that many have power steering...
Scott, who sometimes forgets to think, sometimes doesn't think to think, and sometimes is damn near determined Not to think...but sometimes you gotta...
***EDIT: AND YOU GOTTA KEEP THINKING BECAUSE YOU MIGHT BE WRONG...AND I AM. The attendant forces caused by mechanical trail are resolved thru the steering and do not equate to an understeering moment input to the car. Whatever Shine was onto (or on?) This doesn't appear to be it. I'm gonna keep thinking...despite the risk now so obvious of only having a brain this good to work with.
Follow up post from elsewhere:
You know how amongst the annoying things about nerds is that they keep coming back? And so it is with the inner nerd - it keeps coming back...with "What about This?!" And so I'm back.
Here's the thing - I don't know Dick...Shine...but the picture I've drawn of him is that he got results, was willing to tell what he knew/thought, but didn't/couldn't explain this one thing - his thing about caster on front wheel drive race cars. With not much to go on, and one failed attempt at solution already, I still want to believe him.
I was all hung up on what happens from the contact patch by way of the steering axis and up thru the steering wheel. And my belief in the hypothesis pulled me into unsustainable thoughts (Road Runner Physics). My inner nerd had to retreat into my brains silent workshop.
The other day I found myself drawn to read a thread on the F1 Technical board about driver styles, and found my conscious mind asking "Why am I reading this?". That I kept reading seems to have been due to a work order originating in the workshop. "What about this?"
If you read the chapter in RCVD on Tire Behavior, you find two images of particular interest here: a plot of Lateral Force (call it Fy) against Slip Angle, and a plot of Aligning Torque (call it Mz) against Slip Angle. You can (and have) to imagine the "sum" of those plots as they are expressed at the steering wheel.
As we add Mechanical Trail, the proportion of the Fy signal sensed at the steering wheel increases. The peak Fy still occurs at the same Slip Angle, and so it's easy enough to plot a set of cumulative curves for increasing amounts of Mechanical Trail. I did three of them at half, equal and double the amount of force from Mz.
At zero MT, the force at the steering wheel (AT only) peaks before Fy and tends toward zero as Fy is peaking. By an MT of double the AT the force at the steering wheel peaks at about the peak Fy.
All very fine and well...but So What?
My inner nerd is telling me to tell you to think about how we drive a front wheel drive race car thru a corner. What "style" does the stupid car force on its driver if only thru the narrow set of options it allows?
Do we actually drive a front wheel drive race car at peak Fy very much? Do we want, amongst the three driver inputs (eyes, ass, hands) to be reconciled, Peak Fy to be the one we're sensing thru the wheel?
I think I mentioned how Smith implanted for many of us the idea that we were driving to the "top" of a curve. But upon reflection, I'm asking myself if I am thinking and doing two different things? Sensing a soft curvy top isn't easy. Sensing a significant falling off is easier. An answer has to relate back to the drivers sensitivity and resolution. I've driven both types of curves and am pretty sure I prefer "falling off".
So think in terms of a falling off type curve from relatively low caster. There's a zone from peak Mz to peak Fy. How do I know where I am and what I'm doing? Those other two senses...AND a right foot.
Something should be bugging you about peak Fy. What can't you do at peak Fy? Apply any throttle. You got a problem with that? I got a problem with that.
See where I'm going? When you add caster, and push the feedback curve toward peak Fy, your are inviting a racing driver of average skill and sensitivity to operate the car at or beyond a point at which application of throttle can only result in push and add heat to the front tires.
This reinforces the idea that a key part of making a front wheel drive race car TURN, is that the rear of the car "let go" well before peak Fy at the front. From there we control the rear with the throttle, and the gap between where we are and peak Fy is the drive capacity we have available for the job.
I believe that this is what Dick Shine was on to and couldn't explain. The high caster front wheel drive car actually conveys Less information to the driver and biases that information such as to encourage overworking the front tires.
Scott, whose inner nerd is asking when he gets to drive again...
