Driving epiphanies

(Nik Goodfellow) #21

This. If someone asked me where I brake for any given corner I would not be able to tell you, I just know. My best driving has always been instinctive and it requires so much of that, that I often can’t remember a single lap of the race.

That’s not to say I can’t watch people and tell them where to brake (within a zone), but there’s no point me telling other people where I brake.

Hopefully you won’t hate me for saying this, but actually you want to know when you unload the outside front tire. The kart needs to push the inside front tire into the ground and lift the outside front tire to get that tipping effect, that’s you’re initial turn in, when the front outside tire is loaded (and the chassis has tipped), the chassis’ ability to flex comes to play. Ordinarily you wind off the lock so that the front inside tire is no longer pushing as hard into the ground and rely on the lateral force and the flexure of the chassis to maintain the inside rear wheel lift.

So for any given point in a corner you are relying on different aspects of the chassis to help. That initial turn in (unloading the outside front and loading inside front) is almost entirely front end steering geometry. The transition from turn in to “corner maintenance” is a combination of steering geometry and chassis flex (don’t want to drop that wheel to fast or at all) and “corner maintenance” is all about chassis flex and making sure the wheel drops when you want to, ideally when you are pointing in somewhat of a straight line and are ready to accelerate.

Its helped me immensely to understand the physics, I worked out TJ’s epiphany regarding driving under the rubber by watching top euro drivers and working out the physics in 2001/2.

(Dom Callan) #22

Interesting stuff, Warren. Your thoughts are “out there” but it all makes sense. It’s funny though, as I learn more about driving, it’s more understandable. Maybe it’s sort of like trying to explain physics. In order to do so, it requires using the language of mathematics.
It’s interesting in that your concepts are very similar to the ideas that TJ has given me via some coaching, just totally different in how it’s visualized.

Now, let’s talk about rotation… In my early phases of learning, I have been actively trying to avoid slip. However, TJs under the rubber and your writings, suggest that rotating the vehicle is essential. Basically, inputs to steering while turning are not a good thing in that you mess with balance of kart (lowering rear inside wheel for example). I am guessing that at some point here I will be more steering with throttle and my hands will be doing nothing with wheel after turn in. So in effect, this butt-driving around corner with throttle is rotating the kart?

(Warren Chamberlain) #23

Hey Nik, no hate here; your observations/opinions are just as valid as mine.

For sure you are correct about the ‘process’ of turning a kart. I think we just come at the actual driving element from a different perspective. You enjoy the physics, and I assume thoroughly understanding the scientific basis of turning a kart makes connecting with and performing that task work for you.

I tend to focus more on the interim results that I need to achieve my overall objectives. So if my objective is to run a particular turn on a specific trajectory, my first objective is to get the front end rotated into the turn with enough energy that if follows the desired trajectory, and that the energy builds to the point that the kart rotates around its center of mass at the desired location in the turn. If that happens, then I can use an ‘exit’ tire loading plan to check the rotation and set the inside wheel down so that I can accelerate out of the turn with most of the acceleration energy efficiently pushing through the kart’s center of mass instead of spilling off to the side, scrubbing speed, and burning up the rear tire.

Anyway, I’m more a feel and timing driver than a physics/technical driver, so I tend to lump the whole ‘technical turning’ process into a mental bin I call the kart’s energy cycle. So then my turn entry tire loading plan becomes a game of “when do I need to start my driving input to allow this kart’s energy cycle (the whole geometry, chassis flex, lifting the inside rear process) time to run its course and deposit into my outside front tire the load/traction/forces I need to meet my objectives for corner entry."

That said, if I need to diagnose why the kart is not doing what I want/expect it to do, then I have to crack open my mental ‘bin’ and dig into the science to try and figure out what part of the technical process is not working.

(Warren Chamberlain) #24

OK, let’s talk about rotation, but please keep in mind that I’m speaking from my experience and research to try and convey how things feel to me when I drive, so what I say may not necessarily be correct from a engineering perspective.

First we should define ‘rotation’ (aka yaw rotation, or polar rotation), or more precisely we should define the rotation point in a turn. I guess the simplest definition would be “the point where you stop ‘entering’ the turn and begin ‘exiting’ the turn.” There are a number of more advanced ways of thinking about it, but I won’t go into that now.

First, any reasonably functional and reasonably driven kart will rotate in every turn. It has to rotate or it could not complete the turn… it would eventually just go plowing off the road. This was actually an epiphany I had when reading Piero Taruffi book The Technique of Motor Racing back during my 3rd year of driving… when dinosaurs roamed the earth. It seems so obvious, but for whatever reason, it brought clarity to what I was already doing, and opened up avenues for further refinement and exploration of my craft.

So, rotation itself is not some magic that the fast drivers have and the rest don’t. This difference is that many drivers are unaware (or at least not consciously aware) of rotation as being something separate from ‘line’ so they tend to lump it all together as ‘turning’. Because of this, they ‘accept’ the default rotation location (which it typically at or near the geometric apex of the turn) and therefore they drive within its limitations.

They may make adjustments to their line to try and improve their performance, and that may move their rotation point slightly, and that may improve their performance slightly. However, since they don’t see the relationship between rotation point and performance, they remain ‘line’ drivers who can only accelerate in response to rotation instead of taking control of rotation and using their skills and creativity to move the rotation point to a location in each turn that will allow them to accelerate as soon as physics permits.

Fast racers understand (empirically and/or intellectually) that driving at the limit is more like flying than driving. That is, many drivers seem to think of the kart as a ‘dot’ (the center of mass) that they need to guide around the ‘line’ without recognizing that the kart has length and width and can rotates in yaw like a plane. I believe that fast drivers (again knowingly or unknowingly) relate to the kart as a rectangle (the four contact patches) that they need to orient (rotationally) on the line they are driving to create a trajectory that produces maximum performance. And taking that a step farther, in general, what defines the extents of the rectangle at any given point around the track is the amount of slip (the degrees of slip angle) generated by the loaded tire.

So, butt-driving, is really more like driving a trajectory (an orientation on a line). To do that, you must take control of when/where and how quickly the kart rotates in each turn. To do that, you figure out when/how you need to load the outside front tire to initiate the rotation, and how/when you need to load the outside rear to control and stop the rotation, then you dig into your bag of driving techniques to produce the entry and exit tire loads you need… and Bob’s your uncle.

(Lee Swindell) #25

Warren:

I’m soon to go out for my final stint of the day. I observed during my previous session the time I was wasting in no-mans-land between brake and throttle. I tried booting the gas earlier than I thought the kart could handle - and discovered that it couldn’t just handle it, it was happier that way. I was earlier on the gas and used slightly less road.

I understand that the kart can complete the turn better under power than on a trailing throttle. But why?

It’d be interesting to hear how this observation fits with what you’ve written above.

Lee

(Mike Clark) #26

Lee,
Simplest answer is you induced more oversteer (rotation) to the balance. By putting forward force you took away some of the side force. You can think in various terms such as side force as resistance to rotation, changing slip angle on one tire, which nets to the balance of all 3 or 4 tires, or just picturing a single vector arrow on a clock face (friction circle). You can even picture the arrow remaining @ 12 o clock and the face rotating . . . Think about that.
In a car you can get to rotate by jumping on the throttle or jumping off the throttle. I think you were adding oversteer/rotation, adding forward acceleration and then having the inside rear come down, thereby cutting the rears slip angle about in half with some good timing. (That slip angle cut in half is just a theory, I don’t know the actual values. But it does seem like the side load remains and then you double the tires splitting the load. Hmm?? 60/40 inside outside?? either way seem the rear would have a net value of slip angle being reduced, along with you taking steering out) (this also has something to do with swapping slip angle for percent slip. Sidewall remains twisted & pulled while transitioning from side load to forward load. Think about a friction circle pertaining to only one tire, not a whole car/kart. then think about 4 separate friction circles, 1 per tire all inter related)
I think understanding things deeply helps more with setup as well.I knew a guy from autocross that was pretty darn good. He would be the first to tell you he doesn’t understand any of it. All he knows is when he does this, the car does that. So I took it as he was being honest. I don’t think he was sandbagging on helping others. I believe he just knew the correlation of what inputs = what output in a fairly direct relationship. He was not a setup guy, at all. I was the guy always having knowledge above my skill level, trying to be ready for the next level, if I ever progressed. My usual M.O. is to hit the ground running with a lot of “Beginner’s Luck”, then plateau for a while. TBH I am at the point in life I am only willing to put so much into a hobby, so there’s a nice juicy excuse.

(Mike Clark) #27

FWIW I think slip angle is a bit deceiving of a term. At times i forced myself to think of it as tire wind-up. Think about how much force / energy that takes. You can actually take a kart wheel in your hands and put in in it correct orientation on the pavement and twist it to feel it or take just and tire and hold it distorted to appreciate the effort it consumes.

(Warren Chamberlain) #29

Hi Lee, I hope you had a successful weekend!

In general, I find it best to load the outside front tire on the way in, manage the rotation, which typically involves aggressively transferring load from the outside front to the outside rear tire, and then managing the load/drift angle of the rear tire on the way out to control my line/trajectory.

Unless there is a specific need to have trailing throttle as part of my corner plan, I don’t use it because tires need load to produce traction. With trailing throttle, the kart is not really significantly loaded on the front end or on the rear end, (so neither end has enough energy flowing into it to produce optimum traction).

That makes the kart feel twitchy, or skatey… just not ‘hooked up’, so it feels like doing anything other than waiting a bit until things settle down will result in losing control. When you forced yourself to get on the gas sooner, you brought the outside rear tire to its load ‘sweet spot’ so the twitchy feeling got replaced with the very confidence inspiring feeling of drifting the rear end out of the turn. And the arc described by the ‘drift’ of outside rear is what finished off the turn for you.

Also, that ‘used slightly less road’ at the exit might be saying that you could perhaps carry just a touch more speed to the turn’s rotation point, or perhaps that you could get on the gas just fractionally quicker before the kart rotated as far, so the rear tire would run a slightly wider arc.

(Dom Callan) #30

How do you load the right front, for example? I would assume going into a braking zone, the act of braking will load up the front in general, but how specifically one side versus the other? To load up a specific side, wouldn’t you have to trail brake? So right front would load up under trailbraking into a left turn?

(Warren Chamberlain) #31

Sorry Dom; didn’t mean to confuse this issue. By ‘outside front’ or ‘outside rear’ I just mean that I only care about (invest mental resources on) the most important tire/contact patch at any moment.

So in a sense I kind of do what Mike mentioned…

So my process is very simple (but not necessarily easy):

2. Manage the loads on the outside front tire to create the arc (drift angle), and rotational momentum, I want. For me this typically involves some level of trail braking if a speed adjustment was needed.
3. Manage the timing of the rotational momentum so the vehicle’s center of mass ends up oriented at the desired angel to the ‘line’ at the time/location I need it to be so that I can get on the gas ASAP.
4. Manage the loads on the outside rear tire to create the arc I need to complete the turn. Typically done with slight steering and/or throttle adjustments, or occasionally prayer.

(Warren Chamberlain) #32

I agree 100% with Mike about the terminology used for tire performance. For example, whenever I see a picture of a vehicle with the tail hung out and it’s described as a 4-wheel drift, I always think to myself “um, there are only 2 wheels drifting there.”

Also the definition of ‘slip’ is “slide unintentionally for a short distance” or “to move with a smooth sliding motion” neither of those describes what’s happening between a tire and the track that causes the tire to ‘drift’ some number of degrees away from the vector described by the wheel. So, I like the term drift, because that is what’s happening; the tire (and kart direction) is drifting away from the direction the wheel is pointing.

it’s also really important to recognize that you can ‘drift’ by the same number of degrees in two different ways:

1. By loading the tire properly to produce the traction that deflects the contact patch and sidewall, which produces the drift away from the wheel’s vector (I call this a ‘loaded drift’).

2. By purposefully unloading a tire so that it drifts away from the wheel’s vector because it cannot produce the traction (interlock, sticktion, however you want to think of it) needed to stay on vector (shockingly, I call this an ‘unloaded drift’). This unloading, of course, is done by applying throttle to flow load away from the outside front tire and towards to the outside rear tire.

When executed (timed) properly, you can end up with the vehicle aligned (relatively tangent) with the ‘line’, the outside rear tire in a loaded drift, the acceleration force pushing through the center of mass (instead of spilling off to the side), and the potential over rotation / instability that this ‘dynamic’ could produce being compensated for by the matching drift of the outside front tire.

To me, this is more of a 4-wheel drift than the ‘traditional’ version, because, in fact, all for wheels ARE drifting, it’s just one end is drifting because it’s loaded, and the other because it’s unloaded. In this scenario, you just fine tune the degrees of drift (front to rear) so that the whole vehicle is oriented on the ‘line’ in a trajectory that will allow you to finish the corner on the black stuff.

Anyway, just something to consider.

(Lee Swindell) #33

Warren and Mike:

Thanks for your most recent replies to this, my favourite kart related thread so far, and my apologies for disappearing from the conversation for a couple of weeks. I have a strange condition that destroys my brain function after heavy exertion.

Warren and Johan, I promised you the New Scientist article, so here it is. Mike, this will interest you from the perspective of the ‘exponentials’ that you use in your mind models.

Let’s hear your thoughts on this. Some years ago I drafted an email to the author (yet to be sent) about a steering sensor that accumulates the ‘K’ value in our driving, on the (maybe simplistic, or maybe wrong) basis that the lower this value, the quicker you’ll lap.

Lee

(Graham Reeds) #34

One of the best points is you don’t have to be the fastest to be the quickest. Being ahem heavier I regularly have a slower fastest lap than those around (and more importantly) and behind me. But I regularly beat those who are running 10 kilos lighter.

Is it better equipment? Most of the kart parts are second hand bar the engines and brakes. I have never bought a brand new chassis.

Most of the time I am catching under braking and carrying speed into the corner. Not really an epiphany but something I have honed over the years.

(Lee Swindell) #35

Good to hear from you Graham.

beating dudes while carrying a 10 kilo handicap in heavily used gear is a big deal - you’re obviously doing something very right.

You cite carrying more speed on the way in: just this morning, I read something (in Car and Driver from 1970) related to this that led me to another possible ephiphany.

The author wrote about the scrub that opposes tractive force on the way out of a corner, which is another perspective on carrying corner speed.

When I carry steering lock into a corner in a kart, I’m opposing my engines ability to accelerate me off that corner. Anyone who’s pushed a kart along the ground in other than a straight line knows how huge that scrubbing force is. In a vehicle as underpowered as a kart, that’s a very big deal.

I’ve never known whether the jam my kart hard into a corner to get the tail around, or turn in more gently so as to not ask too much of the chassis (as TJ pointed out here a couple of weeks ago). The sweet spot on corner entry is clearly hit when I’ve carried enough speed to rotate the kart onto a tangent to the apex, without overdoing it (wrecking the line and/ or bogging the engine). What that guy wrote has helped me decide that dilemma, in that the goal in cornering is to hit that exit tangent and the gas, with the wheel completely straight.

This sounds obvious to an experienced driver - but I’d only ever thought about this in term of the geometry, not the interplay of opposing forces (which is your turf Warren, if you’re reading).

That will sound simplistic to some. But it’s given me a new and very productive dynamic to concentrate on while at the wheel.

For me, the black art of corner entry is now greatly less so, thanks to a randomly read 50 year old car mag.

What do you lot reckon?

(Dom Callan) #36

Ok so right about now I am getting terminally confused. This whole discussion seems to be around the idea of getting more exit speed by virtue of rotating the kart via turn-in so that it’s pointing the right way when you get back on the gas.
So, to accomplish this, you are getting a little bit sideways in a sense. Which makes me think of the footage James put up of of Pantanoo driving in the 90s Swedish flicking to accomplish exactly that.
Except I never see this happening anywhere anymore. I assumed this was because karts were DD back then and you had more torque or something which made it viable.
So are we supposed to rotate and pre-turn the kart pre-apex or not?

(TJ Koyen) #37

Karts functioned differently in the 90s. For one, they were much lighter than the are now. You simply can’t flick a modern kart around like that. Frame materials were different as well, and the philosophy of tuning and driving was slightly different as a result. Back in the day, the karts didn’t nearly have as much front end caster as they do now, and the frame material was less springy in general. If you tried to drive like Pantano in a modern kart, you’d find the kart would react pretty violently, as the frame is more compliant now and the steering geometry more sensitive. So modern driving requires a bit more finesse. In the 90s you could really hustle the kart around aggressively and make it work. There are still applications where you can chuck the kart around a bit; some corners require that, but it isn’t nearly as prevalent.

Basically, once you’ve turned in and initiated inside rear wheel lift, you can straighten the wheel back out and the kart should track off the corner on it’s own by virtue of the chassis being loaded with all that cross weight. So you’re making your whole rotation in that first 1/3 of the corner. At apex, you should basically have the wheel straight again.

(Dom Callan) #38

So between itital turn in and apex, you unwind steering to be straight by the time you are at apex, but the kart is rotating around still while you unwind? In other words, you don’t need to hold the angle because the kart will rotate as u unwind?

(TJ Koyen) #39

Pretty much. Obviously there are a ton of variables and every corner is different, but that’s the basic idea,

Here’s two screen grabs from my on-board of testing at Norway at the beginning of the year. It shows the wheel input in the tight hairpin:

I make a firm initial input and then basically have the wheel straight even before apex in this example. There is probably always going to be a little input off the corner, but you’re unwinding as soon as you feel the inside rear wheel peak in most cases.

(Dom Callan) #40

Ok thanks. I’ll mess around with this next practice.

Edit: is this only doable at full chuff? In other words, can this be attempted at moderate speeds or will it only work if you are going fast enough to exceed grip limits?

(TJ Koyen) #41

Well the kart basically has to be on the limit to flex and work properly, so you’ll have to be pushing.

This is something to keep in mind when tuning as well. For example, if you find that you’re having to put more steering in from apex off, make sure you’re on the correct line and your turn-in point is correct before starting to try and tune the understeer out.