# My understanding of what the slip curves essentially describe.

Discussion in 'General Discussion' started by mantasisg, Jul 4, 2020.

1. ### mantasisgRegistered

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TL;DR - just geeking out, sharing thoughts of attempting to understand tires. If you are interested in understanding tires welcome to read, I hope I didn't make myself hard to understand what I mean in here.

Note. Thats not teaching or demonstrating expertise here, I am just interested in understanding stuff. If you disagree with something or find a mistake of some sort logic flaw, then feel free to contribute. It is not meant to be scientific.

Most of stuff I did not invent, except that remaining part of static contact patch at heavy slip angles might experience load sensitivity effect, whic hI honestly just don't know if it is true, just makes sense to me.

In my thinking the main thing that slip curve does is describing how much of a tire contact patch is in static postion and how much in sliding position in reference to the surface. And of course plotting out change of tire performance according to that. Which ultimately is easier to plot down to angle for practical use, but physically the angle does not describe what tire actually does and how friction gets composed.

I was thinking on some analogy recently to illustrate principles of how slip angle influences the capabilities of tire. And I thought maybe two forces lifting an object, for example a beam could work. Two forces are static friction of tire, which is stronger force, and sliding friction of tire which is lower.

Lets say static friction will be represented by 1000N lifting force (at its maximum), and sliding friction will be represented by 600N of lifting force (at its maximum) acting to a single point of a beam at certain position to illustrate significance of area in contact patch of tire. And lets say the weight of the beam is maximum cornering force (because of maximum friction) of a tire, which is 1400N.

Lifting the beam is analogy to taking a corner, so heavier beam means greater cornering force demand.

So how does it work ? All numbers are just for general idea.

_________________________________________ < thats a 1400N weight beam
____________________^1kN <this is a 1000N lifting force

**********************
Slip angle is barely more than 0
_________________________________________
____________________^1kN________________^0.001kN

Beam rests on. Car is either driving straight or barely moving either.

**********************
Slip angle is introduced
_________________________________________
________________^1kN ___________________^0.1kN

An effort to lift a beam begins. Like effort to steer a car begins, tire relaxation length is not taken into account. As slip angle increases significance of sliding friction increases. Bigger portion of contact patch is sliding, non sliding contact patch area is reducing.

**********************
End of linear range of slip curve

_________________________________________
______________^0.99kN______________^0.25kN

Effort to lift a beam keep on increasing (Just like effort to steer a car), a main lifting force reduces a little as new additional force gets better position. Sliding area of tire contact patch keeps on increasing. Rubber load sensitivity starts kicking in the remaining static friction area, could it be the case ?

**********************
Near the end of transitional range of slip curve, close to frictional range start.

_________________________________________
_______^0.92kN_________________^0.52kN

As the tire friction peaks, the lifting power of the beam in this analogy is at its highest, and beam is lifted and held. Car holds on constant turning radius at constant speed.

**********************
Soon after start of frictional range.

_________________________________________
_____^0.85kN_______________^0.55kN

In analogy peak power already started reducing, but still enough to hold the beam. In car handling this is threshold point at where the car begins to feel unstable.

**********************
Further into frictional range

_________________________________________
___^0.4kN________________^0.58kN

Non sliding part of the tire contact patch now has almost disappeared. The net force supporting the beam up is too low and only works as deceleration for falling beam which is of course is pulled down by gravity. In case of cornering vehicle, or a wheel which supports portion of cars weight, it would be its inertia that tries to make it go straight.

**********************
Far into frictional range

_________________________________________
^0.0kN_______________^0.6kN

None of static friction is working anymore, contact patch is at absolute slide. Worth to mention that sliding friction might get reduced further due to heating and it also depends on sliding velocity. In case for the beam to stop falling in this analogy (which would be for a car to stop sliding). Beam needs to be unloaded (speed of cornering reduced or cornering radius) or slip angles reduced (car straightened). It is good thing that sliding friction of a tire slows it down so technically the "falling beam" gets lighter as it falls, but sliding friction can possibly reduce faster than that at high speed of sliding or very late appropriate influence from a driver, such as wrong late adjustments of load distribution and slip angles, slip ratios and speed.

By the way, driving force and braking force are not included here. Obviously with them in a mix tires cornering capabilities decrease.

I hope it is easy to see that the higher the difference between sliding friction and static friction, the more dramatic the frictional limit range will be, the more it will require of drivers concentration and less play will be allowed. Thats the main thing that we keep on arguing all the time, how easy it should be ? Naturally high friction is easy, but great loss of friction is not. Although the progressiveness is as important as the peaks and lows of the slip curve. In my thinking things that takes tire towards the "difficult" direction are: initial sliding area of tire (wider tire has lesser initial sliding contact patch area, drops away faster), tire flexibility (stiffer tire - less play, drops away faster), tread flexibility (stiffer tread - less play, drops away faster), and more things such as rim width, sidewalls....

Last edited: Jul 4, 2020
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2. ### Korva7Registered

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Illustration like that showing the position of the combined force (Static friction + Sliding friction) moving forward on the contact patch could be used to explain why the self aligning torque drops (steering goes light) on some cars (AMG GT3) as slip angle is added at the front.

Do you think that there is actually a strict divide between static and sliding friction, or is it just a simplification?

3. ### mantasisgRegistered

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True, it is the fundamental reason why steering can go light at understeer. As slip angle at front goes into frictional range, resultant lateral force moves towards center of tire, pneumatic trail gets reduced. It starts doing so before peak slip angle. Then it continues to decrease, and after peak angle friction itself starts decreasing too, resulting in further easing of steering weight. But it is only noticeable when built in trail - mechanical trail is low, because it never changes and can give much more weight than pneumatic trail.

I don't really understand what you mean by strict divide between static and sliding friction. They are blended and works both at the same time as tire do it's job, if that is what you think about.

4. ### John R DenmanRegistered

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If you were to dig a deep enough well into this subject you'd reach China.

The properties of the elasticity of the tread compound are overwhelmingly complex. Long chain polymers can be and are often compounded with grip that actually increases with slip angle to a point, and thats tested by varying temperatures on the test stand. Its not the full tire that gets tested, just a sample of the compound on a flat plate clamped to a flat test surface as a stylus on an environmentally controlled tension tester. Years ago Chatillion made a full range of these for polymer testing, they may still do. They can test both static and dynamically and store the data.

Years ago BF Goodrich could take a standard road tire and developed the R1 tread compound using that apparatus. The tire behaved totally different on the rolling tire testing system with reduced heat generated at about 2% slip angle than 0% as I recall.

For the most part you're on the right track.

5. ### Korva7Registered

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Reading the text i got impression that in your thinking there is this strict divide between the amount of friction between the rubber and road when there is no slip and when there is slip (like it is with solid objects) instead of a curve looking similar to this.

6. ### Alex72Registered

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Niels videos on tires is interesting if nothing else. Im sure you've seen them but maybe someone hasnt.

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7. ### mantasisgRegistered

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@Alex72 Yes surely seen them, learning things from him in every videa, all the time. Everybody who is interested at how stuff works, has to check out his YT channel.

Surely static tread cutout experiments could never match full tire experiments in special rigs, I think a tire on actual car at a road would give some different results again, but I suppose no way to make such experiments to plot out some particular data like on test rigs. By the way, little fix, percentages are used to describe slip ratio, angles goes for slip angles.

No, surely didn't mean that at all, even the sharpest tire ever will still have fade out/fade in from one to another. However all these curves are never having one important variable parameter that just happens in practice - time. Surely those transitions from hard slip to hard grip can happen very fast depending on what driver does and many other circumstances. It is interesting thing to watch for footage from real life where cars are being really overdriven, it does happen often that tire appears to be jumping fast under and over the limit, but it is not unusual to see slips with good progression and not so on/off.

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8. ### LazzaRegistered

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As you said yourself, slip curves don't have a time axis. They aren't intended to represent a transition from static to sliding in any scenario.

9. ### mantasisgRegistered

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Yes. They describe change of performance. But that change of performance happens because slipping area grows at contact patch, and static contact area shrinks. So I would suppose slip curve could be traced back to the sliding contact-static contact percentage transition curve. I think that grip from static (adhesive) area can not possibly increase as that area reduces within slip angle growth, if anything maybe its friction gets decreased slightly ? I think tire friction grows with certain amount of slip angle because introduced slip area creates additional friction for a tire. Till of course adhesive area becomes ineffective or disappears completely. Like showed in this picture:

Pic I took from here, interesting statement in that thread, by the way:https://ecomodder.com/forum/showthr...-not-affect-grip-autospeed-article-11433.html

10. ### LazzaRegistered

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@mantasisg something that gets thrown around sometimes, and I've seen it in tyre discussions in racing circles and even from a so-called 'expert' regarding tyre pressure in passenger cars - there's a misconception about contact patch area vs pressure.

Amontons' 3 laws of friction (friction proportional to load, independent of the area of contact, and kinetic friction independent of sliding speed) were established and proven with materials that weren't rubber. If you're sliding blocks of wood against each other they are true (providing you don't go stupidly small on area) but with a rubber tyre they don't hold. I've even seen an apparently authoritative source say the only reason we don't use bicycle-size tyres on cars is for tyre wear. Meanwhile people driving normal cars can relate to the extra grip provided by wider profile tyres and the larger contact patch.

The properties of rubber, and those of various compounds, are why the tests described by John above are relevant despite not dealing with the tyre deformation at all.

And regarding that thread and forum in general: they're hyper-milers wanting to run high pressures to reduce rolling friction. Like anyone else they're inclined to find things that apparently support their point of view. So treat that with a lot of caution.

As far as why a certain slip angle produces maximum cornering force, I'm no authority. The shape of the slip curve and where it peaks, in relation to how much of the contact patch is static (bearing in mind static and sliding parts aren't so black and white in practice), don't really match at all, obviously. It would be reasonable to surmise something about the tyre construction (and resultant forces) in combination with the static area helps produce more force, while some degree of sliding or stretching can produce more frictional force too.

In terms of rF2 especially, we should remember that like real life the slip curves are no longer part of tyre design but a result of tyre construction.

11. ### mantasisgRegistered

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Not really interest to get a low pressures discussion, here, it just happened to be from that thread, I havn't read it yet, I agree it always has to be taken with a grain of salt. But it is pretty basic thing, I am sure somebody like an engineer from Michelin would know how it is. In my opinion pressure is also very important for tires structural properties, that can't be forgotten.

I think the reason why wider tire has better grip is because it has more efficient shape of contact patch, afaik wider cotnact patch has greater adhesion area than narrower contact patch. Thats also why they have less load sensitivity, I think load sensitivity must be all about normal force per area of adhesive contact. Eventually wider tire can also use softer compounds. Wider tire also likely has better lateral stability, simply because of geometry. But despite of being less efficient, narrower tires are more progressve and predictable, I think they drop away less suddenly, and less much. Would be interesting to see data for same tires but just with different tread widths.

Thats why car with narrow tire handles like this, and car with wide tire handles like that:

12. ### LazzaRegistered

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It would make things clearer if you pointed out exactly what you found interesting in that thread. I assumed it was related to pressure because most of that post is.

13. ### mantasisgRegistered

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I put link because I took picture from there.

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15. ### Korva7Registered

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There i was talking about a piece of tread rather than the whole tire.

16. ### EmeryRegistered

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I've been conducting real life tire experiments for my autocrossing and can describe what I've noticed.

In this case, it's same brand, same construction at two mildly different widths. Also worth noting that the car is a relatively high center of gravity, camber-challenged fwd. The narrow tire is 245 width and the wide one is 255 width, both on 8 inch wide rims, same diameter (17 inch), so in classic terms, the narrow tire is mildly pinched and the wide tire is definitely pinched. I use the same tire pressures for both widths as the tire is fairly insensitive to pressure changes over a 10-12% range.

The narrow tire achieves about 1.20 peak G and the wide tire achieves about 1.25 peak G as measured by a datalogger mounted in the car. When I'm competing, I can perceive peak G differences of about 0.02-0.03 G, so 1.20 G vs. 1.25 G is noticeable. The best way to describe the difference is that the narrow tire slides a bit further, another half inch, another centimeter, before hooking up when you're at the limit. On the positive side, the narrow tire will come up to minimum operating temperature a bit quicker in the rear; not that there's much heat generated back there and one wheel is often in the air, but, like the peak G, the narrow rear tires begin working a maneuver sooner than the wide tires.

As a doublecheck on my perceptions, I tried staggering the tires with the wide in the front and the narrow in the rear. Result was a rear that would not hook up as well, but not oversteering like using a stiffer rear swaybar would do.

17. ### Slip_AngelRegistered

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Very interesting discussion going on here.
I personally don't know much about tyres, i just watch onboard videos of race cars that are generally on slick tyres.
Heres my opinion->>

Short version->> In my honest opinion the real tyres on GT cars i have seen looks more in between ACC and RF2.

After playing ACC and RF2 back to back driving similar cars (RSR GTE here and GT3s in ACC) I know that GTE and GT3 are different but they are not quite like comparing prototypes with GT.
So anyways What i noticed in RF2 that the rear tyres have incredible traction off the line compared to ACC.I have tried to do some silly stuff like trying to provoke burnouts from stand still,clutch kick and there is hardly any noticeable wheel spin.
BUT in ACC if you go full throttle in standstill there will definitely be a huge burnout.

Now let's talk cornering
The rear engine cars feels tad bit more sensitive on entry phase of corner in ACC compared to RF2.
In RF2 the entry phase is quite progressive even with slightly harsh inputs.(Not saying that you can't lose it in RF2 but it takes more bad behaviour compared to ACC)

Corner exit->>

This is where ACC and RF2 are quite different.
In ACC the tyres seems to follow a rule which is ,"If you got enough power/torque you can get some wheelspin" which seems perfectly reasonable. And this is why in corners with high speeds appox above 150km/h with normal setup it is nearly impossible to get rear wheel spin or little slide.
Maybe with too much tyres wear and extremely bad inputs from driver in general the the rear tyres are absolutely planted in high speed stuff especially on rear engine car.
But when speed are low enough to get a wheel spin the inside tyre always loses grip first on power.
You can tune it slightly with differential.
In RF2 it always feels like outside tyre loses the grip first on power.
Both completely different results but i have logical reasoning for both.
Basically RF2 loses outside rear tyre because too much load and ACC inside rear tyre loses because not enough load.

Now, In high speed stuff in RF2 the slides don't seems as related to power as ACC but in RF2 it feels like you are sliding because you are at limits of tyres.
Hence in ACC you can smash the throttle in high speed cornering in RF2 you still have to feed the power to modulate slide.

In my honest opinion i think realistic would be results between these 2,i.e not as planted as ACC but not as sliding as RF2 either.

This is why i think the grip levels on rear tyres should be increased in RF2 tad bit BUT at the same time make it lose in more brutal manner.

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18. ### mantasisgRegistered

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@Emery Thats interesting, I'd guess the difference of G force was massive for this amount of width increase, but I don't have other refference. Also I wonder, although it was such small width difference, could you perceive if wider tire was more challenging to maintain smoothness on the limit ?

@Slip_Angel It is always very interesiting to try understand and workout the puzzle of what the car phyics are. Comparing sims brings more confusion IMO, best to compare RL to a sim, but it is hard to resist comparing two different simulations, especially when first hand experience is possible for most, unlike for real life (except very few lucky guys, who also simrace). I have same impression for rF2 tires regarding acceleration, rF2 tires would become less "incredible" in some laps as they degrade in performance pretty fast, they still does right ? IMO ACC tires work more realistically there or how actual practical tire would be manufactured to be by top tires manufacturers. Or used to work - I haven't used ACC in ages, and it is forever changing, juggling likes of users and accuracy to real life, just like all sims does.

In corenring it is all about how fast the tire speaks, I guess rF2 tires are usually in the more optimistic side. I think the high performance tire will always inevitably have a tradeoff for how easy and progressive it is, but perhaps in rF2 the tradeoff is different, maybe wear is the tradeoff, not sure how realistic that is. In iR for example progressiveness is really bad, but it is known that very fast laptimes are possible in iR, so it means tires have high peak of performance, it does make sense to me, although there probably isn't such tire made in reality (perhaps yet ?), if they would make such tire irl, then racers would haaaaate it. Personally to me the very early version of ACC tires and overall physics was perfect, although maybe bit edgier than real life still, I suppose not much. But tire transient, phase shifting speeds and less "suggestive" FFFFffffffffffFffffffffFB was a way off version of reality to what typical AC1 user was used to. IDK if ACC devs would have tried to get slowly back to how it was in early release #1 after hotfix, but I doubt it. To make it even worse for the case, Thimm disapproved, and then they also had PR event with Team Grasser drivers who failed a lot to handle the cars with just Engelhart being somehwat in normal control, and it was bad PR. I still ask myself how much of the trouble was it being too difficult, and drivers not being used to it, I think it was combined.

Corner exit->>

This part is a bit confusing. You mention speeds over 150km/h, at such speeds aero becomes rather influential, and it is first order effect, like actual tire, we can think of aero as part of tire speed sensitivity lol, it is the true speed sensitivity of the tires I don't know how ACC works now, but in very first build it was really sensitive to aero balance, and also there was a very real danger to stall aero, by messing up ride heights, I remember constantly being on the edge at fast turns of nurburgring, many didn't like that as it was bit sketchy feeling, not an absolute confidence in a car, it took good skills and little bit of getting used to.

Some interesting observations you have there about which rear tire looses grip first. I would expect it to be different per different cars, as it will have lots to do with aerodynamics, differential, stiffness and stiffness ratio front vs rear of the car, if there is a pattern that repeats in most cars in one sim in one way and in other sim in other way, then that would be slightly strange. It would also help to match the setups as much as the simulators allow, obviously.

The last GT3 car I drove was 2019 AUDI in rF2 justrace.net Hockenheim online race, I remember feeling quite immersed about everyhting, but online everythign seems a lot better and the handling and physics takes less focus, less than chasing pace and just enjoying racing others. But I think it might be one of the more awesome GT3 cars in rF2, although I don't use them a lot. I remember it being quite alive and drivable, perhaps slightly on forgiving side in the qualy which gave me great confidence during the practice, and then for some reason it was like different car in race.

***
"Now, In high speed stuff in RF2 the slides don't seems as related to power as ACC but in RF2 it feels like you are sliding because you are at limits of tyres.
Hence in ACC you can smash the throttle in high speed cornering in RF2 you still have to feed the power to modulate slide.
In my honest opinion i think realistic would be results between these 2,i.e not as planted as ACC but not as sliding as RF2 either.

This is why i think the grip levels on rear tyres should be increased in RF2 tad bit BUT at the same time make it lose in more brutal manner."

-Well if it works like that these days in ACC and rF2. In both situations it is being at the limit of tires, but the difference is that one is drive force longitudinal limits and other is lateral limits of cornering that you mean, and they work in combination which can be described by friction eclipses. As you described in the beginning you perceived that rF2 tires are hard to wheelspin, which then fits the expectation for them to loose friction because of increased cornering force more likely than because of increase of drive force, although they work in combined way.

The very end of your idea I agree with, although it is not a clear suggestion
, but the way I read it, it is fitting my view. You can have a car with great grip, that is also just as fast (and realistically fast) by limiting its sliding performance and increasing non slip performance. Additionaly similar principles are applied to aerodynamics, but it works for heights and angles, and angles has direct realtion to slip. These things will inevitably make handling more brutal, just because the harder they come, the harder they fall... However as a side effect of making cars more brutal and sharper there is a risk to overdo that, and get accused of being iRacing. Or even much before overcooking you can get such accusations by people depending on their driving skill, perception and judgement. Regular simracers feel like they all should be as fast as best drivers of any era, pro drivers would never ever say that they shouldn't be as good as they were in simulation. Just like IRL, it is not unusual that when there is a win - it is a victory of a driver, when there is a loss, it is a loss of a car. Exactly like that in simulation great performance belongs to awesome skills of simracer, bad performance is caused by anything but wrong simulation. Because of that the difficult stuff side has way less support. I very well understand devs frustration of fearing not making people happy as well. It is good, because bias to mentality of everything being very easy will prevent having it harder than it should be. But it is bad because support for critical thinking is lacking, and pushing things to the more difficult side aren't natural for human perception of technology, even if it could mean greater sport and actually being more true. So I wouldn't ever count of anything official in simracing ever getting more difficult or worse in any way, unless there would be some dead accurate undeniable proof that EVERYBODY would understand.

Last edited: Aug 7, 2020
19. ### Slip_AngelRegistered

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I too remember early days of ACC and how edgier the lamborghini felt.
Especially in fast turns.
Now cars in general are more stable in aero corners. Not all though; old and new porsche GT3 are still edgy.
Rf2 is in way better state than i remember, the new ride height sensitivity, onboard dash, new sounds.It is near perfect no joke but tyres are still holding it back.If you ever play ACC then just for the sake of experiment try it cars on Wet tyres.
ACC wet tyres is how RF2 feels.(obviously not exactly same)

I just need to know how real tyres behave on high speed stuff.
But end result is still same though i have never seen soo much sideways motion in GT cars as i see in RF2. what makes it worse is that it way to easy.
In ACC even if the cars are planted you can still lose it on power and when you lose grip it is more snappy.
So in my honest opinion if i have to choose a compromise i will go with ACC.