Grip modifiers. Tire types

[Daniel3D]

OK. After careful consideration (and in junction with my competition proposal) I want to raise a question....
Different type of cars have different tires for the intended surface they will drive on.

Almost all custom cars have unique modifiers for grip and aerodynamics.
But not all have unique surface modifiers. For most road cars it is not really an issue. They should have road tires.
But for a light car with no profile tires, grass can be more slippery than for an off-road car with deep profile tires, for instance.
We can't change the wheels themselves, but we can modify their behavior to simulate tire type.

What triggered me. (in trying to categorize cars by stats) I can't find an off-road or a rally car by just looking at stats.
An off-road car, or a rally car, should have a dirt grip close to asphalt grip. Right?
Or am I misinterpreting the modifiers?

EDITED: the tone of the post and clarified my goal better. (may 9th)

[Overdrijf]

I don't know about better, more "closer to".

I'm going to assume the dirt in Stunts is meant to model the kind of gravel or compacted earth roads that rallies often use. It's not thick mud, or it would have an effect on acceleration and top speed as well as on grip. A rally car would still corner faster on asphalt than on a gravel road, because the car is still going to be slipping about on the loose gravel, with sand and clumps of dirt flying through the air around it. But the tires and suspension have been optimized that the difference is as small as possible. That's what the modifiers do, if the difference between dirt and asphalt is smaller, the difference in performance is smaller (but you know that). The wiki only mentions the LM002 as having its own set of modifiers, I could have sworn the Lancia and Audi had those as well, and from the custom cars there's the Lotus Esprit. The modifiers are actually used sometimes.

Me personally, as a builder, I tend not to touch them mostly because dirt and ice are not that common on tracks, and people have certain learned expectations about the difference in grip they're going to experience when hitting it. If I keep those modifiers the same those instincts will still work. So unless I was making a terrain car I tend to just leave them as they are. (I might have to make an exception if I ever finish the DAF Siluro, which has some rally pedigree.)

Grass is a bit of a special case, because grass is off the track, you're not supposed to be able to drive there. Grass actually does work closer to thick mud, taking the speed out of the car. So changing the parameters for grass is a bit of a question about how good you want the car to be at shortcuts I guess, particularly relevant in powergear cars.

[Daniel3D]

I don't know about better, more "closer to".

I'm going to assume the dirt in Stunts is meant to model the kind of gravel or compacted earth roads that rallies often use. It's not thick mud, or it would have an effect on acceleration and top speed as well as on grip. A rally car would still corner faster on asphalt than on a gravel road, because the car is still going to be slipping about on the loose gravel, with sand and clumps of dirt flying through the air around it. But the tires and suspension have been optimized that the difference is as small as possible. That's what the modifiers do, if the difference between dirt and asphalt is smaller, the difference in performance is smaller (but you know that). The wiki only mentions the LM002 as having its own set of modifiers, I could have sworn the Lancia and Audi had those as well, and from the custom cars there's the Lotus Esprit. The modifiers are actually used sometimes.

You are right about the way the modifier should work. My description was poorly put, but this is what i mean.
Of all the cars that were available at the end of last year, I have a spreadsheet with all parameters (can make a new one with the others but,, meh..)
about The LM002, it has a 4/3 dirt grip like most cars. i has a better than average grass grip though and way better on ice.
(The melange has the same modifiers, but was modeled on it, so that is explainable.)

Only the dodge Challenger  and several of Ryoma's Ferrari's have a lower dirt grip (the Ferrari's also lower grass grip), No car has a better dirt grip.

Side note: The Lancia EVC1 and 2 have no grip restriction whatsoever, full value for all surfaces. But knowing Ryoma his care, I suspect that to be an accident.

EDIT:  after looking some more. i did see some cars have a lower asphalt modifier. Which makes them better on dirt.
namely Zappers F40, Toyota and Lotus. And Ryoma's F40 and GTO Evoluzione which grip modifiers in the res files are (i suspect) based on zappers F40. (They are the same and very different from the norm)
[Edit: tried to clarify my finding without judgement]

[afullo]

As I already said, the Subaru Impreza in Ultimate Stunts behaves almost exactly on every kind of terrain. But this is probably exaggerated on the converse...

[Ryoma]

No my cars are all based on real torque curve not from another car.

[Daniel3D]

No my cars are all based on real torque curve not from another car.

I have no doubt about that. That is clearly visible in the data.
Only the grip modifiers are  the same. Which makes sense because it's a different version of the same car.
I don't know if it's on purpose or that you didn't know about the strange grip modifiers. That are mostly untouched so I can imagine that you didn't look at them.

[Daniel3D]

Alright,. Back on topic.
Most cars have grip modifiers like road=100%, dirt=75%, grass=50% and ice=25%.
I guess that would be normal tires
The LM002 has 100%, 75%, 75% and 62.5%
So that can be considered off-road tires I think

What would be the modifiers for race tires (F1 type no profile) and rally tires?
Race tires 100%, 50% 30% and 15%?
Rally tires 100%, 90% 80% and 75%?
Ice spiked tires 50% 85% 100% 95%? 

A rally car has of course a different base grip compared to an F1.
this is just the modifiers.
These numbers are meant to clarify the difference between the different road surfaces and not the base grip.
Asphalt should in general be on 100% in this sense. (ice tires with steel spikes have terrible grip on asphalt, so that is an exception)

But if a car should have several versions; like a normal version, a race version and a rally version,
then the base grip should also be modified. (for instance, +20% for race and -20% for rally)

I think we can get more out of dirt and ice road this way. And thus new race possibilities.

[Cas]

I think that playing with grip modifiers can yield interesting results and as long as we remain within normal configurations, say, something that we could find on a normal car, it's healthy to play with these values.

[Daniel3D]

I think that playing with grip modifiers can yield interesting results and as long as we remain within normal configurations, say, something that we could find on a normal car, it's healthy to play with these values.

I agree.
But there is a bit of a problem.
I can't find any information about how tires compare to one another or something grip related.
But I also don't know much about the subject. Maybe somebody else can find it.

My proposal above is mostly gut feeling and a starting point.

[Cas]

I really don't know much about cars in general, so about tyres, I'm pretty lost. But it's clear to me that yes, if one could understand the grip modifiers as representing any physical thing in the game, then that's got to be the tyres, since no other thing makes contact with the surface. I'm sure there must be studies and analyses about tyre types.

After a quick search, I've found this document:
https://www.conforg.fr/euronoise2015/proceedings/data/articles/000532.pdf

Not sure how relevant that could be. Also, some home-made tests could be tried to extrapolate the behaviour of different tyre types on different surfaces, I guess.

[Daniel3D]

Well it was not relevant, but did give me more search ideas.
I found TireRack.com.
The have testing data on a lot of tires. I think I will be able to find values that are reasonable.
Since there are an awful lot of tires, each different the values chosen will never be "real" but realistic.

https://www.tirerack.com/tires/tests/chartDisplay.jsp?ttid=276

[Daniel3D]

Alright. I did a 'secret' field test in the CCC.
Of the Lancia's there was a copy made by Ryoma (the original delta with "the real dashboard")
That was named the same as the original car. I renamed that one to LANR.
I changed the following things in relation to the original car.
A bit less grip on normal road, but better on dirt, grass and ice.
Because a rally car has a roll cage and different suspension, i also gave it a bit more drag and a little heavier.
So slower, heavier but with more grip.
Is it realistic, probably not. That was not the point.

The results in the competition are interesting. Some were faster with the LANC, others with LANR.
For some it made little difference.

[Daniel3D]

The changes in the above test were not extremely big. I tried to keep it balanced and that seems to have worked.
I've been thinking a bit more about how to approach this topic. Why grip is done this way, and I think that I get it.

The base grip is not tire related. It is the it is the result of effects not otherwise specified by the game like downforce. The grip modifiers are how effective the grip is transferred to the road.

So a rally version car with additional spoilers can have a higher base grip but because of the open tire structure a lower effectiveness on normal road.
This could cancel each other out so that the rally version is just as fast as its normal road counter part.
It's just optimized for dirt road which modifier can be standard or better.

It makes quite a complex mechanism. It is mostly about what feels right.
But maybe future creators can use this as inspiration.

[Cas]

My understanding is that grip can be seen as analogous to friction in physics. You have two friction coefficients: a static one and a dynamic one. This results in a very simple linear equation that will just switch coefficients when the maxium static force is applied instead of having to deal with an elaborate hybrid formula.

When the force is less than the maximum static force, you have a friction force opposite that cancels out with it. This maximum force is proportional to the modulo of the normal, which is equal to the car weight if the car is standing on the ground, just in the opposite direction. The proportion between them is called "static friction coefficient". This means that two cars with the same tyres (since it's the tyres that make contact with the ground) will be able to maintain grip up to different maximum forces if they have different weights. In other words, the type of tyre is an integral part of the grip if we see it this way.

Now, there are a couple of things here that general classical mechanics are not taking into account. One is that, at sufficiently high speeds, the air becomes significant and the downforce you mention adds to the normal (in the opposite direction), which increases the grip. The other thing is that, if a car is heavier, it not only gravitates more (weighs more), but also has a greater inertia, so it'll be harder to keep it on track on a curve. In other words, the grip will be reduced. This is a centrifugal, ficticious force, so it makes sense that is left out of the general rule, but it's a lot simpler to calculate it as a force anyway.

In short, without an atmosphere and with a car that's going negligibly fast in comparison to it's weight (we're talking spherical cows here), it's all really simple

[Daniel3D]

Thanks for the clarification. I don't know half of the terms you used but yes. That's kind of it.

The proportion between them is called "static friction coefficient"

That was wat i was looking for.

In stunts it's faked a bit i think because weight is a factor in acceleration. But i do not think that it is used for grip, just as downforce, it is not a variable so grip does not increase with speed.

These are the things you do have to consider, nobody wants an other car that handles like the Countach.