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bmarci

Member Since 05 Jul 2002
Offline Last Active Today, 05:26 AM

Posts I've Made

In Topic: Pacejka MF - deriving peak slip

24 May 2016 - 08:07 AM

you can't do that, or you can but no use. Observe that the curve falls back after the peak, so at least two slip value belong to the same F.

the best method is (I used it too) what Edy mentioned.

It becomes tricky if you use load sensitivity, because the curve changes shape.

I came up with a simple solution:
I sampled the max F's and corresponding slips at every 100N of Fz. That means 100slip values for the 0..10000N load.
In the sim I just interpollated between them and it gave a very good approximation.

For lateral slip it's more fun because the camber also affects the max Fy, but if you think of the bilinear filtering, yo'll be fine :)

In Topic: Engine RPM and wheel angular velocity

21 April 2016 - 11:08 PM

you will probably not find any of those data.
you'd better start with a free/open source sim, like racer and check the available car datas. you'll find a pile of cars for racer and for rfactor, both of them have car setups in text files. very good starting point, and just experiment, experiment, experiment! tweak those params in your sim as long as you don't have the result you want.

In Topic: Engine RPM and wheel angular velocity

21 April 2016 - 01:44 AM

I start to understand some of the things, but it's still difficult for me to compile data from text. I'd be really thankful for some sort of chart or something that gives a nice overview of this topic. Huge thanks already for the help so far! ;)


unfortunatelly some of these things are beyond math or physics. you have to learn how things work and figure out some logic that mimics the real thing. you can write accurate simulation, but many times it's just overkill and no visible difference anyway.

apart from some basic math there are no universal equations that fit all sims. this is why it's difficult, and this is why we like it :D

you may already found this site: http://www.racer.nl

lots of technical info and the source of an early version is still available.

you may also want to check out the rec.autos.simulators google group. around 2000-2003 search for [car physics]
you'll find posts from people who later made test drive unlimited, assetto corsa and such games :)

In Topic: Engine RPM and wheel angular velocity

20 April 2016 - 02:50 PM

Hmm, what happens with angular velocities in 3rd case? Do you only adjust them by local torques and torques transferred by the clutch? I just don't see how they would sync before clutch gets 100% pressed. If they are not getting synced then at the moment of clutch being at 100% you might get sudden change in velocity on the engine or on the gearbox, depends what you choose as a "reference".
I thought about solving it as a friction problem. Generating torque that comes from the friction of the clutch, using relative angular velocity as a "scaling parameter" and exponentially raising spring force as a load on the plate.

 

Actually they can sync when the clutch is not 100% locked. And in extreme cases it can slip when you don't even touch the pedal, but usually this doesn't happen :)

In the 3rd case when the clutch is not locked there are two possible cases. The torque is (1) limited to the amount that the clutch can transfer  or (2) just apply the current clutch torque. For some mystical reason I did the second one. I just found the first one commented out in the code, don't ask why :D

 

for example:

the maximum clutch torque is 400Nm

the engine is producing 200Nm and the pedal is at 25%, so the current clutch torque is 100Nm in case of linear clutch.

In this case the actual torque that goes to the gearbox and accelerates the wheels is 100Nm

In an other example the pedal is not pressed and the clutch is at 100% the max torque is 400Nm

In this case I add all the 400Nm to the gearbox, even if the engine produces only 200Nm

 

First this seems odd, but this only happens for only one iteration and the clutch gets locked instantly and the angular velocities are fixed together.

Handling the clutch lock/unlock scenario is very important in this case. The slipage of the clutch is not related to the pedal position but the torque that goes through it.

So with little amount of throttle the clutch can handle all the torques from the engine even if the pedal is only half way down.

And as you start pressing the pedal so decreasing the clutch torque it only gets unlocked when the transferred torque exceeds it (in any direction).

 

Detecting lock is simple, just check if the angular velocity difference changes sign :)

simplier: when the wheel starts turning faster that the engine (after applying the torque) - or slower in case of engine braking.

 

 

 

The friction model also seems to be reasonable, but harder to find that "scaling parameter", and without proper damping the wheels will always over accelerate and will never sync just oscillate back and forth like a suspension without damper.

Also the clucth will never lock instantly and you'll probably have a nice lag.

I never tried this one so I'm not sure ;)


In Topic: How Does Traction Control Work?

04 April 2016 - 05:14 PM

I want to find out how does the traction control system in car's work. I'd be really grateful if somebody explained it + added some equations :)

I think the traction control has nothing to do with physics and there is no equation, it's just a "programmed" logic that handles the "too-much-slip" situation.

As said above applying brake and cut-off throttle are both used.

I've been only using the last one so far and quite happy with that. The main difference is when you are applying brake, different torques can apply on each wheel, while when cutting off throttle will affect both wheels.

 

 


OK! So that's what actually allows you to have realistic simulation on keyboard without a car that slips like crazy and can't start movement due to slip ratio around 1000000? If so, than this is good thing. I kind of think that it shouldn't be that hard to simulate.

That's something different. You should be able to drive with keys without traction control.

Seems more like low-speed-slip situation, when you divide with ground speed in your slip ratio calculation. Try capping the value between -1..1 or -1..10 maybe :)


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