Re: Basics of a car simulator


Jeff Reid wrote:
> > As I understand it, the bedrock of simulating a car engine is the
> > flywheel.
>
> I'm not sure why you have this impression. The flywheel only
> adds rotational inertia to the engine. The only real effect this
> has is when downshifting and the momentary braking effect it
> causes while rpms are increased.
>
> > At any given time it has a particular RPM and a particular
> > torque. Torque may be derived from RPM and for simple simulation
> > purposes this can be done using a simple graph lookup.
>
> An equation or table look up for torque versus RPM is good enough.
> You can make the simple assumption that response to throttle
> inputs are linear (assume that the car has a real smart ECU).
> Torque peak can range from about 25% to 80% of redline, I suggest
> 60% to 70%. For a racing engine, use a graph from a motorcyle
> dyno chart (you can usually find these in bike magazines), as
> the torque curves are close enough to racing engine (nomrally
> aspirated ones). Scale the rpms for your intended redline.
> If you can find the actual torque curve for the engine you're
> trying to model, use it.
>

Personally, I'd only use a motorcycle curve if I was modelling a
motorcycle, but that's just me :-) There's plenty of engine data out
there for whatever you want to do.



> > RPM is evolved over time as a function of flywheel torque
>
> This is only good for hitting the throttle with the clutch in, or
> in neutral.


Depends how you model the drivetrain. Flywheel torque most certainly
effects every drivetrain part and the tires in my model. As it does in
reality.


Normally you can caclulate rear wheel force (assuming
> rear wheel drive here), from rear wheel torque and diameter,
> then calculate acceleration, and intergete over time to get
> velocity and distance. Use velocity and gearing to determine new
> engine rpm.
>

Yes, but that only works in the absence of wheelspin, really. That's
how my drag racing simulation (main page in my sig) works, but a full
3D sim with proper tire modelling won't work with that approach. Rear
wheel force is actually not a function of engine torque. Instead, a
proper tire model is going to output the force as a function of slip
ratio/load and so on. This force then DETERMINES the "rear wheel
torque," not the other way around.


> > At any instant they have a linear velocity obviously derived from the
> > motion of the main vehicle. Based on the RPM they are being driven at,
> > their radius and the RPM output to them a slip ratio can be computed.
>
> > The "slip ratio" formulation of tyre/surface interaction is an emulation
> > of the real world that seems to be credited to Pacejka. Since
> > calculation requires a divide by linear velocity,
>
> Slip ratio is relative to rear wheel torque and the downforce on the tire,
> not velocity. I'm not sure why so many texts use velocity based equations
> since this is an effect, not a cause.


No, the definition of slip ratio is as described in my first reply and
it's written that way in texts because, well, that's the definition of
it. :-) Torque and downforce are not in the equation. They end up
being whatever they are because of slip ratio, not the other way
around.


The contact patch moves slightly
> slower than the rest of the tire as the tread surface stretches and relaxes
> as it "flows" through the contact patch. The amount of slowing is relatively
> small, and you might want to consider ignoring it.


I recommend NOT ignoring it because that's precisely what causes the
longitudinal force in the first place :-)



You can create a two
> dimensional table, torque and downforce as indexes, containing values for
> slip ratios, and use linear interpolation for values in between.

Again, I recommend using the actual definition of slip ratio. That is
not a function of torque or downforce at all. No such tables needed.


> Another factor is tread squirm, the sidewalls of a tire push inwards on
> the contact patch. You can probably ignore this effect as well.
>

If your model produces lat/long forces as a function of slip
ratio/angle, camber, load, and so forth, then all the physical dynamics
of the tire like tread squirm and so on are already included
adequately.


> > Sideways forces act similarly to the forward relative ones described
> > above, except that RPM isn't a factor - they can be derived purely from
> > the wheel's linear velocity as any sideways motion is pure slip.
>
> Slip angles can occur without slippage. The contact patch is flexible, so
> it's direction is not perpendicular to the tire's axis when there is
> a side load. The smallest maximum slip angles occur on IRL type cars,
> with a working slip angle of around 2%. Modern bias ply tires can go
> over 5%, and the older tires were higher still.
>

Slip angle is just what the term states it is. An "angle." Angles are
expressed generally in degrees or radians, not as percentages.


> > The traction circle limits total traction.
>
> It's usually not a circle, for most tires, forwards/backwards grip
> is a little higher than sideways grip.
>


Yes, this is pretty typical.


> What happens when you exceed the traction circle depends on the tire
> type. For bias ply racing slicks, there is little if any, loss of grip
> while cornering even if optimal slip angle is exceeded. A streat
> oriented radial has significant loss of grip


No data I've ever seen on tires suggests any significant loss of grip
after the peak for either type of tire, except on wet pavement or at
EXTREME slip ratios. 95% of web sites are wrong about the radial tire
force drop off and continue to propogate this myth. The feeling that a
tire breaks away and loses grip is because the force merely stops
rising at some point. I.e., it flattens off more suddenly generally
with a radial than a bias tire. People then interpret this to mean the
grip drops off, and then draw up their diagrams accordingly without
ever viewing any real tire data.

Todd Wasson
Racing and Engine Simulation Software
http://www.PerformanceSimulations.com
http://www.VirtualRC.com

.

Relevant Pages

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