# Re: Basics of a car simulator

> There's plenty of engine data out there for whatever you want to do.

I wasn't sure on this. As posted, if you have the actual engine
data, use it. A lot of race engine manufacturers don't publish
their torque curves, but bike engines have similar torque curves
to racing engines.

>> > 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.

I mistunderstood on this one. I though you meant flywheel inertia,
not engine torque at the flywheel.

> 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.

Agreed. One complication of wheel spinning is that engine acceleration
will depend on inertial momentum of the entire drive train from the
crank to the tires, in addition to the friction losses, and resistance
from the actual torque supplied by the tire.

>> 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.

It's still my belief that the relative rotation rates are the effect, not
the cause of slip ratios. It's a way of defining slip ratio, and can be
used to measure slip ratio, but what's desired is to calculate slip ratio
based on the factors that cause slip ratio. These are longitudinal force,
normal force, and velocity (usually road velocity is used).

A lot of real tire testing varies longitudinal force, normal force, and
road velocity, to create a table of slip ratios. Given a 3 dimensional table
of sample points, the issue with low velocities is eliminated, since this is
just a table lookup with interpolation. Here is a link to one tire testing
program:

http://www.millikenresearch.com/fsaettc.html

I'm not sure where to find real world data for specific tires.

There's also the issue of jerk, or the transitions in force, I don't know
how much tire testing is done regarding jerk.

> 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.

But the tables are more accurate, and don't have a low velocity issue.

>> 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.

Hmm, can't enter degrees as a symbol, sorry. I meant degrees.

> 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.

I'm not sure that this is a myth. Here is a link to a site that claims
that the lateral force curve shape is affected by the normal force on
the tire. At low loads, there is no drop off, but at higher loads there
is somewhat of a drop off. The peak coefficient of friction also lower
at higher normal forces, again according to this web site.

http://www.smithees-racetech.com.au/ackerman.html

.

## Relevant Pages

• Re: Basics of a car simulator
... > adds rotational inertia to the engine. ... Torque may be derived from RPM and for simple simulation ... 3D sim with proper tire modelling won't work with that approach. ... >> their radius and the RPM output to them a slip ratio can be computed. ...
(rec.autos.simulators)
• Re: Engine and reaction torques and inertia
... For the front tires of course you'd just use their ... The net torque, drive - reaction, ... engine determines the rate of angular acceleration. ... but I have a curve relating slip ratio to ...
(rec.autos.simulators)
• Re: Basics of a car simulator
... >>> The slip ratio and the output torque together combine, ... >>> torque scaler, to create a linear force on the tyre and, as a result, ... Tire data is in the form of longitudinal force vs. slip ... like Pacejka's commonly used Magic Tire Model, ...
(rec.autos.simulators)
• Re: Basics of a car simulator
... >>> Slip ratio is relative to rear wheel torque and the downforce on the tire, ... I'm not sure why so many texts use velocity based equations ... Tire models use slip ratio and normal ... the peak in the dry. ...
(rec.autos.simulators)
• Re: Basics of a car simulator
... >> know what the slip ratio is. ... at each end of the car is. ... actual to free rolling velocities at each tire. ... you can add these forces to the car and wheels. ...
(rec.autos.simulators)