rec.autos.simulators

1)what is the formula for rolling resistance? i think it woul dbe something
to do with (for each wheel): the weight on the wheel, the contact area on
the ground (flat tyre etc), the speed, and a coefficient. does anybody know
the real formula?

2) when accelerating, how does the wheelspin affect the acceleration? i
thought the tyre might push with it's maximum ability, and the rest of the
torque would go into spinning the wheels, however, this is wrong - you can
spin the wheels on the spot with enough power, or crappy tyres.

3) is air pressure = 1.3086 kg/metre cubed?

(0.7809x28.0)+(0.2095*32.0)+(0.0093*79.9)
------------------------------------------------   =
                            22.4litres

0.02931227kg / 0.022400metres cubed

is this correct?

thanks for any help here.
Nick.

cheers, Nick.

I'll comment where I can:

Does Pacejka mean anything to you?  If not, go looking...

Atmospheric air pressure?  That's ~101.3 kPa AFAIK... or are you
talking about something else?  (kPa = thousand newtons per *square*
metre)

Jonny

  You want the formula that derives the rolling resistance?  Or do you want the
formula that gives the force *from* rolling resistance?  I believe that rolling
resistance coefficient can be used to get the force that slows the car by:

  Rolling resistance coefficient = Roll_Res_Coeff
  Vertical force = Load
  Force that slows the car = Force

  Force = Load * Roll_Res_Coef

  I'm not 100% sure on that, but that's what it looks like the power equation
would brake down to.  

  >2) when accelerating, how does the wheelspin affect the acceleration? i

  First, how are you getting longitudinal force at the tires (the forward
force)?  

  >3) is air pressure = 1.3086 kg/metre cubed?

  Air pressure is not a "meters cubed" thing..  Perhaps you mean air density?  

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

The rolling resistance is almost independant of speed. Furthermore, I guess
it will mainly depend on how round your tyre is. And this depends on the
load on the tyre and the tyre pressure.
For a typical saloon car, the total roling resistance is in the order of 200
N and slightly DEcreasing as a function of the velocity (40 km/h: 220 N, 120
km/h: 190 N)

Their is a maximum friction between tyre and road, when the contact patch is
not movint relatively to the road (which is the case if the vehicle is
moving without wheelspin).
When wheelspin starts, the friction force will be much less which gives more
wheelspin etc.
athmospheric pressure is 1013 hPa, about 10000 kg per square metre

furthermore: you could use the gas-laws (Gay-Lussac, Boyle etc).

if the absolute (Kelvin) temperature increases, the pressure increases
proportionally.
if the amount of air (moles) increases, the pressure increases
proportionally
if the volume increases, the pressure DEcreases proprortionally.

greetings,
hgrm

That depends on the transmission. Some differentials change the
torque-balance if one wheels start to spin.
Besides, in 4WD cars, the torque is not necessairily splitted 50/50 between
front and rear.

greetings,
hgrm

Same torque to all. I had the same thought when applying suspension
spring force; half up and half down? But it's just the complete force
every which way. It's up to the tires whether they respond to the
torque.

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

Marco's Monster page uses a formula that's based on velocity, but John
Dixon's book 'Suspension, Tires and Handling' uses just
F_rr=coeff*load. It seems mostly velocity independent from most of
what I heard.

Hm, when spinning the wheels enormously (like what Valentino Rossi
does after a win, keeping the front wheel locked and the rear
spinning), the friction goes down quite a lot, probably also due to
heat (it's melting/burning, from the looks of all that smoke).
With wheelspin the force just goes down. According to RCVD, just to
about 60% with reasonably severe spin (SR=1, so wheel rotating twice
as fast as the road below it).

That's air density, I use 1.225, a standard value but a little high
perhaps for high-heat tracks in Spain for example.
Check out http://www.windpower.dk/stat/unitsw.htm
Lots of air densities there.

Cheers,

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

thats what you get from typing so damn quickly coz of the phone bill (in the
UK)

Nick.

what i want is to be able to lift of the throttle, drop the revs and slow
the car down using engine braking and whatever. i wrote a side-program that
used the engine power output (in kW) and the power required to drive the
wheels to see if that was how it works, but i cant figure out a method that
works correctly (or as good as).

cheers, Nick.

Jonny

What are you reading with?  Outlook Express lets you compose
off-line... and ntl give you unlimited, 24-hour 'net access through a
dialer box on a BT line (which is what I'm using).

Jonny

"Modelling of Rolling Resistance Test Data,"
Parmeet S. Grover, SAE980251. SAE, 1998.

You could probably still download this paper online for
a small fee from www.sae.org.

Here is one model discussed in the paper that has/had a
particular use for comparing various tests:

RR = A0 + A1 L + A2 V + A3 (L^2 / P) + A4 L V + A5 V^2

where ^2 is a squaring function, L = vertical load,
P is pressure, V is speed. A0..5 are coeffficients from test data,
and RR is the rolling resistance. See paper for units, etc.

What you will find is that there are a lot of models
from a lot of tests for a lot of tires and no obvious
universal model that meets all needs.

- Matt

--
Matthew V. Jessick         Motorsims

Vehicle Dynamics Engineer  (972)910-8866 Ext.125, Fax: (972)910-8216

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/