rec.autos.simulators

Car physics: wheelrates vs springrates

Ruud van Ga

Car physics: wheelrates vs springrates

by Ruud van Ga » Sat, 14 Sep 2002 21:17:21

Hi all,

I'm getting reports that my Racer sim doesn't deal with low spring
rates nicely (at the suspension).

I use wheel rates to move around installation ratio's. I'm under the
impression that wheel rates are ALWAYS lower than spring rates. But is
this really true?

I have some ideas on this. With low spring rates, might a regular bump
out quickly, or is normally the bump stop on a normal street car never
reached? (say on a Ford Focus with 22500 N/m and 17500 N/m springs
(not wheelrates))

I have a bug I know of where if the suspension hits the bump stop, I
still only take the suspension spring compression (F=k*d) as the
upward forces. It should actually then be the tirerate, which is
transferred more directly to the body.
However, I feel that this situation should normally not arise (hitting
the bump stops) during normal acceleration and braking.

Any ideas?

Ruud van Gaal
Free car sim: http://www.racesimcentral.net/
Pencil art  : http://www.racesimcentral.net/

jason moy

Car physics: wheelrates vs springrates

by jason moy » Sun, 15 Sep 2002 01:27:11



I can't help with your question, but that formula is obcene. =)

Jason

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Ruud van Ga

Car physics: wheelrates vs springrates

by Ruud van Ga » Sun, 15 Sep 2002 02:05:09





>>I have a bug I know of where if the suspension hits the bump stop, I
>>still only take the suspension spring compression (F=k*d)

>I can't help with your question, but that formula is obcene. =)

LMAO!

Hitting the bump stop now sounds *** too. ;-)

Ruud van Gaal
Free car sim: http://www.racesimcentral.net/
Pencil art  : http://www.racesimcentral.net/

Jerry Janss

Car physics: wheelrates vs springrates

by Jerry Janss » Sun, 15 Sep 2002 06:03:58


Hi Ruud

I don't know if I can help you, but my impression is that wheel rates
(don't misstake for tire rate!) in a normal suspension system is
always lower than the spring rate. In certain cases the wheel rate is
equal the spring rate. One such case is when the spring is considered
purely vertical, and is connected directly to the wheel center.

Here's a good link showing how to calculate the wheel rate:
http://www.eibach.com/ERS_Worksheet.htm

J. Todd Wass

Car physics: wheelrates vs springrates

by J. Todd Wass » Sun, 15 Sep 2002 07:22:06

Wheel rate, yes (unless you've got a really bizarre suspension setup).  For it
to be equal to or greater, the spring would have to be on the wrong side of the
wheel.  Or...  If you had a straight axle, the spring rate and wheel rate could
equal each other in vertical movement.

The only time I've ever hit the bumpstops on a real car is while jumping it :-)
 During regular driving, or even racing, you probably will never hit them
unless you've changed the bumpstop.

Well, I guess you're F=k*d ;-)  (Thanks, Jason Moyer) :-P

If you hit the bump stop you're hitting another, much stiffer, *** spring in
series with the suspension spring.  F=k*d then becomes F=k*d^2 ;-)  Kidding
again of course, but you've got another spring rate to add in at that point.

And no, this should never happen during "normal" operation, i.e., tearing
around a race track as hard as you can with a stock car.

If you wanted, you could probably do an impulse collision at the bumpstop
location instead, but I think that would be asking for trouble...  That's not
what really happens anyway so a stiffer spring would be more realistic.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/

My little car sim screenshots:
http://www.racesimcentral.net/

Stefan Larsso

Car physics: wheelrates vs springrates

by Stefan Larsso » Sun, 15 Sep 2002 17:15:05


>>I have a bug I know of where if the suspension hits the bump stop, I
>>still only take the suspension spring compression (F=k*d) as the upward
>>forces. It should actually then be the tirerate, which is transferred
>>more directly to the body. However, I feel that this situation should
>>normally not arise (hitting the bump stops) during normal acceleration
>>and braking.

> Well, I guess you're F=k*d ;-)  (Thanks, Jason Moyer) :-P

> If you hit the bump stop you're hitting another, much stiffer, ***
> spring in series with the suspension spring.  F=k*d then becomes F=k*d^2
> ;-)  Kidding again of course, but you've got another spring rate to add
> in at that point.

The bump spring rates are usually very high which put rather annoying
demands on the numerics. I don't know how the simulation is performed,
but if it is done solving e.g. a state-space using a fixed step
ODE-solver like Runge-Kutta 4th-order, the system will probably get
unstable... Unless the time-stepping is done adaptively (large CPU-load!).

"Cheating" is sometimes the way to go!

/Stefan Larsson

Jonny Hodgso

Car physics: wheelrates vs springrates

by Jonny Hodgso » Mon, 16 Sep 2002 00:02:04


Single-seaters running push- or pull-rod suspension could have
wheel rate higher than spring rate...

Jonny

J. Todd Wass

Car physics: wheelrates vs springrates

by J. Todd Wass » Mon, 16 Sep 2002 14:57:41

Good point, I didn't think of that.

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

My little car sim screenshots:
http://performancesimulations.com/scnshot4.htm

John

Car physics: wheelrates vs springrates

by John » Mon, 16 Sep 2002 15:40:05

goodness
what's on your mind ?




(jason moyer)

> >On Fri, 13 Sep 2002 12:17:21 GMT,



> >>I have a bug I know of where if the suspension hits
the bump stop, I
> >>still only take the suspension spring compression
(F=k*d)

> >I can't help with your question, but that formula is
obcene. =)

> LMAO!

> Hitting the bump stop now sounds *** too. ;-)

> Ruud van Gaal
> Free car sim: http://www.racesimcentral.net/
> Pencil art  : http://www.racesimcentral.net/

Ruud van Ga

Car physics: wheelrates vs springrates

by Ruud van Ga » Tue, 17 Sep 2002 22:09:22


>goodness
>what's on your mind ?

The cinema in my head generally tends to quite interesting. He/she who
invents a device to record brain images to DVD will surely prosper.
;-)

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/

Uwe Schuerkam

Car physics: wheelrates vs springrates

by Uwe Schuerkam » Tue, 17 Sep 2002 23:09:42



>>goodness
>>what's on your mind ?

> The cinema in my head generally tends to quite interesting. He/she who
> invents a device to record brain images to DVD will surely prosper.
> ;-)

Hey Ruud,

just a quick note to let you know your post made it to
a.h.b-o-u fame ;-)

Regards,

uwe

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mail replies to Uwe at schuerkamp dot de ( yahoo address is spambox)
Uwe Schuerkamp //////////////////////////// http://www.schuerkamp.de/
Herford, Germany \\\\\\\\\\\\\\\\\\\\\\\\\\ (52.0N/8.5E)
GPG Fingerprint:  2E 13 20 22 9A 3F 63 7F  67 6F E9 B1 A8 36 A4 61

Ruud van Ga

Car physics: wheelrates vs springrates

by Ruud van Ga » Tue, 17 Sep 2002 23:25:22

On Sat, 14 Sep 2002 10:15:05 +0200, Stefan Larsson



...
>> If you hit the bump stop you're hitting another, much stiffer, ***
>> spring in series with the suspension spring.  F=k*d then becomes F=k*d^2
>> ;-)  Kidding again of course, but you've got another spring rate to add
>> in at that point.

>The bump spring rates are usually very high which put rather annoying
>demands on the numerics. I don't know how the simulation is performed,
>but if it is done solving e.g. a state-space using a fixed step
>ODE-solver like Runge-Kutta 4th-order, the system will probably get
>unstable...

I thought ODE was actually using a modified Euler-type integration,
although I'm not sure what the 'modified' entails. :)

I actually use implicit integrations for the movement of the wheels,
in a fixed timestep (500Hz, can be upped to 1000Hz which is nicer for
F1 cars). The thing is that I have wheels that have mass (it seems
easier for some to see the wheels as having no mass), and for the road
force I use the tire rate (F_road_vertical=tireRate*d where d is the
distance the tire is 'in' the track).

Tire_rates are probably even worse than bump stop rates (hm, or
perhaps just in the same regime). About 10x as high as suspension
rates. Given a F1 or Cart car, normal Euler integration was not really
handy.
With implicit integration this seems to work much more ok (time
reversal).

So in fact I may be able to just include the bump stop rate and do a
combined bumpStopRate and springRate. But as you bump quite
unpredictable anyway, just using the tire rate directly and
hard-locking the wheel position at its outer limit may be good enough.
Once the car really sinks in even then the car chassis bounding box
should deal with the collision. Until I want to surpass Stuntman. ;-)
(nice game, although it's a shame it seems to have digital controls
and some kind of track caching would have been almost vital).

I have an idea also that some cars are given too little space for its
springs to move in (only 5 cm's up & down for example) which can
trigger the problem.

Ruud van Gaal
Free car sim: http://www.racesimcentral.net/
Pencil art  : http://www.racesimcentral.net/

Ruud van Ga

Car physics: wheelrates vs springrates

by Ruud van Ga » Wed, 18 Sep 2002 00:39:38

On Mon, 16 Sep 2002 16:09:42 +0200, Uwe Schuerkamp




>>>goodness
>>>what's on your mind ?

>> The cinema in my head generally tends to quite interesting. He/she who
>> invents a device to record brain images to DVD will surely prosper.
>> ;-)

>Hey Ruud,

>just a quick note to let you know your post made it to
>a.h.b-o-u fame ;-)

My vocabulary unfortunately lacks the full version of that
abbreviation. :) Is it possible to clarify?

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/

Paul Laidla

Car physics: wheelrates vs springrates

by Paul Laidla » Wed, 18 Sep 2002 06:44:50



I wouldn't be suprised if some less sporting road cars do hit the bump stops
when abused, but bump stops tend to be progressive I think so you might
not even notice youve hit them, you wont completely bottom out without a big
jump if that is what you mean.

It should be some inbetween rate depending on the rate of progressiion
of the bump rate, some cars also have rising rate springs.

HTH

    Paul

Dave Pollatse

Car physics: wheelrates vs springrates

by Dave Pollatse » Wed, 18 Sep 2002 12:46:25


I think he meant "ordinary differential equation solver" in the general
sense, not referring to a particular package, so any integrator can be used.
I don't know which modified Euler you're referring to, but one common
variant is ye olde  a' = a + da*dt, v' = v +  a'*dt.
-dave


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