rec.autos.simulators

As I was/am adding a 4WD driveline, I was wondering about deriving
velocity (for the dashboard velocity dial) from the gearbox.

Suppose you have 3 differentials, a viscous one (center) and 2 other
types in front and rear. If the front diff ratio is different from the
rear diff ratio (where ratio is the inherent gearing ratio from ring
gear to side axles), or if the front and rear wheels have different
radii, how would you derive 'velocity' for your dial?

For a single diff, I can solve the relationship between gearbox speed
(=driveshaft speed) and wheels by the formula:

  perceivedVelocity=gearbox->rotVel*diffRatio*wheelRadius

(as rotVel is in radians/sec, the circumference of the wheel,
2*PI*wheelRadius, falls out and you're left with just the above).

The problem seems that for more than 1 diff, if the front and rear
have a different 'diffRatio*wheelRadius' product, then depending on
where the torque flows most, you get a different (inaccurate) velocity
dial.

Does anybody know how this is solved in real cars?
I would perhaps guess that the differential ratios in both front and
rear diffs are the same, but that still doesn't account for
differences in wheel radii.

BTW; A small other question for some clutch analysis. It seems my

and a clutch capable of 650Nm. Would a ratio of about 3:1 when it
comes to engine vs. clutch capacity be a general one? (I'm just using
that to suggest clutch torques to a car builder, and clutches with
very high capacity can make the car drive off a little less easy)

Thanks,

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

In al cars i had the speed was taken in the out of the gearbox. So if
you give full thotlle when the car is in the air the speed does'nt
match with the real one.

Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-44.24   Monster:-124.44

I mean, if the front wheels are half as high as the rears (extreme
example), and you have 3 diffs, what would be factor be to calculate
velocity (in m/s for example) from the gearbox rotation speed?
An average of front+rear?

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

My immediate response is to average, having calculated back
from wheel velocities.

Let's say you have a single diff, driving one large wheel at twice
the size of the smaller one opposite.

Then assuming straight line travel, [damn!  Rice just boiled over]
the diff cage is running at the mean of the two wheel rotvels.
Taking the wheel road speeds as equal and rearranging, I get

roadVel = 2*rotVelDiff * (radiusLarge*radiusSmall) ...
        ... / (radiusLarge+radiusSmall)

You should be able to extend this to include gear ratios in the
"radius", and attach a similar formula between two regular diffs.
You could even have four different sized wheels, but I think the
rearrangement gets a little more, er, interesting!

It shouldn't depend on torque flow since the velocities have a
fixed relationship (until teeth start jumping ;-)  Tyre slip will
affect the answer, but that's a problem real speedometers have too.

I /think/ things are usually arranged to avoid having a speed
difference at the centre diff in normal, straight-line driving
- it saves wear and tear.  Epicyclic diffs can produce a torque
split with equal output speeds.

HTH,
Jonny

I don't see a problem.  you know the ratios and wheel sizes, so just
pick an end and multiply.

--
Gunnar
    #31 SUCKS#015 Tupperware MC#002 DoD#0x1B DoDRT#003 DoD:CT#4,8 Kibo: 2
                 "Det er nok ingen ovn, men fartsm?leren v?r."

one end.

--
Gunnar
    #31 SUCKS#015 Tupperware MC#002 DoD#0x1B DoDRT#003 DoD:CT#4,8 Kibo: 2
                    to err is human -- to forgive is bovine.

Well, I think most 4WDs have the same diff ratios and wheel sizes front and
rear, if they don't, the overall
combination of diff ratio and wheel size will give the same gearing from and
rear. So simply take
the rpm into (or out of) the centre diff, and multiply buy the (same front
an rear overall) ration from there.
I think most speedometers are taking from gearbox output speed, ie before
the centre diff.

If you had a differebt overall ratio front to back this would cause hideous
problems, it would destroy
your transmission and/or tyres in short order.

Of course when you measure speed like this you get the wrong answer with
wheelspin, but this happens with
real vehicles.

Hope this helps.

    Paul

---
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  As the others have said, it seems the combined effective final drive ratio at
each end is usually the same or very close to it (tire radius and gear
together.)  I think I read that Porche used a little different method in order
to move the torque split rearwards though..

  I planned on doing the speedometer by choosing which end the engine was
literally attached to, and using the velocity of that ring gear directly..  Not
sure how it works on a real 4WD car though.  It seems that you have to hook a
speedometer cable to SOME rotating part, so it might as well be the one that
the engine is driving...

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

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

Also.. I'm not sure if that 3:1 ratio sound quite correct...

I just had a look thru AP's product range...  Their 4-plate sintered
Champcar clutch has a torque capacity of 952 Nm...  Now i'm no expert on the
champcar engines - but a 3:1 ration might sound a bit off to me...

Also... are you sure about those figures for you 206?

--
ed_

"Ruud van Gaal" said:

That, to me - suggest a ratio closer to 2:1 for these cars/engines....

--
ed_

In lot of car the speed is taken between the gearbox and the central
differential.
In some motorcycle, speed is taken on the rear brake disc.
Anyway, i don't think some car takes the speed after the diffs.

Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-44.24   Monster:-124.44

In rally cars there's a magnetic captor on 1 wheel. Every WRC codriver
will tell you that on slippy stages the tripmaster give more km than
the real lengh of the stage.

Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-44.24   Monster:-124.44

I.e. the flywheel (and the rotating parts of the engine)
isn't adding as much torque when starting / speed shifting.

      _
Mats Lofkvist

First, thanks to all those that replied!

Oops! :)

...

Hm, yes. Perhaps some iterative algorithm would do better, but I get
the point.

Well, if you have a VC in the middle and it allows some difference in
the frontal and rear feed (which is what a diff does), things can go
out of line a bit (but probably give a lot of frictional problems/wear
though).
The reason I came up with this is someone said a Porsche <x> used a
15/85 central VC diff, so 85% torque goes to the back (under
non-slipping conditions, however they define that). So you quickly get
more velocity at the rear; but that's spinning basically, so I guess
in that case the velocity dial does funny things anyway.

And 85% rear is ok during acceleration I guess, since weight is
shifted there anyway (so it can take more torque).

Thanks,

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

...

I used the actual velocity before; obviously not right in real cars
(although with GPS...). I now take the gearbox, as it's next in my
driveline tree behind the engine, and if you have 3 diffs it's a
little jungle behind that with respect to gearing ratios.
Besides, it seems most cars derive it from the gearbox speed. Also
useful to derive the gear sound (frequency) from.

It seems so. With much clutch torque it's easier to make donuts from a
standstill, with less, you get lots of slipping.
I do notice that compared to other cars, the Peugeot 206's clutch
seems very grippy, so whenever I lift the clutch pedal it has a
noticable grip point which is very hard to get smooth in the lower
gears. So personally I'd prefer indeed less capable clutch.

As my car is not too old, I won't test that. ;-)
However, I think the sound of the grinding would overpower that from
the tire squeak, if you do this in real life.
Too bad free shifters with force feedback don't seem to exist yet. :)

I'll probably propose a 2:1 ratio, which I think is more fluent than
3:1 (in my real-life experience).

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