Coroner says: 4WD licence!!

"Resound" wrote in message
...
What is this based on? When a vehicle is cornering all wheels are under
a
radial load. The front wheels initiate the turn but once cornering all
wheels have sideways loads on them roughly proportional to how much
weight
they have on them.
This will give AWD vehicles an advantage as the remaining tangential
loads
can be distributed better.

I've developed vehicle physics simulations for a few video games and we
had
a cool little vector diagram representing grip circles and loads on each
tyre and it was very interesting to watch. I even had it pop into my
head
as
I nearly lost control of a car going from bitumen to gravel around a
blind
corner. It also helps heaps riding the motorbike and mountain bike to
understand/feel the loads and weight transfers.

PS: I also posted this on cyclingforums.com but it didn't seem to
propogate, sorry if you see it twice


Sure they're all under radial load (with respect to the arc the car
describes as it corners) if there's no drive fed to them, which is kinda
my
point. If you feed torque to the wheels, then they have to deal with that
loading as well as the lateral loading. If the wheels we're talking about
are the front ones then because they break away earlier then the vehicle
as
a whole os more prone to entering an understeer condition.

So lets assume you're cornering and 50% of traction is used up from lateral
forces. In a 2WD vehicle you then have up to 50% of either the front or
rear tyres to use for acceleration[1].
An AWD has up to 50% of BOTH the front and rear wheels to use for
acceleration which is a lot more.

[1] of course RWD has the advantage of acceleration weighting the rear
wheels for more grip but it's a delicate balance. Motorcycles and bicycles
have the advantage from high COMs and short wheelbases that the weight
transfer is so great that they can often become AWD vehicles i.e. All wheels
(1) on the ground are powered. A good reason for plenty of wheelie and
stoppie practice on the MTB.

The slip angle is not measured relative to the car. It is the angle
relative to the velocity of the tyre. A turning vehicle does not have to
be
pointing in the direction of it's velocity which is kinda why they're
called
'slip' angles.

If the lateral forces were not balanced to produce no torque[1] around the
up axis of the vehicle it will start spinning very quickly given the
forces
involved. This goes to show that the front and rear slip angles are
equal[2].

Nearly...it shows that the lateral components of the slip angles are equal.
With appropriate modifications as per footnote.


[1] For a vehicle in a constant radius constant speed turn. Obviously a
small amount of torque is needed to change the rate of turn of the
vehicle.
[2] For a vehicle with a centralised centre of mass. For a non
centralised
COM the slip angles will be shifted accordingly.

So lets assume you're cornering and 50% of traction is used up from
lateral
forces.

Which would mean you're cornering about half as fast as is possible in that
vehicle. This is a scenario that tells us nothing about the vehicles
dynamics. Let's crank it up to near the threshold of loss of traction and
continue the exercise.

In a 2WD vehicle you then have up to 50% of either the front or
rear tyres to use for acceleration[1].
An AWD has up to 50% of BOTH the front and rear wheels to use for
acceleration which is a lot more.

In either a FWD or an AWD at best cornering speed, applying drive to the
front wheels will cause loss of traction and the vehicle runs wide. Again if
you're not at or very close to loss of traction, then you're not trying and
the whole exercise is as demonstrative of vehicle dynamics as the average
trip to the supermarket; that is to say not at all. The only solution is to
back off on the acceleration. In a RWD vehicle, the rear wheels will start
to go wide (oversteer) but will still accelerate the vehicle. The front
wheels don't have any extra work to do and continue to track as before.
Potential solutions are either back off as per FWD/AWD scenario or
countersteer and modulate throttle (the elegant solution). Indeed, you can
dial up varying degrees of understeer or oversteer in the RWD depending on
how close you are to loss of traction and how quickly you apply the
throttle. The only way you'll get this sort of performance out of the AWD is
if the torque split is heavily biased rearwards, so as to mimic the
behaviour of the RWD, albeit with the attendant weight penalty of AWD which
is going to badly hurt your cornering ability in any case. Nothing like
carrying an extra couple of hundred kilograms to screw your mid-corner
speed. Yes it is that much, check the weight of something like a Subaru GX
sedan and compare with a comparable 2WD car.


[1] of course RWD has the advantage of acceleration weighting the rear
wheels for more grip but it's a delicate balance. Motorcycles and
bicycles
have the advantage from high COMs and short wheelbases that the weight
transfer is so great that they can often become AWD vehicles i.e. All
wheels
(1) on the ground are powered. A good reason for plenty of wheelie and
stoppie practice on the MTB.

"Resound" wrote in message
...
So lets assume you're cornering and 50% of traction is used up from
lateral
forces.

Which would mean you're cornering about half as fast as is possible in
that
vehicle. This is a scenario that tells us nothing about the vehicles
dynamics. Let's crank it up to near the threshold of loss of traction and
continue the exercise.

The 50% was just an example. Vehicles aren't going to switch between full
cornering and full acceleration in an instant. There will be a significant
amount of time as the cornering forces are reduced and the acceleration
forces are applied. Getting the drive out of corners as early as possible
is a critical part of racing. Having any wheel weighted but not
contributing to acceleration or cornering is a loss.

"TimC" wrote in message
...
On Fri, 20 May 2005 at 07:11 GMT, Graeme (aka Bruce)
was almost, but not quite, entirely unlike tea:
(still trying to find a spud that will roast properly)

Try Pontiac potatoes...good for roasting...not as waxy as the rest of them

A spud behind the wheel of an errant car? Molotov cocktail thrown
into the car will do it.

--
TimC -- http://astronomy.swin.edu.au/staff/tconnors/
Like most computer techie people, I'll happily spend 6 hours trying to
figure
out how to do a 3 hour job in 10 minutes. --Rev. James Cort, ASR