rec.autos.simulators

by **Wojte** » Thu, 18 Apr 2002 11:20:57

Why do rally drivers drive sideways on gravel or snow? No doubt, on gravel
you get faster laps by driving sideways (e.g. Speedway - motorcycle racing
on gravel ovals). We all know that driving sideways on tarmac is much slower
(but more fun!). There must be a physical explanation for this 'friction
phenomenon'.
I know it's slightly off-topic, but I cannot find an answer to this question
for quite a long time...

by **Sébastien Tixie** » Thu, 18 Apr 2002 16:01:01

Hi,

i try to explain, but i don't know if its true.

Here's the facts :

1)The more load you have on a tire the more grip you have
2)On loose surfaces as snow or dirt, drivers removes the sway bars (
antiroll bars ). to increase lateral load transfert.
3) The tighter the curve is more the driver drive sideway

Here's the explanation:

1) dirvers drive sideway to increase the lateral load transfert.
2) When driving sideways longitudinal forces helps to counter act
the centrifugal force.

well, this is how i explain it, and feel it when driving ;o)

regards,

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

by **remove EATSPAM to repl** » Thu, 18 Apr 2002 16:38:38

As Mr. Tixier said, it has to do with "load transfer".

In a 4WD car, all four wheels generate thrust. The idea is the generate
the
most amount of thrust IN THE DIRECTION OF TRAVEL (which may not
be where the car is POINTING), AND carry enough speed INTO the turn
so you carry more speed OUT of the turn.

On gravel and snow, due to sliding, the direction of travel and
direction
facing would NOT be the same. If you slow to the point where they are
the same, you lose speed. By forcing the tail loose you can carry more
of the velocity INTO and OUT OF the corner.

--KC

by **Gregor Vebl** » Thu, 18 Apr 2002 17:56:25

There are two reasons for this, one of which Sebastien explained already.
The other reason is that loose surfaces act somewhat differently than,
say, tarmac, in that the material gets moved/thrown by the tyre, and this
gives an additional reaction force which makes sideways driving
advantageous. The other reason that Sebastien explained is that in a 4wd
vehicle the tradeoff betwen performance and control is the best ina
sideways situation where you are for the most part quit close to the peak
of performance and can control the behaviour well by the throttle,
preventing understeer which can hamper performance more.

-Gregor

> Why do rally drivers drive sideways on gravel or snow? No doubt, on gravel
> you get faster laps by driving sideways (e.g. Speedway - motorcycle racing
> on gravel ovals). We all know that driving sideways on tarmac is much slower
> (but more fun!). There must be a physical explanation for this 'friction
> phenomenon'.
> I know it's slightly off-topic, but I cannot find an answer to this question
> for quite a long time...

by **Haqsa** » Sat, 20 Apr 2002 09:02:13

I have a different theory. I really doubt that the force from throwing
all that material rearward is very significant in comparison to the
weight of the vehicle. I think what is happening is kind of like
viscous friction. When you spin the wheels on loose ground, you are
shearing off a layer and moving it rearward at the velocity of the
contact patch. At some point lower in the ground, the material is not
moving. Between those two points you have a velocity gradient and loose
material that is behaving like a fluid. Fluid or viscous friction
increases with relative velocity. Spin the wheels faster and you get
more force. Point the car more towards the turn center and the viscous
friction force contributes to the centripetal acceleration of the
vehicle. And this would be a higher force than normal cornering because
you can generate a larger relative velocity difference from a spinning
wheel than from one which is simply sliding sideways across the surface.

> There are two reasons for this, one of which Sebastien explained
already.
> The other reason is that loose surfaces act somewhat differently than,
> say, tarmac, in that the material gets moved/thrown by the tyre, and
this
> gives an additional reaction force which makes sideways driving
> advantageous. The other reason that Sebastien explained is that in a
4wd
> vehicle the tradeoff betwen performance and control is the best ina
> sideways situation where you are for the most part quit close to the
peak
> of performance and can control the behaviour well by the throttle,
> preventing understeer which can hamper performance more.

> -Gregor

> > Why do rally drivers drive sideways on gravel or snow? No doubt, on
gravel
> > you get faster laps by driving sideways (e.g. Speedway - motorcycle
racing
> > on gravel ovals). We all know that driving sideways on tarmac is
much slower
> > (but more fun!). There must be a physical explanation for this
'friction
> > phenomenon'.
> > I know it's slightly off-topic, but I cannot find an answer to this
question
> > for quite a long time...

by **Gregor Vebl** » Sat, 20 Apr 2002 17:49:58

I do believe you are onto something here. Though the physics of such
materials is tricky at best, probably they do behave as fluids to a
certain extent and this contribution is most likely really significant.

Let's see how much the contribution from the material being thrown is;
let's assume that 10kg/s (a high estimate) are being thrown at a velocity
of 10 m/s (36km/h), this only ammounts to a force of 100N which indeed is
almost insignificant as it's probably somewhere around 1% of total grip
(10000N for a 1000kg car cornering at 1g). Indeed there must be other
contributions.

Luckily the resulting empirical equations from both of these contributions
are that the force is proportional to the slip velocity, and since the
proportionality constant is unknown, I believe we have all the info needed
here for anyone attempting a gravel rally sim :).

-Gregor

> I have a different theory. I really doubt that the force from throwing
> all that material rearward is very significant in comparison to the
> weight of the vehicle. I think what is happening is kind of like
> viscous friction. When you spin the wheels on loose ground, you are
> shearing off a layer and moving it rearward at the velocity of the
> contact patch. At some point lower in the ground, the material is not
> moving. Between those two points you have a velocity gradient and loose
> material that is behaving like a fluid. Fluid or viscous friction
> increases with relative velocity. Spin the wheels faster and you get
> more force. Point the car more towards the turn center and the viscous
> friction force contributes to the centripetal acceleration of the
> vehicle. And this would be a higher force than normal cornering because
> you can generate a larger relative velocity difference from a spinning
> wheel than from one which is simply sliding sideways across the surface.

> > There are two reasons for this, one of which Sebastien explained
> already.
> > The other reason is that loose surfaces act somewhat differently than,
> > say, tarmac, in that the material gets moved/thrown by the tyre, and
> this
> > gives an additional reaction force which makes sideways driving
> > advantageous. The other reason that Sebastien explained is that in a
> 4wd
> > vehicle the tradeoff betwen performance and control is the best ina
> > sideways situation where you are for the most part quit close to the
> peak
> > of performance and can control the behaviour well by the throttle,
> > preventing understeer which can hamper performance more.

> > -Gregor

> > > Why do rally drivers drive sideways on gravel or snow? No doubt, on
> gravel
> > > you get faster laps by driving sideways (e.g. Speedway - motorcycle
> racing
> > > on gravel ovals). We all know that driving sideways on tarmac is
> much slower
> > > (but more fun!). There must be a physical explanation for this
> 'friction
> > > phenomenon'.
> > > I know it's slightly off-topic, but I cannot find an answer to this
> question
> > > for quite a long time...

by **Haqsa** » Mon, 22 Apr 2002 00:11:53

One of the reasons I got to thinking along these lines is due to some
conversations I have had with a friend who does a lot of four wheeling.
He doesn't race, but he is in a club that goes on a lot of weekend
trips, and apparently the challenge is to see who can find the deepest
mudhole and still get out of it. ;o) He builds his own Jeeps from the
ground up, and they actually end up a lot like a racing vehicle by the
time he is done with them. Anyway, he had also made the comment to me
about getting thrust from throwing material rearward, although he was
not referring to cornering, he was referring to getting out of a
mudhole. Either way it seems that we are only talking about an
acceleration of around 0.01 G using your numbers, not significant at
all. Another reason I am thinking along these lines, though, is that I
believe he told me once that in really loose material it was actually
better to have narrow tires, so that you would sink into the ground more
instead of floating along over the top. This I think fits with my
theory about getting a behavior from the tires that is like viscous
friction - the farther you sink in, the more surface area of the tire is
in contact with the dirt, and the more total friction force. Now that
would be interesting to put in a sim.

by **Dave Pollatse** » Mon, 22 Apr 2002 07:00:39

I'm guessing the "more load = more grip" thing, if true, is nonlinear; if it
were strictly true, than this would mean that the load sensitivity is
inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55 %
front weight, same tires front and rear, more roll stiffness in front to
make it "safely" understeer) would be oversteer on gravel, while an
oversteer car would become understeer. In my experience, production cars
still understeer in loose surfaces unless the handbrake or some sort of
throttle technique (left-foot brake on FWD, or dipping the clutch on RWD) is
used to degrade rear grip.
Also, I wouldn't expect removing ARBs to change steady-state total lateral
load transfer--isn't that merely a function of lateral force, CG height, and
track width? I would, however, expect removing ARBs to reduce wheel-hop on
bumpy surfaces.
I think the biggest factor is that the friction peak on loose surfaces is at
an extremely wide slip angle, probably due to a quasi-viscous behavior of
dirt building up on the outside edge. I think there's also some threshhold
where for very tight turns, it's faster to rotate the car by sliding than to
actually take the turn "properly"--hence autocrossers use the handbrake even
on asphalt.
Whatever is true, there's no doubt that there isn't enough stuff written
about offroad tire behavior!!!

-Dave P.

by **Sébastien Tixie** » Tue, 23 Apr 2002 17:45:25

On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek"

>I'm guessing the "more load = more grip" thing, if true, is nonlinear; if it
>were strictly true, than this would mean that the load sensitivity is
>inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55 %
>front weight, same tires front and rear, more roll stiffness in front to
>make it "safely" understeer) would be oversteer on gravel, while an
>oversteer car would become understeer.

Well, no. In Asphalt you also have more lateral force with more load.
Just look at the pacejka formula. However that's true that for very
strong load the lateral force start to decrase.

That's true, the car always loose lateral force on the driven wheels
because those wheel tends to slip more.

Rally driver usually removes the ARB on wet tarmac because it's a
really slippy surfaces. Nothing to do with bumps. It's, again, to
increase the lateral load transfert.

I think the optimum slip angle on slippy surfaces is tighter, because
the contact patch on loose surfaces has less friction
so it's deformation is smaller on slippy surface.

IMO, regards,

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

by **Sébastien Tixie** » Tue, 23 Apr 2002 17:49:48

In Rally, on snow, they also use narrow tire, because those tire can
go deep in then snow and reach for the ice / stiffer surface, so the
nails can grip on those surface. Maybe its the same on mud. narrow
tire can reach for the solid ground through the liquid mud ...

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

by **Jonny Hodgso** » Wed, 24 Apr 2002 02:45:47

> On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek"

> >Also, I wouldn't expect removing ARBs to change steady-state total lateral
> >load transfer--isn't that merely a function of lateral force, CG height, and
> >track width? I would, however, expect removing ARBs to reduce wheel-hop on
> >bumpy surfaces.

> Rally driver usually removes the ARB on wet tarmac because it's a
> really slippy surfaces. Nothing to do with bumps. It's, again, to
> increase the lateral load transfert.

Surely the only things affecting total LLT are CG height, track width
and lateral acceleration?

I was under the impression that disconnecting ARBs in low grip is
done to improve suspension compliance, thus making the most of the
grip, and because with less grip available the car doesn't need to
be as stiff in roll to control the sprung mass.

Jonny

by **Haqsa** » Wed, 24 Apr 2002 08:43:25

Lateral weight transfer can be increased slightly by removing bars
because the roll angle will increase, and rally cars have a fairly high
CG, therefore the CG and its acceleration vector are going to point
farther to the outside. But I can't imaging being able to take much
advantage of this without playing hell with the steering geometry. More
importantly I think is that by decreasing roll resistance the peak
normal force experienced during roll will decrease, thereby decreasing
the minimum effective traction coefficient as the car is rolling. In
other words grip during transient maneuvering will improve. Also bars
are just really *** on uneven surfaces. My Jeeping buddy took his off
because it decreased suspension compliance too much. Of course he has a
slightly different requirement than a rally driver - where they see a
large rock as something to avoid, he sees it as something to drive over.
;o) But they still face a similar problem due to the types of roads
they drive on - lots of one-wheel deflections, which are really *** if
you are using ARB's.

by **Haqsa** » Wed, 24 Apr 2002 08:45:19

Now that you mention it I think that is why off-roaders do that.
Sinking deeper doesn't necessarily get you to dry dirt, but it does get
you down to a more hard packed mud.

by **Dave Pollatse** » Wed, 24 Apr 2002 12:05:49

Sorry, I wasn't being very clear... by more grip I meant higher coefficient
of friction, not actual friction force. I guess what it boils down to is
the behavior along an axle (let's assume a single-axle rally car for
convenience on a flat road.)
Let t be the fraction of lateral load transfer, then the normal forces at
the wheels are
mg*(1-t) and mg(1+t). For simplicity, let's normalize everything to mg, so
this becomes
(1-t) and (1+t)
Let the coeff of friction be a linear function of load, f(x) = ax + b, which
will be accurate for these purposes since we're just looking at the behavior
at a given load +/- a small delta.
then the max lateral grip should be:
(1-t)*f(1-t) + (1+t)*f(1+t) =
uhmmmm..... (really, I have a point here, bear with me.... ;)
(1-t)*(a(1-t)+b) + (1+t)*(a(1+t)+b) =
a-at+b - at + at^2 -bt + a + at + b + at + at^2 + bt =
2a + 2b + 2at^2 = 2(a + b + at^2) (whew, stuff cancelled out!)
So this shows that when "a" is negative (which is true of tires on asphalt,
or at least of every tire data I've ever seen), the maximum cornerering
force is at t=0, or no load transfer. This, of course, explains why
increasing front roll stiffness causes understeer and increasing rear roll
stiffness causes oversteer: because the total grip at the stiffer end has
more lateral load transfer, and thus loses more grip.
So if on dirt, "a" would have to be positive to have the effect of lateral
load transfer increasing grip, which would also have the effect of flipping
the understeer/oversteer of a car, which is the behavior I'm having trouble
with imagining or matching up to what I've seen. I think I can imagine this
being true for really wet tarmac, as you say, because increasing the load
would decrease hydroplaning. I suppose the same argument could be made for
a tire on dirt making a "furrow" in the ground, this could cause a "digging
in" effect, which could result in positive "a"...

What we really need is a tire dyno for dirt. I'm thinking of an old school
bus driving down a dirt road with a tire mounted on an arm sticking off the
side bolted to some bathroom scales... throw in some C-clamps and a lot of
duct tape for good measure. I'm still pretty sure that the peak is at a
wide slip angle, although until one of us actually coughs up some real data,
we're just ... (insert colorful aphorism of your choice here)

-Dave P.
PS. I'm actually only *mostly* joking about the bus thing. I just may try
it some day... ;)

> On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek"

> >I'm guessing the "more load = more grip" thing, if true, is nonlinear; if
it
> >were strictly true, than this would mean that the load sensitivity is
> >inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55
%
> >front weight, same tires front and rear, more roll stiffness in front to
> >make it "safely" understeer) would be oversteer on gravel, while an
> >oversteer car would become understeer.

> Well, no. In Asphalt you also have more lateral force with more load.
> Just look at the pacejka formula. However that's true that for very
> strong load the lateral force start to decrase.

> >In my experience, production cars
> >still understeer in loose surfaces unless the handbrake or some sort of
> >throttle technique (left-foot brake on FWD, or dipping the clutch on RWD)
is
> >used to degrade rear grip.

> That's true, the car always loose lateral force on the driven wheels
> because those wheel tends to slip more.

> >Also, I wouldn't expect removing ARBs to change steady-state total
lateral
> >load transfer--isn't that merely a function of lateral force, CG height,
and
> >track width? I would, however, expect removing ARBs to reduce wheel-hop
on
> >bumpy surfaces.

> Rally driver usually removes the ARB on wet tarmac because it's a
> really slippy surfaces. Nothing to do with bumps. It's, again, to
> increase the lateral load transfert.

> >I think the biggest factor is that the friction peak on loose surfaces is
at
> >an extremely wide slip angle, probably due to a quasi-viscous behavior of
> >dirt building up on the outside edge. I think there's also some
threshhold
> >where for very tight turns, it's faster to rotate the car by sliding than
to
> >actually take the turn "properly"--hence autocrossers use the handbrake
even
> >on asphalt.
> >Whatever is true, there's no doubt that there isn't enough stuff written
> >about offroad tire behavior!!!

> I think the optimum slip angle on slippy surfaces is tighter, because
> the contact patch on loose surfaces has less friction
> so it's deformation is smaller on slippy surface.

> IMO, regards,

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

by **Dave Pollatse** » Wed, 24 Apr 2002 12:14:16

Also, wouldn't removing anti-roll bars on a slippy surface also make sense
because on a slippy surface, the lateral forces will be smaller in
magnitude, so the suspension doesn't have to generate as strong of a roll
reaction, and thus compliance could be increased? Sort of the same reason
you move your brake bias rearwards in the wet? (Because less longitudinal
load transfer)