I know this isn't a huge issue but it only really came to light after I decided to let off some steam in the Destruction Derby server last night with the RB4 at Blackwood.

You can normally angle the pitch of a 4WD car in flight by changing the throttle position, in simple terms keep it floored to keep the nose high, lift off to make it nose-dive. If any of you have ever jumped a 4WD R/C buggy or truck or anything then it'll be very obvious what I'm on about - you can almost make them do back / front flips depending on what you do with the throttle mid-flight.

Now whatever I tried in the RB4, lifting off, flooring it, it made absolutely no difference. It doesn't really bother me, just something I noticed and wondered if anyone had a different view on it.

Colin McRae explains it, with demonstrations, very well in this video. The explanation is from about 1:05 on, but may as well watch it all.

http://www.youtube.com/watch?v=dVaqcbBWpI4

maybe it's your suspension settings...

post a set.

did you actually look at the video?

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Minimaxman - interesting. I don't know if LFS supports that behaviour. I hear Colin talk about gyroscopic effect. LFS doesn't have that (not in Z anyway) so it may not be possible atm to recreate the same effect with throttle on and off while in the air.

AFAIK, the only effect you have control over that you'll see in mid-air is from the chassis reacting to engine inertia. Rev the engine in place and you'll see which way it goes. I think for all RWD and AWD cars, it's a rolling motion, and for FWD, it's a pitching motion.

Quote from Victor :

Minimaxman - interesting. I don't know if LFS supports that behaviour.

i nothice the same earlier that year following a discussion on brake size and how you need more braking to make wheels lock up at speed due to rotational inertia
what i found was that you can put a bf1 on its back set the gearing up to make the wheels spin fast enough for the speedo to show 999 (iiirc that was the max) accelerate them some more and then brake hard and nothing happens... the car doesnt even wobble the slightest bit

Quote :

I hear Colin talk about gyroscopic effect. LFS doesn't have that (not in Z anyway) so it may not be possible atm to recreate the same effect with throttle on and off while in the air.

gyroscopic effects have nothing to do with the cars pitch changing due to acceleration (although if the cars rotating round an axis other than pitch or you turn the wheel while jumping it does factor in)
its simply the wheels inertia working against any changes in wheel rotation and transfering torques to the cars body

and an example to illustrate how much of an effect this can have with huge wheels
http://www.youtube.com/watch?v ... 0jbs0&feature=related
and a frontflip using the brakes
http://www.youtube.com/watch?v=nvdAGmuRbiA

It's just the car body rotating in response to the torque on the rear tires as the front ones go airborne. If they're accelerating the torque will lift the front end up (just like a motorcycle or dragster) and if they're decelerating the opposite will happen. Also, the wheels are below the center of gravity of the body so that also adds another source of rotation.

Oops, Shotglass beat me to it.

This kinda reminds me of Trackmania.
Still I doubt that this works on cars as big as a RB4 (I didn't actually watch the video, so there could be a big chance that Im wrong), but it would be still cool if it works in rl.

The biggest effect on the flight of a jump is based on what the throttle/brakes are doing just before takeoff and how they affect the weight transfer.

If you accelerate just before take-off, the nose tends to rise. Brake and it'll dip (like in the OP's video). A constant speed or slight acceleration will be more level. How much each of these will actually affect the car depends on the particular ramp, weight distribution and aerodynamics.

Sadly, it's very hard to control jumps off the autocross ramps in LFS as they have next to zero friction. You do have some control on the dirt jumps however.


Throttle changes in-flight will make next to no difference in the kind of cars in lfs with the kinds of speeds possible. May be possible to test it using big wheels in tweak, I dunno.

If you tweak the engine size to be a bit larger than normal it'll make the car spin around the engine when you rev it. Not sure about the tires though.

Quote from Rotareneg :

It's just the car body rotating in response to the torque on the rear tires as the front ones go airborne. If they're accelerating the torque will lift the front end up (just like a motorcycle or dragster) and if they're decelerating the opposite will happen. Also, the wheels are below the center of gravity of the body so that also adds another source of rotation.

It's not *quite* that, although that will happen too. If you look at the R/C car in the second vid posted above, it takes off, and then a fraction of a second *after* it's left the ramp it begins to rotate, due to the driver (or whatever you call whoever's in charge of an R/C car ) hitting the brakes and dumping the rotational inertia of the wheels into the whole body of the car.

O/T: Sorry just realised I posted this in the wrong section, was meant to be Improvement Suggestions.

I have done R/C racing and once the car is in the air, you can make it pitch any way you want once up in the air. You can start off the jump with the car pitching for a back flip then nail the brakes and do a front flip and still level it out for a perfect landing (sometimes). I ruined a couple bodies practicing that.

This does work with real cars, just nowhere near as responsive. I tried it in LFS and nothing happens at all. there should at least be a little control.

This is an important effect for bikes when jumping in motocross. LFS already adds the engine torque to the chassis, am I correct in thinking it just needs to add the wheel torque to the chassis also?

I imagine it would affect more than just jumping. Even if you're not in the air it's going to affect the load balance of the tyres as the chassis will be trying to pitch up or down, whether or not the rotation is prevented from occurring by the car being in contact with the road.
I'm just guessing here, but if it's enough to rotate a car in mid-air it sounds like it would have a noticable (albeit small) affect on grip as you accelerate or brake.

The speed of the wheels doesn't change as fast when the car is on the ground, so I don't think it would be all that significant.

Ah yes, that's a point.

What about while wheelspinning? I suppose the car isn't really under precise enough control then to feel the effects.

Anyway, implement it for when the car is airbourne and it'll work when it's on the ground too if all that's happening is momentum is being applied to the chassis.

Quote from Bob Smith :

am I correct in thinking it just needs to add the wheel torque to the chassis also?

angular momentum actually but yeah essentially its just that (maybe via rotational energy and some factor to capture eg brake losses)

so the wheel has a rotational energy of

you step on the brakes to slow the wheel down and pitch the car forwars so the wheel now has

of rotational energy
which dumps

of rotational energy into the car

this forum really needs a latex interpreter

Quote from Forbin :

The speed of the wheels doesn't change as fast when the car is on the ground, so I don't think it would be all that significant.

Actually from my experience it is, if you are doing a full multibody sim. AFAIK LFS uses a similar approach to RBR in that the wheels are not modeled as real (full) bodies. That said, RBR dips when applying the brakes in flight..

..When I changed my own model to use general multibody dynamics I first "forgot" to add a "retorque" from the brakes to the chassis.. The effect was very CMR-style handling.. Hardly any pitch from throttle or brake input and way too easy turning. Before that I had a relatively similar approach to LFS's older model that didn't take suspension geometry into account. Then, IIRC, the handling was similar to LFS in that I had correct pitching on the ground but not airborne.. The reason for the difference was that the actual forces between the tire and the road were transferred directly to the chassis (incorrect, but not by a huge factor) and when the car was airborne there were no forces between the tire and the ground to be transferred.. That's how I remember anyway..

Would I be on the right track with this: www.vehicle-analyser.com/files/conservation_of_ang_mom.xls ??

And if that covers during flight, what about when on the ground? As momentum is not conserved during braking if you have grip, but clearly if you're on ice then this should still apply. So how to calculate how much still applies when you have plenty of grip?

Quote from Bob Smith :

Would I be on the right track with this: www.vehicle-analyser.com/files/conservation_of_ang_mom.xls ??

looks alright to me (from looking at it for a minute or 2)

Quote :

As momentum is not conserved during braking if you have grip

momentum is always conserved

Quote :

So how to calculate how much still applies when you have plenty of grip?

exactly the same as in flight? the only real difference that i can see is that the effect will be a lot less significant with the wheel changing speed a lot less suddenly
also most of that momentum passed onto the body will be used up trying to rotate the earth under the car

Quote from Shotglass :

exactly the same as in flight? the only real difference that i can see is that the effect will be a lot less significant with the wheel changing speed a lot less suddenly
also most of that momentum passed onto the body will be used up trying to rotate the earth under the car

I agree, the forces transferred are the same. Locking a wheel on the ground with a brake still transfers all of that inertia or momentum to the car, nothing else. The effect in flight is small, it would be completely unnoticeable on the ground, particularly in the normal braking.

I'm not sure about your last point. Yes the tyres slow the car, and they are on the world, but the world is not attached to the car at the hubs of each wheel.
Like a helicopter on the ground as its rotorblades are accelerated, the same twisting force is exerted to the helicopter itself as if it were in the air. What stops the helicopter from turning is irrelevant, be it friction between wheels and helipad or its tail rotor, the force is still the same.

The road will mean you can't see it happening as tyres and suspension compress and uncompress and the force is relatively so small, but the car will try to rotate around those pivots just the same. Or am I off the mark?

Anyhoo, these are the threads I like, I always learn something

Is a car on the ground a closed system? When we say angular momentum must be conserved, aren't we really just saying that energy must not be lost, only converted, but in the case of brakes we are heating the discs and pads, so isn't the anti-torque reduce by these "losses"?

Maybe the effect is greater on the ground then. Since the car's momentum is continually transferred to the wheel through the brakes, as you brake, affecting the rate of deceleration and the braking force required to decelerate it (compared to a free-spinning airborne wheel).

I find it's easier to understand if you take it to an extreme. Imagine a very heavy and powerful car with 2 insanely heavy driving wheels at the front, which weigh about what the car itself weighs. In the air, when the driver hits the throttle, the car will rotate in the opposite direction, not as quickly as the wheels spin because of distance from the axis, but still a lot.

So on the ground the driver does the same thing, the power exerted on the wheels is the same, but their acceleration is slowed because there is grip from the tyre and the weight of the car must be moved. Since the wheels now do not spin freely, is not a much larger amount of that torque transferred back to the car than if the wheels were airborne and spinning freely?

So when braking, if the tyre has grip (ignoring pitching caused by deceleration itself of course) then the rate at which the wheels' speed of rotation is slowed is being reduced, but the braking force exerted onto it is just the same. Imagine the forces of a car travelling at 100mph transferred to a magically airborne stationary car's wheels (as if it were on a real road mimicing treadmill of some kind), next to a magically airborne car with its wheels simply free spinning at 100mph. Both cars brake with identical braking force. Since the car with its wheels freely spinning has just the wheels to stop, then it cannot exert a great deal of braking force before they do stop. The other car on the treadmill of mysticism has a much larger force to stop and with the same braking force it will take a lot longer. Surely that means the latter car will counter-rotate more than the car with free-spinning wheels, regardless of energy lost to heat/noise etc?

I think the defining factor is perhaps not the weight of the wheel or the rate at which it can be sped up or slowed, but force that can be continually exterted on to it, be it acceleration, or deceleration without the wheel stopping.

Ramble ramble ramble drivel drivel drivel.

Quote from sinbad :

[braking/engine torque is the primary factor]

It's not. The car pitches in mid-air from the momentum of the spinning wheels exerting a torque on the chassis. You don't need very much braking torque at all to have that effect. More torque would just stop the wheels sooner and result in more of a force spike, rather than a gradual force with less torque. The overall energy is the same.

With the car on the ground, braking torque is resisted by the ground and the car body, with the car pitching primarily a function of CoG position, mass, deceleration, and suspension geometry (including spring and damper rates). Likewise, even if you could accelerate/decelerate the wheels at the same rate on the ground as in the air, the effect is so small the ground and springs easily overpower it.

Imagine a dragster accelerating hard from a standing start, but on a frictionless surface. The nose will still try to pitch up (to the point of needing the wheelie-bars if the wheels are heavy enough and the force overcomes gravity, but you'd need comically big wheels to acheive this) as the wheels spin up due to the inertia of the big wheels, even though there is no grip.

It's the same effect on a normal car, but greatly reduced due to the smaller wheels, friction with the ground and much less torque.

Someone needs to work out some figures (haha, not me ), to see how much force this effect would have with a 'normal' car.