Hello .If im right weight transfer is changing by change of weight center up/down. But what happens if you get weight center below wheel axis. This isnt probably posible with most cars, but will be the effect of weight transfer negative? I mean when you are in corner outside wheels are pulled with less force and inside with bigger force?
I think this will transfer more weight to the inside wheels, but you will still have a force to the outside^^ Could be fun to see how a car would react under these conditions.
Correct me if im wrong - my advanced enlish isnt the best
Been a good few years since I worked any of this out but isn't it only the centre of gravity and the axis of rotation for the car? In most the COG is above the axis of rotation causing it to shft to the side when turning and causing the top of the car to lean to the outside of the corner. If, as on a few vehicles, the COG is below the axis of rotation the COG still moves towards the outside wheels of the car but the car will lean into the corners but from thinking about it there should still be more weight on the ouside wheels in both situations.
This could be totally wrong though as I can't remember the actual maths behind it but just thinking about from what I can remember. I am sure someone will correct me if I am wrong.
Load transfer caused by the momentum of the vehicle as a whole, which would place greater load on the outside wheels.
VS
Load transfer caused by the momentum of the sprung mass as it exerts torque along the longitudinal axis of the vehicle, which would transfer load to the inside wheels.
I think for all practical vehicles, the load will still be greater on the outside wheels.
But in totaly extreme situation that cant happen when weight center is long below car, maybe in asphalt? Im trying to look on things from two extremes.
The reason why im asking is because im RC racer and onroad cars have CG somewhere near wheel axis. I think that weight transfer have to do something with roll center and center of gravity. In my opinion there still should be weight transfer if you get CG somewhere under wheel axis but what happens if you get CG alot down (maybe practicaly not possible)?
It's only the absolute distance / geometry from the surface that matters, but that's creative thinking at least . Just because the value is negative in your example in the OP, doesn't matter.
You are right. So if I want negative weight transfer CG should be below surface. That makes sense. I thought the momentum is from the wheel axis but now got that. Thanks
From extreme example: if you have wheels with high diameter with low wheelbase and CG somewhere in mid of wheel radius down it should be really positive weight transfer. Something you cant earn so easily - the way of thinking. Thanks
It would be on the inside wheels in one situation, A Scandinavian flick on dry road, The ending pendulum effect in weight would transfer a slight bit to the inside wheels, In a soft suspention 4x4, It would be enough to do endless spins because the car would lean down into them, Plus downforce or whatever else can effect you like that.
In my epic real life track experience (2 laps at a small track here, But 15 turns is alot.) The weight was always on the outside tires.
However, If you guys remember "Speed" the movie, Moving everyone on one side transferred more weight to the inside wheels (The bus went on the outside wheels) Very interesting movie, And real life situation.
You could make a car roll into the corner by getting the roll centre above the centre of mass, thus making the anti-roll over 100%. I don't think this would be nice to drive though. You would still get weight transfer on to the outside wheels.
Alternatively, you have a vehicle that runs on suspended rails, and you can just lower the centre of mass below the point at which the forces act on the vehicle (typically where the tyres touch the ground ). This would actually cause weight transfer to go in the opposite direction to normal. This however, clearly requires specialist track to run on.
A 4 wheeled vehicle is not much more than 2 bicycles connected together with some tubing and linkages.
To keep things simple, let's always assume left hand turns.
Ride your bicycle and go through a left hand turn. The only way you can keep from tipping over is to shift your weight to the left. If your weight goes to the right, you won't turn, you will be laying on the ground.
The right side tires of a car has the bulk of it's weight to the left. In a left hand turn, that weight is keeping the right side of the car from tipping over. On the other hand, the left side tires have all the weight to the right. In a left hand turn, the left side of the car is very bad and is trying to tip over, but the fact that it is connected to the right side of the car is physically preventing it from tipping over.
It doesn't matter where the roll center is or where the center of gravity is. Weight will always be shifted to the right in a left hand turn. The center of gravity is only important in relation to where the tires are contacting the road surface. The relationship of the center of gravity to the roll center only affects how the car ends up appearing in relation to the road surface, or how much or how little it leans when cornering.
Think about a go-kart with no suspension. Where is the roll center? What does the driver try to do in a left hand turn? He tries to lean to the left of the left side tires if the rules allow it.
On a bicycle, there is no roll center. Or is there? The roll center is actually at the ground where the tires make contact. The whole bike rolls to the left in a left hand turn. If it doesn't, the rider will be all scraped up.
There is a lot involved here. The above is just something to think about.
I dont think so that weight transfer is positive if CG is below surface. Centrifugal force should be aplied at that point so there should be a moment transfering more force on inside wheels if im right.
You are right. The only way to get load transferred to the inside is having the CoG below road surface.
What Bob is saying is that you could make a normal car "lean" into a corner. But that's just body roll. Load transfer is still going to the outside wheel no matter what.
Even If that would be possible you still do not want "negative" weight transfer to go fast. You want zero weight transfer. That's done by having either the CoG at road surface or infinite track/wheelbase.
mopar, roll center is not the point or axis around which a vehicle rotates. I'm pretty sure a two-wheeled sprung vehicle has front & rear roll centers at a certain height and that they only affect squat/dive behavior. Not lateral forces. Leaning to the inside has the same effect in a car & an unsprung go-kart. The effect is just magnified by the relative weight & movement of the person vs the weight & track of the vehicle.
Actually, the roll center is pretty much where the vehicle rolls
The force that rolls the vehicle is the moment arm between CoG and the roll center. But there are several definitions of a roll center, and one is derived from the geometry of the suspension. It works in some instances, but if we imagine the vehicle tipping on it's side and driving on 2 wheels, it will of course be rolling on the bottom of the tyres like a bike, and not at the geometrical point. I guess that's my little contribution here
That is correct. A bicycle's roll center can only be at the contact point between the tires and the road.
Even if the bicycle or motorcycle has front and rear suspensions, they simply allow the frame to move up and down in relation to the tires, the frame does not roll unless the tires also roll. So, the roll center is at the contact patch.
Calculating the roll center of a 4-wheel vehicle is really only meaningful when the vehicle is at a standstill or moving in a straight line or just at the very instant of beginning to turn into a corner. Beyond that, everything changes unless the car is not allowed to roll, in which case, the roll center is still meaningless. Once the car rolls in a turn, everything changes because the control arms which are used to calculate the roll center are all changing angles. Their mounting points are also moving in relation to the distance from the ground. A completely different calculation is needed when the car is leaning.
And the more the car leans or rolls, the less meaningful the roll center becomes, because just at the point where the inside tires are about to leave the ground (assuming enough grip on the outside tires) the roll center is about to become null or based at the contact point of the outside tires to the road.
The bigger the swaybar is, the sooner the inside tire will be to lifting off the ground. A bigger swaybar causes the inside tire to be less useful and the outside tire to be more useful, at least until the outside tire can no longer handle the load.
Whatever point you are describing is not a roll center. Well it is...right until the moment the car starts to...roll. Kinda ironic. There are two definitions of a roll center. None of them is a point around which a moving vehicle's body rolls.
“Roll Center” is the most misunderstood term in vehicle dynamics.
Myth #4 - The Chassis Rolls about the Roll Axis
The chassis moves in reaction to a lateral force: it does not roll about a point or axis. The movement includes chassis roll as well as vertical movement. The kinematic roll center concept clearly describes the roll yet neglects jacking force, which might be small for symmetric cases where both tires contribute equal lateral force. But for racing cars the majority of the lateral force comes from the outside tire. In some applications the inside tire may even be off the ground.
Dixon writes “ … many authors introduce the roll-axis as an axis about which the vehicle actually rolls during cornering, the roll axis being the line joining the front and rear roll-centres. When a vehicle is actually moving on a road, the concept of a kinematic roll axis is difficult to justify in a precise way, especially for large lateral accelerations. Therefore the idea of the vehicle rolling about such an axis, although useful as a qualitative idea, should be treated rather cautiously, except in the special case of a stationary vehicle subject to loads in the laboratory.”
Honestly the geometrical roll center is useless, but for the purpose of explaining what it is, it isn't wrong to say the vehicle rolls around it. As I said, the forces are calculated as a moment arm with the length of the distance between the RC and the CoG. That is its use. I don't know where the 'myth' is, the angular roll IS calculated with this moment arm. Edit: Also, if you learn about production car engineering you will see that the RC is actually considered a static point even at lateral load.
Which is why I said the roll center is almost meaningless. There is much more to making a car handle than worring about its roll center. Which is why I also pointed out that a bicycle really does not have a roll center, yet you can make a bicycle handle quite well. Weight placement is the key.
Lots of ways to describe this and look at it.
There was a time when I too thought that moving the center of gravity below the "theoretical" roll center and causing the chassis to lean in the direction of the turn would make it handle better because it would add more weight to the inside tires. That was a time when I didn't think correctly, or enough. In a case like that, would the chassis be applying more weight to the inside tires? After all, the springs are being compressed and the spring pressure would put more load on the tires. HOWEVER, I have never seen a car appear like this when cornering! So, is it really possible? And if it does happen, maybe the chassis isn't really compressing the springs. Maybe the tires are trying to lift against the chassis and that's why the springs are compressing?
Picture in your mind, the center of gravity being below a roll center, but still above the point where the outside tires contact the road. That center of gravity is headed in a horizontal direction above the contact point of the outside tires. That force will tend to lift the inside of the chassis no matter where the roll center is located. The combination of the chassis trying to lean to the inside and the force trying to pull the whole chassis to the outside and pivot on the outside tires, maybe the car will just lift a little and remain level. That would become a jacking effect on the right side suspension. Not too desirable.
Oh ok well sorry I just thought I previously read here that the roll center of a bike or a car on two wheels was right under the contact patch. I must have deeply misread.
End of the offtopic.
Many RC cars do have the center of gravity below the middle of the tire and run the front roll center below the ground as well. Weight transfer still works the same way. It's strictly due to lateral acceleration, center of gravity height above the ground, and track width (if we're talking pure cornering). The roll center position just shifts some of the weight transfer directly to the suspension arms so it doesn't "show up in the springs and anti-rollbars," so to speak.
Does a car really roll around its roll center? This one is a little tricky to answer because in a sense it does and in a sense it doesn't.
If you were to draw the suspension geometry at 0 body roll and draw the roll center, it's tempting to solve for the geometry at 5 degrees body roll by first rotating the body some tiny amount around the roll center, say 0.01 degree, and then draw out the new roll centers. Then repeat the process by rotating the car around the new roll center, over and over until you hit 5 degrees of body roll. Even though at one instant in time the car does roll around the roll center, this particular method does not really show how things will move. Once you hit 5 degrees of body roll I'm pretty sure you'll have a different geometry than you will in reality.
What that approach misses is the jacking force effect that someone quoted earlier. You do probably have a rotation precisely around the roll center (roll axis), but you also get a vertical motion of the body in addition to it that changes the results you'd get from a purely kinematic approach. In other words, as the body is rolling the roll centers are not moving the same way a purely kinematic approach would predict. So from that perspective it makes sense to say the car doesn't really roll around the roll centers, even though if you look purely at the rotation separately from the vertical motion, you'll probably find that at any given instant the rotation is right around the roll center. In that case the answer is "yes."
For the non-math/physics head I would be happy saying that the motion does not really go around the roll center and answering "no" to the question. If all the car did was roll around the roll center the center of gravity would always get lower with roll (unless it was Bob's "roll center is higher than the CG and rolls into the corner like a motorcycle" car). The jacking can lift the center of gravity.
The key thought is "at this instant it is rolling around the roll center, but it is also moving vertically which we are ignoring."
. Just because the value is negative in your example in the OP, doesn't matter.
). This would actually cause weight transfer to go in the opposite direction to normal. This however, clearly requires specialist track to run on.
