Originally posted by Rayj
Cartman. If your runnung rubbers I would say the roll centers are fine. But foams inherently have much more traction than rubber tires. To tune your suspension with rubber tires you need to soften the suspension abit to get the suspension to do even more of the work. Rubber tires also have much larger diameters than the 59-61mm tires that are typcially run on foams. With rubber tires the roll centers of the car already are quite different than the geometry with foams and the standard front roll center settings work and work quite well. With foams the front roll center mounts need to be 1 to 2mm lower on the upper arm. On super high bite tracks and you see the top drivers running the stiffest possible sway bars they can find It's a dead give away. They don't want to sacrifice turn in with too stiff a front spring. So they crank down the upstops to limit front end roll to 1-2mm and crank the sway bar way up. This is on foams. Rubbers are a whole different story. Lap times go down(that's if you compare the same drivers with foam vs rubbers) Rubbers involve a whole different driving style. You can't drive as hard into the corners with them. The tires will disintegrate if you push them like foams. The higher g-forces you get with foams makes you need a stiffer car taking into consideration tire diameter, cornering speeds and current suspension geometry. Just my thoughts.
Roll centeres do not change with tyre size? if you run the correct rubber tyre and insert they do NOT disintigrate when pushed!
Mugen designed the MTX3 for Foam not rubber!! all we do for rubber is lower the rear top link and stand shocks up more!
tyres or tyre sixe DO NOT and CAN NOT alter the roll center the roll center is built into the car so i fail to see your point!!
take a look here and read the roll center bit http://home.tiscali.be/be067749/58/
to be honest i think your barking up the wrong tree, why doesw no one else want this change????? only you!
Predicting how a car will react when forces are applied at the tires is not easy. The force can be absorbed, split, converted into a torque... by all sorts of suspension components. To avoid all of this you can try to find the roll center of your car and try to predict the reaction of the car from there. A roll center is an imaginary point in space, look at it as the virtual hinge your car hinges around when its chassis rolls in a corner. It's as if the suspension components force the chassis to pivot around this point in space.
Let's look at the theory behind it first. The theorem of Kennedy tells us that if three objects are hinged together, there are at most three poles of movement, and they are always collinear, i.e. they are always on one line. To understand what a pole really is, consider the analogy with the poles of the earth: as earth rotates, the poles stay where they are. In other words, the earth rotates around the imaginary axis that connects the two poles. Now this is a 3-dimensional analogy, in the case of the roll center we only need two dimensions at first. So a pole of an object (or a group of objects) is like the center point of a circle it describes.
If we look at the suspension of a typical R/C car, with a lower A-arm and an upper link, we see a bunch of objects that are all hinged together. These objects include the chassis, the upper link, the A-arm, and the hub. For now we consider the hub, the axle and the wheel as one unit. First, let's look at the chassis, the upper link and the hub. They are hinged together, so the theorem of Kennedy applies. The pole of the upper link and the hub is the ball joint that connects them, because they both hinge around it. The pole of the upper link and the chassis is also the ball joint that connects them. So if we now look at the chassis, the upper link and the hub, we have already found two of the three poles, so if there is a third one, it should be on the imaginary line that connects the other two. That line is drawn in red on the next drawing.
The same applies to the bottom half of the suspension system, the pole of the lower A-arm and the hub is the outer hinge pin, the pole of the A-arm and the chassis is the inner hinge pin, so if there is a third pole it should be on the line that connects the other two. That line is also drawn in red . If your car uses ball links instead of hinge pins, the axis through the centers of the two balls makes up a virtual hinge pin.
If the two red lines intersect, the pole of the hub/wheel and the chassis is the intersection point I . Point I is sometimes referred to as 'virtual pivot', or as 'instantaneous center'. This pole can give us information about how the suspension moves.
The distance from point I to the centerline of the tire is sometimes referred to as 'swing axle length' , it's as if the hub/wheel is attached to an imaginary swing axle which hinges around point I. Having that long swing axle would be equivalent to having the double wishbone-type suspension, but the actual construction would be very impractical. Nevertheless it serves as a good simplification. The swing axle length, together with the angle, determine the amount of camber change the wheel will experience during the compression of the suspension. A long swing axle length will cause very little camber change as the suspension is compressed, and a very short one will cause a lot.
If the upper link and the A-arm are perfectly parallel to each other, the two red lines won't intersect, or, in other words, the intersection point I is infinitely far removed from the car. This isn't a problem though: just draw the green line (in the next drawing) parallel to the two red ones.
The two red lines should always intersect on the side of the center of the car, if they intersect on the outside, camber change will be bizarre: it will go from negative to positive back to negative, which is not a good thing for the consistency of the traction.
The wheel and the ground can also move relative to each other; let's assume the wheel can pivot around the point where it touches the ground, which is usually in the middle of the tire carcass. That point is the pole of the tire and the ground. As it is drawn, a problem might arise when the chassis rolls: the tires might also roll, and hence the contact point between the earth and the tire might shift, especially with square-carcass tires that don't flex much.
Now we can apply the theorem of Kennedy again: the ground, the wheel and the chassis are hinged together, we have already found the pole of the wheel and the ground, and the pole of the wheel and the chassis. If the pole of the ground and the chassis exists, it should be somewhere on the line that connects the other two poles, drawn in green in the next drawing.
The same procedure can be followed for the other half of the suspension, as in the picture below. Again a green line will be found the pole of the ground and the chassis should be on. The intersection point of the two green lines is the pole of the ground and the chassis. (Circled in purple)
That point(purple), the pole of the chassis and the ground is also called the roll center of the chassis. It gives us information about how the chassis moves in relation to the ground. Theoretically, the ground could rotate around it while the chassis would sit still, but usually it's the other way around; the chassis rotates around it while the ground sits still.
The roll center is also the only point in space where a force could be applied to the chassis that wouldn't make it roll.
The roll center will move when the suspension is compressed or lifted, that's why it's actually an instantaneous roll center. It moves because the suspension components don't move in perfect circles relative to each other, most of the paths of motion are more random. Luckily every path can be described as an infinite series of infinitely small circle segments. So it doesn't really matter the chassis doesn't roll in a perfect circular motion, just look at it as rolling in a circle around a center point that moves around all the time.
If you want to determine the location of the roll center of your car, you can either 'eyeball' it by imagining the lines and intersection points, or you can get a really big sheet of paper and make a scale drawing of your car's suspension system.
Now that we know where the roll center (RC) is located, let's look at how it influences the handling of the car. Imagine a car, driving in a circle with a constant radius, at a constant speed. An inertial force is pulling the car away from the center point, but because the car is dynamically balanced, there should be a force equal but opposite, pulling the car towards the center point. This force is provided by the adhesion of the tires.
In principle, the inertia force works on all the different masses of the car, in every point, but by determining the center of gravity (CG) it's possible to replace all of the inertia forces by one big force working in the CG. It's as if the total mass of the car is packed into one point in space, the CG. If the CG is determined correctly, both conditions should be perfectly equivalent.
The forces generated by the tires can be combined to one force, working in the car's roll center.
Viewed from the back of the car, it looks like this:
Two equal, but opposite forces, not working in the same point generate a torque equal to the size of the two forces multiplied by the distance between them. So the bigger that distance, the more efficiently a given pair of forces can generate a torque onto the chassis. That distance is called the roll moment. Note that it is always the vertical distance between the CG and the RC, since the forces always work horizontally.
The torque generated by the two forces will make the chassis roll, around the roll center. This rolling motion will continue until the torque generated by the springs is equally big, only opposite. The dampers determine the speed at which this happens. Note that the roll torque is constant, well at least in this example where the turning radius is constant, but the torque supplied by the springs increases as the suspension is compressed. (See chapter 'springs') The difference between the two torque's, the resultant, is what makes the chassis lean. This resultant decreases because the torque supplied by the springs increases. So the speed at which chassis roll takes place always decreases, and it reaches zero when both torque's are equal. So for a given spring stiffness a big roll moment will make the chassis roll very far in the corners, and a small roll moment will make the chassis lean over less.
So at any given time, the size of the roll moment is an indication of the size of the torque that causes the chassis to lean over while cornering.
Now; a different problem arises; the location of the roll center changes when the suspension is compressed or extended, most of the time it moves in the same direction as the chassis, so if the suspension is compressed, the RC drops.
This little animation shows how the height of the RC changes as the suspension is compressed. The height of the CG also changes a little, because the position of all of the unsprung mass changes relative to the chassis changes. So it's really hard to tell if the roll moment actually increases or decreases.
Also, when the car corners, and the chassis leans over, the RC usually moves away from the chassis' centerline.
Most R/C cars allow for the length and position of the upper link to be changed, and thus change the roll characteristics of the car. The following generalizations apply in most cases. An upper link that is parallel to the lower A-arm will make the RC sit very low when the car is at normal ride height, hence the initial body roll when entering a corner will be big. An upper link that is angled down will make the RC sit up higher, making the initial roll moment smaller, which makes that particular end of the car feel very aggressive entering the corner. A very long upper link will make that the roll moment stays more or less the same size when the chassis leans over; that end of the chassis will roll very deeply into the suspension travel. If not a lot of camber is used, this can make the tires slide because of excessive positive camber. A short upper link will make that the roll moment becomes a lot smaller when the chassis leans; the chassis won't roll very far.
Until now, we've ignored the fact that there are two independent suspension systems in a car; there's one in the front and one in the rear. They both have their own roll center. Because the 'chassis' parts of both systems are connected by a rigid structure, the chassis, they will influence each other. Some people tend to forget this when they're making adjustments to their cars; they start adjusting one end without even considering what the other end is doing. Needless to say this can lead to anomalies in the car's handling. Having a very flexible chassis can hide those anomalies somewhat, but it's a far cry from a real solution.
Anyway, the front part of the chassis is forced to hinge on the front RC, and the rear part is forced to hinge on the rear RC. If the chassis is rigid, it will be forced to hinge on the axis that connects both RCs (purple), that axis is called the roll axis. (red)
The position of the roll axis relative to the cars CG tells a lot about the cornering power of the car; it predicts how the car will react when taking a turn. If the roll axis is angled down towards the front, the front will roll deeper into its suspension travel than the rear, giving the car a 'nose down' attitude in the corner. Because the rear roll moment is small relative to the front, the rear won't roll very far; hence the chassis will stay close to ride height. Note that with a car with very little negative suspension travel (droop) the chassis will drop more efficiently when the car leans over. With the nose of the car low and the back up high, a bigger percentage of the cars weight will be supported by the front tires, more tire pressure means more grip, so the car will have a lot of grip in the front, making it oversteer. A roll axis that is angled down towards the rear will promote understeer. Remember that the position of the roll centers is a dynamic condition , so the roll axis can actually tilt when the car goes through bumps or takes a corner, so it's possible for a car to understeer when entering the corner, when chassis roll is less pronounced, and oversteer in the middle of the corner because the front RC has dropped down a lot. This example illustrates how roll center characteristics can be used to tune a car to meet specific handling requests, from either the driver or the track.
In general, you could say that the angle of the upper link relative to the A-arm determines where the roll center is with the chassis in its neutral position, and that the length of the upper link determines how much the height of the RC changes as the chassis rolls. A long, parallel link will locate the RC very low, and it will stay very low as the car corners. Hence, the car (well at least that end of the car) will roll a lot. An upper link that's angled down, and very short will locate the RC very high, and it will stay high as the chassis rolls. So the chassis will roll very little. Alternatively, a short, parallel link will make the car roll a lot at first, but as it rolls, the tendency will diminish. So it will roll very fast at first, but it will stop quickly. And a long link that's angled down will reduce the car's tendency to roll initially, but as the chassis rolls it won't make much of a difference anymore.
In terms of car handling, this means that the end where the link is angled down the most (highest RC) has the most grip initially, when turning in, or exiting the corner, and that the end with the lowest RC when the chassis is rolled will have the most grip in the middle of the corner. So if you need a little more steering in the middle of the corners, lengthen the front upper link a little. (Be sure to adjust camber afterwards) If you'd like more aggressive turn-in, and more low-speed steering, either set the rear upper link at less of an angle, or increase the front link's angle a little.
Now you might ask yourself: what's the best, a high RC or a low one? It all depends on the rest of the car and the track. One thing is for sure: on a bumpy track, the RC is better placed a little higher; it will prevent the car from rolling from side to side a lot as it takes the bumps, and it will also make it possible to use softer springs which allow the tires to stay in contact with the bumpy soil. On smooth tracks, you can use a very low RC, combined with stiff springs, to increase the car's responsiveness and jumping ability. More about this in chapter 6.