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roll centers on the RC8T front and rear end

Discussion in 'RC8T / RC8T3' started by RC8TE man, Jun 13, 2012.

  1. RC8TE man

    RC8TE man Well-Known Member

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    Have you guys ever tryed diffrent roll centers on the front and rear shock tower other than the stock setting ? And for what conditions would you try a diffrent roll center ?
     
  2. AERacer

    AERacer Administrator Staff Member Moderator Factory Member

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    Here's the most complete information on roll center I can find

    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.
    [​IMG][​IMG]

    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.
    [​IMG]
    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.
    [​IMG]
    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)
    [​IMG]
    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.
    [​IMG]
    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:
    [​IMG]
    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.
    [​IMG]
    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.
    [​IMG]
    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.
    [​IMG] [​IMG]
    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)
    [​IMG]
    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.
     
  3. Grant

    Grant Well-Known Member

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    I honor your info but you lost me at "virtual pivot". I'll have to study this a little bit but it is making a lot more sense to me Thanks
     
  4. Martijn

    Martijn Well-Known Member

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    I found some info once and wasen't that long..
    Will try to find.it ;)
     
  5. AERacer

    AERacer Administrator Staff Member Moderator Factory Member

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    How To Tune With Camber Links http://www.rccaraction-digital.com/rccaraction/201106/?pg=98#pg96
    “It’s all about positioning”

    Camber Link Length:
    Link length is adjusted by altering the mounting positions on the shock tower and/or hub of the vehicle. This adjustment is not used to alter the tire’s camber setting! Instead, once you’ve settled on a camber setting (-2 degrees, for example), the link will be readjusted to maintain -2 degrees of camber after the position change.

    In general, a short camber link increases camber gain (the amount of camber the tire experiences through suspension compression), and produce more vehicle rotation entering a turn and more traction coming out of the turn. As the vehicle’s weight transfers and the suspension compresses in a corner, the increased camber angle of the tire will increase lateral thrust generated by holding more camber in the heavily loaded outside tire. The tire will have less rubber on the road and more cornering capacity when you add camber gain.

    The opposite is true for a longer camber link. Lengthen the camber link by moving to the outer hole on the hub or inner hole on the shock tower, and this will decrease camber gain, which can make the car feel “lazier” and less reactive, while giving the car a more stable feel. Longer camber links are sometimes used on high-traction tracks to prevent traction rolling.

    Formula: Camber link --
    Short camber link =
    + Camber gain /---\ =
    More vehicle rotation entering turns // More traction out of turn.

    Long camber link =
    - Camber gain \---/ =
    “Lazier” Feeling – More stability


    ***********
    Turnbuckle-type camber links allow you to adjust the length of the camber link while it’s still installed on the vehicle. This is done by a standard thread on one end and a reverse thread on the other. A small hex in the middle of the link makes it easy to spin the link with a wrench. A small grove on one side of the hex tells you what side the reverse thread is on.

    ***********
    Changing the length of the link will change camber gain when the suspension moves up and down. Extra holes in the rear hubs and some shock towers give you different camber length options.


    Camber Link Height:
    Altering the camber link’s height (position vertically on the tower or occasionally on the hub) changes the vehicle’s roll center. This adjustment is most often tuned on the rear of the vehicle. Technically speaking, roll center is defined by the SAE as “the point in the transverse vertical plane though any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll.” ---- In other words: Think of roll center more simply as the point around which the vehicle’s chassis rolls in a corner.

    So how do we apply roll center to vehicle tuning? All things being equal, when you move the camber link up the tower, the roll center is moved lower on the vehicle. When the link is moved down on the tower, the roll center is raised. In general, a high roll center (lower on the tower) is better for slippery or bumpy tracks because when you move the roll center really far from the ground level in either direction you introduce jacking, which messes with the ride height of the car, and you have a track width change that can either help or hurt your cornering performance.

    For smoother high speed tracks, a low roll center helps decrease roll, and decrease weight transfer from left to right in a left hand turn and reduces the “tippy” roll-over feeling a car may get in high speed corners on a high traction track. Steering into the corner is increased as the car will “bite” more going in, but the vehicle will feel more stable coming out of the corner.

    You can tune your over/understeer characteristic with roll center. Raising the roll center on the front or rear will make that end wash out first. So raising the rear will wash out the rear and make the car looser. Raising the front roll center will wash out the front first and make the car push.

    Formula: Roll Center –

    High Roll Center: (front) >>>>>>>>> Makes front wash out causing “push”.
    High Roll Center (rear) >>>>>>>>> Makes the car looser washing out the rear.
    Camber link low on tower =
    Roll center is raised =
    For bumpier / slippery tracks

    Low Roll Center: >>>>> Increases steering into corner
    Camber link high on tower = >>>>> Increases stability coming out of corner
    Roll center lowered = >>>>> Reduces roll over feeling
    Smoother / high speed tracks

    So basically with Roll Center you’re either looking for: (in the 2 extremes) ---
    1. Stability – (Low Roll Center)
    2. Responsiveness – (High Roll Center)

    And with patience, trial & error based on your track you will find your sweet spot between these two by adjusting your camber link higher or lower on your shock towers.

    Reference:

    “How To Tune With Camber Links” by Stephen Bess; RC Car Action June 2011: (Pg. 97-99)
     
  6. Grant

    Grant Well-Known Member

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    That would be great I really appreciate it!
     
  7. Martijn

    Martijn Well-Known Member

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    Oke I found it only thing is I have it printed on paper.
    If it's oke, I'll take a picture with my phone or try to scan it
     
  8. AERacer

    AERacer Administrator Staff Member Moderator Factory Member

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    Check what I've posted few posts above, it is well explained.
     
  9. Martijn

    Martijn Well-Known Member

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    It's pretty long, mine is a bit shorter.. And it's almost the same info..
     
  10. Martijn

    Martijn Well-Known Member

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    Oke, found the digital doc on my external hard drive :D

    Hopes this helps you.

    [​IMG]

    CAMBER LINK POSITIONS
    The optional camber link mounting holes alter the rate at which the camber angle changes throughout the suspension’s movement. For the purposes of making only the following changes, you should reset your camber angles after moving the camber link locations.


    Front
    Outside (on the front hub).
    A longer link means the camber will change less as the suspension compresses, which will make the car turn in harder but push exiting the corner.
    Moving to the inside hole will give more camber rise, which smooths out initial turn-in but adds steering through the middle and exit of the corner.


    Inside (on the shock tower).
    Raising the inner mount will keep the front end more flat. On high bite and smooth track, this will smooth out your car’s steering response and make it easier to drive.
    Lowering the inner mount will add body roll and make the car more aggressive. Mark almost always runs the lowest hole available.


    Rear
    Outside (on the rear hub).
    A longer link gives less camber rise, which means less traction. On a high speed track with high grip, this will add more support by eliminating body roll.
    A shorter link equals more camber rise and more traction. Because a shorter link will make the rear of the car feel softer, it will better handle rough sections of the track.


    Inside (on the shock tower).
    Moving the inner camber link mount to the inside or outside hole will have the same effect as changing the length of the link on the hub.
    Raising the link on the rear shock tower will keep the buggy flat through corners and have less camber rise; this is a good adjustment to make on a smooth track with high traction.
    Lowering the link will add camber rise and make the car more forgiving when the track is rough.
    Moving the link out on both the shock tower and the hub, which will keep the camber link the same length, will add support and make the rear of the car feel stiffer.

     
  11. Grant

    Grant Well-Known Member

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  12. Grant

    Grant Well-Known Member

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    The info is great and I can understand it so much more....I thank you very much. If I have any questions I know where to go!
     
  13. Martijn

    Martijn Well-Known Member

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    Oke, yes, just message me and I'll try to help you ;)
     
  14. Carlos cruz

    Carlos cruz Well-Known Member

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    wow great info
     

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