BillyShope
05-09-2014, 04:29 PM
This 3link design is a bit different. Normally, we think of 2 nearly horizontal lower links symmetrically arranged and a single upper link, either in the center (except as affected by driveshaft location) or offset to the right. The upper link is angled down from its rear mounting position, allowing the offset right location to cancel driveshaft torque and equalize tire loading on acceleration.
In this design, however, all links are angled upward from their rear mounts and are parallel. We rely, then, on something you heard in high school geometry, but may have forgotten. Specifically, parallel lines intersect at infinity (http://www.yellowbullet.com/forum/showthread.php?t=853186#). So, if the link (http://www.yellowbullet.com/forum/showthread.php?t=853186#) lines are parallel to each other and also parallel to the no squat/no rise line (also known as the "neutral" line), all these parallel lines will intersect at infinity. The classic hot rod, with its "wishbone" locating arms, will give you an idea of that which I'm describing, but...in this case...we're considering individual pivotable links and, of course, there are only 3 of them.
If this suspension is to provide no squat or rise, the links should be at a rising angle with a tangent equal to the CG height divided by the wheelbase. If, in addition, equal tire (http://www.yellowbullet.com/forum/showthread.php?t=853186#) loading on acceleration is desired (and if, again, there is a single upper link and 2 lower links), the upper link must be offset to the LEFT side, not to the right. This is because, with the upward angle, the tension force in that upper link must be used to reduce left side tire loading and not, as in the normal case, to increase right side tire loading.
In this case, then, the offset would be equal to:
R*W*(B-A)/(A*H*X)
where R is the rear tire radius, W is the wheelbase, B is the vertical distance from the ground to the rear mounting point of the upper link, A is the vertical distance from the ground to the rear mounting point of the lower link, H is the center of gravity height, and X is the axle ratio. The link mounting points are assumed to be on a vertical line passing through the axle centerline in the side view.
While lower link spacing doesn't enter (http://www.yellowbullet.com/forum/showthread.php?t=853186#) into this, it would be convenient to have the upper link directly over the left side lower link. This would allow fabrication of a front bracket which could be adjustable, allowing some squat or rise, if desired. It is necessary, of course, to retain parallelism.
And, we needn't omit a fourth link. In fact, some would say it's highly desirable. But, this fourth link wouldn't be an ordinary link. It would have to be ineffective while accelerating, but effective while braking. In other words, it would be a telescoping unit which would "bottom out" under heavy braking, but would otherwise just be "going along for the ride." I don't believe this is really necessary since we commonly carry poorly distributed loads in our street cars (http://www.yellowbullet.com/forum/showthread.php?t=853186#) and think nothing of it, but, for those who might be concerned, this extra link would return the "stock" braking.
shopeshop.org
https://static1.pt-content.com/images/pt/2014/05/user_offline-1.gif
https://static1.pt-content.com/images/pt/2014/05/quote-1.gif (http://www.yellowbullet.com/forum/newreply.php?do=newreply&p=19493786)
In this design, however, all links are angled upward from their rear mounts and are parallel. We rely, then, on something you heard in high school geometry, but may have forgotten. Specifically, parallel lines intersect at infinity (http://www.yellowbullet.com/forum/showthread.php?t=853186#). So, if the link (http://www.yellowbullet.com/forum/showthread.php?t=853186#) lines are parallel to each other and also parallel to the no squat/no rise line (also known as the "neutral" line), all these parallel lines will intersect at infinity. The classic hot rod, with its "wishbone" locating arms, will give you an idea of that which I'm describing, but...in this case...we're considering individual pivotable links and, of course, there are only 3 of them.
If this suspension is to provide no squat or rise, the links should be at a rising angle with a tangent equal to the CG height divided by the wheelbase. If, in addition, equal tire (http://www.yellowbullet.com/forum/showthread.php?t=853186#) loading on acceleration is desired (and if, again, there is a single upper link and 2 lower links), the upper link must be offset to the LEFT side, not to the right. This is because, with the upward angle, the tension force in that upper link must be used to reduce left side tire loading and not, as in the normal case, to increase right side tire loading.
In this case, then, the offset would be equal to:
R*W*(B-A)/(A*H*X)
where R is the rear tire radius, W is the wheelbase, B is the vertical distance from the ground to the rear mounting point of the upper link, A is the vertical distance from the ground to the rear mounting point of the lower link, H is the center of gravity height, and X is the axle ratio. The link mounting points are assumed to be on a vertical line passing through the axle centerline in the side view.
While lower link spacing doesn't enter (http://www.yellowbullet.com/forum/showthread.php?t=853186#) into this, it would be convenient to have the upper link directly over the left side lower link. This would allow fabrication of a front bracket which could be adjustable, allowing some squat or rise, if desired. It is necessary, of course, to retain parallelism.
And, we needn't omit a fourth link. In fact, some would say it's highly desirable. But, this fourth link wouldn't be an ordinary link. It would have to be ineffective while accelerating, but effective while braking. In other words, it would be a telescoping unit which would "bottom out" under heavy braking, but would otherwise just be "going along for the ride." I don't believe this is really necessary since we commonly carry poorly distributed loads in our street cars (http://www.yellowbullet.com/forum/showthread.php?t=853186#) and think nothing of it, but, for those who might be concerned, this extra link would return the "stock" braking.
shopeshop.org
https://static1.pt-content.com/images/pt/2014/05/user_offline-1.gif
https://static1.pt-content.com/images/pt/2014/05/quote-1.gif (http://www.yellowbullet.com/forum/newreply.php?do=newreply&p=19493786)