Anti-Dive

[fabr]

Antidive is one of those things that can be a little ,none or a lot designed in.It's how you balance the pro and cons that matters for the particular car.

[LiveWire]

Engineer, are you ready to wrap your head around the next set of geometry puzzles?

Tilting the upper arm mounts more than the lower introduces anti-dive which is bascially caster gain on compression. That also means caster loss on droop. Coming in nose down at an angle with zero caster will suck.

Another way to introduce anti dive is to mount the front of the upper A-arms closer together than the rear. From the side of the buggy, the upper and lower arms are at at the same angle. When viewed from above, they are not. When that upper arm moves from straight out upward, the ball joint moves back due to the rake and due to the angle viewed from above. When moved from straight out down, the angle when viewed from above would also cause the ball joint to be moved back. It will be offset by the rake angle though. With a large angle when viewed from above, you would have caster gain on compression and caster gain on droop. So you get your anti brake dive without having zero caster at full droop. You would want the cross over point to be at ride height. It complicates bump steer further. I still got mine down to .1 degree and .3 degrees on 2 setups I did though.

[LiveWire]

Anti-squat was brought up a bit. There is another way to introduce some anti-squat in IRS. If you have ever driven a front wheel drive with bad torque steer, you have felt the effect. A bent CV will resist changing it's shape with torque being applied to it. If the shafts angle forward, they will induce a downward force as the shaft is rotating. If they angle back like on a lot of VW rails with extended arms, they will pull up on the arms. It has to do with rotation, not torque twisting on the suspension. So acceleration and braking have the same effect. That is with inboard brakes.

A front wheel drive car with two different length axles will apply more downward force to one tire than the other causing torque steer.

[Engineer]

Engineer, are you ready to wrap your head around the next set of geometry puzzles?

Tilting the upper arm mounts more than the lower introduces anti-dive which is bascially caster gain on compression. That also means caster loss on droop. Coming in nose down at an angle with zero caster will suck.

Another way to introduce anti dive is to mount the front of the upper A-arms closer together than the rear. From the side of the buggy, the upper and lower arms are at at the same angle. When viewed from above, they are not. When that upper arm moves from straight out upward, the ball joint moves back due to the rake and due to the angle viewed from above. When moved from straight out down, the angle when viewed from above would also cause the ball joint to be moved back. It will be offset by the rake angle though. With a large angle when viewed from above, you would have caster gain on compression and caster gain on droop. So you get your anti brake dive without having zero caster at full droop. You would want the cross over point to be at ride height. It complicates bump steer further. I still got mine down to .1 degree and .3 degrees on 2 setups I did though.

I think I know where you are headed here.  We are gonna have to start drawing pictures to be sure.    I don't know if you are suggesting angling one arm a little bit to get the affect, or are you talking about a suspension that has both arms at an angle as viewed from the top?  Someone was going to coin a term for that when the rear A-arm mounts are wider than the front.

I have seen many cars built with the arms angled front to rear with two basic senario's.

The first is when the front mounts are a few inches apart, and the rear mounts are back on the sides of the car, allowing the driver's and passenger's feet to be in between the A-arms, and making for a more compact design.  It became real popular when they started building 4-seaters while trying to keep the wheelbase down.

The second is when the frame is tapered from front to rear, and the A-arms are mounted to the outside of the frame rails, causing them to be angled front to rear.  In this case the angle is usually less.  This design allows for feet in the middle, but makes for short arms.

WARNING, an opion is about to follow:

I don't have a problem with anyone building a car how they want, but with the first scenario above, most designs like this are knock off's of what someone else has done.  If you keep the arms almost parallel, mabey build in some caster at the spindle, you can get away with a lot, and still have a good working car.  But if you start figuring what the suspension is doing, going in and out of rake, etc.  It gets very complex very fast.  Caster and Camber change become interrelated.

Sorry I have gotten off topic.

LiveWire,  I see what you are saying, and I see how it could be better than tilting the arm....... I have to think about it some more........  Specially when I decide to run front brakes. 

Still waiting for someone to coin that term!

[Yoshi]

. I don't know if you are suggesting angling one arm a little bit to get the affect, or are you talking about a suspension that has both arms at an angle as viewed from the top?

just angeling the top, the bottom would stay parallel to the chassis.....

[Engineer]

Engineer, are you ready to wrap your head around the next set of geometry puzzles?

Tilting the upper arm mounts more than the lower introduces anti-dive which is bascially caster gain on compression. That also means caster loss on droop. Coming in nose down at an angle with zero caster will suck.

Another way to introduce anti dive is to mount the front of the upper A-arms closer together than the rear. From the side of the buggy, the upper and lower arms are at at the same angle. When viewed from above, they are not. When that upper arm moves from straight out upward, the ball joint moves back due to the rake and due to the angle viewed from above. When moved from straight out down, the angle when viewed from above would also cause the ball joint to be moved back. It will be offset by the rake angle though. With a large angle when viewed from above, you would have caster gain on compression and caster gain on droop. So you get your anti brake dive without having zero caster at full droop. You would want the cross over point to be at ride height. It complicates bump steer further. I still got mine down to .1 degree and .3 degrees on 2 setups I did though.

I guess the problem that I see is you get additional positive caster, above and below when the arm is sticking straight out from the frame.  The braking force is going to try to force the spindle to the smallest caster location.  That location is where the arms are sticking straight out from the frame.  On all of my designs the arm is sticking straight out when you are about 4"-6" from full compression.  So in that case, the braking force would want to cause dive, becaue the location it is seeking is 8" below ride heigth.

[LiveWire]

just angeling the top, the bottom would stay parallel to the chassis.....

Right.

The cars with both upper and lower arms angled greatly, the wheels move back as the arms drop. That means when landing nose down on one wheel, the wheel has to go forward and up when being shoved back and up. So I believe it will tend to bind. It is a very copied design.

I guess the problem that I see is you get additional positive caster, above and below when the arm is sticking straight out from the frame.  The braking force is going to try to force the spindle to the smallest caster location.  That location is where the arms are sticking straight out from the frame.  On all of my designs the arm is sticking straight out when you are about 4"-6" from full compression.  So in that case, the braking force would want to cause dive, becaue the location it is seeking is 8" below ride heigth.

If the arm is also higher in the front, then it shifts the cross over point lower in the travel. It won't really be when they are straight out, but somewhere lower than that.

As for the terms, here are some thoughts: Trailing arm is pivot point lateral across frame. Leading arms would be lateral also with the arm facing forward. Semi trailing-pivot has a slight angle. A-arms are longitudinal pivots. So semi-longitudinal or partial-leading?

[fabr]

How about just something simple like----screwy?

[Engineer]

As for the terms, here are some thoughts: Trailing arm is pivot point lateral across frame. Leading arms would be lateral also with the arm facing forward. Semi trailing-pivot has a slight angle. A-arms are longitudinal pivots. So semi-longitudinal or partial-leading?

How about just something simple like----screwy?

Screwy I can remember......