Copied to here from the How's My Numbers? thread for discussion. Most of this will involve antis with the historical understanding of antis and not the force analysis understanding.
Hi
Treefrog, been doing more reading up and thinking about AS/AD and would appreciate some input on my line of thinking here.
You recommended AS/AD be in the 30-40% ish range for my use type (Highway capable moderate rock crawler on 37s).
Please correct any of my thinking that may be wrong here, but having a below neutral AD (<50% at 50/50 drive bias) in the front will cause the suspension to tend to unload and move the wheels down under traction. So, when trying to climb up a rock or ledge, the suspension will have a tendency to push the wheels down into the rock and move the body up in the front. In some ways this seems like a good idea because it may give the tire more traction, and try keep the tires pushing into the ground on steep climbs, but in other ways opposite of what one would want when rock crawling.
Anti = bias is not neutral. It is no movement. 0% is neutral.
I do not think the front moves the wheels down. What I think happens is the side view (SV) torque from the front axle pulling and load transfer causes the nose to rotate away from the axle. Where as in the rear, the links push the wheels down.
When pushing into a rock or ledge to get the front end up, we want to push as much force forward into the surface. More AD/AL comes from a higher IC by definition. A higher IC means that the axle wants to push under the vehicle more instead of moving up and down.
Moving the body up has the downside of raising the CG that plays a part in flipping over.
In my mind, especially if I an climbing one side and not the other, I would think that I would prefer >50% AD in the front so that the side of the suspension hitting the rock will tend to squat, making use of the uptravel available and allowing the wheel to move up the obstacle without having to push the whole body up right away, which I feel would allow the wheel to roll up stuff more easily and comfortably.
I do not know if we want the suspension to compress while driving up a ledge. On one hand lowering the CG will help, On the other, the momentum from the inevitable unloading may cause the cage to be tested. We also want less weight on the front when driving up a ledge, but more weight on it once it is up. I also think that we may want the weight to go up early when we have more traction to push into the obstacle with. And we do not want the vehicle trying to drive over the axle, something that typically comes with a high AL.
The root cause of antis is a moment about the CG caused by the forces in the links. On straight flat ground, our links pass under the CG, causing a nose up moment (pushing rear, pulling front). But when you push the front up against a rock, it creates a nose down moment canceling out the rear's nose up. The torque from the tire climbing a vertical wall predominantly loads the upper link in compression for most offroad setups, though this can not reliably be used to compress it more.
I am also aware that >50% AD makes for better braking performance, and that may just be one of the compromises to make when building a streetable rig.
In theory, yes. But in reality, probably not. The primary function of suspension is to keep the tires in contact with the ground. If you are not touching the ground, you are not braking or accelerating. If it is too stiff it can chatter and hop as you brake. This is more likely the rough the road is.
As for the rear, I would also think having >50% AS would be desirable because on steep climbs or obstacles, the rear wheels pushing down under traction will shift weight onto the front wheels giving them traction, rather than the rear squatting down shifting weight off the front.
The problem that rigs with high anti-squat experienced was the axle kept try to drive under the vehicle. Another issue in hindsight might have been the the rear extending causing the CG to move away from the surface, increasing flip over risk. The IC being higher up means that the point on the travel arc ends up with a larger horizontal component than is desirable. The more the travel is not perpendicular to the surface, the more the force of going over a bump is transferred to the chassis in an uncontrolled manner instead of in a controlled manner through the springs and damper.
I was also wondering more about AS/AD slopes and how opposite sides may interact. You suggest having antis rise with compression, which makes sense to reduce squat and dive more as you approach max compression. But what about drooping out? The way I see it, if you are in a situation such as the video I linked below where you have one side stuffed up and the other side drooped out and you get traction, and your AS curve is such that you have more AS higher up in the suspension travel vs drooped out, the stuffed tire AS will overcome the dropped side AS and push the stuffed tire down as well as the dropped tire up, forcing the body to roll over.
If my thinking there is true, then I would think you would want to make sure the AS curve looks something like this (ignore %, just looking at cure shape):
Where the drooped tire AS will overcome the stuffed tire AS until it has reached a more stable point. If the curve is more of a straight line like " \ " or doesn't curve back over, then the lower tire will always have more AS than the higher one, which may help even more with stability. Maybe?
I think you are overlooking the part where traction results in antis. A drooped out wheel has no weight and therefore no traction. The lack of force means that any pitch resistance it may generate could be considered negligible. As such I do not believe that antis on a drooped out tire are worth much effort in targeting behavior.
One thing to keep in mind is that those plots only show symmetric antis. As an axle flexes, the projected to side view links change their relative end points.
The same sort of logic applies if we are going with <50% AS in the rear, but with the graph going the opposite way like " / ". If the drooped side has a lower AS then the stuffed side in this situation, then it will want to do the same thing and roll the body over, just that it will moreso be the dropped side pulling it over vs the stuffed side pushing it over. Having the graph go like " \ " in this situation would counteract that, but that is the way people's curves seem to go when building <50% AS anyways. So the main thing there is making sure the AS curve looks like " \ " regardless of whether you are running more or less than 50% AS.
But, what about AS crossing over that 50% mark at some point in the curve? If the curve looks something like this:
The same thing as above about weight -> traction -> reaction. I think that you would want the compressed side to have less anti since it will tend to compress the high side more.
then the stuffed and dropped wheels will act oppositely under traction, with the stuffed trying to squat more, and the drooped trying to droop more, which seems to me would be kinda unstable.
That sound like it would be more stable with it trying to lean towards the uphill side. Copied from my shorter response: "The reason to not design near anti = bias is that in theory small changes in traction and travel cause crossover of between anti-squat and pro-rise. This generally results in hopping. Similar can happen with antis over anti = bias where the instantaneous force from acceleration wears off and with it the pro-rise, causing cyclic traction spikes and hopping."
However, with a more vertical curve that doesn't cross over the 50% line, then the stuffed and drooped sides wont have too different of AS values and wont be counteracting each other as much. This may be something to shoot for? A relatively vertical AS curve, whether it is more or less than 50%?
I may be starting to sound like a broken record, but weight -> traction -> reaction. However, I do think that for a trail rig, a steady anti is a good thing. But flex does change the antis.
I could be seriously overthinking all of this though, as I know a lot of this behavior can be addressed with coilover tuning and sway bars. But seeing videos like this makes me think it can have a pretty big effect.
Try not to think yourself into design paralysis. Are you overthinking or are others underthinking?
Again copying from my previous response "The second video is a really good example of something that was prevalent in the early linked days but has generally been designed against. The rear is trying to drive under the vehicle as the front applies resistive force, braking in this case."
