anti-squat and anti-dive
anti-squat and anti-dive
(OP)
I have a couple of questions regarding anti-squat and anti-dive in regards to double wishbone front and rear offroad racing cars. first they have quite a bit of suspension travel 20-30 inches total, low ride frequency, and hit a lot of bumps during braking and acceleration. It would be nice not to use up much of the suspension travel in squat or dive prior to hitting a large bump.
Heres my questions how do you compute anti dive and anti squat for double wishbone suspension systems. I've seen it done several ways in different books, I've searched this forum and several posts reference diagram links which are no longer active, so I'm confused!
Here is how I understand how to compute anti-dive. draw a side view of the vehicle, draw a line through the upper inboard wishbone pivots, do the same for the lower inboard wishbone pivots. where these intersect is the SVIC. Draw a line through the front tire contact patch and the SVIC. where this line intersects a vertical line from the CG to the ground is the amount of anti dive you have in %. Where 100% would have the line intersect at the CG, and 0% would be an intersection at the ground. the same but in reverse would apply to anti squat.
Is this approach correct? it doesnt seem possible when I'm designing the front suspension. the lower wishbone is inclined about 20 degrees to give some wheeel recession for a more compliant ride. if my upper arm is parallel to the lower then I get something like -50% anti dive. if I incline the upper wishbone more than the lower one say 25 degrees I get a very short SVSA and 60% anti dive. If I went further the SVSA would be so short I could get several hundred % anti dive, I guess I dont see how these small changes could have such a large effect.
Any help or graphics would be appreciated. If anyone has ballpark values to shoot for that would help too, is there any reason not to go to 100% if it can be packaged nicely. I've heard about suspension "locking up" with too much "anti" cant figure that one out either.
thanks
Damien
Heres my questions how do you compute anti dive and anti squat for double wishbone suspension systems. I've seen it done several ways in different books, I've searched this forum and several posts reference diagram links which are no longer active, so I'm confused!
Here is how I understand how to compute anti-dive. draw a side view of the vehicle, draw a line through the upper inboard wishbone pivots, do the same for the lower inboard wishbone pivots. where these intersect is the SVIC. Draw a line through the front tire contact patch and the SVIC. where this line intersects a vertical line from the CG to the ground is the amount of anti dive you have in %. Where 100% would have the line intersect at the CG, and 0% would be an intersection at the ground. the same but in reverse would apply to anti squat.
Is this approach correct? it doesnt seem possible when I'm designing the front suspension. the lower wishbone is inclined about 20 degrees to give some wheeel recession for a more compliant ride. if my upper arm is parallel to the lower then I get something like -50% anti dive. if I incline the upper wishbone more than the lower one say 25 degrees I get a very short SVSA and 60% anti dive. If I went further the SVSA would be so short I could get several hundred % anti dive, I guess I dont see how these small changes could have such a large effect.
Any help or graphics would be appreciated. If anyone has ballpark values to shoot for that would help too, is there any reason not to go to 100% if it can be packaged nicely. I've heard about suspension "locking up" with too much "anti" cant figure that one out either.
thanks
Damien





RE: anti-squat and anti-dive
Anti anything is powered by inputs to the suspension. Those inputs are dependant on traction. The time of greatest traction is while encountering a bump. That's when the anti-actions really kick in and why the suspension locks up. Right when you need it least.
It's best to leave the anti-actions low or out, and control things with a bunch of mega-dollar shocks
RE: anti-squat and anti-dive
RE: anti-squat and anti-dive
And, the no squat/no rise line, for an IRS, does NOT pass through the CG, but passes through the rear axle centerline and has a slope equal to the CG height divided by the wheelbase. (Actually, due to the unsprung mass, the line passes slightly below the rear axle centerline.) Roll oversteer cannot easily be avoided (ever be avoided??) with 100% anti-squat and IRS.
RE: anti-squat and anti-dive
Theory is theory, but to my way of thinking, a whole lot of different factors become vastly more or less important. It requires a whole new different set of compromises.
RE: anti-squat and anti-dive
I work from an overall vehicle target, X degrees pitch per g, and x mm rise per g, then I work out the spring (etc)contribution, then the antis supply the rest.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: anti-squat and anti-dive
When the suspension is moving through 20 or more inches, the reactions from the impact of the bump, and the generally moderate level of total traction have a far greater effect than a degree or two of camber or toe. We never even tried to analyse anti dive or squat. The generally low traction and very hard springs kept dive and squat to a reasonable level by default.
Spend your time getting more travel without breaking CV joints if you really want to go faster.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: anti-squat and anti-dive
Secondly, I keep hearing about suspension "locking up" or becoming harsh with too much anti's is this due to the friction in the pivots? no friction=no locking up? if so useing good teflon lined sphericals would do the trick? also if this is the case what about forces on the pivots due to cornering or braking wouldnt those forces cause lock up?
Damien
RE: anti-squat and anti-dive
Firstly, people forget that suspensions have to move up and down, so they chase their geometry program round in circles until the output dissapears up the input, and end up with the inner pivots of the wishbones at some silly angle to each other. Then when you add real joints, there is a tendency for the whole thing to lock solid.
Secondly, with passenger cars there is undoubtedly some influence on harshness from antidive (in particular). I'm not sure I really agree with any of the theories why this is so, it may just be the above situation, to a lesser degree, but I think it is not that simple. Whatever the reason I'd be surprised to see more than 20% antidive on a refined car.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: anti-squat and anti-dive
100% anti dive suggests there can be a substantial additional weight transfer to the front, but zero additional suspension travel. That hints at an infinite wheel rate. It is not quite like that, but the tyres will not be happy. It all becomes very harsh, usually with very sudden premature wheel lockup. Feedback through the steering under brakes is not nice either.
There quickly comes a point beyond which any further anti dive just makes things worse. I suspect the same is true at the rear, but it is usually much less obvious.
Given a choice, I would rather run variable rate front springs, with either no antidive, or very little. I don't know what is optimum, 20% sounds about right, but I can tell you 50% is awful.
RE: anti-squat and anti-dive
RE: anti-squat and anti-dive
thanks
RE: anti-squat and anti-dive
The brake calipers will grab your effective swing arm and pull the front of the car down. For anti dive, the effective swing arm needs to face rearwards and up, and the wheel will then move forwards in bump.
No free lunch, and no simple choice here unfortunately.
RE: anti-squat and anti-dive
This was thought to be one of the reasons for VW beetle type trailing arm front suspension was so succesfull off road.
It increases rather than decreases dive, but the important part of the trade off is bump behaviour and it's effect on durability.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: anti-squat and anti-dive
RE: anti-squat and anti-dive
The choice is free and simple...Don't do it.
RE: anti-squat and anti-dive
Thanks
-Dave
Everything should be designed as simple as possible, but not simpler.
RE: anti-squat and anti-dive
thanks
Damien
RE: anti-squat and anti-dive
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.