anti-squat and anti-dive

[damienmavis]

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it done several ways in different books, I've searched this forum and several posts reference diagram links which are no longer active, so I'm confused!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

 

[Fabrico]

Those here can tell you how to measure, add or subtract it, but not how much you will need. For off-road racing; the anti-squat, rise, and dive you add, will subract from driving and stopping traction. This is especially touchy with 2 wheel drive. There is a fine balance within the suspension between bump handling ability and wheel spin.

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

 

[Warpspeed]

Agree with Fabrico, "anti" suspensions just increase the suspension rates by binding up, and that reduces tyre traction when you need it most. Much better to use the shocks and some really forgiving bump stops.

 

[BillyShope]

My knee-jerk reaction was to disagree with the previous posts, but I then realized this is because of my strong interest in dragrace applications. The "binding up" is due, of course, to the forces generated when the unsprung mass is caused, when a "bump" is encountered, to follow an arc segment more closely aligned to the horizontal (with high anti percentages). The less the unsprung mass, the more anti you could comfortably run.

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.

 

[Warpspeed]

There is probably quite some difference between a design for racing on a dead smooth flat grippy surface, and traction/control over very bumpy and broken ground at speed.

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.

 

[GregLocock]

I get a bit nervous when people discuss 'antis' while ignoring the spring and damper contribution (and bars, if any) tot he overall vehicle behaviour.

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.

 

[patprimmer]

My experience in off road racing taught me that maximum suspension travel is by far the most critical factor, followed very closely by damper efficiency, followed by spring rate, with camber change being of little concern and reasonable toe change also acceptable.

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.

 

[damienmavis]

thanks for the info, from BillyShope's reply I think I'm figuring the anti's improperly, does anyone have any graphics or info or could point me to the right place to get more info on calculating the anti's properly?

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

 

[GregLocock]

I think the lock-up argumant comes from two things.

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.

 

[Warpspeed]

The "locking up" is probably more due to the effective apparent increase in wheel (spring) rate. There are the springs themselves, plus an additional downward thrust developed by the suspension linkage geometry.

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.

 

[BillyShope]

But, as I inferred in my earlier post, if the unsprung mass was nonexistent, there would be no vertical reaction, through the links, to a vertical input. The wheels would simply move forward or back and the springs would absorb the force.

 

[damienmavis]

I still would like some help in the proper method of calculating the anti's for a double wishbone front and rear suspension. any help would be geatly appreciated. I would like to angle the lower front a-arm up in front maybe 20 degrees so the wheel moves rearward during compression for a more compliant ride. the angle of the upper front arm is open. is there any way to get any antidive with an inclined lower arm?

thanks

 

[Warpspeed]

If you angle the forward facing effective swing arm upwards, so the wheel moves rearwards in bump, it will dive even more!

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.

 

[patprimmer]

The wheel moving backward on bump helps to soften the bump by effectivly slowing the forward speed of the wheel as it hits the bump.

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.

 

[BillyShope]

Damien, I started to post the entire procedure, but realized the probability of confusion...with no accompanying illustration...was simply too great. I'd recommend a peek at pages 618,619, and 630 of "Race Car Vehicle Dynamics."

 

[Fabrico]

Re: "No free lunch, and no simple choice here unfortunately."

The choice is free and simple...Don't do it.

 

[Dave K]

Has anyone seen Mark Ortiz's video, "Minding your anti's"? I'm curious if its worth investing in.

Thanks

-Dave
Everything should be designed as simple as possible, but not simpler.

 

[damienmavis]

I have a simple drawing showing my question? I checked out page 618 from race car engineering and the eq. seems to check out w/ my drawings, but they dont seem to pass the "reasonableness" test can I email them to anyone who'd like to give me thier opinion. (I dont know the protocol for emailing other members)

thanks

Damien

 

[GregLocock]

We don't post email addresses, find my website and you'll get mine

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.