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anti - dive geometry
lamhotung - 26/8/03 at 11:55 PM

have any one build a locost with anti-dive?
is it worth having?(may be i just need better spring)

i had brough the book"how to build & modify sportscar & kit car" by des hammill
it make anti-dive sound so good.
but didnt said how to build one.

it just said the pick-up point need to go through the true centre of gravity..

how do i find centre of gravity. arg

thanx for reading

pbura - 27/8/03 at 11:55 AM

There's been some discussion on a couple of threads, so you migh just search "anti-dive".

I haven't seen much in the way of diagrams, etc., on the web, so you might want to do some more reading. Carroll Smith has a good section about anti-dive and anti-squat in Tune to Win. If I was going to recommend one book about suspension design, that would be the one (though I haven't read Hammill).

Best of luck,

Pete

JoelP - 27/8/03 at 12:21 PM

If i were in your situation i would guess the CoG, cos even if you get it a bit wrong the anti dive should still show up a bit (ie less dive). There was also a thread on CoG the other day. Maybe an educated guess since most weight is in the engine and Gbox.

Remember the more anti dive you add in the stiffer the suspension will get under breaking, meaning pot holes etc will feel much worse (jarring).

suparuss - 27/8/03 at 03:18 PM

its pretty simple to achieve really, all you do is tilt the line through your wishbone mounting points upward toward the centre of the car. but it is only really used to make it easier to control the attitude of the car during braking and acceleration, in the name of earodynamics, or to stop the nose from hitting the ground on really low cars.
if you car is a 7 replica i cant see how anti dive or anti squat would benefit the performance.
if you like i can take a picture of an illustration in a book i have to give you a better idea of how to achieve it, and give you little more info.

Russ.

lamhotung - 27/8/03 at 03:37 PM

i am building a book 7 with a 4age.
so i guess i dont need them.

but i really want to know
where is the g centre of the car
is it base on the pick up point ?
also how high is the centre point?

if i do want anti-dive where should i place the coil-over shock? 90 to the flat floor?

suparuss - 27/8/03 at 04:06 PM

center of gravity depends on where the weight is on the car, you can find a center front to back by ballancing the car on a trestle spanning the width of the car, same for a center from right to left (with a trestle spanning the length of the car). but to find the height from the ground you would have to ballance the car on a trestle with the car on its side- or you could just guess it like joel p suggested.
the position of the coil overs is a whole different pack of peanuts, and shouldnt really be effected by anti dive geometry, were only talking about a few degrees of angle on the wishbones.

pbura - 27/8/03 at 04:27 PM

Here's a method for calculating your COG location by weighing your car with the rear at two different heights:

http://members.aol.com/sccacuda/cars/4SmthTa.html

As far as the anti-dive, you could build in a modest amount (equivalent to a production car) by either angling your upper front wishbone down, or your lower up, by 2 degrees.

With 100% anti-dive, all the load transfer is pushed against the wishbones, binding up the suspension. The lighter the car, the more sensitive to binding it will be. A liitle bit couldn't hurt much, as Superperformance has a little bit (http://www.superformance.com/s1_more.asp. You can see that the lower bone is slightly angled.

Pete

Spyderman - 27/8/03 at 06:12 PM

quote:
Originally posted by lamhotung
i am building a book 7 with a 4age.
so i guess i dont need them.

if i do want anti-dive where should i place the coil-over shock? 90 to the flat floor?

The coil-overs can be mounted in the traditional place, but will need pivot joints on top and bottom (rose joints would be ideal) due to the lateral movement of lower mount.

JoelP - 27/8/03 at 09:31 PM

I think it would be possible to use normal bushes still, as the suspension could still be mounted in the same plane as the wishbone pivots. It would have to lean forward though, by the same degrees as the WB was up.

Rorty - 28/8/03 at 04:38 AM

suparuss:

quote:

all you do is tilt the line through your wishbone mounting points upward toward the centre of the car.
Careful there! The wishbones ought to be angled toward the COG. We don't want people to just angle their bones up into the centre of the car willy-nilly. If you angle them up too much, the front of the car will rise under braking!

It's a simple operation to calculate the COG along the car's axis with just 2 bathroom scales and a series of timber off cuts.
It's debatable if it's worthwhile though. What do you want the car for; road or track? If you only want to drive on the roads, then (as someone else pointed out) every pothole will feel twice the size it really is.

suparuss - 28/8/03 at 05:37 AM

yes, should be somewhere in convergance with a line between center of gravity and the contact patch of the tyre, but never higher. i was just illustrating how its acheived, with slight exagguration to aid clarity, along with the fact that its not really neccesary on a road car. the angle is usually a couple of degrees, and as little as 1 degree can make a difference.


Russ.

pbura - 30/8/03 at 03:34 AM

quote:
How does angling the wishbones affect the way the springs hold up the car?


Agreed that anti-dive/squat has no effect on supporting the car. It has more to do with how the suspension responds to weight transferred from the other end.

Think of it this way: If you push on the end of a coil sprig, it compresses. Push on the side, and it does nothing at all. The "antis" do exactly that in directing the load transfer to the side of the wishbones, so the springs don't compress in response to the additional load. It's freaky. I stared at some diagrams for about an hour until I got it.

I'm considering softer springing for my road car, so I haven't entirely ruled out some tiny amount of anti-dive or a bit of anti-squat. I'm thinking in the range of 80-100 cycles per minute for the springs, where a more typical Locost is 100-120. If I choose 100 CPM (firm) out of wheel travel considerations, I won't mess with "anti".

Pete

MustangSix - 31/8/03 at 12:46 AM

Nearly all production cars have some degree of anti-dive built in. In cars that utilize upper and lower arms, it's usually done by angling the upper arm down a few degrees with the rear pivot point lower than the front. In a strut suspension, it is accomplished by mounting the upper portion of the strut further to the rear.

Anti dive geometry works in a similar fashion to anti-squat. If you were to alter the angles of the rear arms you could move the instant centers to induce additional forces between the body and axle. In the extreme, drag racers adjust this instant center so that upon acceleration the rear end often rises up, forcing the rear tires to the ground for more traction. Often this is done differently from side to side to compensate for torque effects.

Anti-dive uses this same instant center reaction to prevent the nose from diving when braking force is applied to the front suspension and the weight transfers. The torque of the brake is moved to the chassis differently causing the suspension to resist downward movement.

But this comes as a compromise. It works well on production cars because they are usually softly sprung. On a tightly sprung and lightly loaded Locost, it's probably not necessary to have much, if any anti-dive. Anti-dive has the added effect of tightening up the front suspension on braking and will cause it to feel too stiff, perhaps to the point of becoming non-compliant at all. In fact, one F1 team has experimented with pro-dive by angling the arms in the opposite direction in order to get the car and front wing lower to the ground for better aero effects on braking.

Anti-dive does have a relationship to the CG, but that's only one consideration. It takes some calculus to figure the full effect of moving the arms away from parallel, but there are several good programs that do so.

suparuss - 31/8/03 at 10:29 AM

quote:
Originally posted by Syd Bridge
It is a very, very rare track car indeed, that you will find this on today. Most modern designers would hold the view that anti dive/squat geometry is an easy way of compromising a perfectly good setup.

i would imagine reactive suspension has replaced antisquat/dive in most racing cars these days, exept formula one cos reactive is classed as a drivers aid and was banned. but anti dive/squat still is a low budget way of controlling the attitude of a car under braking and acceleration so if designed right could give a team an edge against others. but like i said, it only really benefits either keeping the nose off of the floor, hence on a low car youll save the nose from being destroyed. or to keep the car straight (or slightly lower at the front, but under control) to aid aerodynamics.
but sevens are neither too close to the ground or particularly aerodynamic any way, not to the point where ground effect would make any difference so i dont really see the point.



Russ.

Spyderman - 31/8/03 at 11:30 AM

Anti-dive has more uses than just to stop the front scraping the ground or affecting the aerodynamics.
It is also used in the same way as anti-roll bars are used to reduce body roll.
In roll the suspension geometry is upset, so the roll is controlled as much as possible. The same can apply to dive and squat. When diving the front suspension geometry is altered in relation to the ground. The castor angle is reduced and can increase loads on the steering above the normal weight transfer.

Admitedly in a 7 where the weight is much less than a saloon and the springing is higher, it becomes less of a neccessety.

Terry

suparuss - 31/8/03 at 02:51 PM

the car rolling will effect geometry in the same way whether it has anti dive geometry or not, the difference being that anti dive effects the way weight is transfered along the length of the car, in roll weight is transfered from one side to the other. these are two completely different aspects. they will effect each other to some degree but not to the point where you would use anti dive to stop roll.
note also that anti dive will have an oposite effect in acceleration, ie, the front will rise even more than usual. so with that in mind, while cornering, one side will want to rise more than usual thus moving more weight into the opposite wheel. while an anti roll bar serves to keep both sides as even as possible.

[Edited on 31/8/03 by suparuss]

MustangSix - 31/8/03 at 06:30 PM

quote:
Originally posted by suparuss

note also that anti dive will have an oposite effect in acceleration, ie, the front will rise even more than usual. s

[Edited on 31/8/03 by suparuss]


Not necessarily. The anti dive characteristic is applied to the chassis because of the torque of the brakes actig thru the spindle, much the same way that anti squat is applied by the reaction of the rear axle housing. Unless you have FWD/AWD and have a torque effecting the front wheel on accelleration, it doesn't cause a front end rise any greater than you might get from the normal trasfer of weight thru the CofG.

But you are correct that both anti squat and dive cause some tradoffs in cornering because they affect other things like roll steer.

In the end, it's all a series of compromises. You can't categorically say anything is bad or good to have in itself because any change in geometry will affect another geometry. So you shoot for the effect that you would like to emphasize, and compromise around that.

On my car, for example, I'm using fairly soft springs for a compliant straight line ride over rough pavement. But that enhances roll, so I've had to consider a sway bar. Also, I've built in 4 degrees of anti dive, because with the soft springs, I might be nosing the car into the ground on hard braking otherwise. My idea is to control roll and dive using geometry, not stiff springing, but it's a compromise on other characteristics.

[Edited on 1/9/03 by MustangSix]

Spyderman - 31/8/03 at 06:53 PM

quote:
Originally posted by suparuss
the car rolling will effect geometry in the same way whether it has anti dive geometry or not, the difference being that anti dive effects the way weight is transfered along the length of the car, in roll weight is transfered from one side to the other. these are two completely different aspects. they will effect each other to some degree but not to the point where you would use anti dive to stop roll.
note also that anti dive will have an oposite effect in acceleration, ie, the front will rise even more than usual. so with that in mind, while cornering, one side will want to rise more than usual thus moving more weight into the opposite wheel. while an anti roll bar serves to keep both sides as even as possible.

[Edited on 31/8/03 by suparuss]


I think you need to re-read what I wrote!

I was only comparing the need to control dive as being similar to controling roll. Nowhere did I state that one caused or cured the other.

The use of anti-dive or squat will not induce the opposite effect either.
It is purely weight transfer that causes dive or squat, and changing the opposite ends suspension geometry will not make that end move any more than it would have done without the changes.

Terry