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Author Topic:   Anti-dive
340duster
Dirt Full Roller

Total posts: 70
posted May 23, 2002 11:51 PM
How do you measure antidive? I race an A-body dodge and I believe there is a lot of antidive built in. The lower control arm mount points are level but the upper arm is not. The front pivot of the upper control arm is higher that the rear pivot. And what are the effects of antidive on corner entry. Please be as techincal as possible with replies, thanks again in advance for your help.

powerglides
Dirt Full Roller

Total posts: 69
posted May 24, 2002 06:23 AM
Your right 340 the car does have anti dive. What this does is prevent the front end from using the springs to hold it up when you are on the brakes. It uses the front geometry to hold the car up. With stock springs on the road this is a good thing. On the track with heavy springs it is not needed or wanted. What it does is lock up the front suspension when you hit the brakes to go in the corner and that can cause a push. Then when you let off the brake the car suddenly rolls onto the right front, and upsets the handling. I hope this helps a little but I know you will get more and better explained responses on this great website. Don

[This message has been edited by powerglides (edited May 24, 2002).]

snowman
Dirt Maniac

Total posts: 140
posted May 24, 2002 12:16 PM
Powerglides is right on all counts..........

How to measure it ??????

The concept is easy to understand,if you could see a diagram........Could be difficult using only my explanation.......Here goes anyway.......

Lacking a computer program......You need to make a scale drawing of the suspension points.....

Looking at the car from the side....draw a line through the lower a-arm pivots rearward......same for the upper........The convergence point of these two lines form an instant center of the two arms......This instant center could be thought of as a torque arm about which the front suspension reacts...........

Now draw a line from the center of the tire contact patch ( bottom of the tire) to the center of gravity of the sprung mass......

If your instant center were to fall on this line, you would have 100 % anti-dive....If this line is 1/2 between the center of gravity height and the ground,then you would have 50% anti-dive..........That is to say.....The percent of anti-dive is expressed as a percentage of the height of the the convergence points on a line drawn straight down from the center of gravity.....

To say that one more way...........draw a line straight down from the center of gravity....... draw the instant center of the front a-arms..........Draw a line from the tire contact patch through the instant center...........wherever this line meets the line at the center of gravity is the amount of anti-dive..........How high this point is......as a percentage of the center of gravity height..........

Thats how to measure it.........If you have two car builders talking to one another.....They may call the degrees of inclination of the upper arms the amount of anti-dive........So,genericly,sometimes this is called the percent of anti-dive.....It's not really correct.....But it's OK,because they know what they are talking about....But, if you were just listening in....and did not know exactly what they were talking about....You could be confused on the issue........

The effects of increasing anti-dive.....bearing in mind that anti-dive works by using the brake torque forces......
As Don has allready mentioned, when you apply the brakes,your front suspension become tighter......making it less able to keep the tire on the track over the bumps.........More anti-dive also creates more caster change,which can make the car darty and unstable........

Light cars will want no anti-dive.......Heavy cars can use some......Some use as much as 25-30 %.........I personally feel it is best to minimize anti-dive, all things considered.....

Good Luck,
Snowman

340duster
Dirt Full Roller

Total posts: 70
posted May 24, 2002 01:23 PM
Thanks for your help on this matter. I do have a push at corner entry but I think it is before I brake. I need some track time to confirm this. I will draw the suspension point up in autocad and I can figure it out exactly. Should I use the cam centerline for the unsprung mass centerline?

snowman
Dirt Maniac

Total posts: 140
posted May 25, 2002 10:32 PM
340,Duster, I really couldn't say how close that would be........There is a calculator in the toolbox, however,it does not help with the height.........

I am going to pass on trying to explain how to calculate the location with out using diagrams..........If you would like to send your mailing address via e-mail or private message, I will send you a copy of the diagrams and formulas..........

I will send Jammin a note........This would be a nice addition to the toolbox........

If you are wanting to take some angle out of the upper a-arm,and need an answer right now ........Then I reccomend dropping it to 2 degrees........Obviously, I can't say how much anti-dive this will provide for you........But it is known to work well.....

Good Luck,
Snowman

c21
Dirt Freak

Total posts: 346
posted May 28, 2002 01:01 PM
Snowman,
your instructions for finding side view front suspension instant centers were right on .... with the posible exception being if the inner pivot axi (axis's?) are not parrellel to the car's centerline.
Then, in order to find your sideview projected A arms (and the instant center) you need to project the inner pivot axis through the wheel plane and then draw a line through this point (on the wheel plane) and the point at which the respective projected frontview A arms pass through the wheel plane.

clear as mud?
c21

[This message has been edited by c21 (edited May 28, 2002).]

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