Anti Dive

[Steve]

I am currently working in chassis design and wanted to know some opinions on antidive. I noticed that many of the chassis on the forum have little or no antidive built into them. Is antidive something I would desire in a road driven car,or not? Also,does tapering the frame together at the front give the effect of anti dive or not? I see frames built with evenly spaced tubes and some tapered. mAny Info would be appreciated.
Thanks Steve;

 

[Bill D]

I can't answer your question, but would like to know what anti-dive is?

 

[Gary Kadrmas]

Bill,

That's a scuba diver with a snorkel and a low volume set of lungs.

 

[speed220mph]

Steve: There's more than one way to achieve anti-dive. And, yes, it's desirable in a road car simply because it reduces dive, or reduce the tendency of the front end from going into compression during braking. The harder the braking, the more the dive. How do you get it? Start by getting a good suspension book and read up on it . . . Fred Puhn's "How to Make Your Car Handle" is a good one.

Back to anti-dive, the most common method of achieving it is to angle the inner pivots of the upper A-arm down toward the back. This places the rear pivot lower than the front. There's some simple geometry involved in this. Simply put, locating the upper control arm in this manner causes the path of the upper joint at the upright to move back during compression, which causes an action similar to that of a pole vaulter.

I hope that gives you some idea of what anti-dive is and how it's achieved.

 

[speed220mph]

Steve: Check out ERA's web site showing their front suspension: GT front suspension layout

The pictures don't give any dimensions or angles, but one photo shows how the upper arm is angled. Also, the pivots aren't parallel to the car centerline. I suppose it's an understatment to say suspension geometry gets a bit complicated, but all is working to provide the correct camber curve, bump steer (none) and anti features. I've been designing suspension all my life and I'm still surprised by what can be done. Just remember, always use the KISS principle. Don't make it more complicated than it is.

 

[Steve]

The picture shows a design similar to what I was planning. With the pivots not being parallel, what effect does this have on the anti dive. I can see that angling the upper pivot point down on the back gives you anti dive, but will angling the pivots have any effect?
Thanks Steve;

 

[Steve]

I guess I should have said , with the pivots not parallel with the centerline of the car, does this effect anti dive? It looks like the era only has a small amount of anti dive. What percent is recommended in a street driven gt-40?
Thanks Steve;

 

[Bob Putnam]

I was going to recommend Car Suspension and Handling but the price has gone up considerably since 1993...

As you look at the side view of the a-arms, their pivot axis intersect toward the rear of the car. That is the line through which the weight of the car is transmitted. If the line goes through the CG, you have 100% anti-dive. (That's a gross oversimplification...)

The disadvantage to anti-dive is that the tire moves forward as it goes up. That's basically a destabilizing action under braking if the suspension must also simultaneously absorb bumps. That's why most cars design in only about 20%-40% anti-dive.

 

[Steve]

I understand how anti dive is set up ( side view looking at the pivot points of the A arms) . The more the upper A arm angles down at the back the more anti dive. My question is, looking down at the car , if the A arm pivot points are angled in at the front,( as the are on the E.R.A. ), does this effect anti dive.
Thanks Steve;

 

[Tom]

Let's assume that both A arms are located on a horizontal plane on the chassis. If they are mounted on a narrowing tub, as they were on the original, the outboard ends of the A arms will translate backwards under compression (assuming ride height set so that the lower A arm is nearly horizontal) . The upper A arm, due to its shorter effective length, will have greater angularity to its movement for a given compression displacement, and I believe it will translate backwards further than the lower A arm, which would effectively rotate the upright top towards the back. Which should give it some anti Dive, though I suspect a rather minimal amount.

 

[Rick S.]

I was looking at C and D sports racers on the web a week or two ago, and a few of them have anti-dive stuff built into the front suspensions. I am probably going to get this wrong, but the concept is having a third shock absorber/hydraulic cylinder, which pushes fluid to the front shocks as front-end dive occurs.

I don''t know whether this approach could be used with GT40's, but it was interesting enough to read about it.

 

[Bill Musarra]

The Corvette suspension is probably best noted for its antidive characteristics. Here are the pics of my setup. You can notice on the overhead shots the different alignmernt of the mounting points in that plane. The side shot shows the lower position of the rear upper A arm.

Bill

 

[Tom]

Rick,

without seeing the cars you reference, I'd be inclined to think that the 3 spring(?)/damper setups you saw are really more designed for ride height control. Yes, they do have the effect of stiffening pitch resistance, so that would provide some anti dive, but at the expense of a harder ride. Anti dive does it without changing the ride. Flat bottom cars need to be close to road in order to generate ground effect, but if they get too close, the air underneath gets choked off, and downforce goes away, so racers are usually more concerned with managing the aerodynamics.

I seem to remember that Radical has sort of hybrid 3 spring/damper setup called a "Nik" link, which helps to separate pitch and roll resistance. Maybe something similar could be built by an enterprising owner (I think it is patented).

Tom