Front Suspension Antidive Geometry

[doozer]

I am in the process of tubing my front end and fabricating front suspension points but I am curious about building in anti-dive geometry.

I have heard that the spacers that go between the front suspension mount and the a-arm mounts affect the anti-dive geometry.

Any details from the suspension geniuses??

[jpnovak]

I suspect that having the wheel location below (or lower in this case) the leading mount would reduce squat. This is because the momentum of the car will "pull back" on the a-arm extending the travel, thus reducing squat. Conversly, If you raise the leading mounting point this will contrubute to squat as the forward momentum of the car compresses the suspension.

These are relative terms. I wonder how much it would actually effect the system.

[doozer]

Ya me too, I just wonder if there is an ideal or if it is one of those moving targets

[Zeke]

Jamie, I think you've got that mostly right. However, when Chuck from Elephant told me about this, I believe by placing the bushing brackets lower by using the factory attached spacers on the top, you are placing the spindle higher (by a fraction) than the leading mount and raising the car a miniscule amount.

Is this what you mean by "lower?"

[doozer]

I thought the spacers added to the anti-dive

[doozer]

Ahh ok, without the spacers the car sees breaking force like a bump so it compresses the suspension on breaking.

With the spacers the car sees breaking force as a foreward resistance so no dive

[island911]

Lower CG (Center of Gravity) . . . that's thee way.

[Superman]

If I'm understanding this correctly, dive can have its advantages. Generally speaking dive is not good. But some folks know the INCREDIBLE turning response you can get when you crank the wheel during a brake dive. It's an example of the fabled "irresistable force." When you perform this technique, the front of the car IS going to turn around the cone. The only question is whether the rear will follow.

[Superman]

And yeah, I admire the ride heighth of your vehicle, Island. Suspension is next for me. Spring plate bushings and shocks all around. And I'm going to reduce my ride heighth by 1.5 - 2.0 inches. That is going to have a substantial effect on COG, corner and autocross times. I hope.

[arcsine]

I would hazard a guess that dive would contribute to quicker steering as the caster is reduced by the body rotating over the front axle.

[doozer]

On some of the F-cars (race) it is adjustable

How much dive is good dive??

[smdubovsky]

Superman, That is weight transfer. You get that wether the suspension dives or not. (Well, technically you get a fraction of a % more weight transfer w/ dive since the CG moves slightly)

If you set the angle(A) of the a-arm pivot so that the forward weight transfer (X) is equal to sin(A)*front braking force the two will cancel.

I do not know the advantages/disadvantages of low dive. One pro I could think of is no toe change under braking. I guess thats the same reason the 930 uses a better anti-squat geometry in the rear to avoid toe/camber change on accel. Beyond that I don't have a clue...

SMD

[K.B.]

Here's a picture of some spacers to affect the antidive. Anybody got the ideal size worked out?

picture borrowed from this thread

[doozer]

K.B. is this your car?

[jluetjen]

OK, here's my $0.02.

1) Look at the front suspension from the side.
2) Mentally draw a line (line A) through the two inner mounting points for the front A-Arm and continue that line towards the back of the car.
3) Mentally draw a virtical plane through the car's center of gravity (CG)
4) That line passes through the CG plane at a certain point. The location of that point is (generally) below the CG creates a lever arm. When you get on the brakes, the CG will rotate around the point where line A passes through the CG plane. Think of it like a torque wrench were you are pushing forward on the CG and the pivot is where A meets the CG plane. (Note, actually the lever arm is from the contact patch to that point as I've drawn.)
5) If you lower the front A-arm mounting (or raise the rear A-arm mounting, or both), you will create line A'. Note that you have also shortened the lever arm between the CG and the intersection of line A' and the CG plane. So you have effectively shortened your lever arm -- and increased your Anti-Dive. Note that lowereing your CG can have the same affect. Anti-squat incidentally is the mirror image of this process in the back.



Now that you know what it is and how to adjust it, so what? Is more better? No more then stiffer springs are always better. Yes, increasing anti-dive will keep the front end from diving and possibly improve your geometry when the front suspension is compressed. But there's no free lunch. It also makes the suspension act stiffer which will increase understeer under braking, reduce the "feel" of the front end and increase the likelyhood of locking up the brakes. None of these are necessarily good things. Unless your geometry is going out of whack when the front compresses* -- or you have significant ground-affect air-flow under the car, it's not clear to me that more anti-dive will improve the car. It will just make it stiffer.

* Note: Lots of people like to really lower their 911's by indexing the front T-bars rather then raising the spindle. If your car is like that, it very well may have geometry that isn't optimal under dive since you'll be losing camber. Adding anti-dive may help, but it's really a crutch since it doesn't solve the fundimental problem of the car's camber curve. If you want to fix that you need to raise the spindle (returning the camber curve to where Porsche's engineer intended it to be), and will lower the car's CG some and most likely increase the anti-dive to a lesser degree anyway.

[doozer]

Geniuses answer!!!

Thanks a lot!

Yes my spindles are raised

[masraum]

Tyson or Jack needs to chime in here. THe picture of the front end above, I think, is Black Beauty II. In Jack's write up, amoung other things, Tyson added spaces to the front of the front control arms to add extra anti-dive geometry. I believe Jack has even commented on it. Tyson is one of the people that we want to hear from in this thread.

[Bill Verburg]

When the A arms are angled up at the front(shims or whatever) there is an upward component to the force vectors operating through the suspension, i.e antidive

[jluetjen]

Bill -- I think that you have it backwards. What you're describing would be anti-droop geometry.

Up at the front means that the geometry provides no resistance to dive. Up at the back means that more of the braking forces are fed into the suspension geometry rather then the springs. If you add enough anti-dive, the suspension will actually bind up solid because none of the braking forces will be fed into the springs, but rather all through the geometry. If you raise the intersection of A so that it is above CG you will find that the front end actually jacks-up under braking. Some of the old VW buses had this situation in the back and the rear end would actually rise up under acceleration. Given the geometry of the swing-arm suspension, this may not have been a bad thing.

BTW, Carroll Smith's "Tune to Win" and "Engineer to Win" both discuss anti-dive and anti-squat geometry.

[Bill Verburg]

Bill -- I think that you have it backwards. What you're describing would be anti-droop geometry.

Sorry, Your right, i've had one too many glasses of wine