Anti-dive, anti-squat values????

For an IRS, point 1 on your construction line 1-2 is at the axle center, the effect of which is to make it difficult to build in much anti-squat.

From reading elsewhere, I see figures of 50% - 65% being bandied about for stick axle sporty cars (Fox Mustang, F-body) other than those primarily intended for dragstrip service. OTOMH, you might get 25% - 30% or so anti-squat with IRS.

Norm

Thank you NormPeterson for your reply,

I was wondering about what you said, so I've made some research. This is what I have found:

Take the ratio of the CG height from the axle and divide it by the wheelbase.

To achieve maximum anti-squat* on a IRS you must obtain the same ratio by taking the vertical length between the instant center and the axle, divided by the horizontal distance between the instant center and the rear axle.

*Fundamentals of vehicles dynamics 1992. (Assuming that the spring rate is equal Front and Rear)

I don’t know if this is what you meant in your previous post…

But still, how much anti-squat should I put taking account the negative effects on the camber and caster? What about the anti-dive? Any comments?


Alex Laurenceau

In literature that I have, anti-characteristics are usually given a cap due to friction in the suspension joints (rubber type ones), which gives rise to NVH, poor wheel/ground tracking, lockup etc...

For anti-dive around the 30% maximum is a general figure. Although Ive seen and driven in many SUV's which have been lifted and have 80% A-D, although i think there could be some comfort gained in those!. As of interest the Mclaren F1 has about 60% both A-D and anti-squat, although it uses a clever system in that all its supension is on steel bushingss and then a subframe is located onto the chassis with poly bushes.

A-squat of about 25% would be a general cap, pretty much dependant on the amount of power you have, so with 500hp you might need a bit of that to counter squat.

These figures might be a little dated now, I see many examples that go beyond these figures, so it can be done. I guess it would be cool if you can design the suspension with provisions to go higher for testing.

One thing to consider is the handling balance change with the anti-characteristics, these will affect your balance during braking/accelerating times, this can sometimes be helpful or a problem if you didnt account for.

Jakub

Boy, am I confused! You say the car goes from right to left. I'm assuming positive X to the left, then, and that you're showing the front wheel geometry. If you're interested in the no squat/no rise line for a FWD car, during acceleration, it passes through the front wheel hub (as Norm said) and has a positive tangent, calculated from the horizontal, equal to the ratio of the CG height to the wheelbase. The percentage is 100 times the actual tangent to the h/l ratio. You can't define a percentage for braking as it is a function of the front/rear braking effort ratio.

For a RWD non-IRS car, a similar no squat/no rise line passes through the tire patch with the same tangent. For an IRS car, it passes through the wheel hub. Percentages are defined in the same manner.

Good day everyone,
Thank you for your posts.

On Monday I will try to put up on the web a few diagrams that shows how (according to my understanding and what you told me) to calculate the percentage of anti-squat and anti-dive for a rear wheel drive fully independent suspension.

We can discuss our opinions from there. I want to be sure that we all calculate the percentage values in a same manner.

Thanks.

Alex Laurenceau

Okay,
Here are the links for the anti-characteristics diagrams:

While braking:

While accelerating:

According to those drawings and what information I've got from a friend's
2003 Viper, this is what I get:

Anti-dive: approx. 32%
Anti-rise: approx. 40%
Anti-squat: approx. 30%
Anti-lift: approx. 23%

I guess it makes sense, but I would like to know more about what you think.

1 - I know that more anti squat or dive is increasing the camber and caster
angle variation. What camber angle variation is acceptable for a high
performance street car? (500Hp, 3000lbs, 115in Wheelbase).

2 - I have also notice that it is possible to play with the side view instant center length. I know that too short is bad because it can create a jacking effect, but what about to long???

Thanks!

Alex Laurenceau

Alex, in order to use the suspension linkage in this way, they must be carrying traction/braking/inertial forces. So, in the case of a RWD car, you can do absolutely nothing about the front end of the car coming up during acceleration.

As has been pointed out before, an IRS RWD car has a no squat/no rise line which passes through the axle center, not the tire patch. (I use the words "squat" and "rise" to define motion of the driven end of the car during acceleration and "dive" for motion of either end during braking.)

BillyShope,
I have noticed that some books use the term rise instead of lift. Since there is nothing you can do about the front of the car going up I guess it doesn't matter in our case, but I aggree with you statement.

But just like you said, in the anti-squat diagram ( the no squat/no rise line which DOES pass through the axle center, not the tire patch. Do you still believe it is not ok?


Alex Laurenceau

1 - I know that more anti squat or dive is increasing the camber and caster angle variation. What camber angle variation is acceptable for a high performance street car? (500Hp, 3000lbs, 115in Wheelbase).

Alex,

You also get longitudinal movement of the front axle center in bump, which can even be forward. Unless you've got lots of longitudinal compliance you'll tend to end up with an overly harsh ride. You've possibly noticed that harder braking on smooth pavement is less disturbing than braking somewhat less hard over the rough patches and have delayed your braking on occasion for that very reason - at least in part that's your anti-dive working against compliance. This effect tends to be exaggerated with the combination of lots of A-S and a long SVSA; the SVIC is way up and back, so the arc followed by the axle starts out well below the horizontal.

Norm

Alex, the no squat/no rise line must pass through the axle center AND be parallel to a line passing through the tire patch and the intersection of two other lines, one a horizontal line through the CT and the other a vertical line through the front tire patch.

BillyShope, NormPeterson,
Thank you for your response.

Billy,
I understand very well what you are saying. That is why I have expressed the anti-squat diagram with a percentage in the formula. The picture doesn't represent the 100% anti-squat situation! But in the formula, if I respect the 100% anti-squat, the line becomes parallel.

Norm,
Do you think that it would be a bad thing to keep a long SVSA with a low SVIC (a SVIC way back yes, but not up to much?)? Still keeping a 35% anti-dive, this would help reduce the angle difference between the top and the bottom A arm and the arc followed by the axle starts out almost horizontaly. Is there a negative side to this?


Alex Laurenceau

Do you think that it would be a bad thing to keep a long SVSA with a low SVIC (a SVIC way back yes, but not up to much?)? Still keeping a 35% anti-dive, this would help reduce the angle difference between the top and the bottom A arm and the arc followed by the axle starts out almost horizontaly. Is there a negative side to this?

Pretty much by definition, a long SVSA with a low SVIC is a small anti-dive %. IIRC, Mercedes-Benz has tinkered around with angling the LCA upward toward the rear (as seen in side view) and the UCA downward (ditto), apparently to keep a sane axle path without giving up all the A-D. But note that you can't escape the short SVSA that way.

Out of curiosity, are your anticipated ride rates soft enough or are your bump travels small enough such that you need a lot of this kind of help? I'm wondering at least in part because that 115" wheelbase ought to be doing a fair job of reducing the longitudinal weight transfer, particularly in comparison to the pony cars in the 101" range or C4 or C3 Corvettes that were below 100".

Norm

Norm,
The Viper I have studied has around 34% anti-dive but has a short wheelbase: 98.8in.

On our car, the bump travel is going to be pretty short so you're right; anti-dive of 30% should be more than enough in for this long wheelbase vehicle.

Thanks

Alex Laurenceau

Alex, I have trouble interpreting your illustrations without arrowheads. If the "F" value is measured vertically from the axle center to the CG, then, yes, it is correct. But, it surely doesn't look like that's the case. But, again, I'm having trouble interpreting it.

Billy,
on page 253 of T. Gillespie, Fundamentals of vehicle dynamics (1992 SAE). It is is written that 'F' is the height of the IC from the axle line. 'D' is the leght of the SVSA. The ratio F/D divided by H/L gives us the anmount of anti-squat.

Do you define a anti-squat value otherwise?

Alex Laurenceau

No, that's fine. (I meant to type "IC" and not "CG.") I just couldn't see that in your picture. Sorry about the confusion.