# Anti-Squat - Having trouble understanding the "definition"

Discussion in 'The Garage' started by Greg72, Jan 12, 2003.

1. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Anti-Squat - Having trouble understanding the \"definition\"

OK, I've been studying like a good little student... /forums/images/graemlins/blush.gif

I only have a few books on suspension theory, and none are for 4x4 type stuff....it's all racecar related. I figure some of the principles are at least transferrable. I was doing pretty good with the 3-Link and 4-link theory....pros and cons, etc. The problem I'm having is with the way you identify "anti-squat"

I know what the phenomenon IS....but I'm having trouble understanding how you determine it. I know that you start by drawing in imaginary line from each of the links on the rear suspension until you find the point where they intersect (instant center). Then you are supposed to somehow correlate that to the vehicles CG and front wheel centerline. There is also a line that gets drawn from below the rear tire up though the "instant center" point and continues on until it crosses the CG point and front wheel centerline????

So I guess I have two questions:

1. Is anti-squat measured by the intersection of that last line through the front axle centerline?

2. Does anything weird happen if the "instant center" happens to be in FRONT of the front axle centerline? (Not very likely to happen, but I suppose if the rear links were long enough and at very slight angles....it could happen!

Thanks! I'd take a diagram if someone has one. I'm looking at "Chassis Engineering" by Herb Adams p.67 (Figure 9-8) and can't quite tell for sure yet...... /forums/images/graemlins/smile.gif

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3. ### zakk1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

BTW, you should give a quick explination so that others can learn from this thread. I have a neanderthal understanding of anti-squat, and thier math forluma might as well be in chinese.

4. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Re: Anti-Squat - Having trouble understanding the \"definition\"

Well it's not like I'M an expert either (why do you think I'm studying all those books?! /forums/images/graemlins/grin.gif)

But the basic concept is this:

Anti-Squat is the characteristic of a rear suspension that prevents it from "sitting down" under acceleration. I'm not sure how important this will be on a lifted 4x4 vs. a racecar, but I can imagine if you are climbing a steep hill under throttle, it would be better for the truck to maintain it's rear ground clearance as much as possible (anti-squat)....

I don't know if the "climbing of the hill" weight transfer is the same effect as "applying lots of throttle on a flat surface"....but I assume not. The "anti-squat" seems to be dynamically induced as power is applied and torque tries to rotate the pinion housing....whereas climbing a hill seems to be more of a simple "weight transfer" shift from front to back....

SEE? I told you I'm not the best person to describe this!!! /forums/images/graemlins/grin.gif

There are plenty of "brainiacs" around here, so I know you'll get some better answers!

5. ### m j1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

basically anti squat is using the the axle rotation forces to pull on the upper links and push on the bottom links and turn that force into something that helps or at least does the least harm to traction. keeping in mind that under braking the opposite reaction will occur.

people in 4x4 world use terms like 'high' to descibe what not to have, but rarely provide a number.
on the other board there was one gentleman going on about not having a 'high' antisquat, but when I queried him it appeared he had as much as 200% dialed into his Toy.
now if you have been reading the books most will state % approaching 50% is up there.

side note:
there has been some good discussions out there on the common methods of calculating antisquat.
seems there are at least 3 different versions floating around. the image that Zakk posted is the one I found first, then there is the one from the Chev Power Manual, and a third one that is from a stock car publication(could be Herbs, dunno I havent seen it) that is completely out in left field.
sorry to add to the confusion, but better to be aware before you look the fool when someone steps in and says 'fine and dandy but your formula is completely wrong...'(leave that for me to experience)

I believe my webshots also has the Chev power manual pages in it. it is geared heavily to stock car racing and is old school.

6. ### eds77k51/2 ton status

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7. ### m j1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

the front axle centreline is an error in the diagram I believe.
the 11.4 should be at the force vector intersection

8. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

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Re: Anti-Squat - Having trouble understanding the \"definition\"

Is there an accurate way to calculate CG? Everything else can be measured with some degree of accuracy...

Rene

9. ### m j1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

with scales and a few measurements you can get a number.
I think you may have to lock out the springs and am unsure how accurate it will be.

corner wiegh the car, raise it to a 10 or 20 deg angle and rewiegh
formula is in the Chev power manual

10. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

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Re: Anti-Squat - Having trouble understanding the \"definition\"

Any chance of you scanning the formula and emailing it to me?/forums/images/graemlins/grin.gif

I guess I'll have to head over to mission raceway to corner weigh it...or come up with some sort of sling and use the crane and electronic scale at my old work...

Rene

11. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Re: Anti-Squat - Having trouble understanding the \"definition\"

Thanks mj,

I'll check your webshots for other sources there....

I may end up having to post an illustration myself to show what I "think" is right, and just ask for feedback that way too.....

If I start looking at differend diagrams, and each one has at least one mistake on it....it won't take long to become hopelessly lost!!! /forums/images/graemlins/grin.gif

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13. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

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Re: Anti-Squat - Having trouble understanding the \"definition\"

Thanks Mike...

Rene

14. ### m j1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

The author of that book, Mathis, posted in another forum and apologised for the massive omisions and errors the publisher printed.
apparently less then half of his submission made the book and he has no recourse to fix it having sold the work to them.

15. ### 70~K51/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

16. ### Chris Demartini1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

<font color="blue">mj, I have a couple questions. Does the position of the instant center in relation to the wheelbase (how far foward or back the IC is) effect anti squat?

Also, can this formula be used to calculate anti-dive in the front?

17. ### Triaged1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

Sorry for the heep link but this is a good discussion on how to find the CG of your truck with a neat excel file to go with it.
http://www.jeepaholics.com/tech/cog/#_Toc535118705

Do you want to know about just anti-squat or do you care about anti-lift/dive as well?

If we are only talking about the rear, the ring and pinion are mounted to the axle (not the chassis), and we are only dealing with a IC ahead of the rear wheels, then this is what the bible (race car vehical dynamics) has to say about it.

% anti-squat = ((tan theta)/(h/l)) * 100
where theta is the angle between the ground and the line going from the contact patch and the IC
"h" is the hight of the CG
"l" is the wheelbase (what 107" in your case?)

Is that the same equation you have?

Let me step out on a limb here...
If the IC was to be behind the rear wheels than the IC would have to be below ground to provide a pos(+) anti-squat value....
That doesn't sound fun to me!

One other thing to consider is what your anti-squat will be when the suspension is not at a static level.

I need to get a scanner and when I do I will scan that chapter (ch17) and put it online.

18. ### Triaged1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

The reason I think some of those guys can get away with 200% anti-squat is because of their limit straps with 1" of slack on them...a bandaid for a poor design (or at least one designed for clearance and roll axis not IC height).

19. ### Triaged1/2 ton status

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Re: Anti-Squat - Having trouble understanding the \"definition\"

I believe that 11.4 is correct...here is why (May bore some people to death...nerds only)

This is a way of equating the equation in your webshots and the one I posted...

If you call the distance the IC is in front of the rear axle "x" and the distance the front axle is above the ground "y"
Then the equation I listed can be written as:

AS% = ((y/x)/(h/l))*100
Or
AS% = (y/x) * (l/h) * 100

Then by similar triangles y*l/x would be the same as the force vector intersection at the front axle. It gets you away from having the wheelbase in the equation which makes it harder to use (for me) than the one I posted.

Make sence?

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