Percentage Anti-Squat Calculation

[MadBill]

I've run across a major discrepancy that has a huge effect on this calculation! Once you've determined the IC for a system and (for a solid axle) drawn the reaction line from the tire center of contact through it, how is the A-S
percentage calculated?

I have found numerous examples of two very incompatible approaches in suspension books and Internet sites. For example Herb Adams http://books.google.ca/books?id=rY2u...-squat&f=false , Billy Shope http://www.racetec.cc/shope/ and Warren Rowley http://www.rowleyrace.com/publications.htm all say that 100% A-S occurs when said line intersects a vertical through the front wheel center at CG height.



Others, such as the esteemed Carroll Smith and about half the Internet sources I've polled say 100% A-S is achieved when the line of force intersects the actual CG. This of course creates about a 2:1 difference of calculated percentages and casts much doubt on the interpretation of percentage recommendations! Can anyone shed any light on this disparity?

[exwestracer]

While a case could be made for the validity of either method, I think for a solid axle the front axle centerline model is more accurate. Here's why: Any leverage exerted against the chassis in pitch will rotate around the front tire contact patch. Jack up the rear of the chassis (>100% A/S) and what does the front bumper do? It gets closer to the ground. The C/G height affects how quickly the force is transferred for and aft (slope of the neutral line).

Now, if we are talking about an independent rear suspension, there is NO geometric anti-squat from pinion climb. There is only thrust anti-squat from the tires trying to push up under the car through the suspension links. In that case (no direct jacking effect), the C/G becomes a more accurate predictor of pitch effects. Carroll Smith dealt more with IRS than some of the other A/S gurus, so his illustrations may reflect that.

[Bryce]

I have found numerous examples of two very incompatible approaches in suspension books and Internet sites. For example Herb Adams http://books.google.ca/books?id=rY2u...-squat&f=false , Billy Shope http://www.racetec.cc/shope/ and Warren Rowley http://www.rowleyrace.com/publications.htm all say that 100% A-S occurs when said line intersects a vertical through the front wheel center at CG height.

This is correct.

Draw a horizontal line through your CG height, Draw a vertical line through the front wheel centerline. The intersect at a point. Draw a line from that point to the center of rear tire contact patch. That is now you 100% A/S line. If your IC is on that line you have 100% A/S, below is less than 100% and above is greater than 100%.

Can you post a link to Carroll smith talks about this?

[MadBill]

That may explain it, Ray! Smith's illustration (p. 35 of Tune To Win) is for an IRS system, but surprisingly, shows the line of force runing from the contact patch, as is conventional for a soild axle, rather than a parallel to same through the axle centerline as is usual for an IRS. Depending on the fore/aft CG location, this could pretty well cancel out the difference.

Nonetheless, if one Googles many samples of the millions of entries on the subject, there is no concurrence re method of calculation. The concern this raises is of course that a suggested percentage for any specific application may have as much as a 2:1 error if the data is based on the the alternate calculation to what the questioner is using...

The second part of the answer to Bryce's question is that many on-line sources allow viewing of Carroll Smith's work, e.g. http://www.amazon.com/gp/reader/0879...tu#reader-link

[exwestracer]

So stick with the front axle C/L model. Everyone I know of on here uses it, so if you want to discuss A/S percentages, at least we'll all be on the same page.

[MadBill]

Makes sense; "When in Rome..."

[exwestracer]

What cracks me up is when I see the IRS guys using the drag 4 link model and coming up with ridiculous percentages. "So when I hit the gas, my Corvette will flip over it's own front bumper..."

Bottom line is, not very many people REALLY understand this stuff. I teach it, and I'm still learning little things all the time...

[Norm Peterson]

I can't see anything but the Herb Adams/Billy Shope model working, just from the description of what 'squat' and its 'anti' effect are doing.

Regardless of how much A-S is present, the amount of rearward load transfer is known from statics.

This rearward load transfer is defined by [SprungMass] x [ForwardAcceleration] x [SprungCGHeight] / [Wheelbase].

Holding the first two terms constant in order to define only the geometric effect leaves you with [SprungCGHeight] / [Wheelbase].

If you think about what is happening at 100% A-S, all of the rearward load transfer going to the rear wheels is going through the rear suspension linkage. For that to happen, you'd have to have your A-S line passing through the CG height at the front axle line, as that is the rate at which rearward load transfer is occurring


BTW, this model only works for RWD vehicles. FWD cannot develop an anti-squat effect at all, and for 4WD/AWD the percentage of power sent rearward enters into it.


Norm

[exwestracer]

BTW, this model only works for RWD vehicles. FWD cannot develop an anti-squat effect at all, and for 4WD/AWD the percentage of power sent rearward enters into it.


Norm

We should qualify this a bit, just for clarity. FWD vehicles with independent suspension cannot generate pinion anti-squat, anymore than IRS rear drives, can. A front drive stick axle CAN generate A/S, although I realize that point is of almost no value to us here...

[Norm Peterson]

Agreed. Axle torque is removed from being loading that goes through the suspension. The only way that an independently sprung drive "axle" can generate any anti effect under acceleration would be due to the axle height itself being lower than the SVIC and the axle then trying to "drive under" it. It would be fairly easy to get "pro-squat".

If introducing yet another "anti" name isn't too confusing, it might be better to refer to any anti effect occurring at the front wheels during forward acceleration as "anti-rise". While we aren't particularly concerned with FWD here, it's an effect that would also apply to 4WD/AWD, in proportion to the amount of torque/power sent to the front tires.


Norm

[exwestracer]

Agreed. Axle torque is removed from being loading that goes through the suspension. The only way that an independently sprung drive "axle" can generate any anti effect under acceleration would be due to the axle height itself being lower than the SVIC and the axle then trying to "drive under" it. It would be fairly easy to get "pro-squat".

Norm

And it's important to remember that this type of A/S is NOT coming form pinion climb, therefore it has nowhere near the leverage that we see with a stick axle. Typically, getting 100% (calculated) A/S on a street car with IRS will make the suspension just about useless for handling...