S197 Anti Squat Numbers/ Theory/ Real World Results/ Etc

[Mad Hatter]

Thats what I love about the S197. Such a simple set up that's easy to try out different setups. I went through all the squat setups I could think of, nothing like feeling the changes for your self.

 

[Dave_W]

I've been trying to wrap my head around something.

Finding the instant center for anti-squat geometry in side view seems the same as the finding the instant center for a dual wishbone (aka SLA) suspensions in front view, i.e., the Side View Swing Axle diagram for a 3- or 4-link solid axle is equivalent to the Front View Swing Axle diagram for dual wishbones.

You can also define a SVSA IC for McStrut similar to the FVSA - the IC is where a line drawn from the strut top at 90 deg. to the strut intersects a line drawn through the lower wishbone inner pivots.

When doing the FVSA geometry, we draw lines from each contact patch through that side's IC, and then call the intersection of those two lines the Roll Center. And we use the distance between the RC and CG to define the roll and jacking forces.

But for some reason we stop short of doing that in the SVSA geometry - we just use the IC position relative to a line from the contact patch through the CG to determine the anti-squat and anti-dive amount.

It seems to me like we should define the roll center in side view (the pitch center?) the same as we do in front view. And that the anti-squat and anti-dive are really from what we would call the jacking forces in an FVSA diagram - a function of the distance between the pitch center and the CG. Except in side view, increased tire "scrub" due to high jacking forces isn't a detriment, as it's parallel to the direction the tire is rolling. Am I wrong here?

 

[bnight]

I've been trying to wrap my head around something.

Finding the instant center for anti-squat geometry in side view seems the same as the finding the instant center for a dual wishbone (aka SLA) suspensions in front view, i.e., the Side View Swing Axle diagram for a 3- or 4-link solid axle is equivalent to the Front View Swing Axle diagram for dual wishbones.

You can also define a SVSA IC for McStrut similar to the FVSA - the IC is where a line drawn from the strut top at 90 deg. to the strut intersects a line drawn through the lower wishbone inner pivots.

When doing the FVSA geometry, we draw lines from each contact patch through that side's IC, and then call the intersection of those two lines the Roll Center. And we use the distance between the RC and CG to define the roll and jacking forces.

But for some reason we stop short of doing that in the SVSA geometry - we just use the IC position relative to a line from the contact patch through the CG to determine the anti-squat and anti-dive amount.

It seems to me like we should define the roll center in side view (the pitch center?) the same as we do in front view. And that the anti-squat and anti-dive are really from what we would call the jacking forces in an FVSA diagram - a function of the distance between the pitch center and the CG. Except in side view, increased tire "scrub" due to high jacking forces isn't a detriment, as it's parallel to the direction the tire is rolling. Am I wrong here?

Anti-Squat is forward/backwards weight transfer so the Rear IC in relation to the Anti-Squat line (rear tires contact point to front CG) think acceleration/braking forces.

For sideways movement (aka Body Roll) you care of where your Roll Center is in relation to CG and the ground on lowered cars aka S550 at 1" drop the RC got so far bellow ground that you end up needing ether a Roll Center correction kit or bringing the car up.

So the Anti-Squat affect how the car react during acceleration/braking for example my rear is jacking up under braking because of more Anti-Squat than needed when lowered the car this leads to sub optimal brake performance.

I hope this helps.

 

[Dave_W]

Yes, I see that. But what I'm saying is, whether it's in front view or side view, you've got a CG where mass is experiencing a force from acceleration (lateral or longitudinal), you've got contact patches where a countering force is being applied, and you've got suspension links (and wheels and tires) between them.

When there is weight transfer caused by longitudinal acceleration, some portion of that weight transfer happens instantly through the inelastic suspension links, and the remainder happens over time through the elastic springs & shocks (where for some period of the time the force is absorbed as potential energy stored in the springs and heat created in the shock, until steady-state is reached).

This is identical to double wishbone suspension front view, and in this case, it is common to divide the force of the weight transfer experienced at the CG and resisted at the contact patch into vertical and horizontal components based on the CG, RC, and contact patches. The closer the RC is to the CG (technically, the closer the RC is to a line between the contact patch and CG), the high percentage of vertical and lower percentage of horixontal force. The hoizontal portion is what creates the roll of the sprung mass, and the vertial portion creates what's called "jacking force." The horizontal portion creating roll in the sprung mass goes from the CG through the elastic springs & shocks (and sway bar) to the contact patches. The vertical portion goes directly through the inelastic suspension links. The jacking force is generally thought undesirable due to causing lateral tire scrub which reduces grip. In addition, the tire sidewall is really an undamped spring, and the instantaneous application of the force to it can also reduce grip as it oscillates.

in side view, we should see all this the same way -- the same horizontal force resulting in pitch of the sprung mass from longitudinal accleration, and also a vertical jacking force. I'd guess that since the tire scrub from jacking is now longitudinal, it is no longer a concern, but the instantaneous load transfer and tire oscillation still is.

I think the ratio of horizontal pitching force to vertical jacking force is actually what we're trying to calculate with anti-squat, but we're not using the exactly same diagrams (we have a Roll Center but not a Pitch Center; in FV we connect contact patch and IC to find RC but in SV we connect contact patch and CG and see if the IC is above or below it), nor using the same terms (horizontal roll and vertical jacking force components vs. either anti-squat or anti-dive).

 

[RoadRacer78]

@TeeLew

I never got a clear answer on this:

Now that we are maybe on the same page. If you were solving/testing for my "problem" which is where to set the AS% on a 750hp/750hp TQ street car that is only driven on the street. But driven as hard as can be on the street. What would your suggestion be?

 

[JDee]

My suggestion would be to stop playing games on the street and take it to a track. Street racers do nothing but bring disrepute on mustang owners and are bad for the entire mustang community.
We don't need that.

 

[racer47]

I think the ratio of horizontal pitching force to vertical jacking force is actually what we're trying to calculate with anti-squat, but we're not using the exactly same diagrams (we have a Roll Center but not a Pitch Center; in FV we connect contact patch and IC to find RC but in SV we connect contact patch and CG and see if the IC is above or below it), nor using the same terms (horizontal roll and vertical jacking force components vs. either anti-squat or anti-dive).

You're on the right path. I don't exactly know your answer. All of these 2D plots are vehicle dynamics 101. The right way to do this is working with all of the instant centers and lines back down to the tire contact patch in 3D space. I was a tire test engr for a very long time and generated a lot of data that went into these models and talked to several PhD level vehicle dynamics engrs. The real race car models are so much more complicated. I thought I knew stuff until I met these guys.

 

[RoadRacer78]

My suggestion would be to stop playing games on the street and take it to a track. Street racers do nothing but bring disrepute on mustang owners and are bad for the entire mustang community.
We don't need that.

Street racing is done in a straight line. I'm not trying to line out a combo for that.

I'm trying to line out a combo for people that want a nice handling, nice riding car that doesn't break the bank.

 

[bnight]

Street racing is done in a straight line. I'm not trying to line out a combo for that.

I'm trying to line out a combo for people that want a nice handling, nice riding car that doesn't break the bank.

which bring us back to the case that Anti-Squat has limited impact on handling unless you are jacking up the car aka more than 100% AS. You don't want 100% AS and obviously you don't need too little as then the car will squat and what works for drag strip car is not what works for corner cutting machine. What is the issue you are trying to solve with AS ?

 

[RoadRacer78]

which bring us back to the case that Anti-Squat has limited impact on handling unless you are jacking up the car aka more than 100% AS. You don't want 100% AS and obviously you don't need too little as then the car will squat and what works for drag strip car is not what works for corner cutting machine. What is the issue you are trying to solve with AS ?

This has been covered.

I'm not trying to solve an "issue". I'm trying to fine tune the combo by getting it somewhere between 35-75% AS (roughly). You can only get on either side of that with bolt ons. I will need to do something custom.

 

[bnight]

This has been covered.

I'm not trying to solve an "issue". I'm trying to fine tune the combo by getting it somewhere between 35-75% AS (roughly). You can only get on either side of that with bolt ons. I will need to do something custom.

https://suspensionsecrets.co.uk/anti-squat-dive-and-lift-geometry/ Maybe this will help but overall for street car 75% AS should be better. Though as mentioned OEM Mustangs often have a lot less than that which is the reason they rise the nose so much when accelerating. If you look into drag race cars they have very minimal amount of AS because they try to put as much weight on the rear tires as possible during lunch aka lifting the front tires. However if you look into race cars they have a lot of AS as they try to keep the floor as stable as possible. Which is why there is not a good compromise but overall for street car if you like how a OEM Mustang goes you can use it's AS values as a target for example you mentioned you have a GT500 they are often modded to the levels of power you are looking for and they are great no the street so maybe use that AS as a target.

 

[RoadRacer78]

https://suspensionsecrets.co.uk/anti-squat-dive-and-lift-geometry/ Maybe this will help but overall for street car 75% AS should be better. Though as mentioned OEM Mustangs often have a lot less than that which is the reason they rise the nose so much when accelerating. If you look into drag race cars they have very minimal amount of AS because they try to put as much weight on the rear tires as possible during lunch aka lifting the front tires. However if you look into race cars they have a lot of AS as they try to keep the floor as stable as possible. Which is why there is not a good compromise but overall for street car if you like how a OEM Mustang goes you can use it's AS values as a target for example you mentioned you have a GT500 they are often modded to the levels of power you are looking for and they are great no the street so maybe use that AS as a target.

Everything in bold is incorrect.

 

[bnight]

Everything in bold is incorrect.

What you say Test both and let us know.

 

[RoadRacer78]

What you say Test both and let us know.

Click and learn.
Link

 

[ShifterX]

I'm trying to line out a combo for people that want a nice handling, nice riding car that doesn't break the bank.

That seems like an admirable goal.

My experience is mostly cars with F/R double wishbone. All suspension tuned for street to track handling. Generally I have been surprised by the competence of a tuned s197 with one exception, the poor pitch control. Relative to other platforms, it’s inherent. It’s one of the few characteristics that reminds me of my first real project car, my SN95 Mustang.

To @RoadRacer78 point, there may be something there to pick at that is not just a spring/damper solution.

 

[bnight]

Click and learn.
Link

That video perfectly illustrate the concept the 1st setting with Uper bar aimed down is what you need for drag racing. The 2nd setting with both parallel is what you need for a street car. P.S. I took part in learning all of this from late Kenny Brown in his Speed Therapy Academy and that concept was in his learning materials.

 

[TeeLew]

That seems like an admirable goal.

My experience is mostly cars with F/R double wishbone. All suspension tuned for street to track handling. Generally I have been surprised by the competence of a tuned s197 with one exception, the poor pitch control. Relative to other platforms, it’s inherent. It’s one of the few characteristics that reminds me of my first real project car, my SN95 Mustang.

To @RoadRacer78 point, there may be something there to pick at that is not just a spring/damper solution.

I can't really speak to the S197 stuff, but every canned S550 spring package has a front that's too soft, a rear that's too stiff and too much static rake. They're all meant for the guy who takes the last 1/2 of the freeway on-ramp fast every once in a while, not the guy trying to go quickly around a track.

The only company that got it right was Ohlins (no surprise), but that's with their coil-overs, not stock replacement.

Too soft of a front spring allows too much brake dive and makes these things a handful on corner entry. Once you stiffen the front a bit, then you realize additional front spring rate *does not* increase understeer until you get to the point where you've lost all compliance and skitter across the track. If anything, it reduces understeer, I think through better camber control. Further, It gives confidence on turn-in, so you can take a more aggressive entry line and make more aggressive inputs, both will get you pointed toward the exit earlier and allow you to go to gas. You'll find after you increase front spring rate that the rear can be reduced, you won't lose much transient response (because the front is reactive) and you can improve on-throttle traction.

You have to trail brake any GT car to go fast, and front engine ones the most. If you don't have a stable rear end on corner entry, then you won't be able to trail into the corner. I personally do not like having to keep the rear under me on turn-in. It's not (just) that I'm some wanker who doesn't like to get up on the wheel and drive like a man. That type of a setup simply compromises the driving line too much for me and I slow down. Through my adventures, I've come to find that 99% of the people on the planet are like this, but very few admit it. This is not to say that I prefer dump-truck understeer or something like that. I don't. I just like to drive cars with the penalty of over-charging a corner is losing the front and missing the apex by a car width, not exiting the track backwards and loading onto the flat-bed.

Bnight is on this type of a setup (he might have bumped the rear up a tick?). I'm on this type of a setup. I'm aware of others. I've been on this crusade for years (even arguing against Norm). The first thing that people say when they drive it is, "It feels more like a BMW."

 

[Dave_W]

Click and learn.
Link

99.9% of drag racing is launching the car from a dead stop as hard as possible in a straight line, preferably with tires with very soft sidewalls and low pressure. That leads to a very different optimization than road racing (or autocross), where your're also trying to also slow and turn the car on tires with short, stiff sidewalls (for lateral grip and feel), and do so smoothly to maximize time at the edges of the friction circle.

I watched a couple videos from that channel, and while he seems to be very knowledgable in getting a drag race car to "hook" based on adjusting a 4-link and shocks, I'm not sure he explains the actual physics correctly, especially on shock valving.

The only things that affect the amount longitudinal weight transfer due to forward accleration of the car are wheelbase length, CG height, and amount of longitudinal accleration at the contact patch. That's it.

What suspension angles do is change how much of that weight transfer happens instantly through inelastic links, and how much is done through elastic springs & shocks. The elastic transfer happens over time because some energy is used to compress the springs, and thus is taken away from the force at the contact patch, until a steady-state is reached - the springs have reached a new eqilibrium of compressed height for the force they're seeing. Shock valving lets you time how long that transient condition lasts. But the longer it lasts, the less is removed from the contact patch at each instant - you're spreading the same amount of force reduction over a longer period.

An infintely stiff shock takes an infinite amount of time to compress or rebound, so at each instant you're reducing the force of weight transfer an infintely small amount, and you're back to applying the all the weight transfer force (less an infinitely small amount) instantly to the contact patch. With no shocks, the force reduction happens as quickly as the springs can be compressed (a function of the inertia of the sprung mass). Though with no shocks, the springs will also oscillate, causing varying force in the tire and likely loss of grip and traction as the sidewall (another un- or under-damped spring) also oscillates from the varying spring force, and the force at the contact patch varies quickly and wildly.

Now, I think the complication with the force accelerating a car is pinion climb, which creates a force that is different than simply the inertia of the CG being acclerated from the contact patch. The force of pinion climb is reacted at the contact patch, and through the suspension links to lift the sprung mass, plus the front axle unsprung mass if we wheelie.

So a force created at the pinion is balanced by a force at the contact patch and the Moment of Inertia of the sprung mass. I'm not seeing how the CG height at the front axle plays into it, which is what creates the "normal line" for anti-squat. I'm starting to think this whole Anti-Squat Percentage thing is a back-of-napkin approximation of the actual physics. It may be good enough to compare two setups and give you a general feeling about the difference, but I'm thinking any hard numbers from it should be taken with a big grain of salt.

 

[DaveW]

FOr whatever its worth, I have learned more about making a solid axle car work from circle track guys than almost anywhere else. That's a competitive world and they won't tell you but you can look and learn and bring thing to the table to try.

In the end though, its all experimentation and what makes the car faster. I haven't won an autocross by getting 78% AS but I have won many by having the fastest time. No times for street stuff, but you could still translate the driver feel part.

 

[ShifterX]

Quick update to my experience. 11GT on koni and bmr 260/220 springs and oem sways. Setup is a fun street car and track toy. It’s mostly dialed in with my biggest complaint being pitch control. Heavy nose rise and dive on acceleration and braking. Compared to other cars I have driven I would call it sloppy, and this hasn’t changed with 2 different sets of dampers and three sets of springs.

My thought was just get the stiffest springs, which led me to Maximum motorsports. Had a back and forth with them, and they recommended relocation brackets. Mainly because I was happy with the roll balance, brackets are cheaper and all my complaints centered around acceleration and braking.

I went with their brackets, which are very nice, and used the upper mount hole. To note, that hole is 1 7/8s down from the oem hole. You can tell by where the holes were drilled, care was taken to make the upper hole as high as possible. Also they use a 2 bolt design to make room for all control arms. This was close to being an issue with my whiteline arms as they use a large rubber bushing. Once done the rear trailing arms are near level, with a 1 degree down slope to the axle.

Brackets on and on the road in short order. Bottom line is as soon as I touched the throttle I could tell the behavior of the chassis changed. No more sky high nose or front end smash on braking. I was surprised how much the changes to the rear affected the front. The car is way more predictable under power both straight and cornering. Interestingly I am not sure it hooks any better, but it’s easier to launch. It’s also easier to transition to and from power on oversteer. Dynamically the car is just way easier to drive. Plus the suspension is still relatively soft and very streetable.

My guess is others have solved this issue with stiffer setups in general. With enough spring and damper, the pitch can be controlled. I may still end up going with stiffer springs when I get some more track time, but going with relocation brackets definitely addressed my issues.