Technical Can any of you suspension gurus offer some anti-squat advice?

I have plotted this up, and checked it a few different ways.

This is on my 56 Pickup All of the weight on these are up front, so I am not sure how the instant center distance should be positioned. This is just for street driving, not drag racing.

I prefer the upper (83% AS and a shorter IC length) just because it helps with exhaust clearance.

 

There are too many variables at play here to give you an exact answer on a custom vehicle.

If you aren't building a truck with a high torque engine, and sticky tires, the changes will be subtle.

If it that big of a concern, install an adjustable 4-link.

Remember, there are thousands of cars out there with equal length, parallel bar 4-links, that are not adjustable, and do not take into account anti-squat, in any way.

 

I agree with Gimpy. On a street driven car you would be hard pressed to tell the difference. On A pickup with little weight in the back, even less.

 

I agree with @gimpyshotrods as well. An equal length, parallel to ground 4 bar setup is a straight forward and easy option that will give you good handling for street use. It is important to understand that "tuning" a setup will usually move the way the suspension works towards a particular characteristic, and away from others. While tuning in some parameters you think you want, it can introduce other things that are undesirable and unsafe for all-round everyday driving.
For the street, aim for simple, proven and balanced............

 

You're overthinking it.

The purpose of anti-squat [and anti-dive] geometry was to allow manufacturers to use softer suspension without having undesirable pitching.
In the early days of racing [eg: g***ers] squat would help transfer weight to the rear with the limited traction available.
It can be tuned with spring stiffness.
A car with a lot of squat [and front lift] would really plant the rear tyres if it had stiff springs.

Nowadays they adjust "squat" to tune ch***is reaction.

A car with a lot of anti-squat is not desirable when driven on uneven surfaces. A car will "Tram-Line" [or rear steer]
The reason is anti squat causes the wheelbase to lengthen during compression [this is how anti-squat works]
This ^^^^^ causes a live axle to steer on uneven surfaces.

Shorter upper links are desirable for 2 reasons, one is ease of construction/placement.
The other is to counteract wheelbase changes of the lower link.

Most suspension travel is within a 2" compression range. So you really need to set it up for your desired ride height.[if the height changes due to weight change, then re-set or change the springs]

The lower link needs to be 1" higher at the front, so the first 1" of compression lengthens the theoretical wheelbase when it travels in an arc.
The 2nd inch of travel [2" compression] causes this link to arc back to a mirror position [from horizontal]
So 1" compression = longer wheelbase
and 0" and 2" compression is the same [on the lower link]

Now by having a shorter upper link , it will arc much more for the same amount of travel.
This is usually set up level or slightly down at the front.

This is needed so the top axle pivot point arcs forward to counteract the lower axle pivot arcing rearward.
The axle centerline will travel on a vertical path BUT the pinion angle will nose down slightly [which helps anti-squat slightly]



You will still get rear axle steer when the compression extends [all live axles get this] but the tyres are unloaded so it doesn't steer the car.

Set your suspension up so the axle centreline travels on a vertical path.

 

That's exactly what I did in the drawings above...

The question is what is a desirable anti squat value for street driving.

It is setup for 3" of vertical travel at the axle. I live and drive in a mountainous curvy region with bad roads.

 

This kind of topic is always makes for great bench racing but I think the best answer already exists within the aftermarket. For example, look at Fat Man Fabrications' 4-link for your truck. It was designed by an engineer and therefore, I surmise is better than anything I could possibly dream up. You may have more experience in this arena than I, but this is my usual thought process when it comes to stuff like this; someone with more experience, with more training has already developed a solution, why try to outsmart them? That said, I like your approach! Let us know what you end up doing!

 

If I an following your truck with 3" is going to be very twitchy and hard handling on mountain roads

 

I'm going to say about 237.

 

I must be the oddball. I like the rear lower

 

Kind of a different way to decide for handling purposes based on exhaust clearance.

 

3" of travel isn't too much [usually 2" compression and 1" rebound]
It is very normal for a comfortable road car to have 5" of spring load. You need to jack them at least 5" before a wheel lifts off the ground.

You need the geometry to be normal within the first 2" of compression [if you hit a 3" bump you're going to feel it anyway]

Understand this concept...... The Instant Centre [longitudinally] is similar to the Roll Centre [laterally]
A car is 3 dimensional. It will still get lateral acceleration [cornering] and weight transfer [accelerating and braking]
With cornering you would think a car would benefit from a high roll center to prevent bodyroll, but the opposite is the case.
The idea is to harness the lateral acceleration via "overturning moment" to plant more vertical load onto the tyre footprint.
A high RC causes side loads on the tyres [not vertical loads] and what you really want is more grip.
That is why high performance cars have very low roll centres and high roll stiffness.

The same could apply to accelerating/braking
I wrote above "A car with a lot of squat [and front lift] would really plant the rear tyres if it had stiff springs."
Having too much squat can still be controlled by spring stiffness

The formula weight transfer is ..........
Weight x CGH x Acceleration or Deceleration, then divided by wheelbase

With this ^^^^ even a dragster with no suspension still has weight transfer

The formula weight transfer via overturning moment is ..........
Weight x CGH minus IC x Acceleration or Deceleration, then divided by wheelbase

With this ^^^^ overturning moment is a rotational torque of the CGH [centre of gravity height] applied around the IC [instant centre]

Know this above you can calculate how much weight is transfer onto the rear footprint and also how much is transfered via the suspension.[so spring stiffness can be calculated]


Our O/T Mustang road racing car has a very low IC and has a lot of squat [so low that actually promotes roll understeer into corners which is good for side bite]
To compensate for this is has very stiff rear suspension, which would normally cause oversteer so the rear anti roll bar has been totally removed.

 

Manufacturers do this.........everything is a compromise to the parameters available.
It is only purpose built race cars that are designed with a different objective .

 

I didn't say that was the lone decision factor. I said I "like" it. Always has to be one in every group....

 

It depends on what you want to do, what your budget is. I like to learn, I like to understand the principles in play. I am an engineer (I have a mechanical and civil degree, but have spent all my career on the civil side of things, mechanical was/is my p***ion and has become my hobby). Having said that, I can 100 percent ***ure you that not all things designed by engineers are done correctly or without compromise. Say switching geometry on something to make it easier to m*** produce.

What I have also found is that a lot of these after market parts were started by guys like us here, who put something together that just works, and have been able to successfully market it.

For example, I have had a discussion on shocks with quite a few 4 Link suppliers. The shocks I am running have 4.65" of Travel. 3 compression and 1.65 of extension at ride height. My concern was that the shock may bottom out before contacting the bump stop. The companies (Tech guys) I spoke to said it is a possibility, but in the next breath said they wouldn't worry about it. How can that not be something to worry about? Every single 4 link kit I see installed, has no provision for bump stops prior to the shock bottoming out. The majority of the shocks provided with these kits, were not designed to be there own stop.

 

THIS, THIS is what I have been looking for...thank you for the explanation.

If I am wrapping my head around this correctly (taking springs and shocks) out of the equation, an IC closer to the rear of the car would increase the chances of pulling the front tires off of the ground, ***uming you had enough force to do it....

 

Just curious, but are you building a frame or working with a factory frame. Also is it a Ford, Chevy or?
Just had my mind working

 

If I hadn't already fit all the panels to the stock frame I would have had new rails bent. Live and learn.

 

Mimilan, did you post and then delete it? I skimmed over one talking about roll couple and now it is gone?

 

It is slightly more complex than that, but generally Yes

If the IC was low enough and the CGH was high enough for the rate of acceleration ,then yes.
Because the CGH is a rotational torque around the IC , then the IC placement can alter the behavoir of the vehicle.

[example]If the CGH remains constant and the IC is really low[same height] in the following scenarios.
If the IC was in front of the CGH, the CGH would rotate downwards during acceleration.
If the IC was behind the CGH, the CGH would rotate upwards during acceleration. [ this is a simplified "on paper" scenario]

The IC is the theoretical point of aceleration, and the CGH is the theoretical point of resistance [centre of m***]


Now we need to factor in the Anti squat line [used in most computer simulations] with this ^^^ simplified example. The squat line is dependant on CGH and Wheelbase.
[this is the violet line in the drawings provided by @tlmartin84 ]

Anti-squat [and squat] is only applicable to the CG and should not be confused with "overturning moment" [which is front lift and rear squat]
If you had the IC on the anti-squat line but directly below the CGH you would still get overturning moment during acceleration [front lift and rear squat] BUT the height at the CG would remain constant with no lift or squat [in the centre of the car].

Now if the IC is moved forward but still at the same height it would be below the anti-squat line.
There would still be overturning moment during acceleration [front lift and rear squat] BUT the height at the CG would be pushed downwards. [this counteracts front lift ,so the front height appears constant, and the rear squats down as a result of CG being pushed downwards AND overturning moment.

If the IC is moved rearward but still at the same height it would be above the anti-squat line.
There would still be overturning moment during acceleration [front lift and rear squat] BUT the height at the CG would be pushed upwards. [this counteracts rear squat ,so the rear height appears constant, and the front lifts up as a result of CG being pushed upwards AND overturning moment transferring weight off the front.



It gets a bit more complicated with cornering RC's instead of IC [there is 2 Roll Centres]
We can play with each end to get an inside wheel to lift [front wheels on corner exit, or rear wheels on corner entry]
Lifting an inside wheel can be beneficial with vehicles with an "open" differential.

The front and rear roll stiffness relationship to each other is known as "Roll Couple"

 

This post #12 is still there.

My response to your post #16 was copied, deleted and corrected [ just below your reply ] I re-worded it for some simplicity/clarity to prevent misinterpretation.

It is a lot easier to picture in your mind than to put into words once you understand it.

 

Herb Adams suggests a Satchell Link Suspension is the best overall 4 link....mainly because using the converging bars on the bottom of the axle lowers the roll center. I am not sure I understand why though. Can any of you explain this?

 

Can't answer that. But, do you necessarily want a really low roll centre at the rear? most rear wheel drive cars are set up with the static roll centre somewhat higher at the rear than at the front. The reasoning being, when you turn into a corner the roll couple ensures that the outer front wheel has good 'bite' onto the road surface lessening the chances of that scary 'wash off' terminal understeer . As long as there's room to install it, with a four bar setup I would use a Watts linkage to provide lateral location for the rear end -then the pivot point on the axle casing/diff cover is the roll centre.

 

Having the arms converge in front of the axle rather than behind, lowers the CG, correct?

 

Where the links are placed does not affect CG [centre of gravity] that is purely weight placement only.
But link placement does effect dynamic roll centre height . RC

A good example is Triangulated 4 links where the triangulated bars intersect at an "imaginery point" behind the rear end.
This has a cantilever effect on the RC, [as the suspension compresses the RC raises up]

If the triangulated bars intersect at an "imaginery point" in front of the rear end [near the X-member].
The RC would lower as the suspension compresses.

The absolute best [or most consistant suspension design] is the True Triangulated 3-link that is used on the Lotus Cortina.
The triangulated bars intersect at a pivot point [ball joint] making it a true triangle.
This pivot point is mounted under the Diff head for a low roll centre and the RC stays at the same height regardless of suspension travel
[Below]


: No Panhard bar is needed.
: Brake Torque from the Axle places tension loads on the Triangulated Bar.[stronger]
: Pinion Torque Reaction places compression loads on the Triangulated Bar. [minimal]
: Axle thrust places compression loads through the control arms because they are placed directly in front of the axle centreline.

Having a low RC and a relatively high CGH had a lot of "overturning moment" [bodyroll] across the rear.
The Lotus had a relatively stiff front to control roll stiffness .

This is why the L/Cortina was known for "Grabbing Air" [lifting a wheel] in the front when cornering.

Lotus deliberately set up their cars like this because they were running an open differential [and needed to keep the rear drive wheels planted]

 

I actually meant to type RC.....

This truck is heavy up front. Over half of the engine sets in front of the axle. The front suspension will also be stiff, with a stiff roll bar. Having said that I can only vary the rear roll center about 8".

Do you think that will make much of a difference? Should I shoot for a higher RC since the rear is so light.

 

You need the wheel rate of the front suspension to be stiff enough to support the weight upon it. [the same applies with rear suspension]
This is the sprung weight, so the front suspension needs to be stiffer than the rear suspension.

Suspension stiffness should not be confused with roll stiffness even though springs pay a part in controlling both.
The heavier the front the stiffer wheel rate, the more probability for understeer.
So you would be better off with a softer front roll bar in the front and have stiffer roll stiffness in the rear [to balance/remedy understeer]
Because the rear suspension is softer [for weight] you would need a stiffer rear roll bar.

Roll stiffness is a combination of suspension stiffness and roll bar stiffness [softer springs with heavier bar and stiffer springs with lighter bar]

The stiffest end of the car will always slide out first

The biggest mistake with handling is combining Heavy bars with stiff suspension [this causes body-roll to transfer most of the weight onto 1 outside tyre]
This is because the ch***is normally body-rolls the same amount [degrees] at both the front and the rear [which why ch***is designers prefer torsional rigidity]

 

Regarding roll bar stiffness. The stiffness is in relation to the amount of weight on each end of the car. Not necessarily in direct comparison between the two bars themselves?

Does that question/statement make sense?

As in the front bar may be bigger and heavier than the rear bar, but when you look at it based off of a percentage of the load it carries, you want this percentage to be smaller than what the rear is carrying?

EDIT:

I was able to answer my own question. They are not in relation to the bars themselves, but rather the front or rear.

 

Curious, here you say to point the lower link up an inch at ride height.

The 4-Link manufacturer states to do the same.

With the lower links being parallel to one another, wouldn't having these arms pointed up all but eliminate roll understeer? Thus needing the lower arm to actually slope down towards the ground on the frame end?