IC, antsiquat %, and axle torque / vrs chassis seperation

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IC, antsiquat %, and axle torque / vrs chassis seperation

Post by 3pedals » Sat Feb 27, 2016 10:53 pm

I'd like to discuss Axle torque's effect on chassis separation.
If you have an IC on the 100%AS line, can axle torque still cause chassis separation?

I posted the exact same question on another racecar forum, and had only 1 response, and it was not helpful.

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by Brian P » Sun Feb 28, 2016 8:45 am

Explain to the rest of us what you mean by "chassis separation".

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by 3pedals » Sun Feb 28, 2016 9:51 am

Chassis separation:
think of a ladder bar car or a leaf spring car that pushes the tires into the pavement on the application of power, and sees separation between the tires and chassis.

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by In-Tech » Sun Feb 28, 2016 9:58 am

3pedals wrote:Chassis separation:
think of a ladder bar car or a leaf spring car that pushes the tires into the pavement on the application of power, and sees separation between the tires and chassis.
:lol: :lol: I think we all gather that. The tough part is you can see your question clearly in your head, getting that thought into typing is the rub. A clear and concise answer requires a clear and concise question. Not knocking, the interweb is like multiple texts vs a quick phone call. Lots to be misunderstood. :)
Heat is energy, energy is horsepower...but you gotta control the heat.
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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by 3pedals » Sun Feb 28, 2016 10:36 am

In-Tech wrote: :lol: :lol: I think we all gather that. The tough part is you can see your question clearly in your head, getting that thought into typing is the rub. A clear and concise answer requires a clear and concise question. Not knocking, the interweb is like multiple texts vs a quick phone call. Lots to be misunderstood. :)
Agree whole heartedly,
Do you have anything to add regarding the forces at play in a rear suspension?
I have done a lot of research about IC and the force vectors that work through it, BUT I never see axle torque factored into the Antisquat.
example: a ladder bar car
everyone knows a ladder bar car will " separate" at the hit - 99% of the time it is because the IC is well above the car's neutral line.
If the front mounts of the ladder bars are positioned on the exact neutral line of the car, will the car have ZERO chassis seapation at launch, as the theory surrounding plotting the neutral line suggests? - Or will axle torque, lifting at the IC point, still separate the axle from the chassis?

The reason for my wanting better understanding about this, is that I have never seen Axle torque factored in to the forces lifting the chassis at the IC point, BUT it is DEFINATELY there. In first gear, axle torque is first gear ratio x rear gear ratio x engine torque, so it is a significant force.

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by Brian P » Sun Feb 28, 2016 10:59 am

The reaction torque of the axle is always accounted for if the antisquat is being calculated correctly. (If it isn't ... it isn't being done correctly!)

For a counterpoint ... if you have IRS in which the rear axle reduction ratio forces and torques are contained within the chassis (because the differential is bolted to the chassis rather than being part of a live axle assembly), it is much more difficult to have any meaningful antisquat.

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by peejay » Sun Feb 28, 2016 11:01 am

You don't need to factor it in because that is what instant center already takes into account.

Torque on the axle wants to lift the rear of the car up, forward acceleration wants to push the rear of the car down, the instant center's relationship to the car's center of gravity (relative to the front axle and rear contact patch) determines how much antisquat there is, period. More torque on the axle always gets the same amount more forward acceleration so they always balance each other out.

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by 3pedals » Sun Feb 28, 2016 11:04 am

I have yet to see an antisquat line plotted that had any axle torque input. can you please point me to a formula or drawing that shows how this is "calculated correctly"
I agree about the IRS example. I believe it is impossible to achieve antisquat with an IRS

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by Brian P » Sun Feb 28, 2016 11:40 am

It sounds like you need to convince yourself that the derivation of "instant centers" already includes the axle torque reaction.

In that case ... Pick a suspension design for which the instant centers are easy to figure out (e.g. torque arm). Pick a test-case vehicle. Pick its weight, weight distribution, wheelbase, center of gravity height, etc. It doesn't matter what it is, it doesn't even have to be a real vehicle. It just has to be a test case that is easy for you to do the math.

Now, do the traditional instant-center calculation.

Then, forget about the traditional instant-center method of figuring this out, and go back to first principles: force based analysis. Figure out what torques and forces apply to each component and do your vector algebra. (This is why I said you should pick a case where it is easy to figure out what is going on.)

Compare results. If they are different, look for your mistake.

As for IRS, it is possible to have some degree of antisquat but it's not pretty and it certainly has side effects. If the linkage is such that the hub moves back together with moving up, then the forces involved with pushing the hub forward will have a vertical force component on the bodyshell ... and that's what antisquat is. A trailing arm heavily sloping down to the back (chassis-end pivot point quite a bit higher than the hub-end pivot point) will do this.

Roadracing motorcycle swingarms have antisquat because there is a down-angle to the swingarm. (The drag-race folks slam their bikes to the ground and often render the rear suspension ineffective, in which case it no longer matters, but us roadrace types need suspension that works.)

"you can make any bad suspension design work if you don't let it move" ...

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by peejay » Sun Feb 28, 2016 1:05 pm

The 4wd stage rally cars are getting a measure of it in the front suspension (that would be pro squat, wouldn't it?) with heavily angled control arms so the wheel motion has a significant fore-aft motion with travel. They get the antidive back by mounting the struts well forward on the uprights so suspension travel causes the upright to have to rotate. As a bonus, this gets the axle out of the way of the strut so it can extend well below the axle centerline, so they can get heaps more travel so they can run softer suspension and get more grip.

So it is definitely possible in an independent suspension but no, it is not pretty, unless you enjoy the effects! The modern cars look like slot cars even on gravel and sloppy mud. Not as entertaining to watch from a spectator point of view but fascinating from an engineering point of view.

https://www.youtube.com/watch?v=d9EgLA7Sh14

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by 3pedals » Sun Feb 28, 2016 2:07 pm

peejay wrote:You don't need to factor it in because that is what instant center already takes into account.
this is what I disagree with,
IC is the point which the rear suspension rotates around, and the point at which the car is pushed by the force between the tire contact patch/road .
Axle torque is not used to determine the Instant Center point.


Torque on the axle wants to lift the rear of the car up, This I agree with, since every action must have an equal and opposite reaction - if ther is 500lbft at the tire contact patch, there is also 500lbft trying to make the differential case rotate opposite the tires forward acceleration wants to push the rear of the car downnot sure how to qualify this, the instant center's relationship to the car's center of gravity (relative to the front axle and rear contact patch) determines how much antisquat there is, periodAlso fully agree with this. the neutral line runs from the contact patch through a point on the vertical line through the front axle centerline, and center of gravity height. The acceleration forces act in a direction from the contact patch through the IC. if the IC is positioned ON the IC ther should be ZERO antisquat. I think we can agree on that.. More torque on the axle always gets the same amount more forward acceleration so they always balance each other out.
What I dont see in any of the Antisquat calculators is any reference to axle tq. And If it is not acounted for, then how can we say that there is zero antisquat solely by placing the IC on the neutral line? My interpretation is that there is ZERO antisquat created solely by the force acting from the tire contact patch through the IC , but no reference to the lifting force from the axle tq on the IC.
Let me try to clarify what I am getting at. if a car's engine makes 500lbft of tq, the trans first gear is 3.0:1, and the diferential is 4.0:1, we have:
500x3x4 =
6000lbft of torque at the rear axle.
if a car has a 4 foot ladder bar, then there would be :
6000/4 = 1500lbs acting in the opposite direction from tire rotation, at the IC. In other words 1500lbs of force trying to lift the car at the IC(separate the rear suspension from the chassis).
The only way I can see this force NOT causing chassis(rear suspension) separation is if there is no suspension.
What am I missing?
peejay wrote: More torque on the axle always gets the same amount more forward acceleration so they always balance each other out.
can you explain this to me

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by peejay » Sun Feb 28, 2016 3:55 pm

Instant center is the point that the axle is pivoting around. It is the point on the chassis that axle torque is working against.

On a ladder bar suspension, it is the pivot of the A frames. On a 4 link or 3 link, it is a point defined by the links angle and position and it does move with suspension travel. Either way the effect is the same - the point IS where axle torque is working against the body.

It is NOT the point where the car is being pushed from - that is the tires' contact patch! Everything comes from the contact patch. That is why you draw a line from the contact patch through the instant center in the process of calculating antisquat.

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by 3pedals » Sun Feb 28, 2016 4:05 pm

I agree the ladder attachment point is where axle torque is applied to the chassis.
But the forward "push from the tire's is also pushing the chassis AT the ladder rod end.
That's not true with a 4 link, but it is with my ladder car example.
What I'm after is proof, maybe mathematical or otherwise, that the axle tq IS, or is Not, included in the antisquat % that is derived by using IC relative the neutral line

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by Brian P » Sun Feb 28, 2016 5:04 pm

Do the math yourself. It's not that hard. It's the only way you will convince yourself if the mathematical model is correct. If someone else does it for you, you aren't going to buy into the results.

If you don't trust the mathematical model ... you have to derive it yourself from first principles - in this case, force-based analysis.

Draw a free-body diagram of the axle and the torque arm. The torque attempting to twist the axle housing around backwards is balanced by forces applied to the axle in both the vertical and horizontal directions. All the forces and torques need to balance. The ground is exerting forwards force on the tire contact patch - this is the force that is accelerating the car forwards. The inertia of the vehicle body is exerting backwards force that is acting through the torque arm pivot. Because these forces are not applied at the same heights, there is a vertical moment-arm (the separation distance, in height, between the tire contact patch and the torque arm pivot). There is an additional torque in the vertical direction which is the ground pushing up against the tires and the weight of the car pushing down against the torque arm pivot. This force is additive to the static forces through the springs. For the moment assume that the springs are centered over the axle to make life easier.

The torque on the axle from the horizontal force components, plus the torque on the axle from the vertical force components, has to equal the total applied torque at the axle.

You know that total applied torque. With the radius of the wheels, you know the forward contact patch force and that has to equal the backwards chassis-end-of-the-torque-arm applied force. With the height of the torque arm pivot known, you can readily calculate the reaction torque from the horizontal force components. Now you can calculate how much torque the vertical force components have to be in order for the torque balance to work out. Since you know where the torque arm pivot is, you can calculate what that vertical force component is. That vertical force component ... is your "anti-squat".

Now you repeat this exercise on the vehicle bodyshell. Treat the mass as all being at the center of gravity. Work out the torques ... work out the torque and force balances.

If the "extra downward force" on the axle from its applied torque matches the "extra downward force" due to the torque on the bodyshell - arising from the moment arm between the torque arm pivot point and the center of gravity - you have 100% anti-squat and the rear suspension will neither compress nor extend.

The front suspension will of course lift up because there is nothing in the system that can pull it down again other than gravity.

DO THE MATH

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Re: IC, antsiquat %, and axle torque / vrs chassis seperatio

Post by Brian P » Sun Feb 28, 2016 5:11 pm

Once you have completed the above exercise, do it again using an upper and lower link arrangement (in side view). It's more complicated because you need to do a force and moment balance on every part of the system.

If you have links pointed at the previous pivot point of your torque arm system (so that the instant center is the same), you are going to find out that the lower link has a very large compressive load and the upper link has a somewhat lower tension load - the net difference between them has to balance the forward force of the tire contact patch.

If these links are aimed at the same instant center where your torque arm system was, that lower link is likely sloped down towards the axle and the upper link is likely sloped up towards the axle. The vertical component of both of those ends up pushing the axle down and the body up, i.e. the reaction forces are the ground pushing up against the tires and gravity pushing the bodyshell down ...

Do the math ...

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