Antisquat force vs spring K rate force

[sbullet86]

There could be a very straight forward answer to this, but hopefully I can leave here understanding this a little more from someone more familiar with suspension kinematics.
Out of all antisquat material I have read, it seems like all anti squat is described in %. What is this % of (weight transfer force?)? I am trying to apply this in a vehicle by determining the rear spring rates I want to look at for a vehicle.
I also make the assumption that the amount of antisquat and spring rate (rear) are interchangeable when speaking in terms of longitutional acceleration. Is this a safe assumption if lateral forces, and rolling is out of the picture?

 

[GregLocock]

Last question first, yes probably, if you are only concerned with steady state.

I can't remember what the % means, it is in the standard derivation.

Cheers

Greg Locock


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[NormPeterson]

Hint: squat is a displacement, not a force.


Norm

 

[rideway]

I would not say that antisquat even is a displacement, but rather a adimensional parameter.

IMO the most important thing to know from it, its that it states which percentage of the load transfer is transmited to the suspension springs and which one is transmited to the suspension components.

Cant tell you know the formulae by heart but it was a relation between tangent angle of IC point vs horizontal plane, CoG height and wheelbase.

Regards

 

[NormPeterson]

My take is that once you know what squat is, whatever antisquat happens to be, becomes clearer.


Norm

 

[patprimmer]

Norm

I would have trouble with that because I don't know squat. ;-)

Regards
Pat
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[GregLocock]

I should add that in real acceleration events there is a big difference between using springs and geometry to attain a given pitch angle per g, if that is your target.

Generally for traction grip on rough roads and out of turns you don't want much antisquat, and you certainly don't want much antiroll bar, or spring rate.

Cheers

Greg Locock


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[patprimmer]

Greg

I am presuming you mean for a rear wheel drive car.

Regards
Pat
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[GregLocock]

Why wouldn't that apply to the driven axle of a FWD? In fact I'll go further than that, the spring/a/r bar setup for high peformance FWDs is dominated by the traction problem.

Cheers

Greg Locock


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[patprimmer]

I presumed you wanted the compliance in the drive axle so the wheels retain contact.

I would have thought rear wheels retaining contact is pretty irrelevant to front wheel traction, however this is your area of expertise, not mine so I would not presume to argue.

Regards
Pat
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[NormPeterson]

I think Greg was referring to avoiding large amounts of roll stiffness at the drive axle, regardless of where it might be.


Norm

 

[Toddr8541]

Percentage of anti-squat is the percentage of weight transfer created by lateral acceleration, supported by your rear suspension linkages, the remaining percentage is carried by the springs.

Optimum % depends upon tires, road surface, type of driving (IE road course, oval, or straight line)and several other parameters of the car and usage.

 

[Toddr8541]

Another note about % anti-squat is it is a dimensionless rate of weight transfer to the rear tires. It will not change the amount of load on the tires but it does change the rate at which the tires see the full of load of weight transfer. The higher % anti-squat the quicker the load transfers to the tire. The total load of transfer will not be applied to the tires until the rear springs have reached final compression from supporting the final percentage of load transfer.

 

[Toddr8541]

didnt mean to state "lateral acceleration" just "acceleration" sorry for the confusion.

 

[imcjoek]

It will not change the amount of load on the tires

I always thought anti-squat *could* increase the load on the tires, at least momentarily, as the geometry required to get it could result in the body of the car accelerating upwards at time of launch.

So if we had a 2000lb drag car that lifts the fronts on launch, I would expect to see 2000lb at the rear contact patch with 100% anti-squat.

With > 100% anti-squat, I would expect to see 2000lb + M*Ay, where M is the mass of the car and Ay is the vertical acceleration of that mass due to the suspension geometry flinging it skyward.

I didn't draw any fbd's to confirm this (yet). Is this not how it works? There must be a force *somewhere* to push the body of the car upwards (through the suspension links), and I (highly) suspect this force has to start at the earth (rubber, meet road).

 

[NormPeterson]

If you're lifting the front wheels, you'll see 2000# on the rear contact patches regardless of the antisquat %.

Even if you aren't lifting the rear (with over 100% A/S), whatever amount of load transfer that goes through the suspension linkage will develop faster than the load transfer through the springs/dampers or even the F=ma effect, as changes in those forces depend on the suspension actually moving or to have moved. Different stiffnesses, different time constants.


Norm

 

[Toddr8541]

I've never looked at 200 % anti-squat as I've had a car that would withstand that kind of anti-squat. The reason is that it would hit the tires too hard and didn't have enough shock adjustment to reduce the rate of extension.

There are several factors involved in determining the amount of anti-squat needed. First is the tires. The tire construction (radial vs bias ply) and sidewall stiffness (related to size, sidewall stiffness) both of those are related to the spring constant of the tire. The tire acts like a spring. The higher the spring constant the less the anti-squat it can withstand. There is also two different spring constants. One is a vertical and the other is a rotational one (rotational is based on amount of sidewall flex in relation to wheel rotation, usually you only need to worry about this if the car is accelerated from a dead stop)

Other factors involved is the acceleration rate of the car and the power transmitted to the rear tires.

The biggest factor in the rear suspension linkages adding force to the tires is the IC length. The longer the length the less force. However, the longer the IC the slower the rate of transfer from the ENGINE to the rear tires.

The higher the IC the more force to the tires and the quicker it is applied.

I will say this if you go beyond 100 percent shock extension rate and amount of available travel becomes a issue.

The best way to determine it is look at how much traction force is required and rate. Then locate the IC to give you the amount of force and then play with the Anti-squat percentages and shock extention rates to dial it in. You may need to change the Force because lower forces with higher rates maybe whats needed based on the combination of the car. Because there are so many factors its really hard to determine the optimum anti-squat value needed. That's usually determined by alot of testing.

What is the application? Drag racing, circle track or road course? If it's other than drag racing you have a whole new bucket of worms to play with.