# Rear suspension geometries, can they be combined?

Hello all, I am attempting to make an RC car suspension for fun. I would like to know if the suspension geometries can be mixed, such as brake anti-lift and anti-squat. I believe I comprehend the anti-lift and anti-squat geometries, and have already calculated what I want my anti lift to be. Before I progress, I would like to know if I can also add anti-squat. I haven’t been able to find anything that suggests the two are able to be added, or vice versa. Thanks!

Kudos to you. If I attempted something like this, I’d have a stroke. The only thing i remember about rear geometry is that if you have so much torque that you spin the rear wheels on every take-off, try adding traction bars. This is more a question for @Mustangman .

1 Like

I have a stock RC car with a brushless motor, it spins all four tires on payment with the softest, stickiest tires I can find.

Brake anti lift and traction anti squat are defined by the very same side view swing arm geometry. The torque direction and body motion just flip.

If your RC car has an independent rear suspension with the differential mounted to the frame, you have no traction anti squat and you can’t design any in because the torque goes through the differential directly into the frame. (My RC truck was this way). A solid axle drive can have traction anti squat again flipped from brake anti lift.

You can only get brake anti lift if you have wheel mounted brakes. If they are inboard mounted or regerative through the drive motor, you can’t get brake anti lift.

There are ways to seperate the two, with a solid axle and floating brake calipers. Dirt circle track cars do this.

1 Like

So there is no way to implement anti squat with a independent suspension with the differential mounted to the frame… I see… Are there any other ways around this besides a solid axle drive? Is there any possible exception to still implement anti squat with a independent suspension?

You can’t get anti squat with suspension geometry with independent suspension…

However…if you understand how stabilizer bars (anti roll bars) work, you can mount a pair sideways connecting the front and rear suspensions on each side. When the car tries to pitch, the rear drops and pulls the front down at the same time. So you have 2 U-shaped bars running front to rear on each side mounted to the frame with the ends mounted to the suspension.

Stiff spring wire would probably work for an RC car.

1 Like

A little something interesting I found on an RC forum…

By the looks of the image, the anti squat is not taken by the suspension arms in an IRS, rather it is taken by the chassis? In that case it would be a different type of anti squat, or wouldn’t be labelled an anti squat geometry at all.

Apparently from a 2009 Corvette forums thread, somebody says that while a solid axle could have anti-squat when stopped, while an IRS could have a form of anti squat only when accelerating. He also remarked that on an IRS, the trailing arm length and pivot point would play a role in the chassis torque, but to me this isn’t making sense.

This diagram is out of a book called Race Car Vehicle Dynamics by Milliken and Milliken. Page 619 to be exact. Good choice, it corrects some of my thinking.

You can get some anti-lift from an independent rear suspension but it is harder to achieve. (The chart has an error on it, the angle Theta sub R shown on the drawing is replaced by angle Phi sub R in the equation…) But, notice since the angle is through the wheel center instead of the ground contact point, it is much harder to get the angle large enough to get large amounts of anti-squat.

The explanation in the book comes down to this: If the control arms react all of the torque from engine or brakes, the anti is calculated from the tire’s contact point on the ground with solid axles.

Since the independent suspension differential reacts the engine torque, the anti-squat comes from the forward force and is reacted at the tire’s center. The explanation in the book is only one paragraph.

So if you want more anti-lift and squat, if you have a short-long-arm type independent rear suspension, the forward mounting points of the arms must be raised so that in side view, they point to the center of gravity of the car. Keep in mind that 100% anti’s can make the wheels unstable so they start to hop.

1 Like

So the chassis is what truly deals with the torque instead of the suspension arms. Interesting. So in a solid axle, it pushes on one arm and pulls on another to make the rear end either not squat or actually lift up for values above 100%. On an independent suspension, such as one with a trailing arm, does not use this technique, rather it transfers the torque up in the chassis to lift or prevent squating, instead of using the suspension arms. It partially makes sense to me, however, I don’t see how it transfers the torque to the chassis, and makes it lift.

I’ll certainly look into that book, it probably would help me with my project.

Half correct… The diff absorbs some, the wheels absorb some pushing or pulling the car to allow some anti-squat and lift. You need to study the concept of “instant centers” as applied to mechanical linkages. It is not altogether intuitive at first glance.

1 Like

@zinophfragment_163832 - do you have the ability to make large changes in your RC’s suspension geometry? If so, do that, and see what results. You may learn a lot by that approach, to combine with what you’re finding in books and here.

2 Likes

So I can use the anti-lift calculation to decide how much angle I need, as well as the anti squat calculation? From what I have come up with, my current calculations from my current geometry are, based on the calculations that this website provides, https://suspensionsecrets.co.uk/anti-squat-dive-and-lift-geometry/, I have concluded by final brake anti lift and anti squat geometries.
By the calculations, I currently have 16% brake anti lift, and 56% anti squat. I have no clue if these are correct, but I have followed the procedure based on the figure 17.5 in the image I disclosed earlier, for the independent suspension anti squat.

I think you might have that backwards. Since the suspension geometry used to calculate % anti’s is the same, the center of gravity (CG) becomes the difference.

Since the CG shifts forward under braking and that should make the anti lift angle smaller and so it makes rear brake anti lift percentage better.

The rear shift of the CG under acceleration makes the angle needed to get anti squat larger so it makes the anti-squat worse than anti-lift.

If you said you got 54% brake anti-lift and 16% anti-squat, I’d say that sounds about right.