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Suspension Tuning & Optimization: Anti Effects

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Anti Effects

05.59

00:00 - Anti dive and anti squat are terms that you might already be a little familiar with and in this section we'll discuss what they mean and how they're calculated.
00:09 In short, these terms describe how we can use the geometry of the suspension to influence how much travel occurs at one end of the car when we brake or accelerate without changing elements like our springs or bump stops to control the amount of travel we have.
00:25 To be a little more complete in that description, let's take the example of anti dive which relates to how much the front axle compresses or dives during braking.
00:34 Let's say we're damaging the front splitter in a heavy braking zone.
00:39 One way we could reduce the amount the front axle compresses for a given braking force would be to fit stiffer front springs.
00:47 This would absolutely reduce the amount the front suspension compresses but it could have other unwanted effects on the handing of the vehicle.
00:54 Most likely, because it would upset the cornering balance of the car.
00:58 In order to maintain balance, we might then need to fit stiffer rear springs and this could well hurt our drive and traction.
01:06 If you're running at a circuit that has some critical acceleration zones, that may not be a compromise you want to make.
01:12 Anti dive is a tool we can use to avoid all of that.
01:15 To get our head around how it works, we need to turn to the instant centre concept again.
01:20 At this point, I should clarify that there are a few different definitions for anti effects out there and I'm just going to use the one I think is simplest to understand.
01:29 Looking from side on with this double wishbone suspension, we can construct the instant centre for the front axle.
01:35 We can see that running a line from the front tyre contact patch and through the instant centre also runs through the centre of gravity.
01:42 In this situation when we brake, all of the longitudinal braking forces will be reacted through the suspension arms and not through the springs, dampers or bump stops.
01:52 This is what we call 100% anti dive and will result in zero compression of the front suspension during braking.
01:59 As all of the forces are reacted directly to the suspension arms instead.
02:04 Taking a less extreme and more typical situation, the instant centre of the front suspension is shown here.
02:10 If we drop a line vertically down from the centre of gravity location, the height at which the line running from the tyre contact patch to the instant centre, intersects this vertical line, gives us the anti dive percentage.
02:23 If this particularly intersection point is 25% the way up the vertical line, we have 25% anti dive.
02:31 In this case, 25% of the braking forces will be reacted through the suspension arms and 75% through the front springs, dampers and bump stops.
02:41 We can see that by reducing the amount of force applied to the front springs, we'll reduce how much the front dives under braking.
02:49 This sounds like a great solution so why not run 100% anti dive all the time, thereby allowing us to reduce our spring rates? Unfortunately, like everything in car setup, it comes down to compromise.
03:02 By running some amount of anti dive, while it'll reduce the front travel, we effectively stiffen the front suspension at the same time.
03:10 In the case of 100% anti dive, the suspension will essentially be locked solid under brakes and this is going to reduce our tyre grip by taking away the ability of the suspension to follow the uneven road surface.
03:23 The less anti dive we use, the less locked up or stiff the suspension will be.
03:28 It's still a useful and valid tool, but it's just that, a tool, rather than a complete solution.
03:36 In practice, this means we can usually get away with running higher levels of anti dive at smooth circuits while reducing it for rougher surfaces.
03:44 It's important to understand that by adding anti dive to the front axle, we're not reducing the amount of load transfer to it.
03:51 This is governed by the height of the centre of gravity, the wheel base, and the amount of acceleration.
03:58 Anti dive is simply a tool to tun the deflection of the front axle.
04:02 With that covered, let's look at the other end of the equation with anti squat.
04:06 This is used to tune how much the rear suspension compresses on drive and is something you'll most commonly see employed on the rear axle of rear wheel drive cars.
04:15 The construction of the amount of anti squat is similar to that of anti dive.
04:19 We draw a line from the rear tyre contact patch through the instant centre.
04:23 The height of the point that this line crosses the vertical line, dropped from the centre of gravity defines the percentage of anti squat.
04:32 All the same principles apply to anti squat as anti dive.
04:36 Running too much will reduce our rear suspension compliance and therefore grip when we accelerate, but when used correctly, it can be a great way to limit the amount of suspension compression.
04:47 Commonly, you'll find anti squat used to reduce the amount of rear camber gain we get when accelerating which allows for a bigger tyre contact patch to be retained and therefore more traction on drive.
04:58 As to how we modify our geometry to change the anti dive or anti squat behaviour, anything that moves our side view instant centre will affect the anti properties.
05:08 Looking at the earlier example of anti dive calculation, moving any of the inboard wishbone pickup points vertically will have an effect on the instant centre location.
05:18 Generally it just comes down to which ones are most convenient to move and which have the least negative flow on effects to other parts of our kinematics.
05:27 In many cases, the vertical height of the forward pivot point of the lower control arm has the ability to be shimmed at different heights.
05:35 Adding different combinations of shims above the pivot point changes its height and therefore the instant centre position.
05:42 In summary, anti effects can be used at either end of the car to change the amount of suspension deflection during both braking and acceleration.