This is an interesting topic that not many people talk about...rear upper arm affect to camber gain and camber progression.
Yes, everyone pays attention and talks about rear camber....1 deg, or 3 deg, whatever it is on your chassis on the setup board...
However, your camber will never be static, or will stay at that setting running on the track...unless you have rock solid dampers that won't move at all...
UPPER ARM WHAT???
You probably have seen new releases of rear shock tower design recently with more hole options to mount your upper arm.
Higher, lower, more to the inside or to the outside...
Why so many???
This is for setting up your upper arms and setting up your rear "camber gain", or I like to call it "camber progression". The more options you have, you can change the length of the upper arm, and the angle of the upper arm connecting to the rear knuckle.
The length and the angle has a direct relationship to how the camber will change, and progresses as your chassis squats and rolls.
LENGTH AND ANGLE
So here is an extreme example on my D-LIKE Re-Hybrid.
This has the stock shock tower, with a Kamikaze rear knuckle upper arm mount plate with extreme options to mount the upper arms.
If you look at the angle of the left upper arm and right upper arm, you can tell the difference in the angle. Left side being more flat, right side with more angle, inclining towards the rear knuckle. Upper arm lengths are identical.
Just watch these two videos (You tube links) and compare how the camber changes with squat and roll. The first 3 push is squat, then I am forcing the chassis with roll. These plates on the rear knuckles are somewhat an extreme example, but you can add spacers on the knuckle side to add angle, but depends on your chassis setup and what setting are allowable without rubbing the arms to the dampers or the rims/tires.
UPPER ARM LOW ANGLE
There is some camber gain with squat, but more movement in camber with roll.
UPPER ARM HIGH ANGLE
There is more camber gain with squat and less movement in camber with roll.
BUT WHY?
Many may be questioning WHY??? This is your style and preference and how you want your chassis to perform on a certain surface. Some surfaces will gain traction with more camber, or less contact patch. Some surfaces will gain traction with less camber, or more contact patch. Assuming you have good chassis and damper settings and your chassis is able to squat with throttle on and regain ride height with throttle off, and also to be able to roll the chassis in turns, maximizing the rear upper arm to control the camber gain/progression can control your rear traction effectively.
Here's my Kamikaze YD-2S with YD-2EX rear shock tower, with a extension plate by SHIFT. Lowering the upper arm mount, and shortening the upper arm length. I am waiting for a new plate by TN Racing that will bring the mounting position more to the inside to make the upper arms longer.
This is intended to minimize the camber progression with roll so the chassis will have consistent contact patch in an angle, and with a slippery painted concrete, this seems to work well for me to maintain consistent rear traction. It does have extreme amount of camber gain with squat, but the chassis is quick enough that I don't need much push in the straights with throttle on.
SUMMARY - ALL YOU NEED TO REMEMBER
So here is the quick summary of how the rear arm length and angle affects the camber progression. Both arm length and angle affects camber gain/progression, so you will need to find a good balance depending on how you like the camber to progress, and also with what limitations you may have within the setting range on your chassis.
[ Arm Length ]
SHORT - MORE camber gain with squat, LESS camber progression with roll
LONG - LESS camber gain with squat, MORE camber progression with roll
[ Arm Angle ] (Inclining towards rear knuckle)
ANGLED - More camber gain with squat, LESS camber progression with roll
FLAT - LESS camber gain with squat, MORE camber progression with roll
Yes, there are other methods as well to tune the rear for more traction. Active toe is a popular example, but this will also change the behavior of the rear towards how the chassis will maintain a certain angle. Mono shock or a linked shock system in the rear can add roll, but you need to maximize the roll to apply the weight to the tires with camber.
Tuning the rear upper arm is one of the simple ways to tune your chassis with alignments and not introducing any additional components/elements to the chassis. It is also an effective way to tune and understand the geometry of the chassis.
In any tuning, it’s totally up to you on how to tune, and the surface you run.
A simple test you can do is to take the spec tire for the track in your hand, rub it against the surface and see how it feels. Does it have more or less friction with more contact patch or less contact patch? This will give you a good idea on what the tires will be doing with more or less camber when accelerating, or when your chassis is in an angle with a good roll.
Happy Drifting !
April 22, 2018 / Charlotte NC
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