Roll centre adjusters

[totta Crolla]

I looked at how RCA's fit to the car and it seems obvious to me that without further modification to the steering arm, lowering of the joint at the hub by the same amount as the thickness of the RCA, you will end up with bad bump steer.
Am I missing something with this ?

[NoHachi]

You are missing something ;-)

Take another good look.

[totta Crolla]

The RCA needs to exactly the same thickness as the amount the strut is shortened by ?



Zat it ?

[Motax]

The RCA is placed between the strut and steering arm, therefor, there will be no bumpsteer issues.

[totta Crolla]

The tie rod and the track control arm need to operate at the same angle to avoid bump steer issues.

[Deuce Cam]

The rca spaces the control arm and tie rod down an equal amount since it's placed between the strut casing and steering knuckle. As a result the control arm and tie rod remain at the same angles in relation to each other as stock. Overall suspension geometry between the wheel and subframe is changed since the roll center is raised with the rca spacer (this is good).

You don't need to match rca thickness exactly to how much the strut casing is shortened but it's a good guideline. I would keep it close, within 1 cm since final ride height also is a factor (e.g. a 50mm rca will be ok on a 40mm shortened strut casing).

Side note: installing RCA's on non shortened strut casings can cause binding issues at the rack ends near full droop, but it depends on the shock length and the springs you're using. Not recommended.

[Lajoosh]

actually the main theory about rca thickness is that it should be the same thick as the amount you lower your ride height. in reality, while you lower your car you will also have less roll so you wont need that much chamber change mid corner. so you will be okay with any thickness that wont let your lower control arm go up above horizontal level when the suspension is compressed. less than that: you may experience understeer and bad behavour in cornering. more than necessary, you get more camber gain than necessary. rule of thumb: get something like the t3 thickness. will work well
strut casing shortening is closely related to suspension travel. you cut the case shorter, install the short stroke damper and you get more suspension travel compared to a non shortened strut with lowering spring.

back to rca topic: the main goal is to have the the hub spindle higher on your strut when you think of it as a whole.
imagine your strut and lower control arm from the frontal view. imagine your spindle going up along your strut while your strut and lca stays still. the car is getting lower while geometry stays the same. thats what you get with an rca and strut shortening.

[totta Crolla]

Had a mad half hour when I did the original post

[Deuce Cam]

I agree in theory about the drop vs. rca thickness. However...

I tried mounting T3 rca's on my car with stock length casings, tokico blues, and uncut megan springs (shocks and springs are 'long stroke'). Megan advertises a 1.85" drop with the springs, but it's probably actually closer to 1.25". The t3 rca is 34mm (1.34").

With this setup the rack ends would bind with the suspension completely unloaded. I actually tried bolting the rod end to the knuckle with the rca installed, suspension at full droop, and the tie rods wouldn't reach below the knuckle. It wasn't even close - roughly 1" off. I literally had to compress the damper to bolt the tie rod end to the knuckle with the rca installed. FWIW these were heim joint t3 outer tie rods which utilize a separate bolt where it connects to the knuckle, so it wasn't hampering the process like an integrated stud would on a stock style rod end.

Some will say this doesn't matter because the suspension will never be completely unloaded while driving.

I imagine this problem is somewhat common and people just don't realize. Mainly because most people aren't replacing rod ends when they installed their rca.