Roll centre
The imaginary point an axle rolls around. Lowering a car moves it, which changes how the suspension responds to load.
The roll centre is the geometric point about which an axle rolls when the car leans. It is not a physical object - it is where the lines through the suspension pivots intersect - and its height relative to the car's centre of gravity determines how much the car rolls for a given amount of cornering load, and how the suspension loads up as it does.
Lowering a car moves the roll centre, and usually moves it in an unhelpful direction, increasing the distance between it and the centre of gravity. The practical result is more body roll than the spring rate suggests you should have, and suspension that responds to load differently from how it was designed to.
Why drift cars care
A drift car spends its life in a sustained, loaded, high-slip-angle state, and how predictably the front end takes a set is most of what the driver is feeling. Roll centre correction is part of what makes a lowered car behave like a designed car again rather than a compromised one.
It also travels with bumpsteer. The components that fix one usually address the other, which is why they are a single item in our build order.
What to fit
Roll centre adjusters, extended lower control arms, or the corrected arms that come as part of an angle kit. Which one depends on the chassis and on which kit you chose - most angle kit manufacturers specify what they assume is fitted.
In the build
Phase 2, alongside the angle kit, before the alignment. Doing them in that order means one alignment. Doing the kit, aligning, then fitting roll centre correction means two.
How to tell whether yours needs correcting
Ride height is the main signal. A car dropped meaningfully below its factory height has a roll centre that has moved, and the symptom is more body roll than the spring rate should produce, plus a front end that takes a set unpredictably.
Most angle kit manufacturers state what roll centre correction they assume, which is the most reliable answer for your specific combination - check the kit's documentation before buying adjusters separately.
Questions
Is roll centre correction necessary?
On a meaningfully lowered car, effectively yes - and most angle kits assume it is there. It is one of the cheapest parts in phase 2 and it is what makes the coilovers and the kit behave as designed.
Can I just run less drop instead?
Partly - less lowering means less geometry disruption. But drift cars are lowered for reasons beyond looks, including keeping the centre of gravity down and getting the suspension into a usable part of its travel, so correction is usually the better answer.