- Roll Centre Tuning Quick Reference Guide -
The roll centre height is a very powerful (and often misunderstood) tuning tool that has a significant effect on the handling of your racecar. There are many ways that you can tune the roll centre height of your suspension, but each one can have a different effect on the height and it can be confusing to know what each adjustment does. To help with this, I have written a simple excel model to calculate the roll centre height of our B74 and B6 series of Team Associated vehicles and made some simple guides to see how far the roll centre moves with each tuning adjustment.
What is the roll centre? To put it simply, it's the point at which all of the cornering forces of the tyre are concentrated into the chassis. The centre of gravity is the point at which all of the mass forces due to centrifugal force act in a corner. The bigger the gap between the centre of gravity and the roll centre, the more 'leverage' there is to make the chassis roll. A low roll centre means the suspension at that end of the car feels softer, and usually generates more traction. A higher roll centre means there is less roll, the suspension feels stiffer, and there is usually less traction. If the roll centre is the same height as the centre or gravity, the chassis will not lean through a corner. Most importantly, the roll centre forces sxt as soon as the tyre creates lateral grip, so they change how reactive a car feels. A high roll centre will feel very direct but on high grip can make the car feel nervous. With our offroad buggies, the roll centre is usually below the ground level, exaggerating the roll to generate more grip, and reducing the edginess of the car.
To calculate the roll centre height, when looking from the front or rear of the car you draw an imaginary line through the upper link, and another imaginary line through the lower arm. Where these two lines meet is the 'instant centre' of the suspension. You draw another line from the centre of the tyre contact patch to this instant centre, and the point where this line intersects with the Centreline of the car, is the roll centre. I've drawn a simple sketch to explain this.
Generally when tuning roll centres, we usually lower them on a hard / slick track and raise them on a soft /wet track (to help the tyre dig in a little more). Each tyre and surface likes a slightly different roll centre but this is the general rule of thumb we follow. At the rear, a lower roll centre usually gives you more mid and exit traction, but if you go too low the car will feel to 'dump over' and can spin on exit. Sometimes when you lower the roll centre you need to run a slightly stiffer spring or move the shock out on the tower slightly to compensate.
To adjust the roll centre, the most sensitive adjustments are raising and lowering the height of the pills (inner lower arm), the upper link inner ballstud, the upper link outer ballstud, and the height of the axle in the hub (which our cars have had since the B6 series introduced the concept). Changing the ride height also affects the roll centre.
In the diagrams below I have used the setups that Lachlan Munday and I typically start with as a reference point. In this case the roll centre for the B6.2 is around 9mm below ground and the B74 around 7.6mm below ground.
- If you raise the pill by 1 (0.7mm) you raise the roll centre (~2.2mm)
- If you raise the outer ballstud 1mm, you raise the roll centre (~1.9mm)
- If you raise the inner ballstud 1mm, you lower the roll centre (~1.9mm)
- If you raise the axle height (eg going from insert 0 to +1) you raise the roll centre (~3mm).
- If you lower the ride height 1mm, you lower the roll centre (~1.2mm)
There are many other factors to consider when tuning a suspension to include things such as camber change through the travel, and the way the roll centres themselves move as the car bounces and rolls. In this example I've only shown the roll centre height at ride height, which is what you feel when you first start to turn in.
When you tune the upper link length,you make a slight change to the roll centre at ride height, but you change how the roll centre moves in roll and you also change the camber gain. In general, a longer upper link gives less camber gain (the tyres will lean outwards more during cornering but stay more vertical when the car bounces) and the roll centre will become more low when the car rolls (usually giving more mid corner grip but sometimes then causing a sudden spin mid corner). Narrowing the pills reduces how far the roll centre drops in roll.
Some adjustments can be combined to give the same roll centre but with changes to other effects. For example, you can lower both the pill and the upper link inner ballstud to keep a similar roll centre, but you will get more camber change and vice versa.
I hope that this guide helps you to understand your tuning adjustments a little more.