A must for lowered vehicles, Whiteline roll center and bump steer correction kits are designed to correct roll-center geometry using specially engineered ball-joints and tie rod ends. Raising the front roll-center results in a substantial increase to front roll resistance and a significant reduction in suspension compression of the outside front wheel during cornering. The result is more even weight distribution and improved tyre contact ultimately improving front end grip and cornering speeds.
What Is Roll Center?
And How Does Ride Height Affect It?
Our motto here at Whiteline is “Activate More Grip” and our philosophy is to achieve the maximum amount of usable grip on offer. There are many ways to increase the overall grip available on your car however here we will be mainly looking at how to avoid a classic mistake with MacPherson strut suspension and low ride height that can often result in loss of grip.
Many cars in the tuning world run coilover suspension andfor the benefits they give such as adjustable ride height and damping, and whilst it's very satisfying buying tuning parts such as anti-roll bars or anti-lift kits, sometimes the best result can come from a simple setup change. General wisdom suggests that once you've fitted your coilovers you'll be setting the ride height at a low level to reduce the center of gravity. Reducing the center of gravity will theoretically increase the maximum cornering speed of the vehicle, however this is not always the case. And in the case of lowering springs most performance lowering spring kits that are well developed, will take into account a number of factors, one of which being the roll center location, another of course being ideal spring rate.
So What Is Important About Roll Center And Ride Height Then?
The roll center is a virtual point at which the chassis interacts with the center of gravity to create weight transfer and roll. This is particularly important with regard to MacPherson strut suspension. The reason for this is MacPherson strut suspension doesn't have an upper arm, the geometrical equivalent of the upper arm is the top mount. What this means is the typical roll center location of a well-designed MacPherson strut suspension is just above ground level. The majority of chassis engineers will seek a roll center fairly low to ground level, but slightly above it.
"The Problem Occurs When You Lower The MacPherson Strut Suspension Too Far."
With a double A-Arm or double wishbone suspension, where unequal length arms are used, a chassis designer is able to engineer favourable geometry and camber change throughout the functional range of the suspension travel. Unfortunately, on the MacPherson strut suspension the engineer doesn't have this luxury and the camber change and roll center change on a MacPherson strut at very high lateral loads can often result in the loss of grip.
So what does this have to do with the ride height? Well a picture speaks 1000 words, so please consult the image below to see at a glance what we're dealing with.
"Eventually The Car Will “Fall Over” Onto The Outside Front Wheel."
As you lower your coilover equipped car, with MacPherson suspension the lower control arm will eventually start to point upwards at the outboard end, referencing ground level. What this does in reality is drop the roll center below ground. The impact this has dynamically though, is far greater than a static drawing would show.
With the roll center below ground height more weight is transferred via the springs (likewise raising the roll center reduces weight transfer through the springs and increases weight transfer through sprung mass). This makes body roll more pronounced and in turn increases the leverage against the lower roll center, making the instantaneous center move well outside the vehicle causing even more weight transfer and roll. Eventually the car will “fall over” onto the outside front wheel. The tires limits of grip will now have been breached, no more weight transfer can occur. To the driver this is a very frustrating feeling, quite often on a front wheel drive car the rear inside wheel will now be airborne, and the car just understeers relentlessly.
“One Solution Is To Ensure That The Strut Works In A Fairly Defined Range”
There are numerous ways to address this problem but here are some common fixes:
- Keep the lowering to an optimal level to ensure the ball joint pivot point is not above the centerline from the inboard mounting point.
- Fabricate a custom subframe that moves the inboard mounting points upwards or fabricate a ball joint extender.
- Add this extended ball joint system and be DONE.
In summary, if you want your extremely lowered MQB to handle, then you must fix the subsequent geometry problems head on. It's either that or understeer off the racing line (or road and into a curb).
