KPI is another important update in the B7 design. Like caster, it improves straight-line stability by keeping the front wheels pointed forward. Its self-centering ability is notable, and sometimes it’s better for the car to exit corners driving more “square”. This can be an issue if you need the car to drive more “round” on throttle. KPI can also affect scrub radius, but here the focus is on how it impacts the rear.
When combined with caster, KPI pushes the inside front tire down, adding to its load. This creates a “wedge” in the chassis that also puts extra load on the outside rear tire. This wedge can add to the “jacking” of the chassis. While this might seem like an advantage, it’s not always the case. In a fast, sweeping corner, extra load on the outside rear tire usually means more grip. So, the real question is: do you need more grip off-throttle or on-power?
“Square” = More grip on throttle-Car goes straight
“Round” = Less on throttle grip-Easier to turn on throttle
Our RC cars use an open differential, which directs power to the tire with the least load. This can lead to “diffing out” when the inside tire has less load but gets all the power, reducing the car’s forward speed. It can also make the car feel like it’s about to spin out or steer into the pipe. This is where the jacking of the chassis can be troublesome on some circuits.
As a result, many racers have gone back to using the B6.1 caster blocks and spindles, which are lighter than the B7 versions. On tight 180-degree tracks, more KPI can help improve rotation because of the jacking. If you’re running a lot of KPI, and need less rotation, lowering caster can help get rid of some “wedge”. The B7 parts also allow for a steeper front camber link angle, making turn-in more aggressive, but too steep can impact bump, so watch out for that. Overall, we’ve found the older front end delivers the most consistent, linear steering in most situations. If you prefer the B7 parts, the new T7 parts have the ability to run 0 KPI.
Next up, we will discuss “keeping the car flat”.