SPL Parts Suspension Upgrade on the Devine Force Racing VA STi

We all know the saying, “you have to crawl before you walk”…. we’ve been crawling for a bit and it’s time for the Devine Force Racing STI to continue growing and start walking in our racing journey! You may recall our last article in which we upgraded the suspension with some OEM upgrades such as polyurethane bushings and such. That was great for then for the STI as it was still being driven to and from the track at times, and the goal was really to learn the car and work on getting comfortable racing. Now we’re at a part of the journey where I’m ready to take the next step to being competitive and upgrade to the next level of suspension upgrades with SPL Parts!

Devine Force Racing STI exiting the corkscrew at Laguna. 
Photo by Danny Nikkhoo
Devine Force Racing STI exiting the corkscrew at Laguna. Photo by Danny Nikkhoo


The first upgrade we installed on the STI was SPL’s Rear Traction Arms. These were super easy to install and due to being made out of Billet 6061 -T6 Aluminum they were also lightweight, which is helpful when installing parts over your head (my arms get less tired). So, what’s the benefit of these rear traction arms you might ask? If you’ve raced your car before, you’ve most likely experienced wheel hop at one point and it’s not ideal for your times on the track nor does it make your drivetrain happy with the repeated torsional unloading and loading. Just to clarify what wheel hop is – it’s when bushing deflection (movement) causes oscillation (the repetitive torsional unloading and loading) in the rear suspension under hard acceleration and braking.

 

An example of wheel hop is being at the start of a race in a standing start or even slow-speed turn exits where one will be in a lower gear and fighting to get back to throttle as quickly as possible and there’s a harsh jolt or thump feel in the steering wheel or seat. This is jolting feeling is due to the bushing deflection causing the tires to lose contact traction and gain it, then lose it and gain it again a few times in a row.

So now that we understand that’s a huge no-no, we don’t want to experience with our softer bushings anymore, we’ve upgraded to SPL’s Rear Traction Arms which, eliminate the bushing deflection due to their use of their 3-piece FK spherical bearings which offer increased stability and response when getting back on the throttle aggressively.

Aside from eliminating wheel hop, SPL has made their rear traction arm’s length adjustable so we can easily tune out rear bump steer and anti-squat. For those of you unsure what either of those are, bump steer is a measure of wheel steer angle change with vertical suspension travel or when your wheels turn in and out as your car is moving up and down basically steering without you turning the wheel. Since we don’t want this happening on a racetrack we try to get as close to zero or near zero bump steer as we can. To do that we have to have the tie rods on either side of the car to be pointing toward a point called the instant center, which is a point where the inner tie rod pivot is in line with the lower control arm inner pivot point as shown in the picture below. We also need the tie rod to be a specific length which SPL makes adjusting these easy while installed on the car so we can make quick changes to tune out the bump steer.

Instant Center photo explanation





We also mentioned that the adjustable length helps with anti-squat and anti-squat is how the rear in this case, will move vertically and compress under acceleration and braking. Adjusting the length of the traction arm it will enable the weight transfer to the rear to be reduced, anti-squat increased and forces from acceleration to be passed through the suspension and control arms rather than the struts/coilovers. The right amount of anti-squat desired can also help the car maintain a level ride height and allow for the tires to have a better contact patch under acceleration. The ability to adjust this length allows for quick adjustments to reach the desired percentage of anti-squat for your specific application.

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