Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum - View Single Post - This is the roll center, suspension pickup, and steering angle modification thread.

Speed Junkie · 11-01-2021, 08:14 PM

Hi guys. I've been lurking this thread for while and have finally decided to post. I've done a lot of research into 240sx suspension design and put together a decent package for what is primarily a street car that I occasionally drive on the mountains and even fewer times at drift events. After all of this I've come to realize just how compromised the 240sx design is from the factory.

I have an S13 coupe, not very low. Front height is about a finger gap between the top of the tire and fender. Rear is about a finger of tire tuck.

Here are the suspension mods:
Front: GKtech grip/drift knuckles, FA500 coilovers with hyperco springs, GKtech tension rod, Energy Suspenion rack, sway bar, and LCA bushings.
Rear: S14 subframe with risers, FA500 coilovers with hyperco springs, aftermarket sway bar with poly bushings, ISR adjustable and heim joint links.
I also have GKtech rear knuckles and frame reinforcements that I plan on installing this winter.

Wheels are 17x9 +25 and 215 all season tires.
My front alignment is -3* of camber, OEM toe-in, and 6* of caster.
My rear alignment is -2* of camber and OEM toe-in. Trac rod is OEM length but I plan on adjusting it to reduce excessive toe out during compression.

Quick notes:
All of my suspension components are tightened and in good working condition.
All of this is under the context of grip driving. Drifting is super easy in this car and these issues aren't really a problem when sliding the car.

1st and biggest issue: Bump steer. I was hoping that the knuckles would eliminate this but they didn't. They did help A LOT which was great. But far from really any other car. It's really been the one factor that keeps me from truly enjoying the car. Through my research I've learned that this is primarily an issue with rear-mounted racks that naturally produce toe-out under compression. Front-mounted racks (like on Miatas) produce toe in. This is why the OEM alignment specs call out for slight toe in. Despite this, the bump steer is still there. It's not wheel-kickback from an increased scrub radius caused by lower offset wheels. It's the car moving left and right as the front suspension corners flex independently.

2nd issue: the rate at which the car turns in varies as you turn the wheel. Steering between around 10 and 2 o-clock produces less response from the car than steering further. In other words, as you turn in, the initial response from the car is slower than as turn turn the wheel further and past 10 o-clock. The opposite is true of course. As you straighten out the wheel, small changes of steering angle produce a large response from the car. And as you approach straight, the car response is less. I want to make it clear that this is not free play in the steering system or movement in the rack. I think this is due to a combination of Ackerman and the OEM toe in. Since the initial input in steering would first have to get past the wheels pointing in. And as the steering angle increases, you get the inside wheel pointing further and further in due to ackerman (which also adds a moment arm to the car, further helping turn in). Thus, the car turns in more once the wheel is turned past a certain point. It's non-linear and it sucks to drive.

Solutions that I want to run by you guys:
I know that relocating the rack forward will reduce ackerman and if my hypothesis is correct then it will help with the 2nd issue. My only worry is how this will affect every day driving and I don't want to create yet another compromise. Any input?

In regards to bump steer. I can add a spacer/washer to the inner tie rod. Not for more angle, but to get the inner tie rod pivot in line with the LCA inner pivot. I think this should help with bump steer(?)

And at the outer tie rod, do you guys think I should try adding spacers to lower the outer tie rod end further? Again, I thought the knuckles would've fixed this but I can always lower the rod end joint further.

In regards to the rear bump steer. This is less critical than the front but the fact is that it does toe out under compression which is the opposite of what you want. I've ran the car with 0 rear toe before and was surprised at how easily the rear end slides out when I turned in. OEM toe in fixed this but I think I can make it better by adjusting my trac rod.

I'm sure you've all see this from the motoiq article on trac rod adjustment.

EDIT: Last I posted here was 2019.

11-01-2021, 08:14 PM	#7884
Speed Junkie Leaky Injector Join Date: May 2015 Location: Harrisonburg, VA Posts: 113 Trader Rating: (0)	Hi guys. I've been lurking this thread for while and have finally decided to post. I've done a lot of research into 240sx suspension design and put together a decent package for what is primarily a street car that I occasionally drive on the mountains and even fewer times at drift events. After all of this I've come to realize just how compromised the 240sx design is from the factory. I have an S13 coupe, not very low. Front height is about a finger gap between the top of the tire and fender. Rear is about a finger of tire tuck. Here are the suspension mods: Front: GKtech grip/drift knuckles, FA500 coilovers with hyperco springs, GKtech tension rod, Energy Suspenion rack, sway bar, and LCA bushings. Rear: S14 subframe with risers, FA500 coilovers with hyperco springs, aftermarket sway bar with poly bushings, ISR adjustable and heim joint links. I also have GKtech rear knuckles and frame reinforcements that I plan on installing this winter. Wheels are 17x9 +25 and 215 all season tires. My front alignment is -3* of camber, OEM toe-in, and 6* of caster. My rear alignment is -2* of camber and OEM toe-in. Trac rod is OEM length but I plan on adjusting it to reduce excessive toe out during compression. Quick notes: All of my suspension components are tightened and in good working condition. All of this is under the context of grip driving. Drifting is super easy in this car and these issues aren't really a problem when sliding the car. 1st and biggest issue: Bump steer. I was hoping that the knuckles would eliminate this but they didn't. They did help A LOT which was great. But far from really any other car. It's really been the one factor that keeps me from truly enjoying the car. Through my research I've learned that this is primarily an issue with rear-mounted racks that naturally produce toe-out under compression. Front-mounted racks (like on Miatas) produce toe in. This is why the OEM alignment specs call out for slight toe in. Despite this, the bump steer is still there. It's not wheel-kickback from an increased scrub radius caused by lower offset wheels. It's the car moving left and right as the front suspension corners flex independently. 2nd issue: the rate at which the car turns in varies as you turn the wheel. Steering between around 10 and 2 o-clock produces less response from the car than steering further. In other words, as you turn in, the initial response from the car is slower than as turn turn the wheel further and past 10 o-clock. The opposite is true of course. As you straighten out the wheel, small changes of steering angle produce a large response from the car. And as you approach straight, the car response is less. I want to make it clear that this is not free play in the steering system or movement in the rack. I think this is due to a combination of Ackerman and the OEM toe in. Since the initial input in steering would first have to get past the wheels pointing in. And as the steering angle increases, you get the inside wheel pointing further and further in due to ackerman (which also adds a moment arm to the car, further helping turn in). Thus, the car turns in more once the wheel is turned past a certain point. It's non-linear and it sucks to drive. Solutions that I want to run by you guys: I know that relocating the rack forward will reduce ackerman and if my hypothesis is correct then it will help with the 2nd issue. My only worry is how this will affect every day driving and I don't want to create yet another compromise. Any input? In regards to bump steer. I can add a spacer/washer to the inner tie rod. Not for more angle, but to get the inner tie rod pivot in line with the LCA inner pivot. I think this should help with bump steer(?) And at the outer tie rod, do you guys think I should try adding spacers to lower the outer tie rod end further? Again, I thought the knuckles would've fixed this but I can always lower the rod end joint further. In regards to the rear bump steer. This is less critical than the front but the fact is that it does toe out under compression which is the opposite of what you want. I've ran the car with 0 rear toe before and was surprised at how easily the rear end slides out when I turned in. OEM toe in fixed this but I think I can make it better by adjusting my trac rod. I'm sure you've all see this from the motoiq article on trac rod adjustment. EDIT: Last I posted here was 2019. Last edited by Speed Junkie; 11-02-2021 at 07:36 PM..