SLA S30 Front Suspension Conversion

[baby_Carlton]

Hi HybridZ,

Some of you have already seen my Apex Engineered Track Attack information/discussion thread here, I've decided after reviewing the design of the Apex Engineered front suspension kit, the best solution moving forward is to design my own. In this thread I will be documenting my progress and hopefully gaining knowledge on suspension design from you guys (or from what's left, f**k Facebook groups). 

To start...

Why don't you just use the AE kit? It was $4000, you're just gonna waste $4000??? (Yes)
Not enough shock travel, that's pretty much it. Once I received the shocks from Viking I realized immediately that 3.20" of stroke was not going to be enough and thus set out on a fool's errand to find alternative solutions. Viking shocks, from what I've found, have the most shock travel per the physical dimensions of the shock that I've seen so far. They're compact and the adjustment knob is well-designed to take the least amount of space with respect to the rest of the shock body. Very nice, but I will ultimately use either JRi's Mod series DA shocks or Penske's 7800/8300 series DA shocks as now, getting a shock long enough to reach between the strut tower and the LCA is more important.

Design (pictured with 17" diameter wheel)

The most prominent feature of my SLA design is the UCA pickup being located within the engine bay. The shock tower must be cut and a box structure is welded to the frame rail. This design is based on both the Maier Mod 2 design and Cortex Racing SLA kit seen here:

 

Maier SLA Design

 

Cortex SLA Design

The reason I moved the chassis pickup for the UCA inboard was to extend the length of the control arm which should allow for a more compliant spring than the offerings from Apex. Mike Maier inferred the reason for the obscenely high rates was to keep the super short UCA from camming over and ruining the geometry. Fine for a super stiff autocross car but for a road-course car with kerbs, burms, and undulations in the track surface? Unacceptable. With this shock, I'll be looking at around 7-8" of total shock stroke travel. 

 

The remaining part of the design lies with the spindle (not shown) and the boxed-structure (also not shown). The boxed-structure was fairly straightforward to design - shout out to @Ben280 for sending me frame rail measurements while my car's been at the body shop. The boxed structure welds to the chassis frame rail located in the engine bay which the cross-shafts then mount to. The spindle is obviously not straight forward, fortunately for me there's plenty of resources on the LocostUSA forums for me to look at as well as Chassis Design and Race Car Vehicle Dynamics by Milliken and Milliken.

 

Once the completed version 1 of the design is done, I’ll begin FEA with a safety factor of 1.15-2.00 at some lateral load to make sure everything should work without buckling. Until then, I have two separate designs, one utilizing Mustang II style cross-shaft mounting structures and one using Maier’s. The main difference being the orientation of the cross-shaft mounting bolts to the boxed-structure with the Mustang II’s bolts facing down, and Maier’s facing sideways. With the bolts facing sideways relative to the longitudinal axis of the chassis, side load is transmitted into the boxed structure through the bolts on their axial axis as opposed to their shear axis. Bolts do not like shear, however the Mustang II design is insanely popular so I suppose I could be overthinking this. 

 

Maier Cross Shaft Mounting Design

 

Cortex/Mustang II Cross Shaft Mounting Design

 

Methodology

Starting with a 3D scan of the 240z that I re-topologized into ~10000 quads, I modeled an accurate-enough solid body of the fender well and shock tower. From there, I (roughly) modeled the Apex Engineered LCA (will probably try to use the T3 ones I currently have installed on the car as they are boxed structures), the Penske 8300 coilover at ride height, and upper control arm. You may have noticed the bottom of the shock clipping into the LCA, please ignore that, you didn't see that.

 

 

In regards to swaybars, I will try to get the OEM sway bar design to work with my suspension, however due to the bottom of the shock being mounted on the LCA roughly where the swaybar links, this may be difficult to engineer. I could copy Maier’s LCA design, using a bent-tubular lower arm picture here:

 

 

Moving forward, it might be easier to design my own boxed structure and try to mount a blade-style sway bar instead. 

 

Procurement

While there's going to be a lot of custom laser cut metal from SendCutSend, I wanted to keep this kit as OTS as possible. I am not particularly interested in spending buku-bucks on one-off billet parts nor am I keen on dipping my feet into scaled automotive aftermarket product design & manufacturing. That being said, the spindles will likely need to be entirely custom if I can't find an alternative OTS. If I can find an upright on the market that fits my criteria I will definitely go for that, I’m trying to avoid using a Mustang 2 spindle as its shorter than I’d like it to be and completely wastes my steering knuckles and 370z spindles supplied from Apex Engineered (though steering knuckles and spindles are relatively cheap). 

 

Next Steps

The complete version 1 of this design will be complete soon. Once I've finished my spindle and boxed structures the real work starts, articulating the design through all of its axes of rotation and movement will reveal conditions of binding and the dynamic geometries of the suspension. I started work on this project about a week or two after I placed my order for Apex Engineered SLA kit and have been doing it in my free time which is quite difficult to come by. God willing I shall endure. 

Edited Wednesday at 12:25 AM by baby_Carlton

[calZ]

This is awesome. We haven't had a good engineering project on here in a while, so this is a good to see.