Joel’s SL-C Build Thread

Joel K

This is a minor update. For the sake of completeness I wanted to document installing the rear chassis and transaxle triangulation rods.

The triangulation rods serve two purposes. The chassis rods stiffen up the rear of the car to reduce flex. The trans rods are rear trans mounts to hold up the rear of the Graziano and also provide further chassis strength to reduce flexing.

There are multiple ways to configure the rods to increase the area to run a muffler past the k-brace. The plan is for the mufflers to fit before the end of the k-brace so I chose this configuration.

Pic of the rods and placement…

Due to shifting the engine back, the stock location for the trans rods positions them a good 2” forward of the Graziano mounting ears. I did not want to make a 2” spacer which I thought could apply too much torque and bend the mounting ears. Nor did I want to install the rods with an excessive angle reaching out rearward from the chassis to the mounting ears.

So decided to split the baby by adding washers to the ears and mount the chassis rod ends to the back of the mounting brackets instead of between the tabs…

I always find excuses to use the mill so replaced each mounting ear washer stack with a 5/8” aluminum spacer and steel safety washer. Also filled in the empty mounting brackets with 1.5” aluminum spacers…

When installing the rod ends I found it best to keep the jam nuts loose and adjust the rod length as you position and tighten up the mounting bolts. Once the mounting bolts are tight, then tighten the jam nuts on the rods. This insures you don’t torque and bend the mounting ears which are a single shear design and can bend if too much torque is applied.

Here is a side pic showing the spacer on the mounting ear and the slight angle of the rod end…

Pic from the rear…

I’m in a holding pattern on installing the axles so going to mount the diffuser next.
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Joel K

This post covers mounting the race tail diffuser to the chassis.

The SLC has the choice of two different tail treatments, the race tail and street tail. The street tail provides enough body coverage of the rear wheels to pass certain European regulations. The race tail has an aluminum diffuser which serves as an attach point for the massive rear wing and rear clam hinge.

Here is a video on the process…

Starting with cars ordered in 2018, RCR made a slight change to the diffuser so it clears the back of the Graziano transaxle without requiring modification. The 2018 change places the center panel flat at the diffuser bottom vs. positioning it at an angle as the lower outer panels. Here is a pic of the diffuser as supplied by RCR…

The Graziano sits offset to the right and the diffuser wants to sit slightly offset to the right of the chassis as well. You need every fraction of an inch to make room for the wing upright and keep the diffuser centered. Shaving a bit off the side of the Graz rear cap helps. The offending area on the Graz can easily be filed down by hand…

As delivered, the diffuser attaches to the chassis in 4 places. Two brackets are provided for the top corners and two brackets need to be fabricated for the two lower center attach points. The builder is also expected to further secure the diffuser to either the transaxle or the K-brace. Older versions of the diffuser with smaller transaxles connected to the K-brace. Most Graziano diffuser installs attach it to the transaxle.

Since I moved the engine back 1.5” the first step was to trim out the vertical inner plates to clear the transaxle mounting ears.

The diffuser top corner brackets attach behind the top rear control arm points. As supplied, the angle and spacing of the brackets are pretty far off so decided to make new ones. Pics of the trimmed diffuser and supplied bracket…

I found a nice technique to bend up new brackets without a metal brake. Used a hacksaw to cut 1/2 way through the 1/8” flat stock. In this case a 45 degree angle works well…

Put in vice and used a hammer to fold the metal over at the cut leaving the thinner area gap on the outside corner…

Welded up the outside corner to add back thickness and strength. Custom brackets that are strong and fit perfectly. I may box the top of the bracket in to add more strength…

Pic of the new brackets held in place with cleco fasteners. Just with the clecos in place it feels very sturdy…

For the lower attach points I simply used 1/8x2x2” aluminum angles and M6 cap screws and locknuts. I took special care to locate the holes midway in the pocketed area of the lower frame cross member. Pic of the lower attach points secured to the lower chassis cross member…

Followed in Allan and Cam T’s footsteps and fabricated a couple brackets to piggy back off the transaxle mounting ears. This adds a good amount of reinforcement to firm up both lateral and vertical loads on the diffuser. Used 3/16x2.5x2.5” aluminum angle, just some scrap I had lying around and it did the job…

Glamour shot of the rear showing the attached diffuser and rear of the SLC…


JK, You could cut round lightening holes in those vertical aluminum panels without losing any significant strength/stiffness. A flange around each of the holes would be a nice finishing touch, too.


Or you can do this and lose the diffuser altogether. It's really only a body/wing mount anyway
Hmmm...I think I'd replace that round diagonal brace with one made with streamline tubing. It's in the air stream so it would lower overall drag a bit.