38. Winging it

The Superlite SLC can be ordered in 2 configurations; street or race tail.  The street tail incorporates a short duckbill wing to provide aero downforce.  The race tail uses two tall aluminum stanchions attached to the rear diffuser to support an elevated wing.  I opted to go with the race tail – I thought it looked cooler.

Diffuser mounting:

As received, the rear diffuser can’t be mounted to the chassis without modification.  The lower center “wing” interferes with a good portion of the Graziano transaxle so some minor surgery needs to happen.

Patient on the operating table.
The welds are somewhat superficial, there isn’t full penetration – so removing the center “wing” plate isn’t too difficult.  Grind these guys back and you’re all set.
A little blending; I plan to powdercoat the rear diffuser so getting this area decently smooth will be good enough.
Interference between Graziano end cap bolt and right support plate.  Some grinding of the Graziano and plate will be necessary.
Some grinding …
Keeping the center plate removable will make servicing this area easier in the future.  Deleting this center piece causes the diffuser to lose a fair bit of stiffness so it’s important to re-install it.

The RCR supplied diffuser mounts didn’t work for me.  I couldn’t find an angle or orientation that worked without having to put a good amount of strain on the diffuser.  The brackets were either too wide or too narrow depending how you oriented them; so the diffuser outer endplates would have to be bent one way or the other to use the factory supplied brackets – not a good idea.

The diffuser supports the rear wing stanchions so all aero loads will be going through the diffuser and the diffuser’s central mounts.  That’s a fair bit of downward force and likely a good bit of twisting – so the center area of the diffuser will need to be restrained in all axes of motion.  I made a series of L-metal brackets, bolting the two center diffuser uprights to the Graziano support brackets, then another set of L-metal brackets securing the lower edge to the rearmost lower chassis brace.

The diffuser also supports the weight of the rear clam when it’s in the open position and stabilizes it in the closed.  The rear hinge attaches to the diffuser’s outermost plates; without additional supports here the diffuser ends are pretty wobbly – not so bueno.  Much of the loading and vibration will primarily be in the Z-axis (vertical) so that’s what I prioritized for the endplate stabilizer mounts.

Bob helped me out with getting the endplate mounts fabricated.  I wanted it to be very stiff and Bob’s welding comfort is with steel so we came up with the “beefcake” bracket.  I think this takes the cake for strongest diffuser endplate mount ever on an SLC – and likely heaviest too!

Sparks fly when Bob is working!
Passenger side bracket; the two lower nuts secure the diffuser endplate.  That’s one serious looking bracket!

An additional complicating factor is the rear clam swings open until it crashes into the rear wing supports unless the builder installs some type of restraining device.  DCarter came up with a really nice setup where he used 2 folding hinges which prevents the clam from rotating too far back but allows it to swing far enough to let gravity hold it open.  He used Allstar Performance hood prop PN 23620 (2 req’d, sold in singles).  One end attaches to a mount he fabricated which is mounted to the rear of his engine box and another attaches to the engine side shielding of his rear wheel liners.  His rear wheel liners were reinforced with aluminum; see Dan’s build thread, post #294.  I told Dan I was going to shamelessly copy his cool hood prop design.  My plan is to integrate the chassis-side hinge mount with the diffuser endplate mounts.

The clam will swing open until the rear edge crashes into the wing supports.  I used some cushions to hold the clam in the ideal open position (a good bit past the balance point so it wants to continue “falling open”.
Driver’s side bracket; chassis-side mount for hinge shown.  This mount will use a series of washers and thrust bearings to ensure the hood prop rotates freely while still being firmly held in place, see details in rear hinge section below.

Rear hinge:

I haven’t seen a lot of detail in other build threads on how others have done this so here’s some info on how I did mine.  It’s not the most sophisticated hinge but this is one of those git’r’done tasks I didn’t much care to over-complicate.

The RCR supplied hinge has a 1/4″ thruhole; at first I thought about using a 1/4″ dowel and matching drill bushing to get a really tight rotating couple.  Unfortunately the sheet metal used to make the diffuser is pretty thin and I think this type of joint would need more support – so some Aluminum welding would be needed.  My Aluminum expert welder (Lynn) has flown back home for the warmer months so this more elegant solution may need to wait for v2.0 of the car – or never.

Instead, I used a few needle roller thrust bearings to help reduce the friction at the hinge.  I ordered the following:

  • Needle roller thrust bearing for 1/4″ hole, McMaster PN 5909K23, qty 4
  • Needle roller thrust bearing washer for 1/4″ hole, McMaster PN 5909K231, qty 8

I placed a set of washers and bearing on either side of the RCR supplied hinge then bolted the assembly onto the diffuser.  The final assembly will be held together using a nylock nut; you can’t reef down on this bolt because too much preload will cause the hinge to bind.

Close-up of hinge and mounting assembly.  Of course after I’d drilled my mounting holes I discovered my tail was just slightly off, hence the need for a washer stack between the hinge and support plate bonded to the tail.   In hindsight this was actually a good thing as it provides me with the ability to shim as necessary to perfect the tail height in case the bodywork distorts later.  The washer stack will be replaced with a machined spacer once I finalize the height I need.
Another view of the hinge; rear clam in the open position.  The plate attached to the lower edge of the rear clam was first bonded in place using 3M panel bond adhesive then backed up by a series of bolts.
Closeup showing the thrust bearing assemblies.  The order is: bolt > countersunk washer > precision washer > thrust bearing > precision washer > hinge > precision washer > thrust bearing > precision washer > diffuser endplate > washer > nut/jam nut.  Don’t over tighten!  Too much preload will cause the bearings to bind.

To locate and drill the holes in the diffuser I first identified the driver side hole by mounting the hinge onto the underside of the rear clam and drilled a 1/4″ hole wherever the hinge ended up.  To transfer the hole to the other side I used my laser to get an exactly opposing hole – any misalignment between holes means the rear clam will open cockeyed and may bind.

Pro tip: Be sure your rear clam is fully seated on the alignment pins and supported at the rear before drilling your first hinge mounting hole.  Guess who didn’t do that?

My rear clam was *just* slightly off and was sagging at the rear when I made my holes.  To rectify this I need to shim ~0.25″ between my rear clam and the hinge bracket.  Lucky me, I was so accurate in transferring my hinge mounting holes that both sides are off by the same amount.

Wheel liner:

The next step is to install the engine side portion of the rear wheel liners so we know where one of the hinge attachment points goes.

These vertical pieces of the rear wheel well liners primarily shields the rear shocks and bell cranks from road debris.  The shields will need a good bit of trimming so the rear suspension can clear without fouling the shields.  It’s primarily the upper A-arm that need clearancing.  I articulated the rear suspension throughout its full travel (with the rear springs removed) to ensure I had enough travel without interference.  I also had to add a large scallop for my bell crank brace; unless you’ve made one yourself you won’t need to clearance quite as much at the forward relief.

The liners won’t sit flush against the rear clam; as with most other pieces of the wheel well liners, you’ll need to trim it so you can get it as close as possible, then fill the gaps using peanut butter and reinforce with glass.  Except where the vertical piece joints with the curved liner piece, most other areas have gaps which are too large for panel bond.

I started by using construction paper to figure out the reliefs, then transferred everything to foam core poster board for a better representation of the actual piece.  After verifying the clam could open and shut without interference, and suspension articulation was good, I transferred my reliefs to the actual fiberglass pieces.  Be sure to get your liners trimmed and positioned into their final locations before transferring the reliefs.  Note that these vertical pieces are pretty flexy and you may want to trim anticipating some amount of side to side motion as the clam comes down.  Once I verified the actual pieces were good to go, I sanded all contact points down to prep for future fiberglassing.  For the joint that will be getting panel bond, I Swiss cheesed both pieces along the joint to get as much surface area as possible for bonding.  I used several rivets to keep the pieces together while the panel bond was curing.  These rivets aren’t expected to hold any kind of load once everything’s bonded and glassed together.

I wanted to bond the liner with the rear clam positioned and located via the rear hinge points and the forward alignment pins.  There is a risk that bonding these pieces in while the rear clam is out of position may distort the clam and make it difficult to re-install.  Therefore the liner will have to be secured in stages because access to all the necessary joints isn’t possible with the rear clam down.

  1. Bond the two liners together using panel bond, hold using rivets.  Use peanut butter and fill gaps wherever reach is possible.  Let everything cure overnight.
  2. Using CSM, fiberglass all accessible areas.
  3. I had planned on following up with even more fiberglassing with the rear off the body but I was able to get enough of it secured that there really wasn’t any need to add more fiberglass!
Liner temporarily installed and located using rivets (some cleco fasteners would have been handy here!).
Panel bond applied, rivets installed, clam in the closed position.
Rear section of liner, looking through the taillight access hole.  Most of the bent portion of the liner sits too far off the mating surface to get a good bond.  Instead, I opted to cut most of the liner away so I could encase it using peanut butter.
Forward portion of the vertical liner interferes pretty hard with this curve.
A good bit of peanut butter and CSM to bond this edge into place.

Pro tip: add sound damper to the upper wheel well liner BEFORE bonding it in!  It’s pretty tough to add once everything’s closed up (ask me how I know)!

This sheet of sound damper makes a surprising difference in noise attenuation; should be helpful in keeping all those rock pings from traveling into the cabin.
The entire length of the liner will need peanut butter and fiberglass to hold it in place.  This area will need to be particularly strong because this side of the panel will also receive an aluminum plate onto which the hinge will be attached.  So this part of the liner will need to support the weight of the clam pulling on it (hopefully without flexing).
I smooshed in (yes, that’s another technical term) a fair bit of peanut butter to try and bridge the gap between the liner and the bodywork.  I then created as smooth a transition between the liner and rear clam as I could with peanut butter and followed up with a few layers of CSM once the first pass of PB had hardened.  I was originally intending to follow up with a layer or two of glass cloth but this joint is so stout I don’t think there’s any need for more glasswork here.

YES! the last piece of the rear wheel well liner has finally been completed!!!  I did a small victory dance.

After settling on the chassis-side hood prop mounting location, I swung the hood prop while fully extended and marked out where it crosses with the rear clam while in the open position.  The arc created sets all the possible body-side mounting locations.  I used this as a reference for determining where to bond my aluminum reinforcement plate.
Once the plates were bonded I drew an arc with the hood props fully open and picked what I thought would be a good spot to locate the second attachment point.  It works!  There’s a very narrow range of angles between the rear clam’s balance point and when it contacts the rear wing stanchions.  Pick your position carefully!
As you can see, there’s a pretty good gap between the edge of my bracket and the wheel liner.  I used washers to try and figure out the best lateral position for the hood prop.
This stack of washes has got to go!  I’ll weld a small spud of steel and match drill it for a 1/4″ hole.  That way the cantilevered support will have a good amount of strength, more than will be needed for the hood prop loads (it only has to prevent the clam from opening too far, not support its entire weight.  Being close to the balance point means a fairly low load.
Material positioning of the hood prop is important so it doesn’t foul on the contours of the rear clam.  Here, it’s tucked up into the recess between the wheel well liner and the outer “hump” of the rear clam.  Pushing the body-side mounting point further rearward causes the hood prop hinge to push upward, eventually fouling on the bodywork.  Pushing it further forward brings the open position closer to the balance point.  You don’t want to be so close to the balance point that you don’t have margin in case the ground is uneven while you’ve got the rear clam open.  As positioned, the body-side mounting point is just above the bell crank main bolt and the chassis-side mounting point is about 2 inches rearward of the chassis and about halfway vertically between the two bolts securing the chassis bar mount.
  • Diffuser mounted? – CHECK
  • Rear hinge completed? – CHECK
  • Hood prop mounted? – CHECK

So with all that work completed we’re finally able to start mounting the rear wing!

The rear wing mounts are two aluminum plates made from 1/4″ aluminum sheet.  They’re way heavier than they need to be – unfortunately they’re just not very stiff.  Without any additional bracing the wing flops around quite a bit – doesn’t seem like a good idea to leave it as-is.

Tall and kind of boring looking aren’t they?
With the wing mounted the assembly is pretty wobbly.  Push it left to right or twist it and the wing moves a lot.

There’s a pretty wide range of stanchion braces folks have come up with over the years – not to mention a pretty drastic change in the supports themselves.  The most basic is a single rod that extends from one plate to the other and from top to bottom.  This is a nice, straightforward way to give the stanchions additional lateral stiffness – push the wing left or right and that single rod does a pretty good job of keeping things where they should stay.  Unfortunately it doesn’t do much in the twisting department.

Fitting up the support braces so Bob can tack in place.

2 is better than 1?  Yep.  I took our brace design just a slight step further by adding a second bar.  This really stiffens things up laterally!  But won’t this design suffer the same twisting issues as a single bar?  Yes – theoretically … Having a second bar provides a lot more anti-twisting stiffness in one rotation, but doesn’t do much for the other – or does it?

After we fabbed up our 2 bar brace it actually seems like the twist resistance is pretty high – but not quite enough for my taste.  I added another 1/8″ aluminum plate and tied the two lower ends of the bars together, effectively boxing out the lower end of the wing supports.  The wing itself boxes out the top part – so starting very close to where the stanchions turn upward, my supports are fully boxed and cross-barred.  The stanchions are now super stiff – the only flex or motion I’m getting now is at the mount itself.

The options here are less ideal.  I would like to secure the wing supports using quick release pins so I can remove them and open the rear cover without needing a tool – y’know, in case of an engine fire or some other event that would necessitate a speedy opening.  The trouble is that means 2 of the 4 anchor points needs to be removable without any tools, leaving only 2 attachment points per stanchion.  These anchor points can’t be so preloaded/tight that the plates won’t move by hand.  Using the same washer/thrust bearing setup as I have for the rear clam hinge, there’s still just a little slop that I can wiggle the wing back and forth just a smidge.

But I think it’s OK.

With the car parked and just sitting there, it’s not an issue if it can wiggle back and forth a bit.  With aero loading, the wing assembly loads rearward and what little slop there is goes away pretty quickly; I think I can live with that.

X marks the spot!
The lower plate is 1/8″ aluminum and provides anti-twisting strength to the wing assembly.  Starting from there and up to the wing, the assembly is super rigid – less so, below that.  That lower plate also serves as a convenient mounting location for my rear view camera and license plate.

I may come back and try to lighten these stanchions by cutting away material in the centers but that’s a future project likely never to be completed.

  • Wing installed? – CHECK

Last bit – the wing endplates!  The carbon wing has aluminum plates bonded at the very ends for structural stability.  To help keep air flowing over/under the wing as it should, larger endplates need to be added to the wing.  The latest version of the endplates is pretty chunky; it’s 1/8″ aluminum plate and pretty large.  I suppose it’s more efficient because it’s super big and catches more air – but I think it’s a bit ugly; function over form in this case.  I like the smaller, more shapely aero looking pieces used on older builds.  I reached out to some of the old timers for help and HJones was nice enough to send me a template for his endplates – thanks again Howard!

Endplate design that shipped with my car – basically a big “L”.  Apologies, no idea whose car this is so can’t give credit as I pulled this off the web.
The cooler looking, probably less efficient, older version.  Apologies, no idea whose car this is so can’t give credit as I pulled this off the web.

I was a bit concerned about people cutting themselves by accidentally bumping into the endplates.  1/8″ aluminum is pretty thin and even with some careful sanding it’s difficult to get a large enough radius on the edge to make it safe to handle.  I decided I would make my endplates from laminated fiberglass.  I could make the plate thicker while keeping the weight less than the equivalent in aluminum.  I also didn’t want to add too much mass at the very edges of the wing – makes it even more prone to wobbling.

Looks pretty boring, need to do something about that … more to come in a future update!

Phew – that’s a LOT of work just to mount the rear wing!



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