Exocet Update 6: Mating the Powertrain and Frame
- Jan 26, 2024
- 7 min read
01/25/24 Build Update: It Looks Like a Car
Mating the powertrain to the frame posed some challenges. The Miata skateboard is composed of both front and rear subframes connected by the powertrain (engine/transmission connected to the rear diff through the Power Plant Frame (PPF)). There are no engineered lifting points on the skateboard. We needed to get the subframes lifted up and supported in such a way that we could remove the wheels and shocks so the Exocet frame could be dropped onto the subframes.
We chose to use a combination of floor jacks and an engine hoist to lift the front and rear of the skateboard onto stacks of wood placed on dollies. This would allow us to take the wheels off the skateboard, remove the shocks, and move the subframes relative to each other to get everything aligned with the Exocet frame.
Once the subframes were supported and the wheels removed, four of us carried the Exocet frame over the skateboard and dropped it into place. We had to two this more than once, the battery wire harness connected to the PPF interfered with our fuel lines. We bent the fuel lines back slightly and removed the battery wire harness from the PPF to achieve the proper clearance.
If you are familiar with engineered, bolted joints featuring multiple fasteners, you may understand that one fastener sets a component position (the primary), a second fastener arrests remaining degrees of movement (the secondary), and all other fasteners simply provide enough clamping to meet the requirements of the joint (the tertiary). This is sometimes referred to as a pin-slot design.
On the Miata front and rear subframe, one hole in the subframe is a tight fit (the primary) - it has the smallest possible thru hole to accurately position the subframe. Typically, all other features on the subframe will be referenced to this hole. Opposite of this position on the subframe is a fastener hole shaped like a slot (the secondary) - it has low clearance in one direction (front to back) but allows movement of the subframe relative to the bolt in other direction (left to right). These two holes set the position of the subframe and constrain its movement relative to the Exocet frame (or the original Miata unibody). The remaining holes on the subframe (the tertiary holes) are all oversized, they simply are there because the bolted joint requires a certain number of fasteners for a good clamp. These holes are oversized because they don't need to set position. To reduce manufacturing costs, the size tolerance on the tertiary holes is relaxed so the fastener easily passes through despite any alignment variation. This would reduce scrap cost and ensure proper alignment with ease of assembly in manufacturing.
When bolting the frame to the skateboard, we started with the primary bolt position, then the secondary bolt position, then placed the remaining fasteners. Installing fasteners in any other order would potentially cause the primary or secondary to be misaligned since the tertiary holes allow for more relative movement (this may be why some builders struggle with the final few fasteners when mating their Exocet frame to the subframes).
In practice, this worked very well on the rear subframe, there were no issues getting all four bolts into place. On the front, the secondary slot was misaligned significantly to the frame. This may be due to the way the skateboard was sitting on the wood and dollies (the car is slightly raked but the subframes sat flat on the dollies). We used forceful coaxing to get the secondary bolt into place. With a floor jack lifting the subframe from the lower control arm mount, two people pulling on the Exocet frame while pressing down on the lower control arm, and a third prying the frame and subframe into place with a pry bar stuck through the frame and a tertiary bolt hole, we were able to place the secondary bolt. Once the primary and secondary bolt were placed, all tertiary bolts dropped into place with no resistance.
Because the Exocet frame is slightly raked, the subframes did not fit snug against the mating faces on the frame. It is normally not acceptable to seat a joint by running a nut down fastener thread to pull parts together but there was little resistance so we used the tightening of the nut to seat the frame. There were no issues with threads stripping. We went around in a star pattern, first seating everything, then torquing to 70 lbf-ft and finally 90 lbf-ft.
Instead of using the nyloc M12 nuts supplied by Exomotive, we used standard M12 nuts when installing the mounting fasteners. If for any reason we needed to separate the frame and powertrain again, this would allow us to do so. At the point where we know there won't be a frame separation, we will install the nyloc nuts, apply Loctite, and apply the correct torque one fastener at a time - this ensures we apply proper torque and aren't using the lock nut to snug the subframe to the frame.
Finally, we reinstalled the shocks and wheels. On a regular car, the control arms need to be disconnected to have enough install clearance for the shock, the Exocet doesn't have fenders and other things in the way. Simply pushing down on the knuckle to deflect the suspension was enough to pass the shock into the lower control arm and then into the tower mount.
The Exocet suspension geometry may be different than the stock Miata (I actually do not know). The proper way to set the suspension is to loosen all bushing mount positions after installing shocks that change the ride height, dropping the car onto the ground, rolling it back and forth to seat everything, then lifting it off the ground to retighten everything so bushings aren't overly stressed.
Unfortunately, we found an interference between the shocks top-hat and tube frame on the front of the car so we were unable to tighten everything and remove the car from the lift. We also suspect there will be tire rubbing against the frame at the rear of the car at full suspension jounce. Exomotive sells shock tower spacers to address the clearance issue seen at the front, we will use wheel spacers temporarily to alleviate the rear clearance issue. Long-term, we will replace these wheels with a set that has the correct offset.
Time Prepping Donor: 11.5 hours (06/11/2023-08/28/2023)
Total Teardown Hours: 45.75 hours (08/29/2023-11/04/2023)
Total Exocet Build Hours: 40.5 hours (11/07/2023-01/25/2024)
Total Hours on Project: 97.75 hours
Exocet Build Journal:
01/01/2024 - Peter, Paul
- 11:30AM Start
- Discussed how to mate the chassis with the skateboard
- Hit rear subframe brace fasteners on bottom of subframe with PB blaster
- 12:30PM - 1:45PM - Harbor Freight and Home Depot Run
- Removed rear subframe brace
- Mocked up support system for skateboard using front and rear subframes
- 3:30PM End
- 2.75 Hour of Labor
01/07/2024 - Paul
- 3:15PM Start
- Placed skateboard on wood + dollies, need second set of hands
- Removed skateboard from dollies
- 3:45PM End
- 0.5 Hour of Labor
01/11/2024 - Peter, Paul
- 6:15PM Start
- Placed skateboard on wood + dollies
- 7PM End
- 1 Hour of Labor
01/21/2024 - Peter, Paul, Sam, Jon
- 10:45AM Start
- Removed drive pedals from frame
- Using bench vice and dead blow, bent throttle pedal towards brake pedal
- Throttle pedal needs clearance to Exocet transmission tunnel
- Removed shocks from skateboard
- Carried frame onto skateboard (four people)
- First attempt showed interference between fuel lines and wire harness on PPF
- Removed frame from skateboard to fix interference
- Unfastened PPF wire harness, moved out of way; bent back fuel lines
- Carried frame onto skateboard (four people)
- Aligned rear subframe bolt holes - frame is interfering with wood supporting skateboard
- Using engine hoist, lifted rear of car, fixed wood supporting subframe
- Aligned rear subframe bolt holes, placed primary then secondary bolt
- Secondary rear subframe bolt fell into place with no issue
- Aligned front subframe bolt holes, placed primary bolt - secondary severely misaligned
- Jacked front subframe from lower control arm mount
- Used center punch (large diameter, length) in one of the bolt holes as prying leverage
- Two people pulled on the Exocet frame while a third pried on the bolt hole
- Aligned secondary hole on front subframe, placed bolt; placed bolt in tertiary hole
- Remained tertiary fasteners fell into place with no issue
- Going around the car, tightened each fastener with two wrenches
- Tightening nut lifted subframe up to Exocet frame without much resistance
- It is not correct to seat the frame this way but was the only option
- Got all fasteners wrench tight, seating the subframes against the frame
- Using torque wrench, set all fasteners to 70 lbf-ft going in star pattern around the car
- Set all fasteners to 90 lbf-ft going in a star pattern around the car
- Put car on 2-post lift, removed wood and dollies
- 3:15PM End
- 4 Hours of Labor (we took many breaks to sell donor parts during the day)
01/25/2024 - Peter, Paul
- 5:45PM Start
- Installed shocks onto Exocet by pressing down on knuckle
- Left everything loose
- No disassembly of the suspension was necessary to install shocks
- Installed Wheels
- Discovered shocks interference with frame in front
- Determined that rear tire may rub frame at full suspension jounce
- Suspension will be addressed with shock tops / spacers and wheel spacers
- Long-term solution for tire rub will be new wheels with different offset
- 6:30PM End
- 0.75 Hours of Labor
Resources:
Parts Used:
Fasteners:
Tools Used:
General Information:
Other Project Car Builds / Build Guides:
Comments