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-- Use Tesla motor mounted in rear. The Spit has independent suspension, so shouldn't be too hard with custom/modified half shafts to use it this way. Won't need a transmission.
It would need a transmission, but would be using the one which comes as part of the Tesla drive unit (which is an inverter, motor, single-ratio transmission, final drive gear reduction, and differential as one assembly).

Just replacing the Spitfire differential with a Tesla drive unit (or any other common motor+transaxle unit, such as from a Leaf) seems unlikely to work without problems. The axle shafts in a Spitfire are part of the Spitfire's swing-axle suspension, taking all of the lateral loads. The outputs of any modern transaxle are not designed or intended to take any axial loads at all, let alone the entire lateral force on the rear axle in a turn.

You can completely replace the Spitfire's rear suspension, but at some point I think it's time to ask why any part of the Spitfire chassis is being used at all, since all of it is either unsuitable or being replaced. :confused:

I am interested in the tesla motor idea. Would the "small" Tesla motor work well rear-mounted?
I can't imagine why anyone would want a Spitfire frame and chassis with more power than one of the "small" Tesla motors (from either rear or front of a Model S).

Also, do you think I'd have to swap out the forward suspension or just tune it accordingly?
This implies that you think the front will be substantially lighter. With a big pile of battery up front, that might not be true.

Even if the front does end up lighter, I don't see any reason to change anything other than springs and shocks.
 

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Car is in decent shape. About 100k miles, one tiny spot of pitting on the left rear wheel well. Doesn't run only because the owner was replacing the starter (but not a problem for me!). Transmission seems to be functional.
This may not matter, but this situation sounds like a scam to me. If I had no ethics and wanted to sell a car with a completely dead engine, pulling the starter off and claiming that the engine is fine but just can't be demonstrated would be a great scheme. :rolleyes: I would not budget for any significant recovery of purchase cost by selling the engine.

I've also heard that the Spitfire transmission doesn't handle electric motors well in EV conversions.
I suspect that the reason is simply that people are fitting motors with higher torque output than the wimpy little engine which they are replacing. If you're doing that, then you likely need a stronger transmission... and you should calculate what the torque input to the final drive (diff) will be (compared to the stock engine output multiplied by the stock first gear ratio), to see if you should be concerned about blowing that up as well.

I'm considering going no transmission (just straight to a diff). Not too picky on this, but was hoping to get some advice.
If you don't use a transmission of some sort between the motor and the final drive (differential), you only have less than 4:1 speed reduction (and torque multiplication) from motor to axles. The motor will always run relatively slowly, so if you use a typical modern production EV motor the motor will always be in bottom half of its operating speed range, and unable to produce anything close to peak power at most road speeds.

The solution to such tall gearing is a suitable motor. Duncan used an unusually large "forklift" motor in his "Dubious Device" to handle similar gearing; General Motors used an unusually high-torque/low-speed motor (otherwise technically similar to their other AC PM high-voltage motors) in the Spark EV to go with unusually tall (3.17:1 initially, 3.87:1 later) gearing.

To me it seems that a significant advantage of connecting the motor directly to the final drive is that the motor can sit in the original transmission space, leaving the original engine compartment for batteries and reducing the packaging problems that are otherwise encountered. The problem is that a big motor won't fit properly in the transmission tunnel. It is common to see builders cutting out and replacing the entire transmission tunnel; I'm not interested in that level of sheet metal fabrication, or cramping my right leg even more than the stock car. (Note: Duncan's car is custom-built, and accommodates the big motor... even further back than would be possible in a Spitfire frame.)
 

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I'd say my top options at the moment are Tesla Smart batteries (aiming for 20 kWh pack) ...
At first glance, it looks like the Tesla batteries will fit nicely where the gas tank is.
That seems like an extraordinarily small space for 20 kWh of battery. Have you compared dimensions of the Tesla Smart pack or its component modules to the available space?

Baratong's conversion of a 1979 Spitfire put 16 of the 45 large prismatic cells (CALB CA100) in the fuel tank space, which is only 5.4 kWh in 32 litres of cell volume plus support structure, etc. I realize that the Smart EV cells have higher energy density, but still...

The images are no longer visible in Baratong's thread (post #198 had photos) but my notes say that the cells in the fuel tank space were arranged as a 4X4 block with the cells vertical (terminals on top), so that would be a 560 mm (side-to-side) by 268 mm (front-to-back) by 216 mm (tall) block. This is a very tidy installation, with little wasted space.

EV West's listing for Tesla Smart Lithium Ion Battery 18650 EV Module - 57 Volt, 3kWh includes these specs for each module:
  • Capacity: 57Ah, 3kWh
  • Height: 7.25 Inches
  • Width: 2.875 Inches
  • Length: 39.0 Inches
  • Weight: 42 Pounds
  • Bolt Size: M6
  • Voltage nominal: 3.8V/Cell, 57.0V/Module
  • Charge voltage cut-off: 4.2V/Cell, 63.0V/Module
  • Discharging cut-off: 3.3V/Cell, 50V/Module
  • Maximum Discharging Current (10 sec.):150 Amps
That's 13 litres of volume per module, so at least three modules would fit on a simple volume basis; however, I don't think there's any way to put a metre-long (39") module across the fuel tank space. Even if the length works, only one module could be placed standing up normally in that space, and there's nothing like six or seven times the module width available front-to-back.

EV West says
A preferred configuration for a typical AC50 application is 4p2s for a total of 24kWh of energy with a total of 8 modules.
That's 336 pounds (152 kg) of modules, not counting supporting structure, wiring, outer housing, and cooling provisions. It would be a 114 volt, 228 Ah pack, with a maximum discharge rate of 600 amps, and performance limited by cooling.

Even up front, a Smart EV module would extend from the firewall right up to the original radiator location. Several of them side-by-side would fit there, and two layers would fit fine... as long as the motor isn't occupying much of the space. For instance, two layers of four modules wide, all upright, would be
Height: 2 x 7.25 Inches (15.5" or 40 cm)
Width: 4 x 2.875 Inches (11.5" or 30 cm)
Length: 39.0 Inches (99 cm)
... plus support structure, space between modules, outer case, etc.


While I haven't looked at it in detail, it may be possible to place some of the pack in the bottom of the space behind the seats, without preventing the convertible top from folding down. It's probably worth looking at, especially with a front motor configuration, to keep the mass of the components added to the car from making it front-heavy; a Spitfire should be roughly neutral (50:50 load distribution between the axles) with a driver and passenger.


The small size of the Spitfire's tank is a running joke between my wife (the actual owner of our Spitfire) and myself, although published specs say that if you fill it as much as possible it holds 37.6 litres (9.9 US gallons). Between the relatively small tank and the high fuel consumption of our Weber DCOE equipped engine, this car has a shorter range than anything else we've ever owned. :)
 

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If you're doing a Leaf motor why not go with the whole drive unit?
How would you mount that in a Spitfire? I mean, short of completely replacing the rear suspension and cutting out the insides of the rear body and changing the rear structure?

The Leaf gearbox is a transaxle, which means it includes the differential. It is not suitable for taking the suspension load which the Spitfire's swing-axle suspension needs the final drive unit to take. Suspension conversions have been done on Spitfires which remove the axle loading, but the only ones which are reasonably workable with the frame and body adapt the TR6 design, and still leave structure and spring which would not likely be compatible with the drive unit.

I did see a "bolt on" conversion of a Spitfire to a more modern rear suspension; however, the installation method was to entirely cut off the Spitfire frame well ahead of the axle line, discard the entire rear frame and suspension, and bolt on a new system! Sure, if you want to build a new chassis for the Spitfire body, you can certainly use a Leaf, Tesla, or Smart ED drive unit... and custom suspension or suspension from the drive unit donor. While you're at it, I suggest a Miata front suspension for better handling and reliability. For that matter, just buy a Miata. ;)
 

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Compact transmission?

I am also considering a custom 2-speed as an option. Anyone have advice or experience with this? I definitely want some form of transmission for good performance.
It would be nice to have a very compact two-speed transmission which would mount directly to the motor, and output to a splined shaft (for a slip joint) or flange (for a bolt-on U-joint), and allow the motor to be placed into the transmission tunnel. I haven't seen one.

I haven't seen a reasonably-priced and available two-speed transmission with either two reduction gears or one reduction and one direct. There are still separate overdrive units, but they are getting rare and are neither reasonably priced nor equipped with a useful ratio (you need reduction, not overdrive). The similar separate underdrive units seem to have disappeared; it seems that people just use a transmission with a suitable number of ratios now, instead of tacking on more boxes. ;)

I spent some time looking at Lenco's drag racing transmissions (which are modular, built up of one reduction stage per module); however, they are expensive and do not back-drive in the reduction ratio (which would mean regenerative braking would always be in direct).

I also haven't seen any transmission which can readily be mounted to a Leaf motor, other than the Leaf's transmission (which is normal: these motors are configured to drive specific gearboxes, without a clutch, so they are not set up like typical engines with a flywheel flange).


I did find one interesting possibility. It is only a single-speed box, but that's all that most production EVs (such as the Leaf and Teslas) have, and it does provide reduction, which is needed to get the motor into a suitable operating speed range.
ev-Torque Box

This thing is massive (it is "specifically for the electric conversion of late model Ford F-Series trucks"), large for what it does, stupidly expensive (US$3495), and is not set up to attach to a Leaf motor... but it is a reduction box to use between a motor and a driveshaft. They also used it in a 1999 Miata conversion, with a "AC Propulsion 13,000RPM 150kW AC Motor". They even used it in a Triumph GT6 conversion! I don't know what the output shaft and mounting face look like of the UQM motor that they use for the F-150, or the motor in the Miata, and I don't even know what motor is in the GT6, so I don't know how close it is to working with any other motor, such as the one from the Leaf.
 

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If you can fit a Tesla "small" drive unit in there, that would be ideal- but I caution you that the car is narrow and its existing rear suspension design relies on using the half shafts as one of the control arms found in a modern independent suspension. Loadings on the rear wheels when cornering are transmitted THROUGH the half shafts and their u joints into the bearings in the differential, which perhaps explains somewhat why the car seems to chew up u joints and diffs...
Not only that, but below an excellent photo of yours of the Spitfire chassis from behind (from your E-Fire build thread). I took the liberty of downloading and re-uploading this image, so it would display here in a small version.

All of that structure between the frame rails would be in the way of a Tesla drive unit. It could be replaced by something fabricated to arch over the drive unit, but with the spring attached only to the differential, in the conversion this arch would also need to pass directly over the axle line and carry the load of the rear of the vehicle... while fitting in the non-existent space between the drive unit's differential housing (likely bigger than the Triumph housing) and the spring. In the stock configuration, the spring carries the weight of the car through the diff housing, and the diff carries the car by bolts in the middle at the rear (to that box of crossmember) and by a mount that spans across the frame rails at the front.

For those considering a complete Leaf drive unit, I'll note that where those white frame rails start to converge around the front of the final drive unit (diff) is where the Leaf motor would need to sit. :eek:


I would use my own Spitfire (actually my wife's) for illustration, but being lazy compared to all of these fine DIY builders, ours still has the body on it... and a dead engine in it. :D Thanks to those who have shared their build experience! :)
 

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Example of a gent who converted a Spit to use a Miata rear end. Some fabrication for sure, not easy, but not impossible either. Balancing the weakness of the Spit diff when driven by an electric motor, this might overall be a better approach. I'm seriously considering re-working my Spitfire to something like this.... but nobody has ever accused me of doing anything the easy way... :)
Interesting photo - thanks! :)
This is NA/NB Miata hardware (not the later NC or current ND), as with almost all custom use of Miata components. Upper structure has been added to the frame to carry the upper arms; lower arm mounts are bracketed right onto the frame. The upper arms look like OEM units, radically modified; the lower arms are obviously custom-fabricated. The odd frame-under-axle configuration of the Spitfire works well for this conversion. The general design is NA/NB Miata but with geometry compromised by available mounting space, while only the hub carriers and outboard are actually stock Mazda parts. I'm sure it works better than the stock Spitfire suspension.

There's no problem with the spring, because this replaces the transverse leaf spring with coil-over shocks. The roll stiffness will be higher (it is deliberately low in the later Spitfires to minimize swing-axle geometry problems).

The Miata diff is a nice fit in there... much easier than either of the EV drive units mentioned (Leaf or Tesla). As is common in mods using Miata bits, the PowerPlant Frame has been abandoned, and a mount to the frame for the front of the diff housing has been added; the Spitfire has a front diff mount anyway, so that's not a problem.

While not as severe as some chassis modifications - and keeping the whole stock frame intact - this still makes me wonder why anyone would do this, rather than just go with the whole Miata? Yes, the Spitfire is even shorter (for those like me who like small cars), but with excessive front weight bias (the result of most conversions), decades-older front suspension, and a relatively flexible structure (even by 1980 standards), the FrankenSpit won't handle as well.

I think the weird old swing-axle and swing-spring Spitfire is a blast to drive; I also think any Miata is a great sports car. I wouldn't want to make a Spitfire into a half-Miata... but it's good that people can do what they want. :)
 

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On the topic of motors, is it not possible to remove a Leaf motor's gear reduction and couple it's shaft to a transmission (like a general purpose motor)?
Yes, but there are challenges...

A relatively minor challenge is that the face of the motor does not match any of the standards for industrial motors, so it won't fit the adapter plates which are available to match up an electric motor with a transmission.

The bigger challenge is that available adapter parts for the shaft assume that the shaft has a plain end - a fitting is clamped to that to provide a flange, to which either
  1. a flywheel is attached, then a normal clutch which accepts the transmission input shaft, or
  2. a coupler is attached which directly accepts the transmission input shaft
The Leaf motor has a splined end, since it is designed to fit into the a female spline in the transaxle input shaft. Custom fabrication of a female-female spline-to-spline adapter with two different splines is thus required, and for common longitudinal transmissions it has the additional challenge that the motor shaft must locate and support the front of the transmission input shaft

In an ideal world, someone would sell a reduction gearbox with
  • an input side housing designed to bolt to the Leaf motor,
  • an output side housing with a place to bolt a support bracket,
  • an input side shaft with splined socket to match the Leaf motor, and
  • an output side shaft to accept a traditional propeller shaft (driveshaft).
In the real world if we want this one of us would need to make it.
 

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If I had to do it again I would look real hard at grafting on the whole rear ( or front ) suspension/motor setup from a modern car. Especially after two great builds, Baratong and Moltonmetal, report the sane week link. The REAR END.
I understand the logic, but no modern suspension is likely to fit in the rear without cutting out the trunk floor, the floor behind the seats, or both. Triumph's swing axle and transverse leaf suspension design is obsolete, but brilliantly compact, occupying only a narrow tunnel across the car, plus some space very low ahead of the axle for trailing rods.

The custom Miata NA/NB-based suspension shown earlier doesn't look like it fits the floor: the floor's drop into the trunk would likely interfere with the upper control arms, unless they have an unreasonably narrow base.

The front is comparatively easy, as there's nothing but a pair of frame rails to work around.

The real problem is Spitfires are 48" wide. The Tesla 66 wide the Prius is 60 and the Fiat is 55
Those are track widths (rather than body widths), and one source shows rear track up to 50" depending on vintage; yes, that's a problem. The first-generation Miata (common chassis for kits and conversions) has about 55" track width as well, which would be one reason that the custom Spitfire rear suspension shown earlier using NA/NB Miata bits has custom control arms.

I'm guess I'm just not enough of a fan of the Spitfire body to want to build a whole new custom car - powertrain, suspension, structure, and floor - and fit half-century-old bodywork to it.

I don't think Spitfires with original engines (or even modified engines) blow up rear ends very frequently. Maybe the solution is just to manage the motor's torque output?
 

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There's apparently a Subaru diff conversion that you can do, but I think that's limited to the earlier marks.
A Google search for "Spitfire Subaru diff conversion" produces quite a few interesting projects, and even some commercial products.

Themes that I see among these:
  • most relieve the axle shafts of their suspension function, but some appear to add parts to retain bearings at the diff outputs to allow continued use of the axles as suspension links
  • conversions using the Subaru diff typically keep the axles as suspension, applying forces to it for which it is presumably not designed; there have been commercially-produced kits for this
  • suspension changes generally include using coil-over shocks mounted to the stock shock mounts, and upper control arms mounted in place of the transverse leaf
  • production suspensions (from other vehicles) are not used, because they don't fit
  • some custom suspensions are mechanically unsound, but most make reasonable sense
The "Subaru" final drive unit (diff) is typically the Hitachi R160 or similar, also used by Datsun for the 510 and Z-cars (yes, it's that old).
 

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Example of a gent who converted a Spit to use a Miata rear end...
I recently stumbled across the discussion (in another forum) in which this was originally posted:
Forums » Grassroots Motorsports » Triumph GT6

As the builder explains in that discussion, there was a significant amount of fabrication involved, and the geometry still isn't right... and would be substantially more work to fix.

I understand the logic, but no modern suspension is likely to fit in the rear without cutting out the trunk floor, the floor behind the seats, or both. Triumph's swing axle and transverse leaf suspension design is obsolete, but brilliantly compact, occupying only a narrow tunnel across the car, plus some space very low ahead of the axle for trailing rods.

The custom Miata NA/NB-based suspension shown earlier doesn't look like it fits the floor: the floor's drop into the trunk would likely interfere with the upper control arms, unless they have an unreasonably narrow base.
It turns out that the upper arms - which are stock Mazda arms cut apart and re-joined to make them shorter - do fit. Because of all the custom fabrication (not using any of the Miata suspension arms in their original form, and not using the subframe), it does apparently all go under the body: the builder says that "All this fits under a Spitfire body with minimal hammering."
 
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