If I mated a single electric motor to a Ford 8.8" differential, there are dozens of ratio options available to me, ranging from 2.26 to 5.14...
When I first read this, I thought "9-inch", because that was the classic hot rod component:
That's perhaps the best thing about the Ford 9" axle: it is very common and was used in many vehicles, so many ratios (and differential types) are available.
Fortunately, the 8.8" has also become very common in both beam-axle and independent suspension versions, so my comment still applies.
I was switching between threads and lost track of who was building what, so I didn't notice this was a Ford 8.8" proposed for the Civic conversion to rear-drive. I have four concerns with that:
- The 8.8" might be heavier than desired because it is stronger than needed. These things are used in light trucks, mid-sized to intermediate cars, and high-power Mustangs. If you know an accommodating auto salvage operation, it might be a good idea to spend some time there with a scale and tape measure, comparing hardware.
- The pinion shaft may be so long that it will force the motor quite far forward, making more of a packaging problem than necessary; this is one reason a transverse motor setup (like essentially every production EV) is desirable.
- Like every normal ring-and-pinion set in a final drive separate from a longitudinal engine, this is a hypoid gearset - that means a few percent more friction loss than spur gears or a non-hypoid bevel gear set. That directly translates into less power to the wheels and shorter range.
- The hypoid gearing also means that the input shaft is lower than the axle shafts, which is desirable to keep the input shaft low under the car in a conventional configuration, but would place a motor directly coupled to it very close to the ground. At the very least, I assume that it would need to be tilted nose-up a few degrees to raise the motor.
I'm using direct drive to a Subaru diff
When the Toyota/Subaru 86/BRZ/FR-S came out, and I assumed that it had the same final drive unit, I asked some of my friends about its durability - after all, it usually handles only the rear-drive portion of an AWD Subaru's output. They said that people still building Datsun 510s (and there are apparently still many of them) can't get the Datsun units any more, and are adapting Subaru bits and putting lots of power through them successfully.
I now realize that the Datsun/Nissan and Subaru final drive units are from the same series: R160 (160 mm or 6.3" ring gear) for Datsun 510 and old Subarus, larger (R180 - 180 mm or 7" and R200 - 200 mm or 7.9") for newer larger Datsuns, newer Nissans, and more recent Subarus.
As it turns out, the 86/BRZ/FR-S doesn't use a Subaru (or Nissan) diff; it has a Toyota (Lexus) unit. From another forum
(in a post which also lists the ratios available):
The differential is not an R200, it is a Toyota unit. In the past referred to as the G-series (8.0") it is now referred to as the F20SX/F20TX/FD20A/FD20AT (205mm) unit. The same basic differential was used in many Toyota cars and trucks, notably the 86-92 Supra, 01-05 IS300 (with the same Torsen T2 LSD that the Zx6 gets), and the 06-15 IS250. The older models usually have a different outer case (pumpkin), however the internals are still near identical. The newer models like the IS300/IS250 have the same case and internals as the Zx6 in the 86/FR-S/BRZ.
Anyway, no one was asking about hardware from a 86/BRZ/FR-S, but it does introduce another option. This Toyota alternative is an "8-inch"; it may not be any lighter or more compact than the Ford 8.8".
The common Subaru unit looks very long, which is fine in vehicles with the motor located in the front of the vehicle, but bad for this case.
The other final drive unit for light RWD vehicles which should be readily available is from Mazda, used in the past for the RX-7 and RX-8, but now only for the MX-5 (Miata); it has ratios up to 5.38:1 available aftermarket
, and presumably there is some choice (not as extreme) in OEM ratios. There are also the many final drives for the rear of AWD vehicles, in addition to Subaru; however, these often have bulky and unwanted hardware on the input (clutches to control whether or not the rear drives). Any of these might be lighter and/or more compact than a Ford 8.8", but might also offer little choice of ratio.