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Old 03-15-2016, 10:55 PM   #1 (permalink)
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Direct-to-diff conversion possibility?

Hey folks. The car I'm looking to convert has a 4.44 ratio diff in the rear, and weighs 1500kg. I would prefer an AC conversion. Would an AC-50 motor handle that kind of weight? I'm not looking for massive acceleration as such. I'm also open to using a second motor to drive the front wheels :P

Just trying to firm up my plans here.

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Old 03-16-2016, 03:30 AM   #2 (permalink)
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No response in 24 hours. You might try adding a little detail.
  • 1500kg is 3300lb. Before or after conversion?
  • what are the torque and power ratings of the AC-50?
  • Direst -to-diff? No transmission? No clutch?
  • front-engine rear-drive or what?
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Old 03-16-2016, 10:25 PM   #3 (permalink)
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Sorry, yeah. Last minute posts before bed are not so good hah.

The car is a Subaru SVX, factory curb weight of 1,590 kg (3,510 lb) according to wikipedia.

The AC-50 is rated at 120 lb-ft and 71 hp (peak power at about 3,500rpm according to EV West's chart, which according to http://www.angelfire.com/fl/procrastination/rear.html will be at 58 miles per hour, which is fast enough for me!)

By direct to diff I mean no gearbox, yes. I've seen it before once with an Impreza, and they had two motors. The first to the diff between the rear axels, and the second to another diff mounted where the gearbox used to be. They also had a good budget, though!

Ultimately I want to keep AWD, but the gearbox is automatic and I think it would be better to get rid of that dead weight (subaru gearboxes are stupidly large) hence the "direct-to-diff" thing. I suppose that means it would be a rear wheel drive car initially at least. I'm just not sure if such a motor could handle it. It is a fairly heavy car, after all. Though for the price I'd certainly hope so! A second motor would definitely mean good performance but would also mean two controllers (which would be good for Paul). My aim is to have "close to stock" performance, and at least 15km range - which should be easy with lithium cells. This is not a quick weekend project of course so I'd rather do it right.


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Old 03-17-2016, 01:44 AM   #4 (permalink)
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SVX are awesome. I looked at one for sale a while back. My favorite Subaru alongside the 2nd generation BRAT. What color? How long have you had it?

I had a look at https://www.google.com/search?q=Subaru+SVX+cutaway. So, two motors back-to-back in the middle facing the ends? That might could work.

I'd look at the overall ratios of various single-speed motors.
Tesla -- 9.37:1
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Toyota MGR -- 6.86:1
GNK eAxle -- 12.5:1
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Old 03-17-2016, 11:28 AM   #5 (permalink)
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I don't own it yet! But this is the one I'm looking to pick up. So damn pretty.

Yeah I'm hoping to make use of the driveshaft tunnel, or the gearbox space with the driveshaft heading aft. Probably best to do that, since there are already strong mounts there. Not sure yet if the forward motor would be behind the axle or in front of it, yet. Depends on what fits best!
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Old 03-17-2016, 02:06 PM   #6 (permalink)
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I dunno... For direct drive, I'd say that AC-50 would be good for a lightweight (say ~800kg) car, but for a 1500kg car it'll be pretty anemic without a gearbox, especially at takeoff. Assuming your ratio puts the hp peak at 60mph, you're looking at what, 6hp at 5 mph?

If you had a 5-speed in there, you could be seeing around 40-50hp at that speed, much more fun .
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Old 03-17-2016, 09:28 PM   #7 (permalink)
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Look at the orange line. The 'knee' I was speaking of is at ~3600rpm.

Torque for acceleration. Horsepower for top speed.
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Old 03-18-2016, 12:39 AM   #8 (permalink)
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Interesting. If you plan on driving around 58mph, then at that gear ratio you'll have the most wheel torque possible at that speed. The problem is that you'll have the same amount of wheel torque clear from start. If you had a 2:1 ratio before the differential, or an 8.88:1 diff, then you'd have about 2/3 the total wheel torque at that speed but at lower speeds you'd have up to double the wheel torque.

Another "option" would be to use the smallest rims and tires possible. That would effectively increase the force exerted on the pavement by the wheel. Just be aware that your brakes would be more touchy if you did that.
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Old 03-18-2016, 10:44 PM   #9 (permalink)
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Doing the math, you have about 110ft-lbs of torque up to 3500RPM. That would be 488.4ft-lbs of wheel torque. With 225/50 16 tires you get 573.8lbs of force on the pavement. That minus about 130N of rolling resistance equals to a little over 3.5miles per hour per second. But as you speed up air drag takes over until 48mph when your air drag will slow you down to about 3.3mph per second, assuming that you have a .29cd and an area of about 2.32m^2. At that speed acceleration will slow you down considerably from loss of motor torque since I calculated 3500 RPM to be about 48mph with your gear ratio and the tires I mentioned above. Your maximum grade you could climb theoretically would be less than a 14% grade, which isn't that bad, but you wouldn't be able to accelerate on it.
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Old 03-19-2016, 03:24 PM   #10 (permalink)
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Hrm. Thanks for that! I do have a fairly steep hill to climb on my short commute. How do those numbers go with the 2:1 gearbox before the diff? Alternatively having the two motors should mean roughly twice the performance. (and twice the expense!)

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