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Rear motor and differential

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Langutah

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Discussion starter · #1 ·
I live in snow country and average 225" of snowfall per year. I will immediately remove the all-season tires and replace them with snows once I buy my Limited Hybrid.
My question is regarding the rear electric motor for the AWD-I system. I have only seen a relatively vague conceptual sketch of the rear end drive system in the Toyota web site marketing info. Does anyone know if the rear motor which apparently resides about where a rear differential would be located can drive either or both rear wheels? If it is hard mounted/connected to both rear wheels it has to have a differential to accommodate different wheel speeds around turns I believe. Has anyone seen any engineering descriptions or drawings for the rear drive?
 
#2 ·
I can't answer your question but there was a recent post on how it behaved in a couple of different situations needing all-wheel drive. It was in the last day or two so it should be easy to find. With that level of annual snowfall, you may be better off with a "real" 4-wheel drive vehicle. Good luck.
 
Discussion starter · #3 ·
Rear motor and differential discussion.

lmacmil said:
I can't answer your question but there was a recent post on how it behaved in a couple of different situations needing all-wheel drive. It was in the last day or two so it should be easy to find. With that level of annual snowfall, you may be better off with a "real" 4-wheel drive vehicle. Good luck.
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Thank lmacmil, yes I saw the discussion earlier. I am looking for more actual detail and facts - being an engineer type. I've lived in snow country for 30 years and have a second truck - a 4Runner - and have had 20 vehicles here over the years. I've even powered through with a one wheel drive Honda Fit. As long as it has snow tires we can get out (after running the tractor down the driveway). We had to park the Fit for a few days of the winter and get the real vehicle out but it was amazing how well you can handle snow, with practice.

The RAV4 will be more than fine. I even talked the dealer into letting me drive it home and test my driveway (the crux) during an 8" snowstorm. It passed.
 
#6 ·
I had the Rav4 Hybrid on the lift the other day and I can shed some light on the subject.

The rear differential is hard-mounted to the sub-frame and has CV jointed drive axles going to each rear wheel hub. The motor drives the differential carrier through (from what I've read) a 10:1 reduction gear and spins the "open differential" applying torque to both wheels equally until one of them loses grip. The TCS system applies any or all of the 4 disc brakes and modulates the pressure to help prevent runaway wheel-spin but the rear system appears much less capable than the front.

I tried it out today in a New England heavy, wet, slippery-as-snot snowstorm. I live on a big hill and have lots of other hilly territory around here so when I went out for coffee I gave it a real workout.

Using regular 'D' seems fine for plowed roads. Switching to 'S' 4, 3, 2 etc. seems to give you more control and traction without the ABS system aggressively limiting your stopping power. Turning off TCS didn't seem to help much as you lose all directional control when the front wheels spin. It didn't seem to be possible to spin the rear tires more than a revolution or two before they succumbed to the built-in power reduction forced by the AWD-i system. So if you're really suck, you may need a tow-truck if comparing performance to a conventional 4WD system.

Overall, though, I was pleasantly surprised by how well Toyota's engineers got the system working. In normal dry or wet conditions - use 'D'. In severe conditions switch to the appropriate 'S' gear (virtual gear of course) like S4, S3, S2 or even S1 for starting on hills. It appears the engineers set up the system to work very well in the 'S' modes but to be overly aggressive in application of torque limiting in the 'D' mode (which is to be expected I guess for best anti-skid capability).
 
Discussion starter · #7 ·
Excellent info

ST-Bob said:
I had the Rav4 Hybrid on the lift the other day and I can shed some light on the subject.

The rear differential is hard-mounted to the sub-frame and has CV jointed drive axles going to each rear wheel hub. The motor drives the differential carrier through (from what I've read) a 10:1 reduction gear and spins the "open differential" applying torque to both wheels equally until one of them loses grip. The TCS system applies any or all of the 4 disc brakes and modulates the pressure to help prevent runaway wheel-spin but the rear system appears much less capable than the front....
Click to expand...
Excellent info @ST-Bob - just what I was looking for. I think my short list is now down to this vehicle and plan on ordering one end of the month.

That was a great set of tests and test results. It very much simulates my 500' long snow-packed uphill driveway. I live at 6600' and my driveway is the crux of my commute down to SLC. When I tested the hybrid last month in a raging snowstorm with 3" of fresh on top of the hardpacked snow it motored right up in D with no apparent wheel spin. I disabled the traction control with the same result. I motored up it quite quickly, as is my habit, to maintain momentum to ensure navigating a 180 degree turn midway up at the steepest spot.

Do you remember where you read the information you about the differential and its gearing?

Thanks!
 
Discussion starter · #8 ·
ST-Bob said:
The motor drives the differential carrier through (from what I've read) a 10:1 reduction gear and spins the "open differential" applying torque to both wheels equally until one of them loses grip.
Click to expand...
@ST-Bob the motor's shaft is then configured to be in-line to the vehicle's long axis and the differential functions also as a right-angled gear reducer then, correct? Comparable to a driveshaft coming down the the vehicle's axis from a front mounted ICE?
 
#9 ·
Langutah said:
Excellent info @ST-BobDo you remember where you read the information you about the differential and its gearing?
Click to expand...
Nope - not off the top of my head. But it appears the motor is mounted parallel with the output shaft so the differential carrier probably has a helical gear which engages a matching one on the motor with no change in direction like conventional pinion-gear driven differentials. From the relative size of the gearcase around the motor's gear and the driven gear I'd say 10:1 is quite probable. This would keep the motor within its centrifugal limits at the maximum speed the vehicle is capable of.

From my observation the rear motor provided dynamic braking at all speeds I've experienced so it's got to be in constant mesh with the rear axles.

Too bad they didn't include a limited slip differential in the package...
 
#10 ·
Langutah said:
@ST-Bob the motor's shaft is then configured to be in-line to the vehicle's long axis and the differential functions also as a right-angled gear reducer then, correct? Comparable to a driveshaft coming down the the vehicle's axis from a front mounted ICE?
Click to expand...
No, the motor is transversely mounted.
 
Discussion starter · #11 ·
A helical or worm gear rear differential. Interesting. I've only seen ring and pinion rear diffs before. I have had limited slip, open and locking differentials. They probably didn't design in a LSD to reduce costs bowing to the incorrect assumption that nobody would bother taking this unibody vehicle out in deep snow or off-road conditions.

I will be gleefully taking mine out in both :) I don't buy a vehicle that I can't drive to work here in snow country or take off roading into the Utah Red Rock desert when my sweetie has her 4Runner otherwise engaged. I treated my Audi TT and our Honda Fit the same way successfully - always with a backup plan like walking out or shoveling - with no ill effect. Well, except that one flash flood... but the 4WD Tundra was not even up to that task... A different story.
 
#12 · (Edited)
Having owned a couple of conventional 4WD vehicles I can truthfully say the Rav4 Hybrid's AWD-i system cannot compare to conventional 4WD or AWD systems with 50/50 lock capabilities. The MOST the hybrid's power split can provide would be about 65% front to 35% rear (194 net HP - 67 HP [50 kW] rear motor leaves 127 HP from the front and 67 HP from the rear). That's ideal conditions with a fully charged traction battery.

From my experience, the AWD-i system is a poor choice for extended use in snow country. But, keep in mind, it's only snowed once since I picked up my Rav4 Hybrid last week and I've only got 1100 miles on it... So take it with a healthy dose of scepticism and do another test-drive in snowy conditions before deciding if the hybrid is for you.
 
#13 ·
Langutah said:
A helical or worm gear rear differential. Interesting. I've only seen ring and pinion rear diffs before. I have had limited slip, open and locking differentials. They probably didn't design in a LSD to reduce costs bowing to the incorrect assumption that nobody would bother taking this unibody vehicle out in deep snow or off-road conditions.

I will be gleefully taking mine out in both :) I don't buy a vehicle that I can't drive to work here in snow country or take off roading into the Utah Red Rock desert when my sweetie has her 4Runner otherwise engaged. I treated my Audi TT and our Honda Fit the same way successfully - always with a backup plan like walking out or shoveling - with no ill effect. Well, except that one flash flood... but the 4WD Tundra was not even up to that task... A different story.
Click to expand...
When I say "Helical" I mean the ring hear is not cut on a 90 degree angle to its axis (straight cut gears) but with an angle across the face to provide more constant mesh and less gear train noise. Maybe I'm using the wrong term - I'm no mechanical engineer. Conventional ring and pinion gears are hypoid gears, right? Since both gears are aligned on the same axis (transverse) they can either be straight cut or helically cut (to reduce noise).

The configuration is that the differential spider gears and output gears are enclosed in a carrier which has a ring gear on its outer circumference. This gear has angled gear teeth cut into it which engage matching angled teeth on the electric motor's output shaft which is aligned with the rear axles (transversely mounted). The electric motor is forward and slightly above the centerline on the right side of the vehicle. The motor is quite small compared to the rest of the differential gearcase.

I think we may be miscommunicating the differential/motor/gear train terms. I'm an electronics engineer, not a mechanical engineer so it may just be lack of knowledge of the correct terms. Without taking the differential apart I can only guess what's inside but my description above seems to fit what I see when looking at it from underneath the Rav4 Hybrid.
 
Discussion starter · #14 ·
ST-Bob said:
... So take it with a healthy does of scepticism and do another test-drive in snowy conditions before deciding if the hybrid is for you.
Click to expand...
And that is exactly why I did it, @ST-Bob. :)

Looks like you missed my earlier post:
"When I tested the hybrid last month in a raging snowstorm with 3" of fresh on top of the hardpacked snow of my 500' driveway it motored right up in D with no apparent wheel spin. I disabled the traction control with the same result. I motored up it quite quickly, as is my habit, to maintain momentum to ensure navigating a 180 degree turn midway up at the steepest spot."

No matter what it will do better than one of my other car replacement shortlist second place finishers - the new generation Mazda Miata. :)
 
Discussion starter · #15 ·
ST-Bob said:
I think we may be miscommunicating the differential/motor/gear train terms. I'm an electronics engineer, not a mechanical engineer so it may just be lack of knowledge of the correct terms. Without taking the differential apart I can only guess what's inside but my description above seems to fit what I see when looking at it from underneath the Rav4 Hybrid.
Click to expand...
Well, thanks to you I have the info I needed to support my decision to buy my first hybrid for this snow country. Thanks much.

But I am enjoying the dialog so let me noodle on what you've said and get back later.
~Art, a mechanical engineer with an automotive background
 
#16 ·
It would be really cool if we could get an off-the-shelf limited slip to replace the open diff in the rear of the Rav4 Hybrid. I don't want to be the first one to take mine apart though... If the pumpkin has the same mounting system as a conventional ring-gear on a hypoid drive-line then it should, theoretically, be possible to swap the pumpkin with one from a similarly sized Toyota truck that's already got limited slip.

But my guess is that the electric rear transaxle unit is unique and may not be easily upgraded to limited-slip. You want to try it on yours, Art?

Bob
 
Discussion starter · #17 ·
Yes I do Bob! If I could buy a second one to be able to handle it being up on jack stands for two weeks while I fumbled around trying to get that dropped circlip out of the sewer drain where it rolled and descended while disassembling and the subsequent two weeks trying to figure out how it goes back together after disassembling it I'd be totally game :)

I tell you what, you figure out how we can hack the AWD-i system to tune it for off-roading (user selectable for full time/full power rear motor operation) and I will take the case cover off and photograph the layout. You in?

To that end, has anyone here sprung for or seen the factory service manual yet?
 
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