# Help with Differential



## Duncan (Dec 8, 2008)

Hi Guys

I have started to build my chassis (Lotus Seven type but with Subaru bits and fork lift motor) and I have discovered that my Subaru diff is asymmetric.

It has the engine drive-line offset about 30mm to the drivers side (NZ) (we sit on the right)

The diff and drive shafts are on the center line, both wheel drive shafts are the same length 

I am thinking about inverting it, 
The rear diff cover with the mounts and breather is symmetrical so it can stay the correct way up,

This will move the drive-line (and the motor) over to the passengers side giving me more room and helping the balance when I am on my own.

This is important as I am using direct drive and keeping the motor behind the firewall so that I can use the engine bay for batteries

This means the motor is between my legs and my passengers

This will require me to run the motor the other way so I will have to re-advance the motor!!

Is there any reason why I should not do this??

Was this not a common mod on VW gearboxes to change from a rear engine to a mid engined layout??


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## TigerNut (Dec 18, 2009)

Flipping the differential housing upside down will make the rear axle run backwards. With an electric motor that would not be an obvious problem (i.e. you can just set it up to run clockwise) except that then you'll be driving the hypoid gear on the overrun (coasting) side, which it's not intended to do at high loads for extended periods of time.
Another thing is that the hypoid pinion gear sits below the rear axle centerline when it's properly installed, so that it and its bearings will be lubricated by the differential oil. If you flip the diff housing then the input shaft ends up above the centerline, and you will have a lubrication problem. 
Lastly, the engine and transmission are conventionally set on-center relative to the car, as is the differential assembly. The prop shaft intentionally runs at a slight side angle relative to the differential and transmission shafts. This is done to prevent vibration issues that can occur due to U-joint geometry when the transmission output shaft and the differential input shaft become exactly aligned. 

On the VW transaxles, you could indeed run the housing front-to-back, but then you had to flip the ring gear assembly over to the other side so the transmission output wouldn't run the axles backwards.


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## Duncan (Dec 8, 2008)

Hi TigerNut

Thanks for the reply,
I don't think the lubrication will be a problem - there will be plenty of splash!

The drive-line miss-alignment 
My propshaft is going to be about 300mm between joints
A tiny amount of misalignment will give enough angle to keep the shaft happy

Driving backwards
This will reverse the direction of the load on the input shaft, most of these are held in two taper rollers, should be OK - keeping fingers crossed

The gear to gear interface
This I am not so sure of - I will be driving backwards with a different clearance
(because it is moved to the other side of its endfloat)

If you take a beetle engine, turn it around so it is mid-engined - you then need to 
" flip" the ring gear assembly

Does this not reverse the gear to gear interface?
Does it cause any problems ??


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## Woodsmith (Jun 5, 2008)

The ring and pinion problem is particular to hypoid gearing.

In the VW it is spiral bevel and the ring and pinion are on the same axis so it is easy to flip the ring gear round. Flipping the ring gear make the drive shafts turn backwards but the whole transaxle is then turned backwards too it isn't an issue.
The pinion still drives on the same face of the ring gear teeth.

On a hypoid axle the ring and pinion axis are offset with the pinion usually lower. If you flip it over then it will drive backwards BUT you are not turning the axle backwards, you are reversing the drive to it to make the axle go forwards.
There is the problem.

The hypoid axle has a high thrust axial to both the ring and the pinion. this forces the ring away from the pinion while the pinion is trying to screw its way into the axle.
Reversing the drive causes the pinion to try to screw its way out of the axle loading the bearings the wrong way. Some pinion bearings have crush tubes and reverse loading can cause the crush tube to crush further removing the preload from the pinion bearings.

All that and the gear teeth are driving on the overrun side causing wear problems.

Hypoid ring and pinions are set up to allow some backlash in the correct drive direction, reverse drive changes the backlash due to the different bearing loading mentioned above and that causes the backlash to be higher then it should be increasing wear.


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## TigerNut (Dec 18, 2009)

Hi Duncan,

The hypoid gears need more than splash... it's one of the reasons that EP-rated fluids (extreme pressure) are required in the differential. You're really setting yourself up for a lot of serious problems if you mount the diff upside down.

One thing about orienting the diff properly is that the driveshaft ends up about four inches (10 cm) lower in the chassis. This should help a lot with the amount of clearance that you actually need. Because the Subaru diff is not part of the suspension, you won't need as much clearance as a live axle would have.

The offset driveshaft input is a feature of nearly all hypoid rear axles because it packages the ring gear and differential assembly better... chances are that the Seven plans (or Locost if that was your starting point) allowed for the required clearance to begin with.


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## Duncan (Dec 8, 2008)

Hi TigerNut, Woodsmith

Thanks for your feedback

The "plans" consist of laying the Subaru "subframes" and my motor on my build table and designing tubes around them to resist the loads.

The Locost plans are useful for seeing what somebody else has made work, I had origionally intended using more of them but I decided to use the subframes to make all of the alignment easier.

The vertical driveline offset does help as I am planning to run the drive shafts as horizontal as I can.

I am working on a flat bottom with the motor and batteries as low as I dare, this means the diff is raised to get input and output shafts horizontal

I disagree about the lubrication, the diff housing is about half full of oil, one of the gear surfaces is dipped into the oil before moving up to the gear interface,
This will give a full hydrodynamic film.

The EP requirement is not about getting the oil to the interface but about how you treat it when its there.

*With all that and comments about the gear interface and the bearings I have decided to put it back the right way up*, 

I will put the motor on the centerline and have a 6 degree horizontal offset on my drive-shaft

This does reduce the space available for my but, 
This may give me an incentive for a diet


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## Olaf-Lampe (Feb 24, 2010)

I never went too deep in the forklift motor data section, but I feel that you'll need a very high diff-ratio to be able to drive it directly. ( 6:1 ?)
Do you know, if the forklift motor bearings are able to withstand the huge load from the driveshaft?
We went the same way with our Se7en clone and used a motor with 220Nm driving a Ford Sierra diff with ~3.6:1 ratio. Acceleration 0-100kph is in the range of 6-7sec. ( 100kW Inverter )
We were also thinking of putting the motor in the back, with a chain-driven universal diff. Inverter and all the other stuff goes above the motor in the rear and you have plenty of room in the front to put your battery pack. ( Balancing the car with the battery pack is another option instead of inverting the Subaru diff )

Have fun with your Se7en!
Olaf
www.thorr.eu


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## Duncan (Dec 8, 2008)

Hi Olaf

I have an 11 inch motor - weighs 102 Kg 

I am hoping that it will give similar torque to a warp 11 (135 ftlbs - 188 Nm)

With a 4.1:1 diff and 14 inch wheels with cheap road tires I think I will be close to being able to break traction

What did your Se7en end up weighing?


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## Olaf-Lampe (Feb 24, 2010)

Duncan said:


> Hi Olaf
> 
> I have an 11 inch motor - weighs 102 Kg
> 
> ...


11 Inch is a bit larger than our motor, it will work for sure.
We stopped stuffing the frame with batteries at 700kg. There is room for more, but we'll wait to see how handling comes out.
The pics with batteries are from an early stage, where we figured out how much we can put where. There will be 36 in the trunk and 52 in the front ( stacked ) in aluminium-boxes. 
-Olaf


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## Duncan (Dec 8, 2008)

Hi Olaf
I am planning on using 48 Thunder-Sky 160 or 200 AH cells
I have arranged to have 36 "on the floor" in what would have been the engine bay and 12 "on the floor" behind the differential

Most of my mass (battery and motor) is "on the floor" and quite low 
With 120mm ground clearance most of my mass is between 120 and 400 mm from the ground

I expect my car to be uglier than yours as I am using Chapman Struts rear and McPherson Struts front
(Subaru Legacy)
This is because I am a cheap Scotsman and I already have the Subaru bits!

If my calculations about roll angles are correct I don't need the superior camber correction of a double wishbone because it will roll so little

I will find out when its finished (2011)!!


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## Olaf-Lampe (Feb 24, 2010)

You'll have a lot of energy onboard. 
We also discussed about stacking the batteries or not because COG. Usually a motorcycle engine or even a Ford ICE engine is used in these kitcars. At least we are somewhat even to these COGs.
The batteries in the back are the only weak point IMHO: you put them behind the diff and they have a longer 'arm' making it hard to catch the tail or your put them above the diff and lift COG...but anyway, you can't conquer a 6 speed 1300cc hayabusa powered kitcar with any electric version.
BTW: If you have to build your own frame around the Subaru parts, it's no big financial difference compared to our frames. The whole frame (60kg) with fiberglass panels can be bought from 6000€ upwards. There are some upgrade options of course 
A Ford Sierra donor car is almost for free.
-Olaf
Do you have a build blog, where I can read more about your se7en?


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## Duncan (Dec 8, 2008)

Hi Olaf,

Back in the UK or Europe Ford Sierra's are common,
Here not so much but there are thousands of Subarus!

I hope to start a build thread next week


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