Aug 15 2008


This happen to be much more challenging than I have anticipated. I was all over web for many weeks searching for suitable options, and, turn out, there aren't that many. Believe it or not, there was no one to ask who'd really know. Initially I posed a couple of questions on Audi forum. All I wanted to know is highest gear ratio stock Audi A4, A6 or A8 comes with. Well, people seems to be more concerned about engine chips, roaring mufflers, and debate if 5 spoke wheels look cooler than 6 spoke ones. Few had any clue what diff ratio their own cars have, apparently they don't care. No one had an idea about swapping differential options for Audi, or any aftermarket gears to fit, let alone knowing how much torque their diffs can handle. My search so far revealed that the highest ratio I can get in stock Audi diff is 4.38:1 - too tall for the motor shaft speed I'll be running at. I knew I'm going to get my differentials from different vehicle, the question was which one. My Ford 9" trial fit was to see how it can be positioned and whether low pinion in front will work. I was never big fan of that monster which is a way overkill for any normal vehicle anyway. It didn't fit on the rear. It *could* fit in front, but takes too much space. As far as other choices biggest limitation for me was available aftermarket gear ratio - I need something at least 5.5:1 or shorter, and also availability of reverse gear cut option. It's known fact that Ford differential is very common, everyone and their brother seem to have one, and aftermarket support is very good, but, well, I don't do what everyone does, so I'm not using it. Since everybody seem to modify vehicles for either rock crawling or street/drag racing, aftermarket support is geared toward such performance applications. "Normal" differentials, just with different gear ratios, is nearly impossible to find - no OEM will manufacture an aftermarket gear set for a regular car. So where do I get >5.7:1 ratio, independent suspension, straight and reverse cut gears small enough and yet strong enough diffs?? There is chance I can get away without reverse cut ones by flipping the diff up side down, but from what I read it is not the best option.

The only differentials more-less well supported by aftermarket industry I found were Toyota IFS for light trucks and SUVs like 4Runner, and Dana IRS used in Corvettes. But, Dana still didn't have short enough ratio, nor reverse cut gears. As soon as you try to do something little off beaten path, all "experts" turn out to know little to nothing, they don't even pretend to know. Simple question to a couple of differential rebuild shops about how much torque a particular diff can take revealed that even owners have little clue. They can't understand what do I build, how rear diff can work in front and in general can't comprehend why do I ask about Toyota truck diff if I have and Audi Quattro Avant. I wondered what if I have to flip the diff up side down and will overfill it so that pinion ring is still submersed in oil - how is it different (as far as lubrication) from "normal" low pinion option. No clue. I asked if I can run differential "backwards" (on the coast side of the ring gear), and everyone repeats like parrots that it is not going to work, but no one can explain exactly why. Turn out, stock Toyota IFS' for all the light trucks and 4Runner SUVs of about '85 to '91 are set up that way, and apparently Toyota has no problem with it, so this is what I have decided to explore. The only suitable option I found was Toyota 7.5" and 8" IFS'. I settled for 7.5" unit, as it has 5.71 aftermarket relatively quiet Yukon gear option, and overall is very similar to Audi diff in size. This is very desirable detail since if I can maintain the same distance between flanges of the short "stub" axles sticking out of the diff as in stock Audi diff, I won't need to re-make half-shafts and can re-use stock CV joints (on the rear). The Audi CV joints will be bolted to the adapter rings on Toyota's flanges, that is the most elegant and slick solution. All I need is to make adapter rings such that Toyota flanges look like Audi ones as far as accepting Audi's CV joints directly. But for this to work, the width of Toyota diff + thickness of 2 adapter rings should not exceed width of stock Audi diff, and 7.5" Toyota IFS comply with this (I didn't check 8" one). Hopefully, Toyota diff will be at least as strong, but only real life tests will tell for sure. What destroys differential gears and bearings is torque, not power; in other words if I limit motor torque to a safe value (easily done in the software), I can spin gears very fast thus getting high HP without degrading reliability that much. The only problem is to find out this max torque number, and unfortunately it is not black and white distinction. Finding that value will likely cost me one broken differential, but this is fine with me. The key in this case is good lubrication. Once set up is built, I probably will gradually increase motor torque until differential fails (small pinion gear is weakest link here and will give up first), so I'll know how to set up replacement diff. Or, will opt for slightly larger 8" IFS diff. Now the next task is to adapt Audi half-shafts to the Toyota differentials. Here is the situation and what I did:

Comparison of Toyota 4Runner IFS differential (top) and Audi A6 rear differential ( bottom), top view.
 Same, side view.
Same, rear view. I can tell they are virtually identical in overall size.
Removing stub axle from the Audi diff.
Comparison of the Toyota diff stub axle and Audi one.
Same, view from the flange. Except for the flange shape, and the fact that Audi one is held by a bolt instead of C-clip, they are quite similar
Two Toyota IFS differentials, one shown without service cover.
shifter Note the extra thickness of the wall here (between yellow arrows). This is where longer tube of the stock IFS is entering the housing from the left (driver) side. There is no such step on the right side. Because the stock left short stub axle flange is very close to the housing, because of this extra thickness on the right side, same stub rubs against the housing wall. Not good.
shifter So about 6mm of the cast iron material was milled off to make it look like the right side. Now another stub can be inserted here making very standard "symmetrical" IFS (unlike offset to the passenger side Toyota's IFS).

After inserting both stubs into Toyota IFS the distance between their flanges was measured. Luckily, in the rear it is smaller than for stock by about 40mm, so there is room for spacer rings and as I mentioned I could retain stock half-shafts (rear). However, new problem surfaced: because Audi differential has very small offset of pinion shaft, suspension half-shafts are almost of equal length so that pinion shaft is centered in the drive tunnel. Toyota diff's pinion is offset more, so with stock half-shafts lengths is way off center. Situation with this is better if I flip the diff up side down, but then it is below the underbody level. I may end up re-making half-shafts to center the diff. More serious problem is in front: because Audi transmission is very wide, distance between flanges of the stubs coming from it is much greater than for Toyota diff. Accordingly, the half-shafts are shorter. I have no problem re-making them with increased length to accommodate narrow differential, however because the body rails above and below stock CV joint, they will interfere with half-shaft travel. I will have to evaluate the situation and might end up mimicking stock transmission width extending stubs straight out before mounting CV joints on them. Because stubs must be about 80mm longer now, each side might require support bearings. Here is the sketch illustrating the challenge:

Longer half-shafts will bump into the body rails if CV joints move inward too far (to directly link to Toyota's stubs).
Inserting stock stub axle into the housing. White compound is lithium grease.
Rubber mallet settles the stub in - it is distinctive move when retaining C-clip expands and locks the stub in the spider gear.
This is what the differential looks like inside.
Measurement of the distance between centered diff side (no stub shaft) in front and stock half-shaft flange.
Situation with the front diff, view from top. Both stub axles are installed.
Another photo of the front diff with high pinion option. If I flip it up side down, high pinion location results in enough clearance between suspension cross member and the   motor shaft coupler (which is about the same diameter as the pinion shaft flange).
Low pinion is stock position of the diff...
...but it's pinion is so close to the cross member that the shaft coupler will not clear it. Solution remains to be seen.
Examining situation with rear suspension. This is low pinion position (as for stock).

Photo from the side. Flanges are close enough and the diff fits very well.
Photo from the front. If I make distance between half-shafts flanges and stub axle's flanges equal, the problem become is apparent here - the pinion shaft is way off centered. Again, it is not such a big of a deal, just half-shafts will have to be changed for custom lengths to center the diff again, but it would be nice to avoid extra work and expense. Next step will be to see if swapping right and left half-shafts centers the diff enough .

Continued on January 23 2011

My advise to anyone who will undertake a  conversion project: put extra effort to find the vehicle as close mechanically to the final wanted version as possible. Reuse as many stock parts as you can find, and unless you build a custom chassis roadster or a race car or some sort and have access to the specialized shop doing this kind of work, don't modify driveline or suspension parts. The challenge I faced did not stop me from proceeding but boy, but I'm much more handy with anything concerning wires, circuits and electronic hardware in general than with shafts, splines or welding support brackets. I can see what has to be done but don't have right tools or skills, like broaching splines, CNC machining adapters or welding. It was a lot of labor and expense to accommodate non-stock transmission parts and make everything fit. I had no options but to use Toyota truck differential with aftermarket custom gear set because this Audi does not have (and Audi aftermarket does not make) diffs with high enough ratio to be able to use "direct drive"1and take full advantage of high speed 4 pole induction motors. Using stock differential would be the possibility with low speed high torque special multi-pole motor where unlike gearing,  torque multiplication and speed reduction is done electrically, but there are very options for such a motor working with available to me inverters compared to proven 4 pole conventional Siemens induction machines. Besides I can fine tune reduction ratio if I really wanted by choosing a gear set. With 12 pole motor I of course stuck with 12 poles, it is not changeable. Also, re-using Audi rear differential still require custom motor attachment and coupling and still,  this would solve only half of my problem since I have to install identical set up in front. Front drive is custom no matter what differential and motor I use there.

After researching the market for available common (not custom built racing) differentials with desired ratio, the choice was Toyota 1995 truck front differential. This unit has 5.7:1 ratio aftermarket gears manufactured by Yukon gears. So I ordered two remanufactured diffs with right gearing installed. They were painted, but the paint peeled off quickly revealing that the units are in rough shape from outside. Gears inside were replaced, however the wobble of stubs worries me as far as potential vibration at high speeds. Will see, since stock units do not have any additional support of the half-shaft (on one side). On the other hand, Audi diff's stubs have no wobble at all. So I'm reserving the possibility to install additional support bearings.

Below is a series of self-explanatory photos. Basically, I machined off the surface on one side to accommodate the second stub, cleaned up and painted the surface and re-machined the flange to accommodate custom flexible coupler. Here are major steps captured on camera:

Differentials as received. Original paint is off
A wire brush is the only means to get rid of the rust and get to the bare metal.
Dremel tool helps in small cavities.
Protecting parts from rust away spray. Masking tape is better choice for this than packing tape I had laying around. Next time...
First is protective coat preventing rust and serving as primer for the paint.
Both coated and uncoated diffs for comparison.
Close up view - this surface treatment will last for the life of the diff.
High temperature epoxy coating - 2 layers are applied
Peeling off protective tape. Aluminum rear cover does not rust and need no coating
The stubs are reinstalled on both sides (stock diff has it only on one side).
Input shaft flange had to be taken off to be re-machined to match the flex coupler between it and the motor shaft flange.
This is what the flange looks like.

1 It became customary to call "direct drive" arrangement where a motor shaft is linked directly to differential pinion gear shaft without gear box in between. Technically, "direct drive" means running drive wheel shaft directly by a motor shaft (hub motor or when a motor is installed transverse and its shaft is linked straight to a half-shaft with U-joints or CV joints) so the wheel and motor shaft speeds are equal. As soon as you have gears (or sprockets+chain or pulleys+belt) in between, differential included, it becomes single speed transmission with fixed reduction ratio, not "direct drive", even if the ratio is 1:1. By now the wrong term got spread and accepted, so as long as distinction is clear, I will use "direct drive" term here since it has become common understanding. There are many instances of incorrect terminology stuck and commonly accepted in people's minds (for example perhaps 99 out of 100 people will substitute weight for mass and express weight in kg or lb. (which is wrong since weight is a force, N, created by gravitational attraction, unlike mass, kg, which is measure of inertia). So as long as you speak in common even wrong terms with another ignorant fellow, you will be understood just fine ;-).