Planetary torsional differential for A3s by Tim McKinney (European Car – April 2003)
“Do I need one? Why?” These are usually the first questions asked about a limited-slip differential. “We’ve found through rallying on many different surfaces,” said Tim O’Neil, “that a limited-slip becomes important when you get up around 160 to 170 bhp. If you’re on a slippery surface, say 130bhp. If it’s really grippy, maybe 180bhp. It’s when you start to get a little wheel lift [as weight transfers] that a limited-slip becomes important. On a road course, an lsd can help keep that light wheel from locking up under heavy braking. And, of course under heavy acceleration in a corner an lsd will keep the inside wheel from spinning as the suspension unloads.”
If you drive your car hard, skip to the source box at the end of the story and order one of these differentials. If you want to know why, keep reading and I’ll do my best to explain.
When a car goes around a corner, the inside wheel travels a shorter distance than the outside wheel and thus spins more slowly. A solid axle won’t allow this, so long ago the differential was invented. The standard open differential has axles connected to two bevel-cut gears joined by two bevel-cut spider gears and allows the wheels to rotate at different speeds while staying mechanically connected, via the ring and pinion gears, to the transmission. This is good.
However, torque will always take the path of least resistance. Put one wheel on a patch of ice and the other on pavement, and we all know what happens (if you don’t live in the snowbelt, substitute sand, mud, wet leaves, etc.)— one wheel spins while the other just sits there and you go nowhere. Or, accelerate hard out of a corner, and as weight transfers to the outside, the inside wheel gets light and goes up in a cloud of smoke. This is bad.
Lock a differential (make a solid axle) and the traction problem goes away at the cost of the chunked tires, major mechanical stresses and an unpleasant ride. This works for race cars but is a poor solution for the street. Today’s World Rally Championship cars use electromagnet and/or hydraulic (operating at 600 psi) active differentials that react instantly to wheelslip. Technical details are closely guarded secrets, and the diffs are fabulously expensive. Other racers use clutch-pack limited slips, but the more limited the slip, the less a fwd car wants to turn in. A clutch-pack diff will work in the air or if an axle breaks but is still a little extreme for the street. Though it stops working without some load on both wheels (think back to a broken axle), a gear-driven torque-biasing limited-slip differential, invented in the 30’s, is a great compromise for high-performance street cars.
Gary Peloquin has been messing around with transmissions seemingly forever. He was a VW dealer service writer, shop foreman and, by 1984 when he left Volkswagen to start his own business, was the area VW unit repairman for those problem warranty transmissions. A dedicated air-cooled VW fan, Peloquin developed and patented his “mini-slip” differential for rwd VWs after deciding there must be something better than the locked differentials he was building for his drag-racing buddies.
A few years later, he started experimenting with water-cooled transmissions, modifying factory differentials and producing high-quality bolt kits to replace the problematic 020 rivets. Eventually Peloquin decided to produce his own version of a planetary torsional differential for water-cooled VWs. His line now covers 020, 02A, 02J and 02C (Passat 4WD) transmissions.
Peloquin differentials work like any other differential of this type. With any differential, power exits the transmission via the pinion gear, which turns the ring gear, which is attached to the differential and turns the whole unit. The axles, attached to the turning differential, turn the wheels. It’s what inside a planetary differential that’s interesting. The axles attach to a sun gear. Each sun gear turns five planetary (think solar system) gears that live in snug little pockets. Each of the planetary gears meshes with a counterpart from the other sun gear. The two sun gears are not directly connected, though a stack of opposed washers preloads the two gears to keep things operating smoothly. Going straight, none of these gears move relative to each other. hit the throttle, and the differential housing turns, taking he planetary gears in their pockets and the sun gears they mesh with along with it and turning the axles. As long as the resistance from each wheel is the same, the Peloquins diff acts like an open differential.
Go around a corner or have one wheel lose traction, and gears start to turn relative to each other within the spinning differential. When one wheel spins faster than the other, under load, the planetary gears of the wheel with traction try and turn the planetary gears–and the sun gear they are meshed with–of the slipping wheel and are forced out of against the wall of their pocket. As the forces increase, the helical cut of the planetary gears forces them further away from each other and against the ends of their pockets as well. This friction increases the resistance enough to limit the slip of the tire with less traction and keep power flowing to the tire with traction.
Remember, this type of differential only distributes torque under load and unequal resistance. Give it some thought, look at the photos (if you think it is hard to understand, try and explain it in two paragraphs!) and it will all make sense. If not, check out Ians-diff page in the www.gti-vr6.net library (under transmissions) for a lucid primer on various differential types. (Another link to Ian’s Diff Page is found at http://www.houseofthud.com/differentials.htm).
With all that friction, care must be taken with material and manufacturing choices. If the gears and housings were made of the same material, it would weld itself solid in no time. Peloquin uses 9310 aircraft-grade vacmelt steel for his gears and 4620 or 8620 aircraft-grade steel for his housings. The only way these dissimilar materials will weld together is to run the transmission without lube.
One of Peloquin’s first water-cooled products was a replacement set of splined bolts to replace the O20 rivets. HE uses an especially hardened set of bolts to cut splines in the diff housing for the ring gear bolts before the seven -step heat treatment process begins (the housing ends up with a Rockwell hardness of 60 to 65). Without this step, even his grade 12.9 Pelolok bolts run the risk of stripping their locking splines when pressed into places. It took 4 1/2 years to find a manufacturer for flanged serrated self-locking nuts included in the bolt kit, and Peloquin was never able to find the 8mm bolt he wanted to use to hold the differential halves together. He eventually settled on a 5/16-in. bolt for the job (after all, this diff is built in the USA). Peloquin is so confident in his differential, he offers a “no questions asked” warranty.
My recommendation would be to leave the installation to a pro, so one again I was back to see Justin at New German Performance. You’ll need to remove the transmission from the car to replace the differential. Details can be found in the Bentley manual (800/423-4595). But that’s the easy part. Once the tranny is out, it needs to be cleaned and disassembled. A special puller is required to remove firth gear, and you’ll need the factory service manual to go further. Once fifth is off, the case can be separated rather easily. The shift forks come off next. If you want to change the ring and pinion, now is the time. Just disassemble that stack of gears and synchros to get at the pinion shaft and reassemble with the new part. If you are not changing the pinion gear, the stack of gears will lift off as a unit. Remove the stock differential and drill out the rivets that hold the ring gear in place. Press in the aforementioned splined bolts and attach it (or the new ring gear) to the Peloquins diff and reassemble the transmission. Be sure and shim the differential correctly. Like I said, unless you know what you’re doing, this is a job best left to a pro.
The rather modest guys who assembled my transmission–who preferred to remain anonymous–were impressed with the appearance of the Peloquins differential. They were more impressed that the speedometer gear (fragile and difficult to remove from the old diff) and bearings were already properly installed and that the bolt kit, fifth -gear gasket and O-rings were included. With production tolerances held to plus/minus half a thousandth, my differential fit without requiring shims, but Peloquin strongly recommends checking pre-loads on bearings per factory procedures. Figure at least an hour’s worth of hair-pulling, and money, saved.
In real-world driving, the Peloquins differential is totally transparent until you need it and operates smoothly when you do. Unscientifically, I played on wet on-ramps and was surprised at what a difference the diff made. Instead of pushing off the outside of the corner, the car held the line as it accelerated. You’ll find you need to steer out of the corner , though, not just let the wheel unwind. I also pulled to the side of the road, put one wheel in the dirt and dumped the clutch. The result was a straight-ahead launch and a gratifying black stripe on the pavement! A Peloquins diff can’t change the amount of traction available, but it can let you use all that’s there.
I also tried a somewhat more controlled test. Tioga Motorsports Park was kind enough to let me visit. I performed a standing start into one of the roughly 120-ft-radius go-kart track turns immediately and stopped the watch as I crossed the finish line. I experimented with various lines and launches until I was nauseous from the tire smoke. Despite a 36F drop from 70 to 34F between tests, I was immediately 0.40 sec. faster in a 5-sec test–nearly 10% faster–and the car was much more controllable. Launches left two black stripes, and wheelspin in the turn was virtually eliminated.
“In rallying, you can throw a planetary differential-equipped car around, it will rotate very well, and the diff won’t add any understeer. It’s the best of both worlds,” said O’Neil. It’s hard not to agree.