Because I have not been able to find a definitive answer to the question of whether a Torsen differential is safe on brake rollers or not, I emailed the manufacturer for advice. Here's my question and their reply:
Good Morning,
I have a 2002 BMW Z3 2.2i Sport, one of the last off the Spartanburg line, and it is fitted with a Torsen differential as standard. Here in UK the annual safety checks on all vehicles more than three years old (MOT test) employs a roller brake tester which drives each wheel in turn against the vehicle brakes to assess effectiveness and balance. Conventionally 4x4 and LSD equipped vehicles are not tested in this way in order to avoid damage to differentials. Is the Torsen type of differential compatible with this kind of test or should it also be tested in a 'safe' manner like a so-called hard LSD?
Thank you,
Regards,
motco
Motco,
The Torsen differential is similar to other types of LSD in that it generates internal friction in order to resist wheel spin. The reason that the “safe” test exists is that if the dyno forced differentiation at load conditions that the LSD normally prevents. Doing this can burn up the friction surfaces, usually clutch plates.
However, the Torsen does not use clutch plates, and the amount to friction that its gearing creates is proportional to the amount of drive torque is applied to it. This allows the Torsen design a little more flexibility under these conditions. So, if the test applies a load with a roller to the tire, while the other tire is free to spin the opposite direction, it should not cause too much concern. This assume, however, that the torque it applies is relatively low, with a differential rate less than, maybe 100 RPM difference from side to side, and the other tire provides no resistance to differentiation. This is somewhat comparable to making use of mini-spare tire, which Torsen allows but most LSDs do not.
However, if you have any concern about the test being more severe than described, I would suggest defaulting to the “safe” test. The Torsen-type design allows greater tolerance of these types of situations, but there are limits to it.
Regards,
Rick Barnes
JTEKT Torsen North America, Inc.
Rochester, NY
www.torsen.com
585-464-5028
My reply:
Hi Rick,
Many thanks for your timely reply. I have a safety test booked for tomorrow and your advice is clear - avoid the rollers if in the slightest doubt. The tester drives one wheel at a time for the parking brake test and as far as I know the other roller is held stationary so there will be an infinite difference in speeds and high torque applied in order for the brake to resist it. I value my Z too much to take that risk, and I shall pass your advice on to both the test centre and to UK members of the Z community.
Again, many thanks, I am sincerely grateful for your taking the time to reply.
Best regards,
motco
Edited to add that not only did it pass with no advisories, but the tester agreed; no rollers for any LSDs.