MoT brake rollers and Torsen diffs

[motco]

I am a rare visitor to these hallowed pages as a Z3 owner, but this should be of interest to some on here. If the mods decide it is better in another section, please feel free to move it, but I hope it provides some useful information.

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

 

[mmm-five]

I've had plenty of MOTs in the UK over the years and have all used rollers to check brake balance & effectiveness.

I'd like to think that the small amount of time it's on the rollers is unlikely to cause any damage to an LSD, and my Z4MC has over 150k on it...although it has very recently had its 3rd diff fitted (first was a warranty claim as it was clunking, 2nd was accident damage).

 

[motco]

That's a 'hard' LSD and really shouldn't be rollered apparently. :?

 

[BMWZ4MC]

I've had plenty of MOTs in the UK over the years and have all used rollers to check brake balance & effectiveness.

I'd like to think that the small amount of time it's on the rollers is unlikely to cause any damage to an LSD, and my Z4MC has over 150k on it...although it has very recently had its 3rd diff fitted (first was a warranty claim as it was clunking, 2nd was accident damage).

My ///M has half the mileage of yours Tony, but it's still on its first diff. It's never been on the rollers at the MOT test centre. Coincidence? Who knows

OP, I don't think any of the Z4 models left the factory with an LSD of any kind with the exception of the ///M. An interesting read nonetheless

 

[-banned-]

None of my m cars even had brake test on roller, it's not good for lsd.

 

[mmm-five]

My ///M has half the mileage of yours Tony, but it's still on its first diff. It's never been on the rollers at the MOT test centre. Coincidence? Who knows

My first diff was at 80k miles...and I didn't think there was anything wrong at the time, but the conversations on here around the same time got me asking questions everytime I selected reverse and head a clunk.

Had it in for an inspection at a BMW dealer (still under Mondial warranty), and asked the dealer to have a look. The called back and said they'd asked BMW for a warranty replacement...so I didn't argue.

My 2nd diff was fine until my pre-Christmas 2015 accident...and the sideways impact knocked the driveshaft into the diff - proven by the damage to the driveshaft, and the fact that the diff oil was changed a week before and now had swarf in there.

 

[Beedub]

I've had plenty of MOTs in the UK over the years and have all used rollers to check brake balance & effectiveness.

I'd like to think that the small amount of time it's on the rollers is unlikely to cause any damage to an LSD, and my Z4MC has over 150k on it...although it has very recently had its 3rd diff fitted (first was a warranty claim as it was clunking, 2nd was accident damage).

Same...

for the tiny amount of time its tested, its not going to overheat or hurt the ///M variable diff..... doing a donut or track work is putting it under far more strain.