MoT rollers and Torsen diffs

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motco
Joined: Tue 18 Aug, 2009 19:12
Posts: 635

  Z3 roadster 2.2i

MoT rollers and Torsen diffs

Post by motco » Wed 06 Sep, 2017 08:40

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. :)

peter2b
Joined: Sat 01 Nov, 2014 18:47
Posts: 933

  Z3 roadster 2.2i
Location: cheshire

Re: MoT rollers and Torsen diffs

Post by peter2b » Wed 06 Sep, 2017 21:45

I also have a 2.2 sport z3 but 2001 (Y reg) would my car also have the same torso dif ? for the last 3 mot's they put her on a roller to check the brakes
peterz3

gookah
Z Register member
Joined: Thu 07 Aug, 2008 10:51
Posts: 2722

  Z3 roadster 2.8

Re: MoT rollers and Torsen diffs

Post by gookah » Wed 06 Sep, 2017 22:44

I have never heard of anyone having problems with the Z3 diffs, and each of these cars have now had at least 12 MOTs.
Why worry?
Image

Z3 2.8 Progress Journal (Mine)
Z3 1.9 Sport Progress Journal (Wifey's)

I have an element of 'M-styling' on my car, If that's a good enough reason for the manufacturers to adorn a 320 with the M badge, then its certainly a good enough reason for me..

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motco
Joined: Tue 18 Aug, 2009 19:12
Posts: 635

  Z3 roadster 2.2i

Re: MoT rollers and Torsen diffs

Post by motco » Thu 07 Sep, 2017 10:25

Messenger mortally wounded! I claim no deep knowledge, only the sage words of the manufacturer. Personally, I don't risk it even though mine was probably tested four or five times before I acquired it.
Only Sport models and ///M cars were fitted with an LSD unless specified at purchase.

gookah
Z Register member
Joined: Thu 07 Aug, 2008 10:51
Posts: 2722

  Z3 roadster 2.8

Re: MoT rollers and Torsen diffs

Post by gookah » Thu 07 Sep, 2017 16:24

I think you are alone in not risking it.
Everyone else just has a normal MOT.
Image

Z3 2.8 Progress Journal (Mine)
Z3 1.9 Sport Progress Journal (Wifey's)

I have an element of 'M-styling' on my car, If that's a good enough reason for the manufacturers to adorn a 320 with the M badge, then its certainly a good enough reason for me..

peter2b
Joined: Sat 01 Nov, 2014 18:47
Posts: 933

  Z3 roadster 2.2i
Location: cheshire

Re: MoT rollers and Torsen diffs

Post by peter2b » Thu 07 Sep, 2017 21:56

my z3 is a sport //M but when I checked it hasn't got a torson dif, but she does have a nice few added good bits
peterz3

roadvoyager1
Joined: Sun 19 Jan, 2014 18:30
Posts: 65

  Z3 roadster 2.8

Re: MoT rollers and Torsen diffs

Post by roadvoyager1 » Fri 08 Sep, 2017 20:07

I believe LSD was standard on all 2.8ltr cars in the UK.
My MOT tester has never put my car on the rollers because it has the LSD.

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lightning
Joined: Tue 27 Nov, 2007 08:15
Posts: 814

  Z3 roadster 3.0i
Location: Stockport

Re: MoT rollers and Torsen diffs

Post by lightning » Sun 17 Sep, 2017 18:10

I asked this question at my local BMW dealer a couple of years ago, and was given the reply that "a single wheel roller rig may be safely used for speeds of up to 5mph"
as the speed differential between the two wheels is not high enough to cause an issue.

l would guess it's a lot less stress on the drivetrain than doing a burnout or dropping the clutch from standstill with the traction control turned off....and the car has to be designed to survive that kind of abuse.

Also with the traction control turned off you can get wheelspin which will surely cause a much greater speed differential in the driven wheels than putting it on a brake roller.

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motco
Joined: Tue 18 Aug, 2009 19:12
Posts: 635

  Z3 roadster 2.2i

Re: MoT rollers and Torsen diffs

Post by motco » Mon 18 Sep, 2017 12:02

lightning wrote:
Sun 17 Sep, 2017 18:10
I asked this question at my local BMW dealer a couple of years ago, and was given the reply that "a single wheel roller rig may be safely used for speeds of up to 5mph"
as the speed differential between the two wheels is not high enough to cause an issue.

l would guess it's a lot less stress on the drivetrain than doing a burnout or dropping the clutch from standstill with the traction control turned off....and the car has to be designed to survive that kind of abuse.

Also with the traction control turned off you can get wheelspin which will surely cause a much greater speed differential in the driven wheels than putting it on a brake roller.
Re your last paragraph, unfettered wheelspin in which the two driven wheels differ significantly in rotational speed is exactly what an LSD is intended to prevent.
I t was precisely because of this wide spectrum of opinion about the wisdom of brake testing on rollers that I decided to consult the manufacturers, and their advice was to avoid them if in the slightest doubt. I had/have doubts so I avoid them. You may have other views and that's okay of course. :)

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lightning
Joined: Tue 27 Nov, 2007 08:15
Posts: 814

  Z3 roadster 3.0i
Location: Stockport

Re: MoT rollers and Torsen diffs

Post by lightning » Thu 21 Sep, 2017 08:27

The LSD will reduce wheelspin but won't prevent it altogether.
l was told by BMW that low speed roller rigs are not a problem.
But you can ask the MOT garage to avoid using it if you are still worried, with my Ferrari they road tested it instead as it was too low to go on the rollers.

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motco
Joined: Tue 18 Aug, 2009 19:12
Posts: 635

  Z3 roadster 2.2i

Re: MoT rollers and Torsen diffs

Post by motco » Fri 22 Sep, 2017 14:04

I'd be surprised if your Ferrari didn't have a 'hard' LSD anyway, unless it's a new enough one to have an electronic LSD by way of the traction control system.

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