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How to test Limited Slip Differential
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TOPIC: How to test Limited Slip Differential

How to test Limited Slip Differential 4 years ago #48704

What is the basic test procedure for testing a LSD for effectiveness. Seems like there is a method using a torque wrench. Car seems to not be getting traction of the corners like it once did.

Thanks.

K

Re:How to test Limited Slip Differential 4 years ago #48706

  • kgobey
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The rules actually describe the technique that will be used at the event, and that might be the technique best used at home...

9.3.11.3. A welded ("locked" ) differential is no longer permitted.
9.3.11.4. Finned, larger capacity differential covers may be used.
9.3.11.5. Differential lubricant may be replaced.
9.3.11.6 If a limited slip differential is fitted, it must breakaway at 65lb-ft. or less. The lockup will be checked at Impound if a protest is filed. It will be tested at the center hub nut, using a 30mm socket and commercially available ½” torque wrench. The car will be placed on solid pavement, the right side of the car will be jacked up until the right rear tire is free from the pavement, the transmission will be placed in neutral, and the parking brake, if present, will be released. The torque wrench will be placed at the nine o’clock position with the socket on the center nut. The torque wrench will be set at 65 lb-ft. and pushed down. If the differential does not breakaway prior to the torque wrench “breaking away,” a second and similar style torque wrench from an impartial party shall be used in a second test, immediately following the first test. If the differential does not breakaway using the second torque wrench, the car will be disqualified from the preceding race and the DQ will be noted in the car’s logbook.
9.3.11.7 One hose may be attached to the factory differential vent pipe/fitting and must be connected to a catch tank.
Kieran
Midwest SE30 #19

Re:How to test Limited Slip Differential 4 years ago #48708

  • Ranger
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Easier way is to put the torque wrench on a lugnut. Lugnut, torque wrench and wheel center need to be in a straight line. Rotate the wheel a little to get that. Play around with the torque wrench such that you identify the ftlbs that the wheel turns at. Then add 10%. So if you get wheel spin at 45ftlbs at the lugnut, you can call it diff break at 50ftlbs.

The 10% is for the ~2" from the lugnut to the wheel center. Most torque wrenches are around 20" long from ratched center to handle center. 20/2= 10%. Given the accuracy of your average torque wrench, the lugnut method is pretty good.

And it needs to be the lugnut close to you, not the lugnut on that straight line that is on the far side of the hub.

O o-----------------------

Hub, lugnut, torque wrench
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Re:How to test Limited Slip Differential 4 years ago #48709

  • Steve D
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Ranger wrote:
Easier way is to put the torque wrench on a lugnut. Lugnut, torque wrench and wheel center need to be in a straight line.

That still doesn't put the torque wrench on a common rotating point with the hub. Why screw around with fuzzy, highly questionable math when the real reading is a 30 mm socket away?

My sense is that a non-rebuilt diff won't be close to the spec (i.e. it will break away sooner). If you are rebuilding your diff to snuggle right up against the limit of the rules, you will probably also take the effort to remove the wheel and test it like the rule says.
Steve DeVinney
Spec E30 #410 (sold)
Atlanta, Georgia
NASA Southeast Spec E30 Champion - 2011
NASA Southeast Spec Miata Champion - 2012

Re:How to test Limited Slip Differential 4 years ago #48710

  • turbo329is
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I didn't think about the fact that the wheel should be turned backwards. I put the wrench at 3 oclock and pushed down. I wonder how different it is.

Instead of doing any math when you torque something off center just put it at 90 degrees and it will be the same as torquing the center.
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Re:How to test Limited Slip Differential 4 years ago #48711

  • Ranger
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Steve D wrote:
Ranger wrote:
Easier way is to put the torque wrench on a lugnut. Lugnut, torque wrench and wheel center need to be in a straight line.

That still doesn't put the torque wrench on a common rotating point with the hub. Why screw around with fuzzy, highly questionable math when the real reading is a 30 mm socket away?



Because I don't own a 30mm socket. But I have several 17mm sockets.

The math is highly questionable? My HS physics is pretty good. I spent a year teaching it.
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Re:How to test Limited Slip Differential 4 years ago #48712

  • Ranger
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turbo329is wrote:

Instead of doing any math when you torque something off center just put it at 90 degrees and it will be the same as torquing the center.


The math accounts for the fact that my lugnut idea doesn't turn the wheel at the hub, but at a lugnut ~2" from the hub. Yes, the torque wrench needs to be on the same radius as the lugnut, but the angle of the torque wrench doesn't change the need to add 10%.
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Re:How to test Limited Slip Differential 4 years ago #48713

  • jtower
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Ranger wrote:
Because I don't own a 30mm socket. But I have several 17mm sockets.


having the force offset from the center screws things up. so what you need to do is get two torque wrenches and two 17mm sockets, place them on opposite lug nuts, set each to 25 ft-lb and then do the test. that way the force vector cancels out and you're left with just torque. even better would be to use four torque wrenches, one on each lug nut, but that requires two people and ranger has to work alone in his garage (unless his female neighbor drops by unannounced).
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Re:How to test Limited Slip Differential 4 years ago #48714

  • Ranger
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jtower wrote:
Ranger wrote:
Because I don't own a 30mm socket. But I have several 17mm sockets.


having the force offset from the center screws things up. so what you need to do is get two torque wrenches and two 17mm sockets, place them on opposite lug nuts, set each to 25 ft-lb and then do the test. that way the force vector cancels out and you're left with just torque. even better would be to use four torque wrenches, one on each lug nut, but that requires two people and ranger has to work alone in his garage (unless his female neighbor drops by unannounced).


Sigh. For those of you that are unaware...Jason is also a mechanical engineer. Although normal folks would read his post and think "WTF?", in ME circles everyone would be snickering at the fine jape.
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Re:How to test Limited Slip Differential 4 years ago #48715

  • IndyJim
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I think a lot of people have been saying, "Man I wish we had more engineering humor on these forums."

I know I have.
Jim Robinson

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