How to test Limited Slip Differential

#1

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

#2

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.

#3

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

#4

Ranger wrote:

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.

#5

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.

#6

Steve D wrote:

[quote]Ranger wrote:

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?

[/quote]

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.

#7

turbo329is wrote:

[quote]
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.[/quote]

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%.

#8

Ranger wrote:

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).

#9

jtower wrote:

[quote]Ranger wrote:

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).[/quote]

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.

#10

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

I know I have.

#11

Ranger wrote:

Eight engines and not one 30 mm socket?

If you have several 17’s, you should put one on each lug nut then fabricate an adapter plate with four bolts to engage each socket. Then connect the torque wrench to a square hole in the middle of the adapter, then… :laugh:

#12

IndyJim wrote:

[quote]I think a lot of people have been saying, “Man I wish we had more engineering humor on these forums.”

I know I have.[/quote]

a proton walks into a bar and says, “i’d like a beer”. the bartender says, “are you positive?”

#13

I’m a helicopter mechanic. Everthing I torque has to be signed off by technical inspector. We have to torque a lot of stuff with dogbones and crowsfeet. If you put it at a right angle you are not increasing the length of your torque arm. So all you have to do is put the tq wrench on a nut that is directly on the top or bottom and use the tq wrench right at the three oclock.

#14

turbo329is wrote:

I’m trying to visualize what you’re describing. Which ain’t easy because you’re leaving out info. Like “right angle” to what?

As long as the cross product, which is to say, the lever arm, is the same, you can move the application of “moment” wherever you want. The units are ftlbs, but what we are really talking about is the application of a “moment”. Hence “moment arm”.

I’m not sure that I buy your point tho. A torque wrench at 12 o’clock on the top lug nut is going to have a different moment arm relative to the wheel’s hub, then if you applied the horizontal torque wrench to a lug nut at 3 o’clock.

Do this thought experiment. In the case of the 12 o’clock lug nut and the 3 o’clock lugnut, put the horiz torque wrench on each first on one lug nut and then on the other. Is the distance from your hand to the wheel hub the same in both cases? No, it’s not. Therefore the lever arms are different.

I think it’s off by 4%, which is 2" X 1.414.

So if you put a horiz torque wrench on the 12 o’clock lugnut, add 14% to the #.

Time for a beer.

Oh, hey Jim (Robinson). I noticed that no one said any kind words about your post. I lol’d.

#15

I solve problems engineers create…

Why do mathematicians always confuse Halloween and Christmas?
Because 31 Oct = 25 Dec.

#16

IndyJim wrote:

You are being kind calling this engineering humor. This is engineering masturbation.

#17

I was hoping to derail the thread before one of these guys hurt themselves trying to figure out how get the letter ‘mu’ or ‘theta’ out of a standard keyboard.

#18

this requires no math.

[||||]===========o
…oOo
…o

this does.

…o
[||||]===========oOo
…o

Its crude but do you get what I’m saying.

#19

bonus points for typewriter art.

this thread is great.

#20

Ranger wrote:

[quote]turbo329is wrote:

[quote]
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.[/quote]

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%.[/quote]

turbo is right, applying vertical force to a horizontal wrench on a lug at the 1200 position is identical to applying it at the hub. two equal and opposite moments applied to a rigid body cancel each other out regardless of location.