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Can anyone comment on the Differential Break-Away test repeatability and validity of the method? I am not trying to be a pain but we recently checked a few cars “Hot” and “Cold” and had tons of variation from Rebuilt diffs, and junkyard diffs. I suspect that the method itself is to blame. I believe some cars had dragging brake components, old wheel bearings, ect ect leading to results that were unrepeatable and artificially HIGH.
Thoughts? Experiences?
Thank you, Nate.
[quote=“TOOLEAN” post=58624]I believe some cars had dragging brake components, old wheel bearings, ect ect leading to results that were unrepeatable and artificially HIGH. [/quote]In order to have brakes dragging enough to affect the test, I’d have to believe you would see badly discolored or cracked rotors. If the bearings drag that much, don’t worry. They will fail soon anyway.
I’m not a big believer in the repeatability of the test, but it is much better than a tear-down (because it might actually happen). I haven’t found a whole lot of variance hot to cold but I haven’t done much scientifically. I am well below the limit but the diff seems to control wheelspin to my satisfaction.
Thanks for the input Steve.
I found hot to cold variation to be 4-5 ft-lbs. Many cars tested were well ABOVE the limit.
Has the method actually been tested by tech stewards?
[quote=“TOOLEAN” post=58630]Has the method actually been tested by tech stewards?[/quote]In that tech is almost always dictated by the series directors, yes.
The last time we did it I got a little freaked out because we couldn’t get the diff to break away. Then we took the car out of gear and tested well below the limit.
I’ve only had one diff rebuilt so I don’t know what the variance is but my understanding is that the limit was developed by testing a number of competitive cars with relatively fresh diffs.
Neutral helps alot 
Yeah I think rebuilt diffs may test lower than expected, seems backwards but we had one freshly built diff that was the lowest of them all.
I am curious about your assumption that the origin of the rule came from measuring a few cars, that makes sense but can anyone confirm? Again, from the testing I have seen, I question the validity of the spec and method.
[quote=“TOOLEAN” post=58634] Again, from the testing I have seen, I question the validity of the spec and method.[/quote]Let’s not raise too much hell about the spec. We have a spec; it beats the crap out of no spec - even if it isn’t the most unimpeachable, peer-reviewed, double-blind data that produced it.
Any spec is better than no spec. With no spec, you are left with having to tear down diffs, inspect them for non-factory-shop-manual modifications and tolerances, ruin the racer’s Sunday if you tech on Saturday…
The method is the worst. Except for all the other options.
I am with you, we need to have a spec, but I believe that many stock diffs would NOT pass. Frustrating.
Perhaps whoever is rebuilding said diffs is the problem versus the diff break away test? :whistle:
I’m with Steve D on this. My used diff tested no where near the limit yet I don’t detect any inside wheel spin…
How else could it be checked other than measuring the ramp angles? If you are using a craftsman torque wrench i suggest you get it calibrated or get a better one. I went through 4 within the 6 month waranty. 2 and 3 were dead on arrival.
How else could this be tested - This is a good question, but I was just trying to see if the general population was seeing the same results as I have seen; Stock diffs can measure out of spec, results are not repeatable.
You’d need to measure ramp angles, clutch disk thickness, and check for shims. In other words a full tear down of the differential.
The breakaway torque test is useful to weed out any obviously non-conforming differentials.
I would think ramp angle would be more important to overall performance than break away. How can you check that when it’s all assembled?
I guess I would have to agree and disagree to this statement. Yes, it is a very useful test, No it does not weed out obviously non-conforming diffs. I don’t know how else to state my point; Stock diffs can measure out of spec, period. I don’t know what the root cause is, I have not tested enough cars to recommend a better number, just trying hear others results/comparisons.
Thanks, Nate.
Quote from Nate"I have not tested enough cars to recommend a better number, just trying hear others results/comparisons."
Thanks, Nate.[/quote]
Answer to Nate: test enough cars.
While the group that makes the rules do not know all there is about engine rebuilds and horsepower numbers (we are slow learners), we have tested a good number of differentials.Hot versus cold numbers do make a difference.
Perhaps the rule was written as much to dissuade as it was to start a big DQ process.?!
RP