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      12-07-2011, 03:17 PM   #1
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Busting half-shafts and LSD

No, it hasn't happened to me. I'm just curious for those that have broken half shafts, what are the causes? Normally I see it from 1/4 racing with DR. Has it happened to anyone with an LSD too? Does Running a LSD lessen the chance of snapping a half shaft? Does it depend on what type of LSD you run?

I'm curious. I'm confident the next time I run the 1/4, I'll hit 11's and 120+. I just want to know my chances of having a repair bill.
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      12-07-2011, 03:31 PM   #2
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Having an LSD could theoretically reduce the stress on your halfshafts. But that doesn't mean you won't break them. Worst case scenario get the DSS ones.
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      12-07-2011, 03:52 PM   #3
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I am just assuming, since you brought this into discussion: with a clutch-type LSD, since the torque is split immediately after the driveshaft receives torque from the engine, there is less of a chance for breakage at launch compared to a Torsen (torque-sensing, geartype) LSD which begins to work a few moments later, when the driveshaft begins to rotate and the torque difference between wheels is "sensed".
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      12-07-2011, 04:07 PM   #4
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I broke a half shaft with no LSD and hoosiers at the drag strip. I was running 11.9's and I have continued to run it at the track after fixing it without any problems. It broke when i was staging. I started my burnout and the track official stopped me because they spotted oil on the track from the previous car. After they mopped it up they told me to go ahead with my burn out, the tires were already somewhat warm from when I started the first time and it immediately broke my half shaft. Just make sure that you only heat up slicks one time and that is it for that run. Hotrod runs most of the fastest times and I believe that he doesn't have an LSD either so it should not be a problem.

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It cost me $130 for a used half shaft.
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      12-07-2011, 04:12 PM   #5
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Any diff splits torque biasing the wheel with less traction (higher % of available torque). If one wheel can only use X amount of torque the other gets Total torque available X. LSDs limit the amount of torque difference.

When traction is present both LSD and open diffs would distribute torque similarly. Its when traction is reduced that they perform differently and these situations are not such a concern for hardware. So I believe halfshafts would see similar max torque no matter the diff.

I guess if one wheel could utilize >50% of the max engine torque then it could be a concern, but typically this would be low traction and available torque reduced (reduced load). This is probably a wheel hop situation.

FYI: Im a little fuzzy on diffs though.
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      12-07-2011, 04:14 PM   #6
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Good stuff... I suspect most broken half shafts are without LSD but wanted people to confirm or reject this idea.

I think if I were to break them, I'd probably do oem again. Don't know if I want to spend $1k on half shafts.

Any more thoughts on the subject?
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      11-13-2012, 05:28 PM   #7
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I snapped a half shaft today from a rolling start between 1st and 2nd gear with a WaveTrac LSD. The car is at FFTEC now getting looked at.
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      11-14-2012, 09:38 AM   #8
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It would seem to me that breaking half shafts would be more likely with an LSD.

With an open diff, the tire with less traction gets all of the torque. So the side with more traction is seeing none.

With an LSD, torque will be transferred to the side with more traction up to the bias ratio, so now the side with more traction is getting more torque than the side with less traction.

Obviously, more torque is more likely to snap an axle.

Lets imagine a scenario where one side sees 100 lbs of static friction force between the tire and pavement while the other side sees 200 lbs. Lets say for the sake of argument that the axle can withstand 150 ft. lbs. of torque. Assume a bias ratio of 2.5:1. Also assume an overall tire diameter of 2 feet.

The maximum amount of torque seen by the axle connected to the wheel with less traction is 100 lbs. * 1 ft. = 100 ft. lbs.

So with a bias ratio of 2.5:1, up to 250 ft. lbs can be sent to the side with more traction. However, the maximum amount of torque this wheel can take without slipping is 200 lbs * 1 foot = 200 ft. lbs.

Now for the relevant part. If this car had an open diff, only 100 ft. lbs. would be seen by both sides, causing one wheel to spin (1:1 bias ratio) and neither axle to break (100 < 150). Because this car has an LSD, 100 ft. lbs. is seen by the axle with less traction, but 200 ft. lbs. is seen by the axle with more traction, causing the axle to break (200 > 150).

Now if the car in question had an open diff, with the same axles, but one tire developed 200 lbs of friction force, and the other 250 lbs, the axle would still break because the torque seen by the axle with less traction, (200 ft. lbs.) is still greater than the torque the axle can handle (150 ft. lbs.).

But wait, you are saying, wouldn't both axles break?

Not likely. We are assuming both axles can take 150 ft. lbs. but in the real world, manufacturing tolerances lead to axles being able to handle slightly different torques. For instance if one axle could take 149.9 ft. lbs. and the other 150.1 ft lbs, the weaker axle would break before the stronger one.
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      03-11-2013, 11:47 AM   #9
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Quote:
Originally Posted by ajm8127 View Post
It would seem to me that breaking half shafts would be more likely with an LSD.

With an open diff, the tire with less traction gets all of the torque. So the side with more traction is seeing none.

With an LSD, torque will be transferred to the side with more traction up to the bias ratio, so now the side with more traction is getting more torque than the side with less traction.

Obviously, more torque is more likely to snap an axle.

Lets imagine a scenario where one side sees 100 lbs of static friction force between the tire and pavement while the other side sees 200 lbs. Lets say for the sake of argument that the axle can withstand 150 ft. lbs. of torque. Assume a bias ratio of 2.5:1. Also assume an overall tire diameter of 2 feet.

The maximum amount of torque seen by the axle connected to the wheel with less traction is 100 lbs. * 1 ft. = 100 ft. lbs.

So with a bias ratio of 2.5:1, up to 250 ft. lbs can be sent to the side with more traction. However, the maximum amount of torque this wheel can take without slipping is 200 lbs * 1 foot = 200 ft. lbs.

Now for the relevant part. If this car had an open diff, only 100 ft. lbs. would be seen by both sides, causing one wheel to spin (1:1 bias ratio) and neither axle to break (100 < 150). Because this car has an LSD, 100 ft. lbs. is seen by the axle with less traction, but 200 ft. lbs. is seen by the axle with more traction, causing the axle to break (200 > 150).

Now if the car in question had an open diff, with the same axles, but one tire developed 200 lbs of friction force, and the other 250 lbs, the axle would still break because the torque seen by the axle with less traction, (200 ft. lbs.) is still greater than the torque the axle can handle (150 ft. lbs.).

But wait, you are saying, wouldn't both axles break?

Not likely. We are assuming both axles can take 150 ft. lbs. but in the real world, manufacturing tolerances lead to axles being able to handle slightly different torques. For instance if one axle could take 149.9 ft. lbs. and the other 150.1 ft lbs, the weaker axle would break before the stronger one.

what an amazing answer
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      03-11-2013, 11:50 AM   #10
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Quote:
Originally Posted by ajm8127 View Post
It would seem to me that breaking half shafts would be more likely with an LSD.

With an open diff, the tire with less traction gets all of the torque. So the side with more traction is seeing none.

With an LSD, torque will be transferred to the side with more traction up to the bias ratio, so now the side with more traction is getting more torque than the side with less traction.

Obviously, more torque is more likely to snap an axle.

Lets imagine a scenario where one side sees 100 lbs of static friction force between the tire and pavement while the other side sees 200 lbs. Lets say for the sake of argument that the axle can withstand 150 ft. lbs. of torque. Assume a bias ratio of 2.5:1. Also assume an overall tire diameter of 2 feet.

The maximum amount of torque seen by the axle connected to the wheel with less traction is 100 lbs. * 1 ft. = 100 ft. lbs.

So with a bias ratio of 2.5:1, up to 250 ft. lbs can be sent to the side with more traction. However, the maximum amount of torque this wheel can take without slipping is 200 lbs * 1 foot = 200 ft. lbs.

Now for the relevant part. If this car had an open diff, only 100 ft. lbs. would be seen by both sides, causing one wheel to spin (1:1 bias ratio) and neither axle to break (100 < 150). Because this car has an LSD, 100 ft. lbs. is seen by the axle with less traction, but 200 ft. lbs. is seen by the axle with more traction, causing the axle to break (200 > 150).

Now if the car in question had an open diff, with the same axles, but one tire developed 200 lbs of friction force, and the other 250 lbs, the axle would still break because the torque seen by the axle with less traction, (200 ft. lbs.) is still greater than the torque the axle can handle (150 ft. lbs.).

But wait, you are saying, wouldn't both axles break?

Not likely. We are assuming both axles can take 150 ft. lbs. but in the real world, manufacturing tolerances lead to axles being able to handle slightly different torques. For instance if one axle could take 149.9 ft. lbs. and the other 150.1 ft lbs, the weaker axle would break before the stronger one.

Yes but what about people who break axles from wheel hop I dont think all axle breaking stories stem from actual power output. Great theory and logic though. Abusive driving habits, coupled with wheel hop instances would lead to a premature failure too.

So the question then is, does an LSD reduce wheel hop? Some people dont have it, my 335i did with open diff depending on the surface.
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      03-11-2013, 02:01 PM   #11
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That's funny this thread died in November after that mega post, which although it made sense, I think others were like

Jeff is right though, the axles most commonly break due to wheel hop. I have yet to see one break that didn't involve wheel hop.

The solution is to support the diff by anchoring it to the chassis better (i believe). So far, it seems to help!
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      03-11-2013, 04:22 PM   #12
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You should look into Defiv Lockdown kit to prevent any movement of the diff at launch. The rubber diff bushings flex that can break the half/drive shafts.
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      03-11-2013, 04:42 PM   #13
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Quote:
Originally Posted by n54door View Post
That's funny this thread died in November after that mega post, which although it made sense, I think others were like

Jeff is right though, the axles most commonly break due to wheel hop. I have yet to see one break that didn't involve wheel hop.

The solution is to support the diff by anchoring it to the chassis better (i believe). So far, it seems to help!


Quote:
Originally Posted by blisstik View Post
You should look into Defiv Lockdown kit to prevent any movement of the diff at launch. The rubber diff bushings flex that can break the half/drive shafts.
I agree. Although there seems to be an inherent flaw in the suspension geometry for wheel hop but a lockdown kit will definitely aid a lot.
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      03-11-2013, 05:46 PM   #14
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Well I plan on getting lsd with in the next few month should I invest some money into bushings too ? I would like to to get the lock down kit but 600 on top of the LSD might be a reach for the me now is there an alternative ? I was going to have VAC motor sports to do the install I wonder if they could fab one up for less .
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      03-11-2013, 06:49 PM   #15
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^ I would be interested to see if upgrading the bushings would help noticeably as well. Does anyone know?
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      03-12-2013, 12:39 AM   #16
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Turner sells Delrin diff bushings for our car. It's a metal bushing so it should help but I'm not sure if it includes bushing for the diff cover side.

I thought I saw someone selling theirs in the FS threads.

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Well I plan on getting lsd with in the next few month should I invest some money into bushings too ? I would like to to get the lock down kit but 600 on top of the LSD might be a reach for the me now is there an alternative ? I was going to have VAC motor sports to do the install I wonder if they could fab one up for less .
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      03-12-2013, 02:59 AM   #17
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Quote:
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Yes but what about people who break axles from wheel hop I dont think all axle breaking stories stem from actual power output. Great theory and logic though. Abusive driving habits, coupled with wheel hop instances would lead to a premature failure too.

So the question then is, does an LSD reduce wheel hop? Some people dont have it, my 335i did with open diff depending on the surface.
main reason I got a DEFIV lockdown differential kit...I was getting bad wheel hop.
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      03-12-2013, 03:41 AM   #18
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main reason I got a DEFIV lockdown differential kit...I was getting bad wheel hop.
At what speeds?
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      03-12-2013, 11:53 AM   #19
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Quote:
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main reason I got a DEFIV lockdown differential kit...I was getting bad wheel hop.
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At what speeds?
Probably 0-40
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      03-12-2013, 12:48 PM   #20
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Quote:
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Probably 0-40
Surprisingly more Jeff...I had wheel hop when doing a 40 punch up to 70-80....this could have been also due to the worn out differential/ subframe bushings also. I did the lockdown kit, subframe bushings, wavetrac all at once so I can't say which was the contributing factor. I have 0 wheel hop now!
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      03-12-2013, 03:11 PM   #21
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what does wheelhop feel like? i don't think ive experienced it
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      03-12-2013, 03:16 PM   #22
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Quote:
Originally Posted by robc1976 View Post
Surprisingly more Jeff...I had wheel hop when doing a 40 punch up to 70-80....this could have been also due to the worn out differential/ subframe bushings also. I did the lockdown kit, subframe bushings, wavetrac all at once so I can't say which was the contributing factor. I have 0 wheel hop now!
I am still on stock Subframe Bushings. I think as long as you get the differential locked into the actual subframe, it maintains the suspension geometry thus reducing the wheelhop to essentially nil.

I have zero wheelhop and the car track much straighter, so subframe bushing upgrade is not needed for the wheelhop situation.
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