Diff bolt problems

[Winter.M3]

Any advice for broken diff bolts? My diff snapped in half about a year ago, I installed a new diff and bolts and had no problems for a while. The shop that did it said my subframe was bent. A month after my diff broke, a bolt snapped from the diff. I already had a new subframe and went ahead and had a shop install the subframe with akg black poly bushings, and had them replace the bolts of course (the subframe didn't turn out to be bent either...). I should mention that I've only used OEM at this point. A month after this I installed a new exhaust and the bolt broke AGAIN... I replaced it and have had no problems for 2-3 months, until today. My car is in for an exhaust leak and they told me the bolt was broken. They're recommending grade 8 bolts. Are the bolts MUCH better? Will this be the last time I have a problem with them? And why am I having so many problems with the bolt?

I should note I don't beat the car. Normal street driving. Occasional slides and I used to drift it, max 20 minutes of drifting. Nothing crazy. The first time the diff snapped I had TERRIBLE diff wobble, driving like an idiot in the rain. After that I haven't done any insane launches, I never dump the clutch. I do have 285s on and on normal "roll" launches it will have a slight wobble)

I haven't seen anything about people having problems with this bolt. But every few months I feel the knock of the diff on my subframe and it seems like it's extremely common to me.

[amrazM]

if theres a problem a stronger bolt will just make the diff break. if youre going to be sliding the car you need a solid metal bushing, you cant have any deflection in the assembly or youll have issues.

[Redd]

It does not sound like you have done your solid subframe bushings. The diff bolts snap because the subframe deflects under load. Try solid subframe bushings first. Plus there is no NVH penalty to solid subframe bushings.

[Winter.M3]

Originally Posted by amrazM

if theres a problem a stronger bolt will just make the diff break. if youre going to be sliding the car you need a solid metal bushing, you cant have any deflection in the assembly or youll have issues.

That makes sense, but what is causing SO much stress on the bolt that would cause something about the diff / subframe to diff bushing connection to break? I have AKG black poly bushings, just below red which is as close as you can get to solid metal. I didn't go with solid metal because I heard it can crack your subframe. And I'm not going to be needing ALL a metal bushing can get, I'm no pro track driver.

[Winter.M3]

Originally Posted by Redd

It does not sound like you have done your solid subframe bushings. The diff bolts snap because the subframe deflects under load. Try solid subframe bushings first. Plus there is no NVH penalty to solid subframe bushings.

I'd have to pay another 1-2k to get new bushings installed which isn't something I can just do to test. I went with Poly because I read that solid metal bushings can crack your subframe, and that you really only need them for hard track. Besides, if people can drive with OEM bushings and have no problems, my bushings should have next to zero problems I'd hope, no?

[DrFerry]

Originally Posted by Winter.M3

That makes sense, but what is causing SO much stress on the bolt that would cause something about the diff / subframe to diff bushing connection to break? I have AKG black poly bushings, just below red which is as close as you can get to solid metal. I didn't go with solid metal because I heard it can crack your subframe. And I'm not going to be needing ALL a metal bushing can get, I'm no pro track driver.

I agree with Redd when he mentions above, "It does not sound like you have done your solid subframe bushings. The diff bolts snap because the subframe deflects under load. Try solid subframe bushings first."

You mentioned in your opening post:

Originally Posted by Winter.M3

I should note I don't beat the car. Normal street driving. Occasional slides and I used to drift it, max 20 minutes of drifting. Nothing crazy. The first time the diff snapped I had TERRIBLE diff wobble, driving like an idiot in the rain. After that I haven't done any insane launches, I never dump the clutch. I do have 285s on and on normal "roll" launches it will have a slight wobble.

The forces and stresses sent through the diff - when drifting - are also sent through the diff bolt and bushings into the subframe BUT if the subframe is moving around due to potentially worn OEM rubber bushings, the diff bolt will sometimes snap. A nylon or metal diff bushing will still transmit the same or more forces right back into the subframe and its bushings. Hence, solid subframe bushings are beneficial for track use, hard street use and drifting. However, another mitigation approach is to install a diff brace. Linked below:

Various threads exist on this topic:
.
https://www.m3post.com/forums/showthread.php?t=1814900
.
https://www.m3post.com/forums/showthread.php?t=1903759
.
https://www.m3post.com/forums/showthread.php?t=1388619
.
https://www.m3post.com/forums/showthread.php?t=952902
.
https://www.m3post.com/forums/showthread.php?t=666109
.

Diff Brace Thread:
https://www.m3post.com/forums/showthread.php?t=1795104
.
https://www.m3post.com/forums/showthread.php?t=1083365
.

[Redd]

Yeah OP, what you've been changing are the diff bushings.

I'm referring to the rear subframe bushings. The stock rubber bushes allow the subframe to move a lot under load. Because the diff is mounted to the subframe on one end, and the driveshaft on the other, when the subframe deflects, the diff does not move together with the subframe. This causes huge shear forces on the diff mount points on the subframe end - ie the diff bolts.

Installing solid rear subframe bushes greatly reduces/eliminates this subframe movement. It also greatly improves chassis stiffness, improves handling, reduces wheel hop (good for drag launches) and introduces no significant noise or harshness. It's one of those rare mods with zero downsides (aside from the cost to do it).

[North.E92]

Also broke my diff bolts drifting I have yet to do my bushings tho. You’re probably not letting out when you get wheel hop. The second you get wheel hop let out don’t power through it.

[Winter.M3]

Originally Posted by Redd

Yeah OP, what you've been changing are the diff bushings.

I've had the diff bolt break. 4 times. I replaced my bushings myself, and had a shop install the subframe because I don't own a lift.

Originally Posted by Redd

I'm referring to the rear subframe bushings. The stock rubber bushes allow the subframe to move a lot under load. Because the diff is mounted to the subframe on one end, and the driveshaft on the other, when the subframe deflects, the diff does not move together with the subframe. This causes huge shear forces on the diff mount points on the subframe end - ie the diff bolts.

Installing solid rear subframe bushes greatly reduces/eliminates this subframe movement. It also greatly improves chassis stiffness, improves handling, reduces wheel hop (good for drag launches) and introduces no significant noise or harshness. It's one of those rare mods with zero downsides (aside from the cost to do it).

I understand how the bushings work, as I researched and purchased AKG BLACK poly bushings. 2 steps below sold metal; because it's not a racecar, and seldom a drift/track car.

I'm confused as why it's such a big problem because I've already upgraded the bushings. Regardless, OEM bushings shouldn't cause a huge problem like I've had, my OEM ones were in decent condition.

First picture is NEW bushings installed. Snapped bolt.
Second picture is OEM bushing snapped bolt; just before I upgraded bushings.
Third picture is ME replacing the subframe bushings.
Fourth picture is my diff sheared completely from the mount.

I don't have a picture of it but there was one other case of my diff bolt breaking and it being fixed on OEM bushings.

[Winter.M3]

Originally Posted by North.E92

Also broke my diff bolts drifting I have yet to do my bushings tho. You’re probably not letting out when you get wheel hop. The second you get wheel hop let out don’t power through it.

I'm sure that's all it is. I BARELY let it wheel hop, and I don't let it hop period anymore. Very infrequently I'd do a rolling launch; at like ~5mph, and full throttling there would make it barely hop. No more launches I'm sure when drifting/tracking it won't put as much stress as there will be no hop. Taking corners and drifting shouldn't have have a terrible amount of vibration or harshness. Of course more than normal driving though.

I also have 285's on the rear, which is too much grip for the street.

[Winter.M3]

Originally Posted by DrFerry

I agree with Redd when he mentions above, "It does not sound like you have done your solid subframe bushings. The diff bolts snap because the subframe deflects under load. Try solid subframe bushings first."

You mentioned in your opening post:

The forces and stresses sent through the diff - when drifting - are also sent through the diff bolt and bushings into the subframe BUT if the subframe is moving around due to potentially worn OEM rubber bushings, the diff bolt will sometimes snap. A nylon or metal diff bushing will still transmit the same or more forces right back into the subframe and its bushings. Hence, solid subframe bushings are beneficial for track use, hard street use and drifting. However, another mitigation approach is to install a diff brace. Linked below:

Various threads exist on this topic:
.
https://www.m3post.com/forums/showthread.php?t=1814900
.
https://www.m3post.com/forums/showthread.php?t=1903759
.
https://www.m3post.com/forums/showthread.php?t=1388619
.
https://www.m3post.com/forums/showthread.php?t=952902
.
https://www.m3post.com/forums/showthread.php?t=666109
.

Diff Brace Thread:
https://www.m3post.com/forums/showthread.php?t=1795104
.
https://www.m3post.com/forums/showthread.php?t=1083365
.

Referring to my original post. I said I have Akg black poly bushings installed. All of the linked posted have "solved" the problem. They all just tell the OP's with diff bolt problems to get diff bushings. I've done that. Magically it solves the problem for everyone. Everyone says it almost COMPLETELY removes wheel hop; which is not the case for me. After doing bushings the wheel hop is maybe 15% better. Now I didn't get Solid bushings like some; however, I don't want to risk the chance of my subframe cracking like people say.

I might have to look into that diff brace though. Wasn't aware it was something available. Not sure about the price to install though.

[DrFerry]

Originally Posted by Winter.M3

Referring to my original post. I said I have Akg black poly bushings installed. All of the linked posted have "solved" the problem. They all just tell the OP's with diff bolt problems to get diff bushings. I've done that. Magically it solves the problem for everyone. Everyone says it almost COMPLETELY removes wheel hop; which is not the case for me. After doing bushings the wheel hop is maybe 15% better. Now I didn't get Solid bushings like some; however, I don't want to risk the chance of my subframe cracking like people say.

I might have to look into that diff brace though. Wasn't aware it was something available. Not sure about the price to install though.

Sorry, I thought the Akg black poly bushings information - in your original post - was in reference to your diff bushings. With regard to wheel hop mitigation; a couple of things come to mind.

1) If you have EDC, then set the shocks in the softest setting to aid rearward weight transfer during hard acceleration. This helps the car squat better off the line.

2) Reduce your cold inflation pressures by 3 to 4psi to increase the size of the contact patch and aid longitudinal grip.

If the diff bushings and subframe bushings are now nylon, things 'should be' more buttoned down. However, worn shocks can exacerbate wheel hop which is just 'axle tramp' on an independent suspension. From an alignment perspective; are you running a lot of negative camber?

[Winter.M3]

Originally Posted by DrFerry

Sorry, I thought the Akg black poly bushings information - in your original post - was in reference to your diff bushings. With regard to wheel hop mitigation; a couple of things come to mind.

1) If you have EDC, then set the shocks in the softest setting to aid rearward weight transfer during hard acceleration. This helps the car squat better off the line.

2) Reduce your cold inflation pressures by 3 to 4psi to increase the size of the contact patch and aid longitudinal grip.

If the diff bushings and subframe bushings are now nylon, things 'should be' more buttoned down. However, worn shocks can exacerbate wheel hop which is just 'axle tramp' on an independent suspension. From an alignment perspective; are you running a lot of negative camber?

They are my diff bushings, 95A Diff and subframe bushings. Sorry if I didn't mention it I thought it was implied for some reason. (I assume anyone getting subframe bushings also gets the same diff bushings).

Dropping my PSI is a good idea, I'll give that a shot. Unfortunately it doesn't have EDC, but once I get coils I'll definitely figure out the right setup for a good launch; but also a ideal setup to do well on the track.

Thanks for the tips and help with this

[North.E92]

Originally Posted by DrFerry

Sorry, I thought the Akg black poly bushings information - in your original post - was in reference to your diff bushings. With regard to wheel hop mitigation; a couple of things come to mind.

1) If you have EDC, then set the shocks in the softest setting to aid rearward weight transfer during hard acceleration. This helps the car squat better off the line.

2) Reduce your cold inflation pressures by 3 to 4psi to increase the size of the contact patch and aid longitudinal grip.

If the diff bushings and subframe bushings are now nylon, things 'should be' more buttoned down. However, worn shocks can exacerbate wheel hop which is just 'axle tramp' on an independent suspension. From an alignment perspective; are you running a lot of negative camber?

Will more or less negative camber help with this? I have -3 front and rear

[DrFerry]

Originally Posted by North.E92

Will more or less negative camber help with this? I have -3 front and rear

Less negative camber will help a lot. Zero degrees camber allows for a reasonably square uniform contact patch shape which is optimal for longitudinal grip (acceleration and braking).

Whereas negative camber causes a conical shaped contact patch since the tire is leaning on the inside shoulder and is more optimal for lateral grip (cornering).

[pbonsalb]

Wheel hop is deadly. Dump the clutch and you will likely get it regardless of bushings. Unweighting and weighting the drivetrain shocks it too much. Feather/slip clutch to get a smooth launch. And do the things that help traction like not too much tire pressure, not too stiff shocks, not too much negative rear camber.