Are aftermarket diff bushes really necessary?
 

I'm struggling to understand why the diff moving relative to the subframe is that bad.
It won't change the suspension geometry, and how much torque could possibly be lost deflecting bushes?

Will it help prevent axle tramp?

Help me understand.




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I guess there are ones not liking slack in bushings and wishing more direct power delivery to wheels and higher response, when modulating accel, instead of it loading first all joints and compressing bushings.
Problem is though, that half-measures of replacing to less compliant just one or few from all of them often reduce that play only partially, rather transfering that shock to other bushings, but replacing all of them may result to unacceptable overall NVH worsening, at least for daily driven car.
It shouldn't cost anything in performance/torque once that initial load/jump has happened. Just that initial bump, reducing a bit response. But "fixing" it will introduce a lot of NVH all the time. Should have no impact on suspension too. So choose your poison, but i'd probably think of reducing compliance in all of tranny/driveline/diff bushings more in some track only car, instead of daily driving one, where whining/rattling all the time transmission might annoy one quick. Or at most, changing to less compliant bushings only where NVH change is reasonable and gains in some other areas outweigh cons of NVH (eg. gearbox bushings to improve shifting).
I'd probably touch diff bushings only if for other purpose, such as changing out to diff riser ones on overlowered car, to reduce wear on CV joints. Not to get rid of that "jump" when putting in gear. Same for suspension bushings .. they do dampen small vibrations and reduce noise noticeably. If you don't need for specific purpose (eg. i needed powerflex eccentric bushings for extra camber, or some use pillowball camberplates for such), or if NVH impact is not that big of (for me eg. subframe mount bushing inserts, or steering rack lockdown), i'd leave stock bushings alone, not upgrade just for sake of upgrading.

It's just the feel of the thing. Even in my stock engine and stock tuned car I can feel it move around and it's annoying. I can't imagine what it feels like in a car with more power, but I can imagine it being annoying.

You can feel the diff move? Maybe I'm not experienced enough as a driver.

My car runs totally catless, UEL headers on E85, and a 4.67 diff. It makes a shit ton of torque compared to stock. I do hillclimbs with MCA Race Primes coilovers and soft compound Yokahama AO50s and I can't feel the diff moving. I can feel the rear end moving and I use it to rotate the car.

The diff moving can not affect the suspension geometry in any way so I don't know what people are feeling, but it's not the diff moving.

I'm not saying there is no issue, but so far no one can offer a valid explanation.

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everything is a compromise. subaru uses the bushings they use to minimize NVH. those that change the bushings to something stiffer are willing to deal with more NVH as a tradeoff for a slightly more rigid drivetrain, which presents a few less losses.

on the track? it's probably worth it. on the street? probably not.

I can say that with doing hillclimb, autocross, and being turbo, I needed to upgrade my subframe bushings and diff mount bushings. The subframe bushings by far made the biggest impression- the rear end stopped feeling like it was floating if I accelerated in a corner.

During hillclimb, I dont notice the rear being bouncy or anything nearly as much as on flat pavement, and I think it's because it's uphill honestly.

Your car may not need them, how many miles? What model year? I don't think it's fair to say that they MUST be done, it's like anything else; replace if necessary. I also got tires with a thicker sidewall because I had insane hopping if the tires were spinning from a launch.

TL;DR- Diff bushings helped stop my skipping/bouncing from hard launches. The car also responds more quickly to throttle input. My bushings were also cracked and dry rotting though..

It is a "feel" thing. Stiffer drivetrain bushings take windup out of the system. On/off throttle situations the car feel more direct.

Same with suspension bushings. Less deflection, more stable alignment settings through the suspension travel. That part is probably hard to feel for mere mortals. But less deflection makes it easier to feel what the car is doing.

And as NVH tolerance/user preferences/use types/priorities differ from person to person, always worth to rely less on advises from internet, and for all changed parts keep also stock ones and try to avoid irreversible mods. Not all mods/changes can be considered "upgrades" if person is different. Also worth keeping stock ones and not rush with partout for when in future one might sell it off and would wish to revert to stock to up price/broaden possible buyer list. For example - aftermarket exhausts. What might be too mild/quiet for someone, for another one might be too obnoxiously loud & tiring, or not acceptable by significant other/cops/neighbors, so obviously advises that it's ok/not too loud and so on "from internet" should be taken with grain of salt.

Thanks mate. Great info. I get awful axle tramp on launch. I'm happy to do mods that solve an identified problem, but most people say 'the feel is better', and knowing how prone modders are to confirmation bias, I'm always skeptical.


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Whiteline suspension has a video showing how much the stock subframe bushings move around during acceleration and braking vs with inserts. I have the inserts and chassis bracing so the rear end of my car is solid. You definitely feel all the bumps and crevices in the road. https://youtu.be/IDfJ_zYFPcs

That video makes me ill. It looks terrible! I can't stand knowing my mounts are flexing like that.

Did it fix any issues such as axle tramp?

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I have the subframe, differential inserts and the diferential out rigger kit and its alot more solid and no wheel hop, I feel the rear has more grip on corners. Nvh is not bothersome for me since they are not solid mounts. It's good for daily use.

Stiffer suspension bushings do more than just change feel (though that can be nice on it's own). The above video example shows what's happening with the subframe but those types of deflections and shifts can also occur with suspension arms. Result of that is shifting suspension geometry under load. Sometimes that's intentionally designed into a bushing (the voids in the big front control arm bushing), sometimes it's because your bushings are soft from the factory to reduce harshness, and sometimes they're softer than they're supposed to be because they're old and crusty.

See link here for a little extra detail:
https://www.instagram.com/p/CBQUCS9n..._web_copy_link

Driveline bushings have a slightly different job but the whiteline diff inserts do feel really nice.

- Andrew

i can second that i feel so much more planted with them installed. with that said does your rear clunk now?