VR/VS rear cradle bushes/cradle failures, Solid VS soft bushes and mounts????

[Immortality]

I'm trying to comprehend the use of bushes in the mounting of the diff and rear suspension cradle on IRS models (other than maybe noise or vibration from the diff into the passenger cabin?).

The way I see it, the rear suspension already has rubber mounts in the suspension arm pivots as well as the shock absorbers and the spring mounts. Having more rubber mounts for the diff and the rear cradle mounts just allows more flex where it's not needed. Allowing the diff housing to torque up against the cradle mount through the very weak factory rubber mount no doubt contributes to the broken rear cradle problem common to the VR/VS models (what did Holden do to fix this problem on the subsequent VT- VY models?).

I know that the best solution is to use the Harrop sports diff mount, however this is probably overkill for the average build and it's very pricy to boot. The cheaper option seems to be a Nolathane insert that fits inside the standard rubber mount to fill the empty voids but this still leaves a lot of rubber which is prone to flexing and degrades badly with age. At this stage I don't see any other commercially available options.

Now, I'm wondering. What effect would it have to use solid mounts for the rear cradle/diff? If there is no flex than things won't break (I could do this fairly easily at work). The other option is to manufacture some sort of diff mount using some small solid type Nolathane bushes but otherwise uses the factory diff cover.

Thoughts, suggestions or ideas welcome.

 

[VRSenator065]

Hi mate, good thread. This is something I looked at quite a bit when building my rear end.

I think the rubber mounting of the sub frame to the car is there to cope with the twist and flex of the chassis as all cars flex. I am not a suspension designer but it does strike me that pretty much all IRS cars I have seen where the suspension is built into a sub frame are mounted this way. I think if you were to solid mount it, eventually the mount points would fatigue and crack.

The Nolathane vs SuperPro <insert brand here> argument is one of those Ford vs Holden things, everyone prefers one or another. I went SuperPro on my build and they have different grade bushes available, mine are "race" bushes and they are pretty much one solid lump of very hard Urethane, they will flex very very little, so really are almost solid. I assume they will make the car harsher and a little noisier, I guess thats part of the trade off.

This is a pic of them I posted on my build thread as it was a little confusing what parts to use.



Of course the dreaded axle tramp also comes into this discussion, I wrote a long diatribe about that if you were interested, but I would have to try and find it.

 

[Immortality]

Yeah, thanks mate.

The more I think about it, the more I'm thinking that the rear cradle mounts/diff mount are there to reduce noise/vibration from the diff been transmitted to the car chassis but unfortunately that rubber diff mount doesn't really provide any torsional strength in terms of allowing the diff housing to torque up against the diff cradle causing the cradle cracking problems

I agree that I'll happily put up with a little more noise/vibration seeing as the engine will probably mask any other noise/vibration present.

It's funny that you mention axle tramp (yes I did read you thought and I might link them to this thread if you don't mind), the Senator spins up real nice, I can slide it left/right with full control but when I button off to try and actually get some traction it does this weird thing where the ass of the car almost jumps as it gains traction, I've never ever had a solid diff Commodore do anything like it and I can only put it down to the extra flex that occurs in the rear suspension/cradle.

I'm thinking that a new diff mount using urethane type bushes instead of the standard rubber mount would do the job. I'd like the Harrop unit but I think they retail close to NZ$1000 by the time it's delivered to me here in NZ. That's a lot of coin for essentially what is just a simple mount.

 

[VRSenator065]

I actually think the cradle cracking problems are just down to it being poorly designed, i.e. the material maybe is too thin, the section could be beefier, and it would also be very interesting to do a metallurgical analysis on the steel used to see if it changed when they went to the VT, maybe they went to a slightly better quality steel. But at the end of the day basically its not strong enough. Everything will fatigue and crack eventually, but something that's marginal will do it more often and sooner, especially when guys like us put more power and torque through it

 

[Immortality]

It's an interesting thought. I also think the problem is that the original design was actually borrowed from a vehicle that was never intended to have a V8 power plant. They even removed the extra toe links from the original design that HSV eventually put back on on a later model and then Holden did as well in subsequent models.

 

[Deuce]

The cheaper option seems to be a Nolathane insert that fits inside the standard rubber mount to fill the empty voids but this still leaves a lot of rubber which is prone to flexing and degrades badly with age.

Pretty sure Nolathane list the insert to work on VS only, not VR models. Can't for the life of me work out why though??

 

[Immortality]

Not sure, but there does seem to be a hole in the market between the Nolathane insert and the Harrop sports diff cover/mount.

I think the Harrop mount is right in that the bushes are next to the chassis and wide apart, this way they provide much better support, especially from allowing the diff housing to twist against the cradle.

 

[Deuce]

I still think I will get the Harrop one. I really like the idea of the cooling fins and extra capacity of it.

 

[delcowizzid]

in skylines you bolt in solid alloy bushes Nissan RWD Subframe Bushes | Trade Me i reckon they would nearly fit in the commodore front bush too

 

[Immortality]

I still think I will get the Harrop one. I really like the idea of the cooling fins and extra capacity of it.

I'd like the Harrop item as well, but I still think it's probably overkill for a cruiser. Like I said earlier, unless you plan on doing a bit of racing it's probably not needed as oil temps won't be a huge issue. You wanna bring a couple back next time you travel to Aussie? (shipping would be a killer)

edit: Just found the shipping quote from Harrop (AU$115), so by the time you include import duties I don't think you'll get change from NZ$900 for it delivered on your doorstep.

in skylines you bolt in solid alloy bushes Nissan RWD Subframe Bushes | Trade Me i reckon they would nearly fit in the commodore front bush too

Exactly, that is what got me thinking, I've seen solid diff bushings for other models cars as well.

Alloy bushes wouldn't be to hard to turn up and neither would making a solid mount for the diff but I'm not sure that's what I want in the Senator. If it was a dedicated racer or weekend bruiser I would have done it already.