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

Discussion in 'V8 Development and Modification' started by immortality, Dec 29, 2013.

  1. immortality

    immortality Can't live without smoky bacon! Staff Member

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    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.
     
    Last edited: Dec 29, 2013
  2. VRSenator065

    VRSenator065 Now in stealth mode

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    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.

    [​IMG]

    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.
     
  3. immortality

    immortality Can't live without smoky bacon! Staff Member

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    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.
     
    Last edited: Dec 29, 2013
  4. VRSenator065

    VRSenator065 Now in stealth mode

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    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 :)
     
  5. immortality

    immortality Can't live without smoky bacon! Staff Member

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    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.
     
  6. Deuce

    Deuce Super Stock

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    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??
     
  7. immortality

    immortality Can't live without smoky bacon! Staff Member

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    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.
     
  8. Deuce

    Deuce Super Stock

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

    delcowizzid on holiday

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  10. immortality

    immortality Can't live without smoky bacon! Staff Member

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    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.


    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.
     
    Last edited: Jan 1, 2014
  11. delcowizzid

    delcowizzid on holiday

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    ive welded up a few vr cradles if its not cracked ide drop it out and weld on a strengthening patch
     
  12. immortality

    immortality Can't live without smoky bacon! Staff Member

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    Yeah, mine has to come out, but unfortunately with a stuffed arm/shoulder it will be a couple of months yet before I can even consider doing something like that :(

    I like the way VRsenator065 strengthened his so I'll copy his one when I do it.
     
  13. immortality

    immortality Can't live without smoky bacon! Staff Member

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    Had the car up on a hoist this morning and no signs of any cracking in the rear subframe :w00t:

    Plenty of movement in the bushes though, everything else looks like it's original, bushes, shocks (which are shot) etc.
     
  14. VRSenator065

    VRSenator065 Now in stealth mode

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    I reckon the problem with solids is the holes on the chassis will have to line up precisely. Seriously the Super pro's I have are as close to solid as you would ever need anyways. There is a guy on eBay (can't fnd it right now) selling a fabricated diff mount I think to sort of try and replicate the Harrop but without going to the cost.
     
  15. immortality

    immortality Can't live without smoky bacon! Staff Member

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    Yeah, I'll fit Urethane type bushes for the subframe as this car is intended mainly as a comfortable cruiser. I haven't yet decided if I'll go with the Harrop mount or if I fabricate something up myself.

    I think I know what mount your talking about, I think it was all alloy but it looked a but ugly if I remember correctly.
     
  16. VRSenator065

    VRSenator065 Now in stealth mode

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    Yep actually looked pretty rough tbh. I don't reckon it would be too hard to fab one up. Post up if you do, be interesting to see.
     
  17. immortality

    immortality Can't live without smoky bacon! Staff Member

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    If I do it will be steel (I don't have a lot of alloy stock, nor the equipment to weld it). but it should look nice at least.
     
  18. VRSenator065

    VRSenator065 Now in stealth mode

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    Yea and really you will never ever see it. I wouldn't do it in Ally anyway, unless you could lay your hands on some 6061 or something, it will be under a lot of load/stress.

    Might be worth messing around with the height of the cradle if your doing it. You may be able to crib a few millimeters by making the rear of the cradle sit a little higher than stock so that with the car lowered the drive shafts are more horizontal taking load off the cv's Just a thought bubble though, haven't thought it through.
     
  19. immortality

    immortality Can't live without smoky bacon! Staff Member

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    Yeah, I figure If I measure a standard mount before fitting and use that as a base the diff will sit higher as there won't be any sag like you would have with that standard rubber mount.

    You don't really want the CV's to be 100% straight. From my understanding you want at least 1° angle across the CV joint so the bearings turn a little in the joint. If they don't apparently they wear out a lot quicker which I think is why the IRS models have the centre drive shaft mount offset.
     
  20. VRSenator065

    VRSenator065 Now in stealth mode

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    Interesting, I would have thought just regular suspension travel would give you some of that, but I am not Mr CV. Cool, nice little project, post up pics if you do it.
     

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