Bottom Engine Mount - poly hardness

Fantastic write-up Jeff.

Do you think that the 3mm offset on the mounting is a standard fitting, or specific to your vehicle? I would have a look at ours but we're having the tailend of the hurricane outside: :omg:

The materials can be purchased at different shore ratings. high numbers = harder results.
If you can afford to wait for the result to mature, you can mold anything you like very quickly.
Clean out and degrease shocker end, blind one side, secure the inner bush coated in release agent(grease), pour in, job done. A shock absorber bottom mount probably won't take more than 2 days to mature, cos it's not that thick.
Maybe shore70 would be a grade to conside for that application as here we do not seek much compliance under load.

I agree with Clive about coping with engine twist under work - and thought about it a lot before molding my own replacement large shackle/mount..
My method covers vibration and twist concerns by using softer shore60 material and especially permitting the mount to slide on it's bush.
With this replacement fitted I've not attracted any new vibrations and believe I've actually reduced them, yet still achieved improved (less) engine movement.

My reasoning is open to debate:-

The original large rubber mount at the subframe end of the lower shackle is moulded in a softish rubber material in a maner that allows movement in left-right directions whilst absorbing shock front-back. Its metal bush is bonded to the rubber and the bush is nipped tight into the subframe via the through bolt at 100nm. I deduce the rubber is molded with cut aways to allow left-right movement; the engine and the subframe are not always in the same plane with each other during work.

Commercially available poly replacement mounts are in two parts, like tophats and in hard polyurethane. This "upgrade" would seem to be ideal to reduce front-to-back engine movement, however hard poly is not very compliant so vibrations and torque shock might transmit to the car. Also the poly material is tight in the housing as well as the subframe so left-right movement would not be possible. Using this type of poly replacement could strain, and possibly fracture, the entire lower mount/shackle. Actually a standard mount/shackle doesn't naturally line up exactly with it's smaller partner, you have to move it a bit to connect the pair - another reason for the l/r compliance in the large rubber. For these reasons commercially available poly is not an option I wanted to follow - it's too hard and doesn't have a much needed knuckle function

Jeff

cjj wrote: ....I wonder if you can use this stuff to make shock absorber bushes, as I have yet to see any for sale. The rubber bush is a common failure leading to knocking, but on the TF you need to replace the whole shocker, whereas on the F you can buy replacement bushes.

the bottom 'bush' on the TF's shocker is a mechanical device, its actually the same one that is fitted to the rear top arm, and fails for the same reason presumably. My understanding as to the reason why no polybush replacement is available is that to much articulation takes place at the bottom of the shock for a polybush to be used.

David asked about a 3mm offset and yes, I should think it will vary from car to car and under different engine torque loads - and that's actually the point of my method!.

On the standard mount, the bush and the rubber are joined, so the exposed length of the bush will always be 8mm each side. However, the angle of the mount 's metal shackle to the engine sump will not always be a nominal 90degrees.
Because the engine twists on it's pair of weight bearing mounts under load the "at work" position will be at a different plane to the subframe from when the engine is "at rest". Everyone's car will be different as those main mounts wear/soften and acquire a unique resting place when the engine is offload.

With my poly loaded mount, the entire shackle is free to slide on the bush and self centre as the engine twists. At rest I have 5mm free one side and 11mm the other (a 3mm offset). That will dynamically alter during driving - but I've heard no clunks so the shackle cannot be fouling the subframe lugs it's located between.

At some point I'll observe the movement under load, when I get a helper to dab the clutch whilst in gear with the car on ramps (handbrake on!).

petevick wrote:

cjj wrote: ....I wonder if you can use this stuff to make shock absorber bushes, as I have yet to see any for sale. The rubber bush is a common failure leading to knocking, but on the TF you need to replace the whole shocker, whereas on the F you can buy replacement bushes.

the bottom 'bush' on the TF's shocker is a mechanical device, its actually the same one that is fitted to the rear top arm, and fails for the same reason presumably. My understanding as to the reason why no polybush replacement is available is that to much articulation takes place at the bottom of the shock for a polybush to be used.

Yes of course - I have an F so can't really comment on suitability for a TF.
However, my point about shore grades might still apply - it can be supplied at rock hard rating larger than 80.
In laymans terms would this be TFing hard as opposed to Fing hard?

Jeff

hi jeff can you make us bush,as these cars are getting older more issues are arising.

dazlightyear wrote: hi jeff can you make us bush,as these cars are getting older more issues are arising.

It's early days in my trials - will my idea work long term, is it reliable?

The material bonds to the shackle so I need each shackle for 5 days lead time to allow for curing.
If I get a stock of old shackles I could do a batch and then provide an exchange service.

Would that help anyone?
If I got some "and me" replies, I'd know how many old shackles to source in readiness.

Jeff

Borrowing a photo from Clive's Torque tamer bush thread, you can see that the commercial poly bush removes any possibility of sideways movement by filling up the 8mm gaps on the big bush fitment.

Yes, David you are quite right - Clives' pic certainly does fill the gaps between mount and subframe.
With a setup like that, there is nothing left to accomodate engine twist under torque, which is a considerable force - and my reason for concern!
I don't really have a problem with the small bush containing hard poly, providing the large end is softer and has a means of taking up twist. Without it, where does that shock go?

The original design has the cutouts in the rubber for a reason - I think to accomodate twist and variable offset of engine-to-subframe position, whilst doing so as cheaply as possibly. Who knows, the design provided built in obsolescence for ongoing parts revenue [I'm over 40 so quite cynical]. Hence rubber bonded to bush and really flexible rubber at that!

Clive has already said he can feel extra vibration - 'cos he looking out for it, but if that results in stressing something else, a stress fracture may happen someplace.

Idea!! I think I'll put some copperslip onto the visible parts of the bush and go for a spin. On return I shall see how much has been pushed away by the mount sliding along the bush. Then I will know the effect of engine twist at that point.

Jeff

Oh yeh, great idea :thumbsup: . A bit like a flow vis paint run in F1. :yesnod: I would be very interested in the amounts of lateral motion on that point.

there is another issue here,the engine and gear box are heavy,When we take off the sumps,once in a while they seize on,once we had to use a hammer and the sump cracked,it didnt take much force to break it either,
Rover would have spent a long time and money to get this right,they also have loads of data from past models and past problems.
I agree the bottom link isnt on the face of it much good,But the may be the brillance of the design.
Its always intresting to explore things like this,and on the face of it,it seams a good idea,but if there is a risk of cracking a sump and losing oil is that worth the risk.
We have serviced thousands of these cars,and to be honest they where not well made,and the design was limited.But the one car was designed like that.Start making some bits perfect could show the rest of the design up.My son brought up the bottom link to day while we where doing a car.The engine rocks a bit,but all engines rock a bit regardless of car or design.Mg only ever gave the car a face lift,minor changes.May be they good the best out of it.
Long term metal stress with a hard poly bushed link would be the test.and if it affected any other componants.It would mean x raying the sump and checking other related componants.
On the face of it I would like to try one,but its the long term untested issues that would concern me.
But when these cars where brand new,the engine mount was pretty solid with no or little lateral motion.A new car feels very different to drive then an old one,bit like a women really.
Its the shock being transmitted on to the sump casing is the concern.

Better be careful Daz.
I know my wife wouldn't like me discussing the structural integrity of her sump casing! :omg: