Bottom Engine Mount - poly hardness

Here I am again chaps
I painted copper slip grease over the exposed bush that the large mount can slide over (in the style of flow viz paint) and am surprised. There is barely 1mm of slip, after around 50 miles this afternoon throwing the car around at all speeds and directions. With and without SWMBO aboard.

I conclude that the 3mm offset gap difference of the mount either side (within the subframe lugs) is down to the natural self centring point on my car. Could be different on another car.
As my mount can and does self centre on the bush in relation to the engine, the large end mount cannot be carrying any preloaded sideways strain.
There must be some preload in the small mount which explains why my large mount slides over 3mm offset to one side - which off course negates the preload at the small end.

Consider that self centring is not permitted by:-
1) the commercial poly mounts which fill the gaps without compliance.
2) the Rover design where the rubber is bonded to the bush. (It is overcome by flexing the large rubber where the cutaways have be moulded, but at the expense of sloppy movement for and aft. It doesn't stop preload of the small bush)

Therefore either of the above is always affected by preload, unless by fluke, the force centre line through the small mount is exactly 90degrees to the subframe.
Daz's comments about cracking the sump or someplace else, certainly has potential to apply to the above scenarios, especially the first. That's not the case with what I've done as preload is non existent and the poly is not the hard stuff you guys are used to seeing in suspension mounts..

Without preload, both large and small mounts are able to take for/aft movement head on into the absorbing material. Hard poly is not very good at absorbing vibration, but shore60 grade certainly is and still compresses less than rubber, thus providing the required movement limitation. These being the desired properties of any mount.

With preload, the absorbing material (be it rubber or poly) is already compressed with the result that further movement has less capability to accommodate shocks.

Lastly, discussing engine twist - why have I seen barely 1mm of slip across the large mount?
Because the smaller mount sees the twist first and as is no longer preloaded, it absorbs most of the twist, better than with a preloaded setup.
Only when at max absorption will that small mount transmit force into the shackle.
The shackle's large mount contains fairly compliant poly (which is not the traditional hard stuff) and that absorbs most of the residual force.
Finally there only enough force coming through to force the mount to slip just 1m across the bush.
And I really was trying to shift that motor about today on the road, so I must have damped all forces by absorption into the mounts with little arriving at the subframe or the sump - else the big mount would have moved left/right and it did not, bar 1mm.

In my case the small mount remains original and it's in good condition. Before I started this, I observed little movement there - most slop was in the large end. I shall see how things go before changing to a poly filled small end mount. Frankly I don't think it will alter or improve anything.

The major difference is the benefit of a self centring large mount [removes preload stress] and substituting spongy rubber for soft polyurethane.

Jeff

This is fine research, Jeff.(And worthy of a cunning plan award, I think :yesnod: ) It seems that once the initial offset is taken up, lateral movement is indeed minimal. Was the Imm lateral movement a total, or 1mm each side?

Can you approximate what this has cost you to produce the sliding poly bush?

What difference do you find in the car's drive-ability?

hi jeff,as my wife has stepped in a cancelled the uprated demo car to just a demo car in standard colour,body work ,I can let you know how you get air from the front to the rear,
Remove the rad from the front of the car,remove the boot lining.rad is placed into the boot,the rear boot needs venting and scoops placed on the rear boot to force air.
the tunnel where the coolant pipes where is now an air induction channel.
moving the rad is very easy,if you remove the boot floor as well.the engine performs better and cooling issues are sorted out.
you have worked very hard on the bottom mount,the trouble is when we fix a car we have to replace parts like for like,if we fitted something fancy and it went wrong.We turbo charged a car a couple of years ago,bloody nightmare.

David Aiketgate wrote: This is fine research, Jeff.(And worthy of a cunning plan award, I think :yesnod: ) It seems that once the initial offset is taken up, lateral movement is indeed minimal. Was the Imm lateral movement a total, or 1mm each side?

Can you approximate what this has cost you to produce the sliding poly bush?

What difference do you find in the car's drive-ability?

Notes on Sliding mount movement
After mixed driving styles, the [viz grease] indicator showed the mount slid on the bush by 1mm TOTAL in one direction only.
The "at rest" 11mm visible reduced to 10mm at some stage during engine twist conditions.
Thus the mount's offset from the bush centreline reduced from 3mm to 2mm.
The 3mm offset was regained on return to "at rest".

Do realise the term self centering should, perhaps, be self aligning.
The idea is not to get the large mount central on it's bush.
Instead we seek to maintain a 90degree relationship between centre line of the shackle and the front-to-rear engine movement force, in order that both mounts in the shackle are compressed against otherwise relaxed shock absorption material (rubber/poly). Rover could have achieved this if the large rubber mount was not bonded to the bush. Instead a half-assed attempt is provided via the large cutaways in the rubber moulding, with consequent loss of shock absorption property that is marginally adequate only in new rubber material.

Even at rest, engines do not necessarily have a 90degree relationship to this lower shackle. They certainly don't when the engine twists under load. My sliding mount ensures the 90degree relationship is maintained, without relying solely on the twist being accomodated in the large mount. At rest, I find my car aligns these forces 3mm off centre from the bush's centre point.


Costings:
Minimum purchase quantity of shore60 polyurethane chemicals cost £40 + vat and carriage.
Although I drove 450 miles to get some - so that I could "look and learn".
Actual quantity used - probably £3

Cost of time spent - depends how much an hour one's time is worth!
Research and Development - 20 hours perhaps.
Actual production - about 1 hour.

Noticeable Improvements:

1. No judder on drive take up in slow traffic
2. Positive and faster gearchanges (not sure why - guesses anyone?)
3. No engine lurch when moving up/down the box - engine+gearbox correspond immediately to clutch position
4. Less vibration at high and low rpms - no change inbetween

Which all summarise to the unquantifiable "it feels better" / "I smile more"


Jeff

PS I do not worry about metal fatigue or sump cracking, as I would with a hard grade poly large mount, the type that completely covers the bush length. No offence Clive, it's just my own opinion. The small mount in hard poly is probably fair enough, providing the large mount is the original type with new rubber or soft poly on a sliding arrangement, like mine.

So come on then - will you be going in to production?!?!?!?!

:yesnod:

Jackson wrote: So come on then - will you be going in to production?!?!?!?!

:yesnod:

Yep, I'd be up for one! :yesnod:

I am up for one

N

Me too :yesnod:

Now all I need is someone to come up with an equally cunning fix for Hydragas units... :thumbsup:

you can put tf suspension on bit of work but you can

Okay, Okay - production can begin if that's what you guys would like.
I shall make a few - so early takers can try on an exchange basis.
Of course, I shall need to buy some shackles to make an initial stock.
Will see whay I can find on Ebay or call the usual suspects in the morning.

Jeff

Top stuff - remember I asked first