Got a bit of time to do some work on the car yesterday and whilst checking out the rear hubs its obvious that I'll need to replace the two rear suspension lower ball joints before the car is put back on the road (split rubbers etc).

I hadn't anticipated doing this so had a quick search through previous posts and it seems that at some point the thinking was that the offset MGF ball joints were a better option as they gave a more advantageous caster setting at the rear - not the metro ball joint it says I need in the manual.

Should I fit the MGF joint?

Advice appreciated before I shoot over to Brown and Gammons.

Ta, Jamie.

FYI I've read that the MGTFs are also slightly different again. If you can i'd get them to bring out one of each so you can see the difference (if any).

Good tip - thanks.

Is the MGF joint a better option than the metro?

TBH I don't know what the advantage is and wouldn't like to guess without actually seeing what and how much the difference is in the parts.

Might be worth doing a bit of digging on Metro/Rover/MGF forums, articles etc. as you might get more info there.

SAS

I don't think caster matters on rear as there is no steering to self centre. The info I had printed off the rdm gtm site said offset bottom ball joints where introduced at the same time the rear arms where upgraded to pillow ball joints. The offset lower ball joints where fitted to make the wheels toe in under bump so if you fit these you may get the same geometry as the later arms but with the track rod end type pivots ?

I'll have to look into this a bit further. Ive read they offered a small advantage but not 100% sure why.

If the extra offset gives more toe in with bump (or roll) then this would be a good thing as there is a natural tendency for the long trailing arms (and any slack in the joints) to deflect towards reducing toe in under load so would cancel this out. I've found I need to have slightly more than ideal toe in at rest to avoid going positive under load (and thus spinning!). I'll be following this thread with interest!

I've been searching around on TeamGTM and it seems that the offset rear ball joints were used to get a better static axis line which meant a move to toe in under bump (or less tendency to toe out) which would seem a worthwhile step, it was quite a small gain apparently. However, this was a fairly recent recommendation that seemed to go hand in hand with the rear suspension 'upgrade' and I'm not sure it was recommended for use with the Mk1 ball joint rear arms?

There was a load of hoo-har at the time as if you had 17" wheels it meant the tyre was a little close to the rear clam pivot mount. Not a problem with 16's (which I have) apparently. It moved the rear lower hub mount 10mm to the rear which is probably about 5mm max at the wheel centre so not a huge change.

Just wondering if its a worthwhile mod as Im changing the joints anyway? Does it work with the MK1 arms?

Bryn Davies would be the man to ask - anyone know if he's on this forum and still active in the world of Libra's?

As you say it was introduced for the MK2/2.5 suspension when RDM were having some set up work done before the car was reviewed by Evo magazine.

I don't think anyone has tried it with the MK1 suspension.

Thought that was the case Roger thanks.

I'm sure I've read somewhere that the geometry wasn't (intended) to be changed when the new suspension was introduced so in theory it should be worthwhile to the earlier set up as well?

I have done some experiments today on my rear suspension to try to understand how it works. The rear suspension arms travel through a different arc due slight variation in length like unequal length front wishbones change camber angle under compression. As the rear suspension goes up the lower ball joint moves forward in relation to the top one as the steering locking arm fixed to top arm cannot shorten to move with it has the same effect as lengthening it which makes the wheel toe in under bump as Sanzomat said this may only be enough to cancel any deflection at the wheel caused by cornering loads. The rear arm pivots are about 9 inches apart and the arm are about 16 inches long so nearly ratio of 2 to 1 so if bulkhead flex 0.5mm inwards on inner bracket would 1mm change at wheel so 1mm toe in would change to zero. Maybe they realised they cannot get rid of all flex in bulkhead so designed some compensation into suspension instead ?

Nice science Mark, good stuff.

From other posts I've read the offset joint was supposed to help the rear toe in a bit under compression (bump) or at least exaggerate the suspension designs tendency to do this anyway and moving to toe out at the rear under load is obviously a bad idea. I'm sure I read this somewhere and I'll have to go back to what I was reading and make sure I'm right on that.

Be nice to confirm if this is a good idea on MK1 rear arms - has anyone done it on here? At the end of the day I can just replace my joints with the std metro ones but it would be nice to change if its a worthwhile mod - cost is the same

In my manual (date unknown) it mentions metro ball joints but I also have a supplement for the new suspension dated 10.03.05 (not fitted to my car) that states and I quote:

Do not use MGF handed bottom swivel joints on the new suspension set up with 17" alloy wheels!!
These MGF handed bottom swivel joints are only suitable with 16" alloy wheels and are not provided as part of the new suspension set up. However, these can be purchased separately if required.

I have mentioned earlier that there was a bit of a ta-do about not being able to fit them with 17" wheels and this statement would seem to suggest that the MGF joints were being used with the old style suspension? Posts I've read were expressing concern that the MGF joints couldn't be used with the new suspension if you had 17" wheels and more than one person wes not happy as the MGF unit were an improvement on the metro - again this would seem to intimate that people had fitted the MGF joint to MK1 arms??

Hmmm.......more research needed.

Sounds like its time for a practical experiment - let us know how you get on with your new MGF joints!

Yes I think you may be right.

Problem is, I've not driven the car with the standard metro joint so wouldn't know the difference anyway

I have offset but no metro ones to compare against I would take some measurements throughout suspension travel while metro ones are still on then fit one side mgf so you can compare the difference if it is enough to notice I put a long tube through the hole in steering arm near top ball joint then put bottom end in gap where pinch bolt tightened this shows angle change through ball joint centre line long tube makes angle easier to see as suspension is jacked up