There was a discussion a long time ago involving Cooperman and myself, and various others I think, about the tie bar to lower arm fixing. I can't find the thread easily as I don't know what it was titled. Anyway there was some suggestion that a flexible bush at the tie bar to lower arm connection might be a good idea. I think Cooperman and I agreed that it was done on certain cars (I quoted the Hillman Avenger), so it should work.
Well, I was lying in hospital a while back with less than nothing to do and started to think about it again, and soon realised the very big snag. But first let me explain how it is done on the Avenger. This has McPherson Strut suspension, but we are only concerned with the lower wishbone, which is what a tie bar and arm constitute. I found half of my old Haynes Avenger manual the other day, while preparing to move house, and it confirms what I thought, there definitely is a Metalastik bush at the junction. Found a spare bush in the garage too! The tie bar is acuually a channel section pressing, but it fits around the lower arm, a forging, in a fork end with a through bolt, the bush being pressed into the lower arm and the bolt and locknut obviously go through the middle. The Avenger tie bar pivots in a conventional bush on a fore and aft axis, and it is likely differences in compliance between the tie bar bush and lower arm inner bush that necessitate some flexibility where the tie bar and lower arm meet. In other words the base of the triangle defining the wishbone varies slightly, due to quite soft, thick bushes, which would make the bolt work loose, or cause a fatigue failure, if there was a rigid connection. The Mini is much stiffer fore and aft on the bushes.
Thus far, so good. However, line most McPherson strut suspensions, the centre of the ball joint is centred in the lower arm, and that is why it works. The fore and aft forces due to acceleration or braking act on the centre line of the arm, and impart no torque to it, so it does not try to twist around the flexibility of the bush at its inner end. Centre line ball joints make replacing the ball joint difficult. On forged arms the whole arm is changed, on pressed arms, like most modern cars, the ball joint is rivetted in and the new one is bolted, after drilling out the rivets.
Now consider the Mini. The lower ball joint centre is, well you all know where it is, in the common centre of the large and small radii of the spherical end of the ball pin. Now that is, at a rough guess, 30 to 50 mm above the centre line of the arm. (Feel free to check the exact dimension, let us guess at 40mm , 0.04m for now). Now suppose under heavy braking there is a so-called weight transfer to the front wheels, so each is carrying about 300kg, and we have good tyres and a dry road, and achieve 1g braking deceleration. The braking force and hub torque add at the bottom ball joint, and partly cancel at the top. (Don't have exact dimensions, this is only a rough calculation). However we will have at least 300 * 9.81 (call it 10) = 3000N of backwards load AT THE BALL CENTRE, which being 40mm above the arm neutral axis imparts a torque of 3000*0.04 = 120Nm on the arm. Multiply by between 3 and 5 to allow for impact loading etc.
If the inner pivot was a rose joint, as some are, the arm would simply rotate, allowing the hub to move backwards, limited only by the ball joint travel (until it breaks!). The castor angle would be uncontrolled. Substitute the normal rubber bushes at the inner end and the arm rotation will be somewhat restrained, but not satisfactorily.
So why does it work at all? Well, clearly the lower arm is restrained from rotation by the tie bar, which is loaded both in tension and bending by braking forces. It doesn't deform permanently because the deflection is below the elastic limit, and being in tension there is no tendency to buckle. But there will be much more tension on the upper side of the tie rod fork end than on the lower, and although the bolt "should" be tight enough for the load to be carried in friction between the surfaces, in reality the bolt will be carrying shear and bending loads.
Now peak braking force is much the same on a standard car and a highly tuned one, limited by tyre to road friction, so in the braking case you don't need to upgrade anything. But consider acceleration. You will get a bit less longitudinal force than when braking, as the tyre to road friction is not helped by weight transfer to the rear, but if you drop the clutch at full revs you may still manage a short spike of near 1g. This time, the bottom ball joint is trying to go forwards, and the tie bar is loaded in compression, as we probably all knew ever since we first saw a Mini. But, there is still the torque on the lower arm due to the ball joint height, and it is trying to bend the tie bar down in the middle, while applying compression, so it is apt to permanently bend downwards. (They usually bend upwards due to impact for obvious reasons, but I have seen them bent downwards and at the time was puzzled.)
So no, we need the tie bar to react, in bending, the torsional load on thelower arm due to ball joint height, and therefore can't have a flexible bush where the tie bar meets the lower arm. Rigidity is exxential.
Clearly heavy duty tie bars are necessary for highly tuned cars to avoid them bending under acceleration, and you don't get significantly more stiffness, or fatigue capability, by using a higher grade of steel, so that is why they are always thicker. But that adds to the unsprung mass, possibly not too seriously, although a thick walled tube is nearly as stiff as a solid rod, so there is scope for making them lighter. An oval tube, long axis vertical, would make a lot of sense.
Hopefully someone will be along to do some computer modelling of all this. I am not convinced that the suppliers of uprated tie bars have done so, although some of them are decently made, and work. Empirical engineering at its best, hopefully. Some may be over-engineered. Others, beware...
By the way, I have been far too busy lately to spend any time at all on internet forums, so I will likely disappear again for a while. But "I'll be back", as someone well-known once said.... Just got to clear up several very urgent major tasks first, like moving yet again, to an entirely different area.
