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Would lowering the rear of the car mean that the top of the axle is pushed backwards and that having a slightly shorter tie bar would bring it back into the correct position ?
I think it's the other way round! You have to look a the rear axle like a pair of unequal wishbones with the axle as a hub/wheel. So the trailing arms are the lower wishbone and the tie bars are the upper. Because the tie bars are shorter than the trailing arms, as the axle goes up into the body (or you lower it) the angle of the diff moves downwards as the tie bars pull the top of the axle forward. So you'd need to LENGTHEN the tie bars to compensate.
This would, however, only compensate for the difference in nose angle. Roll centre is another kettle of fish. Whilst I don't really understand what it does, the limiting factor appears to be the angle of the lower trailing arm to the road in the "rest" position. On a standard car they are angled slightly down at the axle end. The limit of lowering seems to be when the trailing arm is dead parallel with the road. Which is why Rob recommends raising the axle mounts on the trailing arms, this keeps the trailing arm angle where it needs to be for roll centre conrol whilst lowering the body over the axle. But, as he says, it's a pretty extreme and radical mod to acheive something you don't really need on anything less than an out-and-out race car.
Many years of trackdaying the Carledo makes me confident you won't need it, the stock geometry lowered to the most practical level and with judicial polybushing and some Gaz asps and good brakes, wheels and tyres, had me looking for more horsepower from the 140+ I had cos I was confident the chassis could handle it comfortably. The car's next iteration will hopefully be around 220bhp which I think should be more than enough for a 3/4 (760kg kerb weight) tonne 2 door! That's close to 300bhp/tonne power to weight ratio!
Steve