maupineda said:
I love tech discussion, what data would you have to indicate the rates are off, either soft or stiff, you do realize that despite engine and drive positions, the Z4 is close to 50/50 weight distribution. I think is 52/48 and about 49 cross-car. so it is pretty balanced.
I’m more interested in the outcomes than the technical details, and I’m no engineer but I do enjoy modifying my cars to make them handle better.
I’ve noticed a relative lack of discussion and evidence on topics such as spring rates and weight distribution for the Z4 compared with other makes I’ve owned, which I attribute to an ownership base which tends to be more GT than sports, and because these cars are rarely used on track. I’m not a track person either, but cars that are popular on track tend to have a much deeper knowledge base.
I’ve seen the 50/50 weight claim mentioned frequently. There’s not a lot of available evidence of this, but I have no reason to doubt it. But I note that weight distribution presumably cannot be the same for an S54-engined car whose engine is significantly heavier than say the N52. Meanwhile the impression from my driver’s seat is very much one of front-heaviness, exacerbated by a driving position far from the front axle which magnifies the pendulum feel.
Many road tests and indeed the OP of this thread have described the same symptoms I felt when I first acquired the car, of the disconnect between front and rear. A vague front which does not inspire confidence during hard turn-in, combined with a lack of compliance (and poor damping) at the rear which causes the car and driver to bounce all over the place on anything less than well-surfaced roads.
It therefore made sense to me to look into stiffening the front and softening the rear. And it’s perhaps not a coincidence that this is also the approach recommended by the likes of BC and KW with their default spring rates. I also suspect that BC and KW damping is better-matched to their springs compared with OEM.
maupineda said:
if you match them you now have a soft rear and the wheel will not settle after a bump, and the car will jack down, and ride the stops as a result which will make the ride even shittier.
I can understand this concept, but my experience suggests that you would need massively softer rear wheel rates and massively harder front rates before it manifests.
Using your spring to wheel rate ratios of 0.9F and 0.45R as a guide, the stock BC setup offers a delta of 1.2F:1R and this feels substantially better than stock. In fact on stock 6 kg BC front springs turn-in is still not as precise as I would like, and I aim to experiment with 8 kg springs at some point which would give a 1.6 delta.
My hunch is you’ll need a delta of of >2:1 before the problem you describe actually occurs, and that a delta of between 1.5:1 and 2:1 is ideal for our cars.
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