SVTdriver said:
I probably am over thinking the issue. But I don't think that I could vote for either. The problem being we have no idea how the new suspension will handle. And how much if any the IRS would improve upon that. I can't really vote for IRS as the only choice. Because that to me that says I don't trust Ford to build a good car. So why should I even be looking at it? The other thing I have issue with is tires are nearly as much responsible for handling as the rearend design. And at least the stock tires on my year cobra STUNK. As soon as I changed them things got much better. I appreciate that people think the IRS will handle better. But those people that talk about the car going out of control when they hit a bump. I tend do there after discount what they are saying. The other part of it as an option. Is how would you know if you wanted the option until you test drove them both. Assuming the dealer stocked an IRS model.
I agree...my current Zed4 and the E46/E36 have/had IRS and I would say while it gives great precision for the rear, not to break loose while accelerating round the apex, great ride comfort as well as handling but there are some core needs for IRS to work very well.
IRS requires a very near 50-50 weight distribution because true benefits are reaped when the chassis is tuned to neutral.
It's sensitive to -ve camber adjustments, ride height and it's an "art" to lower the rear or adjust the camber just right to get great straight line acceleration and have it handle well in the corners.
Because the IRS is independent of each arm movement, the lateral forces cause greater stress on the rear shock mounts and rear sub-frame mounting points.
In fact early edition E36 92-95, rear shock mounts are known to tear through; the M-coupes, 3'er coupes had classic tearing up of subframe mount points right throught the chassis. This problem has plagued the E46 models upto model year 2k as coupes absence of the b-pillar contributes to less torsional rigidity.
Metal fatigue is greater in IRS. The chassis has to be really tight and bound together by upper strut/shock bars.
IRS is dead without necessary sway bar linkage and chassis torsional rigidity, it requires advanced lower and upper control arm geometry as well as location of the rear shocks w.r.t to the springs. The two must work in tandem, i.e one can get away by lowering springs without changing shocks in solid axle rear but in IRS such a decision may become a disadvantage.
If limited slip or ABS /electronic traction control is taken out of IRS, it's pretty much game over (another reason why BMW has always offered either LSD or the latter two as standard equipment)
Solid axle:
Because the axle is solid, the point of power delivery (from the drive shaft) is at the centre means that this is the axle's natural axis of lateral rotation.
So if the outer wheel moves up, the axle naturally plants the inner wheel down. It is helped more so by the inner wheel spring which open up. Placing the rear sway bar links to pass below the axle but linking it directly to the chassis say near the lower traverse arms means during cornering the axle will pull down the inner wheel, while the sway bar will pull down the chassis to reduce body roll.
On the other hand an IRS needs advanced chassis dynamics. aggressive spring rates/shocks to help push the opposing end (the inner end) down in a corner. It gets no help from the axle itself. An IRS must be equipped with a anti-roll bar in the rear with simliar stiffness and thickness as the fronts to dial in neutrality. Well because of this it's more comfortable and literally eliminates wheel-hop.
A solid axle does not have to impart a large upforce on the chassis points above the outer wheel because it naturally pulls the opposing end down.
I will put my money on the chassis stiffness, weight distribution and the front l-arm setup. It should reduce nose dive especially during late braking.
