Thread: MY12 D4 suspension stiffness (lack thereof)

Originally Posted by Graeme

Could it be just the front wheels then the rear wheels going over a bump? I recall that I initially noticed the rear-end moving about rather strangely over bumps after having driven live axle LRs for many years.

It's mostly side to side oscillation but there's an element of front to back also. Basically it behaves like my D1 with worn shocks, but maybe 10% less wallow. At times almost like being at in a small boat crossing the wake of a bigger one.

Originally Posted by Brad

It's mostly side to side oscillation but there's an element of front to back also. Basically it behaves like my D1 with worn shocks, but maybe 10% less wallow. At times almost like being at in a small boat crossing the wake of a bigger one.

That doesn't sound right at all.

Took a Trivett Service staff member on a five minute drive around Parramatta. He was of the opinion that the suspension was normal. He did however offer to leave it with them and they would examine a few items. I am supposed to pick it up at 4pm this afternoon.

*Edit* Have now picked up the vehicle and dealer says it's normal. I'll tee up a test drive to compare it against at the dealer next week. If it turns out the ride is the same then maybe the fault lies with me.

tl;dr: Air suspension is significantly stiffer in access height. Access height body roll is excessive due to this. Normal height is fine. My vehicle behaves same as demo vehicle.

I drove a 3.0 (unsure whether SE or HSE) on Friday at the dealership back to back with mine, with a friend and a sales guy along for their opinions. Long story short the ride was almost exactly the same at normal height between both vehicles.

I managed to replicate the bouncing only while in access height going over speed bumps at about 10km/hr. I did not this on the demo vehicle however I'm confident the behavior would be the same.

A friend explained it this way:

1. Shock absorber has a maximum amount of damping it can provide (measured in Newtons). Call it 'Y' newtons.
2. A given spring (or in this case, air suspension at a given setting) will provide a maximum of 'X' force against compression (body roll). This force will be released after maximum compression, to be damped by the shock absorber.
3. If 'Z' is greater than 0 in the equation 'x-y=z', there will be excess force not damped which will translate into a body roll onto the opposite side of the car (where the same process repeats).

Given the above, we assumed that the "stiffness" of the air suspension ( how high 'X' is ) is significantly more in access height than in normal height.

Moving over the same speed bump at 10km/hr at an angle, in normal mode vs access mode, we saw one compress-rebound cycle per wheel in normal height mode vs two per wheel in access mode.

10 kph at access height over a speed bump may have had the front shocks bouncing off their bump stops. The bump stops compress significantly, unlike the normal external bump-stops.