Inertially reacted system & fully constrained system

[mach11]

Hi all,

Quarter Car test rig - the FAW for one wheel - is simulated by adding ballast weights on the sprung mass plate.

Single corner test rig - (3 directional load simulation namely vertical,lateral and longitudinal) here the entire chassis (frame of the vehicle) is made completely rigid by clamping it to a rigid box. The vertical, lateral and longitudinal loading is done using a modular fixture and loading arms connected to hydraulic actuators. Here mass is not added

I would like to know what is the difference between the fully constrained system and inertially reacted system.

thanks in advance.

[DaveW]

I would like to know what is the difference between the fully constrained system and inertially reacted system.

I'm not quite sure that I understood your question. If I can simplify it a little, are you asking for comments on a "fixed" damper dynamometer compared with a "floating sprung mass" dynamometer. Both types are shown in videos listed here.

[mach11]

I would like to know what is the difference between the fully constrained system and inertially reacted system.

I'm not quite sure that I understood your question. If I can simplify it a little, are you asking for comments on a "fixed" damper dynamometer compared with a "floating sprung mass" dynamometer. Both types are shown in videos listed here.

Hi Dave...
Thank you for your reply,

Here are a few pictures of the test rigs that I am referring to. Took it from the Internet.

Here the picture shows the hydraulic actuator being used to simulate road bumps (vertical) to the suspension system. This is an inertial reacted system. Sprung mass are simulated using ballast weights added to the sprung mass plate.

In a fully constrained system, the front suspension system assembled on to the chassis (frame) is fully constrained ( clamped, mounted, bolted) on to the bed plate to remain rigid. Something like the 9 DOF full car road simulator but with only one quarter module evaluated only on 3 DOF system (vertical - Road bumps, lateral - cornering and longitudinal - braking), but keeping the car completely rigid.

[DaveW]

I think your first (inertially reacted) example would almost certainly be controlled be the wheel actuator with a vertical displacement (or velocity) time history, representing a road input. It is quite useful for exploring the characteristics of suspension elements and the effect of (some) parameters (e.g. sprung & unsprung mass, spring & damper values) on the response of a vehicle. It is rather limited (even confusing) when it comes to "setting up" a complete vehicle, however.

Your second example is, in my experience, a test used mainly for durability work. In which case the input actuators would be controlled to maintain specified load time histories. Very useful, but as one engineer admitted to me, it can generate failures that are not found in service, and it doesn't expose some failures that are found in service.

[mach11]

I think your first (inertially reacted) example would almost certainly be controlled be the wheel actuator with a vertical displacement (or velocity) time history, representing a road input. It is quite useful for exploring the characteristics of suspension elements and the effect of (some) parameters (e.g. sprung & unsprung mass, spring & damper values) on the response of a vehicle. It is rather limited (even confusing) when it comes to "setting up" a complete vehicle, however.

Your second example is, in my experience, a test used mainly for durability work. In which case the input actuators would be controlled to maintain specified load time histories. Very useful, but as one engineer admitted to me, it can generate failures that are not found in service, and it doesn't expose some failures that are found in service.

Hi Dave,

Mostly a Quarter car rig is used in product development to provide designers with inputs at an early stage. The advantages of this rig is that with suspension hard points alone one can begin durability trails.

In the second test rig, in my experience I have found failures that like you mentioned have not been found on any vehicle in service. Also, main purpose was to evaluate the suspension with durability in mind.

I would like to know,

In the quarter car rig, the interaction between the sprung mass and the unsprung mass can be studies much more than the other test rig mainly because, the system behaves more closely to the actual vehicle.

But in my other example, this interaction is zero. thereby completely nullifying the effect of the damper and the spring. Am I right?

[Greg Locock]

The quarter car simulator is a technology that is being pushed by MTS, but comes with so many provisoes I'd be inclined to give it a miss. The main issue is that the relationship between measured contact patch loads and the loads you need to drive the rig is heavily modified by the completely diffeerent compliance and damping characteristics of a rolling tire compared with a stationary one. I have seen this exercise done and it is not a simple procedure, or really, any procedure at all, more of a case of we'll try this and see if that works.

The road load simulator in the lower picture can be used to test the durbality of the entire suspension and subframe and even body, with some caution. Since it applies 6 axis forces it tests the suspension properly.

The advantage of both rigs is that they run 24/7 and can use accelerated test schedules (ie cut out the boring bits) or even cleverer ways of reducing the test time.

[DaveW]

On the specific subject of damper durabilty testing, I was once provided with a test spec that was very clearly designed to test "boulevard cruiser" dampers (with very light damping forces). I protested that it was not really suitable for testing serious dampers, & received the reply that the OEM had had no damper issues since the specification had been introduced. Ultimately, the dampers did pass the test and everyone was happy, with the possible exception of the damper supplier, who had to water cool the test dampers, and interrupt the test frequently to let the dampers cool down.

In the quarter car rig, the interaction between the sprung mass and the unsprung mass can be studies much more than the other test rig mainly because, the system behaves more closely to the actual vehicle.

I suppose the quarter car rig would be one way of ensuring that a damper durability test would be more respresentative of a real service use, although a simple model would have served equally well (but perhaps not quite as convincingly as a rig). One theoretical advantage of the rig is that responses adjust as the hardware changes, but that might not be permitted in the test specification.

But in my other example, this interaction is zero. thereby completely nullifying the effect of the damper and the spring. Am I right?

You are correct, but if load histories used to excite the specimen have been measured correctly then there is not much to chose between them. As Greg points out, the advantage is that suspension forces can be applied in combination.