Inertia Damping

[johnc]

From the latest mark Ortiz newsletter:

 

TORQUE TUBE FOR FRONT-ENGINE IRS CAR

 

I wonder if you could consider answering this question on the reasons makers use torque tubes please?

 

Car in mind is a Mazda RX-7 FD model of mid nineties. They run what Mazda term a "Power Plant Frame" which is a lightweight pressed steel frame tying the gearbox extension housing to the front of the differential casing. The car has independent rear suspension by double wishbones. Why would a maker choose this means of locating the rear of the gearbox and the nose of the diff instead of using a gearbox X member and supporting the diff nose of the rar subframe? Some quite low end Opels used a torque tube, as did some of the bigger Peugeots like the old RWD 504. If one were to fabricate conventional mounts for the diff nose using a weld in multi point roll cage to tie the shell

 

together more rigidly, and did something similar with a different but similarly sited gearbox (thinking engine and box change to something with pistons for circuit race usage), what disadvantages can you think of, handling wise? Any? My personal take on this is that it may have been instigated to allow compliant diff mounting to reduce NVH, yet control unwanted diff nose movement to control drive train wind up and wheel hop. I can't find much on the pros and cons of this setup.

 

There are basically two reasons for tying vibration-isolated components together with subframes. The first is to allow soft mounts for good isolation, without incurring undue movement of the individual isolated components. The second is to unite the isolated components as one large mass, which can then be used to achieve a measure of inertia damping by tuning the natural frequency of this mass to interfere with the natural frequencies of the sprung structure as a whole.

 

In the case of a torque tube or similar structure tying a front engine and transmission to a sprung differential, not only do we suppress side-view windup of the diff in response to axle torque, we also create a structure that resists front-view rotation of the engine and diff relative to each other due to driveshaft torque – the same torque that creates torque roll and torque wedge when the diff is unsprung. This relieves the frame or unibody of the need to resist this torque.

 

In most cases, there is a penalty in space efficiency and weight efficiency for using subframes, although to some extent this can be recovered in the main structure by either eliminating loadings as just described, or at least spreading loadings among a smaller number of more widely separated points.

 

When the suspension is softly damped, in pursuit of soft ride, inertial damping from major isolated components can actually help handling as well as ride, as least in terms of the car's behavior on irregular surfaces. But in race cars, we normally use stiffer shocks, which make all tuning of natural frequencies less important, and we want light weight and a stiff overall structure much more than good NVH characteristics.

 

Therefore, traditionally, we build race cars with little or no isolation of the engine, trans, and diff. We solid-mount everything, try to get some structural stiffness gain from the components, and deal with any NVH issues by having a loud exhaust and wearing earplugs. If the shocks are stiff enough for good handling, there won't be much oscillatory behavior from the suspension.

 

However, in recent years race car designers have taken a fresh look at inertia damping, as readers will know who have followed inertia damping's recent introduction, and prohibition, in F1. The reason for the renewed interest in inertia damping is that modern high-downforce race cars have such stiff springing that tire deflection becomes a very significant portion of the total "suspension" compliance: the tires deflect anywhere from half as much to just as much as the suspension proper.

 

The tire deflection does not displace the shock absorbers, so the shocks can't damp it. There is some internal damping in a tire, but nowhere near as much as we'd like. A very stiffly sprung car can

 

bounce or pitch on the tires much like a tractor (although with smaller amplitude and higher frequency), and the shocks can't suppress this – hence the interest in inertia damping.

 

So if the car is going to have ground effects and wings, and very stiff suspension, there could be a case for trying to use compliant mounting to get some inertia damping. This could involve using a torque tube, or not. It is common practice in live-axle passenger cars to use the engine/trans assembly for inertial damping, without a torque tube. It would be quite possible to do that in a race car with a sprung diff, and solid-mount the diff. The only downside would be that the engine/trans assembly will undergo rotational displacement on its mounts when it applies torque to the propshaft, and all packaging, plumbing, wiring, and linkages will have to accommodate that movement. This can be mitigated somewhat by wide spacing of the motor mounts.

 

It should be mentioned that we don't necessarily get inertia damping from compliant mounting of a major mass. If the frequency is not tailored to the tire and suspension frequencies, it is quite possible to get reinforcement of suspension and tire oscillation, rather than interference. So this would not be something to be undertaken lightly. For most of us, the most prudent recommendation is still to go for rigidity, lightness, and simplicity, and solid-mount everything.