MustangForums.com - Torque Bias explained...

...but not by me!

I emailed Torsen inquiring about the difference in driving characteristics between the BOSS 302 and FR500S Differentials and received this excellent reply explaining how Torque Bias (TBR) affects driving characteristics. Also includes an interesting detail regarding the FR500S development history towards the end of the explanation as well. Thought it would be helpful to post it here.
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Thanks for your interest in Torsen. The short answer to your question is that it's complicated.

Much comes down to the nature of the track/course/road being driven, and to the chassis set up of the car in question. What I can offer is that higher torque bias ratios are more aggressive and provide better resistance to inside wheelspin. They also have a greater to tendency to dynamically transfer torque across the axle to influence yaw behavior, in short, it will passively help to yaw a car through a curve. But it does this at greater tendency to induce understeer at turn-in. So, like anything else in a chassis, it becomes a matter of which level of compromise works best for you.

Generally, high torque bias ratios are favored in tight, technical courses or in autocrossing due to the very high dynamic weight transfers that occur. In that situation, you tend to need the aggressive bias ratio to counter the high unloading of the inside tire. But the more resistance to wheel spin that the differential has, the more it will also resist the car's initial rotation about the vertical axis. So, under throttle at corner exit, you'll great management of traction and potentially be able to counteract the car's inherent understeer. But as you set up the corner entry, it will have more of an understeering influence. In an autocross, that can be compensated for with toe settings or tinkering with roll stiffness to levels that you might not do on the street or at an open track event. But generally, corner-exit traction is a higher priority and other compromises compensate. This is really the whole reason we developed the high TBR (4:1) T-2R in the first place. Of course, if you factor in R-compound tires, then the differential doesn't have to work as hard and a high torque bias ratio may not be needed.

On the other hand, on road course - especially a larger venue that is fast and flowing - you can afford to have lower torque bias ratio as there is typically less inside wheel lift. So, you can run a less aggressive differential and have less of penalty at turn-in. To this end, we've often seen TBR levels of 2.5 to 3:1 work well. Less than that and wheelspin rears its head; higher than that and the tendency to push comes back. But again, it is highly dependent on the car and the track. This type of use is why the Boss 302 had the 2.7:1 ratio instead of something close to 4:1. The reason that the FR500S used the more aggressive differential was that the lower TBR version didn't exist yet when the car was developed, so it was tuned around what they had. That might change going forward if Ford Racing develops a replacement based on the S550 chassis.

Best regards,

Rick Barnes

JTEKT Torsen North America, Inc.
Rochester, NY
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