A couple weeks ago there was some discussion about how locking the transfer case differential affects turning radius.

Last week, Doug Z. and I did some field testing, and now have some objective numbers.

Setup:

We did the testing in a fairly level gravel parking lot. We used three Hummers for testing. We measured the diameter of a full circle made by turning the Hummer steering wheel all the way to the stop. All measurements were made of the circle made by the outside front tire.

As might be expected, there were some slight variations in the actual diameters due to the adjustment of the steering stops.

We did not compare right vs. left turning, nor actual radii, as we were attempting to measure the percentage differenct between locked and unlocked t-case settings.

Measurements made with Low (unlocked) vs. High (unlocked) show no significant difference. Low Locked was used for all "Locked" measurements. We did not take measurements in HL, but anticipate that they would be virtually identical to the Low Locked measurements.

Measurements:

With my Hummer, a moderately loaded wagon, we measured about a 9% reduction in turning radius.

With Doug's Wagon, which is slightly less loaded than mine, we measured about an 8% reduction in turning radius.

With Doug's completely unloaded 2-door, we measured about a 3% reduction in turning radius.

All circles were between between 45 and 55 feet in diameter (not radius). All measurements are accurate to about the nearest 1/2 foot (i.e. +/- 3").

Analysis:

When walking along beside the trucks, the front wheel scuffing is plain to see and hear.

Our best conjecture about the difference in % change between the trucks has to do with the amount of weight on the rear axle. Since traction is directly related to weight, we presume that the differences were caused by a difference in traction.

Specifically, the more traction the rear has, the more the front tires must slip/scuff to complete the turn, the larger the difference in the turning radius. The less traction the rear has, the more it can slip/scuff, and the less effect it has on the turning radius.

All of these measurements were made under controlled, nearly ideal conditions. Under some conditions on actual trails I have observed differences that I would (subjectively) say were much worse than the 8% - 9% differences noted here.

Comments and independent verification of these numbers are welcomed.

Dave Breggin
'95 Diesel Wagon
dgb@lucent.com