Overall gear ratios play an extremely important part in the use of your off-road ride. Though 4WD owners have many more possibilities for gear-ratio manipulation, 2WD off-roaders manipulate with their gears, too. The final gear ratio dictates how much control you have over your wheel speed, and how smoothly you can ascend or descend certain obstacles. Lower final ratios (higher numerically) will slow down wheel speed and also produce more applicable power (torque) to turn the tires. Higher gear ratios (lower numerically) will raise wheel speed but decrease the amount of available torque to turn the wheels.
Off-roaders almost always head for lower gear ratios. This will limit the overall top speed (engine rpm will be the limiting factor) but give more power for faster acceleration at any speed until the truck tops out. Not only that, but lower gearing behind the transmission will take stress off the tranny and engine by easing the workload through gear reduction.
In our April 2010 issue, we showed you ways to reduce drive ratios in your 4x4s with two-or-more speed transfer cases. But we missed showing you one very excellent method of gear reduction: The Klune-V extreme underdrive box. The Klune-V is a unique two-speed unit that goes in between the transmission and transfer case in a 4WD vehicle. It uses planetary gearing to offer a reduced gear ratio and is available in two models; The Goliath Klune-V has a 2.7:1 reduced gear ratio and can handle up to 3,300 lb-ft of transient load input torque, while the David Klune-V has a 4:1 reduction gear and can handle up to 2,000 lb-ft of transient load input torque.
And, in fact, a Klune-V was the perfect solution to fix and finish our '79 fullsize Cherokee on 47-inch tires, Matilda. Our project truck suffered from a few different problems that the Klune would solve. The first was that, even with 5.13 axle gears, the 2.6:1 Dana 300 T-case was not geared nearly low enough to properly control 47-inch tires on trails at low speeds. The second problem was that the inadequate gearing led to the Torqeflite 727 transmission working too hard and failing due to overheating. The final problem was that we weren't able keep a front driveshaft in the truck because of the steep angles of the 'shaft. The Klune-V mounts in between the transmission and transfer case and subsequently necessitates a longer front driveshaft (and 6.5-inch-shorter rear driveshaft). The longer front driveshaft would rid us of our 'shaft angle issues (allowing us to use four-wheel drive on the trail again), and the rear 'shaft length would stay more than long enough after the 6.5-inch-long Klune-V was added in.
Greg Miller of High-Impact Transmission and Gear recommended using the David 4:1 Klune-V since our lowest final ratio was still very far off what we wanted and because it has a recently redesigned 4:1 gearset that is much stronger than the original. We couldn't wait to put some abuse on it! The 4:1 Klune combined with the 2.6:1 low range of our Dana 300 would give us a 10.4:1 gear reduction between the transmission and the driveshafts. Miller also recommended upgrading our Dana 300's input and output shafts to 32-spline units (it has a 23-spline input, stock) using an Advance Adapters 32-spline Output kit with a CV yoke. He said we might very well break the 23-spline input with the 4:1 low range reduction of the David Klune-V, considering the tire size, weight, and power of this vehicle.
Our final package made our giant fullsize Cherokee usable on trails and in rocks. It didn't affect the way it drove on the street, and only added 50 pounds to our truck. And so far the David Klune-V (the slightly weaker of the two Klune-V units) has been working out great in our overweight, 47-inch tire fullsize Cherokee with a 400 hp engine under the hood.
So How Does It Work?
We've had the David (4:1) Klune-V extreme underdrive in our giant fullsize Cherokee for a couple months now. We've taken multi-hour roadtrips to trailheads, and had it in some precarious positions in some nasty off-road spots where wheel speed control was ultra important.
We may be using the word "control" too much throughout this story, but that is the one word that comes to mind after using a Klune-V in our ride. Almost all the gear slop seemed to go away (there is a lot with 47-inch tires on the ends of the axles) when in low-low, and we didn't have to feather or throttle the accelerator when overcoming obstacles. A nice steady forward motion was a welcomed addition to our off-roading adventures.
As for the drivability on pavement, it didn't have a single effect. There was no extra gear slop, and no extra noise from an extra gearbox being installed in the drivetrain.
Final Gear Ratios
With the 4:1 Klune-V David extreme underdrive and 2.6:1 Dana 300 low range, we have four different gearing ratios available between the transmission and driveshafts.
|KLUNE-V DAVID 4:1||DANA 300 T-CASE 2.6:1||OVERALL RATIO|
Will Gear Reduction Increase Breakage?
We were curious about this ourselves, as we've definitely experienced more breakage in trucks when putting more power under the hood. Wouldn't adding lower gearing increase torque, and therefore lead to a higher potential for drivetrain breakage? We asked Greg Miller of High-Impact Transmission and Gear his opinion on the subject, and he got back to us with this answer:
"It has been my experience that you break less, not more, when you gear way down. First of all, you can never develop more torque on a component than what the tires will make traction for. And in most cases, you can already make enough torque to break all four tires loose with a standard low range if you punch the throttle hard enough.
"Secondly, leaping, thrashing, and making and breaking traction puts surge loads on parts when you're trying to hurry over an obstacle (due to lack of throttle control). This does not happen when you are geared down and controlling a slow ascent or descent."