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Applications for Extreme Low Gearing |
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First: read the section on Extreme Low gearing in "A Systems approach to 4WD gearing" article.
Some additional applications for Extreme Low Gearing:
Automatic Trannies & Extreme Low Gearing:
When used with an Extreme Underdrive or Dual T-case, compression braking problems are a thing of the past. Coupled with a correct match for the torque converter, and a well designed cooling system, this is a completely road-able option that is wonderful in all types of off road use. A 4spd automatic trans coupled to such an underdrive allows 80 mph on the freeway at around 2500 RPM, while still having crawl gears over 140 to one: with an automatic, this is almost slower than the hour hand of your watch.
Extreme Short Wheelbase:
This tends to limit what you can do. A short manual trans and transfer case is about all that will fit without a super short drive shaft. One answer we have seen that worked extremely well in an early Flat-fender: Chevy V8, a granny SM420 trans, A Dana 18 Tcase with 3.15 gears in it, plus a Saturn overdrive unit on the back of the Tcase. Use about 4.56 Gears with 35” tires. The SM420 is 10.5” long, the Dana 18 is around 10.5 inches long, and because the Dana 18 has an offset rear driveshaft, the overdrive unit can be built to fit on the rear of the Tcase, not using up any driveline length. Engine to rear Tcase yoke is about 28 inches, and you get 16 forward gears. With the extreme low 1st of the 420, and the 3.15 Tcase gears and 4.11 axles, you get a crawl ratio of about 101.2 to one. The over drive can be used to split the super wide ratio gears (1st gear, then 1st over, then 2nd gear, then 2nd over, and so on) so you do not have holes between the gears. And with the overdrive and 4.56 gears, you have good highway drive-ability.
Extreme Underdrive and Close Ratio 4 Speed:
Use the axle ratio that is best for highway driving. With up to four different transfer case ranges to choose from, you can still use a more close ratio trans for excellent road-ability, and have the right gears for any kind of wheeling. Example: A jeep wrangler with a stock 4.0 and AX15 trans, add an extreme underdrive, use 4.56 axle gears and 35 inch tires. The highway RPM is roughly stock, drivability is roughly stock, you still have stock low range for use on forest service trails or sand, and you have 10 more gears below that for rock crawling. A final crawl ratio close to 190 to one in this system!
Wide ratio 5 speed with 3.8 ratio Atlas transfer case:
if you stick with the not-extreme wide ratio 5 speed, you get decent drivability. With the 5 speed in overdrive and Atlas Low range, or Atlas in High range with 5 speed in 1st, there is enough gear overlap that you don’t have a “hole n the middle “ effect
The 4.3 Ratio Atlas really is too low gears for general purpose wheeling. But add a 2:1 dual T-case box or 2.7:1 Extreme underdrive in front, and now you have 4 ranges to choose from. (Example: With the 2:1 dual Tcase you would get 1:1, 2:1. 4.3:1 AND 8.6:1 ratios available.)
Excessive driveline angle on a real short driveshaft:
(Actual Wrangler/Klune install) Clock the transfer case more flat. This allows it to sit lower in the belly pan by 2-3 inches, and points the rear yoke more at the rear axle. Use a CV driveshaft and slip-yoke eliminator on the Tcase. Tip the rear pinion up to point at the rear Tcase yoke. Result: Yes, the driveshaft was only 10.5 inches with the Klune installed, but the angle on the CV joint was only 9.5 degrees (factory design range is 6 - 12 degrees) at ride height with a 4” lift. By getting everything dialed in exactly right, there was no objectionable vibration, and the system ahs worked trouble free for over 20,000 miles at the time of this writing.