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Nuts & Bolts: Double Transmission

Posted in How To: Tech Qa on February 5, 2019
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There has been a lot of talk about which transmission or transfer case is best, but no one is talking about the ultimate selection of gearing. What about coupling two transmissions together before the transfer case? I did that 50 years ago and it was a blast. Remember, double reverse is forward. Gearing selection is incredible and a lot of fun, but remember that double reverse is forward.
Bruce J.

You have an intriguing idea, but also a difficult one to pull off. The two biggest challenges in mating two transmissions is how you would mate two transmissions together and the excessively long drivetrain length once you were done. Truck transmissions are designed to bolt directly to the back of an engine, and there’s not a “divorced” transmission that we’re aware of rated for highway speeds and the torque loads of a typical truck.

With enough time, tools, and machine work, anything is possible, but then there’s the secondary issue of torque multiplication. Say you manage to put one transmission in front of another, and the front transmission has a First gear ratio of 3.50:1. In that gear you’re feeding the input gear of the second transmission 3 1/2 times the torque that it would if it were connected to an engine, as well as everything else down the line. That’s not a big deal poking around on flat ground, but put the truck in a bind and stuff will go snap! fairly quickly. Even with enough beefed up components, you end up with an absurdly long drivetrain combination that could only work on a longbed truck with any kind of lift. There’s also the matter of making all of the shifters reachable from the driver seat. It’s just not very feasible from a practicality standpoint, especially when you have much more compact multispeed transfer cases, underdrives, and range boxes that offer similar gearing combinations as a double tranny but without the packaging concerns. We have to admit, though, that double reverse to go forward would be kind of cool.

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