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Here's How An Old Toyota Pickup Can Climb Anything With A 511:1 Crawl Ratio

Illustration for article titled Heres How An Old Toyota Pickup Can Climb Anything With A 511:1 Crawl Ratio
Screenshot: Husoku (https://www.youtube.com/watch?v=6LoQTme51tA)
Truck YeahThe trucks are good!

This little Toyota Hilux barely makes any power, and yet it can still climb a steep obstacle on the Rubicon Trail with ease. That’s because the ultimate solution for a lack of grunt is gearing. Let’s go over how it all works.

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This 1981 Toyota Hilux doesn’t look like much to most people. And indeed, if you looked under the hood, you probably wouldn’t be particularly impressed. Yes, the original 100-ish horsepower 22R four-banger is gone, but the 3RZ-FE yanked from a T100 isn’t going to light up the tires either with its 150 ponies and 177 lb-ft.

But that’s okay, because off-roaders don’t need a lot of motor when they’ve got an insane Marlin Crawler Ultimate transfer case:

A reader named Paulo sent us this video in tips after this clip went up on Facebook showing the truck doing the slowest burnout in history:

We’ve explained the concept of off-road gearing before, but here’s a refresher. By sending an engine’s power through short gearing in a transmission, transfer case and differential, you can essentially trade RPM (which corresponds to vehicle speed) for torque, which is what is needed at the wheels to accelerate a vehicle over any obstacle.

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Put more simply, if you take this Hilux’s 150 horsepower at 4,800 RPM, and send it through a bunch of gearing like “BigMike” did in this Hilux, you can get an absurd amount of thrust at the wheels. In this case, based on Marlin Crawler’s comment on Facebook, the gears in question are three transfer cases with gear ratios of 4.7, 2.28 and 2.28, a 5.29 rear differential ratio and a 3.95 first gear ratio. Multiply these ratios together, and you get the “crawl ratio,” which is the ratio of torque at the wheels to torque at your engine’s flywheel: 511.

So imagine this truck is turning at 4,800 RPM, where it makes a maximum of 164 lb-ft of torque (this corresponds to a max output of 150 horsepower). Send that torque through the 3.95 first gear, then through the three transfer cases, and through the rear diff, and you wind up with 164*511 = nearly 84,000 lb-ft of torque. Divide that by the tire’s radius to get the forward thrust (because force is a torque divided by a moment arm), and you get about 57,000 pounds. That’s almost literally unstoppable.

Granted, because power—which is proportional to RPM * torque— is conserved through the gearboxes, that increase in output torque is accompanied by a corresponding decrease in angular velocity. So the engine might be turning at 4,800 RPM, but the rear wheel is turning at 4,800/511 = 9.4 RPM. And considering 35-inch tires make about 593 rotations per mile, this means the vehicle will travel 0.95 mph at 4,600 rpm.

God that’s slow. But steady, at least.

Sr. Technical Editor, Jalopnik. Always interested in hearing from auto engineers—email me. Cars: Willys CJ-2A ('48), Jeep J10 ('85), Jeep Cherokee ('79, '91, '92, '00), Jeep Grand Cherokee 5spd ('94).

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DISCUSSION

areisengineer
AreIsEngineer

It is unlikely that the axles are ever seeing 84,000 ft-lbs individually. Let’s assume a ~1.75" axle diameter made out of a nice through hardened alloy @200 ksi tensile. Those axle’s would pop at about 17,500 ft-lbs. Let’s assume locked diffs all around and an equal distribution of torque (with no wheel slip) and you’re at a max of 84,000/4 = 21,000 ft-lbs provided to each axle (at peak torque). In these conditions, assuming he doesn’t want to permanently deform his axles (yield @ .85*200ksi = 170 ksi), he would need to stay at ~ 71% of peak engine torque. According to internet 3rz dyno pulls, this is probably around ~1300-1500 RPM. In this situation (and based on my assumptions for shaft size and material), if he floored (or even went over 1500 RPM) - he would probably twist an axle or two.

Cool!