We'll Show You Everything You Need to Know to Go SlowPosted in How To on November 1, 2001
In every other motorsport ever invented its the fastest guy that wins. Thats just the way it is. As a rule people like speed, power, and acceleration. Its just adrenaline-junky human nature. So it can seem odd to the rest of the world that the top wheelers are usually the guys that crawl the best. When you crawl over obstacles, tires have a much higher probability of getting traction. So the slower you can go the more likely you are to be able to put the tires where theyll grip, and you can even scope out the trail ahead of you as you drive, because hey, youve got time. Youre crawling!
So how do you crawl better? You employ one of the oldest tools ever used by man: leverage. When people talk about having low gears (numerically high numbers), what they really have is lots of leverage on the tires. When you shift into First gear, you let the engine get more leverage on the tires than it would have in Fourth gear. Its this mechanical advantage that gives you more power to chug, climb, and crawl over things some people cant walk over. When people say they have a gear ratio of 4:1, it means that for every four turns you put in to the gears, you get one turn out, but with four times as much torque. This holds true whether the gears are in a transmission, a transfer case, or an axle.
The only downside to low gears is that they will limit your vehicles top speed and they will increase the speed that your engine must operate at for a given road speed. We thought you might like a little reference material to keep in the garage or on the inside of your textbook to refer back to when youre daydreaming about gears.
Want to Know Your Axle Ratio?
You can look for a tag under one of the differential bolts, a sticker with the ratio written on it, or even look at the window sticker for that matter, but if you want to know without a doubt what axle ratio your truck has, try the following simple test.
Chock both front tires and then raise both rear tires off the ground. Shift the transmission and transfer case into Neutral, and, using a piece of chalk, mark a line on the driveshaft (near the rear U-joint) as a reference mark and a corresponding one on the differential. With the help of a friend, rotate both rear tires one full rotation, making sure you turn both the tires the same direction and at the same speed. It may be helpful to mark a line on the tires at the 12 oclock position to ensure you spin the tire the full 360 degrees. As you rotate the tires, count the number of times the mark on the driveshaft passes the reference mark you made on the differential housing. If the driveshaft rotates slightly more than three times, you have 3.08 gears. Close to 3 ¾ turns of the driveshaft would mean 3.73s, and slightly more than four turns means 4.10 gears. This technique works as well in the junkyard as in the driveway, and you dont even have to pull the differential cover. Shoot, you can even use it to approximate transmission or transfer case ratios if you can turn the input shaft while counting the number of turns the output shaft makes.
How to Calculate Your Crawl Ratio
A trucks crawl ratio is the total gear reduction available between the crankshaft of the engine and the vehicles tires. You can figure out the crawl ratio for your combination by multiplying the first gear reduction of your transmission, the low-range ratio of the transfer case, and the ring-and-pinion ratio in your axle. If you have some type of crawler box or gear reduction hubs (like Hummers and Unimogs) added into your drivetrain, you would multiply by that gear reduction ratio as well. An example crawl ratio combination for a vehicle with an SM465 transmission, an NP208 transfer case, and 4.10 axle gears:
6.55 (First gear) x 2.61 (low-range) x 4.10 (axle ratio) = 70 (crawl ratio)
What that means is that the engines torque is 70 times greater at the rear axleshaft than it is at the crankshaft when the transmission is in First gear and the transfer case is shifted into low range. This gear reduction is what helps you crawl over obstacles at a slow and controlled pace because the axleshafts are now turning 70 times slower than the engine rpm. The torque converter of an automatic transmission can add an additional hydraulic gear reduction that is generally considered to be a 2:1 ratio.