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We'll Show You Everything You Need to Know to Go Slow

Posted in How To on November 1, 2001
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When you hear people spouting off ring gear diameters,   this is what they are talking about. Larger-diameter ring gears will usually correspond with a stronger axle assembly. It’s the 9-inch ring gear diameter size that gives the Ford 9-inch axle its name. So immediately you think, “I want an axle with the biggest ring gear I can get!” but that can be a waste. A ring gear with an 8 1/2-9 3/4-inch diameter seems about perfect for maximum durability with a minimum ground clearance penalty. Even with the new generation of shaved housings, too big a ring gear diameter will always reduce your ground clearance. To help illustrate this we called up National Drivetrain and asked them to supply us with a set of 5.13 gears for two different axles. The ring gear on the left is an 8 1/2-inch gear for a GM 10-bolt (a Dana 44 is also 8 1/2 inches). The gear on the right is for a 10 1/2-inch GM 14-bolt (close to a Dana 70). When you hear people spouting off ring gear diameters, this is what they are talking about. Larger-diameter ring gears will usually correspond with a stronger axle assembly. It’s the 9-inch ring gear diameter size that gives the Ford 9-inch axle its name. So immediately you think, “I want an axle with the biggest ring gear I can get!” but that can be a waste. A ring gear with an 8 1/2-9 3/4-inch diameter seems about perfect for maximum durability with a minimum ground clearance penalty. Even with the new generation of shaved housings, too big a ring gear diameter will always reduce your ground clearance. To help illustrate this we called up National Drivetrain and asked them to supply us with a set of 5.13 gears for two different axles. The ring gear on the left is an 8 1/2-inch gear for a GM 10-bolt (a Dana 44 is also 8 1/2 inches). The gear on the right is for a 10 1/2-inch GM 14-bolt (close to a Dana 70).
The 10 1/2-inch 14-bolt has become a very popular axle with guys who want full-floater axle strength that’s easy to bolt into fullsize GM trucks. Typically, this axle’s coolest feature is thought to be the fact that you can get a Detroit Locker for a little over $300 and pop it in. Other trick features are the three-pinion bearing design that the axle uses to keep the pinion from deflecting off the ring gear under high loads. The Ford 9-inch also uses this design, while Dana axles generally space the two rear bearings farther apart to accomplish the same thing. The 10 1/2-inch 14-bolt has become a very popular axle with guys who want full-floater axle strength that’s easy to bolt into fullsize GM trucks. Typically, this axle’s coolest feature is thought to be the fact that you can get a Detroit Locker for a little over $300 and pop it in. Other trick features are the three-pinion bearing design that the axle uses to keep the pinion from deflecting off the ring gear under high loads. The Ford 9-inch also uses this design, while Dana axles generally space the two rear bearings farther apart to accomplish the same thing.
p146194 image large
Here is where the larger ring gear diameter benefits you. Both of these ring-and-pinion sets are 5.13:1 ratio gears. The 8 1/2-inch ring gear on the left uses 8 teeth on the pinion and 41 teeth on the ring gear. The 10 1/2-inch ring gear on the right also has 8 teeth on the pinion and 41 teeth on the ring gear. But because it uses a larger gearset, the 10 1/2-inch gear will have a larger tooth contact and will be stronger, run cooler, and last longer than the same ratio in a smaller ring gear size. Here is where the larger ring gear diameter benefits you. Both of these ring-and-pinion sets are 5.13:1 ratio gears. The 8 1/2-inch ring gear on the left uses 8 teeth on the pinion and 41 teeth on the ring gear. The 10 1/2-inch ring gear on the right also has 8 teeth on the pinion and 41 teeth on the ring gear. But because it uses a larger gearset, the 10 1/2-inch gear will have a larger tooth contact and will be stronger, run cooler, and last longer than the same ratio in a smaller ring gear size.
Can you set up a new ring-and-pinion ratio yourself? Well, for now it’s kind of an “if you have to ask, you probably can’t” kind of question, but some axles are easier to do than others. Unless your axles are from a low-mileage truck, plan on replacing the carrier, pinion, and maybe even outer axle bearings along with all the seals in the axle. You can go down to your local auto parts store to buy all the bearings and seals you need, or you can get an installation kit like we did from National Drivetrain to make it one-stop-shop easy. Even if you are paying someone else to do the install, you can save yourself some money (and ensure high-quality bearings) by supplying all the new parts yourself. Can you set up a new ring-and-pinion ratio yourself? Well, for now it’s kind of an “if you have to ask, you probably can’t” kind of question, but some axles are easier to do than others. Unless your axles are from a low-mileage truck, plan on replacing the carrier, pinion, and maybe even outer axle bearings along with all the seals in the axle. You can go down to your local auto parts store to buy all the bearings and seals you need, or you can get an installation kit like we did from National Drivetrain to make it one-stop-shop easy. Even if you are paying someone else to do the install, you can save yourself some money (and ensure high-quality bearings) by supplying all the new parts yourself.

In every other motorsport ever invented it’s the fastest guy that wins. That’s just the way it is. As a rule people like speed, power, and acceleration. It’s 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 they’ll grip, and you can even scope out the trail ahead of you as you drive, because hey, you’ve got time. You’re 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. It’s this “mechanical advantage” that gives you more power to chug, climb, and crawl over things some people can’t 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 vehicle’s 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 you’re 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 o’clock 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 don’t 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 truck’s crawl ratio is the total gear reduction available between the crankshaft of the engine and the vehicle’s 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 engine’s 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.

Sources

Superior Axle & Gear
Rancho Cucamonga, CA 91730
888-845-0470
www.superioraxlegear.com
National Drivetrain
Chicago, IL 60609
866-427-0080
http://www.nationaldrivetrain.com/
Drivetrain Direct
Corona, CA 92880
909-272-0158
www.drivetraindirect.com
Drivetrain Warehouse
Compton, CA 90220
877-474-4821
http://www.drivetrainwarehouse.com
Reider Racing
877-465-5729
http://www.reiderracing.com

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