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How to calculate your tow truck's towing capacity: One simple formula

Posted by Justin Cruse on July 6, 2017

We're going to teach you how to find out the towing capacity of your tow truck. (Hint: It's a formula. But don't worry. We've put it all into 7 easy steps.)

Is it worth doing the math?

Yes! It all comes down to safety. A tow truck is like a big lever with the fulcrum (or pivot point) at the rear axle or axles. When we load weight behind the rear axle (or axles), we unload weight from the front axle. (Ever seen a tow truck that looks like its front wheels are barely touching the ground? That's a really unloaded front axle, and a very unsafe situation.) Unloading your front axle directly affects the braking, steering and handling of your tow truck. That's the safety part. But unloading your front axle also reduces equipment life, since it risks overloading the frame of the chassis, the lifting devices, suspension components and the tires.

Doesn't my owner's guide tell me my max towing capacity?

Not really. If you calculate the actual towing capacity, you may get a substantially lower or even higher number than the guidelines state, since how you set your overhang will dictate the capacity. You may have your overhang extended as far as possible, but the max towing capacity in your owner's guide will assume your overhang is at its shortest possible length.

 

This towing capacity formula will give you the front axle weight of the casualty you can safely tow under ideal driving conditions.

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STEP 1: Weigh the front steering axle of your truck. Don't use the gross weight figure that's in your truck manual. We need the static weight, which means you'll have to weigh the front of your truck.

STEP 2: Measure your wheel base.
Measure from the center of your steer axle to the center of your rear drive axle.

STEP 3: Measure your overhang. Measure from the center of your drive axle to the actual lifting point for the casualty you're lifting. Note that the point of lift depends on whether you're using your tow sling, the receiver forks or your wheel grid. For example, the lifting point is the center of the L-arm on a wheel-lift, the center of the frame fork, and the center of the tow bar. You'll want to calculate your maximum tow capacity when your overhang is retracted, halfway extended and fully extended, since each position results in a different tow capacity.

STEP 4: Divide the weight of your front axle by 2.

STEP 5: Multiply the result of Step 4 by the wheel base measurement you calculated in Step 2.

STEP 6: Take the result of Step 5 and divide it by your overhang measurement from Step 3.

STEP 7: Confirm that the front axle weight of the casualty you want to tow does not exceed the result of the calculation you did in Step 6.

 

Here's an example. 

STEP 1: Let's take a tow truck with a front axle static weight of 4600 lbs.

STEP 2: The wheel base is 160 inches, measured from the center of the drive axle to the point of lift.

STEP 3: We've measured the overhang, and it's 90 inches.

STEP 4: Divide 4600 by 2 and we've got 2300 lbs.

STEP 5: Multiply 2300 by 160 and we get 368,000.

STEP 6: So now we take 368,000 and divide it by 90.

STEP 7: The resulting number, 4089 lbs, is the towing capacity under ideal conditions at that particular overhang. That means you can safely tow a casualty with a front axle weight (not total weight) of 4089 lbs. We confirm that our casualty's front axle weight doesn't exceed that...and since it's a sunny day with dry roads, we're good to go.

 

How can you increase the towing capacity of your tow truck?

Since your tow truck is a lever, you can increase your tow capacity by reducing your overhang and (or) extending your wheel base. You can also add weight to your front axle to increase you tow capacity, but be careful not to exceed the gross axle weight ratings.

Do these calculations ahead of time, for all the different overhang and front axle weight scenarios for your truck, and keep them handy. That way you can tow safely and efficiently the first time, every time.

Topics: Useful towing tips

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