More on HD Ratings

Jul 20th, 2012 | By | Category: Jack Schrock's Blog

As a follow up to the previous blog, the HD tow truck driver can reduce the load on the rear tandems of his wrecker by simply retracting the underlift as much as possible. This transfers weight from the rear to the front of the wrecker and that increases legal towing capacity.

Now, the most confusing topic is the matter of adding rear axles. To begin, let me say that we add rear axles to “legally handle more towing weight” but in the process we actually “decrease the actual towing weight capacity.” Yes, this is confusing, but let’s examine the facts.

The most efficient HD towing vehicle is equipped with a single rear axle chassis. To better understand, the towing formula is (½ front axle weight times wheelbase divided by overhang equals what you can pick and tow with your rig). When the front axle of your tow truck increases (by adding weight, etc.), you increase towing capacity. By increasing wheelbase you also increase towing capacity. And, adding rear axles such as lift axles or tandems decreases wheelbases and that correspondingly decreases lift and tow capacity. So, we add rear axles to get through the scalehouse and eliminate rear axles to increase lift and tow capacity. That’s why the oilfield rigs are mostly super HD Macks with single rear axles.

Now, along these same lines, too many HD wreckers are mounted on retired road tractors equipped with air ride suspension systems, which means the chassis is sitting on a series of air bags that inflate when loaded. So, the air bags on the forward rear drivers also inflate as the wrecker is loaded and that means the forward axle is now pushing down with even more force and it is that downward force that takes weight off of the front axle of the tow truck and that weight is all moved to the rear … and that adds more weight to the already overloaded rear drivers. Some guys have been known to play games with air bag suspension systems, but that’s tricky and illegal so skip that option entirely.

Well, we’re out of space again, but I just wanted to further clarify a complex topic.