BasicsMar 11th, 2016 | By Editorial Staff | Category: Jack Schrock's Blog
Assuming your HD underlift has the power to do so, its lifting power will depend on several factors — the wheelbase of your tow truck, the front axle weight of your tow truck (not the rating but actual empty weight), and the distance the underlift is extended from the center of the real axle or center of the tandems to the pick point on the casualty.
Remember, the formula for determining lift and tow capabilities is ½ Front Axle Weight (FAW) X Wheelbase (WB) divided by Distance to Load or (DTL). Now a quick comment about wheelbase. It is measured from the centerline of the front axle to the centerline of the rear tandems, or, the center of the rear axle — if only a single axle. With three rear wheels (axles) down in back then the centerline of the middle wheel is the measuring point back to the center of the front axle.
So, the more rear wheels you have on the ground the more weight is removed from the front axle and that decreases the lift and tow capability of your underlift. Also, if you’re extending the underlift to pick at a set-back axle, the lift and tow will be decreased. Retract it and lift will be increased.
Look at it this way, a HD underlift is like a teeter-totter. Have junior sit way out on the end and only his weight will pick up dad, who is sitting in close.