There are three main types of tie-downs: cam straps, over-center lever cam style straps and ratchet straps.
Cam straps use a cam to secure the webbing that is threaded between the cam lever and the housing or frame. Cam straps have a knurled, spring-loaded cam that prevents the web from slipping after you pull the strap tight. The cam mechanism is designed such that the greater the tension in the webbing the greater the grip on the webbing. The surface of the cam lever that contacts the webbing is usually knurled to improve the grip on the webbing. The cam strap is tensioned by pulling the free end of the webbing through the mechanism until the necessary tension is achieved. It is released by depressing the cam lever.
Over-center lever cam style straps rely on the friction between opposing surfaces of the webbing resulting from compression of the webbing. Like the cam straps, the mechanism is designed such that the greater the tension in the webbing the greater the grip on the webbing. When the fastener is “open” you can feed the webbing through the fastener. Because of the complex threading required to make the mechanism function, the length and tension of over-center lever cam style straps are more difficult to adjust. Because of the greater leverage provided by the lever, it is easier to tension the strap.
The tensioning method provided by this mechanism makes it easier than the cam fastener to get more tension into the strap and to keep it there. When the webbing has been pre-tensioned to the appropriate position, “closing” the lever will add tension to the webbing and hold that tension tightly in place. It is released by ”opening” the lever and pulling the webbing back through the fastener.
Ratchet straps use a ratcheting slotted drum and a ratchet crank handle to tension the strap. Ratchet straps make it easier to up the slack in the webbing than the over-center lever cam style tie-down straps. Ratchet straps make it easier to tension the strap than the cam strap style of tie-down. Once the webbing is threaded through the slotted drum, the ratchet crank handle is used to take up the slack and tension the strap to the desired level. The ratchet lever is then rotated to the “closed position”. It is important that there is at least one complete wrap of the webbing around the drum since this type of mechanism relies on the friction between the wrapped layers of webbing to keep the webbing from slipping through the slot when under tension. Too many wraps of webbing around the drum may cause the mechanism to jam. Releasing the tension is a simple matter of pulling the center latch and rotating the ratchet crank handle to the “open” position and pulling the webbing back through the slotted drum.
Replace your straps when the webbing shows sign of wear or when the hooks have become misshaped. In the case of ratchet straps, maintain the ratchet mechanism with periodic lubrication of the moving parts.
It is important to understand the how the positioning of the tie-down straps affect the stability of the VBL, the down force on the VBL and tension in straps.
It is best to have symmetrical or mirror image geometry for the tie-downs both left and right and front and rear with equal tension (Figures 1 & 2.). The VBL will not migrate when transported if the tie-downs are balanced and have equal tension. Since optimal placement is not always possible, we have described some important considerations for tie-down placement. It is more important to have the opposing lateral forces nearly equal, than to have the down forces equal. If you have unequal lateral forces the VBL will tend to move until the forces equalize. There may be times when it is impossible or impractical to have your tie-downs symmetrical. In the case of pickup trucks you may have a tool box or other cargo that dictate the VBL transport position, or in the case of trailers you may have other VBL’s or cargo that dictate that the VBL transport position.
Example: Figures 3 and 4 show the tie-down placements for my ATV in my short bed pickup. I have balanced forces left and right, but since my front straps are angled forward more than the rear straps are angled rearward I have an imbalance that will cause my ATV to migrate forward. To compensate for this I have a fifth strap running from my receiver hitch up behind the bumper and in between the tailgate and pickup bed to the skid plate on the ATV . If I have 150 lbs. of tension in each of the four corner straps as shown in Figure 4, this results in a total forward force of 137 lbs. and a rearward force of only 18 lbs.(Figure 5). To counter this imbalance of 119 lbs. (136 lbs.-18 lbs.) I need a rearward force of 118 lbs. from my fifth strap. View calculations: why tie down placement is important
In this example, the 150 lbs. tension in each strap also results in 138 lbs. down force on each rear corner and a 112 lbs. down force on each front corner for a total of 500 lbs. down force. As you can see from the preceding example it does not take a lot of tension in the tie-downs to put quite a load on your VBL.
Figure 6 demonstrates effect of tie-down strap angles on the lateral and down forces. This example shows the calculated forces assuming that the straps are tensioned to 150 lbs. and are placed straight out to the sides i.e. not angled forward or reward.
If you had a strap on one side that was in position B with 150 lbs. tension and a strap on the other side in position C with 150 lbs. tension, you would have a lateral imbalance of 43 lbs (106 lbs.–63 lbs.). To correct the lateral imbalance you would need to reduce the tension in the position C strap to 89 lbs. or increase the tension in the position B strap to 251 lbs. View calculations: why tie-down strap tension and angle impacts load stability
In the case of cam straps it is easy - the tension in the strap is a little less than 2 times the amount of tension you apply when pulling the free end. View calculations: cam strap tensioning
How do you know how much tension you are applying when you pull on the free end of you your strap? If you have had a lot of experience lifting things with known weights you can probably estimate the amount of tension. For example, lets say you want to pull the free end with 80 lbs. tension, visualize that you are lifting an 80 lbs. bag of premix concrete by pulling on a rope that lifts the bag using a pulley.
In the case of ratchet straps it is also easy - the tension in the strap is about 11 times the force you apply to the ratchet lever. View calculations: ratchet strap tensioning
How do you know how much tension you are applying when you pull on the ratchet lever? Again, visualize the effort required lift the amount you are trying to apply.
You may have noticed that I left out over-center lever cam style straps . This is because there are too many variables for me to be able to explain the resulting tension using a tension formula.
One way to assess if you have too much down force from your straps is to compare how much vertical compression you have when your VBL is supporting the maximum payload recommended by the manufacturer and how much vertical compression results from the straps. If your straps cause more vertical compression than the maximum payload, the down force is too much.
Other Key Points
Q. Why are some ramps arched and some ramps straight?
A. Arched ramps are required when you are loading and unloading low ground clearance vehicles such as Harley-Davidson Motorcycles or lawn tractors. If the ramps are long enough relative to the VBL wheelbase, the arch causes the trailing wheel or wheels to rise enough to prevent the vehicle from hanging up on the tailgate.
The downside of arched ramps is that while the slope is flatter on the upper end than a straight ramp, it is steeper on the lower end. See figure below.
If you are interested in reading an amusing (at least I think it is amusing) story relating to improper securing of a VBL, click here.