Lap BeltDuring a collision the lap belt is normally loaded more than any other belt. Lap belts should never be mounted to the floor or sheet metal, only to the frame / chassis / roll cage. As shown in the install diagram above, the lap belt is to be installed at an angle between 45° and 60° relative to the ground. This angle allows the lap belt to pull back and down, minimizing occupant movement and also allows the lap belt to counteract the upward pull of the shoulder harness. A lap belt installed at too shallow of an angle (< 45°) will allow the shoulder harness to pull the lap belt up off the pelvic area and into the abdominal region with the likelihood of injury. A lap belt installed at too much of an angle (> 60°) will not adequately restrain the forward movement of the occupant's lower torso / pelvis. The two mounting brackets for the lap belt anchor plates should normally be mounted no further apart than the width of the occupant’s pelvis or the outer width of the seat. Mounting the brackets further apart than this will increase the load on the belts, resulting in excessive stretching of the webbing during an impact and possibly exceeding the load capacity of the webbing and / or anchor plates / mounting brackets. In addition to meeting the angle and width criteria, lap belt mounts must be located so that the belt is routed over the driver's pelvic bone and so that the belt will stay firmly and tightly in the crest between the pelvic bone and the upper thigh. During a crash, the lap belt will normally slide into the forward corners of the seat slots. Incorrectly positioned tilt-lock adjusters will interfere with the seat which may cause the adjuster to loosen the lap belt or cut the webbing. Therefore, when using a pull-up style lap belt, the adjuster should be located outside the seat and be well separated from the belt slots in the seat. In all installations, ensure that there are no sharp edges in the vicinity of the belt (e.g. seat frame, seat mounts, frame or chassis, body, etc.) that may tear or cut the lap belt webbing.
| Here a 2" pull-up lap belt is ready for installation. Note that the 3-bar slides will need to be adjusted so that they are as close as possible to the anchor plates and so that there is sufficient webbing length left to wrap-back through the slides. |
| Here a 3" duck-bill lap belt is ready for installation. Because the anchor plates are sewn into the belt their position is fixed and no adjustment is necessary - they simply need to be bolted to the mounting brackets. |
| Here a lap belt mounting bracket is being made from a short section of coped tube, the round slug produced when a 1.75" hole saw is drilled through 1/4" plate, and a nut & bolt. |
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| The centre of the slug is drilled to 7/16" to accommodate the bolt and the nut welded to the back side. |
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| The slug is then welded to the tube with the nut inside. Finally the mount is located in the proper position on the chassis and welded on. |
| The result places the lap belt anchor point at the correct width, the lap belt at the correct angle, and eliminates any chance of the belt rubbing on anything and abrading. |
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| Either a locknut should be used inside the tube-mount, or the bolt should be installed with thread-locking compound. Note the wrap-back of the belt through the 3-bar slide. |
| Here a bracket is being constructed for a lap belt anchor from 1/4" pate steel. |
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| To facilitate the proper angle of the belt, the bracket is cut at an angle. |
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| When welded to the chassis, the angle of the bracket provides for maximum length of weld while allowing the belt to pull at the proper angle. |
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| Checking for fastener clearance from body panel. |
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| 1/2"-NF20 (fine thread) SAE Grade 8 bolt and "Stover" (deformed thread) locknut used for lap belt installation. |
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| When tightening the fasteners on belt installation, be sure to hold the anchor plate steady at the correct angle. Also double check for clearance between the bracket and the webbing to ensure the belt will not rub and abrade. Here the anchor plate needs to be rotated counter-clockwise a couple of degrees. |
| Outside lap belt anchor welded to chassis. Width of anchor position is not perfect - it is slightly wider than the ideal location. Location of seat in chassis necessitates this slight compromise. Belt length and angle are good. |
| Inside lap belt anchor also provides good short belt length and proper belt angle, as well as being more optimally located at close to the same width as the seat. Best practices would have the anchors mounted in double-shear brackets constructed from two tabs each. This installer is apparently confident in the strength of single-shear cold-rolled 1/4" plate steel bracket combined with properly torqued 1/2" fine-thread Grade 8 fastener and locking nut. In NO WAY are the pictures and examples shown here and throughout this article intended to illustrate optimal or perfect installations. They are provided merely as a way to illustrate the theory of best-practice installation rules. DO NOT blindly copy them. Shoulder BeltsShoulder belts are the parts of a restraint system that are most often improperly installed. From the install diagram we can see that shoulder belts should be anchored behind the occupant's seat at a point located from zero to 10 degrees below a horizontal line extending rearwards from the top of the shoulder. Occasionally manufacturers specify a maximum distance, in inches below the driver's shoulder, at which the shoulder belt anchor points can be mounted. To see why the location of the shoulder belt anchor points is such a big deal and to understand why it is a dubious practice to specify a height in inches as opposed to an arc in degrees, let's return to Larry and his seat:
| When we last left Larry, he was safely restrained in his seat, but sadly about to crash. |
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| Properly restrained, with well-chosen anchor points at the level of his shoulders, good restraint geometry, and proper belt routing, the force of the crash imparted by the shoulder belts to Larry's clavicle's acts in a rearwards direction. This is indicated by the purple arrow. |
| The force acts between the two hardpoints - one on Larry's body (his clavicle) and the other the shoulder belt anchor location. Here we see Larry's shoulder belt mounting point is too far below his shoulders. The force still acts in a straight line between the clavicle and the mounting point. The problem is, with the anchor point not directly inline with the shoulder, there is now a downwards component to the force. |
| Physicists often study forces by breaking them into separate components called vectors. The magnitude (direction and size) of any force can be broken down into, and illustrated by, different vectors as long as the sum of the vectors is equal to the direction and size of the original force. Fancy way of saying we can break forces down into different components. Here the original force of the belt on Larry's clavicle is represented by the purple arrow. The yellow arrows are the horizontal and vertical vectors or components that make up the original purple-arrow force. |
| Now, the problem with the shoulder belt mount being too low is that, in a crash, the force on the clavicle (between the two green hardpoints & represented by the purple arrow) has a vertical component - illustrated here by the vertical yellow arrow (highlighted). This vertical force component may act to compress Larry's spine in a crash, even when momentum just throws Larry forward. In other words, mounting the shoulder belt anchor points too far below the shoulders places a risky and possibly unacceptable vertical force on the shoulders and may cause spinal compression - very bad! |
| So now we come to the difference between specifying an acceptable shoulder belt anchor position in inches and doing so in degrees. Those that specify a distance in inches often use the figure of 4". We'll use this in our example. Note that none of these simple diagrams are drawn to scale - they are simply meant to illustrate a concept. The problem with those that specify the mounting point in inches, is that they do not also specify how far back from the shoulders this 4" is to be measured. To understand the problem with this, let's examine two different cases - one in which the shoulder belt is anchored 4" below and 3 feet behind the shoulders and then one in which the belts are mounted 4" below and directly behind the shoulders. Here we draw a line 4" below Larry's shoulders, extending from directly behind the seat to the back of his chassis.
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| If we were to mount the shoulder belts to the chassis 4" below Larry's shoulders and 3 feet behind them, the angle of the shoulder belt would be fairly shallow. Subsequently, the force on Larry's clavicle in a crash, represented by the purple arrow, would also act at a shallow angle. |
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| Now, you can see that the force (purple arrow) and its components (yellow arrows) form a right-angle triangle. If we say the angle between the purple arrow and the vertical yellow arrow is alpha, we can label the triangle as shown: purple arrow = hypotenuse Now, since we know the lengths of the adjacent and opposite sides, we can calculate the angle alpha using trigonometry. tan (alpha) = opposite / adjacent = 36" / 4" = 9 therefore alpha = arctan (9) ~ 84°. This means with the belt mounted 4" below the shoulder AND 36" behind it, it is mounted at an angle of (90°-84°) = 6° from horizontal (within the 10° rule).
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| Now, using a little more Trigonometry, if we assume the purple force is 3000 lbs (purely for illustrative purposes, not intended to represent actual load), we can calculate the size of the vertical component, i.e. the load that will tend to compress the spine. cos (alpha) = adjacent / hypotenuse cos (84°) = adjacent / 3000 adjacent = 3000*cos (84°) |
| Now - let's see what happens when we mount the shoulder harness the same 4" below the shoulder, but this time only 6" behind it. Here we see the load on the shoulder strap once again represented by the purple arrow. Notice the steeper angle. |
| If we use the same labelling conventions as before, this time we calculate the angle alpha as: tan (alpha) = opposite / adjacent = 6" / 4" = 1.5 therefore alpha = arctan (1.5) ~ 56 °. This means that with the belt mounted 4" below the shoulder and only 6" behind it, it is mounted at an angle of (90°-56°) = 34 ° from horizontal (well outside the 10° rule, but within the 4" below rule if we were following that). What does this mean? |
| Well, assuming the crash force (purple arrow) is again 3000 lbs (purely for illustrative purposes, not intended to represent actual load), we can once again
calculate the size of the vertical component, i.e. the load that will tend to compress the spine.
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cos (alpha) = adjacent / hypotenuse cos (56°) = adjacent / 3000 adjacent = 3000*cos (56°) That's 1300 more lbs of force, even though we followed the "rule" and mounted the shoulder harness anchor point only 4" below the shoulders!!!! So it seems that specifying a maximum vertical distance below the shoulder at which it is acceptable to mount the shoulder harness anchor point is LUDICROUS without a corresponding minimum horizontal distance. And since there are an infinite range of " x inches below and y inches behind" combinations that would result in an acceptable installation, it seems as though the only sensible specification is to specify a maximum number of degrees below horizontal at which the belt can be mounted - and according to the install diagram, the maximum acceptable angle is 10°.
| Because of the way many chassis are designed, there may be a temptation to anchor the shoulder belts too far below the shoulders. In this rig, as in many others, there just happens to be a crossbar, about mid-seat height, right behind the seat! Convenient though it may be, mounting the belts too low is wrong and probably dangerous. |
| In order to raise the shoulder belt anchor points to the correct height, without re-doing the chassis, a couple of hoops with double-shear mounting brackets were built. |
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| Remember that each anchor point must be able to withstand the load placed on it, so some reasonable care and attention is required in the fabrication of the mounts. |
| This fabricator could have saved himself a lot of trouble by making the hoop straight across the top which would have made fitting the tabs for the mounts a lot easier... |
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| ...but with some patient grinding and fitting it appears he may have been able to mount the brackets where needed on the hoops and in a manner that may not make them the weak point of the mounting system. |
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| The hoops were then fitted to the crossbar... |
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| ...and gusseted with 1/4" plate. |
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| After welding and painting the result is a mount that is hopefully strong enough... |
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| ...reasonably good looking... |
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| ...and puts the shoulder belt anchor points where needed. |
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| I can't be stress enough how important it is to take the time to mount the shoulder belts at the proper height. You may not have to re-do the chassis to do it, but some fabrication will likely be required. If you are not confident in your ability to design, fabricate, or locate proper driver restraint mounting points - seek qualified professional help! DO NOT blindly copy something you saw on the internet! |
| Having said all this about the importance of the height of shoulder belt anchor points, you may be wondering why, then, do you see some shoulder harnesses with such long belts, and why do you even see them labelled as "floor mount"? Surely mounting the shoulder harness on or near the floor would violate all that we have discussed? Surely this would place way too much force on the spine in a crash? Yes, this would probably be the case if the belt were mounted directly to the floor - BUT, there is a way around this problem if a "shoulder bar" is used. Here we can see just such a bar mounted directly behind Larry's seat, at shoulder height. |
| When we use such a bar, and route the shoulder harness belts around it, it becomes the hard-point, even if the harness is routed lower and secured to the floor. |
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| Since the shoulder bar becomes the effective hardpoint, as long as it is mounted within the 10° acceptable arc, the force on the shoulder straps in a crash will probably be in an acceptable direction, as indicated here by the purple arrow. Having said this, many believe it is not best practice to mount the shoulder harness to the floor or any other sheet metal. The minimum size for a shoulder bar is often considered to be 1.5” OD tube x .095” wall. Be aware that rarely are the tubes of the internal seat frame sufficient to act as a shoulder bar for harness routing. Most seats are neither designed nor tested to carry shoulder belt crash loads. Regardless of the height or angle at which the shoulder belts are anchored, in order to minimize belt stretch and the chance that the occupant may slip out from the harness in a crash, shoulder belts should be anchored as close to the shoulder as possible. For the best restraint of the occupant’s upper torso, shoulder belt anchor points should probably not be more than 8" behind the back of the seat. In addition, with individual shoulder belts, as in an H-style restraint, the anchor points should probably be no more than 3-6” apart behind the driver's neck to prevent slippage off the shoulders. Where the seat has individual slots for the shoulder belts, the belts should probably be anchored so that they run straight back from the seat slots - with one exception: If the shoulder belts are anchored more than 8” behind the back of the seat, the inside edges of the belts should probably converge at an angle of about 20-25° so that the anchor points are closer together than the belts are at the seat. This means that for belts mounted a long way from the back of the seat, the anchor points may need to be placed so that the belts touch or even cross over each other at the chassis anchor point. As a rule of thumb, shoulder belts should probably be crossed when the anchor points are more than 18 “ behind the seatback. The shoulder belts of a driver restraint system will not properly restrain the driver if they are installed with an improper seat. They can not be used with a seat that either has no headrest or that has an integrated headrest but no slots for the belts. Proper quality seats are an integral part of the total restraint system and should be designed and manufactured to work with a driver restraint system. If your seat does not have the proper slots for the belts, or at least an adjustable headrest - it is likely unwise to use it; the shoulder belts can slip off the driver’s shoulders during a crash. The smart user will buy and use the proper seat! Never modify seats to create belt slots – not only is it likely to compromise the seats structure, but sharp edges created by cutting the seat can damage the belt’s webbing. Finally, if the ends of the harness are too far above the shoulder then two problems can occur. First, tension in the shoulder harness may be increased and undue stress may be applied to the harness and its structural mounts. Second, excessive upwards angle of the shoulder belts may allow excessive occupant motion. If the harness belts are too far above the shoulder, they may provide little resistance to forward motion of the driver’s upper torso. Wrap-around Style Installation.Technically, one could attach any or all of the belts in the driver restraint system to the vehicle by wrap-around method. However, many sanctioning bodies forbid wrap-around mounting in anything but the shoulder harness mounts. Given that in an offroad rig the lower mounting points of the anti-sub and lap belts are in a pretty harsh environment (potential for webbing to abrade) and when one considers that it is unlikely that the rig will feature a bar in the optimum location and orientation around which to wrap the anti-sub or lap belt, it can be suggested that only bolt-in mounts should be used for anti-sub and lap belts; and that wrap-around mounting, if used, should be confined to the shoulder belts. Wrap-around style mounting should include some method to prevent lateral or side-to-side movement of the belts. Several methods can be used, such as: roll bar padding secured with hose clamps, aluminum collars, or weld-on tabs.
| The shoulder belts for this rear bench seat use wrap-around style mounting. To prevent unwanted lateral sliding of the belts small stops were welded on either side of each belt. |
| This wrap-around installation appears to illustrate that:
Anti-submarine BeltThe anti-submarine belt should be anchored on or slightly behind the chest line, as indicated in the install diagram. Obviously it should be centred to the driver's body and the slot in the seat. One caution is to account for the direction of pull on the anchor plate / mount. In a crash or impact situation, the anti-sub belt will likely pull forwards and slightly upwards on its mount. The installer will probably want to account for this in placing the mounting bracket. For example, one should probably avoid making the anti-sub belt bracket vertical, as this would likely put undue bending stress on it during a crash. One would probably want to avoid making it horizontal for the same reasons - the upwards pull may well place unnecessary bending stress on the bracket and anchor plate. The exact angle for the bracket will depend on seat height and the vertical distance between the bracket and the restraint system's main buckle when assembled and worn. Some believe that somewhere between 20-45° from horizontal may be about right.
| Anti-submarine belt prior to installation. Note that the 3-bar slide needs to be snugged up to the anchor plate and that, once the installed length is adjusted, the belt will need to be wrapped back though the 3-bar slide. |
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| Anti-sub belt mounting bracket facing forwards and slightly up in the rig. This particular bracket is probably about 40° from horizontal. |
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| Test-fitting anti-sub belt installation under the seat. The theory of the anti-sub belt installation may be relatively simple, but the execution takes some patience as you have to work under the seat - which usually means uninstalling and reinstalling the seat until anchor location, angle, and belt length are all correct. Here once again the 3-bar slide needs to be snugged up to the anchor plate and once the installed length is adjusted, the belt will need to be wrapped back though the 3-bar slide. Belt length should be such that the belt is snug when attached to the main restraint system buckle. Common Installation ErrorsFor the safest possible installation, the chassis, seat, anchor points and driver restraint system should all be designed together and custom fit for the driver by competent, qualified professionals. If/when any design or installation compromise is necessary the individual installer or user will have to decide what is, and what is not, acceptable to them - hopefully with a full understanding of the consequences of any such compromise. Some common installation errors are:
Wearing and Adjusting the Driver RestraintEven a text-book perfect installation will not allow the driver restraint to properly restrain the occupant if the system is not adjusted and worn properly. Remember - the entire driver restraint system is only as strong as its weakest link! To put on the restraint:
| A properly fastened main buckle will look like this. |
Once you have donned the restraint and tightened the belts, have an assistant:
More tips on wearing your driver restraint system:
To exit a driver restraint:
Note: Steps 2 and 4 can be omitted in an emergency. Just undo the sternum strap, rip open the latch & link, and GTFO! Consequences of Wearing the Restraint Too Loosely:Any driver restraint should be worn properly tightened. Some of the possible consequences of wearing the restraint too loosely during an impact or crash may be: Loose shoulder belts result in increased upper torso movement, increased head movement with resulting neck strain, and increased risk of contact with steering wheel or other objects in the cockpit. Contact inside the cockpit leads to injuries. Loose lap belts lead to increased pelvic movement and resultant risk of submarining or ejection from the vehicle as well as placing extra load on the anti-sub strap; possibly exceeding its load capacity and causing it to break. A loose anti-sub belt allows the lap belt to ride up, seriously increasing risk of abdominal injury, as well as increased risk of submarining and ejection. Bottom line, when the belts are too loose the driver will not be properly restrained and in a crash or impact the result can be severe injury or death. Inspection and MaintenanceAlthough they don't require extensive maintenance, driver restraints should nevertheless be periodically inspected and maintained:
SummaryAny driver restraint system will only function properly if:
Below is a summary of some consider some important considerations when installing your driver restraint system. It is not an exhaustive list, nor does it replace the need for qualified, competent, professional advice, design, fabrication, installation, or use:
ConclusionNo restraint system can keep you completely safe from harm. But you need to give yourself a fighting chance. To do so your restraint system must be:
References:
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