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How about seat belts?

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  • How about seat belts?

    It seems that after years of government requirements, most of us, or at least our other 1/2 would feel more comfortable with our rides if they had seat belts. They might even let us take the kids for a ride if there were seat belts installed. The subject of installing seat belts seems to be terrifying to most Rat Rod guys.

    Over the years, I've built a lot of dirt track cars, and somewhere around 10 hot rods/rat rods that I have installed seat and shoulder belts into, some of which have been well tested. I have learned a few things about installing seat belts, so I thought I'd pass some of that info on, for those that may be interested.

    First we need to lay down the ground rules, the disclaimer. What I'm going to lay out is what I have done in my stuff. I have copied stuff from OEM auto manufacturers and have adapted it into my stuff. If you choose copy what I have done, I will not accept any responsibility for your safety. I am not a licensed engineer and I claim no expertise in the field, If you choose to copy what I have done, the full responsibility for any injury or death still falls on your shoulders, not mine.

    The full purpose of seat belts it to keep you in the seat in a crash. If the belt keeps you in the seat, the next step is to keep the seat inside the vehicle, if either event fails, injury or death can occur. Even if both succeed, the outcome still may result in injury or death, the belt and seat staying inside the car increases your chances of a better outcome.

    Generally, a seat and the seat belts are bolted to a vehicles floor pan. If the seat is bolted to the frame, or to a roll cage that is attached to the frame, the seat belts should also be bolted to the frame or roll cage. It is not generally acceptable to bolt the seat to the floor pan, and bolt the seat belts to a frame or roll cage (just don't do it!) In the event of a crash, if the floor pan and the frame or the roll cage become separated from each other, the seat belt could cause a lot more damage to your body then having no belt at all could cause.

    Seat belts are usually attached to the floor pan with 7/16" or 1/2" or a metric equivalent diameter bolt, and often times those bolts are also fine thread bolts. Those bolts screw into 1/8" thick plated with rounded and often curved edges with nuts welded to the bottom side of the floor pan (called nut plates). Most of the factory nut plates are 2" square and are spot welded to the bottom of the floor pan. The nut plates are spot welded to the floor pan to ease the belt installation in the original build process and to reduce the possibility of the bolt tearing the sheet metal in a crash. The bolt passes through a a hole in the steel part of the seat belt, then passes through a hole in the floor, through the plate and into the nut welded to the bottom side of the plate. For that plate to move, it would have to tear the 4 spot welds, and tear a 2" square hole through the floor pan. unless the area is weakened by rust, that would be pretty difficult. It would likely distort the entire floor well before it tears through. As you see the object of the belt anchor is to pull as much material with it as it can if it moves. The other important aspect about the seat belt anchor is its location in regards to the seat. I will cover that in a few minutes.

    Keeping the seat in position is equally important for the belt to work. Nearly all the original factory seats were mounted on a reinforced area of the floor pan. The top of the seat adjusters were bolted to the bottom of the seats, and the bottom of the seat adjusters were bolted to a pair of seat brackets. Those seat brackets were bolted to to the floor at those reinforced areas with a 5/16" bolt on each back at both the front and the rear of the bracket. Over the years, the modern seat brackets were bolted to the floors with larger diameter bolts, first step up was 3/8" bolts, and modern versions use as large as 7 16" bolts (or metric equivalent), or 2 front bolts on each side. Often the reinforcing under the floor has grown more robust then what was used in the 50s and 60s. The most modern seat mounting bolts don't even pass through the floor pan anymore. They are bolted to 11 gauge steel structures welded to the inside of the floor pan, and the front bolts often are bolted to those brackets with the bolts going horizontal. If I still mount my seats with 4 bolts, the minimum I use are 3/8" diameter grade 8 nuts, bolts flat and lock washers, and if I'm just going through the floor pan, I will use as large of a diameter (up to 3" diameter) 1/8" thick metal as a washer as will lay tight against the floor pan at all 4 bolts.

    With the seat bolted in location, the seat belt anchors need to be located so the belt attaches at the floor about 2" forward of where the seat back and the seat bottom intersect. When you are sitting on the seat, the both ends of the belt should go behind where the belt crosses your lap at about a 45 degree angle. you want the belt to lay flat against the side of the seat, and it best if the belt hardware is straight rather then angled to bolt to the floor. I often will make a 1/4" x 1 1/2" x 1 1/2" piece of angle iron about 2" wide with one 29/64" hole (that is the diameter a 7/16" bolt easily passes through a hole) in each leg of the angle (slightly offset so they are not in line with each other) so the belt can be bolted to the angle and the angle can be bolted to the floor pan. If your making your own nut plates and hardware and not using the factory stuff, I use 7/16" grade 8 bolts, nuts, flat and lock washers (course thread works OK, its really hard to find grade 8 fine thread hardware local to me here), and and a 1/8" thick 3" x 3" square plate with the corners rounded off. I do drill the 29/64" hole in the center of the plate and weld the nut to the plate, and I do drill the 29/64" hole through the floor pan along with (4) 1/4" holes around the larger hole and weld the nut plate to the floor (be sure the bolt can pass through both holes and thread into the nut). You will probably need to round off the top of the vertical leg on the angle iron so the belt can not rub against the angle. Pic 1 shows what the angles look like bolted to the floor. Repeat the process for as many lap belts as you want to install.

    That should get you lap belts and seat mount ideas. The next installment I'll tackle shoulder belts. Gene

  • #2
    Shoulder belts are a bit more complex. They have more anchor points, and for them to function correctly, the retractable piece needs to be positioned correctly. There is also a point of mounting that needs to be above the shoulder height, and behind the seated body, and you often need to reinforce the vehicle body at that mounting position.

    The same disclaimer applies as before. You are installing your belts in your vehicle. I have on liability for any injuries or death that may be involved with what you do with you vehicle.

    Like the lap belt, the shoulder belt installation begins with the seat bolted into position. The seat needs to be adjusted to its rearmost position. On the shoulder belt there is a slip collar the belt passes through that has a hole in it. Pic 1 shows the side view of the slip collar. Pic 2 shows the view from the top of the slip collar, with the original mounting bolt passing through it. Notice the bolt is a step or shouldered bolt. When that bolt is tightened into position, the collar will still pivot around the bolt. This is an important thing to maintain, if the collar can not pivot, the belt will bind and could cause damage to the webbing material in the belt. Once installed, the belt must also clear any thing that it may drag against. The placement of that pivot bolt that holds the slip collar can determine the effectiveness of the shoulder belt. That factory bolt is a very high quality (grade 8) 7/16" fine thread bolt.

    The pivot bolt placement.
    With you sitting in the seat, the pivot bolt needs to be 3" - 4" above your shoulder. More distance is better then less distance. It also needs to be behind your shoulder, or about even with the center of the seat backrest. Often time, having something to mount the pivot bolt into will determine the bolt placement. If you have to go much higher the 4" above your shoulder, you should also move the mounting point more towards the rear if possible. If you have to mount the bolt lower then 3" above your shoulder, move it slightly forward. The shoulder belt can cause serious damage to your body if the pivot bolt is below your shoulder height, or forward of your shoulder. Often times a door post or the roof structure behind your shoulder prove to be good mounting points.

    It should be pretty clear that if the bolt location is important, keeping it there in a crash is also important. Like the mounting points on the lap belts, the idea is to mount that pivot bolt in such a way the it pulls a lot of metal with it if it moves.
    I've just completed a seat belt & shoulder belt installation in my 49 Dodge truck. I was thinking about you guys, and I took a few pictures of the process. (the build thread is in 40s Rat trucks under "darn, here we go again")
    The truck is a 5 window cab with a pair of corner windows that wrap around the back corner of the cab. The ideal mounting location for the pivot bolt would have been about the center of the corner window glass, and about 2" from the forward most edge. Pic 3 shows the corner window. The gray primed spot on the door post is the the correct height, and about 3" farther forward of the ideal location. The door post is the best option. The door post, at that location is 4 individual pieces of steel welder together (+ the tin cover). The inside flat surface (under the tin cover) has two pieces over lapping each other a little more that an inch. I have access to the inside of that location from both the top and the bottom. A 15" long piece of 1 1/4" channel fits into the area. At this point, the pictures are all taken from the passenger side of the truck. (someone forgot to take in process pictures of the driver side).
    A 29/64" diameter hole was drilled in the center of the door post at the correct height. The channel was inserted into the door post and clamped into position. there is about 1/8" of side to side movement of the channel inside the door post. With the channel clamped into place (flat side towards the inside of the cab), the pivot bolt hole was marked on the channel. The channel was removed and the hole for the pivot bolt was drilled at the correct height and centered on the channel. Pic 4. The hole was debured, and a 7 /16" fine thread nut was welded on the ear side of the channel so the bolt would pass through the channel and thread into the nut. Once that was cooled enough to handle, the channel was test fit to be sure the bolt would pass through the holes and would screw into the nut. The channel was then used to mark the edges, so 4 holes 1/4" in diameter could be drilled though the double layer sheet metal door post. Pic 5 Pic 6 shows that indeed there were 2 pieces of metal at the hole location. The idea is to bolt the channel inside the post, flat side facing inside the cab, then the channel would be plug welded to the door post through the (4) 1/4" holes. For that nut to pull through, it will have to rip 15" of door post out of the cab. Pic 7 shows the door post welded up and painted.

    The shoulder belt retrace position.
    Once the pivot bolt is done, the next thing that need to be addressed id the belt retract mechanism. These pieces are designed to be free wheeling (allows you to move around a little in the seat) until a sudden impact locks them into position. The locking part is a swinging pendulum inside the retract housing. When the pendulum is centered, the belt retract is free to move, but as soon as the pendulum moves off center, it locks the belt movement. Because of the pendulum action, and because of all the various locations and angles the retract mechanisms need to be located, each retract mechanism is marked to show what angle it has to be mounted at. If you want it to function correctly, it has to be mounted at the angle it was designed to be mounted at. The retract mechanism I used are out of a 88-96 Dakota, and they are designed to be mounted straight up and down and level side to side. I have a pair of rear shoulder belts I pulled out of a car, but those retract mechanisms were designed to mount at an angle on the rear roof pillar, so they have to be mounted at that same goofy angle, or you can't pull the belt out to buckle it. You want to be sure the retract mechanisms you are using are compatible with mounting in your ride.
    The Dakota retract mechanisms I'm using need to be mounted straight up and level, side to side. That means it has to be mounted directly under the pivot point bolt. Pic 8 shows the angle front to rear, and pic 9 shows the side to side angle. (note this is not the correct location for the belt, the pivot was too low when the belt was mounted here. in a crash, the belt would have pulled down on my shoulder. Notice I used a piece of 1/4" x 1 1/2" x 1 1/2" angle about 2" long to mount the retract mechanism to the floor pan. There is also a 1/8" x 3" x 3" nut plate plug welded to the floor under the floor.

    The last mounting place is the floor anchor by the seat. it is mounted just like the outside part of the lap belt was mounted. There is another slide through bracket on the shoulder belt. it is between the door post pivot bolt and the seat belt floor mount. That slide through has male part of the buckle attached and is what you pull across your lap and insert into the buckle.

    A bonus, How to mount the pivot bolt if there is no body structure close?

    When I built my 39 Dodge truck, the seat sat back against the back of the cab, and the door post was too far forward. That cab had good support at the door post, and also had pretty good support around the back window, and a belt line support from the bottom of the back window around the corner to the door post, but it was too low for the pivot bolt. There really was nothing where the pivot bolt would have to go. The distance between the rear window and the door post was around 14" (the truck is no longer here, and I have no pictures, numbers are guesses).
    What I used was two pieces of 1/4" thick x 1" wide bar stock. I bent them into a curve to match the curve of the cab. With the pieces clamped to the rear window and the door post metal, I sat in the mounted seat and marked the location for the pivot bolt, on both pieces of bar stock. I drilled a 29/64 hole in each piece at the location of the pivot bolt. One of the two 1/4" bar stock was fit in behind the belt line cab support and was to be tack welded to the cab support, and would also be tack welded to both the rear window frame, and the door post. Another piece of 1/4" x 1" bar stock was cut about 6" long. It had 3 29/64" holes drilled into it, one hole on each end, and one hole that would have been the correct height for the pivot bolt. 7/16" nuts would be welded to that one piece so the bolts would pass through the bar stock and into the nuts. Pics 10 , 11, & 12 give you the concept, imagine this bar long enough, bent to match the curve of the cab, with 3 holes and 3 nuts welded in place. The 2nd long bar would have been welded to the upper part of the rear window frame, and to the door post. The top of the small bar, and the bottom of the small bar would have been bolted to the upper long bar, and the lower long bar. The pivot bolt would have had the slip collar on it and would have been tightened into the center hole. To come loose, it would have had to bend both 1/4" bars, and the rear window support and the door post at two locations each.

    Bonus 2, What about mounting in the roof?
    Nearly always the lower edge of the roof rail is made of of 2 or 3 formed pieces of sheet metal welded together and is pretty strong. Often the top inside area is open and accessible. Once you determine where the pivot bolt needs to be positioned, its usually just a matter of adding a short piece of 1/8" x 1" x 6" long bar stock with a 7/16" nut welded over a 29/64" hole in the center of the bar for the pivot bolt to thread into inside the roof rail to spread the pull on the bolt out over more material.

    Pic 13, seat belts installed. Gene


    • #3
      Thanks for taking the time with the detailed info Gene. When the mandate first came out saying we "had" to wear seat belts I was part of the "don't tell me what to do" crowd- so I throw no rocks at those who don't add them. Fast forward to say this- I personally am very happy to have (and use) them in my '58 and in the '30 as well. Thumbs up. 1930 Chevy truck build link:


      • #4
        I was also one that resisted the mandatory seat belt laws when they first came out, but I never questioned using them in race cars. Over time, I've become used to them and usually feel pretty naked without them. Most of the stuff I build is old enough not to have belts required, but I put them in so my wife would ride along.

        Years ago I put together a 50 Dodge truck on an 80 Dodge 4x4 chassis. When I built it (late 1990s) I installed a set of lap belts. The 50 Dodge cab is the same cab as my current 49, they have a huge 2 piece windshield. After a few years it occurred to me that in a crash, that lap belt would fold me and put my face in about the center of that big windshield. Having seen the results of such things (my shop was right side of a tow company recovery lot where they stored wrecked cars), I was somewhat less interested in wearing that belt.

        Fast forward to Oct 11, 2011. While driving through town, some lady in a string of cars made a left turn, about 20' in front of me. I hit her head on at about 30 mph. The impact stopped my 4850 lbs truck dead in its tracks, at the point of impact. The Chrysler Lebaron I hit was pushed back 20' from the impact point. The air bags in her car saved her.

        I wasn't wearing that lap belt, so my butt lifted off the seat, my grip on the steering wheel brought the wheel and the column with me until my head got stopped by the metal header above the windshield. Fortunately for me, there was a sun visor between my head and the header panel that softened the blow a little, I also suspect the bending steering wheel and column slowed my movement. The impact with the header knocked me out for a few seconds, and gave me a cut on my forehead just below the hair line that required 11 stitches, and my grip on the steering wheel gave me a sprained thumb on my right hand. I still get slight neck pain from time to time, but otherwise I feel I was very lucky.

        After viewing the truck a day later, I would have been correct with the idea that the lap belt would have put my head at the center of the windshield. I'd post pictures of the truck, but I lost those pictures in a computer crash years ago Since that day in Oct 2011, everything I've built has seat and shoulder belts, and I put them on every time I get in a vehicle. Gene