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I'm thinking seriously about installing 4-point seat belts in my HMMWV. It now has the three point ones. The question I have is about attaching the shoulder belt(s). The roll cage in my truck has a cross bar located about 4" behind the shoulders and about 6" below the shoulders. That's one attachment option. Another is the vehicle's B-pillar, located about 2" behind the shoulders and about 13" above them.
I remember Scott's post about not locating the shoulder straps below shoulder height to prevent compressive loading of the spine in case of hard front end collision. And I have had one experience with an inertial reel shoulder belt located below shoulder height -- it was pretty uncomfortable with every vertical bump because it pulled down hard against the shoulder. Maybe that was because the inertial reel allowed the body to gain some upward speed before abruptly pulling it to a halt.
But if I put the shoulder strap(s) 13" above the shoulder, they sure wouldn't contribute much to keeping me in place if the truck flipped over.
Keep in mind that the shoulder harness is not intended to keep you down, that's the function of the lap belt. The shoulder belt is to keep you from being thrown forward.
My suggestion is this; Get some help and attach the lap belts to the existing seat belt bolts. Attach the shoulder straps with the seat in the normal operating position. Now have someone drape the belts over your shoulders and move them together and apart, up and down and all around until you get a comfortable fit with the straps located at least 4 - 6 inches above the shoulder height of the tallest driver at the highest seat position. (HUMMER installs are made more complex because of this adjustability in height...race cars are fitted to an individual driver). Put your crossbar there. I recommend using the dual strap system, not the Y-yolk, as you can adjust width for less chafing on the neck and you get much more length in the straps if you need to go a long way to an anchor. You must also consider the seat headrest and how the straps drape over it. I found that mine twist slightly on the headrest, which keeps the edges of the belts from digging into my neck.
As for head banging, you'll want to have the crossbar far enough aft so you don't do that, but your headrest should take care of it. If you don't HAVE a headrest (as I recall your pictures), you need one, so take Rod's example and install a well-padded crossbar for that purpose.
I'd recommend the widest belt, because I find them more comfortable, but for some people they are too wide and slip off the shoulders or cramp the gut.
For low-speed driving, you don't need to fasten the shoulder belts at all, just the lap belt, which will hold you firmly in the seat...much more firmly than the stock belts.
The only problem I've had is having the right side strap with the buckle slip down between the seat and hump and get stuck, but this is only a problem when I don't use it for awhile.
The only other thing I'd suggest is that you ask the manufacturer to make your lap belts "reversed" on the adjustment strap. Most belts are set up with the adjuster on the buckle side of the strap and the anchor strap coming up and looping through and back down. With this setup you have to pull down on the strap end to tighten the lap belt. I've found that with the narrow "cockpit" of the HUMMER, and particularly the right side where the hump is, it's sometimes difficult to get a good pull on the strap.
If I had it to do over, I'd order the belts with the adjuster attached to the anchor point, so the belt comes from the buckle, through the adjuster and back up towards the buckle. This way you pull up on the belt to tighten. The strap ends don't lie quite so neatly, but it's much more convienent. They use this method in sailplanes where you don't have any room to pull down, and I like it a lot. The belt manufacturer I ordered from said they could do it, but you have to order it that way.
All the talk about safety belts brings up an annoyance I have with the Hummer seat belts. Anyone else notice that when you are doing serious off-roading and the vehicle is sitting at an angle you can't buckle-up? Whenever this happens I have to continue on without the seat belt until I hit flat ground. I real pain especially when winching as I have to go in and out of the vehicle a lot.
Here's a trick that I use. Required: one of those black spring clip things used for holding a large stack of papers, you know, with the little flip out metal handles.
When on a flat surface, I put the clip on the belt between the guide on the B pillar and me. This prevents the belt from retracting when I unbuckle it. On the flat, the belt will still unspool as usual. On a hill, it doesn't allow the belt to choke you to death and allows you to put the belt back on.
I was wondering, where you would mount the 4 point seat bolts without doing any welding or any other type of modifications
I was going to run in a 'tough truck' competition this summer and bought a pair of 5 point racing harness. The requirements were for a 4 point harness, so you don't need to attach the strap which goes between your legs. I didn't want to drill any holes, and I found out that the side straps will attach very nicely to the bolts which currently hold the factory harness on side of the center hump and the door frame. You may need a few thick washers on side bolt which attaches to the hump or get a bolt that is threaded all the way to the head, because it is the current bolt only goes in 3/4's of the way to accommodate the thickness of the cable which the current harness attaches to (I can't think of a technical name for it). This leaves you with a 'Y' strap which goes over each shoulder. I threaded it under the headrest and instead of bolting it to the floor, I cheated and strapped the to one of the supports under the seat. It was only temporary, and it seemed very secure, but I was also was wearing a helmet, and I did not count on participating in anything which was going to rip my seat out. Besides, with the skid plates on, I had no plans in either taking them off so I could get to the underside of the floor or drilling through them. Now, if you have a '93, there is this great support which runs right behind the driver's seat you could could probably run the strap around. Although the racing harness looked real cool, it was a real pain in the ass to get in. I use my truck a lot, and every time you get in, you have to drape two shoulder straps over your chest, find the two side straps, thread and hook them all together, and then catch up to everyone else.
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