Driver and Passenger Seat Options / Swivel Bases / Seat Belts

Mike Perez

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Oct 2, 2019
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Sorry in advance for the long post. My engine subframe and the floor above it is currently removed. I also have no driver or passenger seat and no seat bases. I'd like to get my seating addressed before I reinstall the engine subframe because the area underneath the floor is very accessible right now, and it is less so when the engine/transmission is in.

I'm running into a few issues with seat choices. First and foremost, I want modern seatbelts with modern retractable shoulder harnesses. I know there has been debate on the other sites before about shoulder harnesses being dangerous in the event of a rollover and body separation. Even if that were the case (which I doubt), the most likely accident in one of these is a slower speed front impact. If you impact something in a GMC at 50+ MPH, you are most likely dead anyway, so seatbelts won't make that big of a difference. What I want to do is protect the occupants at 25-40 mph as much as possible. Check out the video below. I already have it forwarded to the appropriate spot. 1970's sedan crash tested with both lap belt only and lap belt / shoulder harness at 30 mph. See the results for yourself.


This brings us to the GMC. There are a few options for the belts. First, use the original seatbelt anchors on the floor and then mount the retractor to the floor/wall. The problem with this is that the GMC's side window is basically where you would want to mount the harness. Per all the aftermarket seat belt manufacturer instructions, the upper shoulder guide must be directly above the retractor mechanism. There is only one place to do that unfortunately:

20130625_165022 copy.jpg

That won't work well because of how far the side window goes behind the seats. The mounting location will ultimately be too far back and interfere with the dinette or cabinets. That brings us to seats with integrated belts.

In the picture I posted above, the owner installed Toyota Sienna second row seats with integrated harnesses on a original modified swivel type base. I've seen a bunch of seats mounted on Unistrut that were adapted to the original seat bases. Let me know how the ride was through the windshield strapped in to the seat when you come to rest on the trunk lid of the car in front of you. All kidding aside...when I set up my old track car years ago, the seat "anchors" were actually fairly small. Just a few bolts with big washers holding the seat in place. That's because the weight in an accident (you) is restrained by the belts attached to the roll cage. Now imagine what would have been required if the 6 point harness would have been attached to the seat itself, and now the seat was just bolted to the chassis. I'm pretty sure it would have required something much more substantial.

The factory GMC seat bases were designed to restrain the weight of the seat only, not the seat plus the occupant. The lower part of the base is just bent sheet metal that forms a flange. It is not even welded at the corners. Additionally, the swivel mechanism that attaches the seat to the base is limited in it's attachment. I sent some pictures to my dad who is a retired mechanical engineer, and he agreed that the loads on the GMC seat bases would most likely cause them to fail if the seats are equipped with integrated belts.

So the solution would have to be integrated seat belt seats with aftermarket bases. There is a problem there too. I can not seem to find an aftermarket seat base that has both a swivel function and is designed for integrated seat belts unless it comes with tethers. On big rigs, the aftermarket seats with integrated belts are typically tethered down to the original seat belt anchors with straps. Apparently this is done on RVs too when the seats are upgraded to integrated seat belts.

suspension-seat-tether-inst.gif

I don't think I want tethers coming from the floor to the seat bases. They would look bad and be in the way when the seat is swiveled around. This leads me to a dead end. The only thing that I can think of is just fabricating something from scratch. I'm going to make some more calls today, but I'm running out of good options.

Anybody know of any swivel bases rated for integrated seat belt seats that has a swivel and no tethers?
 
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Tom Newell

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Oct 2, 2019
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Hi Mike,

I have been exploring this subject for years for a different context; Scout II, top comes off, don't care for big roll bars. This is not particularly interesting other than to mention I have NOT really explored rotating seat bases and can't contribute anything specific relating to those. As an aside, you are much more restrained than I am able to be when it come to assertions that ANY shoulder belt is somehow not superior to no shoulder belt; a trait to be admired.

The most useful piece of information I have run across has to do with the rated break strength of seat belt material. This is specifically related to another "internet expert" theory that seat belt manufacturer sewing is somehow magic sewing that can never be duplicated (it isn't). Here is a comparison of break strengths of various sewing patterns. The take away is that Seat Belt Webbing has a published break strength rating of 4720 lbs.

An article on webbing break strength testing:

it does not take many grade 8 bolts to hold up to 4720 lbs., either in sheer or tension. Observation tells me that most seat belt hardware is screwed down with one bolt of < 1/2", so the gives an idea of the needed strength.

So the problem becomes, what is needed at the base to withstand 4750 lbs. of force as applied to the top of the seat back where the shoulder belt force levers against the base and whatever hold the seat back in place. I would speculate that the real limiting factors will be either the mechanism which holds the seat back in place (designed by an engineer somewhere; hope he or she knew what they were doing) or the fasteners/fastening points where the base screws to the vehicle. It seems grade 8 or 10.9 will easily take the force, so now we are looking at how the base is fastened together and how is it fastened to the floor. A not homogeneous plywood floor, at that.

I "think" a fabricated swivel base can be designed and built to take the force without a whole lot of drama and agree with you that custom fabricated is likely going to be the only way forward to meet your criteria. I have looked at various boat seat swivel mechanisms, and they don't "seem" strong enough for this use case (to my untrained eye). My real concern will be tearthrough on the floor. At least you have your father to run things past.

Finally, I keep a casual running list of bucket seats with integrated seat belts (driver and passenger; personal preference excludes bulky minivan seats).
- BMW 8-series (the old ones; nice looking; retractor is mounted at top of seatback)
- Mercedes SL's (Fairly nice looking; retractor is mounted at top of seatback; these (the ones I have observed in the junk yard) to not really mount to floor, but rather the vertical surfaces of the driveshaft tunnel and sill; very complex)
- BMW 3-series convertibles (bulky and unattractive; retractor mounts to either floor or base (belt runs diagonally down the back of the seat); lots and lots of these around)
- Chrysler Sebring Convertibles (think I have the model correct) (decent looking, unsure of how the mechanism works but I did see on that had FAILED a junk yard (so that is scary))
These are the ones I know of off the top of my head.

Good luck, and I will be closely following your journey with this. I will think about swivel ideas and share if I come up with anything worthy.

Tom Newell
 
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Hal

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Oct 3, 2019
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I picked up some bases at a surplus RV sales in Eugene,OR a couple of years ago. They 'probably' were used in Monaco product and are really a robust design, certainly used with integrated belt seats. I have a set of new Sienna seats I'd planned on fitting to the basses once I get around to it...they have a side sliding mounting arrangement that will take a little work to mount though. I'll try to get some pictures up to show the details of the basses.
 

Mike Perez

Active member
Oct 2, 2019
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I picked up some bases at a surplus RV sales in Eugene,OR a couple of years ago. They 'probably' were used in Monaco product and are really a robust design, certainly used with integrated belt seats. I have a set of new Sienna seats I'd planned on fitting to the basses once I get around to it...they have a side sliding mounting arrangement that will take a little work to mount though. I'll try to get some pictures up to show the details of the basses.
Funny you mention the Sienna seats. I was on the phone today for a few hours with my mechanical engineer dad and I think I have a solution. Like you said, a custom fabricated base and swivel. Also, I was able to find a set of the reclining second row Sienna seats today and they are being shipped to me. I wasn't going to jump on them, but the color of my coach is going to be silver and black and these just worked for the scheme. The bonus is they seem to already have the adapter bracket from U-joint Offroad that builds bracket adapters for these Sienna seats.
 

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Mike Perez

Active member
Oct 2, 2019
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Hi Mike,

I have been exploring this subject for years for a different context; Scout II, top comes off, don't care for big roll bars. This is not particularly interesting other than to mention I have NOT really explored rotating seat bases and can't contribute anything specific relating to those. As an aside, you are much more restrained than I am able to be when it come to assertions that ANY shoulder belt is somehow not superior to no shoulder belt; a trait to be admired.

The most useful piece of information I have run across has to do with the rated break strength of seat belt material. This is specifically related to another "internet expert" theory that seat belt manufacturer sewing is somehow magic sewing that can never be duplicated (it isn't). Here is a comparison of break strengths of various sewing patterns. The take away is that Seat Belt Webbing has a published break strength rating of 4720 lbs.

An article on webbing break strength testing:

it does not take many grade 8 bolts to hold up to 4720 lbs., either in sheer or tension. Observation tells me that most seat belt hardware is screwed down with one bolt of < 1/2", so the gives an idea of the needed strength.

So the problem becomes, what is needed at the base to withstand 4750 lbs. of force as applied to the top of the seat back where the shoulder belt force levers against the base and whatever hold the seat back in place. I would speculate that the real limiting factors will be either the mechanism which holds the seat back in place (designed by an engineer somewhere; hope he or she knew what they were doing) or the fasteners/fastening points where the base screws to the vehicle. It seems grade 8 or 10.9 will easily take the force, so now we are looking at how the base is fastened together and how is it fastened to the floor. A not heterogeneous plywood floor, at that.

I "think" a fabricated swivel base can be designed and built to take the force without a whole lot of drama and agree with you that custom fabricated is likely going to be the only way forward to meet your criteria. I have looked at various boat seat swivel mechanisms, and they don't "seem" strong enough for this use case (to my untrained eye). My real concern will be tearthrough on the floor. At least you have your father to run things past.

Finally, I keep a casual running list of bucket seats with integrated seat belts (driver and passenger; personal preference excludes bulky minivan seats).
- BMW 8-series (the old ones; nice looking; retractor is mounted at top of seatback)
- Mercedes SL's (Fairly nice looking; retractor is mounted at top of seatback; these (the ones I have observed in the junk yard) to not really mount to floor, but rather the vertical surfaces of the driveshaft tunnel and sill; very complex)
- BMW 3-series convertibles (bulky and unattractive; retractor mounts to either floor or base (belt runs diagonally down the back of the seat); lots and lots of these around)
- Chrysler Sebring Convertibles (think I have the model correct) (decent looking, unsure of how the mechanism works but I did see on that had FAILED a junk yard (so that is scary))
These are the ones I know of off the top of my head.

Good luck, and I will be closely following your journey with this. I will think about swivel ideas and share if I come up with anything worthy.

Tom Newell
I have a plan for the floor. The base will go through plywood floor, but the plywood will in no way anchor the seat to the motorhome structure. There will be pieces added. As far as your seat choices, my issue was never finding a good seat with an integrated belt. The problem was always finding a seat with an integrated belt AND two armrests. My old 2002 Silverado had integrated belts, but only an armrest on the inside. The Ram Promaster has an integrated swivel seat option from the factory, but it only has a single armrest and the parts counter wants $6000 each. That is not a misprint.
 

6cuda6

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Oct 3, 2019
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Ideally to do any 3 point or modern 2 point we need achor points that support the body and the seat under impact.....the old design just let the seat take the body impact which absorbs the crash energy. . (Like an airplane seat)

You would need to rebuild the whole floor frame structure to support the added seat weight plus the load for impact. Its not just add another bar under the floor, the whole deck has to be redone because the plywood is not structual nor is half the floor frame.

Anchor to the left/right pillars you say for the shoulder belt......ummmm last time i looked at the upright beams they were aluminum chanel and not steel tube.

And then want the seats to turn......theres a reason the drivers seat in a big rig doesnt turn and thats safety.

So yes it can be done......tear out the floor and redesign it.....when you think it looks strong enough....double it.

The company i work for builds vehicles that have none seatbelted seats.....they are currently building a unit with seats that have 3 point and had to redo the frame, sidewalls and add gussets to all frame corners (we use 2" square tube floor framing) because the impact loads are so high.

In my opinion, as a mecanic with 35 years experience, an engineering background in racing, 15 years in restoration and fabrication...in order to do it properly and safely, you would have to get it engineered......its one thing to do these things on a napkin but its a whole different ball game when your putting peoples lives on the line.....being ejected through the windshield buckled into a seat is a real possibility here. Just remember, GM crash tested whats there and it is certified...anything one does will not be therefore the vehicle is none compliant and your insurance can and will not possibly payout on a policy do to the none compliance.

Some things people can play with .....others should be left alone.....YMMV.
 

Mike Perez

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Oct 2, 2019
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Ideally to do any 3 point or modern 2 point we need achor points that support the body and the seat under impact.....the old design just let the seat take the body impact which absorbs the crash energy. . (Like an airplane seat)

You would need to rebuild the whole floor frame structure to support the added seat weight plus the load for impact. Its not just add another bar under the floor, the whole deck has to be redone because the plywood is not structual nor is half the floor frame.

Anchor to the left/right pillars you say for the shoulder belt......ummmm last time i looked at the upright beams they were aluminum chanel and not steel tube.

And then want the seats to turn......theres a reason the drivers seat in a big rig doesnt turn and thats safety.

So yes it can be done......tear out the floor and redesign it.....when you think it looks strong enough....double it.

The company i work for builds vehicles that have none seatbelted seats.....they are currently building a unit with seats that have 3 point and had to redo the frame, sidewalls and add gussets to all frame corners (we use 2" square tube floor framing) because the impact loads are so high.

In my opinion, as a mecanic with 35 years experience, an engineering background in racing, 15 years in restoration and fabrication...in order to do it properly and safely, you would have to get it engineered......its one thing to do these things on a napkin but its a whole different ball game when your putting peoples lives on the line.....being ejected through the windshield buckled into a seat is a real possibility here. Just remember, GM crash tested whats there and it is certified...anything one does will not be therefore the vehicle is none compliant and your insurance can and will not possibly payout on a policy do to the none compliance.

Some things people can play with .....others should be left alone.....YMMV.
I'll address one point at a time.

I'm familiar with airplane seats. I fly jets for a living. Let me tell you, there is a reason that the flight attendants put you in a brace position with your chest at your legs before the impact. Because if they didn't, your face would hit the seat ahead of you at a remarkable rate of speed. Guess what the pilot seats have up front? Shoulder harnesses. Flight Attendant seats? Shoulder harnesses. For ultimate safety, the passenger seats would all have shoulder harnesses too. They don't because passengers wouldn't like them and because low speed crashes are unlikely.

The "history" on the crash tests per the "story of a classic" article says they crashed these at 30-35 mph. It doesn't say how they did and to my knowledge, there is no video evidence. (Probably because GM wouldn't want it out there ). Get on YouTube and look up GM Crash Tests. They have been crash testing cars since the 50's. Just because GM crashed them, doesn't mean I want to be in one. To this day, class A motorhomes are still not required to pass the same crash tests that cars and trucks have to. "Certified" by who? The DOT didn't require crash tests on motorhomes.

Regardless of how the seat is restrained to the body, ultimately your body is restrained to the motorhome body through the seatbelt. I have no doubt that the seatbelt would hold my lower body in the seat. I also have no doubt that my unrestrained upper body would fly forward into the dash and steering wheel. I'll take my chances with a modern seat and custom base designed by a mechanical engineer (with load calculations and material analysis) vs the existing setup. I'm not trying to save my life at 50 mph. I'm trying to save my teeth from eating the steering wheel at 30 mph.

Yes, the uprights are aluminum channel. I'm not attaching anything there. If I was, I'd still believe that a shoulder harness anchored would still be better than nothing. So what if the anchor point failed? Would it be worse the no anchor at all? No shoulder harness and your face will eat the steering wheel. There is no way it wouldn't. Again, what is stronger, that aluminum structure or your ribcage?

Clearly non-turning seats would be safer than turning seats. If GM would have built the original seats non-turning, they would have been safer too. It is an RV, so a bit of safety is being given up for convenience. Do you allow anyone to ride in the dinette while driving? What would happen there in an accident? Pretty sure your face would hit the table pretty quick? What about the people riding backwards without high back seats? Definitely not good. There can be minor improvements. Nothing will ever be completely safe short of staying home. At 50 mph, the fridge is going to come through the cabinet and kill whoever is at the dinette if not kill the passenger seat occupant anyway.

The mounting points of the seat don't have to be fail-proof under all circumstances. At a certain number of G's, you will have head and chest injuries that will be fatal regardless. The key is to make a base/floor structure modification/reinforcement that will fail AFTER the number of G's that would have killed you anyway. So the question is.... do you actually think that your face would not abruptly hit the steering wheel in a 30 mph front crash?

I respectfully disagree with your analysis. 'Merica!
 
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6cuda6

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All good points ....my argument wasnt to question your skills etc...it was to raise a point that im seeing way to much in the GMC world, people making modification to vehicle systems that are safe in stock format and making them very unsafe by modifying/modernising them.

People dont even know that modern seatbelts have pyrotechnique reels or of late motor reels in them to pull the belts tight in an accident.

As i said, if engineered properly it could be done....simply bolting in a GM second row seat with intergral 3 point to the bottom rotation seat mount plate of these coaches wont suffice and its being done way to much.
 

Mike Perez

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All good points ....my argument wasnt to question your skills etc...it was to raise a point that im seeing way to much in the GMC world, people making modification to vehicle systems that are safe in stock format and making them very unsafe by modifying/modernising them.

People dont even know that modern seatbelts have pyrotechnique reels or of late motor reels in them to pull the belts tight in an accident.

As i said, if engineered properly it could be done....simply bolting in a GM second row seat with intergral 3 point to the bottom rotation seat mount plate of these coaches wont suffice and its being done way to much.
I very much agree. That's why after running numbers with the engineer dad, the modification of the seat base was completely scrapped and a one-off system has to be designed, engineered, and fabricated.
 

Justin Brady

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Interested to watch here.
We've got lap belts only which I've never been thrilled about but honestly I think we're pretty screwed in an accident any way you look at it so I haven't given much though to it.

I'm a fabricator, so I'm curious what your plan is for the swivel that will take the force of a collision.
The factory swivels are definitely not adequate for restraining the weight of the seat and the passenger in the event of a crash.
 

Justin Brady

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I would think going with a swivel that already is intended for the task would be the most prudent move.
HERE

Then you're just building a base which is trivial.

**After looking though that promaster swivel doesn't have integrated seatbelts so it's not engineered to take the forces we're talking about here.
 

Mike Perez

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Interested to watch here.
We've got lap belts only which I've never been thrilled about but honestly I think we're pretty screwed in an accident any way you look at it so I haven't given much though to it.

I'm a fabricator, so I'm curious what your plan is for the swivel that will take the force of a collision.
The factory swivels are definitely not adequate for restraining the weight of the seat and the passenger in the event of a crash.
Without giving too much away yet....initial design idea could be patentable; the key is that the swivel does nothing except rotate the seat while stopped. It will not be required to carry any load at all in an accident.
 

Mike Perez

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I would think going with a swivel that already is intended for the task would be the most prudent move.
HERE

Then you're just building a base which is trivial.
What you posted there is exactly what gets people in trouble. 100% off the aftermarket swivels are not designed for seats with integrated belts. The additional loads on the seat with a 200 lb occupant belted to it would not work. All of those aftermarket van swivels are designed for vans with seat belts bolted at the factory location.

I see you went back an edited....
 

6cuda6

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Without giving too much away yet....initial design idea could be patentable; the key is that the swivel does nothing except rotate the seat while stopped. It will not be required to carry any load at all in an accident.
I agree with you Mike, done right this has huge market potential not just in the GMC world but the whole industry.
 

Mike Perez

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I would think going with a swivel that already is intended for the task would be the most prudent move.
HERE

Then you're just building a base which is trivial.

**After looking though that promaster swivel doesn't have integrated seatbelts so it's not engineered to take the forces we're talking about here.
If you look at the OEM Promaster swivel seat option.....every component is different. Different base (probably just lower to adjust for height), a extremely beefy swivel mechanism, and a different seat with integrated belts. It could possibly be modified to work, but four problems:

1. $6000 each
2. Seats swivel on center and would most likely not work on the GMC anyway.
3. GMC Floor would need to be modified anyway.
4. $6000 each!
 

Mike Perez

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Take a look here:

From the site:
The kit includes the Turnout swivel base, vehicle specific mounting bracket and a seat with integrated belt. The kit has passed both crash and pull tests.
I've seen those kits that are designed to get people out of the vehicle to a wheelchair or for other disabled people. They have lots of parts and are very expensive. Not really setup for this application, but it can give you ideas of what is possible.

tlv_heart_770x500.jpg
 

Mike Perez

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Oct 2, 2019
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Also see

I can't tell if the "reinforced tether belts" are internal or external?
External. I called that manufacturer. See my original post with the "trucker" seat setup. You take the tether belts and attach them from the original seat belt anchor points and then they are bolted to the seat base. It may work fine, but would be ugly.