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Old 05-03-2015, 01:37 PM   #1
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Crown Owners/Engineering Question/Seat/SeatBelt

Hey all,

I purchased two captain's chairs from a junk yard from a town and country mini van. They are seats for my kids. I am placing them rear facing on top of the wheel wells in the front of the bus. I have all grade 8 bolts however my question is how wide of a washer is sufficient so the seats do not rip the wheel well out in case of a crash.sudden stop. Granted I highly doubt this bus could make a sudden stop even if it hit something. The wheel well metal is not as thick as other metal parts of the bus. I am bolting it down in 4 places. The metal looks to be 3/32's thick. I know crown used the high tensile steel for the construction, and probably did here too.

Seems to me since I have the bolts in 4 places. That would equate to about two bolt pulling on slightly thicker than 1/4 metal. Which seems sufficent. I have grade 8 washers that are 7/8th inch wide.

So half of 7/8 is .4375 squared is .1914 time 3.14 x 4 bolts = 2.4 inches of surface area.

Now I am sort of loss in determining the tensile strength for that thickness of steel and how to plug that in to get a number.
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Old 05-03-2015, 03:12 PM   #2
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flat bar stock

I assume the bolt holes for the captains chairs are about 12in from each other,
I would say get two pieces of 1/8in thick flat bar stock, about 1in wide by 16in long, and use these pieces as long washers on the underside of the wheel well.
I would also want to confirm all the edges of the wheel wells are welded on and not just a few spot welds holding them on, might be just a mess of some type of rubber insulation keeping the air out.
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Old 05-03-2015, 04:26 PM   #3
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bubb,

funny you say that I actually just purchased some of that just yesterday for some under support for some cedar shelving that sounds like a plan. Also good point for checking if its welded.

Unfortunately I have painted up under the wheel well so its hard to say if it was welded there. I can see on the outside part (towards the wall of the bus) there is an inch lip that holds it on all the way down on the wheel well so that side is secure. From the inside the top of the wheel well and the semi circle side are very close together and I tried prying at it with a crow bar and it didn't loosen. Though I could not get the lip of the crow bar in very deep. I did not notice any rubber sealant. Seems like if there was some it would be 1/8th inch thick or so It. I will try to investigate a little further though as that could be concerning.

Anyone else have a crown that has explored this idea or messed with their wheel wells before?
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Old 05-03-2015, 06:53 PM   #4
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You should really read through gmarvel's build. He had a seat mount made over the wheel tub.
http://www.skoolie.net/forums/f11/jo...n-8600-29.html
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Old 05-03-2015, 07:01 PM   #5
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one other piece of the puzzle to think about, the kids or adults weigh X, so in an accident they will achieve a forward motion in relation to stopping power

If you hit something that will stop your 18000 lb bus instantly...you are screwed as well as the front 10 ft of the bus, if you are going 45/55 mph there will not be anything left

so look at very fast stops, my bus will lock up the back duals and stop way faster than you would think

I would actually tie into the side rail and floor with a couple seat mounts, the wheel wheels are designed to stop a tire from coming in, not to mount a seat (would actually probably be okay)
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Old 05-03-2015, 07:22 PM   #6
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What happens when a tire blows in that wheel well? Nice little bounce?

Just add a nice 1/4 plate underneath. No need to weld it there, just bolt through the wheel well and through the 1/4 inch plate. Now there is no way the bolts will rip through the wheel well.

My seats will be on swivels with only 4 bolts, spaced 12 inches apart.
I too will be adding a 1/4 inch plate to the underside.

Nat
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Old 05-04-2015, 04:54 AM   #7
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Hv

I Had been looking through his conversion before but missed that part thanks.

Bansil,

I agree that if there is enough stopping power to stop my more like 30000lb bus enough to rip the wheel well off and put the seat through the window then whether or not it was bolted down enough to not be thrown through the window anyone sitting in that seat is gonna be toast anyway. I can't really imagine that happening except for hitting another bus or semi head on in which case you are facked. The G force of that alone would probably shatter your bones or snap your spine if you were that close to it

Nat,

The seat has its own mount that is about 1/4inch thick and was quite a bit more difficult to drill through than the wheel well; that should suffice for the 1/4 plate you recommended I would imagine.

Thanks for your input. Everyone. Does anyone disagree with my response to Basil?
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Old 05-04-2015, 08:38 AM   #8
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Quote:
Originally Posted by HunteR0se View Post
Nat,

The seat has its own mount that is about 1/4inch thick and was quite a bit more difficult to drill through than the wheel well; that should suffice for the 1/4 plate you recommended I would imagine.

Thanks for your input. Everyone. Does anyone disagree with my response to Basil?
Nat was recommending another piece of 1/4" plate on the wheel side of the well, to help distribute the load and help keep the bolts from tearing through the sheet metal. Unless it came with two pieces.

Pics would help for the visualization
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Old 05-04-2015, 09:05 AM   #9
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You guys need to do a little math here - don't calculate the force of an impact ripping off the wheel well. Calculate the force of an impact that wants to flex the CHAIR, then the total weight of chair and occupant, then multiply by the length(s) of the mounting leg(s). A plastic chair holding a doll could probably survive the disintegration of the front of the bus. A 300lb guy belted into a single-post captain's chair like off a boat might tear out the bolts through 16ga sheet steel just from a hard stop.

The object in the chair wants to stay in motion, and the force of decelerating it will be transmitted up (yes, basically "pulling back" on the chair to slow it down) through the mounting leg(s) via the mounting points. That basically makes the leg(s) a lever, which multiplies the force applied to the floor. Also, remember that the force involved here is torsion, not shear. Sheet metal is weakest in torsion.

There's a summary of the math involved here, you can just adjust the numbers to suit:
http://hyperphysics.phy-astr.gsu.edu/hbase/carcr2.html

There's a cute reference to unbelted occupants and windshields, which has nothing to do with this chair mounting case - but it IS a good reminder of what force-multipliers can do. A chair leg is a force multiplier.

An adult occupant sitting atop a 2' leg (lever) will have their weight multiplied by that distance in terms of the force they apply to those bolts. Add in a 2g+ hard stop (not even a collision) and now you have several hundred pounds of force involved. Now imagine hitting a tree even at 30mph - even at 10mph from a parking break not set! - and you could be talking several tons of force. Four bolts through 16ga sheet metal - that chair's occupant is going to be a missile. Sixteen bolts - they'll probably make it.

But now, put a 1/4" backing plate under there. This does more than just provide thicker material to bolt to. It's also going to help transmit more of that torsional force laterally, converting it to a shear force. To tear out the wheel well, the force is going to need to "rip" a much longer line of it, and it'll be doing it much further away from the mounting point. Where this happens, the force is now pulling mostly sideways, and sheet metal is much stronger in this direction. (You can prove this to yourself. Rip a sheet of aluminum foil in half the usual way - easy. Try pulling it apart sideways and it's much harder to do.)

Even without doing the math further (no idea what chair you're installing), the 1/4" backing plate sounds like a great idea here.
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Old 05-04-2015, 09:08 AM   #10
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Oh, and where you put it does matter. It's not the thickness of the material that matters, although thicker material can redirect those forces better without bending. Putting it on top of the wheel well means as soon as you have ANY separation (even a thousandth of an inch), any part of the mounting plate not in contact with the wheel well is basically doing nothing to redirect those forces. Putting it underneath keeps it in contact at all times during this period of stress.

I'd still put it underneath. But since you have a 1/4" flange on top here, you might get away with 1/8"-3/16" underneath.
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