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Old 05-16-2013, 07:12 AM   #1
Bill Goddard
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Default Sudden stop shock load

Can someone tell me how to calculate the shock load in lbs of a sudden stop from 60 mph? Or maybe the G - load? I am trying to design a seat belt anchor system. Thanks Bill G
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Old 05-16-2013, 07:40 AM   #2
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Default Re: Sudden stop shock load

Acceleration is change in velocity divided by time. How fast are you stopping?
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Old 05-16-2013, 08:55 AM   #3
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Default Re: Sudden stop shock load

Assume worst case against a wall. Bill G
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Old 05-16-2013, 09:34 AM   #4
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Default Re: Sudden stop shock load

It's difficult to give an accurate answer without knowing the crash characteristics of the Model A, but I can show you how the calculation is done:

Assume a deceleration time of 0.5 second from 60 mph to zero. THIS IS A BIG ASSUMPTION!

60 mph = 5280 ft/min = 88 ft/sec, so deceleration = 88ft/sec / 0.5 sec = 176 ft/sec^2

Assume a 180 pound person ( mass of 5.6 slugs);
F=MA =5.6*176 = 984 pounds. This is the force on the seat belt; you can divide by two for the force on each attachment point.
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Old 05-16-2013, 09:53 AM   #5
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Default Re: Sudden stop shock load

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Assume worst case against a wall. Bill G
@60mph you will not need belts, just an undertaker. If you do not have a shoulder harness the steering wheel will likely kill you at 20mph.

I have belts and shoulder harnesses in my 160B, but I realize they really only help in low speed events. Above 20mph all bets are off.

I used the anchor plates available from the belt companies. Yes, you could rip them through the body, but that amount of force would probably cut me in half with the belt anyway.

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Old 05-16-2013, 10:13 AM   #6
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Default Re: Sudden stop shock load

Maybe some of the folks from "Down-Under" will respond. From what I understand, seat belts are required and the attachment points have to be properly engineered and subsequently inspected so as to meet specs.

For my own use, I'm pretty much in line with MikeK in that they are basically there to keep me from sliding out when going around a corner and the door pops open. (And my wife requires one when she rides with me!)
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Old 05-16-2013, 10:22 AM   #7
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Default Re: Sudden stop shock load

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Maybe some of the folks from "Down-Under" will respond. From what I understand, seat belts are required and the attachment points have to be properly engineered and subsequently inspected so as to meet specs.

For my own use, I'm pretty much in line with MikeK in that they are basically there to keep me from sliding out when going around a corner and the door pops open. (And my wife requires one when she rides with me!)
Same here. Since I don't have any doors (or windows...or roof...) my seatbelt is there to keep me from bouncing out if I hit a big pothole.
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Old 05-16-2013, 11:39 AM   #8
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Default Re: Sudden stop shock load

Restorer magazine May Jun 1993 has a multi-page story of belt install with pictures and measurements. Coupes are mentioned in the text and drawings and a Tudor seat is in one of the pictures.

I installed seat belts in the Coupe in 1963. Attachment was to the body sub-rails in Rumble seat area where your feet are when you ride in the Rumble.

In 1966 this anchor system was tested when I hit the rear of 56 Plymouth at 45 MPH that was stopped in the road.

Belts and sub rails held just fine and did not cut me in half. Only some brusing of my shoulder as it hit the steering wheel.

Last edited by Benson; 05-16-2013 at 11:52 AM.
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Old 05-16-2013, 11:52 AM   #9
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Default Re: Sudden stop shock load

Does this help, rear anchor point.. Mike K is correct.. Your belt in a model a is ONLY going to keep you in the car when the doors fly open "IF" your lucky.
here they are secured to the rear.

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Old 05-16-2013, 04:23 PM   #10
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Default Re: Sudden stop shock load

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...my seatbelt is there to keep me from bouncing out...
Haha! Mental picture!
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Old 05-16-2013, 07:28 PM   #11
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Default Re: Sudden stop shock load

MikeK, Could you tell me where you bought those shoulder belts? thankx Bill G
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Old 05-16-2013, 09:34 PM   #12
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Default Re: Sudden stop shock load

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MikeK, Could you tell me where you bought those shoulder belts? thankx Bill G
Bill, It's a complete three-point retractable inertia lock (ELR) system like in a modern car. The door post mounting for the retractor and upper swivel took some engineering on my part for attachment and strength. They are available from several vendors, many are USA made and assembled, some are cheaper chinese LINK
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Old 05-17-2013, 08:01 PM   #13
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Default Re: Sudden stop shock load

MikeK, Thanks for your help. Bill G
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