Sunday, September 29, 2019

Vehicle-buggy accidents, Part 4: Seats

The fundamental challenge in designing a system for the Amish buggy rocket sled is the potential variation in ballasting.

Longitudinal rails that generate a 15G pulse when accelerating 800 pounds of humans and 200 pounds of apparatus generates a 37G pulse when the only occupants are 2, 100-pound boys. 37G is not survivable.

Dialing back the longitudinal rail structure so it produces a 15G pulse when the two boys are riding in it produces a 6G pulse when carrying 4, 200-pound men and requires over four feet of crush space. If you run out of crush, then the accel spikes and you kill the men.

Twelve inch links tipped 30 degrees forward
Providing each occupant with their own seat is one way to address differences in ballast. That would be a change from the current Amish practice of one, common bench seat.

Same links tipped 30 degrees aft. Total rearward travel of 12". That is 50% of the 24 inches we need for the 15G, 30 mph rocket launch or a third of the 36 inches we need for the 10G launch.
I used the symbol for a spring in the image but it needs to be constant force spring. One way to create a constant force spring with the required degree of extension is to have a lubricated, tapered mandrel expand the inside-diameter of a tube as it is pulled through the tube. The free end of the mandrel would be attached to bottom of the seat and the free end of the tube is attached to the top. It is prudent to fill the tube with grease to prevent rust.

System tuning
We still have some significant, technical challenges. It is not possible to provide adequate protection for all occupants with one constant force spring. There needs to be at least three of them and we need to design a system that can turn them on-and-off.

To illustrate the challenge:
Suppose we want to stay within the 10G-to-15G window. A spring that produces enough force for a 15G, 50-pound girl (a typical, seven-year old girl) will produce a 10G pulse for a 75-pound girl (a ten-year old girl) and a 4G pulse for a 180 pound man.

Suppose it were possible to design a mechanism that locked a seatbelt buckle beneath the seat when a person who was sufficiently heavy sat on the seat. Furthermore, assume the seatbelt webbing ran parallel to the other constant force spring. Occupant restraint systems already use tear-out stitching to limit the forces on occupants and to manage energy. The tethers on fall-hazard harnesses use the same technology. Ripping stitching is proven technology to manage loads and energy.

If a sufficiently robust system were found to latch-unlatch the webbing, then next range would be from 76 pounds (still a ten-year old girl) to 115 pounds (a fifteen-year old girl, although Amish tend to run small).

A second belt (third spring) can be added which could be tuned to keep 116 pounds to 174 pounds in the 15G-to-10G window respectively.

Word picture
Imagine a full grown man sitting in one seat and a second-grade girl sitting in the seat next to him.

They are rear-ended by a pickup truck.

The full grown man's weight depressed the seat cushion and inverted, seatbelt buckles engaged two lengths of webbing in addition to the default, diagonal constant force spring.

The man gets a rocket launch of a bit less than 10G if he weighs more than 175...rather rare for an Amish man. They DON'T tend to fat.

The little girl's weight is not sufficient to depress the seat cushion and the only diagonal member that is engaged is the default, constant force spring.

The little girl get a 15G rocket launch.

Part 5


  1. The other option would be a pressure switch in the seat cushion calibrated to various weights vs. depth of compression.

    1. The issue is whether the Amish would trust them and use them.

      There are reasons the Big Three don't use weight sensing technology in seats (beyond infant sensing to turn off air bags). There are too many ways it can malfunction. Right now, there are automotive engineers dry-heaving because I suggested seats with multiple locking features. The litigation exposure if it was in a factory-built unit is staggering.

      Something as simple as a squirrel wedging a hickory nut beneath the seat will cause malfunctions.

      It is a whole different kettle of fish if Billy-Bob whips one up beneath a shade tree and then lets his family ride around in the buggy.


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