TheSamba.com :: View topic - BusDepot front cot WITH 3pt seatlbelt on jump seat

Wed Mar 09, 2022 10:56 am

Spreader wrote:
For now, the Mrs. has given me approval to NOT put in the rear facing 3pt seatbelt. So I'll be installing the cot as per usual - blocking the lower seatbelt bolt hole.

Sounds good.
Here is another thread that has measurements and tips for installation.
https://www.thesamba.com/vw/forum/viewtopic.php?t=701257&highlight=child+cot

Wed Mar 09, 2022 11:09 am

Moving seat belt anchor points is not a big deal. I wanted a jump seat and I also wanted the PS to be put in facing backwards The seat belt retractors are in the way so I moved them.
In this case it was the lower. Cut a piece of ½" aluminum, drilled and tapped. Removed the retract and eliminated the plastic lipstick cover, drilled holes for the new position. Through the fresh air vent in the B pillar, put in the aluminum plate and held it in position with string and bolted it up. Would be no problem to move or install ones at the top either.

Duncan

Thu Jul 14, 2022 9:49 am

Spreader: Any updates? Are the wife and kids happy?

Sun Sep 18, 2022 5:51 pm

JimmyJagged wrote: Spreader: Any updates? Are the wife and kids happy?

Wife still thinks I'm nuts, but the kids love the van.

I bought the Westfalia repro cot kit from BusDepot. I did NOT buy the 3pt seatbelts for the rear-facing jump seat.

Buying one less part for the van was an easy sell.

And they're already rear facing... so inherently safer than either kid sitting on the back bench! Now I still have to get the 3pt belts installed for the bench, but don't remind my wife.

Mon Sep 19, 2022 7:19 pm

DuncanS wrote: Moving seat belt anchor points is not a big deal.
In this case it was the lower. Cut a piece of ½" aluminum, drilled and tapped. Removed the retract and eliminated the plastic lipstick cover, drilled holes for the new position.

Duncan
1/2” aluminum is not strong enough due to lack of engagement length.

Mon Sep 19, 2022 9:08 pm

A secure attachment for the retract seat belt mechanism. !/2" aluminum is not good enough? What do you use for criteria to assume that? In a crash, the loads are in shear, not withdrawal. Ultimately what determines the strength is the thickness of the B pillar steel and how the load is distributed. If it were 20 gauge, with a small load area, the bolt could deform the tin and tear the tin and create an oblong hole, angling the bolt and changing the load from pure shear to a combination including withdrawal. By having a large backing plate "nut " which won't allow the steel to deform, the loads will always remain in shear. My aluminum backing plate measures 3" x 7", 21 square inches of load transference. The factory used a welded captured nut with a distribution of around a single square inch. Even in a roll over, the likelihood of their being a withdrawal load is remote. Thread depth and the material is a function of withdrawal strength, which is not what is needed here. I used ½" aluminum as I wanted a large "nut" to spread the load over as much of the B pillar material as possible. In addition, the aluminum is secured to the factory tin with 3M 5200 which will make elongation of the hole almost impossible.

Duncan

Tue Sep 20, 2022 3:46 am

I think the question is the thickness of the plate bring 13 mm, meaning it’s engaging 13 threads of the bolt. I would want to figure out the thread failure point of a steel bolt in that plate and be 5x above expected loads

The rough engineering rule of thumb is that depth equal 1.5x bolt diameter approximates tensile strength of bolt, but for a safety belt you want more fingers on that calculation.

Tue Sep 20, 2022 10:02 pm

Abscate wrote: I think the question is the thickness of the plate bring 13 mm, meaning it’s engaging 13 threads of the bolt. I would want to figure out the thread failure point of a steel bolt in that plate and be 5x above expected loads

The rough engineering rule of thumb is that depth equal 1.5x bolt diameter approximates tensile strength of bolt, but for a safety belt you want more fingers on that calculation.

Did a bit more looking. First the factory captured nut is about 3/8"thick. I have ½" so there are 30% more threads than factory has.
The chart shows the factory bolt would have 150k psi stripping strength. If mine were steel then I'd have 195k, but at 40% I'm down to 78k. This is a straight withdrawal capability. And I have a larger surface area behind the B pillar tin. My guess is if you put 150k of withdrawal on the factory nut, it would pull straight through the B pillar. So would 78,000 psi. So at what point does the human body fail? Surely way under a 78k psi load.

Caveat-- I have compared my system with an SAE grade 8 bolt against the factory metric components without trying to calculate the difference in diameter of the bolt nor the thread pitch. I'll let the nitpickers rework the math, but the differences can't be very great.

But as I said before, there is only a very small withdrawal force. Because of the "pulley" component of the attachment above the shoulder, almost all of the load is in sheer. When the belt stops, then the lever arm of the remaining amount of belt will try to lift the bolt out of the hole. I'm guessing here, but I'll bet the roller would fail long before the bolt and just rip the end of the belt free. No doubt, Wolfsburg did all of this testing to failure many times before they were satisfied. No doubt, these numbers are for failure. For a safe working load of 1/5 of failure, we are still way ahead what my aging bones and wasting muscle mass can withstand.

Given my greater surface area, I'm guessing in a failure test, I would out perform the factory attachment method.

This has been instructive and interesting for me, so thanks for encouraging my curiosity to discover some of the science behind what for me was obvious to all as seat-of-the-pants engineering. Or is it top-of-the-shoulder engineering?

Duncan