Seat Belt Mounts

[BDA]

Nice installation, Brian! Now it looks good and it's safer! 

[Grumblebuns]

Very neatly done. I was wondering just how large an area your 3 mm backing plate covers. FYI, I've attached a couple of pictures of how the federal seat belt take up reel is ultimately attached to the chassis by using two intermediate braces.

Joji Tokumoto
Fallbrook, Ca

[EuropaTC]

Hmm, that's interesting and had me scratching my head and looking for the Parts Manual.  The engine bay on my car has pretty much the same arrangement of tubular cross bars across the bulkhead and a wire stay that goes to to the rear wheel arches, but not the one you've shown inside the wheel arch and presumably going to the top damper mounting.  And I don't ever recall seeing one, so I pulled the parts manual and found yet another Lotus Variation 

It seems on the inertia reel belts Lotus decided to fit not only a different mounting structure for the reel (quite reasonable) but also an extra strut linking back to the rear dampers. Looking at the parts manual (image below) it's quite obvious that for some reason they made this change just for the Federal versions. Your seat belt regs must be tougher than ours ?   

I'm surprised at the difference in design though.  I'd have thought if you managed to get enough force to pull the two tubular struts plus the rearwards tie strut through the bulkhead by the action of your chest on the seat belt, then not having that wheel arch tie strut would be the least of your worries !

To get back on track;  I didn't take measurements, it was a case of "try it in place, mark it, cut it" engineering      The steel plate is the width of the reel and it's mounting plate, which incidentally looks to be a wimpish 1mm steel. It is mounted inside the cabin because it ties together the upper belt guide bolt and the bottom mount for the reel assembly.   

My thoughts were that in an accident the first point of restraint would be the belt as it passes through the top guide and that's going to pull vertically on the reel to lock the belt. The plate I added is just there to hold the reel and allow it to lock, it doesn't take any horizontal or forward loading. It will take some vertical load & putting the plate in compression but as it's all in line I could see the webbing snapping before the plate starts to bend.  I suppose I could have just bolted the reel to the fibreglass bulkhead with some large washers, but I get a warmer feeling knowing I have a steel plate rigidly linking the 2 mounting points rather than 3mm of fibreglass.   

Once the reel locks the main force diverts into the belt and it's upper guide which is on the Lotus OEM mounting bolt & strut/tie arrangements as for the static belts, so I'm back to trusting the Lotus designers.

I really wanted to mount the reel down on the sills using the lower belt mount plate, but there just isn't enough room. This seemed the best compromise whilst keeping in mind the design principles on how the kit was intended to be installed.
 
Brian

[BDA]

You could be right about the efficacy of the earlier design. Certainly the later design is much stronger. I highly suspect that Lotus is either soothing some bureaucrat or being defensive so as to avoid dealing with one. This just points to one reason the Europa went out of production - lack of structural steel as in door beams and bumpers - and also why it is so light!

[blasterdad]

Looking at the parts manual (image below) it's quite obvious that for some reason they made this change just for the Federal versions. Your seat belt regs must be tougher than ours ?

Does anyone know when they changed to the inertia reel system? Mine is a '71 Federal S2 & it doesn't have them...
 Just curious. 

[BDA]

My guess is it was with the TC or maybe the Special. Maybe somebody else has better information.

[EuropaTC]

I think BDA has it on the head with "soothing some Bureaucrat" because I couldn't find the additional strut listed for the S2 variants either, it just appears in my TC parts manual as for the TC & TCS Federal versions.  I can hear the conversation now after Lotus announced they were introducing the TC engine in the 70s....

"you've put a heavier engine back there on the same engine mounts. What happens in an accident when it moves forwards and collides with those seat belt cross braces ?"
"hmm....   dunno. The braces stop the engine ? "
"can you show us the calculations that prove the fibreglass wheel arch supporting the bulkhead brace to wheel arch strut is strong enough to prevent the engine squashing the driver ?"
"hmmm....  calculations.... that's sums, right ?"
"yep"
"hmmm....  how about we add another strut and tie it back to the steel chassis ?"
"ok".......

    Idle speculation fueled by a beer of course, and based on the observation that the additional strut forms a bolted "cage" from the suspension cross member down the sides of the car and to those bulkhead cross braces.

Moving slightly off topic, before I looked at mounting these belts I did some digging to see if I could find what was generally accepted to be the design forces were involved and found a few "worst cases" on physics websites.   Here's one for example, a 30mph crash into a solid object.   http://hyperphysics.phy-astr.gsu.edu/hbase/carcr2.html#cc1

Over 2 tons ? gulp....   I now understand why we have airbags in cars.

[BDA]

"you've put a heavier engine back there on the same engine mounts. What happens in an accident when it moves forwards and collides with those seat belt cross braces ?"
"hmm....   dunno. The braces stop the engine ? "
"can you show us the calculations that prove the fibreglass wheel arch supporting the bulkhead brace to wheel arch strut is strong enough to prevent the engine squashing the driver ?"
"hmmm....  calculations.... that's sums, right ?"
"yep"
"hmmm....  how about we add another strut and tie it back to the steel chassis ?"
"ok".......

Since we've gotten on the subject of design, has anybody pointed out the brilliance of using the driver and/or passenger to protect the valuable frame? Something to think about when crossing a big intersection! (sort of like Gurney keeping in mind that the Eagle F1 car had a magnesium tub when worrying about getting a flat tire!) Keep alert and drive defensively!

[TCS4605R]

Brian,

The seatbelt mounting reinforcement in the Twin Cam is weird.  If you follow the seatbelt reel bracing back to the rear of the car, it terminates at the top of the shock mount - a long way to go to find a firm anchor.  I guess you saw the photos of some of my restoration work.  My attention to detail on my restoration has added up to 22 years of on and off work.  I'm finally ready to paint the fiberglass shell after DA sanding off six layers of old material and doing fiberglass repair work.  Need to complete the work before I'm too old to get into it.

Tom

[LotusJoe]

Weird is a good way to put it. The double bracket from the seat belt mounts to the shock tower bolt is strange. I suppose the angular bracket going from the belt mount to the frame helps....but, I bet not much. The reality is if you got hit hard in the rear, you are going to get hurt. To me it looks like a last minute engineering job to conform with the seat belt laws in the States in the 70's. And how many people have removed the cross bar between the seat belt mounts when they got rid of the vapor catch can.

[BDA]

Lotus' response to the bureaucratic regulations is a bit weird, but there was almost no other option. Your point that if you get hit or hit something solid in the car, it's probably going to hurt and given that, the lengths they had to go to to beef up the restraint system is a bit silly (in the real world). But I don't think I've seen anybody who had done away with the cross bar between the seat belt mounts. You'd be left with only a diagonal strut to attach the seat belt retractor to unless you also kept the rod arrangement to the upper shock mount. That would be even weirder!

When door beams were mandated, there wasn't even an engineering kludge they could use so one reason for the end of the Europa.

[blasterdad]

 
Let's face it, this would be our only hope!