To my other TR6 pages

February 5, 2017

Seat Belts

My car was fitted with 3-point inertial seat belts.  This must have been sort of a big deal back in the day, since I have seen some early 70s ads bragging about it.

I was a little overwhelmed at first when I dumped out the box with the seat belts, until I realized there was a set of GT6 belts in there, too.

The heart of an inertial system is a spring loaded reel which pays out the belt freely except when there is a sudden motion of the car, in which case it locks the reel.  The reel, spring, latch, and acceleration sensor are all in these aluminum housings mounted at the base of the rear wheel wells.  My units were dirty, crusty, and frozen.

Under the cover at one end of the unit is the sensor and latching mechanism.  The sensor is just a weight suspended from a plastic ball joint.  During any kind of acceleration, the weight's inertia will cause it to deflect from vertical.  When it does this, it slightly raises a plastic lever so that a pawl on the lever catches one of the teeth of a plastic ratchet gear tied to the belt reel.  If there is outward tension on the belt, the plastic gear will move the lever further, such that it presses on a metal pawl which engages with the main ratchet gear on the belt reel shaft.

Under the cover on the ther end of the housing is the reel spring. It is a "constant force" spring, meaning that the torque it exerts isn't affected much by how much it is wound up. This is due to the fact that most of the turns of the spring are coil-bound, and only the free turns in the middle section of the spring are active in generating force.   If I lifted my thumb off the spring, it would instantly escape in a frenzy of exuberant springy energy.

To disarm the spring, I pulled all of the webbing out of the reel housing.  This exposes a pin that captures the end loop on the belt to the reel spindle.  Removing that pin frees the belt from rotating with the spindle.  Before pulling the pin, I put a clamp on the spindle so I could control the unwinding of the srping.  Mainly, I wanted to count how many turns it took to relax the spring so I'd know how much to wind the spring up in case I was lucky enough to get everything back together again.

The relaxed spring comes out with its own little plastic carrier.

With the spring and other parts gone, the actual reel spindle can be removed.

With everything now taken apart, it was time to inspect and renew parts as necessary.

The pendulum assembly seemed mostly OK, but the weight was really corroded and had flaking plating.

With some new plating and a new tubular rivet, it looked a lot better.

Next up was the webbing and its various fittings.   Some of the fittings were a real mess.  The worst of them were stitched into the webbing, so the web would have to be cut or unstitched to get at them for renewal.  I unstitched the end fitting since I hadn't decided whether I was going to try to reuse the webbing.

The fitting cleaned up and plated nicely.

On closer inspection of the webbing, I really had no choice.  It was dirty and faded, and in places, it was pretty worn.

So I bought some seat belt webbing.  Now the only problem is that I don't sew.

Well, I didn't sew, but I guess I do now, sort of.  I didn't really want to invest in a sewing machine, especially a heavy duty one man enough to sew multiple layers of this webbing.  Some research revealed to me that sailors sometimes have to repair their canvas sails without benefit of a machine, and there may even sometimes be places on a sail that a machine can't reach.  In these cases, they are forced to revert to the ancient art of hand stitching. Pictured below is one tool apparently popular in sailing circles.  It makes a lock stitch, just like a machine, but slower.  I sewed up a few practice pieces and eventually decided that I was certinly no Betsy Ross, but I could make a strong seam.  I definitely got better as I went.

So with most of the component parts renewed, it was almost time to reassemble.  Just needed to make the main housings more presentable.

I bought some silver hammertone powder coat powder.  It turned out to be darker and more contrasty than the original hammertone paint, but I'm not going to fret about it.

I was even able to clean up and reapply the original manufacturer's logo.

Installed the spindle and the spring and cover plate.

Then, to wind up the spring, I made this little crank thingy that fits over the end of the spindle.  The picture doesn't show it, but the end loop of the webbing has to be installed on the spindle before tensioning the spring.

I gave the spring about 20 turns to wind it up, inserted the keeper pin to capture the web, and let the web slowly wind back onto the spindle.  It only takes about 12 turns for the web to fill the reel cavity and stop.

Now on to the inertia mechanism.  I applied some light grease in places where I found dried up grease on disassembly.

Put the cleaned up cover back on, and this dude is pretty much done.  I still wonder why the mfr made that red plastic piece looks so much like a pushable button.

Now just have to do the other one.

I realized after I was done with both units that there was originally a plastic cover piece on the back side of the reel housing.  I found the broken cover in the bottom of the box the belts were in.  The cover attaches by a fragile little pin at each corner that fit into tiny holes in the housing.

I'm not sure this cover is really that necessary, but I wanted complete closure.  I made a couple of aluminum plates that fit pretty snugly.  I didn't think I could recreate the pin mounting arrangement (that didn't really work that well anyway), so at least for now, I just fixed the covers in place with strips of packing tape.

On the shelf with these guys.

Though the inertial mechanisms are done, there is another part of the seatbelt system.  There is a latch mounted on a stalk that sticks up on the inboard side of each seat.  The latch accepts the tang on the seatbelt and hold it securely until the release button is pushed.  One of my latch pieces had some violence in its history.

Now I can fix a lot of things, but I just didn't see any practical way to fix that plastic shell.  I looked online for some stalks for sale, but didn't find any available by themselves.  Next, I appealed to the LBC community on one of the popular forums.  A gracious and generous member sent me a couple of sets of stalks.

The stalks were not from a TR6, but a Jaguar, I believe.  They were very similar to mine, but did have some differences.  The most significant difference was the actual stalk part that the latch mechanism attaches to.  The new parts had shorter stalks with a different mounting angle.

Now on the original parts, the latch connects to the stalk with a roll pin, and the pin is accessible through a plugged hole on either side of the latch shell.

Unfortunately, the new parts didn't have the access holes.  Assuming the innards of the new parts were the same as the old, at least in how the latch attached to the stalk, I drilled a small exploratory hole.  Bingo--the roll pin was right where I hoped it would be.  I enlarged the hole and drilled one on the other side so I could drive the pin out.  Then attached the original stalk to the new latch and plugged the holes with the original plugs.

Can't wait to install these.

This was a fairly involved project, but pretty cheap.  Though aftermarket seatbelt systems are available and reportedly work well, I'd probably do this again given the chance.  At least my sewing would be better on the next one.

Comments to Ed at

To my other TR6 pages