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What causes a retractable seat belt to lock up during hard braking ?

Kern Dog

Life is full of turns. Build your car to handle.
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I've searched online but maybe I'm asking this wrong.
On a regular, properly functioning seat belt system, when the brakes are applied, the belts extend slightly then lock, preventing an occupant from coming out of their seat.
I want to know what causes the retractor to lock.
This is in regards to older cars before air bags. Is there some type of inertia mechanism or pendulum that swings and locks it up? Are they just designed to lock if they are extended/pulled out too fast?
 
I kind of remembered there was a part in them called an inertia reel so I looked quickly and found the following:
An inertia reel seatbelt stops
through a mechanism that is activated by either a sudden deceleration of the car or a sharp tug on the belt itself. When a crash occurs, the car stops abruptly, but the occupant's body continues to move forward due to inertia, pulling the belt out of the reel at a high speed. This rapid unwinding triggers a lock in the retractor, often by a mechanism with a swinging pendulum or a centrifugal clutch, which then prevents the belt from extending further.

So I think you guessed it.
 
I found this:

*************************************************************************************************************************

Extend and Retract​

YiLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjI5MH19fQ==.png
A spiraled spring rotates the spool to keep the seatbelt webbing taut.
In a typical seat belt system, the belt webbing is connected to a retractor mechanism. The central element in the retractor is a spool, which is attached to one end of the webbing. Inside the retractor, a spring applies a rotation force, or torque, to the spool. This works to rotate the spool so it winds up any loose webbing.
When you pull the webbing out, the spool rotates counterclockwise, which turns the attached spring in the same direction. Effectively, the rotating spool works to untwist the spring. The spring wants to return to its original shape, so it resists this twisting motion. If you release the webbing, the spring will tighten up, rotating the spool clockwise until there is no more slack in the belt.
The retractor has a locking mechanism that stops the spool from rotating when the car is involved in a collision. There are two sorts of locking systems in common use today:
  • systems triggered by the car's movement
  • systems triggered by the belt's movement
The first sort of system locks the spool when the car rapidly decelerates (when it hits something, for example). The diagram below shows the simplest version of this design.
YiLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjI4NX19fQ==.png

The central operating element in this mechanism is a weighted pendulum. When the car comes to a sudden stop, the inertia causes the pendulum to swing forward. The pawl on the other end of the pendulum catches hold of a toothed ratchet gear attached to the spool. With the pawl gripping one of its teeth, the gear can't rotate counterclockwise, and neither can the connected spool. When the webbing loosens again after the crash, the gear rotates clockwise, and the pawl disengages.
dpZiIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6MjAwfX19.png


The second kind of system locks the spool when something jerks the belt webbing. The activating force in most designs is the speed of the spool rotation. The diagram shows a common configuration.
The central operating element in this design is a centrifugal clutch— a weighted pivoting lever mounted to the rotating spool. When the spool spins slowly, the lever doesn't pivot at all. A spring keeps it in position. But when something yanks the webbing, spinning the spool more quickly, centrifugal force drives the weighted end of the lever outward.
The extended lever pushes a cam piece mounted to the retractor housing. The cam is connected to a pivoting pawl by a sliding pin. As the cam shifts to the left, the pin moves along a groove in the pawl. This pulls the pawl into the spinning ratchet gear attached to the spool. The pawl locks into the gear's teeth, preventing counterclockwise rotation.
In some newer seat belt systems, a pretensioner also works to tighten the belt webbing.
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The reason that I asked about this was that I was curious if it mattered how the retractors were mounted.
For example: If a set of belts and the retractor were mounted like so:

RSB 25.jpg


......Would they still work the same if they were rotated 90 degrees and mounted in the C pillars? I'm curious if they are directional.
 
Yes it matters.. Ever been in a car with seat belt retractors when the car was going down a steep grade? There's no deceleration but the seat belt locks due to the angle... Because the pendulum gets far enough out of plumb with the mechanism to lock the spool...
 
Yes, I have seen that....belts that lock up in the wrong conditions.
The 73-76 A body 3 point belts are hit and miss. They sometimes lock even when you're sitting still and just getting into the car.
I've thought of getting belts from a newer Charger or Challenger but they mount the retractors sideways. If I needed a new headliner that might be cool to integrate in the C pillars but the red car of mine has a decent headliner in place.
Rich was commenting on how he saw a video telling of how some belts cinch up when the air bags are deployed so those must have some manner of wiring to them to sync up with the reaction of the air bags.
 
Also if memory serves some new cars have a section of belt stitched onto itself designed to pop and give a bit in an accident, somehow that works in conjunction with the bags. So if you use those in an old car I guess you have to cut the stitches.
 
Yes, I have seen that....belts that lock up in the wrong conditions.
The 73-76 A body 3 point belts are hit and miss. They sometimes lock even when you're sitting still and just getting into the car.
I've thought of getting belts from a newer Charger or Challenger but they mount the retractors sideways. If I needed a new headliner that might be cool to integrate in the C pillars but the red car of mine has a decent headliner in place.
Rich was commenting on how he saw a video telling of how some belts cinch up when the air bags are deployed so those must have some manner of wiring to them to sync up with the reaction of the air bags.
There are designs in recent decades which have an air bag triggered one-time small explosive charge that tightens the seat belt in the early stages of an impact/air bag event.
 
Also if memory serves some new cars have a section of belt stitched onto itself designed to pop and give a bit in an accident, somehow that works in conjunction with the bags. So if you use those in an old car I guess you have to cut the stitches.
That makes no sense IMO.
 
That makes no sense IMO.
Which part? I double checked just now (yeah it was Chat GPT) and it steered me to old posts (2012ish) on a miata forum where they would do that mod after removing their air bags. That is probably where I first saw anything about removing the "rip stitch" since I was into miatas back then.
So at least I'm not hallucinating this time.
 
Belts will stretch when subject to load and if there's a 400 lb body in that seat, the belt will stretch even more.
Yes it matters.. Ever been in a car with seat belt retractors when the car was going down a steep grade? There's no deceleration but the seat belt locks due to the angle... Because the pendulum gets far enough out of plumb with the mechanism to lock the spool...
I've had pickups that must of had a heavy pendulum because it would lock me in place when I tried to grab something on the dash to keep it from sliding to the far right on even a mild turn to the left. Another pickup was perfect in the fact that didn't happen but would lock up on a harder stop or turn.
 
I drove an Infiniti a while back( 2010 G35? ) and when you really hit the brakes it would automatically suck in the seat belts, almost like going into sport mode. Now there's a nanny system no one wanted.
 
I drove an Infiniti a while back( 2010 G35? ) and when you really hit the brakes it would automatically suck in the seat belts, almost like going into sport mode. Now there's a nanny system no one wanted.
75% of the crap on the new vehicles is not wanted by me! I've been driving a 22 Colorado and even though I use the back up camera, I'm better with the mirrors but that camera is pretty good backing up to a trailer.....and that's the ONLY thing I find good in it. I have used it to see how close someone was to the truck at a stop light. I'm surprised they didn't blow their horn when the backup lights came on.
 
New car crap kind of ruined owning a 2019 Scat Pack for me. Between the electric throttle and a delay valve in the clutch it was the worst stick shift I ever had. I could never get comfortable with it the way I did with my other stick shift cars.
And with all the nannies it still wanted to spin around if the ground was slightly wet. But I agree on back up cameras, I don't rely on them solely but that with the cross traffic beep when backing out of the space are nice safety features that don't feel intrusive to me.
 
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