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MG MGA - Seat belt mounting - improving this

Here is the design which I am improving on, showing where it is mounted.

John


John Francis

Have you checked if in interferes with top stowage_
dominic clancy

John

Looking good from the prototype you showed me at Stoneleigh.

Any idea yet on price?

Steve
Steve Gyles

Good question Dominic. I (and some MG brothers) would like to know the same thing.
Gerry
G T Foster

The bracket itself does not interfere with the stowage. I plan to use the quickfit MGB upper belt clip which unclips from the bolt very neatly. The bolt for the belt may be a slight awkwardness but the bracket is up in the space behind the cockpit rail.
John Francis

Clarke in the USA sell a replica seat belt mounting kit for early As. I bought one a couple of years ago and never used it. Cost about $200 from memory... Sean
S Sherry

I did something similar although without a bracket. I drilled out one of the mounting bolt holes in the nut and tapped it for the larger (I believe it is 7/16" NF) bolt supplied with the Moss kit. I have the clip on the floor mount to re-route the seat belt around the hood frame, when it is raised or lowered. It has worked out well. It is tough to adjust the belt length when the hood is down though, but it can be done. I had seen the brackets that weld to the rear shroud, but I wasn't going to attempt that on a finished car. I like your idea John.

Ralph


Ralph

Ralph

I have something not dissimilar on my car, but John informed me that it wont have the required strength in an impact. Hence his arrangement that incorporates both welded nut locations.

steve
Steve Gyles

I have an arrangement similar to John's current set up, but with a further locknut below the tapped plate for increased threaded contact area. I have no doubt it is strong enough and it holds the belt over my shoulder without ever slipping down. The height of the belt cannot be increased anymore as it comes out under the rear cockpit rail. The factory fit option was simply a token and would not have held up in 'anger'.

Steve, You could fit your top mounting to a spreader plate fastened to both the wing bolts for added strength. In the event that that failed under load, i would suggest that the shunt would be so heavy as to become academic.

regards
Colin
C Manley

Hi Steve,
The bolt passes throgh from the wheelwell
and threads into the nut as original, just oversize. It is a long grade 8 bolt, and the Moss supplied bushing and nut thread on to the exposed end of the bolt. I was concerned about strength so put a heavy hardened large OD washer under the head of the bolt. I think on a heavy impact it might bend the metal a bit, but It can't see it pulling that bolt and washer through.
The original 5/16" wing mounting bolt was not going to be strong enough shear strength, so that's why I changed to the 7/16", which is also what the Moss kit had.
At that point the rest of the car, but hopefully not me, would probably be toast anyway.

Ralph
Ralph

This shows the bracket tested to failure with 3 tons load applied. The bolt (nut) thread stripped rather than the bracket failing. The orientation may look a bit odd but it suits the test machine which applies the load vertically.




John Francis

And this shows the hole deformation. In practice there will be a nut welded to the back of the plate so I would not expect this deformation.



John Francis

JF I've come a little late to this discussion but there are a few points that I grappled with when I installed my fixings in a coupe.
One was that I think there's UK legislation regarding a spreader plate on the restraining (external)side of the body requiring a plate of something like 3x1x? " -can't fully remember since its been about 8 years since I did it. This is the same plate found in various after market seat kits. Its job is to spread the load to the body rather than on concentrated point loads. Also does your attachment use the wing fixing screws? If it does then this means they have to be torqued up very tight but this is contrary to the needs of fixing the wing where its problematic to squeeze the plastic wing piping over tight. I'd better apologize in advance if you've sorted this out but my two-penneth may be of some value.
J H Cole

Hi JH

All thoughts are valuable as I have realised, eg Steve's point about the corner of the tonneau cover when its folded when driving.

There is no legislation on spreader plates, just a test of 13KN on the belt, that's 1.3 Tonnes. The body is allowed to distort with that shock load but it must not fail.

The spreader plate will obviously stop a bolt pulling through thin sheet metal. I was thinking that the large oval washers on the wing mounting bolts should be sufficient to spread the load over three bolts and that most of the load will be in shear anyway so will be tending to rip the wing and body tub rather than pull the bolt through. I have noticed that the new oval washers supplied by most suppliers are thinner than the originals and distort. Todd Clarke has supplied me with a set of 1/8 thick washers which are much better.

My thoughts on the tightness of the nuts on the bolts is that I will tighten them normally for the wing mount but will then torque the inner nut up against the weld nut on the body to clamp the bracket to the wing nut. This may not be ideal given that bolts need to be torqued up to provide maximum stress capability, however, I think it should be satisfactory and a capable solution.

John Francis

John

Any idea on time scale before you hit the market? I may need to start saving! And more importantly, sort my bulkhead out.

With all these new ideas and products, are there issues with liability How do you ensure it is fit for purpose and cover yourself?

Steve

Steve Gyles

Hi Steve

Very good question. I'm waiting for Bob West to give some feedback on the the steering bushes. James was fitting a set to a twin cam recently. Mine have worked really well so far.

On the seat belt mounting, if they are are a sensible cost/price they will definitely be sold with no guarantee. All I can do is supply the test information and give mounting advice. I might also ask MIRA to have a look at the design when it's in the car. I might have to specifically ask for an indemnity from buyers!

I'm not really looking to make a business out of this, its a bit of interest using my professional engineering skills which don't get used at work these days and solving problems which might be of use to others.

John
John Francis

Shown here is the inboard and outboard sides of what I came up with for the upper shoulder mount bracket.

Construct and install follows:
(2) 1 1/2" x 6" x 1/8" cold-rolled steel flat stock
Along the center line, drill (2) 5/16" holes @ 5" o.c. (12.6mm). On your car, confirm the center distance of the two fender bolts in front of the bulkhead panel. Use the points of the original fender bolts as a guide.

Along the center line, drill a 15/32" hole centered 2" from one of the 5/16" centers.

Tack weld a self-locking (not Nylock) 7/16-20 nut centered on the 15/32" hole

Remove the two previously mentioned fender bolts and replace with (minimum) Grade 5, 5/16-24 x 1 1/2" and re-tighten to secure the fender using the large, oval fender washers.

Install a 5/16-24 jam nut on the protruding portion of the fender bolt. This acts as a spacer for the 7/16-20 self-locking nut between the bracket and the fender.

Slide the bracket onto the protruding fender bolts and against the jam nuts.

Place a Grade 8 (hardened) 5/16" flat washer on each of the fender bolts and secure the installation with
5/16 x 24 stainless nuts.

Use a 7/16-20 x 1" Grade 8 bolt with a thin, dished washer on either side of the shoulder belt attachment.
Tighten the bolt in enough to just allow the belt to swivel on the bolt.

I obtained BEMIS 3-point shoulder belts from J.C. Whitney, Inc. but had to modify the design. I relocated the shoulder belt adjuster as their design would place the shoulder belt adjuster in an unusable position behind the occupant's shoulder. I was unable to come up with a way to install an inertial reel belt assembly.

The resulting installation is very secure and comfortable and allows the easy adjustment of the upper portion without disturbing the tension of the seat belt portion. The angle of the belt is across the shoulder without a lateral pull towards the neck as occurs with the factory upper mount location.

Additionally, the upper belt does not interfere with the earlier design side curtain storage pouch and the installation is completely reversible.

You will need to use the original-style, quick release lower outboard mount since the belt must be detached and repositioned when the soft top is raised and lowered.

When using, adjust the inboard portion of the seatbelt belt as short as possible before latching the seatbelt and adjusting the length of the shoulder belt. You ABSOLUTELY want the buckle mechanism to be on the SIDE of the hip bone and NOT in front of you as you would wear a seat belt. The shoulder belt must go across the body starting at the hip bone.

If anyone wants photos of the final installation,
send me an email.




TJP Pollak

Many moons ago I decided to design and install an inertial reel sysem and came up with the following ..
1) Load distributing brackets with their fronts against the bulhead behind the driver/pax and taking the load to two strong points on each side of the boot .See pic below
2)The inertial reels are mouted on a heavy steel brackets which are bolted through the bulkhead to the bracket in 1) above as high as possible to get the seat belt easily over the seat top and onto the shoulder.see pic on 2nd post
3) The outboard disconnect behind the seats is a heavy marine link shackle connected to a floor ring which is connected through a load spread plate under the floor. This allows the soft top frame to be erected very quickly.See pic on 3rd post below.
The system has been on my car for 10 years and works without fault. I had it installed professionally.It stays on the shoulder ( I am 6ft 2in ) and the high location of the reel does not interfere with the stowage vol required for the softop. Very pleased with it. I had a fixed length 3 point before and it was a time consuming nuisance.



Neil Ferguson

2nd pic


Neil Ferguson

3rd pic ..sorry about the quality


Neil Ferguson

Here is the final installation of my seat belt bracket design.

The design has been tested to failure in the laboratory and has at least a x1.5 safety margin, ie takes 2 tons. The bracket uses the Quickfit bits quick release top mounting used on the MGB so it is easy to put up the hood.

The bracket is bolted using the standard wing bolts x 3, 2 inside the cockpit and 1 inside the boot/trunk so there is no modification to the body,

The mounting is high up so the belt doesn't slip off the shoulder and does not interfere with stowing the hood.
Couple of other photos to follow


John Francis

Plates before installation and showing one tested to failure. Note the hole failed and in the final design the 7/16 nut is welded to the plate around the hole so it will take a higher load. There is slight plastic deformation around the central mounting hole but this is allowable since it does not actually fail.

This was an interesting design exercise which we set a masters level student at my university for his project work.


John Francis

Final photo.


John Francis

Good work John!

So when do you go to market? Or do we have to wait until another student gets that task next year!?

Great job!
N McGurk

Looks very neat, but I presume you still need a bracket welded to the chassis and a modified transmission tunnel for the lap belt.
John Bray

Neil, I have made an extra pair of brackets so if anyone is interested in buying them I would be happy to sell them - no guarantee but I can share all the test data. Not sure if Barney would be interested in a video of testing and the load/displacement graph or I might put it on youtube.

John, no need for a welded bracket for the lower mounts. Bob West fitted those for me using standard eye bolts and backing plates. They just need a hole drilled through the outside corner of the floorboard rail where there is a triangular gusset and a hole through the tunnel for the inner mountings. The eye and backing plates are available from several places and you can get FIA approved mounts which are racing approved to a very similar design.

John Francis

John, I would be very interested in purchasing the spare pair of brackets. my mga should be back on the road soon and seat belts would be a real bonus. i am located in milton keynes, sp not far away. I can be contacted on 07875 177743 or 01908 271068. Yours, Dave
DM Gibson

I have loaded some videos onto YouTube of the laboratory load testing on the brackets. If you search under "MGA seat belts" on You tube they come up. http://www.youtube.com/results?search_query=MGA+seat+belts&aq=f

The orientation of the bracket in the test rig is so that the load in the bracket is vertical in the machine but is correct when installed in the car.

The second two videos show an animation of the computer aided stress analysis of the bracket. The colour changes show the stress in the part, red showing highest stress levels. The part is show deflecting which is shown x10 the actual movement. Those interested in engineering design tools may like to see these.

John Francis

It is an interesting test, showing that the bracket passes the test with 1.5 safety factor. However, the bracket is bolted to a substantial piece of steel that is obviously stronger than the bracket, for this test. What it does not test is the strength of the MGA body shell in the area of the bolts. Will the bolts perhaps pull out of the body shell before the bracket fails? How about before the bracket reaches the standard minimum test force? Food for thought.
Barney Gaylord

It appears to me that the weakest part of the seatbelt attach is the attachement at the tunnel. The bracket that John made is stronger than both the stock upper attach that it replaces and the center tunnel point still used. Once the upper attach point is stronger the "Weakest Link" theory comes in to play. The seat belt will fail where it attaches to the tunnel before Johns part will fail. Using this as the criteria I believe that the part is more than adequate to the job. This picture shows Clarke Spares upper attach piece. It is like the original and is spotwelded only.


R J Brown


just visible on the right on the attached picture shows how I reinforced the fixing point at the tunnel. just 4mm sheet metal with the threaded inserts from Todd. Not tested as I didn't fit seat belts.

Siggi


Siggi

Lower anchor plates for seat belts are available on ebay to FIA standard. These are 65mm square 3mm thick and are intended to be welded to the floorpan of a car.

Non FIA standard are 50mm square and 2mm thick.

To answer Barney, I entirely agree that we tested the bracket alone on a rigid frame. In the car the wing bolts go through two layers of sheet and are backed by the large oval washers, 25mm x 50mm x 3.1mm thick (original ones are 1/8", Moss ones are only 2mm thick). My design relies on the three oval washers to spread the load under the bolt heads and I think this is more than equivalent to the recognised standard for a single anchor plate 50 x 50 x 2mm. The single plates are intended to be welded on though.

We did test one even stronger bracket design to failure and the nut and bolt on the middle hole failed at 3 tons load so I am confident that the bolts will take the maximum load likely to be seen, test spec 1.2 tonnes, which I was more concerned about.

The key is that the bolts do not pull through the wing flange and the body or the bolts break. In the worst case I would expect the body to deform slightly and the plastic deformation helps to absorb the energy without failure. Its similar to modern car design where you have crush zones which are not too strong but absorb energy.

With these accident only applications providing none of the mountings fail in an accident they have done their job and can be rebuilt again. So the key design issue is making sure they are just strong enough.

Anyway I better stop hogging the board and enjoy the car now!


John Francis

This thread was discussed between 27/03/2011 and 18/06/2011

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