fabricating new seat belt brackets - materials question

[mikey]

Seeking input from those with the requisite knowledge base..

For my Lola T91-50 I needed to design and fabricate new seat belt mounting brackets for both the lap and shoulder belts. I modeled the bracket shape off of my old PFM, except that I increased the material thickness from 0.087" to 0.125".
The PFM brackets were thin steel. My machinist suggests making these thicker new brackets from billet T6061 AL. Given the use of billet and the material being thicker, will they be strong enough? Belts are 2" OMP type. Bracket drawings below. Shoulder belts will wrap around 5/16" AN cross bolt that goes between each pair of the smaller bracket type "ears". Lap belts will attach to the single bolt hole at the top of the triangular bracket.

Appreciate your thoughts/input!

[tlracer]

Why do you need to redesign these and will the new parts attach to the chassis in the same way as the standard ones?

6061 should be okay or you could use 7075. In either case, do you plan to machine from solid? I've not checked but there may be L-section extrusions that would reduce the amount of machining required.

[GAC]

You are probably looking at going from 0.087” 4130 steel to 0.125” 6061. Take a look at how the strengths will compare. I would say absolutely do not use 6061. Maybe, maybe you could use 7075-T6, but 4130 steel does better in impact-energy testing, which is exactly the type of load you will need them to hold. I would highly recommend making them from 4130 if bending sheet or 4140 if machining from billet and keep the original wall thickness. What material is all the approved belt mounting hardware made from, ie the plate that is bolting to that mount? It is steel.


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[jrh3]

[QUOTE=GAC;641879]You are probably looking at going from 0.087” 4130 steel to 0.125” 6061. Take a look at how the strengths will compare. I would say absolutely do not use 6061. Maybe, maybe you could use 7075-T6, but 4130 steel does better in impact-energy testing, which is exactly the type of load you will need them to hold. I would highly recommend making them from 4130 if bending sheet or 4140 if machining from billet and keep the original wall thickness. What material is all the approved belt mounting hardware made from, ie the plate that is bolting to that mount? It is steel.


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[Rick Iverson]

You are probably looking at going from 0.087” 4130 steel to 0.125” 6061. Take a look at how the strengths will compare. I would say absolutely do not use 6061. Maybe, maybe you could use 7075-T6, but 4130 steel does better in impact-energy testing, which is exactly the type of load you will need them to hold. I would highly recommend making them from 4130 if bending sheet or 4140 if machining from billet and keep the original wall thickness. What material is all the approved belt mounting hardware made from, ie the plate that is bolting to that mount? It is steel.


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Good stuff here.

[mikey]

Thanks guys. They will attach to the carbon tub using the same threaded inserts as the old brackets. Redesign was needed due to new seat and belts and overall seating position of me vs the prior owner.
My machinist actually already made the parts from billet 6061 so now we're trying to figure out if they're usable or scrap. He doesn't work with steel so if that's what I need I will have to find another fabricator.

[tlracer]

It depends on what grade of steel was used for the PFM ones you're using for reference:

If it is 4130 then 7075 aluminium is really necessary but, if not, then 6061 is acceptable.

Maybe not ideal as a further point of reference, but 4130 or T45 only came into common use with racecar chassis around 30 years ago. Swift were, I think, the earliest to go that way, with Van Diemen and others following suit. Before that time the most common material was CDS, with brackets, etc from bright mild steel.

[Rick Iverson]

Thanks guys. They will attach to the carbon tub using the same threaded inserts as the old brackets. Redesign was needed due to new seat and belts and overall seating position of me vs the prior owner.
My machinist actually already made the parts from billet 6061 so now we're trying to figure out if they're usable or scrap. He doesn't work with steel so if that's what I need I will have to find another fabricator.

Wow, nicely done.

It depends on what grade of steel was used for the PFM ones you're using for reference:

If it is 4130 then 7075 aluminium is really necessary but, if not, then 6061 is acceptable.

Maybe not ideal as a further point of reference, but 4130 or T45 only came into common use with racecar chassis around 30 years ago. Swift were, I think, the earliest to go that way, with Van Diemen and others following suit. Before that time the most common material was CDS, with brackets, etc from bright mild steel.

Good stuff here

[EYERACE]

1. How well anchored to chassis/car..............

2. Appear to be thicker than "standard" such that safety is more likely..............but we all know, including yourself, (and that's why you're bringing this to everyone else's attention for comment) there's no guarantee.

............so build it and run it........but not into a wall

[LJennings]

I would look at the load path through the shoulder belt brackets. You are creating a moment due to the distance from the thru bolt hole and the first mounting screw.

Probably should do the same with the lap belt anchors.

Also look at the stress concentrations in the 90 deg corners.

Nice work, but better safe than sorry.

lrj

[mikey]

Here are some photos mounted in the car. Ignore the bolts holding belts to brackets. I will have correct AN hardware. This is also the first time I’ve tried any H&N device in this car. I don’t think I want to use the NexGen after trying it today. It doesn’t secure enough. What degree HANS should I try?

Regarding the belt brackets, it seems like 7075 AL in this configuration would be preferable based upon comments thus far. I really appreciate all the input!

[tlracer]

7075 would be preferable to 6061, for the added safety margin. My only observation on the design is the distance between the axis of the belt-loop bolt and the upper fixing. I guess this is necessary to raise the belt anchor point?

With the scalloped edge to the mount I'm seeing a potential bending issue under load. Would it be possible to replace the scallop with a straight taper, to increase the cross-section at that point?

[mikey]

Yes I agree the ideal cross bolt location for the belt loop would be between the bracket mounting bolts, but I needed it higher than the tub anchor points would allow in order to keep within the max 20 degrees down angle for the belts from the top of the shoulder (with H&N device) to the belt loop anchor.

Several ways these could be strengthened against the bending moment concern mentioned if necessary, one being more material on the "scallop / fillet" as suggested.

[patman]

Regardless of material they should be designed to hold a minimum load of 3000lb per anchor point. This is based on US, euro, and fia published standards. I personally don't see any issue with 6061 as long as they exceed that strength requirement.

[LJennings]

Regardless of material they should be designed to hold a minimum load .

[mikey]

Ok moving forward with making new brackets from 7075 and adding material on the shoulder belt tabs to strengthen against potential bending. Proper AN hardware from Pegasus. Thanks again for all your inputs. Big thanks to Carter Thompson for the machining!