craig1410
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 posted on 3/4/04 at 11:22 PM |
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Crush tubes for harness mounts - How big?
Hi,
I've just finished welding on a horizontal cross bar onto my rollcage so that I can mount my 4 point safety harness top mounts onto it.
I wondered if anyone knows what wall thickness of crush tube I would have to weld through the rollbar to provide an adequate mounting point. Obviously
I can either thread the crush tube or just drill it out and use a nut and bolt right through.
In any event, does anyone know where I can source a suitable piece of tubing either threaded or not. Alternatively, does anyone fancy making up four
mounts for me? If so then please give me a price. They need to be 60mm long to pass through my 48mm rollbar and leave enough at each end (6mm) to
easily weld to. I think the minimum wall thickness would need to be 3mm and if I use the likes of an M12 bolt then I'd need an 18mm or greater
outer diameter tube.
Comments/advice most welcome.
Cheers,
Craig.
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type 907
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| posted on 4/4/04 at 05:43 AM |
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Hi Craig
I think you have to thread them.
Nuts on the back aren't liked by Mr Sva.
I'v made 22mm bosses x M12 , A4 80
bolts, but they havn't passed SVA yet.
(all stainless chassis)
Paul G
Too much is just enough
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craig1410
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| posted on 4/4/04 at 08:55 AM |
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Hi Paul,
22mm would certainly be more sturdy than 19mm and would probably remove any doubt over suitability but I'm surprised at your comment about
threading to be honest. I had thought that given the mounting bolt "must" be mounted so that seat belt loads will be across the bolt in
shear then the threading isn't really loaded much and thus a nut welded onto the bottom would be sufficient. Of course it would be tidier if it
was threaded...
Any idea where I can get some 22mm bar with either a 7/16" or M12 thread already cut? I had considered getting some threaded inserts from Rally
Design as they have them for use with their rod ends. One of these would not be long enough but if I put one in from each end and made sure they met
in the middle then it should be fine. They do them for 7/16UNF threads.
Cheers,
Craig.
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britishtrident
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| posted on 4/4/04 at 09:21 AM |
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16+ mm od seamless tube with at least a 2.5 wall thickness will give sufficient strength, 18mm with a 3.5 wall thickness would be better.
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David Jenkins
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| posted on 4/4/04 at 10:43 AM |
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The latest SVA manual has clarified the issue regarding nuts for fastening seatbelts - it is perfectly OK to use 'loose' locknuts behind
the belt fittings, as long as they are the correct type (i.e. HT steel) and as long as they're easily accessible for belt maintenance.
If the bolt is going through a tube then they will be looking for anti-crush support for the tube.
I would be worried about using nuts on the rollbar though, as they may fall foul of the projection rules - you will have to cover them in some way.
cheers,
David
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craig1410
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| posted on 4/4/04 at 10:49 AM |
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David,
Thanks and , yes, my interpretation of the SVA manual is the same. As for exterior projections rules for the bolts I would probably cover them up with
nut covers anyway but thanks for the reminder.
Does anyone know a good source for internally threaded rods or indeed for non-threaded thick wall tubing of between 19 and 22mm o/d? You'll
laugh but I had a go at drilling a piece of 19mm solid bar with a handheld drill and you won't be surprised to learn that I didn't get
very far...Doh!
Cheers,
Craig.
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britishtrident
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| posted on 4/4/04 at 03:58 PM |
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I am sure some size of seamless tube used for boilers will be suitable , There is a steel supplier near me that specialise is small ammounts of steel
(I think they buy in off cuts and surplus stock) they are quite happy to cut even 9 inches off a bar. They operate under the "Metal Super
Markets" brand which seems to work a bit like Spar does for groccers, take a look through yellow pages thier might be one near you.
As for thickness there are two main critera, (1) The tensile strength of the bolt, as long , the anti crush tube has a greater crossectional of metal
than the nominal diameter of the bolt the bolt will fail first (should apply even if the tube is only a fairly ordinary mild steel and the bolt is HT
8.8)
(2) The thickness of the roll bar tube you are welding to, getting a good weld is easiest if the roll bar tube and the anti crush tube are roughly the
same thickness.
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craig1410
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| posted on 4/4/04 at 05:12 PM |
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Hi again,
I do have a Metal Supermarket near here (Glasgow) but they didn't have anything suitable on Friday when I visited. As you said they seem to buy
assorted bits of steel in and it's pretty hit and miss most of the time. Maybe the bigger branches have greater selections.
On your other points, my rollbar is 3mm wall thickness. Cross sectional area for an 18mm o/d 3mm wall tube is greater than that of an M12 bolt
(141.5sqmm versus 113sqmm) so 3mm wall thickness should be ideal from that point of view.
The only other relevant point which you didn't mention is that the flat surface on the top end of the crush tube should be large enough to
adequately interface with the harness mounting plate. This is where I see the 22mm o/d tubing being advantageous as it will give a nice big face to
support the harness mounting plate. That makes me wonder - are the harness mounting plates designed to pivot or do the bolts actually
"grip" the plates and prevent this? Obviously normal seat belt mounts tend to have crush tubes to allow the plate to pivot to adjust the
angle to suit the occupant.
Cheers,
Craig.
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type 907
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| posted on 4/4/04 at 07:00 PM |
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Hi,
David, a "locknut" is a nut that is tightened against another thread, so
IMHO tubes must be threaded.
Craig, if your stuck I could make them.
U2U me.
Too much is just enough
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craig1410
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| posted on 4/4/04 at 07:17 PM |
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Paul,
The SVA manual says, "The bolt may be secured into an alternative fixing, eg. a lock nut of suitable strength, where access is provided to the
rear of the mounting to enable separation/re-attachment of the belt." (Chapter 5, page 4 of March 2004 edition.) I think therefore that an
unthreaded crush tube with a bolt on the bottom would be allowed even though it would be better to use a threaded crush tube from a cosmetic point of
view.
Cheers,
Craig.
ps. You have U2U or will have in a moment...
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type 907
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| posted on 4/4/04 at 07:49 PM |
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Hi Craig
I havn't got the 2004 ed. It's that word
"interpretation" again.
Again, IMHO, a case of "Belt & Braces".
Paul G
Too much is just enough
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David Jenkins
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| posted on 4/4/04 at 08:07 PM |
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I think their definition of 'lock nut' is a bit suspect - I think they mean something like a Nylok nut. However Paul, you are quite
correct - a lock nut is one nut tightened hard against some other threaded thing (often another nut).
rgds,
David
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Mark Allanson
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| posted on 4/4/04 at 08:28 PM |
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I used 20mm solid bar, lathed out a 9.7mm hole ready to tap out for 7/19 UNF thread, gives an approximate 5mm wall thickness. These are welded into a
1 7/8 x1/8 crossbar welded into a 2" 1/8 fully braced rollbar (more 1 7/8 1/8 used for braces).
When you look at production car seatbelt mounts (puny in comparason) the SVA should be a doddle.........
 
Rescued attachment BeltMounts.jpg
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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craig1410
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| posted on 4/4/04 at 11:42 PM |
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Mark,
Yes if only I had a lathe it would be easy as I would also be able to make my own tubes. Are you planning to use 3 point harnesses or just double up
on the single mounting? I'm planning to use 4 point harnesses and fitted four crush tubes for the four upper mounts.
Cheers,
Craig.
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Mark Allanson
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| posted on 5/4/04 at 06:36 PM |
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I already have 3 point harnesses, as for the lathe, I didn't do it myself, I found a model engineer - they are usually itching to make stuff for
people, just to show their skills, cost me a pint!.
I used to use CNC lathes at an engineering firm is used to work for, I just don't have a lathe!
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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craig1410
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| posted on 5/4/04 at 06:50 PM |
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Mark,
Yes I think I have found an obliging fellow builder who is going to make me up some tubes sometime later this week. I'd love to get a lathe of
my own but the cash is better spent on my Locost at the moment and to be honest I don't really have enough room for it. Maybe in our next house
there will be room for one...
I've ordered my harnesses today from MAC1 Motorsports. One of the guys on the forum here (Bob I think) recommended them, they are Sabelt SVA
friendly 4 point harnesses and come with 7/16" fixings. They have the push button release which should without doubt be okay for SVA.
Anyway, thanks again for your help.
Craig.
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