I had this plan to weld 45mm box 2.5mm wall between the rear vertical roll hoop. On the back of the box i was going to weld the seat belt mounts based on the haynes roadster design.

Problem - despite seeing 1"7/8ths with 2.5mm wall box being a standard size along with 45mm, Metal Supermarket don't do it and it seems neither do the online metal places.

So do you think 1 1/2" 1.5mm box will be strong enough (personally despite people welding seat belt mounts to 1" 1.5mm top chassis rail, i'm nervous about it being strong enough)

The other option is to get some new seat belt bosses, 1 7/8ths CDS tube with some holes drilled in for the bosses - just seems a lot harder to do, more hassle and expensive. Plus i'd like to do it this weekend so i'm running out of time.

Personally, I would like the harness bar to be round to better fit in with the rollbar. It will be something you will hate on the car if you do not do it properly first time round; this is not something you can change easily lateron, is it?

ah, but square would be easier to fish mouth, easier to weld in the roll bar mountings and ....

by mounting the mounts behind the centre line of the roll bar therefore meaning the bolt length isn't going interfere with the diagonal! Its a cunning plan and totally gets round the problem i've seen one or two other people have

nope neither are strong enough. at an absolute minimum you need 38mm x 3mm thick cds.

each belt is rated to 2tones so two people thats 4 tones now to be safe you double it which roughly equates to 3mm cds.

you can get away with something much thinner but you will need a third brace to the chassis in the middle of the bar

remember in a 30mph crash your weight is around 12 times more than normal so for a 100kg driver you will be putting 1.2 tonnes on the belts and mountings.

its a fair bit to think about

[Edited on 2/3/10 by ashg]

doesnt the 'standard' haynes design use 33.6*2.6 CDS tube?
IIRC the ron champion chassis was 25*25*1.6 square (but it is a while since I looked at the plans.

Regards
Hugh

quote:

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Originally posted by ashg
nope neither are strong enough. at an absolute minimum you need 38mm x 3mm thick cds.

[bits removed]

its a fair bit to think about

[Edited on 2/3/10 by ashg]

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Just checked on the mcsorley site - the rail on the original chassis is 1 ich sq 16g.

If I remember correctly (try wikipedia'second moment of area' the force to bend a round tube is proportional to its diameter cubed, and for square tube the 'diameter' is the length of the side. The force to bend the tube is directly proportional to the thickness and also directly proportional to the sheer strength.

So if you use 2" square 2.5mm thick square welded tube, that is twice the diameter and about 1.5 times the thickness, for material of the same strength compared to uncle Ron's. E.g 2cubed*1.5 = 12 times as strong for bending resistance.
If you use the structural type CDS rather than 'hydraulic' there is quite a difference in the material strength too (a factor of about 1.5 I think).

Please check all this information though!

Regards
Hugh

Edited to add:
50*50*50*2.5thk about 25% stronger than38*38*38*3thk*1.5(strength) for CDS if the above is right.

[Edited on 2/3/10 by hughpinder]