Seeking engineers advice

[Jamieg285]

Time for the crude drawings again.

I'm in the planning stages of building a rack to carry cycles on the back of the Camaro.

I've got the basic design worked out, but am looking for advice on what stock to use in one location.  In the diagram below, all of the blue bits are 1" round tube.  It's the red bits that I'm debating.  These will slide into an appropriate receiver under the back of the car.   Up to 3 bikes (Estimated Max 40Kg) could be hung along the top blue bars.  



What do you think would be the best thing to use for the red bits (bearing in mind I'm trying to keep it from weighing a ton) - The options I have thought of so far are:

* Round tube (same 1", maybe bigger)
* Square tube
* Rectangle tube
* Angle (sqaured - i.e. L )
* Angle (45 degree - i.e. ^ )

Or doesn't it really matter?

p.s. If it makes any difference, the receiving end will be the appropriate shapes welded to flat stock which will be bolted to the bottom of the rear bumper.

[Roadkill]

The potential failures to this would come from wind resistance at speed, not from the load itself.

I would use 50x50 RSA (angle) [EDIT - Or 40x40 RSA] for the red bits, but also use that on the lower blue bit, too - this would give you more surface area for a weld on the tubular uprights where the fracture stress is likely to be highest.

[Andy]

Angle, angle, angle!

Always use angle. It may end up looking home made (cos it is) but the contact areas for welding are greatly improved and the rigited vs weight imo is better.

Looking at the design I'd be wanting 45 degree plates on the main bottom two brackets and the tubes that the bikes sit on if that's what you're using.

[Roadkill]

Here ya go -

Quoting: Andy

I'd be wanting 45 degree plates on the main bottom two brackets and the tubes that the bikes sit on if that's what you're using.

Was gonna suggest that also, but I think the contact area is good at the bottom if you do it as above . . . may be a good addition at the top, though.

[Jamieg285]

Interesting.  Angle it will be then.

What thickness?  Quick scan of ebay for supplies suggests 3mm or 5mm are readily available.

On building the base section, do you recommend the 45 degree cut and butt weld, or overlapping and welding all the seams?  

I was envisioning the angles being the other way up, with the tubes welded into the corners.  Does it make any difference?

By 45 degree plates, do you mean gussets, reinforcing the tube to angle joint?


The top tubes are slightly more complicated than my diagram suggested.  The arms are from an existing bike carrier.  These terminate with 1" ID box section, which slide over 1" ID box section on the main frame.

I also want to put these arms on a horizontal slider (tube over tube or box over box) to give some adjust-ability to the arm width.



Part A can slide left and right and will be held in place with a bolt/quick release pin.
Part B is the Pre-made arm that will slide onto Part A, also held in place with a bolt/pin.

What recommendations have you got for this section.

[Andy]

Can't draw anything at the mo, but I would make the whole thing from angle. It's just easier to work with imo.

I'd just do a standar 45 degree cut frame like RK has drawn with three pieces in a rectangle going vertical without the crossing over bit.

Having a larger area behind the bikes would help keep the from smacking the car when loading and in transport.

[Roadkill]

Quoting: Jamieg285

What thickness?

I'd Go for 5mm for the base.

Quoting: Jamieg285

45 degree cut and butt weld

Yep.

Quoting: Jamieg285

I was envisioning the angles being the other way up, with the tubes welded into the corners. Does it make any difference?

Yes, but not enough to be a problem for what you're doing.

I drew it lke that as the other way up would be a dirt/rust trap.

(EDIT - Also assumed it'd be easier to mount to the underside of the car that way)

Quoting: Jamieg285

By 45 degree plates, do you mean gussets, reinforcing the tube to angle joint?

Yes.  Also called "knee" bracing.

[Roadkill]

I think a 1" seam of 1/4" well penetrated weld can hold over a ton, by the way.

Worth baring in mind.

[art b]

gussets....!

[Andy]

Stick weld the mother too!

[F Body]

Quoting: Jamieg285

These will slide into an appropriate receiver under the back of the car.

I'm not saying this for a wind up, but are the rear chassis rails still strong enough to support this sort of load, they are a known rot area

[Incursus]

I assume you've tried the strap on ones from halfords?  Worked great on the Mustang and parents Jag.

[art b]

Quoting: Incursus

I assume you've tried the strap on's from halfords

keep bikes of the maro

and put em on ya other car...

[Jamieg285]

Quoting: F Body

I'm not saying this for a wind up, but are the rear chassis rails still strong enough to support this sort of load, they are a known rot area

The rear frame rails are still in pretty good condition (had a good look when I had the petrol tank out earlier in the year).

Beside that, I'm planning to hang this of the rear bumper (The metal bit behind the cover) and that bolts through the body, into areas that were repaired a couple of years ago.

[Titsy]

Quoting: art b

keep bikes of the maro  

and put em on ya other car...

[Jamieg285]

Quoting: Incursus

I assume you've tried the strap on ones from halfords?

I thought you had to go to more specialist 'private' shops for that sort of thing

I have looked at the regular cycle carriers, but the spoiler gets in the way, and there isn't any where really suitable on the rear panel for the lower support - it's either on the lights or on the plastic cover.

At least doing it this way I keep all contact points away from the paint.

[Jamieg285]

Quoting: art b

keep bikes of the maro  

and put em on ya other car...

If I have use of the other car then yes, and I already do.  Problem is if I want to drive somewhere to go cycling that leaves Debs + kids with no transport.

[Roadkill]

Again, the stress will come (mainly) from wind resistance so make sure your anchor points are good and that the lower portion of the assembly is . . . err . . . stiff.

Quoting: Jamieg285

If I have use of the other car then yes, and I already do. Problem is if I want to drive somewhere to go cycling that leaves Debs + kids with no transport.

Fair comment.