zilspeed
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| posted on 27/12/09 at 10:05 AM | |
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quote:
Originally posted by mr henderson
BTW calculating the position of a bird's mouth joining two square tubes in different planes is NOT basic chassis fabrication technique, and I
have yet to see such a joint in the various professional and amateur chassis that I have seen so far.
Have you seen any such joints?
I did one last week. Not on mine, but on a friend's non locost chassis which I am assisting with.
I'm sorry to say that it really is no big deal. We worked out the position of where we wanted it, I cut it with the hacksaw, the TIG guru tigged
it into position. Really, no big deal.
I'm originally a joiner goign way back, so birds mouth joints are a fact of life when doing traditional roof construction, so it's just a
joint.
If I might add, it pales into utter insignificance when compared to a compound mitre where the third member of a round tube chassis comes into two
existing ones and has to fit in.
There's no magic formula, just lots of cutting, grinding and patience.
(Actually, that's not strictly true, there are magic formula, I can draw such a detail in cad, but it won't isolate the existing members to
allow fitting, it's simpler to just cut it and grind it.)
Here's a small example.
 
Roll Bar
Very very easy, and all of the mitres are spot on and withstand scrutiny, but at the end of the day, you just have to start cutting and grinding and
edging up to the finished job.
[Edited on 27/12/09 by zilspeed]
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indykid
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| posted on 27/12/09 at 12:03 PM | |
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i imagine you'd measure between the corners of the two tubes at the points you want them to intersect and make that the apex of your cut. you
could use a square on the faces of the tube for ultimate accuracy or measure the radius of the tube edge and subtract.
if it's angled, you mark up one face, set tube at said angle and bandsaw it.
i'd personally rather weld to the face than the corner
tom
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mr henderson
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| posted on 27/12/09 at 01:10 PM | |
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This sketch might help to show what I mean. It's pretty much to scale
Using a simple cad program I found that the joing tube would need to be 16mm square if the welding was only to be on the face of the end on tubes if
they are at 45 degrees to eachother as shown here
http://img22.imageshack.us/img22/598/birdsmouth.jpg
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MakeEverything
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| posted on 27/12/09 at 02:23 PM | |
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I knew what you meant, but i dont think its as strong as having a birdmouth on square tube.
Good practice would be to use smaller tube to avoid the 'gap' as you said.
If its load bearing, i wouldnt birdmouth square tube, but if its not so heacily loaded (Rear basket) then maybe id consider it.
Just my thoughts. Good luck,.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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mr henderson
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| posted on 27/12/09 at 02:32 PM | |
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quote:
Originally posted by MakeEverything
I knew what you meant, but i dont think its as strong as having a birdmouth on square tube.
Good practice would be to use smaller tube to avoid the 'gap' as you said.
If its load bearing, i wouldnt birdmouth square tube, but if its not so heacily loaded (Rear basket) then maybe id consider it.
Just my thoughts. Good luck,.
The joining tube is square, I should have made that clear in the diagram.
If we suppose that this joining tube needs to be there in order to strengthen the chassis, then it's my belief that using a 25mm tube cut and
welded as shown is stronger than using a 16mm tube.
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MakeEverything
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| posted on 27/12/09 at 03:27 PM | |
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quote:
Originally posted by mr henderson
quote:
Originally posted by MakeEverything
I knew what you meant, but i dont think its as strong as having a birdmouth on square tube.
Good practice would be to use smaller tube to avoid the 'gap' as you said.
If its load bearing, i wouldnt birdmouth square tube, but if its not so heacily loaded (Rear basket) then maybe id consider it.
Just my thoughts. Good luck,.
The joining tube is square, I should have made that clear in the diagram.
If we suppose that this joining tube needs to be there in order to strengthen the chassis, then it's my belief that using a 25mm tube cut and
welded as shown is stronger than using a 16mm tube.
Sorry, i meant to say not as strong as a birdmouth in ROUND tube.
I think birdmouthed square is weaker than round, so i wouldnt do it. Under load, i believe that the birdmouth is a weak point, and weaker than a flat
mating onto its top or side surface.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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mr henderson
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| posted on 27/12/09 at 03:36 PM | |
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quote:
Originally posted by MakeEverything
Sorry, i meant to say not as strong as a birdmouth in ROUND tube.
I think birdmouthed square is weaker than round, so i wouldnt do it. Under load, i believe that the birdmouth is a weak point, and weaker than a flat
mating onto its top or side surface.
That may well be true, but that isn't available in the situation described above, where the choice is 25mm with a birds mouth, or 16mm without.
Unless there is another way of doing it that I haven't thought of?
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Richard Quinn
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| posted on 27/12/09 at 04:56 PM | |
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Would the 16mm have to be box? A few of the triangulation pieces in my chassis that join the main 25mm box at a jaunty angle are tube rather than box.
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MikeRJ
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| posted on 27/12/09 at 05:10 PM | |
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quote:
Originally posted by MakeEverything
I think birdmouthed square is weaker than round, so i wouldnt do it. Under load, i believe that the birdmouth is a weak point, and weaker than a flat
mating onto its top or side surface.
Not that I'm disputing this, but do you have a reference for this? Is it weaker under any type of load or just one specific loading?
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mr henderson
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| posted on 27/12/09 at 05:18 PM | |
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quote:
Originally posted by Richard Quinn
Would the 16mm have to be box? A few of the triangulation pieces in my chassis that join the main 25mm box at a jaunty angle are tube rather than box.
Well, there are those that say that box section is stronger that the equivalent round tube, and it is an area that I would like to be as strong as
possible.
In the particular case that caused this question to arise it's not really a problem because I can get a print out for the tube ends, and may
well get them laser cut anyway. But I did think that it was an interesting problem that people would like to discuss.
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scootz
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| posted on 27/12/09 at 05:46 PM | |
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I thought it was generally accepted that round tube is stronger than box section?
It's Evolution Baby!
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mr henderson
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| posted on 27/12/09 at 05:51 PM | |
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quote:
Originally posted by scootz
I thought it was generally accepted that round tube is stronger than box section?
This has cropped up before, there's a phrase that engineers use to describe the strength of tubes, can't remember what it's called,
but I expect someone will be along to remind us.
Was it something like 'moments of inertia'?
IIRC the consensus the last time this was discussed was that the reasons posh chassis are made with round tubes are-
a) there's a much wider variety of fancy alloys available in round
b) it looks nicer
[Edited on 27/12/09 by mr henderson]
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zilspeed
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| posted on 27/12/09 at 06:22 PM | |
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quote:
Originally posted by mr henderson
quote:
Originally posted by scootz
I thought it was generally accepted that round tube is stronger than box section?
This has cropped up before, there's a phrase that engineers use to describe the strength of tubes, can't remember what it's called,
but I expect someone will be along to remind us.
Was it something like 'moments of inertia'?
IIRC the consensus the last time this was discussed was that the reasons posh chassis are made with round tubes are-
a) there's a much wider variety of fancy alloys available in round
b) it looks nicer
[Edited on 27/12/09 by mr henderson]
polar moment of inertia is the term which you are looking for.
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mr henderson
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| posted on 27/12/09 at 06:34 PM | |
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Here it is-
http://www.locostbuilders.co.uk/viewthread.php?tid=113423&page=1
somewhat inconclusive, though.
In the particular example I offered above, though, the choice would be between 16mm round tube and 25mm square (or round) tubewith a bird's
mouth, and I know which I would choose.
Cutting the bird's mouth itself probably no more easy or difficult in either, as the cut out would need to be square to fit onto the square tube
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MakeEverything
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| posted on 27/12/09 at 07:49 PM | |
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Why was Zilspeed edited by Mr Henderson?? I didnt think you could do that? What did Zil say?
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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mr henderson
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| posted on 27/12/09 at 08:00 PM | |
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quote:
Originally posted by MakeEverything
Why was Zilspeed edited by Mr Henderson?? I didnt think you could do that? What did Zil say?
I didn't/couldn't, it's just a slip in the formatting, he's missed some ['s
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gazza285
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| posted on 27/12/09 at 08:53 PM | |
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At what point in this thread do I take umbrage at being called a dickhead and delete the whole lot, useful or otherwise?
A rhetorical question as I'm just not that childish. Perhaps if people had not reacted as badly when questioned about their motives, and phrased
the initial posting better, then any friction would have been diverted. That, and a lowering level of pomposity from more than a few on here would go
a long way toward a more harmonious community.
DO NOT PUT ON KNOB OR BOLLOCKS!
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mr henderson
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| posted on 27/12/09 at 09:02 PM | |
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quote:
Originally posted by gazza285
At what point in this thread do I take umbrage at being called a dickhead and delete the whole lot, useful or otherwise?
.
Any time you like. Seeing as you started this thread deliberately to take the mickey and for no other reason it's rather laughable you trying to
take the moral high ground now.
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Peteff
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| posted on 27/12/09 at 11:41 PM | |
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quote:
Originally posted by gazza285
At what point in this thread do I take umbrage at being called a dickhead and delete the whole lot, useful or otherwise?
The sooner the better as it's just pointless and boring.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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indykid
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| posted on 28/12/09 at 12:02 AM | |
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second moment of area is the term you're looking for.
i strongly suggest some reading. once you understand that, it's fairly logical to see which section is better in which scenario.
once you get into higher strength steels, you have to use round because they don't make it in any other section and will still be
stronger/lighter/both despite the suboptimal section.
tom
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Fozzie
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| posted on 28/12/09 at 02:13 AM | |
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As I have sent 3 u2u's to one person in particular
regarding this thread, and, although continuing to post in this thread, has not had either the courtesy or manners to either read or respond, this
thread is therefore now closed.
Any person going back to edit will be dealt with....
Fozzie ..... Admin
'Racing is Life!...anything before or after is just waiting'....Steve McQueen
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