alright geniuses, (i actually mean that) I'm welding up a dirt bike carrier for the jeep and the hot rolled, mild steel 2" square tubing (that goes into the receiver) has a wall thickness of .125" Without getting too detailed (because there will be two stabilyzing tongues as well, then there's the length of the tube, weight of the bike, etc) but how much force (or weight) APPROX. does it take to bend a piece of this stuff? The details that i do have: the bike platform weighs about 30lbs, and the bike weighs about 215lbs. (length from reciever to channel is 30")
It would be just as easy (and only about $10. more) to use .250" but if .125" is more than strong enough, making this lighter would be nice for lugging it around when i'm not using it. I have a habit of adding all kinds of extra supports and brackets on stuff i make but sometimes it's just overkill. I guess i was really wondering how much weight or force it would take to bend the .125" stock. Thanks for all the comments so far fellas.
as a calculation load 250#, and got just over 1/8" of bend, and 13.6 ksi bending stress at the receiver.
With a yield stress of 36ksi, A36 would seem to be OK--BUT it is not unusual for bouncing and other momentary stresses to cause the actual forces to exceed that, so a common rule of thumb is to use four times the load for a safety factor.
If we re-run the calculator for a 1000# load, we get just over 1/2" of bend and a stress of 54ksi, which would put you over. So my quick "back of the envelope" calculation would agree with your other two responses, use the thicker section.
I was going to make a luggage rack / platform that would fit into my 2" receiver and was planning to use 2" square x .125 tube also. Just to get an idea of the strength my son and I stood on the tube about 2-3 feet from the hitch and we bounced up and down on it, tube did not bend or flex, (as far as I could tell and we weigh about 325 lbs combined). So based on that, you should be fine. If you don't have the tube yet, buy the 3/16 of 1/4 inch stuff. I'm a big fan of overbuilding when it comes to safety issues.
Walt, Having spent the better part of my 25 years as a welder/machinist in the army "field engineering" (usually field fixes for poor design "real" engineers put into play) I also am a fan of over building. Now...having said that...the good captain here has a point that stands at the very root of field engineering...find an equivalent field force test. That principle has NEVER failed me (when combined with my 0ver build habits. There is most definetly a down side to over building...namely stuff gets too %$#@!! heavy. In today's "over sue" world, you probably won't find anyone (myself included) who can promise you 1/8" wall tube will be O.K.. Especially when we don't have an actual detailed design to look at. I will tell you that if I'd done the test the good captain did & got the same results, I'd go ahead & use 1/8" (with gusseting & bracing whereever possible). Also, I'd tell you...next time look at using some 1/8" wall chrome-molly (4140) tube...it's good enough for stock car frames! Hope this helped
Like you said 13.6ksi until he hits the first bump with the resulting equivalent acceleration load of 3g's. Then the bike is left dragging behind the car. Even 4 times may not be enough.
Before you go off and buy some tubing with thicker wall, do a bit more thinking. The max bending is at the receiver. So maybe a six inch piece of smaller tubing to go inside the tubing that you have and then weld a foot or so of that tubing that you have under the other tubing. Since that changes the tubing from 2 inch square to tubing that is 2 inch by 4 inch, it will increase the strength more than going to .250 wall.
Will not be so bad, because the bike has damping. But the load that is frightening me most is torsion. The way the rail is connected to the car (one tube in the center) is -in my eyes- the second most stupid way. It would be OK for a bicycle, but not for the wheight of a motorcycle.
In my eyes, the construction is complete rubbish and doesn't need to be calculated.
None. The "tongues" will only help (partial) with bending in the horizontal plane. But you have torsion in the vertical plane.
Scenario: The bike is on the stand, you go around a sharp corner. Now some math: CG of bike is about 500mm off the stand, bike wheights about 110kg. You do have a moment of 550Nm if g is 10m/s^2. Now I'm too lazy to calculate the stress, because it is a waste of time. Don't forget that you have to add it to the bending forces.
BTW: If it helps, you can laugh, but it won't stiffen the sick construction. It will only work if you add 2 more connections (rear of bike + handle bar connected to the car).
Nick, There is no way you have enough information from what has been said in this group to know where the center of gravity is (I am laughing)...also, you don't know enough about what he is trying to do to call his design crap.....you'd need to see detail first (i.e. does the bike mount at just one point...or more? and...do the "tongues" indeed mount in such a fashon as to help with the stress at all....or do they play as much a part as the hitch mount? Don't do a bunch of "assumed math" & call a design you know almost nothing about crap buddy. It makes you look silly.
Haven't seen a motorcycle recently? No? So I bet you don't even know what a MX looks like, how much it weights (it was posted) and where the CG is. I have lifted (during 8 years of MXing and Hare Scramble racing) my bikes (Husqvarna, KTM, ATK, Honda, if that helps) enough times to make an estimate where the CG is. Trust me!
I can see pictures, I can c&p links. If you can't I'm sorry for you.
Well, I bet the bike won't just mount on one point. No need to think about this. YMMV.
Picture, see above.
I've seen the picture + there is just one location to connect the whole frame to the car.
I called it a 30" cantilever beam with a 250 lb load multiplied by four for bouncing, etc. and get BeamOut(0 15 30)(30E6,1 I_sqr 2 .25)Beam 0(30 1000) pos'n pt. couples shear bend. slope def'l force stress stress
Better to just use heavier wall. Also note that with a cantilever beam the greatest bending stress is right at the point of suspension so welding another piece underneath won't help.
That was my thought as well. The only thing that gave me hope was the fact that it was a 2" receiver which is usually mounted properly now days.
About 1 1/2 months ago I had a similar situation roll into my shop. A older couple from Arizona where coming through with the smallest camper I've ever seen in my life behind there car. Somebody had put a
1 1/2" receiver on the back of the camper and they had a bicycle rack in it. However due to the curve on the camper the bicycle rack had to be extended about 1' so that it stuck out from the receiver about the
30" mentioned here. The rack was holding up fine. However the receiver was just welded to a 2"x3" piece of about 16ga wall rectangular tubing which ran across the back of the camper frame. Of course it pulled and buckled in till the rack was able to drag the ground.
I had to extend the receiver so that it went to the next cross member up the frame and reinforce the back cross member to repair it.
This is the second absolutely lousy poorly thought out welding job to come from Arizona to my shop. The other was when somebody with no welding skills put a 30' dry box off a bob tail truck onto another camper frame made from 2"x6"x14ga rectangular tubing. The tongue pulled out twice before it got here and neither place that repaired it did as good a job as my dad could do welding it back (and my dad is NOT a welder by any stretch of the imagination). I ended up making a new frame from 6" channel with proper 6" channel cross members (the original had light weight C purlin type cross members which where broke) and a reinforced tongue before I would let it back on the road.
The original welding on that frame was so lousy that I was able to take the 3" I beam off that they'd welded down the length (so they could U-bolt the box to the frame) by grinding one weld. The other welds where not stuck to anything.
I'm starting to get nervous when I see a Arizona tag. :-)
And well you should... To describe just one example, a neighbor came up to my place (in AZ) to use my equipment. I just watched, somewhat amazed while he stick-welded two identical sledge hammer heads as spacers between the springs and axle of a pickup bed converted to a water hauling trailer (2400 lb. load). The rest of the project was equally ah, interesting. And he never touched the wire brush and chipping hammer I supplied. I keep waaaaay back when I see this kind of stuff on the road, no matter where it's from.
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