material tubing thickness

[willieworld]

terry  the rules permit you to use any tube size anywhere on your bike period--other than aero. there is no advantage with the smaller tubing ---you can use streamline tubing if that is a concern--it comes in C M  if you want it in mild steel there are some companys that will make it but it will cost the same--i gave it some thought when i built sheris and my bikes but decided the expense wasnt worth it --as far as weight savings we both run 70 pounds of ballest on our sidecars ---it doesnt matter how light and fast it is if it breaks before you get to the finish line ( 2 times ) well you know the rest -----willie buchta

[willieworld]

when i started this thread it was because someone asked about material thickness on the outside of a bend (thought they might sonic check the outside of the bend )---i was cleaning the shop today and was looking at those bends and remembered the rule of bending--if the outside stretches the inside has to shrink (compact ) that would make the tube thinner on the outside but thicker on the inside ---any one have any guesses as to how much--none--as much as the outside thinned--more than the outside thinned--  i will post the results tomorrow     willie buchta

[Sumner]

.......---any one have any guesses as to how much--none--as much as the outside thinned--more than the outside thinned--.........

d.) all of the above,

Sum

[panic]

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[Cajun Kid]

willie,

I would say on a 90 degree bend, the inside would shrink and become thicker than the outside. 

The inside would become thicker by 33 to 50 % of the amount the outside thinned.   

So on your 1 1/2 DOM, .125 it was .126 actual and when bent the out side thinned to .122  or .004... so the inside would grow to a max of 50% of the .004 so add .002 to the .126 and the inside would be a max of  .128 but I wold say .1275

Just my logical deduction (not based in any fact or experience LOL)

Charles

[Geo]

On the previous page we talked about having the tubes meet correctly.  Here is a pdf marked to show the rays of the angles and how they should meet.  Except for one that is about 3/8 inch off which creates the eccentricity and the levering between the connected rays at the vertex and the non-connected single ray.  It's a short lever however the forces are huge.

Found out how to markup a photo to post 

Geo

[panic]

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[Rex Schimmer]

Using the type of bender that is most common in motor sports the thickness of the outside of the bend has to get thinner and the inside has to become thicker. The amount of the thickening and thinning is related to the bend diameter and the diameter of the tube. i.e. a large diameter tube bent around a small radius die will stretch more on the outside, therefore thinning the metal and will compress more on the inside, therefore thickening the metal. If we looked at a .75 inch diameter tube bent 180 degrees around a 6 inch bend radius bend block (the 6 inch radius is at the tube center line.) the lenght of the outside surface of the tube is 6.375 x PI= 20.0276 inches, the inside length is 5.625 x PI=17.6714 inches and the length at the tube center line, which does not change is 6 x PI= 18.8495 inches. So the % change for the out side is 20.0276 div. by 18.8495= approx 6.3% longer and on the inside the number would be 17.6714 divided by 18.8495= approx 6.3% shorter. So the material on the outside of the bend should be about 6.3% thinner and the inside should be about 6.3% thicker but if you measure the thickness this is not what you find because as you bend the material the outer gets thinner and the inner gets thicker so this moves the neutral axis of the tube closer to the inside radius so what happens is the outside actually thins more than the 6.3% and the inside thickens less than the 6.3%. Also these numbers are only somewhat close on very thin wall tubing. If the tube was .050 wall the amount of increase and decrease would be around .003 inches. It becomes pretty obvious that if you go to larger diameter tubing the outside radius has to stretch a longer distance and therefore thins more. On a 2 inch tube it would be almost 17%!

Now there are machines that can actually bend thin wall tubing at very small bend radii and actually increase the thickness of both the inside and outside walls. This is all part of the "art" of tube bending. These benders will have a mandrel that will wipe the inside of the tube to keep the tube from collasping and also a "pusher" cylinder that grips the tube just infront of the bend dye and wyper and actually forces the tube into the area being bent and can actually force extra material into the area of the tube that is being bent and prevent thinning of the tube on the outside radius! The F1 guys bend 50 mm dia (2 inches) with .5 mm wall (.020 inch) tubing made from 625 inconel at less than 2 inch bend radius and make perfect bends! That is tube bending "art"!

We also need to remember that when the tube material is stretched or compressed it can actually have enhanced material properties because of work hardening which usually means an increase in the materials yield and tensile strength.

I will be interested in Willie's information on his testing of wall thickness after bending. Willie be sure to give us the bend die radius and the tube diameter and wall thickness.

Rex

[willieworld]

well folks i checked the inside of the bends today and i must say i was a little suprised--also im not a very good speller and was also suprised that someone would make fun of that--i think thats why i know and do what i do i dont spell so good and i dont rely on someones opinion without some facts ---any way enough of that--when i get the chance i will go back to the first post and add this in----the 1 1/2 CM was .098 on the inside--the bend radius was 5 1/2 ( center of tube )  ------ the 1 1/2 dom was .137 on the inside -5 1/2 radius----the 1 5/8 EW was .129 on the inside -6 1/2 radius-----the 1 5/8 DOM was .136 on the inside -6 1/2 radius----the 1 1/4 EW was .135 on the inside -4 1/4 radius----now remember all of the bends were not exactly 90 degrees ( close though )--this was not a perfect experiment but i think we all have a better understanding of the bent tube--im sure that different benders will have different results  sometimes i oil the tube to bend that might even make a differance also but i dont really think that is important--i think that us all thinking about it is the important part---i would like to thank charles for THINKING about it and posting his answer --GOOD ON YOU CHARLES----willie buchta

[Peter Jack]

Saved you the work Willie. This one blew me away. I've bent a lot of tubing and I always assumed that there was more effect on the outside than the inside. There's usually a slight flattening on the ouside of the bend with the types of benders we commonly use. I guess that's partially how the thickness is maintained. Bye the way most of us know how assume breaks down. (ass-u-me) Oh so true!

Outside dia.       Advertised wall        Actual wall          outside   inside      radius
1 1/2 od              .083 cm                 .083                 .081    .098      5  1/2

1 1/2 od              .125 dom               .126                 .122   .137      5  1/2

1 5/8 od              .120 ew                 .118                 .115   .129      6  1/2

1 5/8 od              .125 dom               .126                 .123   .136      6  1/2

1 1/4 od              .134 ew                 .124                 .122   .135      4  1/4

Pete

[willieworld]

thanks pete --im not to good at columns---there is one more thing maybe you could do ---all of the bends had an outside length of 8 inches and a inside length of 6 inches( or close to it ) so thats a 4 to 3 ratio--using that ratio could some one figure the average wall thickness for the bends --that would be interesting to know ---the 1 1/4 tube was 4 inches on the outside and 3 inches on the inside ---thanks all for everything   willie buchta

   this is my bender       http://www.jd2.com/

[Rex Schimmer]

Peter and Willie,
Great info and it is always great to get the real hands on information!! Pretty dramatic to see the amount that the inside wall thickness increased! This is probably a result of the neutral axis of the tube moving closer to the bottom of the bend .

Willie, I am like you never a strong speller but somehow both of us have mumbled through! and I am of the age right now that I really don't give a sh-t if I can't spell!

Rex

[interested bystander]

Some of this is much ado about nothing,

So whatta ya gonna do if the outer comes out thinner than the inner on MOST any bender that u and I can access?

If yer worried, build it with thicker material. SCTA/BNI, to my knowledge doesn't YET have a sonic tester-even one  like Willie's several year old one.

Structural design may be more important.

By the way, the neutral point (a general statement-meaning MAYBE -not exact- is about 41% of the radius.)

[Dean Los Angeles]

Willie - I will take spelling errors and non-aligned columns any day as long as you keep good stuff coming like this.

If you look at a single straight tube you can determine the strength of that tube very easily. If you assemble that tube into a girder with proper supports and gussets it gets  more difficult. Put that all together as a complete frame and the analysis becomes a problem. Solving for partial differential equations is beyond the calculator or spreadsheet.

To really detemine frame strength you need Finite Element Analysis software.
Here is a source that you can look at to get an idea.
Nastran Finite Element Analysis
Wikipedia entry:
http://en.wikipedia.org/wiki/Finite_element_analysis

I am pretty sure that the Ack Attack and Speed Demon streamliners were designed with FEA. There may be a few others, but it requires huge buckets of money.

This also assumes that you can correctly predict the stresses in an accident.

[mkilger]

Scta does have one and from what I understand it will be used, so over the winter people can fix there  cars before next year. I think they may even have a list of cars they what a closer look at .