How to calculate how much load something can take?

[gibsonc]

Hello,

This is my first post here so I am sorry if this is in the wrong place!

I am looking to build a bike repair stand out of mild box steel. I think I need to make it out of 40x40x3mm box steel.
If I have done my calculations correctly I think a 1.6M length can hold 20.96kg.

I have drawn a very rudimentary sketch of the product on my iPad, to demonstrate the lengths etc.

Generally speaking a Full world cup DH bike is around 17.7kg, this stand won't be holding more than that so I would say that is the weight it needs to hold.

I would really appreciate if someone could help me work out if the piece was mig welded together would the product hold the bike?

IMG_0060.jpg

Thanks
Chris

[PinkertonD]

Hi Chris,

Welcome to the forum and this is the correct place for your question, but we are not too rigid on which Forum anyway.

Is the stand when in use, to be static? As in, not mounted on the rear of your car etc. If it is just for working on the bike in the shop or at the track, then without even calculating anything, I would say get the hacksaw and mig-welder out.

If there are going to be "bounce" loads, then maybe a few gussets would be a good idea at the tube connections.

[gibsonc]

Thanks for the quick reply!
This is just going to be a static stand for the workshop. Im sure that it will take the weight as I have built one out of wood and a microphone stand and that holds by bike!
It is for my GCSE project so I felt that I needed to prove that it will hold it rather than just using previous experience that it will!

[jboggs]

What is GCSE? Is it an educational program? Is it one in which you are expected to know how to do the necessary calculations for your project? I'm really asking if this a class assignment.

[gibsonc]

GCSE's are exams all people aged between 15-16 take in the Uk, it dose not look like you have equivalent exams in the US. You have to take English, Maths and Sciences (be it triple where you do separate exams in chemistry, physics and biology or double etc...) You then have the option to take other exams. For example I am taking Design Tech, Economics, Geography, Graphics. But the amount you take and what you take depends on your school.

This is for the final practical part of my DT exam, Its not essential that I put the calculations in the project but it will help me get the high A* grades. I won't be tested on the calculations, plus I would like to know them as I am possibly thinking about taking Engineering further!

[PinkertonD]

Hi Chris,

We do have a no-homework policy here but I think this falls outside that scope as you were not asked to do the calculations as part of the project. I applaud your desire to go for the gold and not just get a passing grade, but... ain't there always a "but?"

In order to help you, please show us what you did to arrive at the 20.96Kg figure and we will then confirm or refute. That way we are not supplying the solution to you, merely making sure you are on the correct track. If not, then we will guide you to the correct answer.

[gibsonc]

Hi,

I looked up an equation for the max weight that a box eel section can take. This was width x width x 0.00000785 x length.
So it was 40mm x 40mm x 0.00000785 x 1600mm = 20.096kgs by my calculations.

This is just the weight that the main upright section will hold, I assume I have to calculate the weight that the clamp will hold as well?
I also guess I have to take into consideration the weight the wilds hold, am I right in saying the weld is stronger than the metal itself?
I think it may be a good idea to pop onto solidworks and model the product, I can then do some stress tests on there!
And thank you for the previous answer, I have always been one to wonder why something works not just accept it and move on! I wouldn't be supprised if this is possibly to large a project to take on, I may end up building an iPod dock or something!
But I have gone to far in my design process, if it is not possible at school the I will build it at home! Nothing like proving our teachers wrong!

[PinkertonD]

Hi Chris, the calculations should be fairly simple once you look at it in the correct way. What you have are two cantilevered beams. The vertical beam (post) is cantilevered off the bottom legs. For this design, we can safely assume that the legs are infinite and therefore the base of the vertical beam is essentially fixed to the floor.

Next, you have a cantilevered beam (the arm for the bike) off the top of the vertical. First, calculate the deflection of that arm-beam relative to the vertical-post. We can assume that the vertical is permanently fixed to the wall for this calculation.

Once there, mentally turn the whole thing on it's side with the arm pointing down and calculate the deflection for vertical-post which will be the weight of the Arm +the weight of the bike. While that is not actually the totally correct method, for what you want it will provide an extra margin and save a bunch more calculations.

Generally a supporting structure is always assessed with some Safety Factor which can be anything practical juggling weight (cost) and relative risk in a collapse. I would never go below 2.5. So, the theoretical weight of the load will be 17.7Kg x 2.5 = 44.25Kg plus the weight of the arm.

Check on this website for the beam calculators and you will find one for cantilevered beams. Then get the properties of the material you intend to use and plug them in to the calculator to solve the equations.

You may want to start with very small tube (25mm x 1.5mm) as the deflection figures you get for 67mm x 3mm square tube will probably have about 46 zeros before the first value. OK, maybe not 46, but quite a few.

p.s. Make the legs about 50% longer than the arm to prevent easily tipping over.