How to calculate the load capacity of my project?

[ajlelectronics]

Personally I wouldn't work under a car on just 4 stands...it's inherently unstable if a smallish sideways force is applied to the car.

It would be as, if not more stable than a two post lift.

 

[Pigeon_Droppings2]

It would be as, if not more stable than a two post lift.

If the 2 post lift is bolted to the floor and the stands aren't then I'd say not as stable IMHO.

 

[tigdlo]

Personally I wouldn't work under a car on just 4 stands...it's inherently unstable if a smallish sideways force is applied to the car.

The sideways force required to push the vehicle over would be relative to the stand height, base width and load, it's standard practice in commercial workshops to lift vehicles 2m high and place them on 4 stands to work on, the stand base is about 1m wide and with a load of about 10 ton or more they aren't going anywhere.

 

[Pigeon_Droppings2]

the stand base is about 1m wide

Yup...but the base has to be in proportion to the height raised otherwise ain't a good idea.

4 naff folding stands...at max height...no thanks...but each to their own!

 

[8ob]

Got a small pile of axle stands if anyone needs them

 

[tigdlo]

The setup isn't the issue, but like anything you have to use the right tool. I built my stands with a height to base ratio of 2:1 even with very little load as I have nearly a bare shell, I can't push it hard enough lift a leg on the stands. I also shimmed one of the stands so they are all loaded, that helped too.

 

[Pigeon_Droppings2]

My mate came round and asked me why I had about 6 different supports under my car...my response was if I used 5 and it failed then the safety report would likely state..."if only he'd used the 6th support instead of just leaving it sitting in the corner of the garage doing nothing".

Think I go over the top...but I'm still here and still have all my fingers, toes eyes etc etc .

Pretty sure if I had a serious accident in the garage the missus would put a stop to all the fun...that alone keeps me very safe!

 

[JOEPRO]

Wow, thank you all for the responses! There is quite a lot of information and advice here - I'd like to learn more, so I'll try to respond to everything, to help me get a clearer picture.

The general consensus seems to be that my design does not have a large enough footprint to ensure stability. So then what should the footprint be, given the height and load of the project? There must be a calculation for this? I came up with the blocks of wood, based on using the jacking points that a workshop would use when putting the car up on a 2-post lift:
View attachment 455463
After the blocks of wood worked so well for me, I have basically based this new design on the size and shape of the wooden stands I was using. But I realise that the difference with a 2-post lift is that the whole thing is connected a bit like the suggestion from atomant to brace the 4 stands together. Actually, I quite like that idea of being able to bolt-in some angle once the stands have been placed.


Is this because the weight is centred, and then the feet of the stand protrudes in different directions? In theory, if one created a block of steel the same length, width & height as an axle stand, wouldn't it have the same stability, just with a lot of wasted material?

You're absolutely right, and I have thought about this. My design definitely relies on accurate square cutting of the box section, to ensure even weight distribution - it is definitely a disadvantage of that design.

I found those, and they look nice and strong. Can you confirm the size of the footprint on those?

It's true that many people use 4 axle stands, but in the safety instructions that the manufacturer provides, it always says not to do this. That's the only reason I have concerns about it.

Those premier stands look sturdy - but the footprint appears similar to my suggested 200mm x 200mm - I'd be interested to know the exact dimensions.

This could be an option. I have something similar to that already actually - https://www.etamfg.com/shop.html?store-page=Rennstand-Each-1-Piece-p252276669 They make different adaptors, and I bought several of these at the time, the specific adaptor for my car is this one: https://www.etamfg.com/shop.html?st...er-920-Mercedes-Benz-Pair-2-Pieces-p255070130

Just on a side note, those stands are good for getting the car off the ground, and doing little jobs, but I wouldn't trust them enough to be underneath the car, as the footprint is not 'square' enough. They are very wide in one direction, but not the other. I also own their other stands, but these are only good if you're keeping the wheels on.
View attachment 455464View attachment 455465View attachment 455466


Thank you for these calculations. I will need to take some time to understand them better, but this is the first step to being able to calculate the loads, so thank you.

Thanks for that - although the lifts in those videos are only good if you're keeping the wheels on, and they need to come off in my case.

These are the dims of the Premier 6 tonne versions. Pics taken in the garage with very poor lighting apologies for the quality!

 

[JOEPRO]

Yup...but the base has to be in proportion to the height raised otherwise ain't a good idea.

4 naff folding stands...at max height...no thanks...but each to their own!

The jump up from 3 tonne stand size to 6 tonne stand size when working on a car does make a considerable difference to the stability. When working on old S class mercs that weigh well over 2 tonne I won't use anything less. I ain't going out getting crushed by a car! (The 6 tonne stands are larger and therefore much more stable)

 

[Novice_MIG]

Biggest stability problem is the square flat base, the area of which is made less by welding distortion, and a perfectly flat floor needed for them, whereas three legs are very stable on rough surfaces. Even milk maids knew that.

Ok, this makes sense - thank you.

A 3 legged axle stand will have a variable height/base ratio depending on the direction of the force, it's good practice to have them oriented 180 degrees to each other.

The lighter the load the less side force that would be required to push it over so it would need a wider base. Compac axle stands look well proportioned to me and appear to have a wider base than most.

The Compac CAX 12S looks excellent!! I really like the idea of having a threaded rod to adjust the height, as one of the things I dislike most about axle stands is the rattling around of the support arm, especially when it's at full extension - it just feels so unstable. However, the cost is around £250 per stand, and I'm not going to spend that kind of money if I still have to manufacture my own adaptors once they arrive!

A three legged design will be inherently a lot more stable on a workshop floor than a flat base.
The reason why the wood cribbing works is it will give and adjust as the load is put on – your steel stand won’t do this and there will always be some slight wobble, multiplied by four stands and I suspect you will find it a good deal less stable.
The larger the footprint of the base, the more awkward it will be to work around too.

As above, I’d recommend some decent axle stands, and make some adapters for your jacking points.
Depending on how the jacking points work, something with a bit of give (hockey pucks work well) should absorb some of the difference and improve stability.

Its not really worth the time and effort to mess about with it, especially if you are not 100% confident in the welding to risk getting under it.

You could get the four legged axle stands if you are worried about tipping over, although again they will be a little more wobbly on an uneven surface.

I’ve put vehicles on four stands many, many times never a problem providing you are careful jacking.
Getting it on and off is the most dangerous part – once it is secured on the stands and passes the “rock” test, you should be pretty safe unless you are really trying to push it off.

The only other thing to be aware of is if you drastically change the weight distribution – I’ve seen cars jacked up on lifts, which have toppled over once the engine has been removed.
Most of the time it shouldn’t be a problem, but on some cars the balance point and front jacking point can be quite close together.

Thanks MetalMonkey, I'm in agreement with you on the most dangerous part being the lifting and lowering of the vehicle, as well as the importance of weight distribution - these points are worth remembering for everyone! I understand what you and Gritineye mean about the flat base now.

These are the dims of the Premier 6 tonne versions. Pics taken in the garage with very poor lighting apologies for the quality!

Thanks for doing this, they have a bigger footprint than I thought!

So, based on the figures from Joepro and tigdlo, it seems that a safe ratio is between 1.5 and 2 to 1, or to put it another way, the height of the stand should not be more than twice the width of the base. And, it seems better practice to have 3 legs as the most stable setup. The trouble is, I really don't want to buy axle stands. Apart from the rattling support arms, that irritate me, and the fact that every manufacturer of axle stands says not to use 4 of them at once, I don't see an easy way to make a secure adaptor to fit onto the top of an axle stand. As I don't have a requirement to adjust the height, or to lower them down for easy storage, I think I can come up with a more stable design based on the advice of a 2:1 ratio, and the suggestion of a 'tripod' base. The downside to making my own design, is that I am not able to calculate reliable 'max load' figures - which brings me back to the original question of how to calculate this. I won't be satisfied in just knowing that 'it seems strong enough', I would like accurate figures before I go crawling around under the car! I'll come up with another design, and see what you all think.

Thanks again for all the advice, this thread should be useful for anyone wanting to support a car in future!

 

[Novice_MIG]

Something along these lines perhaps....

This has a height of 505mm and the distance from the centre of one circular foot to another is 350mm. It's basically a fixed axle stand, but will need some finesse to make the top a bit of a better fit. No bloody clue how I'll weld pipe on to the central column at such a precise angle mind you! It could probably do with some bracing between the legs too.

Like this:

 

[tigdlo]

No bloody clue how I'll weld pipe on to the central column at such a precise angle mind you!

Flat bar and angle iron legs are well tried and tested as well as being easier to make accurately, these styles are worth considering, https://www.ebay.co.uk/itm/116227776249 and https://www.ebay.co.uk/itm/285822315639

This is how I made my tallest stands, not suggesting you should too as I'm not qualified to design anything.... I supported the front of my car with one stand with extra weight in it then removed the weight and supported it on 2 so they have been tested to more than twice the weight they are currently holding.


The circular top plate, triangle mid plate and feet were laser cut from 5mm mild steel, £70 for 4 stands.

 

[Bornfree]

Got a small pile of axle stands if anyone needs them

View attachment 455514
View attachment 455515

30 odd years ago when the HSE used to do farm visits every two years. We used logs under every trailer drawbar always had done, until a new inspector turned up and gave us a notice to fit stands. I spent over a week making stands.

 

[jimbo84]

Something along these lines perhaps....
View attachment 455549

This has a height of 505mm and the distance from the centre of one circular foot to another is 350mm. It's basically a fixed axle stand, but will need some finesse to make the top a bit of a better fit. No bloody clue how I'll weld pipe on to the central column at such a precise angle mind you! It could probably do with some bracing between the legs too.

Like this:
View attachment 455550

You'd mentioned in your first post that you're a novice welder and your original design wouldn't be relying on the welds to be load bearing.

Your above design will rely on the welds to hold it together, they'll be load bearing.

Even if the manufacturers state not to use 4 stands it will still be safer than relying on something home built with questionable welds holding it together.

With stuff like this that ultimately your life depends on unless you're very competent you're always better buying something designed and tested from a reputable manufacturer.

In no way having a pop at you, just my 2 cents worth.

 

[Gritineye]

Many years ago a friend of mine had a car fall on him when he was working alone in a remote shed, he was lucky, but his head was about twice it's normal size for quite a while.

Be careful out there, stay safe.

 

[Pigeon_Droppings2]

I would start with one of these...builders acro prop...already rated at way more than you need + height adjustable via screw thread.

The feet have holes already...you could use them to bolt on a support system and come up with a design that doesn't rely on welding.

Props can be bought very cheaply in a range of sizes.

....this was my original plan but in the end I bought a drill powered car roller so didn't need to bother.

 

[Novice_MIG]

Flat bar and angle iron legs are well tried and tested as well as being easier to make accurately, these styles are worth considering, https://www.ebay.co.uk/itm/116227776249 and https://www.ebay.co.uk/itm/285822315639

This is how I made my tallest stands, not suggesting you should too as I'm not qualified to design anything.... I supported the front of my car with one stand with extra weight in it then removed the weight and supported it on 2 so they have been tested to more than twice the weight they are currently holding.
View attachment 455553View attachment 455555

The circular top plate, triangle mid plate and feet were laser cut from 5mm mild steel, £70 for 4 stands.

Those eBay items are interesting. As you say, this is obviously tried and tested, but they don't look as strong as they must surely be! I like your drawing - which software do you use for that? I also like the idea of testing the stands - excellent idea. I could find someone with a shop press, and then test each stand for several tonnes before use - that would make me feel better.

You'd mentioned in your first post that you're a novice welder and your original design wouldn't be relying on the welds to be load bearing.

Your above design will rely on the welds to hold it together, they'll be load bearing.

Even if the manufacturers state not to use 4 stands it will still be safer than relying on something home built with questionable welds holding it together.

With stuff like this that ultimately your life depends on unless you're very competent you're always better buying something designed and tested from a reputable manufacturer.

In no way having a pop at you, just my 2 cents worth.

You're quite right jimbo, I don't really want to rely on my welding if I can help it. Going back to my previous design, I'm now thinking to run the central cylinder down to the ground. This would result in a 4 legged design, and put the majority of the load onto the central column, with the welds and 3 posts only being stressed if there is a sideways movement. I think I would have enough welded area to feel more confident about this setup, and as mentioned, I will make sure to over compensate with the testing, just to be safe.

 

[Novice_MIG]

I was wondering...in order to overcome the difficulty of getting a 4-legged stand to sit perfectly flat on a workshop floor, how about placing a piece of steel plate underneath the stand? A sheet of neoprene is another option, but this would cause an issue if the stand moves slightly when loading/unloading the stands, the rubber would bite in and prevent movement. But on sheet metal, there would be less friction, and the stand could slide a few millimetres if it needed to. Same principle as with a trolleyjack needing to have free moving wheels.

 

[Jack_K]

If you think car axle stands are unstable, have a look at mobile truck column hoists. They don't look like something I'd design thinking it's stable.

 

[Ed.]

Hi all,

I would like to make some stands for my car. I have previously used blocks of wood to put my car up in the air for long periods of restoration. More accurately, I used blocks of railway sleeper (L)200mm x (W)200mm x (H)100mm - these were stacked 5-high, with each stack of blocks having a 10mm thick mild steel rod going through the centre, to ensure the blocks did not slip against each other. This setup allowed use of the factory jacking points, and gave 500mm of additional clearance.
View attachment 455431
View attachment 455432

I made up these stands, and they supported the car well for 3 months (axle stands were used for added insurance). This was a few years ago, and now that I need to carry out a similar job on another car, I see that the wood has become dry, warped and cracked - definitely not safe to use. So, now that I have a welder I would like to make something safer and longer lasting. As a welder I am a novice, but the designs I have in mind will not rely on any of my welds carrying the load, so I don't anticipate any major issues there. I believe this would be the safest design:
View attachment 455433

This uses 4 x 500mm lengths of box section (100mm x 100mm x 5mm wall) sandwiched between two 10mm thick mild steel plates (200mm x 200mm). Some issues with this design are very high weight and cost, and also I'm not sure exactly how/where I would weld the pieces together. Perhaps making the end plates 220mm x 220mm would then give me a lip to weld the box sections to.

My main concern is ensuring the design can support the car safely. The car weighs 1600kg, and this will be shared by 4 stands, but during jacking, the car's weight will be put onto two stands at a time. To feel safe, I would like each stand to have a capacity of at least 2000kg. It might be possible to reduce cost and weight by using box section with a 3mm wall.

Here's another example using mild steel tube and angle.
View attachment 455434

I'd appreciate any advice or tips on this project!

My two cents on this is that it is way overkill for a car, each one of 100mmx100mmx5mm SHS at 500mm high would probably support at least 6T plus, the issue I see, is the height and the stability that will cause you the main problem. You need a wide foot print and some sort of stability support to stop toppling it over. Years ago I made a couple of stands for my boat, had some 10mm off cuts lying around so welded them in to a square got some "40mm x 3mm SHS" welded in to the middle of the base and added some 10mm gussets, welded a large nut to the open end and put in a largish bolt to adjust the height. The boat and trailer weighed over 3.5T. Total width of the plate was 230mm and the height was about 400mm with the bolt giving more height. Each one of these could support the whole boat and trailer, however if you look at the pics you will notice that I used a big welder to weld it all together. These welds aren't my greatest but I was in a rush and they still did the job. I feel safer using these ones then the store bought ones I have.

But as far as using the 4 x 100mm SHS x 5mm thick per stand is concerned you could support a prime mover on a single piece of SHS that size without the other three pieces. The issue is the design , foot print, stability and the welding of it. I have attached some pics to show you what I did. As I said it was made just out of the scrap metal I had lying around. If I was going to push the car from the side then obviously I would need to build more stability into it including a bigger footprint and more and bigger gussets. But static downward load more than ample. But if you don't have the right welder or welding skill , then it's probably easier, quicker and cheaper to just go and buy some ready made. You will be surprised how cheaply and how much thinner material they use in store bought ones.