I recently bought a pair of ratchet axle stands to find out if they were a lot more convenient that my pair of ordinary stands. I was also planning to use two pairs of stands to elevate the four corners of my MX5 which I thought would make jobs like renewing brake fluid a lot more convenient.
However, the instructions with my Halfords Advanced Ratchet stands are clearly not to use 4 stands at once! Is this good advice? What are the dangers of using 4 at once?
Nick
 
Long discussion on this previously
The dangers is the car falling off the stands. There have been incidents of people being killed or seriously injured because of a car being supported by 4 stands. The issue is its difficult for the car to be balanced safely while getting it up onto stands, or when taking it off. People have done it, I have done it, but I wouldn’t recommend it. Someone just has to lean on the car, and it will fall off the stands. A car can still fall off 2 axle stands, because they are not very good when it comes to sideways loads. And then, occasionally, an axle stand will fail, either because of a manufacturing fault, or because it becomes momentarily overloaded during removal/placement operations.
4 stands not so bad when you are using the lowest height setting, but things get a bit larey as soon as you want to start raising the stands a bit.
If you just want a way of changing brake fluid quickly/bleeding, I can recommend Speedbleeders; these are inexpensive replacements for the nipples on calipers, which includes a non-return valve; the bleed, slacken off, pump brake, retighten. I fit some tubing on the nipple into a jar, to save a mess, but its perfectly possible to change the fluid/bleed the brakes without use of jacks this way.
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If you must get the car off the floor totally, I would say a combination of stands and ramps is the way to go, short of more expensive scissor type lifting equipment.
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There are multiple other threads on the subject, where people have aired a variety of opinions, including the view that the HSE, who has issued safety warnings on the use of axle stands, only exist to nanny people, a view that I firmly do not subscribe to.
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You will probably get a bunch of people saying it perfectly safe, as they’ve never had a problem. Probably a large number of people who’ve not had a problem working under a car supported by a scissor jack, but you wouldn’t want to be one of these fools.
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Probably safer not to renew the brake fluid at all than to try and get the car on and off of 4 axle stands 
 You can, it works and its a free world, sort of, however, i would not, indeed i dont.
Safty first.
The stand comes with safety advice to cover the manufactors ass and to suggest to you that you think about safety. Its funny how the comman man lacks comman sense when working on his car, cant sratch her although dont mind 1 ton of metel crushing my thin crainian.
I own two sets of axle stands, 3 pairs of ramps , one hydralic jack.
Never trust a scissor jack alone. i dont like using them although interesting i need to use the scissor to raise the very low 5 a little in order to get the main jack under and away. Rather anoying, but hey saefty first.
To get the car off the ground fully i raise one end and place her on ramps, then the other end, again on to ramps, and repeat until even and high enought to work under safely. Take a break dont run under, think about what your doing. Give her a bloody good shake. Take your time.
Obviously this aint no good for brake bleeding. Here i remove one wheel at a time,work the erea and replace before moving on to the next wheel. IÂ use Hydralic jack, place wheel under sill and put a ramp under the car not to suport but to take it should the jack go south. To be fair some would argue this is a little bit cheeky so i very good jack is required.
Alternatively use axle an stand with said additional suport and good quailty jack. You could use two stands to suport one end for two wheels, say front or rear ends however, the effort required, my way, well just as well leave three wheels on the ground and work one wheel at a time. Yes its time consuming and you may well have  to repeat.
Use /buy good quality equipment, have a friend work with you, take your time. If in doubt dont do it.
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This is only my opinion and im not suggesting you follow it.
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Halfords have to cater to all levels of intelligence with their health and safety advice, if it is done correctly then using 4 stands at once is not a problem. The problem arises when people to not understand all the peramiters and do not realise that a trolley jack will cause the car to be pulled towards it unless the jack is used on a smooth level floor.
Another potential danger is when trying to undo a very tight or rusted nut/bolt, we tend to get a larger breaker bar, and with this amount of leverage you could upset the balance and pull the car off axle stands, just somthing to bear in mind
Many thanks to everyone who has helped me with their comments; I will play safe!