Not a good idea at all sorry guys because at first look it looks great.
1 The manufacturer of these jacks advises they are only there to raise the object not to hold for prolonged periods in that case you need stands for the load.
2 At 500 mm lift the allowed load is 25% of the advertised load.
3 These jacks are designed for a load pushing down vertically at the lifting arm.
What is happening there is a bending moment caused by the load being applied on the tyre tracks say on average 400mm from the centre of the upright and going on to the crossmembers at some unknown point.
There is nothing keeping the car from falling off the platform if the hand brake is not put on or if the brakes are being worked on.
What is the safe point load if you jack a wheel on the ramps and someone will if they build one of these!!!!!
Where is the warnings about max loads at different heights displayed especially for the person who built it and forgets these numbers that most people will.
Sorry to be down on the idea but I looked at this and was just about to do the same as it was such a great idea.
If anyone disagrees by all means correct me factually rather than say it is OK just for the home garage.
eddie
