New answers tagged safety
Regardless of weight, the advisory height limit of 50 cm will likely be violated if the plan is to carry the two kayaks on their sides. I'd advise against doing so for this reason alone.
Would this be safe? tl;dr: No. It's hard to say exactly how unsafe or how bad the consequences might be. Assuming the rails themselves weigh on the order of 5-10 kg, you're talking about almost doubling the advised load on the whole system. From reading that advisory, it's hard to say exactly what failure modes their mechanical engineers are ...
Look at the tire straight on so you are looking at the tread (cannot see the rim). Look at the central 3/4 of the tire (~1/8th of the tire's edges on either side). This is the central 3/4 of the breadth of the tire. The tread in this area must be at least 1.6mm deep across this entire width, going all the way around the tire. This would include where the ...
In general, anectodal evidence would show that there should be no issues, but there is one caveat which I didn't see addressed in the other answers: With age, some vehicles can have shock absorber seals fail if they are raised with the load of the wheels still present, rendering them useless (e.g. '07 Camry). If the vehicle is known to suffer from such a ...
On 4 jack stands is a bad idea, unless the ground is perfectly level, since you have no tyres, or suspension travel to even out irregularities. I wouldn't do it.
It may be safe for your car, but it isn't safe for asphalt. Once I did this in an apartment parking lot and with the hot sun bearing down, the jacks melted about a half inch into the tar. It was only for about a day, actually.
If where you are putting the vehicle up at is level ground (driveway, for instance), there is plenty of support under the jack stands (like concrete pad), and you are putting the jack stands at secure locations under your vehicle extended the same amount (so vehicle is level), you will have no issues what-so-ever. Vehicles which are suspended like this are ...
We have a few project cars and have left them on jack stands for long stretches of time while we were rebuilding stuff. I'll second Tariq that if the vehicle is held up at proper jack points you should have no problems. It's also a good point to have the vehicle somewhere secure, if only to keep curious people, pets, etc from crawling underneath and ...
I have seen a lot of qualified mechanics leave the car on jack stands for months without any problems. It is more a question of where the car is being stored while on jack stands. In a garage where it is safe from accidentally being leaned on and possibly tipped over would be best (and out of sight from vandals/hooligans that might want to cause havoc, ...
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