Most useful when building custom vehicle suspension setups, a rose joint is a linkage used to keep two points at an exact distance apart from each other, while allowing quite a free degree of movement otherwise.
The joint typically comprises two parts which are joined together and supplied as a whole. The internal part is a sphere with a hole drilled through the middle. The outer part is a ring with a threaded shaft attached. The sphere is held within the ring, but is free to rotate in any direction. The interface is lubricated with grease.
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On the above diagram, the central circle is the hole in the sphere. The next circle out is the hole in the joint's ring. Next is the outside edge of the ring.
When used to build a suspension system, all rods attached by rose joints are free to move up and down with the bumps of the road. They also allow the steering to operate if this is on the front of a vehicle. The important part is that the castor and camber of the wheel is also adjustable, by screwing the joints in and out of the suspension rods to adjust the exact lengths of each component of the setup.
A lot of work, time and money has gone into designing the suspension setup for your car, and while fitting rose joints may have some benefits, there are certainly disadvantages. The argument that they're used on racing cars so they must be better is fundamentally flawed for a road car.
The first disadvantage is the cost and maintenance schedule. Race cars tend to be rebuilt every race, so the fact that rose joints wear quickly is not such a problem. On a road car, particularly with more dirt and grit around, they will wear far too fast, even if rubber boots are used to prevent ingress of water and dirt.
The other main disadvantage is that unless you really know what you're doing with your suspension geometry, adjusting it is not generally a good idea. Leave it to the experts.
When building a kit car, rose joints allow the suspension to be built easily without having to fabricate specialist parts. Also, if you really do know what you're doing, they give the quickest route to setting up suspension geometries for testing.