They sometimes rely on ride manufactures to construct the ride. Imagineers are involved in building the buildings, the queues, the ride story, but the actual ride track and ride vehicles (also ride software) are usually an outside company. Granted these outside companies design the ride exactly how Disney wants it, they are the ones who do the math and calculate everything out.
Rock N Roller Coaster - Designed and built by Vekoma
Expedition Everest - designed and built by Vekoma
Primevil Whirl - designed and built by Reverchon
Matterhorn (
Disneyland) - Designed and built by Arrow Dynamics
California Screamin (Disneyland) - Designed and built by Intamin
Mullholland Madness (Disneyland) - Designed and built by Mack Rides
Maliboomer (Disneyland) - Designed and built by S&S Power
I'm sure there are others, but these are ones I'm sure of.