Wheelbase
The distance between front and rear axles. Longer usually means a smoother ride.
Wheelbase is the distance between the centers of the front and rear wheels. It is distinct from overall length: two vehicles the same length can ride very differently depending on where their wheels sit.
A longer wheelbase smooths the ride over bumps and opens up rear legroom, which is why limousines stretch it. A shorter wheelbase tightens the turning circle and quickens handling, useful in cities and off-road.