Portland triMET MAX Red Line shimmy

[NW cannonball]

Rode the trimet Red Line inner city to airport last week.

On the high-speed parts of the route, the train did a disconcerting - shimmy? wiggle? - ~1HZ cycle on the yaw axis when on curves.

Is this account of the railcars and their trucks, or the track design?

 

[the_traveler]

It's been a few years since I've been on the Red Line, but I don't think anything of that shimmy. No matter what form of transportation you take (including trains, cars or even planes), there will be some slight movement. Some more slight than others.

No form of transport is 100% smooth.

Acela is "more modern" than regionals (using Amfleet I cars), yet I think regionals ride smoother than Acela.

 

[DET63]

I've noticed it more on light rail (MAX and S.F. Muni Metro) than on BART (which uses Indian gauge; perhaps that makes the difference).

 

[Bus Nut]

I believe what you're referring to is called "hunting" or "truck hunting". Essentially it happens because the consist is not perfectly rigid, so some of the kinetic energy gets translated normal to the desired direction of motion and the oscillation begins. It's much more noticeable at higher speeds. Oscillation can begin with very little initial force when a system is in astable equilibrium. A certain amount of hunting is to be expected on light rail.