If you want more immersion...

[teeds]

When I set my output to sub-woofer the shaker constantly made some strange noises, no idea why? Impedance issues maybe. But as headphones it seems fine.

Also found that the vibes faded out as a bike revved up and i'd like to feel them throughout, so I whacked up the treble. I can now hear sound from the shaker but I can just about feel the bike at higher revs, think the cushion on my chair is soaking this up a bit unfortunately. Maybe another shaker attached to my handle bars might work, hmmm...

[h106frp]

Problem you have is limited audio 'stage' in the game, the only bits that ''rumble' are the kerbs, the weird LF is possibly the wind noise or a noisy output from the amp. Your seismic shakers will not like driving higher frequencies for long and might get a bit warm.
This is why I started to use the telemetry accelerations to drive the shakers in a more realistic way.

I did start a shaker circuit (eccentric motor) that ran off the rpm signal  for engine vibes:)

[teeds]

When I got this constant hum/buzz/noise when selecting a sub-woofer output, it was while not in game and also while no noises were being generated by windows? Not checked for heat but nothing has stopped working yet, fingers crossed.

This is why I started to use the telemetry accelerations to drive the shakers in a more realistic way.

I did start a shaker circuit (eccentric motor) that ran off the rpm signal  for engine vibes:)

Would be good but I imagine starts getting a bit more complicated.

Maybe one day sound cards will get a dedicated shaker output usable by applications.

[h106frp]

SimVibe is the only API driven system I know of... but pricey

[teeds]

Variable simulated wind would be another great addition to the immersion level. Just a standard fan in front of you works to some degree, as i found out on a hot day in summer