Which techniques/filters do improve sensation of speed on a motorcycle?
I agree with my fellow German dude. Lowering the framerate (until 24) can increase the feeling of uncontrolled speed and momentum. If you already did that and didn't get the effect you were looking for, here's how you can use some trickery to fake it:

Motion blur! It helps selling the effect big time. I recommend using a radial blur which is going outwards from a defined point. Put this point in the center of the screen and use a feathered mask to cut the effect from the center, so the blur only effects the edges of the screen. This way, it appears as if the road is flashing by way faster than it actually did.
Also, when it comes to filming vehicles, it's surprisingly easy to get away with simply speeding up the footage. Don't overdo it obviously, but 125%-135% should look just fine.

Combining all these effects, lowering the frame-rate to 24/25, adding a radial blur towards the edges of the screen and speeding up the footage slightly should give you a convincing effect.