What's that Trombone sound?

[fish]

500 miles on my Forza so far...and I think I've decided that mini-trombone sound coming from the left side is NOT a reflected exhaust sound.
Is that noise at throttle coming from the air intake?
It seems louder than the muffler!
If everybody is not hearing that noise from the left side ( a bit annoying actually) then maybe I better pull the cover off an have a look to see if something is adrift in there.
?
Fish

[BRed]

The engine air intake is tucked under the body, but the CVT air intake is a separate port on the Forza (unlike the Reflex and BR which shared an air inlet.)
If you slide your hand along the front of the CVT blower cover you can feel the forward facing ports.

But most of the sound on the left side, including the rather noisy 64-70mph transition is from this:



That's the new final drive cross-shaft support bearing....it's mounted in the CVT cover and slides over the end of the clutchbell nut.
at about 65mph, it resonates the face of the CVT cover in some sort of harmonic vibration just for a few seconds....
to be truthful, if you play with the throttle you can hover at ~68mph and stay "in the zone" long enough to confirm that the loudest sound you hear (or I hear?).... is from the CVT cover.

The slight rumble sound of the bearing is always present, but rises to a crescendo @68mph then fades away as speed increases (on my Forza, anyway).

This is especially apparent if you take a short run with the plastic and foam noise barrier off...it's the black plastic CVT cover cover?.
Once I figured out where the sound was coming from, it was easy to ignore it.

[MichRocks]

BRed-
Great post!

[fish]

Not sure I understand...the sound I got in the first 50 ft pulling away from the dealer's lot on this brand new scoot is from a bearing?
No, I don't think that is what I'm hearing - I think it's the airbox howling as the air rushes in for acceleration.
In any event....it's louder than the pipe on the other side!
Fish

[BRed]

The "turbine" whine sound is from the CVT's air intake (as best as I can tell)....
it uses a series of small inlet ports along the circumference of the blower cover and has a unique whistling sound that follows the engine revs as it blows air though the CVT case.

The outboard cross-shaft support bearing is mounted in the CVT cover (relatively thin cast aluminum) and it definitely is the source of the 70mph transition noise (or vibration) that many have reported with the Forza.

This bearing is new for the Forza and has never been used on any other Honda single cylinder scooter CVT, as far as I know.

[blackcat54]

Would it be fesable to spray the inside of the CVT cover with some kind of sound deadening material? The automotive audio industry uses all kinds of spray and sheet material for sound and resonance supression.

[BRed]

It may be possible....there's also a hollow cavity behind the bearing mount that may be responsible for much of the resonance.

The CVT's out-board bearing almost seems an after-thought, like the designers finally realized the engine had more torque than they thought and the driven pulley needed some additional support to avoid thrashing the final drive.

It will be interesting to see if modding the variator for a different upshift rpm will also move the speed the vibration occurs.
I suspect the vibration is caused by the contra-spring loading the belt at some point during shifting, more so than just the engine revs...
if that's the case, changing the variator weights will move the resonance to a different speed.

[beegmouse]

I think I'm getting this at 4300rpm, about 35mph, v distracting.

If it is the plastic CVT cover, what should I do to silence it?

Chris

[beegmouse]

Something else to note, the PCX also has that cover mounted bearing, but no strange noises.