Modern Vespa

I have noticed on my 2023 GTS Supertech 300 when the rear brake lever is pulled that the engine loads up under a strain and there is a slight vibration. Anyone have any idea why?

mine does it too. i just assumed it's a power-assist off vacuum or something that causes just a tidge of extra load at idle.

hasn't caused any issues (at least not yet), so i can't imagine it's a major thing.

Do you mean that when you pull one/both brake levers while stationary, the noise changes and slight vibrations may occur?

Then this is absolutely normal. Pulling the brake creates a frictional connection, the noise changes and vibrations from the engine can be transmitted (differently) to the body via the brake, brake caliper, chassis, etc. When the brake is released or applied, the path of transmission of the vibrations changes / the "oscillating" mass changes.

Like the OP, I interpreted the vibration as load/strain on the engine, but was told it was normal.

However, I was never told why. Your answer is very interesting, I never thought of that. Thanks

Answer: FAQ: Why does the engine sound different with the brakes on?

When the brake is applied, the pressure in the brake line forms a "solid" connection between the engine and body allowing the vibrations to get through, bypassing the suspension.

Only while the handle for the rear brake is pulled while stationary. Doesn't do it when the front brake only is applied. It's not causing any problems, I just thought it was odd that it done that. The dealer never noticed one doing it and didn't have a clue why it does it.

I never thought of that as a possibility. Very interesting and a likely reason.

It IS the reason. The suspension becomes locked by virtue of the brake being applied, and the amplitudes of the harmonics of the main engine frequency alter relative to one another.

^^^ This! On my old chopper (long front end) the feeling when stopped at a light changes dramatically when I apply the front brake or release it. With that long front fork, you get a lot of oscillation when the brake is not applied. Grab the brake and it goes from being an oscillation to a buzz like vibration. Changes the whole feel of the bike.