Modern Vespa

Created article FAQ: The engine sounds different when I hold the rear brake

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Changed the name of this topic to better reflect a question.

I would like to offer a different Theory. The engine is isolated from the frame by rubber bushings whether the rear brake is applied or not. The axle is under load from the weight of the scooter and the rider. The force of the brake pad transferred to the rear axle is minimal when the scooter is setting still.

So IMHO the clamping of the brake pad to the rear disc does little to nothing to change the sound and vibration. What does go from an isolated to non isolated state is the hydraulic fluid and line activating the rear brake. Unpressurized the rubber hose and fluid dampen sound and vibration transferred from engine to frame. Pressurized, the fluid and the line become almost solid ending the damping and create a conduit for the sound and vibration to be transferred from engine to the frame. If the rear brake was activated by a cable, the change in sound and vibration would be less because of the cable being made of many stands of wire and having more flexibility than the hydraulic line and fluid under pressure.

I hadn't heard that theory before, but it's a good one. I never really thought of the hydraulic line becoming like a solid, but in a sense it really does.

I don't think there's anything at all wrong with adding this theory to the existing article. It's at least as good as the one that's already there.

Instead, couldn't you test that theory by hooking up a brake line, master cylinder, and lever setup that's not touching the frame in any way (maybe just run a long brake line to a second bike's master cylinder) and just applying the brake? That way, there'd be no conduit to transfer sound and vibration to the frame of the test bike. Just a thought if somebody wants to get all empirical and stuff.

Also, don't forget the brake applies the stop light. This high wattage bulb loads the generator at idle and increases the load (albeit lightly) on the engine, and possibly further changing the engine note. This will be even more noticeable when the battery is not fully charged. Just another thought! It's probably all of the things mentioned in this thread above put together.

TOTALLY AGREE ON THAT!

Couldn't that be tested by applying the rear brake, then the front? Either lever activates the brake light. The electrical load is the same. Does the engine sound different depending on whether the front or rear brake is applied?

Yes it does, that's the whole point. The front brake doesn't lock the rear suspension. The rear brake does, and thereby changes the resonant frequencies of the whole engine and frame.

A 20W bulb sucks up a whopping 0.025hp. Arguably this might make a noticeable difference if the charging system is weak and the battery is shagged, but I doubt it.