Modern Vespa

I know that too many miles at flat out starts to limit a scooter's top speed. And I understand this is because a groove forms. Would that also affect the speed at which the rev limiter kicks in?

After 3 days of solid interstate driving I noticed that the rev limiter was kicking in earlier and earlier. The last I noted it was kicking in at just under 75 mph. If it isn't caused by the groove, what could cause something like that.

Thanks!

Yes. The groove effectively alters your gearing -- only it’s not actually gears. The principal is the same, though. For the same RPM (which is what the rev limiter is based on) you will be turning the rear wheel more slowly. Thus, the rev limiter will kick in at a lower speed.

The rev limiter works on revs, not speed. Several things will affect the relationship between revs and speed when in top gear, especially after some long days at WOT.

The tyres gradually get a smaller diameter, so top revs mean a lower top speed.
The belt gets worn. It can't go so far out in the variator, so effectively you're in a slightly lower gear - less top speed at top revs.
The variator may get a groove - effectively reducing how far out the belt can go in the variator - see above.
The rollers (or Dr P sliders) can wear down. Rollers especially can get flat spots. This will reduce top speed as above, as the variator won't push out so far.

So they all conspire in the same direction, giving you less top speed at top revs.

That's good. I've got a new variator, belt, and rollers to get the scooter back into commuter shape. Glad to know for sure that there isn't a separate issue to chase down.
Thanks!

How much top speed have you lost?

I noticed the same thing on my GTS. The rev limiter was kicking in 3-4 mph lower than normal.

A week later the belt broke.

In my case the pulley was not groved and the rollers were not flat spotted.

It had been 9,500 miles since my last belt change. It looks like I pushed it a little too far

It would only get to 67-68 on the flat. But the torque took a major dive. I know the poor scoot was overloaded, but that was in comparison to the same overloaded state. Did changing the belt fix the problem?

Indicated per speedometer or GPS?

Those particular numbers are GPS, as compared to 75 on the flat GPS. The before and after speed ratio is similar if I go by indicated.

That’s more than I would expect from the groove alone. I don’t think it warrants concern, but it may be a combination of wear factors, as Jim articulated.

Also, if there was extra luggage on the scooter, that would add to the drag. As the power needed to overcome drag is proportional to the cube of the speed, this also might be a factor.

I think drag force is proportional to the square of speed? Power would be proportional to force, so also square of speed. Or am I missing something?

(Not that it changes the point you were making)

Yes, the drag force is proportional to the square of the speed. Power is proportional to the force times speed. Therefore proportional to speed cubed.

Yes. YMMV.

In fact, this is a good warning sign to get the belt replaced. I found this out the hard way a bunch of months back, but my belt gave up roughy 1k miles before yours did.

I think the horsepower is subject to the cube root. Sorry I'm not a mathematical genius.

Top speed = Aerodynamic constant x the cube root of horsepower. I think
or TS=Cx (HP ^1/3)

If it’s an older bike and it’s a very spooky feeling it might be the engine cut off switch on its way out. I had one of those go bad, they don’t go all the way bad every time, mine would randomly kick on and off while driving highway speeds, not a fun feeling