Modern Vespa

I 100% agree with the statement above, but I also think that there are more variables at play. Mostly, the weight of the tire (on a Gilera Runner 13") is 57% of the overall wheel/tire weight (133 oz. / 232 oz.). I have not measured a 12" wheel/tire, but would suspect the tire weight percentage to be even higher. If ture, this would mean that the 12" wheel/tire could have greater angular mass...

It is a bit backwards thinking for those of us used to car sized wheels & tires... but then again, so is everything else on this bike.

Without a sound bench mark one has no idea whether one has made a difference.

"Look, I did the tire thing and it goes 0.02 mph faster."

Um, let's forget the tail wind.

Um, and the down grade.

I'm not sure the GTS's drive train is consistent enough to get repeatable performance data, even with the testing method you provided. I don't think the wheel/tire weight differences are enough to get a measurable acceleration difference.

At the end of the day a wheel with less angular mass will always out-accelerate one with more, even if it can't be proven in a test on the GTS (provided overall diameter & friction are equal etc.). Less angular mass also usually coincides with less overall mass, which helps reduce vehicle weight & sprung weight... again, good luck measuring the improvements.

Hmm... is the facepalm in reaction to Sully's perfectly apt technical phrase a symptom of a non-STEM education, or is it just "who gives a shit" snarkiness?

Whichever it is, I'm not sure I understand why you are commenting in this thread if you don't appreciate someone sincerely trying to apply a little technical analysis to the situation at hand.

So, I didn't weight the 12" wheels before the swap but the I did compare them and the two wheels were the same knowing the real diameter diffrence is 2mm. The bike "feels" quicker and more responseive, I say "feels" because I have not data to support a "felling". My uneducated guess for the Lively-ness is the result of the higher softer sidewall (130/70 12 vs 130/60 13).
On a side note, with the 13s the bike sometime was fast and sometimes sluggish so this could that or the combined with affects of the stiffer sidewall.
Thank you for all the help, I am still curious about the physics of it all and not sure I fully understand it...

https://en.wikipedia.org/wiki/Jerk_(physics)?

Thanks, I remember my physics and rotational acceleration but is it the larger bead or something else I am missing?

There's no way to put a 13" tire on a 12" rim and make it seal and hold air!
You'll essentially have a 1/2" gap all the way around between the wheel and the tire.

So I took my bike for a long ride and for the first time it reach the speed of 80 mph - 82 mph indicated and acceleration is generally better. Before 75 mph indicated was the max. So, the words of Scooter West ring true: "not sure why but the 13s make the bike feel flat"
I suspect to improve performance on the 13s upgrading to a variator kit is the way to go but then again with a variator kit the 12s will still perform better. Stock wheels are best but the 13s feel more stable

Well...it appears I spoke too soon, the bike is bogging down on acceleration again with the 12" wheels so I assume the setting in the ForceMaster 2 is in correct or something else entirely...

your thoughts are greatly appropriated

the stock vespa 12 inch wheel weigh about 97.6oz (5 spoke black with silver lip)
and the 10 spoke MP3 12in wheel in silver was around 100.7 oz.

but even if the 12 and 13 wheel weigh the same the 12 will be easier to turn or stop. they have less rotational mass as the rim section is 1/2 inch closer to axle.

To make a direct comparison, you'd have to determine the polar moment of inertia for each wheel/tire combo. It is not so much the weight, but where it is located in relation to the axis of rotation. Frankly, with a vehicle/passenger combo of 400 lb. or so, I doubt the inertia of the wheels is the determining factor.

In theory, if the 12" wheel/tire has a smaller rolling circumference, that incrementally lowers the gearing. If they are about the same, there may be differences in rolling resistance.

Frankly, if you intended to actually do tests, you'd want to get a VBox or something similar to remove any bias, conduct multiple passes in opposite directions, adding fuel as needed to maintain the same vehicle weight, allow the engine to cool between runs, etc., etc. I don't care enough to bother.

I will say that when I changed the rear tire, my 'other scooter' picked up a couple of MPH on the top end. Same size, so I can only attribute the change to less rolling resistance.

When you are dealing with very limited power, slight changes can make a noticeable difference.

Thank you Jimding and jerryd, given the theoretical, the actual real life differences are very small, no?

So, I weighed both wheels and the 12" wheels with City Grips 130/70 12 came in at a little less than 2lbs lighter than the 13" wheels with Power Pure 130/60 13. The 13s feel better and the issues I was having were not wheel related, my OEM exhaust graphite washer was almost completely gone....