Modern Vespa

I see a lot of references to replacing the rear stock 130mm GTS tire with a 140. The often stated reasons seems to be "more traction" or "better handling" or "higher speed".

From my motorcycle experience, though, putting a wider tire on the same rim results in a more curved cross section, less contact patch, and a tendency to "fall into" turns at low speeds.

Is there instrumented, objective evidence the 140mm tire offers an improvement in top speed, lap times, and/or braking distance? Or is it all "seat of the pants"?

I put a 140 on my old GTS and didn't notice it back there and will do it again on my new scoot when due. I don't buy that curved cross section/fall in stuff at all!!!
It makes the contact patch slightly larger and throws off a speedo thats off to begin with but not by enough to matter.
I've read too much evidence from more experienced Vespa ownners here that the front needs to be left at 120.
Do it if you want more rubber on the ground. Lap times? Who does Vespa laps? Braking distance is a pure guess as is top speed unless you GPS the scooter.

I guess you missed my point. COMPARATIVE braking distances, lap times, top speeds, acceleration times, maybe a Dyno run, under controlled conditions, measured and quantified, are the only way to ACTUALLY know if a change has made a difference good or bad, and is worth spending my fixed income on. "More experienced Vespa owners" is anecdotal, or hearsay, information, not data.

You don't actually know your contact patch is bigger unless you measure it and compare it to the contact patch of a 130 tire with you on the bike with the tires on the ground. It might actually be smaller because of the rounder profile (which is geometry, not faith based) of the wider tire.


https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=11&cad=rja&uact=8&ved=2ahUKEwii7MPdxYfgAhVFPK0KHSvTD_QQFjAKegQIAhAB&url=https%3A%2F%2Fwww.motorcyclistonline.com%2Fmotorcycle-tire-width-and-traction-video&usg=AOvVaw3j7IfhCK0NlCc6ajwjBOdK

I have to think the engineering data is out there, probably at Piaggio, to reveal how the GTS achieved its current factory configurations.

I have thought about this topic somewhat... but am not an expert by any means. Everything below is my opinion and not fact.

I can absolutely see how a wider tire could lead to a more rounded profile. This is the exact opposite of what happens when using a Gilera 13"x3.5" wheel instead of the regular Piaggio 13"x3.0": the wider wheel stretches the tire more, reduces the curvature, and increases the contact patch. A wider 140 tire on the narrower 3" wheel will "balloon" more.

Currently I have the 3.5" wide wheels with a 130/60R13. Is the handling noticeably different when riding? Yes, but I am not experienced enough to say weather it is better or worse. I was also initially concerned with the reduction in the max lean angle, until I realized that the center-stand/exhaust will scrape long before I get to the edge of the contact patch.

I think it's also worth noting that the speedometer will be far more accurate with a 130/70R12 or 130/60R13. I suspect that Piaggio originally designed the GTS to have a 130 in both the front and rear, but changed to a 120 due to handling/cost...

One more thing: The wheel/tire package on Vespas are so small that the tire weight is more than the wheel weight. Adding more rubber is probably not the solution for better performance, as it will increase the unsprung weight & rotational inertia much more than a car's tire.

Gee whiz, I am in over my head on this one...