Modern Vespa

I know we as group have discussed it endlessly, but I have the 13 inch wheels on my bike and scootewest tells me that if I go back to the 12 inch wheels my acceleration will improve. How is that possible when the diameter of both is roughly the same (about 2mm difference)?

Did you explain to them that the diameter of the tires is nearly the same?

Yes I did and the response;
"Yeah, I don't know why but the 13s make the bike feel flat"

Now to be fair, I find the 13s to be more stable I suspect due to the shorter stiffer tire side wall but I don't understand how that will affect acceleration given the similar diameter?

If anything, all else being equal, the 13" wheels could make for faster acceleration because of their lower moment of inertia, if there's less heavy tire rubber at the outer circumference of the wheel.

Reduced inertia with 12" wheel/tire combination.

+1
The aluminum of the wheel is heavier than that part of the tire. Smaller wheel = less moment = better acceleration if the overall tire diameter is equal between the two.

This also assumes similar mass distribution of the wheel, etc....

Thank you both, need to dust off the good old physics text book...

Yep, figure out how many HP it takes to spin a wheel up to 60mph in 1 second.

As the guys are saying, the 12" wheels offer better acceleration, even although there is only a tiny difference in overall wheel circumference (the 12" being slightly smaller). The smaller size lowers the overall gearing of the final drive. The 13" wheels raise the overall final drive gearing and can affect top speed, and depending on how good your motor is, it may slow you down on top speed or actually give you a tiny increase in top speed. Generally, unless you have tuned your motor 12" wheels offer the best performance.

What width & profile tyres are you talking about?

I am running 13s with 13 130/60 front and 13 140/60 rear with the full Malossi kit w/force master 2 and Malossi suspension

Budz, remind me to chat to you soon about your tuning mods. I'm wanting to do the same in the not to distant future! I'm assuming you are very happy with the way it goes? Did you fit the upgrade kit?

Sorry to op for stealing a bit of your thread!

I've got 13's on my gts 250. Like you it's kitted with forcemaster+ cam and upgear kit. No stock 300's I've ridden with are faster off the line....at speed it is so much more stable feeling. I would never go back to 12's regardless of a loss of initial acceleration.

While not kitted yet (if ever) but I've been debating going to the larger tire size on my 300. Thank you for clarifying. I'd be willing to loose a little off the line for a more stable ride once cruising or at high speed! Thanks for the input!

Super dumb question, I know but I thought I saw or recall hearing Robot say on one of the Scooterwest videos that you can throw the 13inch tires on the 12 inch rims.

Correct?! Someone mentioned on this thread and I like the idea the next time I need tires of just getting the bigger ones and slapping them on the 12 inch rim. Instead of getting 13 inch rims and new tires.

Please help, I'm being dense here I know it.

has anyone put a 13" wheel & tire setup on a scale? I cant imagine it being more or less than a few ounces in overall weight difference between either. If I remember correctly the OD of the 12 & 13 tires are almost the same sans a couple mm.
Ive read some "subjective" impressions of increased stability.
I suspect those were garnered on the crown of the tire vs the sides.
In stock sizes on a large frame vespa you ain't touching the last 10mm of the tire tread unless you crash or hit it with an orbital sander.

Anyway... I would imagine variator & clutch tuning will start to play a role
in you wanting to blast off from a stop.

I'll go out on the limb here and say they'll leak air, 13" on 12"

13" rims is the way to go. I have them on my Vespa GTS 300 ss and it's so stable at high speeds. Also have done some mods on my Vespa and honestly it's so quick off the line. So much quicker than other Vespa's I have "raced"

Best decision I made putting 13" rims on.

Super easy to seat though.

I know I was being dense and I didn't relook at that video of Robot going through Scooter Works Tire shed again to see what specs and all he was talking about.

I just saw a new video where Robot talks about "1 size bigger" tire on the 12. I'll relisten to it to get the specs. It was a 12 inch tire but I think thicker.
I'll try these first before I totally switch up to a 13 inch rim and tire combo.

Thanks for the input and review! What tires are you using? Pirelli, Michelin...what?

so you still have the stock cylinder head just a larger lift/duration camshaft?

Any port or other top end work?

Were the "other" 300s actually racing? Did you trade bikes with the other rider to eliminate that variable?

I guess the question here is... how do a tire/wheel combo translate into more stability when the OD measurement is about 3mm at best?

Is the stability from less revolutions per mile?

I would suspect the 140 size in the michelin is a "plus 1" over the 130 stock size. The 140 michelin is much taller than the 130 size pirelli.

It's the lower stiffer tire side wall that makes the difference in the "feel". The tires are 130/60 13" vs 130/70 12"

I'm using pirelli diablo

I get that... but I was more curious about the weight combos between both.

the OD measurement of the 12 & 13" (michelin) tires are pretty close.

okay, I switch back to the stock 12" wheels today and the problem persists. I think it could me a mapping issue with a defective ForceMaster 2 unit, is that possible?

There appears to be 3 things wrong with this project...

1. The shiny metally bit of the wheels have a different DIAMETER O.D. in their size (13" vs 12")... therefore don't match in SIZE

2. The black rubbery bit of the wheel is a different DIAMETER I.D. in their size (12" vs 13")... therefore don't match in SIZE

3. The 2 completely different tyres have the exact same folia good distance ( they take the same REVOLUTIONS over distance ... ie the O.D. Or Circumference of the black thing are identical, therefore will have NO EFFECT on speed nor revs, not anything else for that matter

https://www.tacomaworld.com/tirecalc?tires=130-60r13-130-70r12

Unless you are involved with timed competition. I doubt you will notice any appreciable difference between 12 and 13.

If you go with a lower aspect ratio tire there might not be any difference.

I thought so all along, local shop claims the larger wheel "kick the bike into the higher gear" also makes no sense to me since the circumference difference is 2 mm not enough given the over all size/diameter...

The rate of angular acceleration will be impacted by mass, and where the mass is. Assuming the two setups have the same mass, the larger diameter wheel with its corresponding larger diameter tire bead will put mass further out from center, resulting in more torque being required for the same acceleration. If the 13" wheel is actually heavier, it will only increase the difference.

As to the comment about the 13" wheel feeling more stable, if its mass is at a larger diameter it would have stronger gyroscopic effect.

While I agree with Sully's reasoning in theory, in practice the inertia of the scooter/rider combination is so large compared to that of the rear wheel as to render the latter pretty insignificant... except perhaps for the difference in the gyroscopic effect.

Also, with the minuscule 0.1% difference in the tire circumference, you'd need one helluva well-calibrated butt dyno to detect any performance difference due to the difference in overall gearing. A bit of tire, belt or roller wear would likely overwhelm any theoretical difference.

if you want better acceleration and higher top speed go buy a more powerful vehicle.

not sure how your post is helpful

Not meant to be. It is just an opinion.

OK. Here is a more helpful one. This is paraphrased from a post I made a few years ago and could not find.

So you want to go fasterer? Here is what you do:

A. Establish an acceleration bench mark:
1. With the stock scooter you are starting with find somewhere safe and appropriate.
2. Measure the 0 -50 times as best you can. (or 0 - 60, or whatever)
3. Take numerous readings on both directions to even out any wind or gradient effects.
4. Write down the readings.
5. Average the results.

B. Establish a top speed bench mark:
1. With the stock scooter you are starting with find somewhere safe and appropriate to run the scooter flat out.
2. Measure the max speed of the run times as best you can. All my Garmin GPS units, if you touch the speed area on the main panel, will display a data panel. One datum herein is Max Speed. Reset the GPS.
3. Take numerous Max Speed readings on both directions to even out any wind or gradient effects. Reset the GPS between each run.
4. Write down the readings.
5. Average the results.

C. Now:
1. Check that both tires are set to the manufacturers recommendation.
2. Remove both wheels, remove wheel bearing grease, clean and re-pack the bearings with fresh, new grease.
3. Drain, flush, and replace the engine oil.
4. Remove and replace the gearbox oil.
5. Examine the brakes to ensure that the shoes are not binding on the discs.
6. Remove everything from the pet carrier and glove box
7. Remove all bling: front rack, top case, rear rack, crash bars, etc.
8. Lose about ten percent of your body weight.
9. Remove wind shield
10. Wear smooth, aerodynamic clothing and safety gear.
11. Wash the bike thoroughly to remove all dirt and crud from the engine and body.

D. Now repeat A. and B. above using the same equipment and comparable weather conditions. Compare results.

E. Finally, make whatever mods you want. Repeat A. and B. after each mod so you can tell which one had an effect. Repeat with and without the wind shield.


Tabulate your results and post them on Modern Vespa for all to enjoy.

We'll see you in a few weeks!

Have a nice day.

Oh. And the above post was inspired by an article entitled "The Mythical Miliwatt" I read in an Ham Radio magazine many decades ago. The article basically said in order to improve one's radiated power one should examine every aspect of the equipment from the antenna to the microphone and speaker. Clean or replace every fitting and every contact, remove any rusty wire, and generally give the rig a thorough birthday. Each step may only improve things by a miliwatt but the aggregate total can add up to a louder, clearer signal.

There's also such a thing as an overkill reply.

I did Chrome Stuff:

Powdercoated Gilera Runner Wheel: 98.90 oz
Chromed Gilera Runner Wheel: 101.90 oz
Michelin Purepower 130/60R13: 132.91 oz

I haven't measured the OEM 12" GTS wheels, but I suspect the overall weight to be very close to the 13" Gilera Runner Wheels. Other 13" wheels are probably heavier though. I think you are on to something here... the contact patch on a 13" would be wider than a 12" (of the same overall diameter), due to the lower sidewall profile.