I just measured my tire and it is somewhere between 17.5 to 18. ''4.00-10 Zippy2's'' I base my rough numbers of the area I like to ride. If you have all flat areas my #s should be close. I run 16-33 and can pull any big hill in 4th with my stroker 125 ''hi compression'' build that can rev to 12 grand ''billet clutch'' equipped, in first second and third.
17-31 would be a BIG OLD dog or a big steaming pile of
with a L125... Agreed? 6 teeth off from racers math. Enjoy!
If that combo works...it works. Replicate it, i.e. build another engine of the same configuration (bore x stroke, cylinder head, cam, piston, etc) and you'll know precisely how it needs to be geared, from the get-go.
What I am saying here is that changing a single parameter can affect gearing, via sprocket combo, dramatically. So, be careful about making assumptions. While "displacement equals torque" and "HP equals MPH" are both valid, neither quite tells you everything you need to know to realize optimal results on the first try. The most obvious variable is gearing, from crank-to-countershaft; this is why I state final drive gearing (crank-to-wheel) in mph/1000rpm. Less obvious is the fact that two engines of the same displacement can have significantly different power curves...and with them, gearing requirements (i.e. optimal mph/1000rpm gearing).
To be clear, I am NOT disputing your results. I am attempting to further clarify/explain for the benefit of those either new at this, or are about to begin tuning a different engine combo. Since the first Chinese 110s & 125s hit the market, I've seen optimal sprocket combos ranging from 16/35 to 17/33, paired with Trailwing tires. Those differences are bigger than you might think and displacement is the least of them. It is possible to get the same displacement by increasing bore, or stroke, or both. Increasing bore size has a modest impact on torque & gearing. OTOH, increasing stroke length is like night & day. For example, the ever-popular "88cc kit", a.k.a. 52mm bore, increases torque enough to allow upsizing the C/S sprocket by a single tooth. Retaining the stock bore paired with a 51mm crank yields 89cc...and gearing requirement gets pushed close to what you're running with your 125cc tune. The more typical 108cc tunes, based on Trailbikes 52mm bore-up + 51mm crank, work well with a 4-speed tranny paired with 15/31 cogs...and Trailwings. 16/33 = 2.063, 15/31 = 2.067...a difference of 0.2%. That's negligible and too close to explain why your 54x54 tune wants the same gearing as a relatively mild 52x51tune. I know of at least one 110cc motor that works best with 17/31 sprockets, mine; changing from Trailwings to Bridgestone road tires, both "4,00-10" required changing to 17/32 to maintain the same mph/1000rpm. At the ragged edge, where you want a road machine to be geared, it is that critical.
No need for a deep dive into the physics, here. The concept remains simply that any given engine can only pull so many mph/1000rpm. Beyond that, it's overgeared and will suck out loud. The "shortcut" is going in knowing what a specific engine can pull, after someone else does the homework for you. The next best approach is an educated guess based on stroke length, displacement and gearing (primary drive + top gear combo), unless one has a tachometer. Here's a "cheat sheet", expressed in mph/1000rpm final drive: stock CT70 gearing ~4.78mph, w/88cc kit ~5.0mph, 52x51/52 (108-110) ~6.6mph, L110-125 ~6.8-7.0mph. Use these figures as starting points; the further you deviate from stock, the more they'll diverge. And, Chinese engines have come in many different flavors, over the past 18 years, so be aware.
