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Aftermarket Engine/Parts
Lifan
New 125 Sputters through all throttle/idle
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<blockquote data-quote="69ST" data-source="post: 181646" data-attributes="member: 5"><p>Optimal gearing boils down to one thing...what the engine can actually pull. This can be affected by more than just horsepower and displacement. The key parameter is crankshaft revs per mile; that is much easier to understand when expressed as mph per 1000rpm, in top gear. You're heading in the right direction, applying some basic math to figure out where things stand...and which direction they need to be moved toward. Knowing the actual horsepower & torque numbers it's easier to guesstimate a starting point and limit the trial & error testing. This is where I usually cringe when someone makes blanket recommendations about sprocket combos. Ideally, you want to be within 1 tooth at the wheel sprocket; being 1 tooth off at the countershaft will cost you. But, the L125 is a known quantity...more or less. Thus, we can skip a lot of the theory and deal with known values. A garden-variety L125 motor should work best geared around 6.8-7.2mph/1000rpm in 4th gear. Taller than that, it'll fall flat on its face...shorter and it'll be busier than it needs to be, i.e. wasted revs.</p><p></p><p>Going by your stated numbers:</p><p></p><p>15.5" tall tire = 48.7" circumference. A brand new Trailwing will fall between 56-56.5" circumference. I dunno where one would find a tire with a 48.7" circumference; that's smaller than any 3.50-10 that I have ever seen. 18.6" diameter = 58.4" circumference; that's closer to the 120/90-10 tires I've seen. You really want the gearing to be within 3% of optimal, for a road bike. That's a lot easier to achieve by measuring tire circumference; being off by as much as an inch will introduce less than 2% error...easy precision just about anyone.</p><p></p><p>I would strongly suggest slapping a measuring tape over your existing tire(s). Assuming your numbers are good enough 58.4/48.7 = 1.20. So, if you had the gearing optimal with the existing tires, you'd multiply that sprokcet ratio by 1.20 to compensate for the taller tires. </p><p>Thus, if 17/31 was optimal with those short tires: </p><p></p><p>17/31 = 1.82</p><p>1.82 x 1.20 = 2.19</p><p>2.19 x 17 = 37.2 would be (mathematically) the matching wheel sprocket. But before you order a 37t wheel cog, let's get a little more accurate. </p><p></p><p>40mph @ 6000rpm = 6.67mph/1000rpm, pretty close to what I'd expect this motor to pull. 40mph @ 5000rpm = 8.00mph/1000rpm I'll guarantee your motor won't pull that very well, except going downhill with a strong tailwind. </p><p></p><p>Best guess, your motor will run out of power around 55mph and out of breath around 8000rpm, with peak power coming on ~7500rpm. OLD CT gave you the quick summary and I agree with all but one of his points, especially running the largest C/S sprocket that will fit (16t or 17T) to extend chain life...and provide swingarm clearance. Once you move away from a stock size tire the mph/1000rpm ratio is changed. You're now talking about 20% taller gearing...from your tire size change. His numbers are based on stock-size tires, which also are shorter than your bike's new shoes...by enough to make a noticeable difference. If you want to dial-in your gearing perfectly, get some accurate measurements, then break out the calculator. At this moment, I don't know that your tire size numbers are accurate as they could/should be...</p></blockquote><p></p>
[QUOTE="69ST, post: 181646, member: 5"] Optimal gearing boils down to one thing...what the engine can actually pull. This can be affected by more than just horsepower and displacement. The key parameter is crankshaft revs per mile; that is much easier to understand when expressed as mph per 1000rpm, in top gear. You're heading in the right direction, applying some basic math to figure out where things stand...and which direction they need to be moved toward. Knowing the actual horsepower & torque numbers it's easier to guesstimate a starting point and limit the trial & error testing. This is where I usually cringe when someone makes blanket recommendations about sprocket combos. Ideally, you want to be within 1 tooth at the wheel sprocket; being 1 tooth off at the countershaft will cost you. But, the L125 is a known quantity...more or less. Thus, we can skip a lot of the theory and deal with known values. A garden-variety L125 motor should work best geared around 6.8-7.2mph/1000rpm in 4th gear. Taller than that, it'll fall flat on its face...shorter and it'll be busier than it needs to be, i.e. wasted revs. Going by your stated numbers: 15.5" tall tire = 48.7" circumference. A brand new Trailwing will fall between 56-56.5" circumference. I dunno where one would find a tire with a 48.7" circumference; that's smaller than any 3.50-10 that I have ever seen. 18.6" diameter = 58.4" circumference; that's closer to the 120/90-10 tires I've seen. You really want the gearing to be within 3% of optimal, for a road bike. That's a lot easier to achieve by measuring tire circumference; being off by as much as an inch will introduce less than 2% error...easy precision just about anyone. I would strongly suggest slapping a measuring tape over your existing tire(s). Assuming your numbers are good enough 58.4/48.7 = 1.20. So, if you had the gearing optimal with the existing tires, you'd multiply that sprokcet ratio by 1.20 to compensate for the taller tires. Thus, if 17/31 was optimal with those short tires: 17/31 = 1.82 1.82 x 1.20 = 2.19 2.19 x 17 = 37.2 would be (mathematically) the matching wheel sprocket. But before you order a 37t wheel cog, let's get a little more accurate. 40mph @ 6000rpm = 6.67mph/1000rpm, pretty close to what I'd expect this motor to pull. 40mph @ 5000rpm = 8.00mph/1000rpm I'll guarantee your motor won't pull that very well, except going downhill with a strong tailwind. Best guess, your motor will run out of power around 55mph and out of breath around 8000rpm, with peak power coming on ~7500rpm. OLD CT gave you the quick summary and I agree with all but one of his points, especially running the largest C/S sprocket that will fit (16t or 17T) to extend chain life...and provide swingarm clearance. Once you move away from a stock size tire the mph/1000rpm ratio is changed. You're now talking about 20% taller gearing...from your tire size change. His numbers are based on stock-size tires, which also are shorter than your bike's new shoes...by enough to make a noticeable difference. If you want to dial-in your gearing perfectly, get some accurate measurements, then break out the calculator. At this moment, I don't know that your tire size numbers are accurate as they could/should be... [/QUOTE]
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New 125 Sputters through all throttle/idle
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