Gearing for Road Driving

[SS_canuck]

Hello all,
I'm awaiting the arrival of the Lifan 140cc engine I've ordered for my CT70. The bike will be plated for road use this spring. I've been reading through past forums and have come to the conclusion that my bike's stock 15/38 gearing may not be optimized for street riding. I'm not overly interested in building a wheelie machine. There are many very helpful threads on selecting gear ratios for top speed. My ideal set up would be one that would reach and maintain a cruising speed of around 50 mph (80 kph) or slightly higher without revving the engine to high. With that in mind how would one go about choosing sprockets to match these requirements. Do you want to be cruising around at the engines torque peak? In my case that would be at 5500 rpm. Or do you want your cruising rpm to be somewhere between to peak and hp peak?
Thanks in advance,
Matt

[racerx]

There's no single, one-size-fits-all, answer to this one. Optimal gearing is a blended range of hard numbers, rider preference and capabilites of the specific engine in question. If you want to do things the easy way, then you'll need to know the top speed potential, peak hp rpm & engine redline, transimission & primary ratios, tire rolling circumference. Max cruising speed will be 80-90% of top speed potential. (how fast the bike can go with the gearing optimzed for top speed under ideal conditions, not necesarily how fast it will actually go)

The formula:

• mph = .006 x engine rpm x wheel radius (in inches)/final gear ratio

mph/1000rpm simplifies mental calculations and the equation, which becomes: 6 x engine rpm/final gear ratio

Measure the actual rolling circumference of the wheel and divide by 3.1416 and you'll have an accurate wheel radius value.

Final gear ratio: (primary gear ratio x transmission gear ratio) x sprocket ratio. Calculating engine gearing from crank to countershaft simplifies things as sprocket ratios are changed.

If that strikes you as too much theory, consider the alternatives: buying a lot of sprockets to test, purely trial & error, or taking someone else's advice which will likely fall short, unless the advisor has your same setup, knows what he's doing and shares your riding preferences. It's either a bit of applied science or luck, take your pick.

With next to no info on your engine, aside from stated displacement and torque peak rpm, my best wild-assed guess is that gearing for 7.2-8.0mph/1000rpm is the best place to start. Use countershaft sprocket changes to determine the range, then fine-tune with the wheel sprocket. A one-tooth change on the countershaft is roughly the same as two teeth difference on the wheel, allowing you to "split the difference". Starting with the largest countershaft sprocket (probably limited to 16 or 17t) and smallest wheel sprocket that will get you close to the 8.0mph/1000rpm point - plus a wheel sprocket one tooth smaller, should take care of your wheel sprocket requirements. With 14, 15, 16 & 17t countershaft sprockets you'll have 8 ratio choices. CT70 wheel sprockets can be had as small as 28t, however, without hub machining, the limit is 29 or 30t. From there, it's up to you whether you want to leave the bike in fourth gear all the time or use fourth gear like an overdrive and downshift into third for ascending grades.

Reader's Digest version for those with A.D.D:

If the clutch must be feathered from a dead stop then the bike is overgeared, even if it runs out of revs before reaching top speed potential. In that case, top speed & cruising speed are limited by transmission ratios and there's not much to be done unless you can source different tranny gears. These engines don't take kindly to low-speed lugging; it's brutal on the clutch and bearings. Select a sprocket combo that just allows for reasonable takeoffs and you'll be at the workable high limit. Gear down from this point to suit your preference.

[SS_canuck]

Thanks Racerx!!
I was sort of thinking that picking up an array of sprockets would be the way to go.
I'll give the calculations another try to at least build a starting point to work from.

Regards,
Matt

[racerx]

There's no magic to any of this. Even if you have an accurate engine dyno printout and a good understanding of gearing, there will still be enough trial & error once you hit the road. The goal is to get as close as possible, using theory, to keep that mean ol' Mr Reality from wearing you down in the real world, once you hit the road and start getting it dialed-in.

It's all about determining ranges. The rider will care more about specific gearing than the engine will. Fewer revs per mile equal less engine wear, but mostly it's about sound & feel. For what it's worth, I like gearing on the high side (numerically low values). With fourth slightly overgeared for steep grades, normal cruising is more relaxed (It's surprising just how much difference there is between cruising along at 7200rpm vs 7700rpm, for example, even though it's only about 6.5%.) and there's some headroom for the occasional downhill blast. Real-world results are cruising speeds up to about 60mph, mid 60s toppped-out and 50mph possible in third, which is handy when ascending a long grade into a headwind & riding two-up. For those occasions when top speeds won't exceed 55mph, a one-tooth smaller countershaft sprocket is installed...a 5-minute job.

[chris_s]

what's a good source for sprockets?

is the ct70/lifan/zongshen front sprocket a pretty generic piece?

[racerx]

Best place to find out which countershaft splining your engine has is the seller. There seems to be more variation between Chinese engines sold over the past couple of seasons and some run #428. Aftermarket offereings have expanded to the point where odds of finding the right spline with #420 pitch are good. You just need to know what you have.

[MSZ]

Or you can play with the gear ratio calculator I've developed and put online.

http://honda50.com/speedcalc/speedcalculator.html

This calculates mechanical speed.

Keep in mind your engine has to have enough horsepower to propel your bike to your desired speed so don't overgear thinking a 120-140cc chinese engine is going to get you to 80mph.

The chinese 120's go around 60-65mph depending on the size of the carb and the quality of the exhaust and state of tune.

Since I don't have chinese engines listed in the drop down charts, you'll have to manually enter the values in the entry boxes.

[John Pond]

Originally Posted by racerx

Measure the actual rolling circumference of the wheel and divide by 3.1416 and you'll have an accurate wheel radius value.

From the obnoxiously picky peanut gallery:

That gives you the diameter, not the radius. Divide the result by two for the radius.

Back to your regularly scheduled programs...

Cheers!

John

[SS_canuck]

That is awesome. Looks like I'll be crusing at 55mph around 8000rpm.

John looks like you put a lot of work into not just the calculator but the entire site!!! Great resource.

It's my newest bookmark!

Regards,
Matt

[John Pond]

Originally Posted by SS_canuck

John looks like you put a lot of work into not just the calculator but the entire site!!! Great resource.

It's my newest bookmark!

Actually, PonyExpress (Dan) and racerx (Bob) both put an awful lot of their time moderating and helping folks: I just kibbitz annoyingly. (Is that redundant?)

It should be your newest and bestest bookmark.

Cheers!

John