Sprocket Talk

[bigdano711]

It has recently come to my attention that a 14/33 (.424) combo might work better than the often recommended 17/45 (.378). The figures in decimal are direct representations of the ratio...it helps my brain to see it that way. To me, it just looks taller, like going with 17/40 (.425)


What I'm trying to understand is what the YouTuber is trying to explain, but either I'm missing it or what he's saying simply does not compute. I was hoping to get you guys' expert opinion on the matter. He does explain that a similar Honda from the factory comes with the 14/33 combo, which I find intriguing.

[TominMO]

When he is saying a 233 gear ration for the 14/33 sprockets, he means 1:2.33. (Actually it is 2.35 though.) Rather than dividing 14 x 33 as you are doing, he is dividing 33 x 14. Same difference, just a different way of doing it.

You are correct, it is taller, but that seems to work for him and how he rides on the highway.

[Sport Rider]

I'm curious to understand if there is value to one or the other approach. seems like from an overall gear ratio perspective they would both achieve similar speed/RPM numbers. But I wonder if the larger sprocket in the back on a 17/40 would provide better torque to the rear wheel under load. I have absolutely no clue. hmmmmm....

[ProDigit]

A taller front will reduce the torque on the chain, and therefore allow the chain to last longer.

It'll also reduce engine vibrations through the frame, acts a bit like a flywheel, not the sprocket itself, but the sprocket driving that chain.

Generally speaking, there are only 2 forces exerted on the chain, 1 from the engine to the wheels in terms of thrust and acceleration,
And 2, from the wheel to the engine in terms of engine braking.
The former being much larger than the latter.

In terms of rotating mass, usually the mass of sprockets don't significantly contribute or detract from acceleration; if you were for instance to compare a steel 34t to an aluminum 34t.
But if you're counting in the realm of milliseconds, you might as well get the smallest sprocket setup you can get, and using an aluminum rear sprocket, to reduce weight.

In terms of reliability, using taller sprockets will usually result in longer lasting drive train.

Ideally, for longevity and mpg reasons, you'll keep the stock rear sprocket, and go up on the front one to a 17, 18, or 19tooth, or more if you can.
Ideally you'd be increasing the front sprocket in the same amount as you decrease the rear sprocket, to the point where both sprockets are nearing equal size (if gear ratio wasn't an issue).
For instance a 15/45 will have a lot more sprocket and chain wear than a 20/40 or 20/35 setup.

In terms of what gear ratio is ideal for touring or even top speed, you'll want to have at least 4th or 5th (or with a 6spd 5th or 6th) gear as a top speed gear, and if you can, use the final gear as a true overdrive gear where you can't reach top speed, but usually can still be able to reach a respectable speed for downtown, resulting in extended mpg.

The ideal engine rpm to ride in at any time, is the rpm where the torque of the engine is highest, if you need it. Otherwise it's the lowest rpm your engine can push, without lugging.

On small bikes, they usually tune it to ride at the hp peak, which quite often is near the redline. But this is quite inefficient.
A 50cc doing 45mph at 8k rpm may use just about the same fuel as a 250 doing the same at ~3-3.5k rpm.

The absolute worst you can do, is put a small front and a large rear sprocket on. It'll be bad in terms of mpg, sprocket and chain reliability, engine reliability, cush drive and bearings,and the reliability of the wheels and tires (especially if you are running spokes).

[Megadan]

Nobody mentioned what chain size between the two. Still 428? I run a 520 chain and 13 front and 33 rear, which is equivalent to a 17 front 43 rear on a 428 chain setup.

I only say this because a 13T front 520 chain sprocket and a 17T 428 sprocket are almost exactly the same size. A 14T front 520 sprocket barely fits on some bikes, and I can't run the sprocket cover on mine. It's huge.

[bigdano711]

The YouTuber is on 428, but I think I see what you're getting at. I was trying to justify buying this setup, but it seems to be the same as going with a 17/40 setup, which I may try. Like, less rolling mass, fewer teeth=less resistance or the motor doesn't have to work so hard...some kinda weird science that will let me eek out another .5 horsepower.


Time to buy the Stage 2 kit from MotoCult!

[XLsior]

You might end up increasing friction on the swing arm and wearing out spockets, chain and output shaft faster with smaller sprockets.


So give and take.

[Megadan]

Originally Posted by bigdano711

The YouTuber is on 428, but I think I see what you're getting at. I was trying to justify buying this setup, but it seems to be the same as going with a 17/40 setup, which I may try. Like, less rolling mass, fewer teeth=less resistance or the motor doesn't have to work so hard...some kinda weird science that will let me eek out another .5 horsepower.


Time to buy the Stage 2 kit from MotoCult!

There is no science to "eek out .5hp"

Science says hp = torque/5252. Engine ratings are done via brake dynos, and brake dynos function by applying a maximum load to an engine. The number you get = all the horsepower it has.

What you are doing is shifting where the engine sits in the rpm range at a given speed. This is why a complete stock Hawk can barely do 59mph indicated (more like 56-57 actual) with the 15/50, and then 64-65mph with the 17/45.

Just for fun, some comparison data that should help prove that point. A carbureted Hawk 250 is rated at 14.1hp@7000rpm and 12.1ft-lbs @5500rpm. Stock top speed being 56-57ish actual MPH, and generally can cruise at 45mph comfortably all day long. Would you like to guess what RPMs those speeds happen at? 56mph is at 7500rpm, right at the point the HP peak rolls over and falls off hard. 45mph happens just past torque peak at 6000rpm, still in the "meat" of the torque peak.

17/45? I think we can reasonably agree that an otherwise completely bone stock - no exhausts or air filter shenanigans - with a 17/45 is pretty much maxed at 63-65mph depending on the rider, and can comfortably cruise along all day at 50-55mph. I bet you can't guess the top speed RPM... 65mph is more or less exactly at 7000rpm 55mph? That's right at about 5800rpm.

With a full exhaust and the carb jetted for it, the power peaks both leveled out a bit and carried the power up a few hundred RPM - call it about 300. and my top speed walked up a couple mph on the 17/45 and I found myself able to stretch out to 67mph

17/43? I ran this on the same bike as the other 2 combinations and on the same exhaust only upgrade as the 45 rear. I actually did so as a test to see where the reasonable limits were as far as gearing was concerned. My repeatable top speed was 68-69mph and it absolutely cruised along at 60mph without screaming it's head off. Bet you can't guess what RPM 69mph lands on, yep 7000 exactly. 60mph? just a tad past 6000rpm. I did gain some speed, but I started to drop off on the tail end past the peak, despite the exhaust giving me a bit of extra room. That more or less says "We have reached maximum velocity for power output."


Have you noticed a pattern? Power at top speed and the best cruising speeds all happen at the same RPM point - the peaks.

I even tried a 40 tooth just out of curiosity and I actually started to lose a little on the top speed. In terms of power, a Hawk with an exhaust and a 6ft4in tall 260lb man sitting on top of it has exactly enough power to just barely hit 70mph (actual mph)

I have tried a few other combinations over the years, and I keep going back to running an equivalent of a 17/45. It's hands down the best comination for a mostly road going Hawk. The 43 is definitely what I consider "street only" use, and it starts to lose a little climbing ability when up against a proper incline. JerryHawk ran a 17/40 and was able to hit in the low-mid 70mph range, but he's also not Megadan sized, not even close.

Going beyond a 17/43 starts going into heavier and heavier diminishing returns.

On the plus side, a lot of these sprocket combinations can run the same number of links, so you can experiment for relatively cheap and see what works best for you and where/how you ride.