Performance vs. Economy  By  phssthpok (Read 1,063 times)

The one thing missing is where you ride. If you are always on open long stretchs of road that you use to get to places then I would suggest taller gears. The thing that saves you on mpg is lower rpms. If your in city driving going from stop light to stop light or stops signs you need the low end power to get you moving with the traffic so not much saving there.
Alleyoop

I live in Miami, FL, so I kind of know what terrain you're riding.

On bobber motorcycles it's very easy to maximize fuel economy.
First thing you will not want to have is a CVT.
A CVT is tuned for only one style of riding. When you change the sliders to heavier ones, your fuel economy will go up, but your acceleration will go down.
When you change them to lighter rollers, your acceleration will go up, but your fuel economy will go down.
And in either one of the cases, unless you tune your bike's rollers/sliders 'perfectly', you'll never reach top speed.
That, and a CVT eats about 10% of power, or 10MPG compared to a manually geared bike.

For that reason you must go manually geared, or at least, semi manual (the manual type of gears where the clutch is worked in the gear pedal, and doesn't have a separate gear lever on the handlebars).

If you're one of the lucky people where both sprockets are large (eg: the front (engine) sprocket has 17T, and the rear sprocket has 46T), you will have a lot of play to improve fuel economy. Worst case scenario, a 15T front sprocket and 41T rear will give you some room for improvement but not a lot, because once the sprocket gets too small, the rear fork will be in the way of the chain.

It's easy to find rear sprockets of all sizes. Front sprockets usually are made out of hard steel and usually can only be found in 15 or 16T.
So the most common thing to do, if you can't find a front sprocket is change the rear sprocket of a bike.

Also, it looks to me that you want a rocket, a fuel economy monster, not an actual rideable motorcycle in traffic?
But even if,
The math of these things is very simple, and it doesn't really matter how much you want to improve your fuel economy, the principle is still the same.

1- Most Chinese motorcycles are not tuned very well, and automatically need a smaller rear sprocket. A large rear sprocket gives lots of torque, easy starting from a standstill, high acceleration, but acceleration somewhat gets lost in too many shifts.
Changing a sprocket makes gears longer, which means there's less acceleration per gear, but also less time lost in shifting to do to get to a certain speed.
Most chinese bikes are tuned for 'towing' rather than for cruising, so they have a lot of leeway to play with.

2- Most motorcycles that are tuned well from the factory, are tuned in such a way that in their final drive, their max RPMs (where they make max HP), corresponds to the top speed that that bike can go, for 2 skinny riders, or one big rider. If you happen to be a small and skinny rider, and mainly ride by yourself without luggage, you can easily change 3tooth down on the rear sprocket, on most bikes, and gain some top end speed and MPG too!

3- Here's where the real MPG numbers come in; tune your bike for fuel economy and ride-ability:
Instead of making your final gear the gear where you reach top speed, you can make your second-to-last gear your top speed gear. The last gear you can make as an overdrive to significantly reduce RPMs while cruising.
There's little math involved.

Suppose you have a 15T front sprocket, and a 44T rear sprocket,
And your final gear has already been optimized for top speed (in 5th gear you get to ride at the max speed this bike can handle);
If that's the case, and in 4th gear you do 8k RPM, and when you shifts to 5th gear the RPMs drop to 6k RPM, then you can safely conclude if you change your rear sprocket's tooth, by the RPM differential (6000/8000), that 4th gear will become your top gear.
Basically by changing the rear sprocket by dividing the rear sprocket tooth count, by the final gear drive RPM differential, you will make your new 4th gear the same gear ratio as the stock 5th gear; and essentially add a top gear (like a 6th gear).
What I mean with it is this:
Change 44T to a 33T (44*6000/8000=33); that's a change of 25% on the rear sprocket's tooth thus changing your overall gear ratio by 25%, and will more than likely result in 20-25% fuel improvement when riding in final gear ratio.

Doing this, will drop your top speed in final gear, but the bike will still be able to reach for the same top speed you had before in a gear up from final gear, 4th gear.
When before you where doing 6k RPM to reach to 60MPH, now your RPMs will lower by 6000/8000 as well, so in final gear you'll be riding at 4500RPM (=6000 * 6000 / 8000) when going 60MPH.

4- This is where it gets really tough.
Increasing gear ratio by a larger than 25% amount, might result in hard starts from standstill, requiring some advanced clutch skills for the rider to get the bike to go from a standstill.
But the above gear change (numero 3) is great for still having a rideable bike, but this paragraph is an even more improvement on gas mileage on paragraph 3-.
Basically, instead of focusing on getting to top gear, in a gear below the final gear, you'll be focusing on maximizing MPG in the environment you ride in most.
These calculations depend on your engine size (cc), your weight, your passenger/luggage, terrain (lots of hills, or level ground), wind (lots or mostly windstill), and speed.
Suppose that gas mileage is your top priority, and you will want to move from point A to point B, with maximized fuel efficiency, some minor math is needed:
a) It's a known fact that the most economical speed to transport yourself in is between 30 and 40MPH, as wind resistance is easiest to overcome there. Any faster, and wind resistance will increase. Any slower and you might as well walk or use a bicycle, or get a 33cc engine for maximum fuel efficiency.
b) Once you determined your most frequent riding conditions (and let's say you will want to move between 30 and 40MPH most of the time), you can start determening your engine CC.


So suppose you are a single rider of 180 pounds, and mainly will be riding a manually geared bike at the speeds of 30-40MPH, and never exceed 50MPH, then your best bet is to go with a 50-125cc engine.
Let's suppose you choose a 125cc engine bike, and in final gear ratio the bike does 6k RPM to go to 60MPH, and it has a 17T front sprocket, and a 47T rear sprocket, that means that the engine will be doing 3-4k RPM @ 30-40MPH. You can halve that, meaning 1,5k RPM @30mph, 2kRPM @40 MPH easily, as a 125cc engine will be able to pull that off. A 50cc engine won't but any +100cc will.
So in order to halve RPMs, you will have to halve the rear sprocket's tooth. Since it has 47T, you'll have to choose between a 23 and 24T rear sprocket. Usually your rear fork won't allow you to choose for such a small rear sprocket, as the chain will eat the rear fork, but when measuring clearance you will find that quite often a 30T is the smallest you can go, on just about any motorcycle, without having the chain eat through your rear fork.
Doing the math, you changed from a 47T to a 30T, so your differential is about 36% (1 - 30/47).
That means your RPMs will drop by 36%, and fuel economy is improved by about 36%, and that will be the top of your ability, unless you can increase the front sprocket more, which in most cases is not possible.
Your RPMs will go
from 6k RPM @ 60MPH to 3,83 RPM @ 60MPH, or
from 3,5k RPM @ 35MPH to 2.23RPM @ 35MPH.

If your gas mileage before was 65MPG, you now get 101MPG tops.

Generally speaking if your speed is below 40MPH, and you can expect 100+MPG, means you don't have a lot of acceleration in final gear drive.

120MPG is about the borderline of MPG you can expect out of any bike, and the limitation of what's physically possible, Beyond that it's impossible to accelerate; unless you start combining with other technologies, like regenerative braking!
Regenerative braking might add you 10-20MPG, especially in the city.

If you're more specific on which bike you want to choose, (like cc's, and sprocket teeth count), I can do the basic math for you.
But from the looks of it, you will be best off with a 110cc engine, second best a 125cc engine, third best a 50cc engine with BBK, fourth a 50cc, and lastly an engine size of 150 or over! For your riding conditions you don't need a 150 or greater to pull it off, unless you do plan on going on the highway now and then.

I live in S-Florida, and I never thought I'd be on the highway, when I bought my first 50cc; but still find myself often on it. With a 110cc you still have the speed (50-55mph) to get to the nearest exit in case you accidentally end up on one.
But if you plan on regularly going on the highway a 250cc is better.

BTW, if you're interested, I'm selling my MC-05-127. It's already equipped with a 31T rear sprocket, and is getting just over 100MPG, top speed close to 60MPH.
If you look at my sig (saying 97MPG) that is because I had a 40T on before, I switched to 36T, and 34T, and now 31T, and doing mainly rides above 60MPH. Riding at 40MPH, it gets closer to 105MPG.

www.fuelly.com/driver/prodigit/mc05127


If you're interested in it, pm me!

howdy, you don't say where in eastern washington and whether you will ride the freeways, but if you intend to ride the freeways in eastern washington, be aware that you will have significant hills no matter where you are. that says that you need something at least like the hd200 sym which from experience i know stock, will give you about 80 mpg. on a scooter at speed, your greatest fuel mileage stealer is aerodynamics. very few changes will make much difference other than that. that sym or the 200 kymco would be minimums in my opinion and experience for that area. that is where i have lived and ridden everything legal and some that aren't. by the way, i found the hd200 to be so near the 250's in performance as to be negligible. i had the kymco 250 grand vista also and other than having more storage onboard, there was negligible difference in speed up the mountain passes. i did put 8,000 miles on it and a great bit of that on the freeway. i now have the bv350 piaggio and find it a great traveling machine but now down just over 60 mpg but that at 70 mph or speed limit everywhere. i've put 5500 miles on in 9 months including a 960 miles round trip to boise idaho.

lotsa miles and smiles to ya
ken

Durability is different from brand to brand. If all engines where made in the same way, with the same materials, it could be compared,
however some brands have hardened materials on the cylinder walls and pistons, while others have only aluminum pistons. Some engine designs are optimized, like some brand's asymetrical piston and cylinder designs, fuel injection, or port injection, while others are based on 1980's carburetor technology, and basic engine design.

So if you would compare the same design, chinese 50, vs 150, vs 250 cc engine, performance vs durability, the answer is pretty simple:
The higher it revs, the lower it usually lasts. When riding at 30MPH, a 50cc usually revs to ~5-6k RPM, a 150 to 4k RPM, a 250 to 3k RPM.
A 250 therefor should last twice as long.
Not only that, but a 250 would reach a certain mileage usually twice as fast as a 50cc.
That, and the fact that most 250cc bikes are mostly used in the suburbs, and on the highway, while most 50cc's are used in the city, with a lot of stop and go traffic.

So not only does a 50cc wear faster because of the continuous stop and go traffic (braking and Wot riding), but in the time difference it it takes a 50cc to get to 20k miles compared to a 250cc, a lot of that 50cc will be rusted already. (meaning, with my style of riding, a 250 gets to 20k miles in 1,5 years. A 50cc would get there in 3 years. The extra 1,5 years in the elements will do no good to the 50cc).

I agree, I never thought of it that way !! That really is a bargain.

Oh, you'll get no argument from me that a direct chain drive is more efficient from an engineering standpoint. I'm just trying to work with what I actually have, not what I'd like to have.
Although, that has sparked an interesting concept in my brain.

Imagine in your mind a typical GY-6 CVT transmission. On the front end (the variator assembly) replace the outer face with a 'puck' of chain-gears, and have the inner face acting upon a shift lever for a derailleur.

The derailleur would be placed at the mid-point between the drive side (variator) and the driven side (clutch), and a counter spring added between the front 'puck' and the engine side variator face to cause down-shift as engine speed decreases (essentially moving the counter spring from the driven side of the transmission to the drive side). The clutch assembly would remain the same and allow for the gears to spin freely at idle (and thus downshift into the lowest gear).

The clutch counter-spring would be eliminated, and the outer face of the clutch pulley would have a similar sized 'puck' of gears oriented in the opposite direction, such that at idle speeds the smallest gear up front/tallest gear in back would be engaged.

As engine speed increases, the variator face would move away from the engine, actuating the derailleur to 'shift' to the next step (moving the chain on both gear-sets at the same time owing to the short distance between the derailleur and either gear-set). This would have the same effect as the CVT (insomuch as the gear ratio would improve as speeds allowed the engine to reach it's power-band) yet have the added efficiency of a direct chain drive, though admittedly with not anywhere as smooth of a transition.

All of this is presuming, of course, that one would want to keep the 'automatic' transmission of a scooter. If you are into manual shifting there's no reason to retain the front variator assembly...simply mount a gear-shift lever someplace readily accessible, and remember to downshift at the stoplights (IMHO the 'automatic clutch' is just too bone simple and convenient to give up though )


Thoughts?

(Bear in mind this is mostly a 'thought experiment'...the odds of actually getting into something like this are slim to none owing to a lack of funds and/or a sacrificial testbed machine)