Sachs Madass 125 ?

[ProDigit]

And here is where we differ.
If you need 4.5HP, and your 110cc only makes 6hp, you'd be running it at the 6k RPM, where the torque and MPG is less.

If you do this on a 250cc, it makes 4,5HP at 2,5k RPM, where MPG is more.

[user removed]

Best bike for mileage of all that I owned was a 1965 Honda Super Cub at 170 mpg. Best bfsc in smaller engines is at higher rpm. The cub got that mileage running at 6k rpm and it's top speed was 42 mph.

Larger engines will have best bfsc at lower rpm, but super high mpg vehicles are generally not high speed vehicles. I have always liked to see people who get top mpg list their average speeds. In most cases those average speeds are close to half what I average, and not many people will tolerate a 22 mph average speed, while mine is much closer to 40 mph.

regards
mech

[Grant-53]

Also consider that a low drag human powered vehicle will let you travel in the 20-30 mph range. Sometimes it helps to look at the volume of A/F mixture per unit time to compare different engines as well as compression ratio.

[PeterS]

Originally Posted by ProDigit

And here is where we differ.
If you need 4.5HP, and your 110cc only makes 6hp, you'd be running it at the 6k RPM, where the torque and MPG is less.

If you do this on a 250cc, it makes 4,5HP at 2,5k RPM, where MPG is more.

9.5 hp actually ....so 4.5 isn't asking to much.

The lower total swept volume seems to be an advantage for the smaller engine.
I'd be happy to use a 250 but the feet forward position is easier to arrange on a CT110 Honda.

[sendler]

Aside from the standard bike (non low seat/ non feet forward) riding position, The CBR125R is a great starting point for a highway capable streamliner project. Fuel injected, chain drive 6 speed for good performance and an easy gearing change, water cooled so no over heating problems with full bodywork, true highway capable speed from the perfect amount of power for a barely enough streamliner.
Never sold in the USA though. An otherwise perfect 2007 with scratched body work can be had all day long from both sides of Canada for $1,000. Europe, Aus, and Asia should have simlar used bike availability.

[ProDigit]

The Honda Grom would be a good bike to do projects on.
It's a newer engine, with fuel injection.
Newer engines mean smaller tolerances, which in turn means higher efficiency than the older engines.

Some 100cc's of the days of old, wouldn't even make 4HP stock.
With some tuning, they could do 5 to 6 HP.
The Grom has 9HP, just like Sachs Madass 125.

A 125cc is right on the mark for a fuel sipping eco commuter.
A 110cc would in my opinion be too small, when trying to get to speeds over 40MPH; unless you want to ride a scooter with fairings like that of a UFO.
I would be too embarrassed for it.

The drawback of a 250cc is that it uses a lot more fuel than a 125cc or lower CC, by just idling.
On the other hand, a 250cc can easily get 120MPG or more, cruising at 30MPH, yet if need be, it can go 80MPH at half that MPG.
A 110cc would have better average MPG (with start-stops), and similar MPG @40MPH, but lower MPG at speeds like 55MPH.
By design, larger engines run more efficient than smaller ones, because the losses on small engines are much greater (heat loss, friction losses, etc..).

I think both Sachs Madass, Roketa MC-05-127, and the Honda Grom are 3 platforms that are excellent for city and town commuters!


This forum will become really interesting, if more of the current technology engines are being mass produced, like the wave disk engine, rotary engine, and the duke engine. All supposedly are more efficient than the standard 4stroke engine!

[sendler]

Smaller engines have less surface area in the combustion chamber and less heat loss.
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And UFO styling is the only way to get the best efficiency.
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[ProDigit]

But the smaller engines still need to overcome almost the same wind resistance, tire and friction resistance, as a larger engine; and still need to push the same 200-350lbs bike + rider.
On a smaller (aka 33-110cc) engine, these frictions would equate a higher percentage of their output power than for a larger (aka 125-250cc) engine, resulting in less remaining output power to the wheels, and overall efficiency drops compared to a larger engine.

If both large and small engine where made with the same hairline tolerances, this would result in the larger engine being more efficient than the smaller one, as (to simplify a 0.1mm tolerance on a 10mm piston head equates 1% tolerance, vs the same on a 100mm diameter piston head, equates to 0.1% tolerance.
In both cases the measured piston gap tolerances are identical, but the larger piston will have less losses than the smaller one.

[sendler]

Originally Posted by ProDigit

But the smaller engines

So using your logic a 650cc engine running at 6hp will always be more fuel efficient than a 125cc engine running at 6hp. But in reality this is not the case due to the increased heat losses in the larger combustion chamber and the increased pumping losses due to the throttle being closed more with the bigger engine. The first thing for you to learn about here is the concept of Brake Specific Fuel Consumption versus rpm and load.
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https://en.wikipedia.org/wiki/Brake_...el_consumption
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[jkv357]

I would say you need to determine what speed and what type of riding you want your design to work best at. That's going to determine what would be the best engine size.

If you want the best mileage for short trips at 45 mph and under, your engine choice will not be the same as it would be if you want to run Interstates at 70 MPH. If you want to run in something like a Vetter Challenge, you are going to need more power than you would for just tooling around on backroads.

I personally wouldn't go below a strong 125cc engine, and probably would lean more towards a 250. There are trade-offs, but a newer F.I. single in the 125 to 250cc range is going to give you some pretty good numbers.

Look at what the 250 Ninjas can get with aero mods and the right rider. That engine is not the best design for mileage, but it still can produce some amazing numbers - and it's not running anywhere near its peak efficiency range.