How to achieve top speed - gear to wheel ratio

Discussion in 'Frame Mounted Engines' started by Earth Mechanic, Aug 8, 2007.

Tags: Add Tags
  1. Top speed is achieved by maxing out the rpm's of your gas motor while also maxing out the rotational rpms of your rear wheel. If you have a light rear wheel, you will reach max motor rmp's faster than if you have a heavy rear wheel (but you're wheel is weak). If you have a heavy rear wheel then you need a motor with more torque to achieve max motor rpm's with your heavy (but sturdy) wheel. Heavy wheels just dont spin fast, and its better to have the weight as close to the center of the wheel as possible, rather than in the rim. This is the law of enertia.

    So, I chose the 56 tooth on my 4 stroke because i felt like downhill i was maxing the motor rpm's (7800rpm, honda gsh50). But i might be wrong as i don't have a tach on my motor. I suppose i should add a tach...anybody got one?

    btw...this thread was created as a result of my 4stroke info post.

  2. npk1977

    npk1977 Guest

    Is that really true? Heavy wheels do not accelerate well, but their spin speed should be independent of their weight. Tires are incredibly important, but really, the #1 most important factor is aerodynamics. If you want to go fast, get a recumbent with a wind-shield.
  3. Heavy wheels

    Thanks npk, good point. heavy wheels definetly dont accelerate as fast as light wheels, and having less resistince from your tires (ie. slicks instead of knobbies) is definetly a big factor. Also, once the wheel is rolling, heavy wheels carry more momentum than light wheels, thus will roll out longer than light wheels once the throttle is off.

    Overall aerodynamics is also huge, as you say. Did you catch the show on the learning channel about the german aerodynamics designer Luigi Colani that took a standard 18 wheel rig and made it 50% more fuel efficient just by making it very aero, no motor mods.

    My point on the top speed and wheel weight though, is that i think there is a relationship between the ability of my 50cc motor to hit its max rpms and the weight of my rear wheel. It seams like there should be a weight threshold at which point the motor cannot spin the wheel any faster because it cannot overcome the weight. For example, if i put on a motorcycle rear wheel, the bike's acceleration would be slower, and probably the top speed. But now that i say it and think about it, im not sure that is so. perhaps max motor rps and max wheel speed are more related to total bike wheight and aerodynamics than the weight of the rear wheel.

  4. gone_fishin

    gone_fishin Guest

    momentum is an important factor with 2-strokes, which seem to me to rely on "speed of flow" for their power...i understand that a 2-stroke will just keep going faster until it blows up or is limited by intake (?)

    it will take more power to get a heavier bike to speed, but heavier wheels would then contribute to top end, and cruising efficiency.

    those are opinions, as i never did get my rocket-scientist degree.

    i didn't see the show you speak of, but i remember reading (maybe in an old hot rod mag) a long time ago about an engineer saying something like "we completetly ignore the most areodynamically important part of the american automobile...the underside."
  5. Not a physicist either...

    I agree with the higher cruising speed efficiency with heavier wheels, and smoother ride...

    so i guess the real questions are:

    1. With two rear wheels, one heavy and one light, will top speeds be the same but acceleration time to top speed differ?

    ...if that were the case i could run a sportster rear wheel on my 50cc motor and eventually reach the same top speed as my current lighter weight wheel.

    2. Is there a rear wheel weight threshold where the engine displacement cannot overcome wheel inertia at a certain rotational speed .

    ...if there is, then we could make a formula that would tell us max recommended wheel weight for engine displacements.
  6. npk1977

    npk1977 Guest

    For top speed, the #1 most important issue to deal with is aerodynamics.

    See that image? The y-axis shows the total power required to propel some bike forward. The top curve is the "total" power required to achieve a certain speed. Our bikes put out ~1HP, which is ~750W, which means that on the bicycle in this plot, our top speed is ~30mph (extrapolated.) At that speed, 95% of the resistance is from wind. No matter how you gear the bike, if you're not pedalling, you'll not go faster than 30mph.

    Forget about your weight and drivetrain, figure out how to decrease wind resistance. A 10% change in wind resistance will get you 9% faster. A 99.99% change in drivetrain will get you 10% faster.

    ps -- look at the following image:
    You see how different frames affect the wind resistance. Notice that a racing bike's aerodynamics increases your top speed 30% over a cruiser.
    good luck
    Last edited by a moderator: Aug 10, 2007
  7. gone_fishin

    gone_fishin Guest

    i lean over when i want to go faster...does that count? :p

    well, the problem we also face is that we need a heavier bike to withstand the increased sustained speeds...

    i'll state another opinion...i get a whopping 32mph with S-2.1, the engine doesn't struggle and is overall very reliable...i'm plenty happy with that :)
  8. Technical is good

    Thanks for the chart npk. It looks like i need to build a fairing.:D