GUIDE: Why scooters dont go 90-100mph+.

Discussion in 'Scooter Guides' started by tErr0, Feb 20, 2011.

  1. pekos

    pekos New Member

    Messages:
    7
    Rides:
    R6
    I have read all of the original post and all of the rest. Some are just talking crap, some make perfect sense.
    I fully understand what the original post is saying, but it is incorrectly titled.
    172cc bikes can and some do top 100mph on the road. Your bhp figures do not make sense, as the speeds acheived by all sorts of peds/bikes on the road do not need to produce the bhp figures you quote.
    I have personally modified a Yamaha jog with a stage 6 70 kit, gear up and Stage 6 variator, a few other minor mods but still using the original carb. This bike genuinely reaches 80mph on the road, (this was clocked by several other bikes including my R6) The speedo did read higher on the jog, but I have taken this error into account.
    Admittedly it was ridden by my son who is near 10stone in weight and yes he was crouching down, but this bike was nowhere near modified to a race standard and didnt even have a race crank at the time.
    " This however is pretty much never the case no matter how many parts you bolt on or tuning you get done. In this topic I will explain why this happens and try to get you to understand what sort of tuning you will need to do to reach a certain top speed". Your words not mine.
    Anyone reading this from Portsmouth who is a moped tuner/enthusiast will probably know my son or know someone who does and I'm sure they would confirm this if they have ridden with him.
    I'm not claiming with more mods that it would acheive 100mph, but its not a well set up/ tuned 172 either.
     
  2. tErr0

    tErr0 Administrator Staff Member

    Messages:
    5,816
    The title and some statements are journalistic.

    You said you understood the topic and then explained your self that you agree with it by saying a small rider crouching down? I'd say with about the power figure and standard over reading people get to it would be spot on. About 70ish real speed with head down.

    I myself have owned and ridden many scooters and bikes in different states of tune and yet another person claiming the impossible is just fine to me. Doesn't bother me.
     
    Mark Emerson Trentham likes this.
  3. creamsodauk

    creamsodauk Ped Ped!!

    Messages:
    707
    Rides:
    Runner 125 SP
    thats some 70! my 172 clocked on car clocks was about 70-80 flat out on 125 gearing

    and it pulls like a tank from the line with the front wheel lifting on its own and even when its reading 80 on the clocks which is the last number before it goes round again it still pulls hard from 80 to off the clocks
     
  4. pekos

    pekos New Member

    Messages:
    7
    Rides:
    R6
    I have nothing to gain by lying. So perhaps we can just agree to disagree. :cool:
     
  5. Snake_Plissken

    Snake_Plissken New Member

    Messages:
    21
    I thought the Idea was to balance speed with gas mileage. 60 to 65 is about as fast as you should go on a bike with 10 to 12 inch wheels. If all you want is speed and don't care about mpg then why a scooter? Just get a V-twin
     
  6. Gazz

    Gazz New Member

    Messages:
    7
  7. scubabiker

    scubabiker NITROJUNKIE

    Messages:
    7,321
  8. scubabiker

    scubabiker NITROJUNKIE

    Messages:
    7,321
    The size and shape of a motorcycle, together with the rider and any aerodynamic aids such as a fairing,
    windscreen or legshields, affects its drag and lift, and hence its power requirement, the more so as
    speed increases. Many complete books have been written on the subject of vehicle aerodynamics and
    so we can only scratch the surface of the subject in one chapter. Most texts and articles on
    aerodynamics usually concern themselves with extolling the virtues of low drag and the accompanying
    effects on performance and fuel economy. However, the creation of slippery motorcycles is probably the
    easy bit.
    In common with all medium to high speed vehicles the effects of wind pressure on stability and
    controllability must be considered, but not unexpectedly this becomes very complex on a bike compared
    to self balancing vehicles such as cars. Trail, gyroscopic reaction, yaw and roll coupling and their
    interaction with the steering, are the cause of many stability and control problems not present on cars,
    lorries, etc..
    Fairing design for roadsters and road racers usually leaves much to be desired, for most designers
    concentrate their attention on the front end and neglect the rear. In racing, this is a result of the tight,
    post 1957 framework of FIM rules to which fairings must conform. In touring, it is due to designers either
    copying racing designs or thinking only of styling or weather protection for the rider.
    Here we’ll briefly consider some elementary factors regarding drag and then look at some of the
    aerodynamic considerations unique to single track vehicles.
     
  9. scubabiker

    scubabiker NITROJUNKIE

    Messages:
    7,321
    Drag
    Firstly, let's consider just what causes air drag -- which is after all the largest thief of our engine power
    output at all but the lowest of speeds. Drag is a force trying to prevent rapid movement of a bike though
    the air, this force is generated through a difference in pressure between the front and rear of the
    machine. This pressure difference acts on the frontal area of the bike to give the drag force, and hence
    the larger the frontal area the greater is the drag. It is not always appreciated but it is the viscosity of air
    that is responsible for drag. Without viscosity even the most un-streamlined shape would create zero
    drag at high speed.
    According to the well established Bernoulli theorem, the total pressure in a streamline of gas is made up
    of two parts, one is the static pressure and the other is the dynamic pressure due to its longitudinal
    velocity. The total pressure (the sum of those two) remains constant, so if the gas is speeded up, its
    dynamic pressure rises and the static pressure goes down. As the air meets a moving object such as a
    motorcycle, it will be speeding up and slowing down as it flows over the various shapes, and so the
    pressure felt on the surfaces of the bike and rider will vary from place to place. If we sum all the
    individual components of this air pressure over the whole machine then the resulting force will comprise
    the drag and lift forces. If there is a side component to the air stream there will also be a side force.
    However, using the classic laws of physics with what’s called an “ideal gas” would result in a calculation
    that gave us no drag at all. In other words the drag force from gas pressure acting on front facing
    surfaces would be exactly balanced by a propulsion force due to the gas pressure acting on rearward
    facing surfaces. This apparent lack of drag is sometimes known as the d’Alembert paradox. Experience
    tells us that we have drag and so we need to look for other causes.

    Near to the surface of the bike and rider the air does not behave like an ideal gas, viscosity causes
    internal friction. Right at the surface the air speed will be that of the moving machine and the internal
    gas friction will tend to drag adjacent layers of air along with it. As we get farther from the surface the air
    will be largely unaffected, so there is a relatively thin layer of air near the surface which has a strong
    velocity gradient. That is, at the surface the velocity of the air particles is equal to that of the vehicle and
    outside of this layer their velocity will be that of the surrounding air. This layer is known as the boundary
    layer and the behaviour of this layer is so important to the aerodynamic properties of a vehicle.
    When the local airflow over part of a body is slow (either because the vehicle is travelling slowly or
    because of its shape), the velocity gradient is small and adjacent layers within the boundary layer slip
    over each other in an orderly fashion and the flow is known as laminar. However, at higher local
    velocities the velocity gradient is likewise high and the friction between layers causes them to trip over
    one another (like ocean waves breaking on the shoreline), and create eddies. This type of flow is known
    as turbulent. Except at very low speeds we will have a mixture of turbulent and laminar flow over a
    moving vehicle. Generally speaking, the frontal aspect of a vehicle is such that the air has little choice
    but to follow the shape, but as the air passes the widest cross section the boundary layer becomes
    thicker and when the shape of the body departs too much from the ideal, the air can no longer follow the
    shape and the boundary layer becomes detached from the body. The object moving through the air then
    leaves a turbulent wake behind. The size and shape of the object and the speed are the most significant
    factors affecting the size of this wake. The pressure within the wake tends to be constant and less than
    normal atmospheric pressure, and certainly less than the average pressure acting on the front of the
    object and hence there will be a nett drag force. Studies have shown that the drag force is
    approximately proportional to the size of the wake.
     
  10. scubabiker

    scubabiker NITROJUNKIE

    Messages:
    7,321
    (foale, Motorcycle Handling and Chassis design, 5-1)
     
  11. sean85730

    sean85730 Member

    Messages:
    43
    Rides:
    Pulse lightspeed 2
    this thread is highly interesting, using this and some creative balancing, modification and blind luck, I got my Pulse Lightspeed (8HP from stock 125CC) t an average road speed of 55MPH, and some moments to 63-4 MPH, thats when stability goes out the window, (dry weight of 105KG) I have had her a little faster still, but down hill with a good tail wind, BUT at that point its hard to keep her stable, however, she is happy to plod litterally all day at 50.

    Great thread with lots of interesting points made :D

    Sean
     
  12. motomatsu

    motomatsu New Member

    Messages:
    4
    Buying a maxi scooter (e.g Burgman) gets you up to around 90-105mph without having to do much or any work on it at all.
    It's still a scooter, automatic transmission, just bigger.

    And in the end you save more buying a lightly used maxi scooter than tuning the crap out of a small one.
    In my opinion, a normal vespa isn't meant to go faster than 53mph (85km) really isn't all that safe.
    I have a 125cc scooter in Thailand that is the same size as that of my Typhoon, and it tops at 95km, and that doesn't always feel safe in turns.
     
  13. scubabiker

    scubabiker NITROJUNKIE

    Messages:
    7,321
    80mph on their in factory dynomometer, which doesnt include drag caused by rider and bike.....
     
    scootzmadness likes this.
  14. scootzmadness

    scootzmadness Left the forum.

    Messages:
    4,235
    Rides:
    None.
    Lets run one next to my car with multiple GPS units running and I have a radar gun too.

    THey dont hit 80mph. If they do then Im the popes son.
     
  15. Mark Emerson Trentham

    Mark Emerson Trentham Well Known Member. Staff Member

    Messages:
    8,553
    Rides:
    NSC110/ ET2.
    Would'nt think why not,my Piaggio hit +70 on the way back from Hornchurch...then stopped dead on that,did'nt even feel like it was stressed,think S/B might of mentioned awhile back.sounds like it was limited.;).
     
    scubabiker likes this.
  16. scubabiker

    scubabiker NITROJUNKIE

    Messages:
    7,321
    yep, bang on mark, they have a rev limiter to stop valve float, but i think cam timing limits it before valve float anyway....
     

Share This Page