Where is the structural weakness that keeps...

Discussion in 'General Questions' started by Unhappy Time, Sep 20, 2008.

  1. Unhappy Time

    Unhappy Time New Member

    motorized bikes from safely doing higher speeds? I have some ideas - what is your experience?

  2. Pablo

    Pablo Motored Bikes Sponsor

    The big nut behind the wheel?
  3. Happy Valley

    Happy Valley Active Member

    Nominated: Best comeback......2008!
  4. Tinker1980

    Tinker1980 Guest

    In my experience with my first motorized bicycle, these are the weaknesses I've observed:

    First and perhaps most important, the brakes. I have a 70 cc HT engine mounted to a 14 year old Trek 820 mountain bike. It has the center pull cantilever brakes on it. With my 44 tooth sprocket, I can get it going fast enough that it will not stop quickly and in a safe manner.

    Tires. Bicycle tires are not made to go faster than (I'm guessing) 40 MPH. I've once gone as fast as 50 MPH (Without engine, downhill on Skyline Drive in the mountains in the western part of Virginia. More exciting than a crotch rocket.) There are tires for bicycles that are rated at something like 70 MPH, but I can't remember what they are called or who makes them. Maybe Google can remember.

    Wheels. They just aren't heavy enough. I could taco a rim so fast... the bearings aren't really made for high speeds either. They would heat up, thin out the grease, which would cause more heating. You'd likely not have a failure the first time going too fast, but after a few heating-cooling cycles the metal would fatigue and start to crack. Not to mention the axle could lose it's temper and get soft.

    A lot of motorized bikes (Including mine) don't seem to have suspension on both wheels. When you get going fast enough, you need your wheels to be able to track the ground better. You don't want to just hit the high spots, and you don't want it to shake around so much you get thrown off.

    Last, but not least: Police. If you've got your motored bike flying about at 70+ MPH on the highway, or even 45 on a street, you greatly increase the amount of attention you will get. That's not good. It's a novelty to see a bike with it's own engine. (Especially around here, I have THE ONLY ONE.) We don't want it to be a nuisance.

    Can anybody think of anything I've left out??

  5. Merlin

    Merlin New Member

    center of gravity?
    on a normal motorbike, you have a great honking heavy piece of metal under you, pretty much lower than the height of the axles. On a bicycle, the heaviest thing is you, perched up there above the top bar...
  6. Alaskavan

    Alaskavan Guest

    I've seen a few broken frames. Usually either at motor mount points, or where the bottom tube is welded to the head tube.
  7. kerf

    kerf Guest

    I think all the specifics have been pretty well documented here, so let me generalize. Bicycles were designed for human power, in the world of power, that ain't much. Due to this, weight is a major factor, I would not care to pedal a bicycle that weighed 200# very far. Bicycles are designed to be as light as possible and hold up to the strains of human power. Weight is much less a factor on a motor cycle due to the power of a 20+ HP engine, they are designed in a much more robust manner and designed for speeds that are out of reach with human power. Obviously, one can add a motor to a bicycle but in doing so we are pushing the envelope. If we push that envelope too far into the motorcycle realm, somethings gonna break, more than likely that will include our bodies.
  8. SimpleSimon

    SimpleSimon Active Member


    Ain't it the truth?
  9. Unhappy Time

    Unhappy Time New Member

    Thanks for your thoughts! Especially the bad pun :)

    This is kinda what I thought too. I was musing about whether dirt bike motorcycles wheels and tires would solve some of the weaknesses.
  10. Other than the possible structural strength issues I think the biggest thing holding bike speeds down (other than engine power) is the lack of suspension....too easy to get thrown around by road imperfections when running hard tires with no suspension....get a good suspension and I think our motored bikes will be alot smoother AND faster with a greater degree of safety....Of course that is all relatively speaking....

  11. kerf

    kerf Guest

    I'm running a full suspension and it's the only way to go. The sad truth is, however, the frames of motorcycles are made from larger tubing with heaver walls. The axles, bearings, hubs, spokes, wheels and everything else is bigger/heaver. Remember, a row boat will float just fine, take it in the ocean, you're gonna die.
  12. ZnsaneRyder

    ZnsaneRyder Member

    I highly recommend FULL suspension and good tires for high speeds. I can go 40+Mph and feel fine because the little bumps are absorbed, even despite my trailer having no shocks.

    Also, if you have rubber brakes PLEASE get the extra-length pads, then you will be able to slow down and stop from any speed. I SWEAR by them, and they cost ~$5 at Walmart or wherever.

    Biggest weakness IMO is rims, and bike tire tubes. Rims bend easy and wobble and have poor balance. My bike has no trouble at high speeds, even with warped rims, but I've shredded tubes and even a tire before if you happen to have it go flat while pushing 30+mph. Changing the rear tire to a solid-foam "no-flat" tube fixed that. My front tire still has a normal tube for now, but the front doesn't have near the stress of a rear tire with the rider's weight on it. Also, the foam makes the tires less bouncy, and safer than an air tube.

    If you could have a bicycle with resin-plastic or alloy mag rims and solid foam for both tires, you could probably do ANY speed you are crazy enough to try. Regular spoked rims can handle 50+mph speeds in my experience, but they are weak and WILL warp over time.

    A 6.5HP trike I built for a friend has 3 heavy duty 20" resin-plastic 3-spoke rims with solid foam inside the tires, and have had NO issues at any speed, even though he has NO shocks.
    Last edited: Sep 21, 2008
  13. SirJakesus

    SirJakesus Guest

    You can get around the wheel problems by getting premium downhill mountain bike wheels or parts from them to compliment your current setup. Double walled alloy rims can hold up to just about anything. Even when I popped 4 spokes on my rear wheel (that has a ton of weight on it) I still didn't get any hop or wobble. I only noticed the broken spokes doing an inspection. Dual suspension frames are the way to go IMO. You do get a ton more stability even on rough roads.
    I think the biggest danger people have on MB is not doing enough inspections of their rides OR not having enough skill or roadsense to avoid or survive a quick maneuver without dumping.
  14. kerf

    kerf Guest

    Inspections are at the top of the list, MBing takes a toll on the machine. Better finding that spoke or tire issue in the garage than on the way to the pavement.
  15. Mountainman

    Mountainman Active Member

    yes -- more MB inspections -- may save lives !!!

    I took off down this mountain the other day
    just taking for granted that everything was fine
    bout the only thing I checked good were the tires

    this is one heck of an E-ticket ride coasting down this mountain
    fast -- 4 or 5 mile drop

    at the bottom I noticed
    bracket had come loose
    holds my rear brake and support for engine !!!


    Ride That Thing - Mountainman
  16. kerf

    kerf Guest

    Asphalt face plant, don't go there!
    Last edited by a moderator: Sep 22, 2008
  17. duivendyk

    duivendyk Guest

    I think if you are **** bent on pushing the envelope you'll end up with something resembling a small motorcycle/scooter.If you live that long.I think anything above 30 mph is pushing your luck.A suspension allows higher speeds in more comfort,but I don't think it's any safer.
  18. Alaskavan

    Alaskavan Guest

    My trike has a surprisingly stable and smooth ride at 30+, considering that there is no suspension. I have considerable confidence in the 3/16" walled 2" aluminum tubing. However, a pothole at 30+ could be terminal. I slow down considerably when on roads that I'm not VERY familiar with.
  19. Leonardo

    Leonardo New Member

    i hope not to ruffle any feathers here...
    but is this what you mean?

    for me, it is a two step process...

    build the bike with the best parts and with keen atention to every detail (you covered.)
    then build the power to match it.

    much the way we built sprint cars 20 yrs ago.

    i started with the largest frame i could find
    with top notch suspension parts (had to be K2 EVO)
    those are 8 inch hydrualic discs on 30mm front axel and 20mm rear
    the bearings up front are greasless.

    phase two will be power...
    i will go slowly now,
    my thoughts pull me in so many directions.

    perhaps this forum may be the catalyst..

    tell me how you would power this bike?

    i am testing this "platform"

    need to lear to walk before i run...

    so far i have broken a crank,
    had the disc bolts work loose,
    and gone over the bars twice (at slow speeds, thank god.)
    (hydraulic brakes on something this light, are very "touchy")

    does it look like it matches the "qualifications"
    i am seeing a quility helmet and leather britches in my future.

    Attached Files:

  20. Unhappy Time

    Unhappy Time New Member

    You are going to have to use a rack mount so I would use staton gearbox and and the suzuki ~2.5hp 4 stroke. buts thats just me :)