My rear engine rack mount, finally done!

Discussion in 'Performance Mods' started by Pyrotechnic, Nov 7, 2011.

  1. Pyrotechnic

    Pyrotechnic New Member

    http://www.motoredbikes.com/showthread.php?t=23898

    I posted that about two years ago. This project has been in the works for a very long time now and this summer it finally became a reality. Between college and work things just never came together but with gas prices and me just wanting to make this bike happen I've finally got something.

    I've never seen a bike done like this before. The reason? There was really nowhere else to put the motor. Everyone told me right over the top of the rear tire, but I didn't want the center of gravity up that high and I didn't want the motor right by my back if it came apart.

    Hows the balance?

    Well you can lift the front end of the bike up and it will sit back the rear frame rails without any help. Very rear heavy but I knew that would happen. Once I sit on the bike though it's not bad.

    How does it run?

    It's a blast to ride! There's nothing like going through the gears on this thing. Usually I just shift through the front chain rings first, then build my speed with the close ratio cogs out back. I can also shift a combination of the chain rings and cogs to get some really close gearing, kinda like driving an 18 wheeler. The belt clutch works great too, it's like a motorcycle with a ton of gears.

    Right now it's getting about 60-70 MPG and that's running full throttle all the time with a lot of stopping for stop signs through residential streets.

    These are just some quick pics I took. I've replaced the entire rear wheel since then and I've also built a front rack for my backpack. New pics to come soon.
     

    Attached Files:


  2. Happy Valley

    Happy Valley Active Member

    Well, you posted because I assume you want comments and opinion.

    IMO, the motor is too big, if I were to ride it where I live it would be a cop magnet.
     
  3. 5-7HEAVEN

    5-7HEAVEN Member

    I applaud your dogged determination!

    It took me 6 months just looking at my bike, before I figured out how to secure the Scooterguy mounts on my fat top and down tubes.

    How fast and how much does the bike weigh?
     
  4. 5-7HEAVEN

    5-7HEAVEN Member

    I am very curious as to how your belt drive connects to your chain drive.

    Is that the elusive left-hand derailleur?:tt1:

    Why did you extend both your exhaust and intake tubing so far?

    What are your gear ratios, especially first gear and final drive?

    I have an 8-speed transmission on my bike. I usually skip every other gear, and you could prolly do the same for yours, maybe use every third or fourth gear.

    Can you explain in detail how your engine applies power, from the crank to the final cassette gear? I'm very interested.

    Is that a gas pedal on the foot peg? What's that pull lever on the center of your handlebar? Is that a pulley tensioner?

    :likelots:

    How much hp? Top speed? Cruising speed? Are your brakes enough to slow down this heavy bike?
     
  5. CroMagnum

    CroMagnum Member

    It needs one of these -


    [​IMG]
     
  6. Pyrotechnic

    Pyrotechnic New Member

    Where you see the large pulley, there is a jackshaft right there. On the other side of the pulley is a small sprocket. The derailer is just a regular one that I use to route the chain around the wheel hub and it also acts as a tensioner.

    The intake is long because putting a filter right on the carb would make the bike even longer, it might suck hot air blowing off the motor, and it adds weight in the wrong place. Where the filter is now, it gets fresh air. That air filter is actually a walmart oil filter that I turned into an air filter...all for less than $3.

    I made the exhaust long because I had read somewhere that a longer pipe will naturally quiet things down and take the edge off the sound. With the layout, the pipe either had to be really short or really long. It actually sounds really nice, the long pipe has a nice deep tone you wouldn't expect from a motor this size.

    As far as gear ratios, the gear reduction is around 17:1. First gear is 1:1 and the top gear is 0.29. That would make my final drive 4.93.

    As far as gear shifting, from a stop all the way up the top speed I've found that it works best to shift the front chain rings first. This gets the bike going pretty fast and you only have to wind through 3 gears. The wide ratios work fine at these lower speeds. After that, I start shifting the close ratio cogs out back. Acceleration is harder when fighting wind and rolling resistance and the close ratios really start to shine. The bike is a 21 speed, but shifted like this it is a 10 speed.

    The power transfer is pretty simple.

    The engine pulley drives the belt that goes to the pulley on the jackshaft which turns the small sprocket. The small sprocket drives the chain that goes to the sprocket on the left side of the bottom bracket. The pedals are cut off, so that basically "pedals" the stock drivetrain.

    The controls are just like a motorcycle. I re-routed some of the cables to make this work. The left brake lever controls the clutch, then right brake lever controls the front brakes. The black center level I added is for the throttle, and the bottom foot pedal is for the rear brakes.

    The engine is rated at 2.5HP from the factory, maybe I've gained some with the exhaust an intake? Given a 3500 RPM governed speed, and considering the gearing this gives me a theoretical top speed of 50 MPH. I have no idea what the governed speed is now, it came off an edger and I have not touched the governor. I need to get a tachometer to see where the governor is holding it at. I'm estimating it does 30 MPH pretty easily and that's only shifting the front chain rings.

    Everyone always asks me about the brakes. When it was just a bike, the rear brakes could easily lock the rear wheel and the fronts if squeezed hard enough would be enough to send you flying over the handle bars. All this tells me that they far exceeded the needs of a bicycle. The brakes actually work great. Between the rear and front brakes I can stop pretty quick. I can actually squeeze the fronts pretty hard...with all that weight out back I don't think that hard front braking could even lift the rear.

    The bike is heavier now, but as a bike it was extremely light. The 24" rims and aluminum frame really don't weigh much.

    Here's the problem I've been facing all along: I just can't keep the rear tire inflated! I knew that there would be a lot of weight back there but I gambled on the tire being able to hold it.

    The issue is, it seems with all that weight back there, the tires really gets pushed hard into the ground, and into anything that might puncture it. Not only that, at lower air pressure I get pinch flats. When running max pressure, I blew up a tube while reinflating, which tells that the high pressure weakened the tube.

    When running on nice clean pavement at moderate pressure, it does alright as long as I don't run over anything nasty. Part of my ride involves going through grass for awhile, which is the tire killer.

    This tire situation has been a little discouraging, considering that everything else about the bike works great. I'm going to try the ghetto tubeless method as a last try. This gets rid of the weak point, the tube. The sealant will protect against any punctures as well. Seems like it will solve my problems...if I can get it to work.

    That was a long post.:sweatdrop: Hope my explanations are clear. Any more questions feel free to ask. Future plans are a tachometer, speedometer and some real numbers...if I can get the rear tire situation sorted out. :-/
     
  7. motorpsycho

    motorpsycho Active Member

    I applaud your persistence.
    But man, what a contraption!
    it reminds me of the Wright brothers' first airplane / flying machine!
     
  8. 5-7HEAVEN

    5-7HEAVEN Member

    "As far as gear ratios, the gear reduction is around 17:1. First gear is 1:1 and the top gear is 0.29. That would make my final drive 4.93".

    JMO, 17:1 isn't that low, and 4.93:1 might not be too effective. Using a much smaller chainring sprocket might work more effectively. In comparison, my lowest gear is 46.32:1 and final gear is 15:1. I'm running a 2.8hp Tanaka 47R engine w/pipe. If yours is 4-stroke, your gearing requirements might differ from mine. But hey, if it works for you...good!

    "As far as gear shifting, from a stop all the way up the top speed I've found that it works best to shift the front chain rings first. This gets the bike going pretty fast and you only have to wind through 3 gears. The wide ratios work fine at these lower speeds. After that, I start shifting the close ratio cogs out back. Acceleration is harder when fighting wind and rolling resistance and the close ratios really start to shine. The bike is a 21 speed, but shifted like this it is a 10 speed'.

    I hear you regarding close-ratio gears for headwinds and short hills. That's when my first four gears shine too!

    "Here's the problem I've been facing all along: I just can't keep the rear tire inflated! I knew that there would be a lot of weight back there but I gambled on the tire being able to hold it.

    The issue is, it seems with all that weight back there, the tires really gets pushed hard into the ground, and into anything that might puncture it. Not only that, at lower air pressure I get pinch flats. When running max pressure, I blew up a tube while reinflating, which tells that the high pressure weakened the tube.

    When running on nice clean pavement at moderate pressure, it does alright as long as I don't run over anything nasty. Part of my ride involves going through grass for awhile, which is the tire killer.

    This tire situation has been a little discouraging, considering that everything else about the bike works great. I'm going to try the ghetto tubeless method as a last try. This gets rid of the weak point, the tube. The sealant will protect against any punctures as well. Seems like it will solve my problems...if I can get it to work".

    Your major tire problem MIGHT be solved by using a motorcycle tire. A few members here or at the other forum are posting about that. For them, it's a "want". For you, it's a "need.

    Are all your spoke ends unexposed and protected from the tire tube?

    What an EXCELLENT project!:bowdown:
     
    Last edited: Nov 13, 2011
  9. Pyrotechnic

    Pyrotechnic New Member

    motorpsycho - Yes I'll admit it is quite the contraption. The bike, I got it in the 5th grade and I've had it ever since (I'm 23 now). The engine was given to me for free a few years ago. Cheap or free is the name of the game here.:grin5:

    Yep mine is a 4 stroke flathead. I figure it can probably run about 3500 RPM reliably? The gear ratio actually works pretty well...remember this is a 24" tire. 1st gear could be just slightly lower but it runs fine as is. I found a smaller engine pulley that should put me around 20:1 and I think that will feel just right.

    That's some crazy gear reduction you got going on yours, but you probably have an 8000+ RPM redline?

    Motorcycle tire sounds pretty expensive...I don't think I can afford that one. I was hoping to get this tire issue solved for around $20-$30.:lol: I put a brand new rubber strip in to cover the spoke ends so that shouldn't be the issue.

    Thank you! :grin5:
     
  10. 5-7HEAVEN

    5-7HEAVEN Member

    Yeah, my engine buzzes 7000-8000rpm during the cruise, but it runs well @ speed.

    If you're blowing tubes like toilet paper, you NEED a stronger tire/tube. Imagine blowing a tire @ 35mph? Your life is on the line. The motorcycle tire is meant to carry heavy loads. Your bike is totally unsafe without solving this issue.

    Put your bike on a bathroom scale. How heavy is it?

    What size is your tire? A bigger/wider tire might do the trick.
     
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