Hensim 97cc (Baja "Doodlebug" engine)

Discussion in 'General Questions' started by CanadaGlass, Nov 5, 2013.

  1. CanadaGlass

    CanadaGlass New Member

    I have a minibike that I was going to swap out the motor on, and switch it up to a 200cc job. This will leave me with a 97cc motor, complete with a clutch for a #35 chain. Building a mounting bracket for the motor should be easy, and allow me to move the motor between bikes. That only leaves the rest of the drivetrain to think about...

    Anyone work with one of these motors before? I did a search on the forum for "Hensim", but it came up blank.
     

  2. CanadaGlass

    CanadaGlass New Member

    I have decided to go with a belt drive. All seems straightforward to me, from swapping the #35 chainring for a pully, a pair for reduction behind the seatpost, and a second belt going to the wheel. I am good with math, and there are plenty of threads on ratios. I plan on going with the smallest practical pully I can find that will fit the clutch, solving the riddle of the pully that attaches to the wheel, then choosing belt lengths/reduction pullys to fit it all together...
     
  3. CanadaGlass

    CanadaGlass New Member

    Still haven't picked up a wrench on this project yet, but I have been thinking. Here is where I am at...

    Having decided I want to do this with buying as little as possible, and I am abandoning the belt drive plan. I have a stack of old bikes around, but nothing nice, so I want to be able to move this between frames easily when I do find something. Also, I have decided I want to add a homemade jackshaft to it, so the plan is a piece of 90% stock coming up from the platform for the engine with a bracket on top for a rear wheel hub to fit in behind the seat post tube. The engine has an 11 tooth #35 sprocket on the clutch, and I have the 70 tooth sprocket on the rear wheel of the minibike I can scavenge. The idea is mounting it to the left side of a rear hub turned jackshaft, and a gear on the right side will drive a chain to an internally geared rear wheel. Another chain from the pedals to the freewheel on the jackshaft completes the drivetrain.

    I don't know the ratios inside the geared hub, but I know second gear is 1:1, and that is enough for me to work with. Shifting into top gear when I hit 40 km/h sounds about right, so assuming an 8000 rpm "redline" (*see footnote) I still need around a 4:1 gear reduction between the jackshaft and the rear wheel. Does this sound right to those of you who do this math regularly?

    Footnote: I have no information on this engine in particular, I picked the number 8000 based on what I have read here about other 4 strokes. The circumference of a 26" bicycle wheel is double that of the rear wheel of the minibike, and the minibike goes about 46 km/h as it is currently configured, so that approach to estimating says I need a 2:1 ratio between jackshaft and wheel...
     
  4. butterbean

    butterbean Well-Known Member

    I have a 98cc Lifan flathead, which I've been told by people on the minibike forums is identical to the 97cc. I am using a chain drive, but my final ratio is 12.44:1, and I have a top speed of about 30-32mph. It's a 5k engine, not an 8k, and that is with the governor removed. I recommend ordering the black clutch spring from www.hotrodminibike.com. The stock clutch spring is either pretty light and will engage too early, making takeoffs jerky, or is made of piano wire, and will overheat and kill the clutch. This engine does not have the power for a variable pulley system. If you want more than one speed, it will most likely have to be chain driven. But I can tell you that I weigh 300 lbs and have no problems climbing hills with this thing with single speed.
     
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  5. butterbean

    butterbean Well-Known Member

    Disregard my comments about variable pulleys clearly you're not going that route. I had failed to read your whole post the first time around.
     
  6. CanadaGlass

    CanadaGlass New Member

    5000 RPM sounds more in line with my estimate based on how it performs on the minibike. I will update this thread as things progress. Not in any rush on it, as winter is here, and I have 2 HT bikes to ride (or one to ride and one for parts) until it is ready...
     
  7. butterbean

    butterbean Well-Known Member

    These engines make a great winter project. I started building my bike last winter and had it finished by spring. Stone reliable too. Just change the oil and keep gas in the tank, provided your bike is built solid, miles of trouble free riding.
     
  8. butterbean

    butterbean Well-Known Member

    My bike has had its share of issues, but the engine has always started on the first or second pull.
     
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