Hydraulic Drive Rack to Front Crank

Discussion in 'General Questions' started by powdersummit, May 15, 2011.

  1. powdersummit

    powdersummit New Member

    My ultimate goal is to build a kit for a full suspension downhill bike that has climbing ability and a decent top speed on the flat. I love the idea of electric and running it through the crank derailleur system but I don't like the idea of having to drop $400+ on batteries to get any range. A small 4 stroke is cheaper, has more range and doesn't involve expensive electronics that can go "POOF!" Now the problem is how do I get the power to the crank when I can't place the motor in the frame because of the size? A series of belts and chains would be complicated and would be bike independent.

    Solution: Hydraulic drive! I've thought it out and here's how I'd do it. Honda GX35 or EHO35 on a rear or front rack setup, geared down 2:1 with the clutch still being used to a small hydraulic gear pump. From there run hydraulic lines down to the front crank area where you have another larger displacement motor with a freewheel on the shaft, from there have a 2:1 reduction to a freewheel crank. The return line from the pump/motor would then go through a small filter into a small reservoir to be run back through the system. There would be no valves in the system due to the fact you are using the clutch on the motor to regulate if the system is moving or not and the bike speed would depend on rpm and which gear you were in on your derailleur.

    You would have to work through the numbers on the total reduction depending on what size pump/motors you were using and how much reduction to have from the motor to the crank but that isn't that difficult.

    The idea would work in theory in my screwed up head but I'd like to hear what some of you have to say. I still need to work out the numbers for fluid velocity and reservoir tank size.


  2. DougC

    DougC Guest

    Warning: I am not a professional engineer-

    Random thoughts:

    1) why keep the regular engine clutch, if the hydraulics can do that with a valve, and be totally-burn-proof?

    2) most true hydraulic pumps/motors are only rated for maybe 3000 RPMs but you might be able to find an oil pump that you could run directly off the engine's 6000-7000 RPM shaft speed. The question is, how bulky will it be?

    3) if you used off-the-shelf hydraulic motors, they're going to be way thicker and heavier than what you really need. Most hydraulic components I've seen are rated for 5000 or 5500 PSI, while an appropriately-designed bicycle-drive setup might not reach more than 100 PSI in use. If you had the machine tools to build the hydraulic motors yourself, you could make ones that were plenty strong enough for this, but still much more compact and lightweight than anything you'll find off-the-shelf.

    Generally speaking, you can use a gear-rotor-type hydraulic pump as a motor and vice-versa.

    What websites are you looking at for hydraulic components?

    http://www.surpluscenter.com/ (USA) is the main one I know of that usually has some smaller hydraulic pumps and motors at discounted prices.
  3. powdersummit

    powdersummit New Member

    Doug, thanks for your insight on the idea. I hadn't thought about trying to use a valve system instead of the clutch on the motor. I'll have to look into that one a little more. I'm not sure how you would do it to make it so the valve would open when the throttle opens so you would get power to your pump. I guess you could do some sort of electrical solenoid valve but it would add complexity. Some sort of RPM activated valve would be ideal. Great idea though getting rid of the clutch all together though, it does make things more simple from a manufacturing side of view.

    As far as the engine RPM goes the fastest pump I have been able to find is about 6000 rpm. I figured on using a belt reduction off the motor to drop the rpm down for the pump. If I could find something that was able to run at 8000 rpm it would be a great candidate for direct drive. I'll have to look into it some more.

    I do have the ability to manufacture quite a bit of the parts. I'm a graveyard shift supervisor in a machine shop. I worked for a while at a hydraulics manufacturing shop where they specialized in gear pumps and large radial piston hydraulic motors. It was a few years ago when I first started my machining career. I really didn't do much of note there but I did learn some stuff. We did outsourced our gears for the gear pumps though. Other than the gears for the pumps I could machine all the housings if I had to. It would be nice to find an off the shelf solution for the pump and motor. I had planned on using another gear pump for the crank motor. Surplus center was the main place I was looking at for the pumps. If I can find a decent price on gears I might have a go at making my own pump and motor.

    Thank you again for your ideas. I really think this idea is doable and there is a market for it if someone was to figure it out.

  4. loquin

    loquin Active Member

    Max torque on an EHO35 is at about 5700 RPM, I believe...
  5. professor

    professor Active Member

    You also will need to cool the fluid.
  6. powdersummit

    powdersummit New Member

    Well I spent about half the night last night working on the pump displacement numbers while the lathe I was running was running itself. I think that volume is going to be my enemy so I was trying to figure out which numbers for the gears was going to create the smallest volume and create higher pressures. Having a higher pressure and lower volume/flow rate will create a system that is smaller and lighter. So far the numbers have not been working for me.

    I'm not sure if this idea is going to pan out just like the half a dozen or so other ideas I've had for a motorized bike.