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Hydraulic drive for trike.? Answered

I am in the (long term) process of rebuilding my tricycle recumbent bike (a tadpole configuration). At present it has the usual long chain driving the rear wheel.

I would like to do away with the long chain for several reasons and am toying with the idea of using a hydraulic drive.
Cylinders at front and rear with the front driven by my feet and the rear driving a standard pedal set with the normal short chain/gear set - in face the rear frame of a standard bike with the hydraulics rams in place of my legs..

I can't find anywhere that this has been done and this worries me as generally what can be done is done.

In metal weight would be a problem but I hope to be able to machine the rams in plastic tubing giving me a light energy transfer from front to back.

Hopefully it will be electric assist as well but that is another matter.

Any comments would be welcome before I commit to building it so I can see why this is such a bad idea.



Best Answer 3 years ago

Interesting idea, but unless it is a pressurized system, you won't get a full one to one transfer of energy from front to back. Home built plastic cylinders will not be able to handle much pressure and commercial cylinders, if you can even find any that small, are going to be very expensive. You will also have the resistance of the fluid pumping back and forth which would make for one heck of a workout. Some type of shaft drive might be a better option if you can figure out how to set it up properly. I have seen regular bikes with shaft drives and that might be a good starting point. Someone may even make a shaft drive conversion that could be modified to work.

Toothed belt drive is an alternative option. I can get plastic that will go to 10 bar.

Because this isn't commercial or mission critical I have fewer worries about failure, If I go ahead I intend to try using water as a fluid to minimise viscosity issues and for pure cheapness.

We used to set up simple hydraulic systems in school using plastic cylinders and water - This was a commercial education system for learning about hydraulics.

If there isn't full 1:1 energy transfer where are the losses:



Mechanical losses in bearings/connections etc.

I expected that assuming the cylinders are the same size/areas the effort would be similar to pedalling directly? Hydraulic experiance very limited as you can tell.

There are many reasons why you would not get a 1:1 energy transfer. Compression of the fluid is one. Air in the system. Expansion of the hoses under pressure. Also, water is a very poor hydraulic fluid for this type of use. You need something that will provide good lubrication to your seals or they will deteriorate from heat buildup and wear out quickly. You also get friction from the movement that will build up heat that could actually boil your water. Water also expands when heated which could blow your lines or cylinders.

Here's a simple test you can perform to show why effort would be greater. Take a syringe and fill it with water then try to squeeze it out as fast as you can. It takes more effort to push a large volume of liquid through a small orifice. It's called restriction and unless your hoses are the same size as your cylinders then you will always have that creating resistance to movement. An even better example is to take two syringes of the same size and connect them together with a piece of tubing. Fill one syringe with water before connecting while the other is empty. Now push the water from the full syringe into the empty one. Odds are the tubing will pop loose before the empty syringe moves, but if it doesn't you will see the amount of effort it will take to move it.

I'm not an engineer, but I was an Aircraft Hydraulic Systems mechanic for 20 years. I may not be able to explain all the formulas and laws of fluid dynamics, but I can tell you that in my experience it won't work like you hope.

Firstly to all who have replied, I am not being argumentative, I am just trying to get a handle on an idea to see if it is worth following before I spent a lot of time and effort on it.

I fully respect everyone's experience and knowledge.

In passing the syringe idea actually works really well and is often used in schools to demonstrate the principles of hydraulic systems.

See this instructable


1. I intend to use low friction plastics, PVC or polypropylene and Teflon to reduce friction as far as I can.

2. At 65 the best i can pedal at is about 50 -80 rpm so I doubt I will be boiling water. I also intend to use an electric assist thus reducing the strain on the mechanical system even further.

3. Plastic will reduce the weight considerably, My current 12 feet of chain is actually fairly heavy in it's own right. If I am carrying batteries then a little extra weight isn't really of consequence.

4. I will try to use common plumbing components, I am told they are good for 10 bar, this will mean the pipe is a good proportion of the anticipated cylinder size.

5. I am unable to find any reference through google that shows me that the efficiency of such a simple system might be. Some formulae ask for information I don't have like hydraulic constant for the fluid??

6. I was hoping someone could show me that this has been tried before or that there was some profound reason why it isn't worth pursuing.

Any further thought are welcome.

Even with light weight plastics, you still have to have o-ring seals or it will leak all over the place. This creates resistance. Also, as I mentioned before, you are pushing a large volume of fluid through a small orifice which also creates resistance, friction and heat. There are electric bike kits with the motor in the front wheel hub that might be a better option unless it's just a project you want to try without any specific goals to accomplish. If I haven't dissuaded you then all I can say is give it your best shot. There's a guy in Europe that builds giant beasts made from cheap plastic plumbing pipe that walk around on a beach using wind power and pneumatics, so you never know what you can accomplish until you try.

Theo jansen :-) I have built some of his beasts as well although not as large.

Thanks for your input anyway - This will be a long term project will advise of results some time in the future.

You have a dual use system, you'll be able to draw off the working fluid to make a nice hot cup of tea to recover from riding it....

A cuppa is always welcome. :-) I don't think i put out enough energy these days to boil the water though!. Even pumping the Primus is an effort!

Why use pistons. Use a set of hydraulic motors. One attached to your peddles and the other attached to your drive wheels. You can keep the existing gearing in place and use a vary small chain between the gears and the gear on the hydraulic motors. As you pedal the motor spins pumping fluid from one motor to the next spinning the drive motor then the fluid returns to the peddle side again. Will likely be hard to get it moving. But it will be a vary interesting build to chow off. Sometimes it doesn't have to be logical or even a good idea to turn some heads. It would be something i'd like to see at a maker fair. May even win you a contest here if you can find one that relates to the project.


Possibility, I had looked at although they seem heavy and expensive. Building a rotary hydraulic engine isn't out of the question, i have built rotary model steam engines before.


3 years ago

Weight: Hydraulic systems are markedly heavier than chain drives.

Power efficiency: Hydraulic system power loss is around 25%. Chain drive power loss is 5% to !0%.

Costs: Hydraulic systems are more expensive to construct, maintain and repair.

weight: I will try to build from plastics, as this is not commercial and partially an experiment.

Cost: DIY I should be able to at least fund a test bed.

Efficiency: High losses (25%), what cause them? the long, (12 foot) chain also has issues as the chain run on a recumbent isn't a direct line. This is a known weakness of recumbent bikes, sometimes overcome by making a front wheel drive system.

Friction in the cylinders ? Viscous losses in the fluid movement ?

See above. I am cautious as my hydraulic experience is weak (under statement) but this seems like a useful approach.