HowdySki

Hey Everybody,
I am working on a biomechanics project and need to make an exercise bike crank out a ton of watts. The nice thing is, it has mostly actual bike parts on it, so changing it should be pretty easy.
So here are my questions:

1. Does anybody know of where to find huge time trial rings? The largest I've found is 60t, does anyone know of anything bigger?

2. Does anybody know where to buy dicta single speed freewheels?

Thanks, if you have any info, please reply to this thread or email me at Jester@dublin.com



Joe

Little Darwin

I believe that sometimes recumbents and/or folding bikes use large chain rings due to smaller wheels, so you may want to search for some recumbent specific dealers for large chainrings.

However, the size of the chain ring doesn't impact the wattage, the wattage is determined by the engine driving the system... You may be able to tweak torque etc...

If you are trying to maximize the wattage generated by someone on an exercise bike, then the only thing you can do is increase the efficiency of the generator or decrease the drag of any components between the person and the generator... driving a huge gear isn't necessarily the right answer unless it addresses one of those two issues.

If the generator you need to drive requires a certain minimum or optimal input RPM, then you have that limitation as well.

DieselDan

I'm assuming your trying to generate wattage, not measure wattage. Then you may get a better result with a fixed gear to optimize output.

jccaclimber

You can put a pass through assembly to up the ratio. Really that won't make any more power than a normal bike as humans are not efficient at low RPMs. Huge chain rings are used to compensate for small wheels, and low drag. If you increase the drag load at the other end of your system you should not need to have a huge chain ring to regulate the input RPM. I've seen people put out 1200 W on a 53x14 combination, you just have to up the resistance.

HowdySki

Right, thanks guys.

To address both Little Darwin's and iccaclimber's responses:

The problem is I have a finitie amount of resistance, and certain regulated speeds, so in order to increase the wattage, I need to increase the torque (as you said) we are looking at doing this by getting a large chainring and short cranks, I'm just trying to find some that will give us more of a mechanical disadvantage.

Thanks guys!

Bill Kapaun

Short cranks may increase the mechanical disadvantage, but they will make the "engine" less efficient if they have long legs. Slow "mashing" is just knee damage waiting to happen"!

Sturmcrow

The conundrum of engineering. Sure you can make your design put out more power within the design constraints, but you blow out your client's knees. What's a designer to do?

triplebutted

Ask these guys, they have a 68T photo.
Not sure what John Howard used but I think its pretty huGE!

https://www.bikecult.com/works/chainring.html

jccaclimber

What are you willing to pay for this? I can pass your info to a guy I know who runs a CNC place and has made custom ones for his recumbent. Huge rings, as in 90+teeth.

Edit: here is one of his chain rings. Not the largest as far as I know, and I think this one is actually on a Rotor crank, but you get the idea. Note the length of the crank arms in comparison to the chain ring diameter.

meb

Big chainring, small freewheel, small cranks are all features that will reduce torque and increase rpm on your generator.
Are you trying to increase torque or rpm?

Hostel Shoppe is a source for manufactured large chainrings-up to about 65T:
www.hostelshoppe.com

Highpath Engineering in England will custom make large chainring -up to 110T:
https://www.highpath.net/

Warhawk used to custom make large chainrings, (up to 125T, $324.95 was the old price for a 125T) they may again after they finish relocating:
www.Chainrings.com
Right now they seem to have website problems.

Here are some templates for making your own large chainring:

https://www.ihpva.org/Builders/Sprocket/

https://www.ozhpv.org.au/shed/sprocket.htm

The former is a file of a printed template of 3 different large round chainrings.

The latter is an executable program letting you specify teeth, bcd pattern, eccentricity to generate a template.

In both instances, a drill press is the only tool needed.

triplebutted

My knees hurt just looking at these numbers!!

jccaclimber

If you can find (good luck) a Shimano Capreo cassette on the back that may help too, although your chain losses start getting bigger as you try to round a 9 tooth cog.

HillRider

I looked at Bike Friday's web site (they use big chainrings to compensate for 20" wheels) and the largest they offer is 60T too.