Efficiency, getting real data

[FransS]

Hi Oran,

Great test you're doing!
Wouldn't it be an idea to keep the heart rate constant and then measure the resulting time/avg speed rather than trying to keep the speed constant?

Cheers, Frans

[Oran]

Hi Frans,
Good to see you have joined the forum. Having thought about the data I have come to the same conclusion as you. I shall be repeating the test sometime soon but a little differently.

Normand
The equation I have used is from Wikipedia http://en.wikipedia.org/wiki/Bicycle_performance section 4.1 Power required. I say the power calculated is approximate because I’m using typical values for constants K1 and K2 as I’m not sure how they are calculated.

[Normand_Nadon]

I was thinking about the best test to obtain the most stable, repeatable and reliable data to compare NuVinci vs derailleur setup and I came up with this:

Needed:

-Bike A, equipped with a well maintained derailleur and dual chainring (relatively known power losses)
-Bike B, equipped with a nuvinci N360 drivetrain (initial break down done)
-Weights to match both bikes on the rear end and simulate a rider (say a 150 pounds cyclist + or - the added weight to match the weight on both setups.)
-Interior trainer with power meter and variable resistance
-A gearmotor with loads of torque with a sprocket mounted on. Calculations have to be done to ensure that the ratio of the motor vs crankset mimics a pedaling cadence of around 90 RPM
-Good quality Ampmeter

What I would do:

Mount Bike A (derailleur) on the trainer.
Monitor current flow in the driving motor with the ampmeter. Voltage is known (120Vac) so we can multiply by current to obtain watts.
Monitor current on the trainer to.
Run the motor on any gear for 10 minutes to get it to a stable temperature.
Set ratio to minimum on the bike and run the bike. Then try all resistance levels monitoring the power Input (motor) and power Output (trainer). Then, do the same at all resistance level and ratios.
Redo that routine twice to ensure repetability is ok.

I know that some of you will tell me that the power meter on a trainer is not that precise, that I forgot reactance of the winding in my power Input calculations and so on and so on...Getting exact NASA style values is not crutial here as we are only going to mesure the DIFFERENCE between the two bikes, not the absolute power losses on both systems. We will still have a general idea of the absolute power losses, but that is not what I am looking for here.

Now the complicated part begins! Obtaining precise ratios on the NuVinci bike (B) to compare with the derailleur bike at same ratios. I will not elaborate on this for now but in short, using marks on the ground at 5 full revolutions of the rear wheel on a perticular ratio on the geared bike, and then trying to have the same result on the nuvinci bike should provide really accurate ratios.

After a ratio has been set, mount the NuVinci Bike on the trainer and test it at all resistance levels. Unmount the bike and find your next ratio, then remount the bike on the trainer and start again! Do the same with all ratios (long!!!).


This test will give us a really precise set of data to compare NuVinci vs Derailleur bike!

A tacometer could be added to the test to see if the resulting speed is the same on both bikes at all ratios and give even more precise results.

[Normand_Nadon]

What do you think Oran?

[Oran]

I’ve been thinking and I recon this is exactly the sort of test that needs to be done. I would like to verify that the graph provided by Fallbrook is accurate. Questions such as, is the drop in efficiency in underdrive due to increased torque, could be answered.

However this shouldn’t be the only test as I think real world data is just as important even if it can’t be as accurate. Having read the Rohloff website and then realised that a CVT adds another level of complexity I began to wonder if real world data isn’t the best.

I totally agree with you on the method for getting data and I also agree that the exact power loss isn’t important. Unfortunately I don’t have all the equipment needed. What I do have is a homemade permanent magnet alternator that’s waiting to be made in to a wind turbine. I believe I’m right in saying that because the alternators stator contains no steel the mechanical power can be calculated. Some sort of large variable resistor would be needed to regulate the power dissipated. The best motor I have with any kind of speed control is an electric drill and the power will have to be monitored with a plug in energy and power meter.

[Normand_Nadon]

For the alternator, all you need is to put something like a heating element at the end to dissipate some power... like a car 12v heater per example. Resistance will be hard to varry though... Adding some heaters in serie could do it...

I do not own half of the equipement I listed, but I can borrow most of it from friends around... In summertime, indoor trainers are not used often! I can probably get a good motor from an electronic supply shop nearby... Maybe a wiper motor... that could work!.. I will look around in the next few weeks to see if I can collect all the needed equipment.

[Oran]

It would be good if we could both do the test then we can check results. My method won’t be very high tech but I think I might be able to do it.
If I connect the drill to the alternator I can get an idea of its efficiency over different speeds and loads. This is something we need to be aware of, motors don’t have a fixed efficiency. Then I just need a better ammeter, a way of measuring rpm (cycle computers can be good for this) and make a variable resistor able to dissipate the power.

[Normand_Nadon]

Just remember that dissipating more than 1/4w in a resistor will require large size resistor... dissipating 200w will require the mother of all resistors!!!

[Oran]

I might try a liquid rheostat as they have the ability to dissipate large amounts of power due to the high thermal mass of water. It could be as simple as putting two wires in a bucket of sea water or something more refined like Grant Thompson’s Scariac.
The other option is to use the wire from some old heaters and put lots of short lengths in parallel. The resistance could be varied by adding or removing lengths of wire.

[Normand_Nadon]

Never heard of liquid rheostats... I will google that!

The heater wire is a really good idea! Resistance should be really stable once temperature is stabilised. You have a lot of good ideas Oran!