# Bike trainer concept I've been messing with in my head

Discussion in 'General Electronics Discussion' started by Solidus, Sep 4, 2012.

1. ### Solidus

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4
Jun 19, 2011
This is a concept for a bike trainer I've been toying around with in my head for awhile, but lack the electronics specifics to really make anything more of it.

This isn't something I'm attempting now, it will be in the future aways, when I actually have a need for it or have the time on my hand to do it. The reason why I posted it in electronics chat as opposed to projects forum is I have no timeframe as to when I will do this and just want the information on a food-for-thought basis.

My idea was to build a frame assembly and have my rear wheel spin an alternator (as in from a car). That alternator would generate power, and a wattmeter would pick it up.

Thing is, most wattmeters I see simply plug in an outlet, you plug devices into them and they tell you the draw your device is putting on the wall. I need a wattmeter that would tell you the total amount of power created.

Now, it bears saying that for any of you familiar with cycling, this is not designed to be a replacement for a power meter, as I would also have to take my temperature and monitor that, the change in room temperature, and go through thermo calculations to determine overall power expenditure. To assume I would be 100% efficient at transferring muscular power to mechanical would be a GROSS overestimate - metabolic mechanisms are only ~40% efficient, and that's a hard-to-achieve number (car engines are I believe around 17%).

What I would be doing with that wattmeter result is taking it, applying it through a formula, and getting distance.

Here's my logic, tell me if I need additional considerations:

1. A single revolution of the alternator generates a potential and power. Whether or not it is measurable depends on things such as the gauge of the winding and other factors.
2. The wheel turns at a fixed ratio with regards to the alternator pulley. For every turn T of the wheel you get x spins of alternator pulley, and if one revolution generates power P,

T = x * P.

3. Concurrent to the circumference of the wheel, the wheel has to turn a certain number of times to make a mile. Therefore the amount of total power can be used to reasonably calculate distance.

I'd like to hear what some of you think.
My ideas always come out better when it's not just me thinking about them.

2. ### Laplace

1,252
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Apr 4, 2010
If you only just spin the shaft of an alternator, you will generate a potential but no power. In order to create power one needs to dissipate power in a load such as a filament bulb. That is the usual setup for an introductory science demonstration: an alternator driven by bicycle pedals and driving a lamp. Most such demonstrations include a wattmeter to show how much power is being created/used.

So what parameter could you measure as being proportional to velocity? And then integrate that parameter to show distance? What would be the best load to use? What about internal power loss within the alternator? How would internal power loss (which is also power created) affect the calculation of distance?

3. ### Solidus

349
4
Jun 19, 2011
Agh, I feel like an idiot, lapse of mind.

Power is current * potential, so of course no power would be generated. To have power one must have a circuit first.

I probably should've asked this first, but do alternators behave in the same manner as DC generators in that there is a direct correlation between voltage and spin rate (r/min)?

In terms of power loss - I only need to have the unit accurate to say, a tenth of a mile. Working out the math:

27" tire = 27 * pi inches circumference = 84.78" = 7.065 ft.
Wheel rotation per mile = 5,280/7.065 ft = 747.3 rotations
Wheel rotation per tenth of a mile = 74.73 rotations.

1.75" diameter pulley = 5.495" circumference
Wheel to pulley rotation ratio = 1:15.4
Alternator spin per tenth of a mile = 1,150.8.

Internal losses, as long as they're not horrendous, most likely wouldn't cause enough of a loss in power to really effect my distance calculation.

4. ### BobK

7,682
1,688
Jan 5, 2010
If all you are trying to do is measure the distance, counting wheel rotations is much easier and more accurate.

Bob

5. ### Solidus

349
4
Jun 19, 2011
I know I can just count the wheel rotations with a wheel magnet and sensor, but I was trying to develop a use for an old alternator, as I have one that's just been gathering dust.

Also, there are other practical considerations to why I chose to use an alternator rather than simply devising a way to count wheel rotation.

Felt resistance at the pedal on a bike is a combination of gear ratio (relevant to torque) and ground resistance, as well as incline.

On a bike trainer, where the rear wheel is elevated and not touching the ground, you lose that element of ground resistance and incline and much of the practicality of using a trainer at that point is lost. That's the reason why trainers nowadays incorporate electromechanical, hydraulic, or magnetic resistance systems. Simple rotation counters have none of those provisions.

Having the alternator in contact with the rear wheel would provide resistance and a varying load on the alternator output would provide varying resistance to the motion of the rotor.

6. ### BobK

7,682
1,688
Jan 5, 2010
Funny that you didn't mention that you intended the alternator to provide mechanical resistance. Now it makes more sense. It still does not seem like a good way to measure distance. The output will vary with speed. You would have to calibrate it and integrate the output power over time.

Bob

7. ### Solidus

349
4
Jun 19, 2011
Well actually, I may use a different method for measuring distance after all.

I was mainly deciding to use the alternator to provide both and to harness the energy off it in a useful manner, but I may just connect it to a load to vary the resistance.

Is there a set relationship between electrical load and the mechanical resistance imposed on the rotor?

The main use of the alternator will be to recover the element of ground resistance that would otherwise be lost.

8. ### duke37

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Jan 9, 2011
A car alternator produces AC which is rectified to DC on its output.
You could get at the AC directly to count the revolutions to get distance.

9. ### Solidus

349
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Jun 19, 2011
Would that entail opening it up and removing the rectification diodes?

How would I analyze the AC output to count revolutions?

10. ### duke37

5,364
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Jan 9, 2011
Yes, opening up an alternator should be fairly easy to someone who wants to build a machine.

The diodes do not need to be removed but you will need a connect a wire before a diode.

The cycles will need to be counted to get the distance just as they will need to be counted if you use a reed switch on a wheel.

A car alternator needs some power to energise it. Whilst it can produce this power when it has got going, it will consume power at low revs.

11. ### Solidus

349
4
Jun 19, 2011
Going down a different tangent, would it be easier to graph the voltage coming out of the alternator, and analyze that to gain wheel RPM (and therefore speed)?

of which integration would yield distance?