Minimalist engineering for bicycles

[jawnn]

I want to climb a 16% grade (3.2” over 20 inches) at 17 gear-inches. I climb this hill at 10.5 gear-inches with my .25 HP legs pushing about 400 lbs.

Wanting to keep the system as light as possible. I was thinking about a 24-volt motor, but then thought that a 48-volt motor would allow me to use a smaller motor because of the increased power at the low speeds????? I do not want to go fast.

What gear ratio is needed? I would really like a formula so that I can calculate other bikes and cargo weights.

[safe]

Increasing voltage on a motor while keeping the current limit the same means that heat increases a lot. If you reduce the current limit with the controller then the heat goes away, but so does your power.

Generally speaking I'd suggest increasing the voltage (which increases the maximum rpm) while reducing the gearing. However, I've been pushing the limits of these brushed motors and have found that the harder you drive them the more likely the commutators will fail. (well, on the motors I've used)

You need specifics for us to give you specifics in return...

[jawnn]

please tell me what kind of data you need? I realy do not know what I am doing.

ok I will try to reformulate this question so that you can understand it. Maybe it is in the wrong place? I don't want to insult any one, maybe I should just forget the whole idea.

You need specifics for us to give you specifics in return...

[safe]

The essential ingredient to be able to say anything about a DC electric motor that uses permanent magnets is it's KV value.

For instance....

If you have a motor that will rev up to 4000 rpm when it uses 48 volts then you will get a KV value of:

4000 rpm / 48 volts = 83

Once you know what this value is you can then begin to establish things like gear ratio and performance on things like hill climbing.

But you aren't out of the woods yet...

In order to be able to make some statement about efficiency of the motor you need to know what the resistance of the motor is. For some motors the resistance can be very high... like 0.300 ohms. However on those motors they usually have low KV values which sort of balances it out. Motors like the brushless RC motors that Recumpence uses can have resistance values as low as 0.050 ohms (50 mohms) and this can mean that you can run some serious current through them and get great power. In the brushed motor world the Magmotor also uses this high current / low resistance design and also performs well.

So the two things to know are the KV and the resistance... if you can know these things (and many low quality motor sellers are afraid to tell you because they are so bad) you can begin to figure out the motor.

Often you can backtrack and estimate the KV and resistance if you know the rated load, rated voltage and rated current. It's a little harder to do, but it can be done. (I've done it many times)

[stude13]

hi jawnn, this is not my field. but i am sure there is a solution here.

[jawnn]

After some thought into the subject I realized that I should turn the question into one of minimal speed.

What is the minimum speed one can ride a bike with out falling over? Then tell me what gear inch you’re in. Well MTB’s have a 17gi for the lowest because that’s the average tipping speed. But what is it?

But now I don’t know what I don’t know and may not understand the formula any how.

[jawnn]

This I found at wikipedia...there must be more to it?

Relationship with torque:
For a given torque and speed, the power may be calculated. The standard equation relating torque in foot-pounds, rotational speed in RPM and horsepower is:

P[hp]=9T[ftxlbf])(RPM)/5252?????


Where P is power, τ is torque, and ω is rotations per minute. The constants 5252 comes from (33,000 ft·lbf/min)/(2π rad/rev).

so now all I have to do is study this for a year or two...
http://en.wikipedia.org/wiki/Torque#...rque_and_power

[safe]

so now all I have to do is study this for a year or two...

I started back in 2006 learning all the formulas and applying them. It takes time... don't let anyone that is ahead of you (in the process) make you feel like it's not possible to learn all of this. It just takes patience.

If you want I can help you out, but it's going to involve a step by step process of building up the background on all of this. Figuring out gearing and motor powerbands takes some effort.

[AussieJester]

I strongly suggest you sign up at Endless Sphere jawn it's a forum dedicated to electric only bicyles some extremely talented members there that can advise you step by step. I have gearing calculators also if they are any help to you happy to email them too you.

KiM

[safe]

The problem with Endless Sphere is that they have banned some of the most enthusiastic folks in ebikes. (myself included)