# Minimalist engineering for bicycles

Discussion in 'Electric Bicycles' started by jawnn, Aug 24, 2009.

1. ### jawnnMember

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.

Last edited by a moderator: Dec 15, 2015

2. ### safeActive Member

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...

3. ### jawnnMember

what kind of data?

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.

Last edited: Aug 24, 2009
4. ### safeActive Member

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)

5. ### stude13Active Member

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

6. ### jawnnMember

This is the worse one I have thought of yet....?

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.

7. ### jawnnMember

Is this it?

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#Relationship_between_torque_and_power

Last edited: Sep 2, 2009
8. ### safeActive Member

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.

9. ### AussieJesterMember

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

10. ### safeActive Member

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

11. ### johnrobholmesMember

No, we just banned you, safe.

I am not familiar with your gear inches formulation. What are you using for the inputs? In simple terms the motor size determines your power. The slower you want to go, the less power you need. If you want a very small motor you will need much geardown to take advantage of high motor RPM.

Last edited: Sep 3, 2009
12. ### jawnnMember

What gear ratio for half hp?

Can any one tell me what gear ratio it would take to drive 450lbs up a 16% grade 3 to 5 mph with a 24v half hp electric motor???

I assume that a higher voltage motor would have more torque. So it may be necessary.

Some one told me 45 to one at 5000 rpm for a gasoline motor, but don’t electric motors need to run faster?

The show me the formula that you used???

Last edited: Sep 12, 2009
13. ### safeActive Member

It's best to start with a Bicycle Calculator:

http://www.me.psu.edu/lamancusa/ProdDiss/Bicycle/bikecalc.htm

...from this you can get a general idea about how much power is needed to achieve various results.

This other Bicycle Calculator is really good at figuring out the gear ratio you will need:

http://www.arachnoid.com/bike/index.html

...with these two you should be able to arrive at an exact gearing.

Example:

Motor Rpm (pedal cadence) - 4000 rpm
Front Gear - 10 tooth
Rear Gear - 450 tooth

Resulting Speed - 6.9 mph (11.1 kph)

About 45 to 1 is probably a pretty good guess. It's hard to take a motor that spins at 3000 - 5000 rpm and gear it down to something usable for an ebike. In most cases a multiple stage geardown is required to get anything usable.

This is (in part) why I'm looking at the experimental usage of AC Induction motors for ebikes because they can be run at as low as 1800 rpm and actually behave well at speeds below that for the 4 pole design.

Anyway... those calculators should do the trick... if you still feel frustrated let me know and I'll manually help you through it.

Last edited: Sep 12, 2009
14. ### jawnnMember

good thinking,but.....

These calculators are great, but they dont tell me how to get the data I want. Maybe I have play with it a few days.

The idea of using an AC motor is very good, just the kind of thinking we all need.

15. ### safeActive Member

I try many often bold and somewhat crazy ideas at times and sometimes they work well and other times they don't. I do learn with each mistake and have incremetally improved my bikes so that they now perfrom really well. (I just managed to pull 58 mph downhill starting with a simple 600 watt motor)

As for AC Induction motors applied to ebikes... well... it's a pretty wild idea, but I can see that if it does work it could really spark a totally new way of building these bikes. (chain driven, but without complex geardown units)

You just need to think about what you want to do. Insert the weight that you are going to be using. Select the slope you want to climb. Decide how fast you need to climb. From these starting conditions you can get a result about how much power you need to achieve those conditions.

...the calculator can't read your mind. And as they say:

"garbage in, garbage out"

...so your results are only as good as how well you define the question.

16. ### jawnnMember

Minimum RPM's?

The only problem I see with the calculators is that they do not let me add the sprockets I think will work, ie: 17 front and 34 rear, twice. Well I don't see the formula even on the "scource".

So just keep us informed how the experiments go, and I want to know how much power they produce at minimum rpm, and what that minimum rpm is.

I realy think I finnaly asked the most simple question possible. Not that I understand any of this. But I do know that I do not want to use a motor large than half hp and volts more than 24, because it just adds too much weight. That also why I dont mind not driveing fast, I just want to get up the hills with out a major strain on my legs...even at 10.5 gearinches.

17. ### jawnnMember

This is the missing link that I needed for calulating the power needed:
=
Figure out how fast you move with the weight and grade you need and then calculate the power you need.
=
Speed calculation:
18 front / 34 rear sprockets = 1 to .53 ratio .............
60 rpm’s at the crank x (.53) = 31.76wheel rpm's..............
62.8” circumference x 31.76 rpm's = 1994.82 ipm ..............
Divided by 12 = 166.235 fpm ..........
166.235 x 60 minutes = 9974.117 fph ............
Divided by 5280 = 1.889 mph.........
=

My second gear, with 28 sprockets on the crank, is 3 mph.

All I need now is a geared hub motor with a 50 to one gear ratio.

Last edited: Oct 24, 2009