Motor adjusts to gear ratio?

[veloman]

I changed up the gear ratio on the front cranks, going from a 1: 3:33 to a 1:4 (think more torque). But since i can choose different gears on my cassette, then end ratios are going to be same if I adjust select the gear accordingly.

So, now my motor sounds different, and seems to have less power. IDK, maybe I just need to get use to it. By changing the crank gears tonight, I accomplished: lowering my pedal cadence when assisting the motor. That will help a lot on hills so I won't be spinning like crazy.

My question is, do motors "wear" into a certain range? I have 260 miles on it with the other gearing. Specifics on my bike are in the below link.

 

[dogman dan]

What motor?

A direct drive motor's tourqe will get lower and lower as it approaches it's no load speed, so yeah, you will feel less acceleration as you get faster. At no load, the dd motor will get the most silent.

At slower speeds, if you are in a higher gear, you will feel a lot more pedal resistance, as you supply more power than you once did.

Edit, now I see, did you change your motors gears or just the pedaling gears?

 

[gogo]

There have been a few threads about the geared MY1016 motor and if I remember correctly, the noise of the gear mesh can change. You can physically adjust the mesh by adjusting the motor cover that houses the gears, and you can lower the noise a bit by making sure there is adequate grease on the gear.

A loss of power and sound change could also come from overheating the magnets.

 

[veloman]

I just got home from work, riding it. It's definitely lost it's top end speed, even though I am compensating for the lower front chainring rpms by using say the 20 or 23 instead of the 26 on the rear cassette. The bike clearly is noticably weaker above 15mph. But it accelerates a little better from 3-12mph. Climbing is a tad slower too.

I don't think I overheated the motor on Friday, as it's never gotten too hot to touch. It does get quite warm in these 90 deg temps though.


The only way to figure out if it's the gearing or the motor is damaged is to switch back to the other gearing I was using. I'm doing that right now.

About 2 weeks ago I got caught in the rain for a mile. At that time, and for the ride the following day, the motor sounded quieter, but didn't feel weaker. Since then it's seemed to return to it's normal whine.

I might take apart that gear box and add some grease if necessary. It'd be great to reduce the wimpy whine it makes.

 

[vanilla ice]

Overheating magnets does the opposite.. you will lose tq/power and no load rpm rises.

 

[amberwolf]

I changed up the gear ratio on the front cranks, going from a 1: 3:33 to a 1:4 (think more torque).
<snip>
So, now my motor sounds different, and seems to have less power.

Well, if you change the gearing ratio, you end up with either more torque and less speed, or more speed and less torque. Same power, though.

However, if the motor was optimized for one gear ratio, then changing it for more torque means that it now spins faster to give you the same wheel speed you had in the "same" gear before (meaning, shifted into the same position on the HB shifters).

So the motor will be noisier, most likely. But it is easier on the motor to have it spin faster to provide the same output torque. It should run cooler now than before for the same speeds.

Motors don't exactly wear into a speed range, but they do have a powerband where some spots are more "powerful" feeling, meaning that they have a higher torque. The faster they spin, the closer the back-EMF voltage built up in the coils becomes to the voltage supplied by the battery and controller, and the less torque you get out of them.

So changing the gearing "moves" the curve of the powerband around.


Increasing the torque by changing the gear ratio does reduce your overall top speed, though, by the same percentage as the ratio difference. If I did the math right (unlikely :lol then you have about 83% of the top speed you did before, right?

 

[veloman]

Believe me, I'm quite experienced with gearing and bikes, and I spend hours some days playing with different ratios in my calculator. I am definitely beyond the simple - "it is now undergeared and won't go faster, why is that?". The total gear ratio, taking into account the rear cassette, has not changed. Only the gear ratio inbetween the motor and the cranks has. I am making up for that difference by using a smaller cog on the cassette.

Now, with riding essentially the exact same total gear ratio, my top speed is 2-4mph lower. It is baffling.

Here I'll type it out:

Before:
9 motor shaft : 30 crank
then 44 crank : 23 cassette
total ratio is 1.74

Today:
9 motor shaft : 36 crank
44 crank : 20 cassette
total ratio is 1.82

Yes, they are off by about 5%, but the same thing happens in nearly all my gears. I am climbing hills 1-2mph slower (very odd considering I have even more torque available now).

What you said about there being a power band - I noticed that before today's gearing. It was very clear there was an optimal throttle position at about 3/4. Going past that point gave no increase is power, and just before that point there was a lot of power 'range'. I feel like i don't notice that power band anymore, and hence the much lower top speed. I had trouble getting to 19mph on the flats today. Normally I can cruise at 22, that's a 15% difference in speed. Where is all that power going? If i switch a smaller cog in back, the motor just bogged down. Motor got just as hot, if not hotter since I tried using smaller cogs to make up for the difference, it just made the motor lug.

Anyway, I decided to do an experiment and run a 24 sprocket on the cranks, so right now it's
9 x 24
44 x cassette

I won't be able to assist it at anything above half throttle, but I want to see how my top end is. Hill climbing will be likely very bad, I'm expecting that since I won't be able to help the motor. But I never pedal on the flats, and I want to see if the motor is capable of cruising faster. (Since it was slower going from a 30 to a 36, maybe it's faster going from a 30 to a 24?)

This was a good learning experience though. It's taught me that a motor isn't necessarily producing it's best power at it's highest rpms. It seems to do best at 70-75% of its no load max.

 

[amberwolf]

Ah, I missed that you were still running about the same motor-to-pavement ratio.

Then I don't know what is causing it, because if I understand correctly, the motor will still be in the same part of the powerband. Something else probably changed coincidentally with the sprocket change, but I am not sure what.

Is it possible there is a connector not fully making contact somewhere, so that you can't get full power to the motor now? If it is so, that connector probably gets warmer now than before. It is also probable that such a problem would slightly reduce the total amps being drawn from the pack, noticeable with a wattmeter if you have statistics from before the change vs after the change at the same speeds/conditions.

Regarding the powerband, it'd be different for different motors, so if yours responds best to the previous conditions, then that helps you determine it's best powerband for your application. But a different motor in the same conditions otherwise might have a totally different powerband.

Motors produce the most torque at the lowest RPM. As they speed up, back-EMF prevents full current flow thru them, which reduces the torque available. AFAIK they are still outputting the same power, but not in the same way. Somebody that knows better than me should come along anytime now and correct me.

 

[veloman]

Thanks for your input here, i'm learning a lot.

So i just did a day on the new gearing with the tiny 24 sprocket, making crank rpms much higher.

Today's setup:

9 x 24
44 x 30 (never needed to go to the 26, I was hitting 24mph in the 30 cog on the slight downhills before coasting.

Turns out I was able to push through a decent headwind, with a huge plastic bag/box on my back (fan I bought while on lunch :lol: ), at 20-21mph with no motor lugging. Acceleration from 13-20mph was pretty good compared to the other ratios. The verdict is that the bike is clearly faster above 17mph with the odd high crank rpm gearing.


This has me completely baffled. Also, my motor is slightly cooler today than it was yesterday, with the same 90 degree temps outside.

With the above ratio, at 20mph, my motor shaft should be spinning 470 rpms

with 9 x 36
44 x 20
shaft rpms should be: 470 rpms

These are the same exact total ratios. But yesterday, with the 36 sprocket, I couldn't get above 19mph without the motor lugging.

Today with the 24 sprocket, I was doing 21mph into a headwind with no motor lugging.

I can't figure it out. Can anyone else?

Maybe I need to do some more controlled tests, with my watt meter on. i still need to get that hooked up properly.


But I'm happy about this. If the Currie XYDJ 1000watt motor I have acts in the same way, it means I won't need to reduce it as much as I thought. Or, rather, play with the crank gearing to maximize the motor's performance, without altering the total gear ratio.

:lol:

 

[amberwolf]

If you are also pedalling with the motor during the times that it is different than you expect for the same total ratio, but the actual pedal chainring is different, giving you different speed/torque powerband placement with your legs, then the results you get with the motor might make sense.

If you have only motor input and the overal ratio is the same from motor to ground then I don't get why it is different, unless there are other environmental factors such as wind (even a slight difference in breeze, perhaps) or air pressure that are causing changes.

For instance, if the local air pressure changes, not only does your aero resistance change, but so does your rolling resistance, unless you pump up or down your tires to match this.

If the temperature is the same, including at the road surface (meaning same time of day in most cases) then it should not matter. If it's hotter at the road surface it will decrease your rolling resistance a little, because it will heat up your tires and expand the air inside more, increasing tire pressure. Riding longer at faster speeds does the same thing, as does braking a lot if you have rim brakes or if you tend to skid during braking a fair bit.

If it's lower, the converse is true, and you end up with higher rolling resistance as your tires deflate a bit from lower pressure. The difference in either case may be insignificant, depending on the temperature difference, the original tire pressure, and the total volume of air in the tires, plus the hardness of the tire itself.

 

[veloman]

Well, the experiences haven't been scientific, but I am quite of aware of those things you mentioned. I am a 'serious road cyclist', so yeah I am very in tune with power requirements and resistance.

I am curious to figure out why, but I don't mind having the extra top end either.

 

[dumbass]

If you are running a motor system simular to how a Cyclone motor single ring kit is mounted ( the motor actually directly drives the crank sprocket) then your changing of the crank sprocket does not effect the motor to hub sprocket ratio. However, if your setup is simular to a Cyclone 3 ring motor kit then your changing of the crank sprocket will alter the gear ratio between the motor and the hub sprocket.

 

[amberwolf]

I just figured I'd bring up anything not yet mentioned, in case it might help trigger a thought. If I had any good ideas, I'd post those too, but I ran out already.