Undervolting, motor selection, etc

[woahdae]

First of all, thanks to all you endless-sphere-ers, this is an incredibly useful forum! I think I've learned enough to ask the right questions.

TL;DR, ~180lb rider sometimes hauling up to ~40lbs up Seattle hills, currently totally happy with a Bafang SWXK 24v 250w given how stealth it is (I shoulder-carry my bike multiple times / day). My current setup is a "hill topper" sold by a Seattle company: http://www.electric-bike-kit.com, but I want to switch to rear drive so I can:

a) Stop breaking my fork. The torque from the anti-rotation washer is levering my (steel) dropouts apart! There is now ~2mm of slop between the washer & dropouts, where 1000 miles ago there was hardly space for the frame paint.
b) Use a hub dynamo.

According to this forum, the Cute 100 seems to be the ticket, specifically Q100C: https://bmsbattery.com/ebike-kit/618-q100c-cst-36v350w-rear-driving-ebike-hub-motor-ebike-kit.html#/rpm-201.

My questions:

* Is the Q100C really my best bet? Priorities: durability, light weight, at least as performant as my current 24v/250w Bafang SWXK.
* Can I use my 24v (unsure about amps) controller & battery with a 36v 350w motor and get no worse performance than I currently have?
* Are there any advantages to running the engine under its capabilities? Greater durability? That would be highly desirable.

Thanks!!!

 

[motomech]

I think I've learned enough to ask the right questions.

Not really.
You should have been using a torque arm from day one!
Now you have a problem that putting a new motor on the rear is not going to solve.
If you mount a rear motor, what are you going to use for a frt. wheel?
You can't put the original back on. Aside form the spread drop-outs, you must have filed them out to fit the Hill Topper.
Right now, I think you need a new pair of forks with a proper TA.
And why would you need a dynamo? You have a battery.
To see what a TA on a Hill Topper style motor looks like, click on my second build link and scroll down.

 

[woahdae]

You should have been using a torque arm from day one!

Clearly the evidence bears out that conclusion, but The Hill Topper website and staff (I picked up the motor in person) is sort of a zombo.com for electric bike kits, claiming it all just works.

From the Hill Topper FAQ:

Q: Do I need to use a torque arm with the CR hub motor?
A: No, a 250 Watt motor is safe to use without one.

If you mount a rear motor, what are you going to use for a frt. wheel?
You can't put the original back on. Aside form the spread drop-outs, you must have filed them out to fit the Hill Topper.

I dremeled the engine axle flat about a half-mm on each side under the anti-rotation washers, no way I was filing my forks.

The original has always had a bit of slop when not tightened down, and fits mostly the same as it always has at the top of the dropout. Not ideal, but I'll keep an eye on it. Seems to still be usable.

And why would you need a dynamo? You have a battery.

Technically true, but it's a big nice-to-have for a bike that replaces a car.

You know it gets dark here as early as 5:00 in the winter? It's weird. I go through a lot of charge cycles on my main light. Not a huge deal, but generally speaking I can still ride my bike if the motor battery dies, but I can't or shouldn't ride my bike if the light dies. Yeah, backup batteries etc, but you would think it was crazy if a car had rechargeable or battery pack headlights, right? IMO it's a similar situation on a commuter bike. Plus, I like the headlight-style optics most dynamo lights use. Plus plus, dynamos are great for multi-day unsupported trips.

To see what a TA on a Hill Topper style motor looks like, click on my second build link and scroll down.

Thanks for the reference TA pics. Seems like the simplest option from where I'm at now, but long term I still think it's ideal for me to have the motor in the rear.

 

[woahdae]

Meant to attach this, since it's kinda interesting.

 

[woahdae]

OK, don't know how I didn't find this earlier, but check this out:

http://ecmweb.com/content/how-save-your-motors-during-brownout

For an electric motor, torque changes as the square of the voltage applied. A 10% increase in voltage, for example, will boost torque 21% (1.1×1.1=1.21). Conversely, at 90% of rated voltage, the motor suffers a 19% reduction in torque (0.9×0.9=0.81). A more severe undervoltage condition, 20% below rated value, would reduce the motors torque to only 64% of rating (0.8×0.8 =.64). The effects are a 156% overload and catastrophic failure.

Excess heat is a problem for motors because insulation life is halved for every 10°C increase in temperature (Fig. 2). At the same time, the temperature of the winding will rise 10°C to 15°C for each 10% drop in voltage. That means the insulation life of a motor that operates on 10% lower voltage will decrease to only 50% to 75% of its expected life.

So I should definitely match my motor voltage to my supply!

 

[motomech]

Ok, I have a Q100CST and a MXUS frt. The MXUS is more or less the same as the Hill Topper and it is larger and able to take more power than the Cute and has been very reliable.
The Cute CST has a few issues. The wheel build needs extreem "dishing" to center the rim. The beauty of the CST, is it's ability to use the donor's cassette, in my case, a nice quality Delore 9-speed. Your bike doesn't have a super nice drive system worth reusing.
But, IMO, that's a moot point as I can't for the life of me, see why you would want to ditch what you have for a one of lesser quality.
To use the original wheel in your forks now, IMO, is unsafe. But it's your face.
Why not take the $300 to $400 USD that want want to spend and work toward a quality bike to convert?
That Hill Topper kit, with a 36V or 48V battery, mounted on a quality mountain bike would be much the same as my Idrive/MXUS combo and it's an outstanding Ebike that I ride every day and still enjoy after three years.
For the short term, I would recommend dual torque arms(easy without a disc)and a $25 Walmart AA powered H.lite as a back-up.

 

[motomech]

double post

 

[woahdae]

> For the short term, I would recommend dual torque arms(easy without a disc)and a $25 Walmart AA powered H.lite as a back-up.

Agree, maybe I'll never get past this stage. And I'll start shopping around for a new fork too. Maybe you saved someone's face today!

> Why not take the $300 to $400 USD that want want to spend and work toward a quality bike to convert?

I'm attached to my porter/gravel bike (saving up for this one: http://www.elephantbikes.com/stock/), it's perfect! And not particularly cheap, just not "overbuilt" (a relative term, for sure). I just want a little boost sometimes, ya know?

 

[motomech]

That's what I'm talking about!
Think of all the batteries that would go in that Triangle.
New fork with TA would be best, although I think a decent welder could weld on some reinforcment steel.

 

[amberwolf]

Plus, I like the headlight-style optics most dynamo lights use. Plus plus, dynamos are great for multi-day unsupported trips.

A catch for a dynamo light: in-use, it's going to rob some power from your motor, taking more power to move you forward than without it.

As long as it's not engaged it won't be a big deal, but depending on how much power your light takes, it could make some difference to your range when engaged.

* Can I use my 24v (unsure about amps) controller & battery with a 36v 350w motor and get no worse performance than I currently have?

If the motor is wound for the same speed you already have, but you use it with a battery that's 1/3 less voltage, you'll also get 1/3 less top speed out of it.

It'll probably also have significantly less torque than what you have now.


If it's wound for a different speed, then "it depends".


I recommend going to http://ebikes.ca simulator and playing around with the same setups at both 24V and 36V, with no change other than the voltage, so you can see what I mean. THere is an A / B function to let you do two and compare.

* Are there any advantages to running the engine under its capabilities? Greater durability? That would be highly desirable.

Under-current it'd run cooler. Under-voltage may mean under-current, and at same current means lower power.

 

[chas58]

My questions:

* Is the Q100C really my best bet? Priorities: durability, light weight, at least as performant as my current 24v/250w Bafang SWXK.
* Can I use my 24v (unsure about amps) controller & battery with a 36v 350w motor and get no worse performance than I currently have?
* Are there any advantages to running the engine under its capabilities? Greater durability? That would be highly desirable.

Thanks!!!

I have 24v and 36v Q100's. They are basically the same motor, so yeah you can run the motor at either voltage. I run the 24 volt at 36 volts, which gives me a rotational speed of 150% what is quoted. So, the 200rpm 24v version runs at 300 rpm in my case.

In your case, the 328rpm version 36v would run at 2/3's its specified rpm or 218rpm (maybe 15mph motor only)
The 201rpm version would run at 134 rpm if you run the 36v version on 24v.

See if you can find the no load speed of your current motor, and try to match that if you are going to undervolt it.