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PostPosted: May 24, 2016, 7:29 pm 
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Location: Toronto, Canada
I am working on an chain drive R1 Midi and I am trying to figure out a good Electric Reverse solution.
Something like this from the UK forum.

Image

My questions are:

1.What gear ratio do I need? How fast do I want to go in reverse? I'm guessing about 1 ft per second. Is that too fast or too slow? What is the output RPM of a typical starter motor?

2. What starter motor to use. Obviously, small and light is good. I think I would prefer CCW rotation for ease of installation. What are my choices for CW and what are my choices for CCW? What is everyone else using?

Thanks


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PostPosted: May 25, 2016, 2:23 pm 
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By my reckoning 1fps = about 0.68mph. I've met faster snails. ;)

Walking pace is typically a little over 3mph, which equals 4.4fps.

Motor RPM will depend on load, so you need to do some calculations based on the tech spec of the motor you have in mind.


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PostPosted: May 25, 2016, 5:21 pm 
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Why not put a sprocket on the starter motor (another possibility is a winch motor; see "Cool Idea for Reverse" thread) to engage the chain directly?

Simpler and more freedom of location?

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PostPosted: May 26, 2016, 7:41 am 
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The issue with the sprocket on the winch/starter motor is bearing wear on that part. Those parts are not specd to have continuous operation at the speed they would see on a direct chain setup.

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PostPosted: May 26, 2016, 9:00 am 
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And I thought the issue would be the minimal engagement of the starter sprocket to the chain. In normal use, there is ~180 degrees on both the drive and driven end. Add the reverse motor along the flat of the chain, and the sprocket would have only a few degrees, if that. the chain would slip off the sprocket and create wear on both the sprocket and chain.

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PostPosted: May 26, 2016, 10:07 am 
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Sam_68 wrote:
By my reckoning 1fps = about 0.68mph. I've met faster snails. ;)

Walking pace is typically a little over 3mph, which equals 4.4fps.

Motor RPM will depend on load, so you need to do some calculations based on the tech spec of the motor you have in mind.


Maybe 1 fps is a bit slow but I'd hate to try backing into a parking spot at 4.4 fps.
Yes, I could do calculations. If I knew the exact rolling resistance of the car, the exact technical specs of the motor (resistance, voltage, torque, etc, etc. I could probably figure it out. But with all those variables, my calculation would likely be way off.

I was hoping someone would jump in and say something like "Hey, I sued a ford focus starter with a 20:1 reduction and it worked great but if I had to do it again I would probably go with 25:1 ratio. 20:1 is a bit quick."


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PostPosted: May 26, 2016, 11:15 am 
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rx7locost wrote:
And I thought the issue would be the minimal engagement of the starter sprocket to the chain. In normal use, there is ~180 degrees on both the drive and driven end. Add the reverse motor along the flat of the chain, and the sprocket would have only a few degrees, if that. the chain would slip off the sprocket and create wear on both the sprocket and chain.


One would assume that this would be put on the backside in what would be an idler tensioner state. Given the chain is of the 520/530 variety and only ~1hp is needed for a reasonable reverse, a fully 180 wouldn't be required.

The force required to move a ~1000 lb car 4 mph isn't much

From my back of the envelope calculations (1000lbs ~450 KG goes to 500 for easy math. 4 mph ~2 m/s and lets say it is reached at 2 sec) gives me 150 Nm required with a 24" tall tire at the axle.

Take that into a standard 42/18 (8.56"/3.6") for a 530 chain reduction for the drive motor and you only need 150/2.4=63 Nm or 47 lb ft of torque (obviously you could gear it lower or higher as needed. The motor would need to turn ~150 RPM to get 4.4 mph at 42/18.) HP is 1.3. I bet the teeth on the ring gear set up would be under similar stress.

I still worry about the bearings. You could tear the unit apart and put in good bearings...if you can find some to fit with a decent life span.

Ohh Here is an idea. forget the bearings and connect the sprocket to a one way coupler. That way the winch only turns when it is being used. Maybe cheaper and easier.

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PostPosted: May 26, 2016, 12:43 pm 
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Don't know if it's helpful for your needs, but Martin posted up his electric reverse here some while ago using a Small Block Chevy starter and a snowmobile flywheel.. said it works well, but he rarely needs/uses it..

viewtopic.php?f=10&t=17852

https://youtu.be/TdObULcKL94


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PostPosted: May 26, 2016, 1:15 pm 
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Thanks, ccrunner. You're more organized than I am. I had to go look for my video. :rofl: However, I also found this one, which may be helpful.
https://www.youtube.com/watch?v=7Q7yFT_ehVU
Thumper, unfortunately, I have very little in calculations. My whole reverse was somewhat experimental, as I had to wait for the final test to see if it was correct. The two criteria that I had to consider, were the gear size and the starter motor bendix. I wanted as small a gear as possible, to reduce rotational mass, and to fit it into the engine bay. Once I found the gear, I looked for the most powerful starter motor, that had a bendix that would match the gear. As it turned out, a Skidoo starter motor has the nearly same bendix dimensions as the SBC (I think the difference was around 0.030" in diameter.) Those two factors made my decision. As to the RPM, I honestly don't know. I think in your case, you will need to use a reduction gear, because you are not going through a differential. I have seen some units like that on some UK suppliers websites for racing applications.
As to using the reverse, it is an acquired taste. Because there is no clutch, backing into a parking spot, wouldn't be my first choice. I see it as an emergency backup, when I'm strapped in, and need to do it fairly quickly. Otherwise, it is a lot easier to just push. And having a full cage, makes that so much easier.

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PostPosted: May 26, 2016, 6:15 pm 
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Thumper wrote:

Maybe 1 fps is a bit slow but I'd hate to try backing into a parking spot at 4.4 fps.


Could I recommend that you fit your car with a newfangled innovation we have on this side of the Atlantic called 'brakes'? :D

The manufacturers of the motor will have the specs you require on its performance, if you ask them nicely.

Rolling resistance at that sort of speed is trivial, on the flat, but I'd suggest you base your calculations on having enough torque at the tyre contact patch to pull the car up a reasonably steep gradient (perhaps 1:6?) even if it's only at your snail's pace.


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PostPosted: May 27, 2016, 12:36 am 
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Sam_68 wrote:
Walking pace is typically a little over 3mph, which equals 4.4fps.
I've been using reverse a lot these last couple weeks, backing my Toyota Hilux (mini pickup) into the old shop to load it up with stuff, and y'know? I don't drive backwards at a walking pace very often.

I'd be quite happy with 1fps in reverse...particularly considering that electric motors typically have max torque at min speed and will accelerate to full speed (whatever you gear it for) like Usain Bolt coming out of the blocks at Beijing.

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PostPosted: May 27, 2016, 3:26 am 
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JackMcCornack wrote:
I'd be quite happy with 1fps in reverse...particularly considering that electric motors typically have max torque at min speed and will accelerate to full speed (whatever you gear it for) like Usain Bolt coming out of the blocks at Beijing.


The easy analogy is with a torque converter auto: typically geared for around walking pace at tickover, in reverse. On the rare occasions you want to go much faster than walking, you give it gas. Otherwise, you hold it on the brakes and you can creep as slow as you like.

If you want to be clever (and safe), you can even use the autobox trick of a solenoid that won't allow you to select the electric reverse unless both the bike gearbox is in neutral and your foot is on the brake.


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PostPosted: May 27, 2016, 9:49 am 
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FlightService wrote:
rx7locost wrote:
Ohh Here is an idea. forget the bearings and connect the sprocket to a one way coupler. That way the winch only turns when it is being used. Maybe cheaper and easier.


That won't work.
When driving forward, the one way coupling would engage and spin the electric motor.


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PostPosted: May 27, 2016, 4:16 pm 
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Thumper wrote:

That won't work.
When driving forward, the one way coupling would engage and spin the electric motor.


Stop being correct!!!! :lol:

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PostPosted: May 29, 2016, 4:38 am 
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Location: Hawkes Bay, New Zealand
Thumper wrote:
I am working on an chain drive R1 Midi and I am trying to figure out a good Electric Reverse solution.
Something like this from the UK forum.

Image

My questions are:

1.What gear ratio do I need? How fast do I want to go in reverse? I'm guessing about 1 ft per second. Is that too fast or too slow? What is the output RPM of a typical starter motor?

2. What starter motor to use. Obviously, small and light is good. I think I would prefer CCW rotation for ease of installation. What are my choices for CW and what are my choices for CCW? What is everyone else using?

Thanks



I have used this setup for my reverse gear using a Honda Odyssey ring and starter motor. I'm using a Motobatt MTX16 battery. The speed is like goldilocks....Not too fast, not to slow but just right but the problem I have is that the starter motor pulls too much current for the battery and shorts out internally.


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