My RC Mid Drive with Single Stage Reduction

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FZBob   100 W

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My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 10 2018 3:00pm

I thought you guys might be interested in my RC Mid Drive conversion. I ride frequently in the hills, and I wanted to try just a bit of climbing assist without changing the character of the bike. I ran a Cyclone on and off about 7 years ago, but the benefit to downside ratio was not there, so i shelved it.
https://endless-sphere.com/forums/viewt ... ne#p412252
I did determine that about 250 watts of assist was a good balance between power and battery capacity.
I had been interested in RC motors since then, and last spring came across the Turnigy 125KV skateboard motor.
Reign Motor z sm 10-10-18.jpg
Reign Motor z sm 10-10-18.jpg (43.61 KiB) Viewed 4820 times
With 10:1 reduction, this motor could meet my performance criteria. I laid out a few gearboxes in Autocad (I’m an old Mechanical Engineer), but then determined that I could do it with #25 chain and 9T and 90T sprocket from electricscooterparts. The pics show a few different drive configurations I tried.
Reign Motor c sm 10-10-18.jpg
Reign Motor c sm 10-10-18.jpg (81.79 KiB) Viewed 4820 times
Reign Motor d sm 10-10-18.jpg
Reign Motor d sm 10-10-18.jpg (230.79 KiB) Viewed 4820 times
Reign Motor a sm 10-10-18.jpg
Reign Motor a sm 10-10-18.jpg (125.95 KiB) Viewed 4820 times
Reign Motor b sm 10-10-18.jpg
Reign Motor b sm 10-10-18.jpg (114.6 KiB) Viewed 4820 times
I have been riding the bike hard for the last 3 months - 250 miles and 25,000 feet climb - and I really like it. Everything has been reliable, and I have not snapped any chains at up to 470 watts. The chain has been stretching slightly, but if I need to replace a couple of chains a year that’s OK. The VESC torque control is far superior to the old Cyclone RPM control, making pedaling under power much more fluid and less obtrusive.
The bike is a ’08 Giant Reign with a big fork, really nice shock, wide rims and a 170mm dropper post. It rips downhill! It weighs 33.4lb without the motor, and under 38 with motor and one 4S 5800 pack. (the old cyclone system added 12lb!). This is good for a VERY PLEASANT assist on a 13 mile, 1250 foot climb, aggressive dirt ride with a friend who is 20 years younger, and a natural athlete. I am able to stay ahead of him on the downhills without motor, which means that the bike’s character is intact!

Edit 6-14-2019: Note that this system is a low power, light weight assist ASSIST. It is not intended or designed as a stand alone (no pedaling) drive. The motor, running at low RPM, is under powered for stand alone use. The drive system is too light to handle a big power increase.
Last edited by FZBob on Jun 14 2019 1:15pm, edited 1 time in total.

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Re: My RC Mid Drive with Single Stage Reduction

Post by flat tire » Oct 10 2018 3:39pm

Now that's a nice build. Couldn't agree more with your statement on the Cyclone benefit / downside ratio. Everything about cyclone is junk.

Bike chains can take a lot of power and torque if you are ok with frequent replacement and occasional breakage. I've run over 6kw.

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 10 2018 4:42pm

flat tire wrote:
Oct 10 2018 3:39pm
Now that's a nice build. Couldn't agree more with your statement on the Cyclone benefit / downside ratio. Everything about cyclone is junk.
Thanks! Yup, I suspect the cyclone gearmotor was an industrial conveyor drive or something similar. I only used the motor and throttle, the rest was not acceptable. The motor weighed a ton (7 1/4 lbs!). The Turnigy motor weighs 780gm.

I'm still using the Cyclone throttle, unfortunately. It has about 20 degrees of mechanical deadband which I can't fix in software. I would love suggestions on a better unit with no deadband.
flat tire wrote:
Oct 10 2018 3:39pm
Bike chains can take a lot of power and torque if you are ok with frequent replacement and occasional breakage. I've run over 6kw.
My concern is just for the cute little 1/4 inch pitch #25 chain to the motor. I'm probably over the recommended operating tension. (I suspect that's not too uncommon in this forum.) However, 1/4" pitch allows 10:1 reduction with a reasonable sprocket diameter, so I can live with that compromise.

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Re: My RC Mid Drive with Single Stage Reduction

Post by flat tire » Oct 10 2018 5:20pm

Throttle is easy to fix. For the hardware go Domino, high quality reliable precise throttle that blows the chinese stuff out of the water.

To remap the input range with a controller that won't let you program that you can run your throttle thru a cycle analyst.

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 10 2018 5:34pm

flat tire wrote:
Oct 10 2018 5:20pm
Throttle is easy to fix. For the hardware go Domino, high quality reliable precise throttle that blows the chinese stuff out of the water.
That looks like a very nice throttle! Any suggestions for the more budget minded consumer?
flat tire wrote:
Oct 10 2018 5:20pm
To remap the input range with a controller that won't let you program that you can run your throttle thru a cycle analyst.
I've got a VESC, so mapping is easy. Unfortunately, that Cyclone throttle is dead for the first 20 degrees. I'm tempted to see if I can make a mechanical stop on the closed end.
By the way, setting an exponential response curve in the VESC is sweet.

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 10 2018 5:37pm

I forgot to mention that I’m currently running with no freewheel. The motor is chained directly to the crank.

I wanted to eliminate the Sick Bike Parts HD freewheel and square taper bottom bracket, as that saved almost a pound (400gm). I thought about the concept of an “electric freewheel” - running the motor at 5 or 10 watts any time I’m pedaling without assist. It turns out this is not really necessary. The motor is easy to turn over, and the drive seems very efficient. Running the motor on the bike stand draws 7 watts, including turning the back wheel. When I was running the left side freewheel, 5 watts would bring the motor up to speed while I was pedaling.

I recently did a 20 mile flat ride in this configuration with a friend (who it turns out is irrationally opposed to E-bikes), so I pedaled the whole way with almost no use of the motor. I didn’t really notice the motor drag.

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Re: My RC Mid Drive with Single Stage Reduction

Post by flat tire » Oct 10 2018 6:40pm

Magura is half the price of Domino but not as high quality.

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Re: My RC Mid Drive with Single Stage Reduction

Post by Kepler » Oct 13 2018 7:37pm

Very interesting build FZBob. Subscribed
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Re: My RC Mid Drive with Single Stage Reduction

Post by ScooterMan101 » Oct 14 2018 6:08am

FZBob,

I like very much what you have developed here, as much as I like the simplicity of a Friction Drive, I want to change out tires for the different rides I want to do, meaning street tires, then aggressive mountain tires for riding in the loose dirt riding conditions where I live, most MTB riders use the very widely spaced knobby tire the Maxxis Minion. and they run them tubeless at very low tire pressures . That tire or others like it that are necessary for the conditions here , just would not work with a friction drive.
Also,
Since I must give my legs and cardiovascular system a much needed rest during rides, I must use a freewheeling crankset.
There is an option,
You do not have to buy the heavy square taper freewheeling cranksets , the lighter weight version is the iSis version . you can just buy the freewheel/crankarms/spider/and BB from a place like Sick Bike Parts,

http://sickbikeparts.com/bottom-bracket ... 48mm-isis/


or buy the iSis BB from a regular mail order bike parts vendor. I see the one from SBParts is 148 mm wide.
My first conversion ... Sold

viewtopic.php?f=3&t=71378&p=1077497&hil ... 1#p1077497

It's 2018 already, lets get some real , improved e-bike / e-velomobile / e-motorcycle designs .

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 14 2018 1:54pm

ScooterMan101 wrote:
Oct 14 2018 6:08am
FZBob,

I like very much what you have developed here, as much as I like the simplicity of a Friction Drive, I want to change out tires for the different rides I want to do, meaning street tires, then aggressive mountain tires for riding in the loose dirt riding conditions where I live, most MTB riders use the very widely spaced knobby tire the Maxxis Minion. and they run them tubeless at very low tire pressures . That tire or others like it that are necessary for the conditions here , just would not work with a friction drive.
Also,
Since I must give my legs and cardiovascular system a much needed rest during rides, I must use a freewheeling crankset.
There is an option,
You do not have to buy the heavy square taper freewheeling cranksets , the lighter weight version is the iSis version . you can just buy the freewheel/crankarms/spider/and BB from a place like Sick Bike Parts,

http://sickbikeparts.com/bottom-bracket ... 48mm-isis/


or buy the iSis BB from a regular mail order bike parts vendor. I see the one from SBParts is 148 mm wide.
So I tried using my freewheel crankset without pedaling. (See the pic above leaning on the bike rack) Running with just the motor it felt very weak, and sucked battery life. This was with 3S, 4S might be a bit better, YMMV.

I set up this system for a very specific corner of the performance envelope. Design changes will probably have a cascade effect. If you do not want to pedal you will probably want more power. The best way to do that is to spin the motor. It is rated for 2000 watts at 12S after all. However, then you will need about another 3X reduction, about 30X total, which will require at least 2 stages of reduction. At that point, the #25 chain will be too weak. You will also need a bunch of battery capacity which weights a ton. That all kind of puts you back into a heavy bike...

I have found that adding 80 to 160 watts of assist to whatever I am doing, and 250 watts up very steep hills, really takes the effort out of my rides. It feels like I'm not putting much effort in, but that really adds a lot to the motor.


I looked at ISIS, but a bit of research was not encouraging. Great for connecting Crank to Spindle, bad for bearing life. It is almost universally disliked on all the mountain bike forums...

Sickbikeparts and http://electricscooterparts.com/ are both fun resources!

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Re: My RC Mid Drive with Single Stage Reduction

Post by ScooterMan101 » Oct 16 2018 9:04am

FZBob,

With your last post I see you have designed your RC mid-drive for much lower power settings than I have ever seen a RC design listed here on E.S. , do before.
With such low power assist I can now see why you have now discarded the freewheel.

I am looking at RC mid-drives, or perhaps a friction drive if a tire could last at least 1,000 miles.
For me much higher speeds and power are necessary . I use my bike for transportation , on the street where the faster you go the more the 5,000 pound or more weight vehicles coming up behind you have time to look up from their cell phones , and / or pay attention , and even see you before they are at the same spot on the road as you.

So for street/commuting , etc. use higher power , without stages of reduction is preferable. This is where the freewheel crankset comes in.

What is desirable is a RC drive like yours that would give assist like you have for when exercising, and much higher power levels when needed/wanted.

What would be needed to be done to do that with only one reduction , at the crank/chainring ? Run the Drive on 4s pack when on a bike path , then switch out to 6 or 7 or 8 s pack When on the street ?

Or ... would just different throttle settings allow the slower speeds/lower assist power levels on low throttle, then higher speeds/and amp draw at the higher throttle positions ? there by allowing for use of a 6/7/8s pack to be used all the time.

Do the VESC's have the ability to run on different amp draws by way of on the fly/ride adjustment on smart phone ? or do all we need is adjustment up to a certain amp draw. ?
Since the lower the cell count the higher the amperage needed.

What are the MTB forums saying on BB bearing life on the i sis system ? and how long/many miles do they last ?
My first conversion ... Sold

viewtopic.php?f=3&t=71378&p=1077497&hil ... 1#p1077497

It's 2018 already, lets get some real , improved e-bike / e-velomobile / e-motorcycle designs .

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 16 2018 2:54pm

ScooterMan101 wrote:
Oct 16 2018 9:04am
FZBob,
I am looking at RC mid-drives, or perhaps a friction drive if a tire could last at least 1,000 miles.
For me much higher speeds and power are necessary .

What are the MTB forums saying on BB bearing life on the i sis system ? and how long/many miles do they last ?
If you need high power, unfortunately this is not that... The key to my drive is the #25 chain, which allows the 90T sprocket in a reasonable diameter, and therefore 10X reduction. That, along with the 125Kv motor and 4S gives reasonable crank RPM. At 100 to 300 watts, the little #25 chain is relatively happy. Much more and it starts to complain.

Reduction ratio is bounded by the following. Minimum teeth on the drive sprocket, 9T is pushing it. Maximum teeth on the driven Sprocket, determined by space (diameter) available and chain pitch. For example if I used 1/2 inch pitch bicycle chain, I would only get 5X reduction in the same space. But In a high power application, I would need that chain strength

The best way to get more power is to spin the motor. It is rated for 2000 watts at 12S after all. However, then you will need about another 3X reduction, about 30X or so total, which will require at least 2 stages of reduction. At that point, the #25 chain will be too weak. You will also need a bunch of battery capacity which weights a ton. You will want to put some freewheels back in, as 2 stages of reduction at 30X will be hard to backspin. There are lots of high power builds on this forum.

ISIS - Google "ISIS bearing life" or similar - MTBR is a good forum.

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Re: My RC Mid Drive with Single Stage Reduction

Post by ScooterMan101 » Oct 17 2018 8:55am

Not until a couple of posts above did I find out you have designed this drive for low power,

I ( and I am sure others ) am still interested in what you have done here for its simplicity .

The high power RC drives up to this point in time use expensive custom machined parts for the reduction , those are way out of my budget.

Each thread has some great information, and from your's here I can see that for one using a # 25 chain is not desirable for speeds, it looks like you are saying that a regular bike chain like a 8 speed would work but there just are not chainrings big enough to give the reduction necessary.
However the more threads that show DIY RC Mid-Drives that show up here on E.S. the more likely someone is going to start making Chainrings that are large enough and sell to the DIY Builders. for use with 8 speed chains, and now the new Sram Eagle NX 12 speed chain , which is made for use with e-bikes.
Making a large chainring for the front would help greatly in making simple RC mid-drives,
the chainring can be made in a dished way to give some more room to clear the rear chainstay on the bike frame. ( Luna is machining some chainrings , However it is made for the BBSHD, and goes only up to 48 now and only up to 56 tooth in the future ) so larger ones are needed for the DIY .

Also
look at the bike frame designs that are copies of the Santa Cruz Superlight/Bantam/Julianna, Diamondback Atroz, and all the other bike frames like that.
With that kind of frame there is room for a very large chainring.

Two things to note:
From what I have read in the past here , it is better to have 12-13 teeth on the motor shaft sprocket , and you know what that means more reduction needed.
and
With the new 10/11, and more important the new 12 speed Eagle drive train you can get a rear cog that is 50 tooth.

It looks like you have a 36 tooth cog for your largest cog on the back ?

I am still waiting for someone to make a simple jackshaft for another stage of reduction , using off the shelf large 53/54 tooth chainring and perhaps a disc front hub to bolt it to , drilling 6 holes in the chainring in order to bold up the disc side of the hub.
Simple off the shelf, inexpensive parts, to add another stage of reduction.
My first conversion ... Sold

viewtopic.php?f=3&t=71378&p=1077497&hil ... 1#p1077497

It's 2018 already, lets get some real , improved e-bike / e-velomobile / e-motorcycle designs .

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 17 2018 1:22pm

ScooterMan101 wrote:
Oct 17 2018 8:55am

the more threads that show DIY RC Mid-Drives that show up here on E.S. the more likely someone is going to start making Chainrings that are large enough and sell to the DIY Builders.
Making a large chainring for the front would help greatly in making simple RC mid-drives,

Two things to note:
From what I have read in the past here , it is better to have 12-13 teeth on the motor shaft sprocket , and you know what that means more reduction needed.

It looks like you have a 36 tooth cog for your largest cog on the back ?
This chainring might just be big enough for a medium power single reduction with a bicycle chain. Serious. Might be OK on a road bike as long as it didn't drag in corners. There are custom sprocket makers out there who can make these...
Big Chainring.jpg
Big Chainring.jpg (59.73 KiB) Viewed 4373 times
My bicycle gearing is 22-40 on front and 11-40 on back. I climb a lot.

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Re: My RC Mid Drive with Single Stage Reduction

Post by ScooterMan101 » Oct 18 2018 7:25am

Funny,

Have you seen a jackshaft ?

A jackshaft can be put in the main triangle of the frame, location dependent on the style of full suspension design.
A jackshaft will allow a very large amount of different gearing options.
On the jackshaft two chainrings. to the motor a large road chainring , to the crankset s a small mountain chainring .
Using off the shelf standard BCD's / Chainrings that MTB's and Road Bikes use. ( with a spider adaptor for the larger road chainring and/or down to the smaller BCD of the Mountain Chainring.

You say you are using 11-40 on the rear , wonder how much easier it would be to go to the Eagle NX System with 50 tooth large cog on the rear ?

Just think what you can do with a 50 or 53 tooth road chainring on the crankset and a Eagle NX Cog on the rear .
Or is there a 8 speed cog set that has 50 tooth as it's largest cog ?

Those of you that can do the math, what speed motor and volt battery pack ( number of cells in series ) would work without a jackshaft/second stage reduction ?
and
What speed motor and volt battery pack could be used by using a second stage reduction using standard size chainrings on a jackshaft ?
My first conversion ... Sold

viewtopic.php?f=3&t=71378&p=1077497&hil ... 1#p1077497

It's 2018 already, lets get some real , improved e-bike / e-velomobile / e-motorcycle designs .

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 18 2018 12:26pm

ScooterMan101 wrote:
Oct 18 2018 7:25am
Have you seen a jackshaft ?

Those of you that can do the math, what speed motor and volt battery pack ( number of cells in series ) would work without a jackshaft/second stage reduction ?
Jackshaft = 2 stage reduction = a bunch of other builds on E.S.

For single reduction, this is kind of the box you're in. I did the math (I'm a Mechanical Engineer - this stuff is very basic... :D ). Power (watts, horsepower) is proportional to Torque (Amps) X RPM. If you increase motor RPM you will need more reduction, as otherwise your crank will spin too fast. Unfortunately, the biggest practical single stage chain reduction is about 10:1. Using #25 chain - Can't go smaller than 9T, Can't go bigger than 90T. If you try to source a smaller pitch chain, you will have less strength. Larger pitch chain = very large chainrings or less teeth (see above pic). If you increase Torque, you will tear up chains. This build exists in a niche - It's happiest around 160 to 250 watts, with occasional excursions to 400 or so. Even at that level, the chain stretches a bit...

If I didn't mention it above, I'm using one 4S pack, 5.8 Ah.

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Re: My RC Mid Drive with Single Stage Reduction

Post by larsb » Oct 18 2018 12:59pm

FZBob wrote:
Oct 10 2018 5:34pm
That looks like a very nice throttle! Any suggestions for the more budget minded consumer?
flat tire wrote:
Oct 10 2018 5:20pm
To remap the input range with a controller that won't let you program that you can run your throttle thru a cycle analyst.
I've got a VESC, so mapping is easy. Unfortunately, that Cyclone throttle is dead for the first 20 degrees. I'm tempted to see if I can make a mechanical stop on the closed end.
By the way, setting an exponential response curve in the VESC is sweet.
Love the build!
I don't know what cyclone uses for the throttle, i guess it could be wuxing as they are cheap and common. If so, check out my mini-guide:
viewtopic.php?t=77735#p1156644
Ride on :D

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 18 2018 10:47pm

larsb wrote:
Oct 18 2018 12:59pm

Love the build!
I don't know what cyclone uses for the throttle, i guess it could be wuxing as they are cheap and common. If so, check out my mini-guide:
viewtopic.php?t=77735#p1156644
Thanks for the info! Here's a pic of my Throttle. I wouldn't be surprised if they are pretty similar internally. When I get a bit of spare time, I'll try to duplicate your fix.
Throttle.jpg
Throttle.jpg (213.71 KiB) Viewed 4263 times

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Re: My RC Mid Drive with Single Stage Reduction

Post by Dogboy1200 » Oct 19 2018 9:48am

FZBob wrote:
Oct 10 2018 3:00pm
I thought you guys might be interested in my RC Mid Drive conversion. I ride frequently in the hills, and I wanted to try just a bit of climbing assist without changing the character of the bike. I ran a Cyclone on and off about 7 years ago, but the benefit to downside ratio was not there, so i shelved it.
viewtopic.php?f=3&t=28621&p=412252&hili ... ne#p412252
I did determine that about 250 watts of assist was a good balance between power and battery capacity.
I had been interested in RC motors since then, and last spring came across the Turnigy 125KV skateboard motor.

Reign Motor z sm 10-10-18.jpg

With 10:1 reduction, this motor could meet my performance criteria. I laid out a few gearboxes in Autocad (I’m an old Mechanical Engineer), but then determined that I could do it with #25 chain and 9T and 90T sprocket from electricscooterparts. The pics show a few different drive configurations I tried.

Reign Motor c sm 10-10-18.jpg
Reign Motor d sm 10-10-18.jpg
Reign Motor a sm 10-10-18.jpg
Reign Motor b sm 10-10-18.jpg

I have been riding the bike hard for the last 3 months - 250 miles and 25,000 feet climb - and I really like it. Everything has been reliable, and I have not snapped any chains at up to 470 watts. The chain has been stretching slightly, but if I need to replace a couple of chains a year that’s OK. The VESC torque control is far superior to the old Cyclone RPM control, making pedaling under power much more fluid and less obtrusive.
The bike is a ’08 Giant Reign with a big fork, really nice shock, wide rims and a 170mm dropper post. It rips downhill! It weighs 33.4lb without the motor, and under 38 with motor and one 4S 5800 pack. (the old cyclone system added 12lb!). This is good for a VERY PLEASANT assist on a 13 mile, 1250 foot climb, aggressive dirt ride with a friend who is 20 years younger, and a natural athlete. I am able to stay ahead of him on the downhills without motor, which means that the bike’s character is intact!
Is that an RD350 back there. I had one when I was in high school, it was a rocket!
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Re: My RC Mid Drive with Single Stage Reduction

Post by neptronix » Oct 19 2018 10:04am

Not many things impress me these days because i feel like i've almost seen it all by now.
This? this is super duper cool and i'm rather surprised it works so well.
I always thought it made sense to kind of gimp out a high power density RC motor, and run a simple to build single stage. Just haven't seen anyone really take it on.

Have you thought about adding a tooth or two to the primary gear on the motor, then upsizing the crank gear appropriately? seems like the extra tooth contacting on the primary gear would be a benefit for chain durability. If you have 2.5 teeth contacting, then you could bump it up to 3 at once this way.. Hey, that's 17% more contact.. not huge, but anything helps..
Efficiency is everything :bolt:

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Re: My RC Mid Drive with Single Stage Reduction

Post by falcongsr » Oct 19 2018 1:28pm

Would using a 60kV motor such as this one: https://alienpowersystem.com/shop/brush ... 0kv-1800w/ potentially open up higher voltages or simpler reductions?

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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 19 2018 1:32pm

Dogboy1200 wrote:
Oct 19 2018 9:48am
Is that an RD350 back there. I had one when I was in high school, it was a rocket!
Close - It's an RD400. I used to road-race it back in the early '80's. Yup, it ripped. I used to walk Kawasaki 900 Z1's regularly. Guess I've slowed down just a bit...

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Re: My RC Mid Drive with Single Stage Reduction

Post by thepronghorn » Oct 19 2018 1:41pm

falcongsr wrote:
Oct 19 2018 1:28pm
Would using a 60kV motor such as this one: https://alienpowersystem.com/shop/brush ... 0kv-1800w/ potentially open up higher voltages or simpler reductions?
Nope, changing Kv does nothing to change a motor's torque capability, it only changes the voltage/current combinations which produce those same capabilities. A smaller reduction means less torque amplification which means you need a torquier motor which is a bigger motor (or a much much more expensive motor of the same size).

falcongsr   10 mW

10 mW
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Location: California

Re: My RC Mid Drive with Single Stage Reduction

Post by falcongsr » Oct 19 2018 1:45pm

ok I don't quite get it. The motor's stated torque in N*m is much higher than higher kV motors of the same size.

FZBob   100 W

100 W
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Re: My RC Mid Drive with Single Stage Reduction

Post by FZBob » Oct 19 2018 2:00pm

neptronix wrote:
Oct 19 2018 10:04am
Not many things impress me these days because i feel like i've almost seen it all by now.
This? this is super duper cool and i'm rather surprised it works so well.
I always thought it made sense to kind of gimp out a high power density RC motor, and run a simple to build single stage. Just haven't seen anyone really take it on.

Have you thought about adding a tooth or two to the primary gear on the motor, then upsizing the crank gear appropriately? seems like the extra tooth contacting on the primary gear would be a benefit for chain durability. If you have 2.5 teeth contacting, then you could bump it up to 3 at once this way.. Hey, that's 17% more contact.. not huge, but anything helps..
Thanks! I've been riding it every few days, (I alternate with my non Ebike), and I'm really enjoying it. It has just the right amount of assist to make the climbs feel much easier, and the power blends in very smoothly. I think the VESC controller really contributes. FOC mode is nearly silent, and putting Expo in the throttle response makes it really nice. Once i'm out of the heavy climbing, I don't use assist at all. The bike pedals on flats and slight uphills just like it did without the motor. (I wonder how the 55lb commercial ebikes pedal slight uphills without assist...) I often ride with a guy who is much younger and fitter, and he has taken to requesting that I bring this bike, as I can keep pace with him on his preferred (long) rides. I just installed a 170mm dropper seatpost and wide rims, so downhill flow trails are a blast.

The 9T is definitely right on the limit, it makes more noise than the motor. I have 10T and 11T motor sprockets in the parts box. Tried the 10T, but I liked the 9T more for power. The largest sprocket I found is 90T. I would be curious if there is anything bigger out there off the shelf. I don't know if I would go bigger on a dirt bike, as the 90T is about the size of a 45T bike sprocket. I don't want to bash it.

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