sub 4 pounds, sub 500w RC mid drive

R

rastamax

10 mW
Joined
Jan 29, 2009
Messages
22
Hi guys, I have been lurking around here for quite a number of years now and have enjoyed learning all I can about ebikes.

So my background is mountain biking. As in ride uphill for an hour, ride down for 5 min. Rinse and repeat. I love it and I have spent quite a bit of money on a high-end bike that is both reliable and lighweight and can take a good beating. Obviously one can only do so many runs like that before being too tired. As much as I like the high power setups that I see here, it doesn't really suit my purpose.

What I am truly after is the ultimate lightweight sub 500w mid drive. One inspiring setup that I have seen is on the montanara volta bike built by Jean-Pierre Schiltknecht.

So basically I was thinking of using a astro 819m-9t and a neugart ple40 at 80:1 reduction and 2:1 going to the cranks for about 100 rpm cadence at 12 volt. Final reduction would have to be adjusted after experimenting a bit I'm sure but it would be close.


Motor is listed at 120gr (?? seems low)
battery:nanotech 3s at 4500mAh/280gr
ple40-80 at 550gr
13t freewheel at 200gr
controller ??30gr
frame mount 250gr ish
total: about 1700gr but it's missing a few parts like throttle, extra chain/chainring...

Anothe option would be Astro 860m-5t and PLE40-40:1 but run at lower voltage to halve Kv of the 819m. Bigger motor/lighter gearbox but I suspect lower efficiency. Also maybe a 100 gr heavier .


So let's say goal is to stay below 4 pounds WITH 4,5 AH battery.

So what I want out of a system like this is obviously to keep the lighweight feel of my bike when I charge down the hill but be able to ride uphill twice as long using the motor and my usual leg power.


Do you guys think it could be a viable setup for it's intended purpose or should I just splurge on a setup like tangentmotors.com which has too much power and is about 2-3 pounds heavier but works reliably in the end.
 
Hello Rastamax,

looks like you have a firm grasp of what you're attempting. I looked up the 819 motor series & see they use cobalt magnets....that's a min requirement to handle the heat you will likely be generating in climbs. (even intermittently)

I like the weight constraints you're chasing. the only other point that will need some review & testing will be your battery capacity. if you have to go heavier than goal, I suspect it will be in battery capacity. Better to err a few extra watt hours to keep the pack safely in its range of max life expectancy. (maybe I am just cheap)

I guess I have nothing else to offer but best wishes & look forward to any updates & maybe a build thread.
T
 
Hi Thud,

I agree I overlooked battery capacity. At 12V x 4.5ah= 54Wh i doubt that would get me to the top of my local trails. Doubling that would mean 100W for an hour of climbing so if I normally output 200W when climbing would that mean it would be twice as easy or twice as fast if I pedal like usual? It would also mean switching packs at the bottom after each hour of climbing but thats fine.Other than that do you see any problem with that setup. I'm aware that being in the wrong gear at the wrong time can strip the gearbox without amp limiting but other than that I wonder if there is something I overlooked.
 
this is exactly what i am looking for. from a bit of research i think the tangent motor is going to be very loud although perhaps not at the loads you are looking for?

I think a set-up like tangent with near silent operation at power levels that are much more 'bionic' legs than minimoto is pretty ripe to sell alot of units to the mtb scene. bbs units i suppose are the answer but it would be nice to get something that mounts inside the triangle and has better parts quality
 
rastamax said:
So what I want out of a system like this is obviously to keep the lighweight feel of my bike when I charge down the hill but be able to ride uphill twice as long using the motor and my usual leg power.
Click to expand...
Have you considered using a small backpack for the batteries?
 
just chucking some numbers into a calculator like this (bottom of the page)

http://www.cyclingascents.com/alpedhuez.html#.VpY-HPmyNBc

are you sure sub 500w is going to deliver what you are after? to hit 15mph on a steep fire road i would think you are talking around 800w?
 
I've been wanted to do something similar, but I got a hold of a 85kv gt4020 motor so i can do away with a heavy, expensive, and probably loud gearbox.

You left out the Freewheel crank, thats probably going to add some weight over a nice mtb crank.
also all the mounting will take the torque is going to add some weight.
 
I think a set-up like tangent with near silent operation at power levels that are much more 'bionic' legs than minimoto is pretty ripe to sell alot of units to the mtb scene. bbs units i suppose are the answer but it would be nice to get something that mounts inside the triangle and has better parts quality
Re: sub 4 pounds, sub 500w RC mid drive
Click to expand...

I also think it would sell like hot cake after the initial OMFG Ban all ebikes reaction that is common at the moment.

Have you considered using a small backpack for the batteries?
Click to expand...

Yes that is the plan.

are you sure sub 500w is going to deliver what you are after? to hit 15mph on a steep fire road i would think you are talking around 800w?
Click to expand...

All I'm looking for is to double my leg power . I don't mind if I crawl up the hill at 5 mph like I usually do anyway, I just just wanna get there half as tired as if I was under my own power.

I guess you could do without a freewheel crank for a relatively low power system, where weight is a priority...
Click to expand...

I'm sure I can do without yes. Only planning on a one way bearing or freewheel at motor output.

So after some more research, planetary GB are really expensive but might be worth it over the long run. And of course noisy but that is the price to pay for a motorized lightweight bicycle at the moment. Noise isn't much of a concern to me cause I don't ride in crowded areas but I totally understand how it can bother a lot of people.

A low Kv motor like saul mentionned would be really nice. Can you tell me more about it? because in every mid drive, reduction is always the core of the problem. If I could find a proper motor that would allow for only a 2:1 reduction, that would be ideal but I suspect is impossible or impractical.

One thing that I've been looking at is the newer brushless cordless drills, those are hall sensored, they come with a nice finger throttle and controller. They also have a clutch to prevent damage to GB and output more than 500w and quite a bit of torque. Most are under 3 pounds and are quite affordable to boot.Some of them even have temperature control to prevent motor damage.

Now they spin a bit fast at 400-500 rpm so I was wondering about lowering voltage by half. How would that affect them? Only half power or also way less efficiency and more heat buildup. Oh yeah they also have a fan to cool the motor :lol:
 
rastamax said:
All I'm looking for is to double my leg power . I don't mind if I crawl up the hill at 5 mph like I usually do anyway, I just just wanna get there half as tired as if I was under my own power.
Click to expand...
So, you only need around 250 Watts.....
 
So, that's an additional 24 Nm at the cranks.

You could just about get 8:1 from of a single stage reduction.

So, you'd need 3 Nm continuous from the motor.

A decent outrunner weighing 800 grams or so should manage that.

No need for a gear box....... :)
 
But how much would the single stage reduction weigh? Recumpence V4 drive weighs around 1800 gr I'm sure. Remember I wanna stay under 2000 grams if possible. I know most of you couldn't care less about a few pounds here and there but to me going from a 30 pound bike to a 34 pound bike makes a noticable difference and is a setback. So I'm looking for a lightweight solution and by setting some design goals we have some parameters to narrow down the options.
 
Sure. I understand.

If you're referring to the "feel" of the bike then, only the weight on the bike itself is what you should be concerned about.

How about a goal of 1600 grams for all the components on the bike?

I'm exploring the simplest option first. A motor with an 8:1 reduction, direct to the cranks. So, the weight additional to the motor would be for two sprockets, chain, overrunning clutch and mounting bracket. Plus the controller and wiring, of course.

12t to 96t using #25 chain would keep the sprocket sizes acceptable? The 96t sprocket could be laser/water cut from Aluminium alloy and hard anodised.

Weights:

#25 chain is circa 100 grams a meter.
 
+1 On a single stage reduction, however, requiring a long run for #25, I have done nearly the same with a 5.5 : 1, 17T to 95T, but such a long run (assuming you will have a similar set-up as the one in my signature) will be hard on the 12T.

With that long of a run, go with #219 kart chain instead, or you will probably run through a lot of 12T sprockets.
 
It's a single stage to the cranks, in this case. So, the run can be quite short, with a tension arm/jockey sprocket. 12t sure is a compromise but the only way to keep the chainring size down.
 
Oh! and here is a great resource for making these decisions, I wish I had read this part before making my current #25 set-up as final drive a couple years ago. :roll:

https://endless-sphere.com/forums/viewtopic.php?f=16&t=46630

From respected fabricator and designer "Thud"

Bike chain: fine for pedaling...I wear it out pretty quickly powering it with electrics...there are better grades than the budget BMX stuff I use but I have options.

#25 is great for small size & semi-robust applications such as primary reduction.Too light weight for final drive on a bicycle (my bicycles anyway)
the only limitations are really large rear sprockets & adapting them to things.

#35 chain is my favorite for 2 reasons..
1st) its common. You can get chain & masterlinks & a variety of sprockets at any Tractor supply store/lawn & garden dealer & master links at any menards & ace hardware.
2nd) Its incredibly affordable & easy to adapt, the .375" [3/8"] pitch gives you a finer range of adjustment to gearing. You can also get #35 racing chain in superb alloys that will last a long time.

I have no issue making #35 fit a parallel drive on the right side..even with 2.5" wide tires.

#219 chain.
Its a great chain developed by the Kart racing scene. it is .306" pitch, so even finer adjustments in gearing are available...
One of the big advantages is the super quiet "Extron" composite sprockets available for it (inexpensive & standard 5.25" BCD)
the other side of the coin is the limited selection of drive sprockets...expensive if your adapting a kart drive cog....or limited to a fat .750" shaft diameter & need a small 12T cog
You have to pay the man for #219 chain, as there are no cheap sources. There are no "masterlinks" for #219 chain so you'll need a chainbreaker to work with it.
Click to expand...

I found this to be true running a 650W Inrunner from Golden Motor, and targeting around the same wattage you are talking albeit mine a much larger less efficient motor.
 
Following :)

I'm a MTBr and Downhiller at heart...so you are speaking my language with light motors.

I love my Intense/Tangent 3210 setup, only 7 lbs (plus battery)....and a heap more power than you need.
Maybe go with the AstroFlight 3205...smaller and lighter than the standard kit, with power to spare.

(see sig for pix)
 
Miles said:
Sure. I understand.

If you're referring to the "feel" of the bike then, only the weight on the bike itself is what you should be concerned about.

How about a goal of 1600 grams for all the components on the bike?

I'm exploring the simplest option first. A motor with an 8:1 reduction, direct to the cranks. So, the weight additional to the motor would be for two sprockets, chain, overrunning clutch and mounting bracket. Plus the controller and wiring, of course.

12t to 96t using #25 chain would keep the sprocket sizes acceptable? The 96t sprocket could be laser/water cut from Aluminium alloy and hard anodised.

Weights:

#25 chain is circa 100 grams a meter.
Click to expand...

Ok, I guess I missed something here?

I was guessing that the 96T sprocket you mentioned in the 12 to 96 tooth would mean a 96T sprocket on the rear hub?

Not sure where else a sprockt that large would fit?

***edit*** Ok so the 96T would be on the crank? that's probably going to cost some ground clearance for a MTB?

And yes, for a short run through the cranks, #25 is just fine, I just had the impression it would be going to a wheel similar to I had done (I am now most likely going to rework my bike for a MXUS 3000w mid drive, but in the mean-time, I might convert to 219 chain to continue with what I have :) )
 
LI-ghtcycle said:
***edit*** Ok so the 96T would be on the crank? that's probably going to cost some ground clearance for a MTB?
Click to expand...
Hence the need for #25 chain for which a 96t sprocket would be equivalent size to a 48t bike chain one.
 
First time I come across (or pay attention to) #25 chain and sprockets. Very useful thanks Miles, proves I can learn stuff every day.

So I'd be willing to go to a 32t bike size equivalent chainring cause anymore than that sacrifies ground clearance quite a bit. About a 60t #25,that would give 5:1 to the cranks.
What about using one of those 100Kv motors like this
http://www.hobbyking.com/mobile/viewproduct.asp?idproduct=54888

What happens if it's run on 2s instead of 8s-12s with the amps limited to 40. I can't seem to find a definitive answer.

That would bring no load RPM to about 150 at the cranks. Wonder how much it would be while under load. If anything I can always compromise to a 6:1 ratio.
 
I don't think that motor would work on 2s, and if you lower the voltage too much you'll get more heat and less efficiently, but at very low power like this it probably won't be an issue.

I got my 85 kv for much less on here https://endless-sphere.com/forums/viewtopic.php?f=31&t=71188&p=1089276#p1089276
from another user's custom motor run.

I've been doing calculations for 12s, but on 6s with a 5:1 + ~2:1 on the crank it should work nicely.
Only other motors i've seen with low enough kv are from alienpower, 90kv 63mm, i think it saw a 60k once.
 
rastamax said:
What about using one of those 100Kv motors like this
http://www.hobbyking.com/mobile/viewproduct.asp?idproduct=54888
Click to expand...
I've entered the data that Hobby King give into the Motor Comparison spreadsheet. The second no load figure is just a guesstimate.
 

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