Thoughts on new drive system/s. What would you like?

[recumpence]

There is, indeed, a bit if a hole between the high revving RC motors and the larger BMC and similar styles.

Remember, the main reason most people go with RC drives is the small size/high power. That is due to high efficiency as well as high RPM geared down for greater power from such a small motor. Any time you decrease motor RPM you decrease pwer per pound (assuming all other factors are equal). It is not problem making a system for a BMC or other motor. But, the weight is higher while reducing power. That being said, the reliability would be good and it would run quieter because of the lower RPM. It is all a trade-off.

The first thing I am working on is a small RC outrunner system that is 1,000 watts, and about half to 1/3 the weight of my standard reduction, and much smaller. It will also be about 40% to 50% less expensive than a 3210/V4 system. After that, I will turn my attention to other ideas.

Matt

 

[bobc]

I don't have a turnigy outrunner on my bike because it's lightweight or powerful, I have it because it's cheap (1kW looks like about $30 at the moment). That's why I have an aeroplane ESC on there, that cost $20. The custom engineered bits on my bike could be mass produced for peanuts. I think it should be makeable complete for $100 + batteries.
The hall throttle to RC ESC interface board should be cheap enough in bulk - (parts cost is <$1) -

There's the rub, the + batteries bit - I have 4 x 5Ah6s lipos on the thing, that's $160 at least.... OK for 15 miles at 20mph ;^)

 

[Kurt]

Any time you decrease motor RPM you decrease power per pound (assuming all other factors are equal). It is not problem making a system for a BMC or other motor. But, the weight is higher while reducing power.

I agree with this except I don't think the intention is to keep all factors equal.

Looking at things in a very simplistic way ....

Lets say motor A is a 2kg RC out runner. It produces X amount of power at 10,000rpm. take the same motor and 1/2 the RPM to 5000rpm like you said we will loose power... tourqe X rpm = hp

If we then take motor B that's also a out runner style but is is double the size of the RC motor and weighs 4kg. It has double the torque due to its size and can then produce the same power at 1/2 the rpm of the rc motor.

When you look at it this way you haven't sacrificed hp Just weight and size. Really even if you look at the weight and size the average Astro or turnigy RC motor that people are using they are only 1.5 - 2kg and 1/2 the size of a coke can. so doubling or even tripling weight-size isn't a huge thing. Especially when mounted in the frame.

When you consider the reliability and noise advantages. I think the advantages would more than outweigh the weight savings of 2kg or so. When you think about it the people riding the bikes don't all weigh the same. I'm only 70kg some my mates are 100kg and everything in between. The 2kg or so extra mid mounted gets lost in the equation of total rider weight.

To to use a car analogy.... what would you be sacrificing if you took a 1lt 4cyl motor with 100hp at 10,00rpm and then swapped it for a 2lt 4cyl motor with 100hp at
5000rpm?

I guess what I am getting at is. Are we sacrificing to much by limiting ourself to tiny motor size?
Kurt

 

[recumpence]

Your analogy breaks down when running at the motor's potential. Gearing a small high revving motor down and running it at its power peak (continuous peak or burst peak is still simiar in this), will net you more power per pound than a larger motor running at lower RPM. I used to be impressed by a 35 pound hub motor delivering 9kw. However, I see the same amount of power from 10 pounds of RC equipment (motor, reduction, and all related bits required for that system to properly compare it to the hub motor).

That is a 3.5 to 1 increase in power per pound.

Now, you are absolutely correct on sound. I could not agree more. For people with tenitis, or any other sound sensitivity, a hub motor sure is apealing.

I am not against hub motors at all. You never hear me referring to them as "Frock" motors. I am all for them. I feel they fill a niche that is important for many (most?) people looking for reliable electric bicycles, especially if used for daily transportation. RC systems, or any other small high revving systems, are for the "Sport Bike" types who do not mind a little added sound and increased complexity, but want the ultimate in power per pound performance.

It is somewhat "Apples and Oranges". They are both food, but very different.

Now, that being said, I have put thousands of miles on RC systems without so much as a loose set screw (not counting torture testing for R&D purposes). So, they can, indeed, be reliable.

They all make more sound than a hub, however.

Matt

 

[Kurt]

I wasn't referring to a Chinese hub motor. I to agree that a heavy and pore quality hub motor that's original intention is to be laced into a rim. It's the extreme opposite of a quality RC motor and a bad example of what a larger motor can do. I am not a fan of hub motors for many reasons they were never designed with ultimate power or performance in mind and adding all that extra weight for no gain.

I was referring to a middle ground motor. Picture a motor of similar build Q to a Astro motor but double the size- It still wouldn't be a big or heavy motor by anybody's standards. Ultimately a motor with lower rpm. perhaps even low enough that could run directly to the rear wheel without a primary reduction.

Yes power-pound RC is king but is mid mounting a extra 2kg to a bike with 5 or 10kw going to make much difference? Where we given regulations that limit the motor weight to 2kg? I guess we have chosen this particular size because that's what was available?


To sum it up

Take the weight of a average electric recumbent trike(or even MTB) with a RC drive system and average size 15ah lipo battery. mine is around 30kg + average male rider 80kg this gives you a total driving weight of 110kg Now take the same trike and +2kg(double the size motor) and make it 112kg. 99.9% of people would struggle to see any difference in performance. Blind testing would prove this as we are talking about less than a 2% weight increase It's the same as adding 20kg to a 1100kg car you couldn't tell. If some one told me I could + 20kg to my car's weight but in return I get to rev the motor 50% less I would be happy.

Ok handling perhaps 5% if your lucky could tell the difference especially if the extra 2kg is mid mounted where the riders weight is.

Giving your self 2kg of extra motor size of the same quality as your little RC motor to play with could open up a lot of breathing space and help with complexity, noise, and heat dissipation-copper mass for longer sustained hp handling. Is 2kg that much of a price to pay?

Restricting capacity is one of the biggest limits you can put on anything that has to produce power. Be it gun, rocket, ice motor, wind turbine, solar cell, Its the old saying size dose matter LOL

It may not be so cut and dry with the percentages in the real world but I still think its something to think about. I am not trying to take anything away from the currently available rc drive systems. I admire the bikes matt has built a lot. I am just thinking where things could go if the same creator worked with some different materials... bigger motor.


Kurt

 

[liveforphysics]

I agree with Kurt.

If I've got to have say 5kw of power, I started out wanting it from the smallest lightest motor spinning at a zillion RPM, and gearing it down to work.

After playing that game a bunch of times and never having something I was happy with for various reasons, my "perfect motor" ideas have shifted.


Now I like a motor that weighs 2-3x more, takes up a bunch more space, to make the same 5kw, BUT, gives me something that doesn't burn up, and enables a simple drive system to power the wheel.


It seems like taking a step backwards in performance, and in respect to power-to-weight, it definitely is.

However, in real world operation, the extra few pounds is something I gladly carry along to just not have all the heat issues and high RPM drive related issues that I used to have to deal with.

 

[Thud]

Aw Luke, you prolly start your day burning up a motor in your ellectric razor. :lol:

& what your desribing is: Reasonable expectations using 50-75% of a motors potentials to deliver reliability....instead of running a tiny motor at 99.9% (or more in most cases of an rc motors potential)for the power it can produce for a limited time.

So what i hear is:
Every one is looking for a low rpm, tourque monster motor to rid the non hub set ups of the (some say) obnoxious noise generated. For a hundred bucks!

 

[liveforphysics]

Aw Luke, you prolly start your day burning up a motor in your ellectric razor. :lol:

Well, I'm not about to run it on the 1s pack it came with Thud, I don't have time to waste shaving slowly.

JK, I actually use an unpowered razor, and generally shave on intervals spread out long enough that I look like a hobo for most of my life.

 

[Miles]

Kurt,

This isn't exactly something that hasn't been discussed before.... I think Matt understands what your saying....

If you go to the Power-Assist group, you'll hear it ad nauseam from Randy Draper.

If you skip a reduction stage, you multiply your motor weight by the reduction ratio lost and vice versa. Pretty simple.

Bigger motors are more expensive.

Choose your compromise.

So what i hear is:
Every one is looking for a low rpm, tourque monster motor to rid the non hub set ups of the (some say) obnoxious noise generated. For a hundred bucks!

 

[Kurt]

Yes ,sorry about the rambling. I know I am not teaching matt anything new. I was just breaking it down for some of the readers who might take the power to weight thing to serious.

back on track :wink:

ps, happy to delete the posts if they were derailing the thread to much.

Kurt

 

[rodgah]

Ironic isn't it thud .....not this discussion again LOL

So is there a motor out there that fills this niche......maybe not in the right Kv, but when has that stopped us. Something to go to Matt to design around. I have wondered about those BMC motors for a while, they are hulks of things so why is their rating so low.

 

[Miles]

The problem at the moment is, lack of choice in mid-sized motors. There's very little available at the size/form factor of the one that I used in the Moulton.

 

[drewjet]

That 4" Astro would be ideal! How about it Matt, get it off the back burner and back up front and on FIRE!

I would be first in line to get one.

 

[Whiplash]

I agree on the size/power thing, I would be happy with a low RPM motor about 4 or even 4.5" in diameter that can handle say 3000 watts for long durations and still be at least decently efficient, hell even as efficient as a DD hub would be OK... I just need the small form factor! 2-300 RPM on 12S would be killer, but I don't know if that is even possible... That's why my MID/MAC plan is going to rock if I can make it work right! Quiet, powerful and pretty reliable too!

 

[Miles]

I agree on the size/power thing, I would be happy with a low RPM motor about 4 or even 4.5" in diameter that can handle say 3000 watts for long durations and still be at least decently efficient, hell even as efficient as a DD hub would be OK... I just need the small form factor! 2-300 RPM on 12S would be killer, but I don't know if that is even possible...

Errr, no. You're talking about a continuous torque of more than 100Nm at the motor shaft.......

 

[Kurt]

Miles, The Moulton bike has to be one of my favourite e bike projects. The drive system and motor size looks great. I guess using a small 406 - 20" wheel opens up more options in regards to gearing with a single stage belt drive.

Perhaps a Large motor sold by the likes of Asto would cost more than may are willing to pay. Thinking about it though. Even though I went with a $99 motor for my rc build. I would be more than happy to pay $1000 for a motor if a new it was goign to last a very long time and I was 100% content with it's performance.

kurt

 

[AussieJester]

Weeeell...you may be in luck Kurt, if Astro
Ever make that 4in diameter job mentioned above
available to the "public" IIRC it was well over a grand :wink:

KiM

 

[johnrobholmes]

Maybe the question of how much reduction is acceptable would be well to discus? Of course the least amount possible is best, but the bigger motor cost goes up so fast that we can't ignore the balance between motor cost and reduction cost.

Matt, I think a great first step would be to develop a smaller alternative to the drives you currently offer. Something good for a HP or so, the real legal ebike conversion that could be driven either left side drive or through the chains. A "little" outrunner spinning 4k could have enough reduction to do this and still stay in the efficient zone. Motor and reduction weight of about 4 or 5 pounds (less battery) should be pretty easy to hit. The lightest 20mph hub motors I have used are about 7 pounds, so we are still at a power to weight ratio advantage.


Next step, pursue a better motor design. Next time we talk I will pass some ideas by you that I have had that will allow for minimum tooling cost, and we can run the idea by your new factory contact. I'm sure you don't need the help, but I'm willing to vouch for you and help in the development any way I can.

 

[Miles]

I guess using a small 406 - 20" wheel opens up more options in regards to gearing with a single stage belt drive.

That's true. It would have been tricky to get a single reduction greater than the 6.25:1 I used.

It's a shame that the alternator that motor was based on is no longer available...

 

[recumpence]

Well, I have lots of parts on order (both off the shelf and custom parts). This new project is quite a research and engineering challenge.

Keep your eyes peeled. I will have something in prototype form to show before Christmas.

Matt