Teenagers e-skateboard for Christmas - On a budget

[WeeChumlee]

Ah well, dull day at work.
I have here at my disposal here a whole box of dc motors, mostly brushed, and a 24V power source to drive them.
Just as a bit of fun I got my skateboard from my car (standard pool skateboard so smaller and harder, though not very hard, 90a wheels). Longboards are at home so unfortunately could not try them right now.
The motor I got from the box was only 3000r/min so nowhere near the speed of the Banggood motors suggested. I attached a bronze bushing to increase the shaft diameter from 4mm to 15mm - basically to get a better cohesion and increased wheel speed. This worked a lot better than just the blank shaft.
I spun the skateboard wheel up with it, trying to get the best connection between the two as I could.
Without load it of course it works lovely, unfortunately I could easily overcome the torque transfer with just my hand. Grabbing the wheel I could instantly stop it.

I am not saying this is a very representative test, but it was fun and gave my colleagues in the office a laugh so still worth it.
Still in the opinion that a 3mm (or 5mm) blank shaft on a urethane wheel would not work on anything other than flat and smooth ground - once you have kicked to speed.
Not my idea of an electric skateboard, but hey - if that is your thing then this is a free world (cough cough), go ahead and make it happen.


Ah, as an afterthought I wrapped the bushing with packing tape for better friction, helped of course but not an awful lot.
If I made the bushing really rough it would work pretty well with softer wheels - but then that is really just a form of gearing. Rather put a cog and pulley on myself.

 

[astilwell611]

Lets put it this way. If friction drive was more cost effective and offered the same performance level as belt drive, there would be a lot more builds on here using friction drive. Being as this is a DIY community almost everyone is looking to build something on the cheap. If everyone could drop the $20-$50 they spend on belts and pulleys they defiantly would. The only application where direct drive offers even remotely similar performance is with hub motors. But you aren't talking about hub motors. There was a company on kickstarter that were selling friction drive kits that were removable. The only difference is they were using the barrel of the outrunner and not the motor shaft. I don't see the shaft having enough surface area to do anything, the barrel is another story though. Still with their kit you had to kick to speed then the motor took over. As WeeChumlee said that isn't really the point of an electric skateboard.

 

[WeeChumlee]

Hi astilwell611

Yeah indeed, I did see those friction drives you mention. Actually was not a bad idea how they did it.
A clip on/off idea could be a bit of fun, especially to spread the good vibes of Eboarding. Guys could use their boards, clip the thing on and try it out, all without any other modifications.
The spring loaded tensioner to hold the outrunner to the wheel would probably work OK for flats or very slight inclines, if you had a bit of speed already. I can't remember what sort of surface the outrunner had, I presume they put some urethane around it or something like that?
Do you know if they actually were financed and if they are in business? (Just out of curiosity)

 

[MagiicHat]

This is a question purely of surface area. Notice in the picture of all those urethane rollers that none of them are tiny? Usually those rollers are pressed (rather forcefully) against another large roller or a flat surface, resulting in a large contact patch. A 2mm (or even 5mm) shaft is going to have a contact patch in the neighborhood of just a few sq milometers.

I'd wager that this setup would have enough friction to maintain, once the board has been bodily kicked up to speed. It certainly won't take you up a hill.

I saw you throw out 8-10mph earlier. When I am strolling down the street on my (unpowered) longboard, 10-12mph is a pretty casual pace. 15-18mph is probably pretty standard. I can push around 22-23mph on smooth flat ground, and regularly reach upper 20s with some minor incline. If I am really going for it, I hit 40mph on the regular, and top out at about 48-50mph bombing hills. I don't say this to belittle your project, but to put the speed into perspective. I'm certainly not the fastest guy, and there are plenty of grade schoolers who can blow me out of the water.

 

[NyOliver]

I used a design envelope, and you do not fit it. You're not 8 to 12-year-old under 100 pounds. And I specifically mentioned that a kid should be able to run out Whatever top speed the board goes. You're an adult, not a kid, why would you want to fit in this design envelope? It's like saying, I don't want to ride on that teeter totter. Do you regularly get on top of kids toys? I know it says teenager on the title but, the OP asked about a preteen actually. I actually don't believe you sustain 23 miles an hour on level ground, I have a hunch that you are guessing or boasting, big time, but any case anybody 12 years old and pushing 25 MPH, to blow you out of the water, is also out of the design range.

The friction is not just one level, steel and urethane already have appropriate coefficient of friction, since the motor is rigidly mounted, as you increase the shaft pressure you've increased ability to drive. Until you start bending the shaft, cutting into the urethane, or overheating it you can increase the vehicle load. You could certainly carry a full-grown adult at speeds up to the motor wattage limit, even up an incline, but, drive wheel might not Make a whole trip. (5mm shaft)

My design envelope includes a push-off, and you think that's a dealbreaker? OK.

If I had a reason to make this, that would be great, but I don't fit in this design envelope. I know I'm repeating myself there.

Wee, you held the motor in your hand, and you call that the best connection you can have, lol. Do I need to repeat what desirable duro you would use for friction drive? Because I made it clear what it would need to be.

Astillwell, not everything scales, tricycles are direct drive exclusively. Does that mean all bicycles should be direct drive? Or tricycles don't actually work, because bicycles do work and they all have gears.

You're making an equivalence where it doesn't exist. No one has put together a design to fit this guy's budget, and I did. Is it better than other ways of doing things? Well, in that it fit the exact design envelope, it's better. Pretty sure I mentioned the downsides.

If the forum wants to only copy proven designs, I totally approve of that. I don't appreciate it if somebody doesn't want to understand there is data that tells you a polished Steel shaft will drive a urethane wheel, where a kid can cruise around his neighborhood at above jogging pace. Because, and I'm repeating myself, this only takes a few tens of watts.

This is a question purely of surface area. Notice in the picture of all those urethane rollers that none of them are tiny? Usually those rollers are pressed (rather forcefully) against another large roller or a flat surface, resulting in a large contact patch. A 2mm (or even 5mm) shaft is going to have a contact patch in the neighborhood of just a few sq milometers.

I'd wager that this setup would have enough friction to maintain, once the board has been bodily kicked up to speed. It certainly won't take you up a hill.

I saw you throw out 8-10mph earlier. When I am strolling down the street on my (unpowered) longboard, 10-12mph is a pretty casual pace. 15-18mph is probably pretty standard. I can push around 22-23mph on smooth flat ground, and regularly reach upper 20s with some minor incline. If I am really going for it, I hit 40mph on the regular, and top out at about 48-50mph bombing hills. I don't say this to belittle your project, but to put the speed into perspective. I'm certainly not the fastest guy, and there are plenty of grade schoolers who can blow me out of the water.

 

[MagiicHat]

Just trying to help by giving you a baseline. I know I'm bigger, but I've been awfully impressed by some rugrats.

 

[NyOliver]

Ok, would love to see video of you pushing at 23mph for even 3 min, You lost all credibility with me on that.

8-10 mph measured MPH is squarely in the design envelope range of scooters, hoverboards, under $400. ( technically most of those are listed at 13 and over as well. )

And it's actually feels very fast on a penny board (something I am repeating), but maybe you know 10-year-olds on penny boards, pushing at 20+ mph.

 

[WeeChumlee]

Well, lets just be happy that the OP is not going to try the possibility and decided on gears.
We are pretty sure it won't work as described and NyOliver "doesn't have a reason to make it" to try and prove it works - so the argument is pretty moot.
I could use my longboard wheels and press it against them. and yes with my hand I can exceed the axial load of the bearings on the mentioned motors - very easily actually, it will not improve the result much.
And then there is the question about braking - well you won't have any.
And you ride over a damp leaf, wheel gets moist - hmm polished metal against urethane. (Oh I forget, kids would never do that and only ride in perfect dry and dust free conditions)
The idea is bullshit. It was bullshit from the start but I wanted to take it in a more lighthearted way. That Ny keeps pushing a point that is just not feasible, with a lot of wind, zero practical backup but with lots of patronizing attitude makes me wonder why.

 

[NyOliver]

I do agree that, talk is so much wind, real world results is where the urethane hits the road.

I'll could make it, why don't you give me some incentive?

You name the parameter of a design test, I'll let you know, if I think i can do it, and my small fee for the effort and video demo.

You're either someone with zero engineering sense, or far better sense than me, so send me (or escrow) a few bucks and At minimum it will be entertaining.

By few, I mean less than 50USD.

Well, lets just be happy that the OP is not going to try the possibility and decided on gears.
We are pretty sure it won't work as described and NyOliver "doesn't have a reason to make it" to try and prove it works - so the argument is pretty moot.
I could use my longboard wheels and press it against them. and yes with my hand I can exceed the axial load of the bearings on the mentioned motors - very easily actually, it will not improve the result much.
And then there is the question about braking - well you won't have any.
And you ride over a damp leaf, wheel gets moist - hmm polished metal against urethane. (Oh I forget, kids would never do that and only ride in perfect dry and dust free conditions)
The idea is bullshit. It was bullshit from the start but I wanted to take it in a more lighthearted way. That Ny keeps pushing a point that is just not feasible, with a lot of wind, zero practical backup but with lots of patronizing attitude makes me wonder why.

 

[NyOliver]

Hmm, where did I say I had no experience?

Here.

Hi
A nice polished shaft on Urethane is not going to give any traction. It will, I imagine as I have obviously never tried this, spin (very high KV motor) like crazy, get warm and melt the urethane. (I know there are direct drive, basically clip-on modules out there - if you are going this route get them...

I showed the data where a longboard wheel has good characteristics with polished steel also. The coefficient of friction is a known and subject to dispute.

 

[silviasol]

In the spirit of christmas I could send you a vesc for $50 plus shipping. That is cheaper then the cost of the components but I have a few extra that I am not using.

 

[markz]

They are in their early teens so I am skipping this as a Christmas gift, instead bought some Nerf guns and making a midway style target, plus PS4 and games.

 

[made_in_the_alps_legacy]

u mean this Christmas ? you might be interested in the one I built, link to it in my signature