Why not friction drive?

[etard]

Well I think that Holy Roller is too wide for my rear triangle, so I think I will try these:

But these look interesting too:

But they only come in 700 size

I hope they are a hard enough compound while still interlocking.

D,
You are right about the soft compound, I will seek out the hardest compound that fits my requirements while still conforming to my needs. I guess I will have to make the trek out to the Jenson bicycle store locally and match it up.

 

[deecanio]

Hi etard,

no worries, im guessing you're after 26" then - i asked as i have 2 brand new holy rollers in 20" which i would have sent you if you needed them.

Cheers,


D

 

[vanilla ice]

I wonder if you couldn't carefully wrap the hub with a section of the tire tread, and shoot in some hard urethane casting compound. You would have a perfect match up. I would imagine the angles at which the knobs ramp up from the tire are going to be important. Each tire probably will work best in a narrow hub diameter range.

If you make sure to use an inexpensive easy to find tire.. your worries about wear go away, just swap tires.

 

[etard]

Thanks for the offer D, but I do need 26".

Vanilla, casting compound might be the way to go, do you know the strength of this material? I will have to search it up.

Well, my order from Mcmaster arrived FAST, like yesterday. So here is a more complete picture of my setup.

Sorry for the vision impaired, I took these with my phone. Anyway, you can see that I might have to machine that motor pulley to fit onto the shaft, unless overhang is ok. If I can, I will tap out a hole in the teeth for (2) 90 degree keyways. I really need a longer shaft ( ) so I will look into that, I would really like to have a support bearing on the outside. My biggest concern right now is the strength of that snap ring to keep the SRAM (screwed onto the 18 tooth sprocket)mounted pulley on the hub. I will also have to drill those steel pulleys out as much as possible to keep the weight down.

 

[Grinhill]

Hi guys, some great ideas on this thread.

Here's a few more ideas to add for the dual-roller sandwich style on the sidewall (French system discussed early in this thread):

1. What about a motor on each roller, but have two different diameter rollers (each with a one-way bearing). You would also need dual ESC and dual throttles, but you would effectively have a two-speed system, depending on which of the two motors was switched on. Obviously one motor would be set up to run in reverse. The non-driven roller would provide a squeeze to maintain good contact.

2. You could instead set up a two-speed system using a single motor by having a belt or chain between the rollers, and using forward and reverse to use the two gears. I haven't thought this one through completely yet, does anyone see a problem? It's a similar concept to the retro-direct (see Miles 3D pdf) system with double-sided belt (link?) but only requires one belt.

I'm not quite sure how the reversing would work, do you just need a heavy-duty switch to swap two motor wires, being careful that motor isn't spinning?

 

[gogo]

@Grinhill
I think the French system drove against the wheel rim and not the sidewall of the tire? This would not affect your ideas, I'm just clarifying.

I like your idea #1. I have a double sidewall pinch drive with two motors. The only potential shortcomings might be that by using only one roller for drive at high speed, you would need more pinch pressure on the sidewalls than if you were driving with both. Also, you have the extra weight of the motor you aren't using. On my setup that's 10 pounds!

I also like your idea #2. In fact, that is quite an ingenious implementation of retro-direct drive. Certainly one worth testing, especially since it wouldn't need any reduction stages. I just wonder if the chain speed might be a limiting factor that would make it more practical on a 20" tire, and/or with a lower RPM motor with larger drive rollers.

 

[etard]

Somebody was saying there would be alot of inefficiency in the way those rollers are applied to the rim. I think it has to do with the rim always in a turn as opposed to the roller going straight, although now that I think about it.... If the roller was conical and matched to the rim size, problem solved !

Originally, I wanted to do this same thing, but geared by sewing (or glueing) a timing belt to the sidewall of the tire and using a non-flanged timing gear against it. I still don't think its a bad idea, but I have committed to my current design, so I will procede.

Taking Vanilla's advise I have looked into epoxy resin to accomplish a tread pattern on the roller to match the tire. I have also gotten a 3-speed Sturmey Archer hub ($22 on ebay) that I will use as the prototype roller because I don't want to ruin the SRAM. I will be setting up the platform for this drive this week.

 

[EVTodd]

Etard,

I'm really interested to see how the 3 speed hub as a roller idea turns out. It's nice to see someone coming up with some good friction drive ideas.

I do, however, have my doubts about being able to glue tire tread to the hub and I still think a metal knurled roller is plenty good (for me anyway) unless you're in the rain. But if it works, great, I'll be the next one trying it .

Anyway... I'm just starting to piece my next friction drive project together myself. I'm looking forward to comparing notes when we're done.

 

[etard]

Todd,

I thought I lost you somewhere back there, I have been wondering how you are proceding. It's nice to find someone else doing the friction thing too, when it seems so rarely executed.

So I was brainstorming on securing the tread to the tire and it all became obvious!!

1. Cut out tire to match the 3 speed hub.

Here is where it gets interesting...

2. a) Lace tennis racket string through spoke hole, into the sidewall of cut up tire, routed underneath tire tread contact patch, and out the other side to spoke hole, tension, and repeat 36 times.
b) Use long thin screws, to accomplish the same thing.

Will tennis string be strong enough? Will the flex be good or bad? Dumb idea

 

[Grinhill]

Etard
Sounds like a fiddly job with the string, but best option since screws near tyre sounds like a bad idea. You would probably need some epoxy as well to stop it wriggling around.

You could also add some inch-wide rubber bands around the hub either side of the contact patch to help secure it - slice up an old inner tube!

 

[vanilla ice]

You can get urethane compounds in a wide range of hardnesses. I bet theres something that would work out. I would try cutting out a section of which ever tire pattern you will use, secured with treadside facing the hub surface, oh, spray tire with release compound, contain the urethane in between the tread and hub somehow until it sets up. I've not worked with the stuff before so I can't give any more detailed instructions unfortunately. Its used for skateboard wheels so its pretty durable stuff, but still with some give unlike aluminum.