adrian_sm wrote:When I say via geometry, I don't mean locking the drive to a position, and hence fixed losses.
I am still talking about a free to pivot progressive contact pressure just like your design, with the motor torque dicatating the contact pressure. I just want to limit the maximum engagement via a deadstop so I can stop too much contact pressure. I think you acheive the same thing but by using a soft-stop, ie. your foam dampener. This applies more and more reaction force as the drive engages, until a balance is found. I was think of a more defined stop to limit excess contact pressure, and hence losses.
EVTodd wrote:I've been a big proponent of variable pressure friction drive but honestly... I recently made a plain ol' friction drive with no movement for my beater mountain bike and it works great. I think maybe we're all over thinking this stuff.
Sure, the variable pressure systems do help with making the motor more efficient but I'm starting to doubt that it's by a huge margin. Now that I have a watt meter I'll have to do some testing on that theory.
My current opinion (and keep in mind that this changes day by day ) is that for lower power systems you simply don't need a mount that moves. It's really not that hard to get the correct pressure on the roller. Yes, you'll need a clutch bearing in the roller, but man, it's so much easier.
If I wasn't afraid of blowing up an esc I wouldn't even use a clutch bearing. A smaller rc motor doesn't put much drag on the tire at all.
Kepler wrote:Problem is without a damper, the motor will typically go to the stop when put under load. Because of the length of the pivot, you wont be able to control this no matter how much you play with the geometry, or at least I never could. It will either be on or off. The damper solved this issue.
Kepler wrote:I think you have a fair point in relaton to over complication but I suppose it depends on what different people find complicated. I personally find the process of coupling up a roller to a motor just as complicated as making a swing arm for a direct drive setup.
adrian_sm wrote:I would be really interested to see what sort of drag your "plain ol' friction drive" has. If it spins down faster than a hub, then personally I would probably just stick with a light weight hub instead. The fact that the variable pressure systems can totally disengage is the only reason I was keen to try it, and the only reason I would put up with some of the down sides of friction drive.
EVTodd wrote:You have a good point about being able to totally disengage the motor. I think you missed the part of my post that said I'm still using a clutch bearing though, so the "plain ol'" version freewheels anyway. I have tried it without and the drag really isn't bad but it would depend on which motor you're using. Never had a hub motor so I can't compare the two.
Don't get me wrong. I'm still using the sliding mount on my main bike. That roller would slip like crazy if I didn't. I'm just talking about low power "assist" type systems.
I think your drive is turning out great btw.
Thanks for the pics. I love how simple these friction drives can be.
Have you done the spin down test on the "plain ol" freewheeling version? So lift the rear wheel, go full throttle, then see how long it takes for the wheel to come to rest. This for me is a quick and dirty test for how much drag the system has.
For example my crystalyte 408 hub motor takes about 8s.
EVTodd wrote:I know that with the clutch bearing it takes a lot longer than 8 seconds to stop spinning.
drifter wrote:Adrian, try this.
Solcar wrote:Excellent analysis. One thing I am familiar with is "Solution 1b: current limiting controller, so I can drop the power but not the top speed." Well said. My present homemade controller is this way and it's great. I have yet to get my friction drive completed and have only used this type of controller on a through-the-gears drive.
Solcar wrote:Excellent analysis. One thing I am familiar with is "Solution 1b: current limiting controller, so I can drop the power but not the top speed." Well said.
umejopa wrote:I have now make a controll loop in the wattmeter that take down speed if current go to high only thing can not test it on E-bike before the snow is gone ......... I now check current and adjust the speed evry 20mS
jbond wrote:How about using the CA to do this. Modify the CA to have a variable current limit on a pot. Then have the CA control the ESC throttle to keep the power below the current limit.
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