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Front Hub Motors Dangerous?

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  • ynot
    replied
    Spent yesterday morning machining the dropouts for the freighter trike, machined them in a stack so all four are identical, in a fit of utter stupidity made them too sloppy. Oh well they only took me about 4 hours to make & I have four more I can cut today.

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  • Tommycat
    commented on 's reply
    Don't have specific torque specs, but very pleased with its performance. Gets me where I want to go!
    Wish I could join in on the hill climbing trials. ;-) Great to hear that your wife is having fun, and your weather is getting warm enough to ride!
    Cheers.

  • ynot
    replied
    Thanks for that TC, pleased that you are getting into this, So you tightened to 70nm +/- bet your motor at 750 watts is putting out more than 70 mn,

    An O/T, Favorite wife was out yesterday on her trike, using the help you gave she is starting to tune the parameters and is closing in on getting to where she is most comfortable, and is enjoying herself. Looking fwd to taking a few rides together.
    Cheers,

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  • Tommycat
    replied
    Just from my experience with keeping the motor axle in place...

    Rear mounted, direct drive, 1000w (derated to 750 watts) hub motor in a steel frame, and at the time using excessive regeneration braking. Not enough to throw you over the handlebars, but enough that you would want to brace for it. Turned it down to less than half now.


    Having nice axle shoulder nuts with the serrated flat side that goes toward the dropout locking the nut in place. Having them securely torqued down at 50 ft/lbs, which seemed the maximum I wanted to go. What with the percentage of threads removed for the flats... And with the anti-rotation washers with the tab that sits in the dropout. I felt confident that all would be well. About a week later I removed the motor assembly for other reasons and spied this.




    An inboard spacing washer that showed signs of the axle rocking back and forth. :-( Now the nuts were still tight to specs, and that axle was STILL moving!

    Now the axle flats seemed to fit nice and tight...




    So at first I tried a store bought type.



    But with it's gear type design that allowed for different axle orientations, which was sloppy. And something about a worm gear clamp around a frame member just turns me off.

    So the "Prowler" TA was manufactured and installed. ;-) And added jam nuts for fun.


    Bottom line point. With my axle nuts securely held in place, the axle was still rotating. Enough to cause damage in the future...? Perhaps not, but I feel better about it now.
    Could one have increased the nut torque? Perhaps, but the last thing I wanted to do was strip the axle or nut threads...

    Now I know we are not "apples to apples", but just wanted you to have another data point.


    Difficult to offer possible helpful suggestions without "seeing" your installation. Such as flattening the axle on the inboard side and putting a TA there (no room?), or perhaps "clamping" the existing dropout tight to the axle...?

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  • ynot
    replied
    My misteak <sic> the calipers take the load not the spindle.

    In the absence of regen.
    Last edited by ynot; 03-15-2022, 12:39 PM.

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  • AZguy
    commented on 's reply
    LOL - not trying to pummel anyone... just clarifying ;-}

    As another point the permanent magnet brushless DC motor controllers can deliver more current to the motor windings than they draw from the battery so they can provide more torque at lower speeds since as the motor speed gets higher how much more current they can deliver drops an will have a direct relationship with the motor rpm and the maximum motor rpm at full voltage... for a DD hub this isn't likely to happen until you have a really big head of steam going... geared hubs will often be speed limited by the battery voltage and the max torque may drop off more with speed (depends on the controller) but they will still be torque limited by the max output current the controller can deliver at a standstill or very low speeds and most controllers on the geared hubs I've been on were without hall sensors and have a hard time delivering much current at these low speeds for rather anemic torque at a standstill.. And there I go digressing again since a front hub is most likely a DD motor and it will really come down to the controller at this point - some DD controllers can deliver a *lot* of current at stop/low-speed however...

  • AltaBrad
    commented on 's reply
    Excellent rebuttal! I'm Liston on the ropes, you be Clay pummelling the crap outta me :) Time out while I regroup, not easy after 68 trips around the sun, you at 78 (respect), know what I mean. But don't even think I'm reaching for the towel, nope not yet.

  • AZguy
    commented on 's reply
    There's a big difference between accelerating and braking with a hub motor in the forces on the axle when that energy is transferred to/from the bike

    During acceleration there will be a force applied as a torque to the axle and that torque will also create a resultant lateral force in the direction of travel also applied to the axle... the torque to lateral force will be proportional to the wheel diameter, larger wheels more torque for the same lateral force - the lateral force is what accelerates the bike

    During braking there will be some torque from regen if the motor has it and the rest will be a lateral force opposite the direction of travel - if no regen then no torque

  • ynot
    replied
    One other thing we have not evaluated is the force of braking on the spindle, in theory (that word again) if it takes 50mn to get you and the bike up to speed, it is going to take the same amount of energy passed through the spindle to stop

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  • AZguy
    commented on 's reply
    FWIW permanent magnet brushless DC motors don't have infinite torque at startup... not even close from an "ideal" perspective... the torque is limited at all times by the amount of current the controller can supply to the motor windings and so max torque is relatively flat throughout most of the RPM curve and is actually lower at startup (and at higher speeds)

    For super high startup torque a series DC motor (the magnets are replaced by field windings that are in series with the rotor wingdings) is going to win that battle and while it is limited by the current the controller can deliver unlike permanent magnet DC motors where torque vs. winding current is relatively close to proportional across the speed range the series DC motor torque to winding current is greatest at zero speed and drops of as speed increases - this is what they use in electric dragsters and other high startup torque applications like train locomotives... but I digress...

  • AltaBrad
    commented on 's reply
    Warning: never weld a shock lower unless you know what you're doing. If done wrong they can and likely will rupture causing severe injury or death

  • AltaBrad
    commented on 's reply
    I buy nothing but steel lowers. Because I can weld to them with the equipment I have. I'd buy aluminum lowers if I had a nice Tig machine but I don't. I would never buy magnesium lowers, won't even ride 'em. I acquire them from donor bikes and immediately scrap them, don't trust them at all.

  • AltaBrad
    commented on 's reply
    Thank you for not retreating from our debate/discussion. I'll keep this brief and say you understand better than I thought you did. Your years of experience and broad range of experience has served you well.

    Removing the wheel and replacing it with original would be one (but not the only) responsible thing to do. Explaining to buyer what we have been discussing thereby alerting them to the potentials is equally as responsible. The right price and a proper understanding between buyer and seller is the sweetest deal.

  • ynot
    replied
    @AltaBrad:
    Now you have me up way past my bed time thinking about your comment that the hub motor has infinite torque at start up.
    In theory I guess that is true, as the RPM goes down the torque goes up, so at 0 rpm you could have infinite torque.
    I if that were true, in practice you could not stall the motor, if there was a way to hold the bike in space you could spin the planet.
    The zero rpm only stays that way for a split second, as soon as motor begins to spin the torque is reduced.

    Bafang claims a max torque of 45mn for this motor, and recommended wheel nut torque for a 12mm with flats on shaft, range from 60 to 120mn
    In practice then if you secure the nut from spinning by using an external holder, tightened to 60 mn, you have 15 mn plus, for each nut, in friction on the spindle threads, more than you need for stopping spindle rotation, over and above the holding power of the dropouts etc.
    Please shoot me down if wrong, as it is rather important for safety, and I am in no position to do an engineering study.

    PS That no one has reportedly tried it is not part of the equation, if I felt that way would not have applied for and received my patent.

    Leave a comment:


  • Dshue
    replied
    Fork lowers are nearly always magnesium. Not aluminum.

    Leave a comment:

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