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Thread: Yellow GPR-S rehab

              
   
   
  1. #1
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    Yellow GPR-S rehab

    Now that my original GPR-S is back up & running, I'm focusing on the yellow GPR-S that I picked up from Jeff. It has an Etek/Mars motor, Alltrax controller and EIG cells. It ran well initially but over time developed some issues. The highlights in chronological order:

    1. Periodic motor squeak, noticeable at low speeds.
    2. Chain jumping off the sprocket, occurring more & more frequently.
    3. Power rapidly cutting in & out at lower speeds.

    The motor squeak was just a nuisance. I investigated it and put some graphite powder in the brush holders. That seemed to remedy it for the time being.

    The chain jumping happened rarely at first but towards the ends was a frequent occurrence, especially if I accelerated through a turn. Putting the bike on a stand and freewheeling the rear wheel, it looks like one of sprockets has warped. The chain shifts side to side during a rotation of the wheel. Thinking back on it, I may have warped the rear sprocket when I gunned it turning out into traffic from a stop. The horse nearly threw me off the seat.

    During the last few months of riding, it developed a strange power pulsing problem. At lower throttle positions and lower speeds, the power would pulse at around 3-5 Hz. My best description of it is a really poorly implemented PWM. I replaced the brushes for about $100 to no avail. The motor felt stronger but the pulsing remained.

    Initially I focused on the motor. After thinking about it some more, I'm starting suspect that maybe the Alltrax controller is at fault. I don't know how to test each component in isolation without buying new parts and swapping out. A bit of an expensive way to test. Any recommendations how to proceed? Here's what I'm looking at:

    • Diagnose motor, controller, chain & sprockets. Unknown methodology to me.
    • Toss the whole shebang in the trash and switch to a hub motor. I'm really sick of that #@$! chain. What's a reasonably priced hub motor that can do 65 mph @ 80v?

  2. #2
    teddillard
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    yellow = GOOD!

    (photos?)

  3. #3
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    Quote Originally Posted by chef View Post
    During the last few months of riding, it developed a strange power pulsing problem. At lower throttle positions and lower speeds, the power would pulse at around 3-5 Hz. My best description of it is a really poorly implemented PWM. I replaced the brushes for about $100 to no avail. The motor felt stronger but the pulsing remained.

    Initially I focused on the motor. After thinking about it some more, I'm starting suspect that maybe the Alltrax controller is at fault. I don't know how to test each component in isolation without buying new parts and swapping out. A bit of an expensive way to test. Any recommendations how to proceed?
    Have you tried changing the throttle response curve?
    "obstacles suck but challenges are fun"

  4. #4
    Seņor Member podolefsky's Avatar
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    Re: Yellow GPR-S rehab

    New sprocket is a lot cheaper than a new motor. Have you checked that the sprocket is true? Could also be that the sprocket carrier is misaligned.

    Pulsing could be the throttle. If its a Magura, they tend to get flaky after a while. Test it with an ohmmeter, see if its smooth from 0-5k ohm, and if you can hold it steady in one place at low throttle.

    Pulsing could be related to the chain. Does the pulsing change with speed?

    There are ways to check for good PWM if you have an oscilloscope.

    Could also be a messed up commutator.
    - Noah Podolefsky -
    The GSX-E

  5. #5
    teddillard
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    As far as the sprocket/chain thing goes, my money is on them being out of alignment, because I've had that happen... I've never had a worn front or rear throw a chain, and I've never had a chain that was frozen throw itself. (Break, yes, but throw itself, no.) I've never had a loose chain throw itself. I've also never had a sprocket warp, other than one that got (literally) hammered on.

    Even this sprocket did not throw the chain:


    It's not a case of the alignment being "close". It has to be aligned.

    If they are perfectly aligned, then just replace all three. (I always replace the entire set together anyway, so they have equal wear... but some claim it's only necessary to replace the sprockets every other time.)

    On the pulsing, I'd go after the throttle first. Can you swap out the one from your other bike to test it? Just sidle them up together and move the wires?
    Last edited by teddillard; 07 April 2013 at 0525.

  6. #6
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    Thanks for the ideas. The throttle is a Longma Hall-Effect inductive throttle. I replaced it for an unrelated reason... the throttle wire was crushed due to an unfortunate routing of it above a bar. Both the old and new throttle had the pulsing problem. I planned on reading the controller settings this weekend but other tasks have eaten up all my time. I'll report back when I can carve out some time for that.

    Regarding the chain & sprocket alignment: Based on Ted's feedback, it looks like it's time to replace the set. I've tried tweaking the left & right position of the rear axle for alignment purposes. No matter where I align it or what tension I set, I can see the chain shifting as the wheel spins.

  7. #7
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    Perhaps the axle is not parallel with the motor shaft? I have seen lots of axles shift in the swingarm/chain adjuster brackets when tightened. One thing is for sure, you can not depend upon the markings on the swingarm. It would be better to verify that the motor shaft is parallel to the swingarm pivot and then measure carefully between the center of the swingarm bolt and the center of the axle shaft to be sure that everything is properly aligned when you adjust the chain or tighten the axle nut. Motion Pro makes a chain alignment tool that you sight down to verify by eye that the chain is properly lined up between the sprockets. The Motion Pro tool uses a rod to check this alignment. There are also tools that use a laser to perform the same task. But I find that measuring between the two shafts works just as well - assuming that you have clear access to both ends of the swing arm pivot and the axle bolt.
    Richard - Current bikes: 2018 16.6 kWh Zero S, 2016 BMW R1200RS, 2011 Royal Enfield 500, 2009 BMW F650GS, 2005 Triumph T-100 Bonneville, 2002 Yamaha FZ1 (FZS1000N) and a 1978 Honda Kick 'N Go Senior.

  8. #8
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    Quote Originally Posted by chef View Post
    No matter where I align it or what tension I set, I can see the chain shifting as the wheel spins.
    Are you sure the motor is mounted securely enough to prevent misalignment from chain tension? Watch or put a dial gauge on the brush end of the motor when you throttle-up to check for any movement. Also, it's best to mount sprockets as close to the motor as possible to prevent excess overhanging loads and misalignment of the motor shaft. If you can, post some pics or a video.

  9. #9
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    Here's a less than flattering shot of the bike in my cluttered garage--
    YellowGPRS.jpg

    The motor is mounted securely as far as I can see. Not sure how well it's aligned though.
    P1100333.jpg

    I forgot to mention that the motor sprocket has a habit of working loose. I have to re-tighten the hex nuts regularly, even after apply locktite. I suspect that the chain wobble is working the nuts loose.

    Here's a video of the motor free-spinning.


    After reviewing my notes and previous threads, I need to troubleshoot a bunch of things that I put off since last summer.

  10. #10
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    Off-topic: This video showed up when I finished watching mine above. The guy made his own controller and tries to stop the motor with just a glove on I'm impressed by his circuit design and implementation prowess.

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