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  1. #1
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    Front vs Rear-Wheel Drive: Tradeoffs?

    Speaking as somebody with zero experience, two come to mind:

    - Drive train compatability: No issue with front, cassette-vs-threaded issues w/Rear.
    Also, 9-speed click shift on existing bike is going to have tb replaced w/Rear.

    - Traction: Going uphill, my front wheel tends to get really light.

    Are either of these "Real-World"?

    Any others?

  2. #2
    put our Heads Together cerewa's Avatar
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    Quote Originally Posted by PeteCress View Post
    Speaking as somebody with zero experience, two come to mind:

    - Drive train compatability: No issue with front, cassette-vs-threaded issues w/Rear.
    Also, 9-speed click shift on existing bike is going to have tb replaced w/Rear.

    - Traction: Going uphill, my front wheel tends to get really light.

    Are either of these "Real-World"?

    Any others?
    Regarding traction: it should never be a problem on paved roads except in extreme cases. Like snow and ice. (Or, uphill with a layer of wet leaves on top of pavement, uphill with substantial layer of sand on top of pavement, or a steep slope that a car is barely capable of climbing.)

    Most rear hub motors require you to use a 7 speed freewheel (or fewer speeds) (freewheel is not quite the same as a cassette). 9 speed shifters generally don't work right on a 7 speed cassette.

    Another significant thing to think about is balance. A bike with batteries on rear and motor on front will balance fairly well. If you put batteries and motor in the back, the bike will feel off-balance (unless you spend a little while getting used to it, and you're a person who has fairly good balance to begin with).
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  3. #3
    Senior Member nwmtnbkr's Avatar
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    Quote Originally Posted by PeteCress View Post
    Speaking as somebody with zero experience, two come to mind:

    - Drive train compatability: No issue with front, cassette-vs-threaded issues w/Rear.
    Also, 9-speed click shift on existing bike is going to have tb replaced w/Rear.

    - Traction: Going uphill, my front wheel tends to get really light.

    Are either of these "Real-World"?

    Any others?
    Safety is the biggest issue. Hub motors cannot be placed on aluminum or suspension forks because they have cast drop outs that fail without notice (due to the affects of torque), causing loss of control and serious injury. Torque arms won't help stop such failures.



    Steel drop outs can fail to over time (the more torque the motor generates, the more quickly the fork will be stressed). You need to inspect them frequently. You need torque arms on front or rear installations. There is a vigorous debate in the Endless-Sphere e-bike forums about whether any hub motor is safe. If you choose to install a powerful hub motor, you'd be much better off going with a rear wheel installation on a steel bike. That said, you can still have frame issues develop due to high torque.

    There's a reason that Congress mandated that ready-made e-bikes that can travel over 20 MPH and/or have electric motors over 750W must comply with moped and motorcycle safety standards set by the National Highway Transportation Safety Administration rather than the safety standards for consumer bicycles set by the Consumer Product Safety Administration. The National Highway Transportation Safety Administration's standards for mopeds and motorcycles address issues of the affects of torque and mandate safe frames, good brakes, etc., as well as mandating other safety features such as lights (headlights, turn signals, brake lights) and horns. At present, conversion kits aren't caught up in the safety standards issues. However, as more 1000W (and higher) hub motors get sold online I look for federal regulators to seek legislation that will prohibit the sales of these motors for use on electric bicycles.

  4. #4
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    Quote Originally Posted by nwmtnbkr View Post
    Safety is the biggest issue. Hub motors cannot be placed on aluminum or suspension forks because they have cast drop outs that fail without notice (due to the affects of torque), causing loss of control and serious injury. Torque arms won't help stop such failures.
    That one's been bugging me since the day I started looking into eBikes and realized that they transfer the torque by that little spline thingie against the dropout slot itself.

    Seems like a fatal design flaw to me. I've got Rohloff hubs on two of my bikes and they do it right: no spline, but a torque arm connected to the chain stay. I weigh over 200# and have hammered those two bikes pretty hard with no hint of failure.

    I'm no engineer, but it seems like eBIke hubs sb set up to use dual torque arms and not load up the dropout's slot.

    Awhile after posting what's above, I stumbled on what seemed at first (to my uneducated eye) tb a pretty good discussion of the tradeoffs: http://tinyurl.com/y9vjjbo

    But a quick read indicates it says nothing... nada... zip... zilch.....bupkis about the obvious safety issue around fork integrity.
    Last edited by PeteCress; 12-14-09 at 06:07 PM.

  5. #5
    Senior Member Snowsurfer's Avatar
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    Quote Originally Posted by PeteCress View Post
    That one's been bugging me since the day I started looking into eBikes and realized that they transfer the torque by that little spline thingie against the dropout slot itself.

    Seems like a fatal design flaw to me. I've got Rohloff hubs on two of my bikes and they do it right: no spline, but a torque arm connected to the chain stay. I weigh over 200# and have hammered those two bikes pretty hard with no hint of failure.

    I'm no engineer, but it seems like eBIke hubs sb set up to use dual torque arms and not load up the dropout's slot.

    Awhile after posting what's above, I stumbled on what seemed at first (to my uneducated eye) tb a pretty good discussion of the tradeoffs: http://tinyurl.com/y9vjjbo

    But a quick read indicates it says nothing... nada... zip... zilch.....bupkis about the obvious safety issue around fork integrity.
    That article does seem biased. Like he was arguing something. Look at how long the write-up for Front-Pros is.
    Rear-mount is safer. The drop-outs are much more resistant to failure. But have two torque arms.

  6. #6
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    Quote Originally Posted by cerewa View Post
    Another significant thing to think about is balance. A bike with batteries on rear and motor on front will balance fairly well. If you put batteries and motor in the back, the bike will feel off-balance (unless you spend a little while getting used to it, and you're a person who has fairly good balance to begin with).
    For better or for worse, I went with the front-wheel drive.

    Found a deal on Surly's 1x1 singlespeed frame ($265 w/fork, tb delivered on 12/22).

    My plan is to do a boltover from a Rholoff-geared bike whose frame does not fit me very well to the Surly.

  7. #7
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    I may be well off the mark here, but...

    Doesn't a front hub motor drive put more lateral stresses onto the headset bearings (other than the normal instantaneous braking stresses)? Wouldn't the bearings be designed more with continuous longitudinal (vertical) forces in mind, rather than continuous — even light — lateral (horizontal) forces?

    I imagine that the front fork sort of ends up "pulling" the bike and its rider's mass along. Isn't that vaguely similar to the reasoning behind not using rear-drive hubs on bikes with rear suspension? In other words, don't you need the structural rigidity of the rear triangle to resist transmitting the motor's torque to other "movable" parts of the frame. A bit like anti-tramp bars on a car's rear axle maybe?

    When you think about it, the bike's headset has to resist turning around its short axis — with a front drive — via the bearings. Extra wear?

    Another point that may be relevant is that rear wheels always tend to lock up first due to shifting mass distribution under heavy braking. With my bike and its (heavy) rear hub/rear battery positioning, this tends to be alleviated a bit. Hence somewhat better braking... well, in theory at least LOL.

    Personally, I'm happier with the rear-wheel drive.


  8. #8
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    Good point by ausGeoff. I would add one thing that I read (perhaps on endless-sphere?) ... what if the magnets fail? You could experience an immediate lock-up of the wheel. I'd much rather that happen to a rear and not the front wheel.
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  9. #9
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    Quote Originally Posted by lucasgo View Post
    Good point by ausGeoff. I would add one thing that I read (perhaps on endless-sphere?) ... what if the magnets fail? You could experience an immediate lock-up of the wheel. I'd much rather that happen to a rear and not the front wheel.
    Ouch!... I have no clue what magnet failure is, but that sounds like a deal-breaker to me: face-plants at 20+ mph can be fatal.

    Meanwhile, after a day of riding around on my new FWD, I've found another con for FWD: it compromises the ability to lift the front wheel.

    Mine, at least, is *heavy*... think 20 lbs.... When ascending a curb, one needs to do a little bit of a wheelie to avoid hitting the curb. When descending, same thing: one needs to raise the wheel enough so that the bike goes off the curb flat and lands either both wheels simultaneously, or rear-wheel-first.

  10. #10
    Senior Member Snowsurfer's Avatar
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    Quote Originally Posted by PeteCress View Post
    Ouch!... I have no clue what magnet failure is, but that sounds like a deal-breaker to me: face-plants at 20+ mph can be fatal.

    Meanwhile, after a day of riding around on my new FWD, I've found another con for FWD: it compromises the ability to lift the front wheel.

    Mine, at least, is *heavy*... think 20 lbs.... When ascending a curb, one needs to do a little bit of a wheelie to avoid hitting the curb. When descending, same thing: one needs to raise the wheel enough so that the bike goes off the curb flat and lands either both wheels simultaneously, or rear-wheel-first.
    If you have a front hub seizure, the results are far more dramatic. You get bucked off by your bike on top of the sudden deceleration. While on the rear, there is far less bucking force that throws you off.
    Also, if the rear has the hub, the back is heavier, which makes it even more resistant to bucking.

    Hub motors work by alternating push - pull magnets. If a magnet is pushing while it should be pushing, it can cause seizure. This isn't failure due to the magnet itself, but the electrical energy adjusting the magnet's polarity. (+) charges repel other like (+) charges, and pull in (-).

    Magnets can also become detached if you run them at high enough voltage and current. Total power. (P=VI)

    The glue can melt and the magnets come loose. It has happened to a guy who ran 72V, but it was an Aussie summer. I would advise for a rear mount, not trying to be preachy.

  11. #11
    xtrajack xtrajack's Avatar
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    I am running a Xtracycle. I went with a front wheel motor, I have the the battery in front as well. I figured with the weight I would have in the back, being a cargo bike, that some weight in the front wouldn't be a bad thing.. This seems to be the case, as I haven't had any handling issues.

    I also thought that having two wheel drive in the winter would be a good thing. Unfortunately, at this time I haven't had an opportunity to verify this thought/theory. Some yo-yo, who shall remain nameless, managed to poke a hole in one of the cells in my battery, and I haven't got it back yet.

    I will admit that I permaybehaps didn't do as much research as I should have before buying my motor. I had never even heard of torque arms before I started reading this forum. I have ordered one which will be shipped with my battery when I get it back.

    I am not seriously worried about catastrophic wheel/motor/fork failure for a few reasons:

    1. My average speed is around 10mph. Sometimes I go faster, but, as a general rule I am not a fast rider.
    2. My bike weighs enough (~112 w/work stuff) that it would be very hard to get the rear high enough to throw me over the bars. I would probably wind up going over sideways as opposed to over the bars. I could be wrong and if and/or when it happens we will find out.
    3. I also wear a helmet. I figure it should help minimize the damage if I do manage to go over the bars.
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  12. #12
    Senior Member Snowsurfer's Avatar
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    FWIW...THE Doctorbass, the one and only, runs a rear motor on his Drag Set-up.

  13. #13
    Senior Member Snowsurfer's Avatar
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    Peter.

    I just wanted to clarify more.

    Doctorbass explicitly discourages front conversions.
    Also, Dogman explicitly discourages front conversions.

  14. #14
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    I have no problem with front hub motors as long as forks are steel and 'don't run much over 20A controller. Under those parameters - a front flat is just about as likely as a magnet failure. I'm much more nervous about 300-500lbs sitting over a laced rear wheel.

  15. #15
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    Quote Originally Posted by Snowsurfer View Post
    Peter.

    I just wanted to clarify more.

    Doctorbass explicitly discourages front conversions.
    Also, Dogman explicitly discourages front conversions.
    Point taken.

    My plan is to keep fooling around with the FWD that I now have until I get a chance to assess the noise factor of the eZee motors firsthand.

    Once I've done that, and I haven't killed or maimed myself along the way, I'll get a RWD motor - either sticking with 9C or, if it's not too noisy, get an eZee motor.

    I had my first FWD issue a few days ago. Being clueless about the relationship between rim width and tire size, I mounted one of my 55/55 Mutano Raptors on the 19mm el-cheapo Brand-X rim that came with the 9C. All my MTB rims are about 19mm and I never, ever had a problem. This one, however, seems tb somehow smaller bc the 55/55 went on too easily.... and peeled off a few days later at (thank G_d) walking speed. Would have been a fatal or serious injury descending a hill at 20 mph...

    For the RWD bike, I will need to get another frame. All my bikes' rear drops are alu except for a Surly 1x1 - but that one lacks a der hanger.

    One of Electrabike's cruisers is on my short list: the plan being to have a "Shore Bike" ("Beach Bike" for Left Coasters...) that's also my eBike.

    Lacking that, maybe an el-cheapo used MTB. I'm 6'5", so frame size is a factor....
    Last edited by PeteCress; 12-30-09 at 02:58 PM.

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