Danger of plug braking?

O

OneWayTraffic

100 W
Joined
Aug 4, 2008
Messages
175
Having read the thread about plug braking in front hubs:

http://www.endless-sphere.com/forums/viewtopic.php?f=3&t=8212

And the thread where methods came off a bike which exhibited unexpected regen at 25mph, spining the hub out of his front drops;

I was wondering what the real danger of this is.

Lets assume that you have a hub properly secured with torque arms so the wheel coming loose isn't an issue. Then somehow the phase wires short circuit together turning the front hub into an electromagnetic brake.

How could one calculate the braking force, as a function of speed?
 
Speed = RPM*circumference.
Kv = RPM/V -> V = Kv*RPM= Kv*(speed/circumference)
I = V/R (R being motor resistance) = Kv*(speed/circumference)/R
Torque = k*I = k*Kv*(speed/circumference)/R.
Force = Torque/radius = k*Kv*(speed/circumference)/(R*radius) (radius would correspond to the middle of axle and the end-point of the axle that secures to the drop outs - basically, the axle's radius).

There's a relationship between the motor's voltage-RPM constant (Kv) and torque-current constant(The k) that's eluding me - . If you find that relationship, you can get rid of either k or Kv (R might play a role too). It's explained on one of ebikes.ca's page. So, anyways, it appears that the force during "plug braking" is linear with speed.
 
Lock you front brake while riding along. See how that goes. That's the same effect you would feel.

It looks something like this:

http://www.youtube.com/watch?v=KV50Z6v4teI

I love that this guy reaches up and takes off his helmet in the second before he endos. Outstanding work!
 
Heh Heh, gotta laugh all that much more at that one. You can also test the experience by putting a water bottle between the forks and the front wheel like I did last summer! Anything stops that front wheel and torque arms are not the answer, you will be pointing feet to the sky.
 
liveforphysics said:
Lock you front brake while riding along. See how that goes. That's the same effect you would feel.

It looks something like this:

http://www.youtube.com/watch?v=KV50Z6v4teI

I love that this guy reaches up and takes off his helmet in the second before he endos. Outstanding work!
Click to expand...

That's obviously true if you have a X5 and are travelling fast. But it's also completely untrue if you're travelling at slow speeds. So I wanted to know where the danger area is.
 
dogman said:
Heh Heh, gotta laugh all that much more at that one. You can also test the experience by putting a water bottle between the forks and the front wheel like I did last summer! Anything stops that front wheel and torque arms are not the answer, you will be pointing feet to the sky.
Click to expand...

Well torques arms might save the bike. I can grow new skin and I don't hardly ever use my brain anyway. Won't miss it.

The sad thing is I have a degree in maths, and studied physics for two years. Never did more than touch on electric motors though. It's all black magic to me.
 
If you have the right amount of resistance in the shorting wire, the braking force will not be enough to lock the wheel. It is a function of speed however, so the faster you're going, the more braking force there is.
 
I'm just saying that anything stopping the front wheel will send you flying. The front hub - torque arm issues are only one of the many ways to do the flying W as you see on the video. Near as I can tell, it was the end of the race or ride, and he was taking off the helmet with one hand, braking with the front wheel with the other, and OOPS! He wasn't going very fast but still flew over the bars.

I went flying over the bars last year at about 10 mph and broke both collarbones, and have shoulder joints that may never be the same again. Actually might have been better at a higher speed, then I might have flown further and slid some. As it was I just pounded my head into the curb straight down and got hurt pretty bad. Would have died without the helmet for sure,,,,, at 10 mph! :shock: Never let anthing get in the front wheel.
 
dogman said:
I'm just saying that anything stopping the front wheel will send you flying. The front hub - torque arm issues are only one of the many ways to do the flying W as you see on the video. Near as I can tell, it was the end of the race or ride, and he was taking off the helmet with one hand, braking with the front wheel with the other, and OOPS! He wasn't going very fast but still flew over the bars.

I went flying over the bars last year at about 10 mph and broke both collarbones, and have shoulder joints that may never be the same again. Actually might have been better at a higher speed, then I might have flown further and slid some. As it was I just pounded my head into the curb straight down and got hurt pretty bad. Would have died without the helmet for sure,,,,, at 10 mph! :shock: Never let anthing get in the front wheel.
Click to expand...

Oh agreed and I've been having second thoughts about front hubs for a while now. I don't have any logical reason to prefer them over the back for my kind of riding.
 
fechter said:
If you have the right amount of resistance in the shorting wire, the braking force will not be enough to lock the wheel. It is a function of speed however, so the faster you're going, the more braking force there is.
Click to expand...

So a short may or may not result in disaster?

If I had a rear hub I'd guess that this whole failure mode would be far easier to recover from. A lot easier. Rear braking tends to move weight off the rear wheel, in motorbikes, for example, the front brake provides twice the stopping force.

Just how likely is it that one would actually see this kind of failure? Low quality (or non potted) controllers aside, the only thing I can think of is a poor hub install pinching the wires.

So if one was to have a high quality controller, potted to remove any chance of internal shorts, and had a good solid wire path to the motor then the chances of this would be virtually nil?
 
The FETs in a controller can always fail in a way where the current grid contacts the backplate, which results in a short, which results in throwing you over the bars :) Powering the back wheel makes a lot of sense.
 
liveforphysics said:
The FETs in a controller can always fail in a way where the current grid contacts the backplate, which results in a short, which results in throwing you over the bars :) Powering the back wheel makes a lot of sense.
Click to expand...

How common is this mode of failure?

With all the people on here with ebikes I've only heard of one guy who ran into this problem in a real sense.
 
Are brushed motors safer because they don't have phases that can short?

I can't tell from what video what made their front wheel lock can you?
At first I thought the strap from the helmet itself which would have been super-ironic but it's not close enough to the wheel. Maybe they had some really sensitive brakes.
(laughing at the person though as they are doing on youtube is asinine however)

Years ago before I was motorized, I hit a concrete bus bench at full speed from a downhill at night rushing home from work to try to catch the bus (with bus bike rack). Bike stopped instantly and I did a flip over the bench (length wise) and landed smack on my butt. Front wheel and fork were destroyed. I was strangely fine and rolled what remained of the bike half a mile to the bus and made it home. I was younger and stupider then, but somehow more nimble fortunately, I don't think I'd survive that today.
 
Let's not forget my delightful accident with rear wheel lock-up on my tadpole! See my Sig. Only 8 broken ribs from being flipped at 20 mph! Unexpected lock-up, be it from mechanical (my stupidity) failure or electrical can hurt you even with "just" the rear locking!
otherDoc
 
needWheels said:
Are brushed motors safer because they don't have phases that can short?
Click to expand...

No. In fact I think they are more dangerous since the most common failure mode is for the FETs to short, resulting in full power to the motor with no current limit. I was seriously injured by such an event on my Zappy scooter.
 
fechter said:
needWheels said:
Are brushed motors safer because they don't have phases that can short?
Click to expand...

No. In fact I think they are more dangerous since the most common failure mode is for the FETs to short, resulting in full power to the motor with no current limit. I was seriously injured by such an event on my Zappy scooter.
Click to expand...

Details, please! Focus on the injuries if possible.

I'm kind of interested since I thought an unmodded Zappy scooter could only go 12 mph at max, and shorting the motor wouldn't make it go faster, so it's curious how you became seriously injured.
 
swbluto said:
Speed = RPM*circumference.
Kv = RPM/V -> V = Kv*RPM= Kv*(speed/circumference)
I = V/R (R being motor resistance) = Kv*(speed/circumference)/R
Torque = k*I = k*Kv*(speed/circumference)/R.
Force = Torque/radius = k*Kv*(speed/circumference)/(R*radius) (radius would correspond to the middle of axle and the end-point of the axle that secures to the drop outs - basically, the axle's radius).

There's a relationship between the motor's voltage-RPM constant (Kv) and torque-current constant(The k) that's eluding me - . If you find that relationship, you can get rid of either k or Kv (R might play a role too). It's explained on one of ebikes.ca's page. So, anyways, it appears that the force during "plug braking" is linear with speed.
Click to expand...

Another implication that nobody here seems to have hit on is that the same principles you cite mean electric braking is inherently anti-lock. You can't lock a wheel when lockup = zero braking torque and low wheel speed = low braking torque.

Hard and sudden electric braking at high speed might be enough to bend the fork, tear out the dropouts and/or torque arm, or unseat the rider. But it can't lock up the wheel.
 
swbluto said:
Details, please! Focus on the injuries if possible.

I'm kind of interested since I thought an unmodded Zappy scooter could only go 12 mph at max, and shorting the motor wouldn't make it go faster, so it's curious how you became seriously injured.
Click to expand...

Who said anything about unmodded :twisted:
My Zappy was running 24v with a highly modified motor. It can go about 20mph on flat ground and can climb a 20% grade.

I was just starting out from a stop sign when it blew and it tried to do a wheelie. I jumped off, but I did not let go of the handlebars. The rear wheel go traction and it tried to rip my arms out of the sockets. It tore a rather large section of back muscles and it knocked the wind out of me. It took over 2 years for the constant pain to subside, but I still have issues with my back as a result.
 
I think I experienced plug braking once last fall just after I started ebiking. Was riding in rain and a FET in my controller got wet and shorted. My front wheel violently braked me to a halt with a very strong jerking vibration. I was going about 15-16 mph on a 408 motor. The wheel never locked up and I never lost control. No fork/motor damage but shook the hell out of me.

I imagine that a similar situation on a 530x series at higher speed could be far worse. The braking torque in that case would be higher because of the greater motor radius and wider magnets.

Regards, Bill
 
The described brushed motor controller failure kinda scares me if the typical default is full throttle.

Is there anything I can do to try to prevent that? I guess a 35A fuse on my BD36 wouldn't help?

I do have a on/off switch below the seat but if I had to find it in a couple of panic seconds, I doubt I could.

I wish the throttle actually disconnected the battery when it is at zero. That would be a handy mod.
 
needWheels said:
The described brushed motor controller failure kinda scares me if the typical default is full throttle.

Is there anything I can do to try to prevent that? I guess a 35A fuse on my BD36 wouldn't help?

I do have a on/off switch below the seat but if I had to find it in a couple of panic seconds, I doubt I could.

I wish the throttle actually disconnected the battery when it is at zero. That would be a handy mod.
Click to expand...

My Zappy has a kill switch right on the handlebar which is easily reached while riding. I was waaay too slow.

I've heard reports of Alltrax/Etek combinations doing the same thing. Just try to stop an Etek stuck on full throttle with no current limit. This is one of the reasons I have switched to brushless motors.

The idea of disconnecting the battery when the throttle is zero is a commonly used technique in golf cars. There is a small switch in the throttle that opens the main contactor when the throttle is zero. This works. The only complaint I hear is the noise from the contactor can be annoying. I'm not sure what happens if the controller shorts out either. It seems like the next time you hit the throttle, the main contactor will weld itself on and you'll be flying anyway. I'm sure there is a way to make a 'shorted controller' detection circuit that disables the main contactor.

With smaller motors, you can simply use the brakes to overpower the motor and hopefully blow the fuse or at least stop the thing long enough to jump off.
 
fifthmass said:
I think I experienced plug braking once last fall just after I started ebiking. Was riding in rain and a FET in my controller got wet and shorted. My front wheel violently braked me to a halt with a very strong jerking vibration. I was going about 15-16 mph on a 408 motor. The wheel never locked up and I never lost control. No fork/motor damage but shook the hell out of me.

I imagine that a similar situation on a 530x series at higher speed could be far worse. The braking torque in that case would be higher because of the greater motor radius and wider magnets.

Regards, Bill
Click to expand...

So if you'd had a waterproofed controller, then this wouldn't have happened?

Aside from water what could make a fet short? I won't be running voltages higher than 36v.
 
Chalo said:
swbluto said:
Speed = RPM*circumference.
Kv = RPM/V -> V = Kv*RPM= Kv*(speed/circumference)
I = V/R (R being motor resistance) = Kv*(speed/circumference)/R
Torque = k*I = k*Kv*(speed/circumference)/R.
Force = Torque/radius = k*Kv*(speed/circumference)/(R*radius) (radius would correspond to the middle of axle and the end-point of the axle that secures to the drop outs - basically, the axle's radius).

There's a relationship between the motor's voltage-RPM constant (Kv) and torque-current constant(The k) that's eluding me - . If you find that relationship, you can get rid of either k or Kv (R might play a role too). It's explained on one of ebikes.ca's page. So, anyways, it appears that the force during "plug braking" is linear with speed.
Click to expand...

Another implication that nobody here seems to have hit on is that the same principles you cite mean electric braking is inherently anti-lock. You can't lock a wheel when lockup = zero braking torque and low wheel speed = low braking torque.

Hard and sudden electric braking at high speed might be enough to bend the fork, tear out the dropouts and/or torque arm, or unseat the rider. But it can't lock up the wheel.
Click to expand...

This is a good point. I wonder just how much braking force a 9c motor puts out at 20mph. If it's not enough to unseat you, then the force will rapidly decrease as motor speed decreases.
 
I had a bit of a spark of inspiration and tried the ebikes simulator.

The 9c produces more than 10 pounds of braking thrust per 5km of speed. I used a zero resistance short, and zero battery voltage.
 
OneWayTraffic said:
I had a bit of a spark of inspiration and tried the ebikes simulator.

The 9c produces more than 10 pounds of braking thrust per 5km of speed. I used a zero resistance short, and zero battery voltage.
Click to expand...

:shock: That's quite an excellent divination.
 
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