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Current demand versus battery voltage


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1 minute ago, zlymex said:

From energy point of view, if the load is fixed say at 800W, the battery must provide at least the same power at 80V * 10A. If the voltage has dropped to 40V, the current must be increased to 20A in order to maintain that 800W of power.

Yeah, but what I think @Carlos E Rodriguez was pointing at, is that if you use for example 400W the actual way the torque is achieved is by how long the pulses are. So at 80V, the pulses will be half as long as at 40V.

Feel free to correct me if I misunderstood Carlos.

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14 hours ago, DaveThomasPilot said:

Great way to clarify things!

I've been referring to battery voltage (Ub) and battery current (Ib).  It's important to differentiate from Ud and Id.

It's harder to talk about Id, since there are multiple conductors at different phases.  So, there's not just one current to consider, but three (or more)?  That's why I was trying to stick with just input current (or Ib) and battery voltage (Ub).

14 hours ago, zlymex said:

Quite true. If we read the current from an app(such as Wheellog), it is only one value. I suppose they just converted to single phase motor. There are two current sensor ICs on the board in series connection with 2 of the 3 motor wires, the 3rd current may be calculated from the two.

In a real bldc motor used in the EUC there are 3 phases. They are internaly connected as a star, so the motor voltage is always applied over two phases in series. For the commutation sequence the voltage is applied in a fixed sequence (for example A-B, A-C, B-C, then again A-B, ...) so at any time there is voltage applied over two phases and the current flowing through them. To make it more complicated this voltages to the phases could be sinusodial or trapezoidal - but then again one could take the effective voltage...;)

If voltage is applied to i.e. A-B then I A = - I B, in the internals of the motor one coil of phase A sits aside of a coil of phase B, so A attracts the permanent magnet and B pushes the same magnet...

For my calculations the resistance of this two phases and just look at one (effective) voltage and current, which should be for our purposes a good enough approximation.

14 hours ago, DaveThomasPilot said:

I think that the average current in each winding will be (first order approximation) proportional to the torque, or at fixed speed, proportional to the mechanical output power.  I think this is what many are thinking about when they assert the input voltage doesn't matter for motor current.

Battery voltage does not really matter for the torque, since it is pwmed (stepped down) to the right motor voltage to get the correct current (proportional to torque) by I motor = (U motor - U back emf)/Rcoil... (as long as the battery voltage is greater than Umotor)

14 hours ago, DaveThomasPilot said:

But, as I've tried to point out, battery current (Ib) MUST be inversely proportional to battery voltage (Ub) at a fixed motor speed and load.

Since the load and speed determines the needed power and P=U*I thats prooven!

 

8 hours ago, Carlos E Rodriguez said:

I will try to simplify.

  A motor is running at 10mph, constant grade, no change in rolling resistance. The battery is 80 and the current shows that to maintain this speed is 10AMPS.

  Now lets say same conditions but the battery is at 40 volts.  The current will still be 10AMPS to make the wheel go 10mph.

Yes, considering the 10A are the motor current and not the battery current!

In this example battery current will stay the same (constant power demand).

8 hours ago, Carlos E Rodriguez said:

The numbers are academic and not real but only to demonstrate the relationship.  

So why is this so!  Well at the time when the battery is 80volts, the computer was putting let say 10% duty cycle to generate 10amps to produce the mechanical torque required to maintain 10mph.  When the battery is 40volts the computer then increases the duty cycle to generate 10amps to produce the same mechanical torque required to keep the wheel spinning at 10mph. To produce a mechanical torque on a motor only current is the parameter that determines how much torque is produced.

Inductance, Resistance, Capacitance, Voltage are also important but as secondary items on the complete design.

Motor coil resistance is also very importand, since with the currents it adds a voltage drop on top of U back emf and by this makes a higher battery voltage needed to reach a certain speed.

The Inductance (togheter with the motor interia) mainly averages the pwmed motor voltage - so i looked only at "static" situations where it has no "influence" beside this. Once accelerations/load changes happens so that the motor current changes they presumably play a big role and are not to be neglected again - they'll also contribute then to a negative or positive voltage on top of the ohmic coil resistance voltage drop and back EMF. (They are working against the change, so higher voltages are needed to get the current for the required torque flowing to balance the wheel...)

Capacitance ... pffff ... I assume the big capacitors on the mainboard are needed to support the peaks the battery cannot deliver with their internal resistance and stray inductivities?

 

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15 minutes ago, Chriull said:

Capacitance ... pffff ... I assume the big capacitors on the mainboard are needed to support the peaks the battery cannot deliver with their internal resistance and stray inductivities?

i have driven a Ks16 with broken capacitor wires... the pedals got weak....idling wasn't possible anymore..and on the next hill the batteries where so stressed that the fuse blows... so Yes...without capacitors no peaks possible

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2 minutes ago, KingSong69 said:

i have driven a Ks16 with broken capacitor wires... the pedals got weak....idling wasn't possible anymore..and on the next hill the batteries where so stressed that the fuse blows... so Yes...without capacitors no peaks possible

Had exactly the same - was somehow like driving on super soft mode ...;)

... And then the fuse blew up on an incline - fortunately it was my brother testing (at low speeds) :ph34r:

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10 hours ago, Chriull said:

In a real bldc motor used in the EUC there are 3 phases. They are internaly connected as a star, so the motor voltage is always applied over two phases in series. For the commutation sequence the voltage is applied in a fixed sequence (for example A-B, A-C, B-C, then again A-B, ...) so at any time there is voltage applied over two phases and the current flowing through them. To make it more complicated this voltages to the phases could be sinusodial or trapezoidal - but then again one could take the effective voltage...;)

If voltage is applied to i.e. A-B then I A = - I B, in the internals of the motor one coil of phase A sits aside of a coil of phase B, so A attracts the permanent magnet and B pushes the same magnet...

For my calculations the resistance of this two phases and just look at one (effective) voltage and current, which should be for our purposes a good enough approximation.

Battery voltage does not really matter for the torque, since it is pwmed (stepped down) to the right motor voltage to get the correct current (proportional to torque) by I motor = (U motor - U back emf)/Rcoil... (as long as the battery voltage is greater than Umotor)

Since the load and speed determines the needed power and P=U*I thats prooven!

 

Yes, considering the 10A are the motor current and not the battery current!

In this example battery current will stay the same (constant power demand).

Motor coil resistance is also very importand, since with the currents it adds a voltage drop on top of U back emf and by this makes a higher battery voltage needed to reach a certain speed.

The Inductance (togheter with the motor interia) mainly averages the pwmed motor voltage - so i looked only at "static" situations where it has no "influence" beside this. Once accelerations/load changes happens so that the motor current changes they presumably play a big role and are not to be neglected again - they'll also contribute then to a negative or positive voltage on top of the ohmic coil resistance voltage drop and back EMF. (They are working against the change, so higher voltages are needed to get the current for the required torque flowing to balance the wheel...)

Capacitance ... pffff ... I assume the big capacitors on the mainboard are needed to support the peaks the battery cannot deliver with their internal resistance and stray inductivities?

 

I technically agree. I just think talking about voltages confuses the essence of how torque is generated in a magnet/coil magnetic field interaction. The only thing that matter is being able to generate enough current-amps values to maintain a  wanted torque. The computer will adjust the PWM duty cycle to generate the necessary amps to maintain the necessary torque to reduce the tilt angle error or the wheel put by the ridder. So as long and the voltage is not lower than needed to generate amps , it can be as high or as low. Of course at some point is too low because 100%duty cycle volts / coil resistance - back emf is the max amps that can be generated. But battery volts can vary widely before it can not be a positive delta to generate positive amps. 

So i try not to confuse people with volts. High or low batter volts is not the control variable. The control variable is amps. And yes without volts there is no amps. But amps is the key parameter. 

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The torque the motor produces is (to a first order approximation) " is the parameter that determines how much torque is produced. "

Yes, absolutely!  But the average battery current required to drive the motor is higher when the battery is lower.

Made a simplistic model will help.  This isn't really what's going on, but its valid in an approximate way.

Think of ESC as a buck/boost inverter that adjusts it's output voltage to drive the wheel at the speed you want.  The motor speed, as I think you agreed, is set by the applied voltage.  So, the ESC does PWM to effectively adjust the average voltage (Vm). 

For low speed, it's acting like a buck convertor.  So, the duty cycle reduces the average current the battery sources (Ui) by the duty cycle.  At lower battery voltages, the duty cycle is higher, which results in higher (cycle average) battery current.

So, I think the point you're missing is that average battery current does not equal the  time average current in the windings.  You need to apply the duty cycle, which varies inversely with the battery voltage.

 

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So, at this point, is there any disagreement?   At a fixed motor power, lower battery voltage means high battery current (easy to prove by conservation of energy).

The "effective motor current" does NOT change, since power output and speed is constant.  The ESC adjusts the duty cycle for the speed required.

That duty cycle affects the battery input current--reduces (bucks) it when the winding voltage (sort of Um, but there isn't just one winding) / Kv of the wheel motor is lower than the battery voltage and boosts it when the  the Um / Kv is higher than the battery voltage.

In Marty's case, someone said the melted insulation was on a wire from the ESC to the motor.  The average current in those wires doen't vary (much) with battery voltage change.  But, the wires from the battery to the control board will have current inversely proportional to the battery voltage (at constant battery power).

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14 hours ago, DaveThomasPilot said:

In Marty's case, someone said the melted insulation was on a wire from the ESC to the motor.  The average current in those wires doen't vary (much) with battery voltage change.  But, the wires from the battery to the control board will have current inversely proportional to the battery voltage (at constant battery power).

High current in battery wires have until now never been an issue. On the other hand, the note increased battery voltage can be used to drive higher currents through the motor, getting higher torque/output power. So the 84v gotways are melting the cables because of the higher voltage, but because of the higher output power, which demands higher currents. All assuming the motors windings are the same for 67 and 84 volts.

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On 6/23/2017 at 6:23 PM, DaveThomasPilot said:

So, at this point, is there any disagreement?   At a fixed motor power, lower battery voltage means high battery current (easy to prove by conservation of energy).

The "effective motor current" does NOT change, since power output and speed is constant.  The ESC adjusts the duty cycle for the speed required.

That duty cycle affects the battery input current--reduces (bucks) it when the winding voltage (sort of Um, but there isn't just one winding) / Kv of the wheel motor is lower than the battery voltage and boosts it when the  the Um / Kv is higher than the battery voltage.

In Marty's case, someone said the melted insulation was on a wire from the ESC to the motor.  The average current in those wires doen't vary (much) with battery voltage change.  But, the wires from the battery to the control board will have current inversely proportional to the battery voltage (at constant battery power).

OMG!  Are freking kidding me!  Stop thisnvoltagebnon sense.  

Torque is amps. Nothing else matters. Voltage is just a potential to allow amps. The darn battery volts are PWMed so you can not do a simple I = v/r.  It just about how many amp you need. Period. The battery volts are pwmed into three phases so the batery amps will be higher than on an individual motor phase . 

From the control boat there is a short pig tail of high quality wires. I believe 12 gage. 

Then the motor has another set of wires with thinner insulation. These two wires are connected via bullet connectors. The bullet connectors insulation melted as well as the motor wires. The board pigtail look like the held up well. 

 

 

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I'm about to give up Carlos.  One last time, since there's not much point in going into more subtle effects like battery internal resistance and winding resistance until the fundamentals are grasped.

Quote

The darn battery volts are PWMed so you can not do a simple I = v/r.

Where did that come from?  No where is v/r referenced as a way to calculate current.   The point is you can use output power and input power calculations to easily show time average battery current is higher when the battery voltage is lower.

When a current (or voltage) is PWM, you do a time average over the cycle time.  Take a 60% duty cycle example.

Input power  = (Ub * I1)  * d + Ub *I2 (1-d)   where d is the duty cycle

The average of the two different power consumptions seen in each cycle.  If I2 is close enough to zero ,then it's just

d*Ub * I1

If there more phases involved that have significantly different currents, then you just weight them by the period of time each applies:

Ub * t1 + Ub *t2 + Ub * t3 + ...  /  (t1+t2+t3+ ...)

Quote

The battery volts are pwmed into three phases so the batery amps will be higher than on an individual motor phase . 

Whether the motor winding currents are higher or lower than the battery current depends on the ratio of the effective motor voltage set by the ESC and the battery voltage.  But, for the battery voltages and battery windings used in EUC, yes, the winding current will be higher the battery current.

Motor winding current (to a first order approximation), is independent of battery voltage (under constant output power).  But, battery current is inversely proportional to battery voltage (again, to a first order approximation).

I think you were pointing out that the resitive losses in the motor windings, interconnect wires, and battery internal resistance can limit the current to less than what would be required for a given torque.   So, higher battery voltages will provide the ability for more current before these resistances limit winding current.

Yes, that sounds plausible and we can get into that in more detail.  But, that analysis starts with understanding of what current flows where so associated IR drops can be calculated.

I'm not trying to say a lower battery voltage caused Marty's issue or other Gotway issues--it's pretty clear it didn't.   But, the voltage drop due to battery internal resistance and wiring from the battery to the control board is probably not negligible either. 

 

 

 

 

 

 

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On 22.6.2017 at 5:17 PM, Chriull said:

...

I finshed my formula set (so everything can be calculated in excel without iteration): https://drive.google.com/open?id=0B6-Hr1aVKjLLaE4tbDhubGVpdzQ (1)

...

I had my first encounter today with ltspice:

Tpw2w10.jpg

Should be the same parameters as in the excel sheet linked above with a 6kHz PWM signal - just

- a mosfet is included instead of an ideal switch (i took just some and changed Rdson to 10 mOhm)

- a big fat capacitor is parallel to the motor - without it there are spikes in the kV region...

- 0.1 mH for the coil - @esajmentioned this value once for e-bike motors...

outcome:

EGlS2Mb.jpg

V(n001) ... green ... is the battery Voltage (after R1... the internal resistance)

V(n003) ... blue ... is the motor voltage (including L1, R2 and the Voltage source==Back_Emf)

I(R1)      ... cyan ... negative battery current (don't know by now how to change the direction ;)

I(R2)      ... red   ... motor current

U Battery Average = 63,925V

I battery Average = 65,271A

U Motor Average = 20,125V

I Motor Average = 63,674A

Also don't know why the battery current gets so big (250A max!)! Got the LC circuit resonating?

That above was the "stable" state, but it needs some time to settle:

dGphqC6.png

 

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5 hours ago, Chriull said:

outcome:

EGlS2Mb.jpg

V(n001) ... green ... is the battery Voltage (after R1... the internal resistance)

V(n003) ... blue ... is the motor voltage (including L1, R2 and the Voltage source==Back_Emf)

I(R1)      ... cyan ... negative battery current (don't know by now how to change the direction ;)

Look at the direction of the arrow when you take the mouse cursor above R1, you'll notice it's pointing in the opposite direction, that's why it's showing negative values. You can just correct it "in your head", or change the expression to say "-I(R1)" by right-clicking on the I(R1) -text on the top of the graph, or turn the resistor around. To turn around a component, enter move- or drag- mode (either click the hand icons in the top row, or use shortcut keys: F8 for drag, F7 for move). Left-click on the resistor to start moving/dragging it, but instead of changing its place, either click on the rotate-icon on the top row (showing "E" turning to it's side), or use the short-cut key (CTRL+R). After you spin the resistor around 180 degrees, the arrow showing the current measurement direction will also have turned.

Quick cheat sheet of shortcuts & directives, these make it really fast to use LTSpice (once you otherwise get more used to it ;)) and the directives allow you to run more complex simulations with (for example) time-varying parameters:

zuVEHez.png

 

5 hours ago, Chriull said:

I(R2)      ... red   ... motor current

U Battery Average = 63,925V

I battery Average = 65,271A

U Motor Average = 20,125V

I Motor Average = 63,674A

Also don't know why the battery current gets so big (250A max!)! Got the LC circuit resonating?

That above was the "stable" state, but it needs some time to settle:

dGphqC6.png

 

Not sure, but the problem might be that you've made the motor back-EMF constant... it's probably varying as the phases turn on and off, having it as a constant value will cause large current shoots when the fet is switching on and off?

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6 hours ago, esaj said:

...

Not sure, but the problem might be that you've made the motor back-EMF constant... it's probably varying as the phases turn on and off, having it as a constant value will cause large current shoots when the fet is switching on and off?

Yes - seems to be just a bit above the max possible current to flow : (76-13,81)/(0,185+0,1+0,01)=210A. The "missing" ~40A could be from "recharging" the capacitor...

With 0,3Ohm coil resistance this value is down to 125A - around the "magic" number we heard here once from GW. The max current limit from the circuit?

With an additional 0,3 Ohm series resistance for the capacitor also ltspice shows this max number ~125A...

So the absolutely constant back_emf means, that the motor is running at an absolute constant speed - which is not true, once the "big" current comes while the PWM signal is turning the mosfet on it will accelerate (or at least try to) and then decelerate again -> so we have our whining sound/torque ripple...

Seems i have to extend the schematics to the second of your ltspice motor simulation link (https://www.precisionmicrodrives.com/application-notes/ab-025-using-spice-to-model-dc-motors) to get "real" numbers...

 

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After some more time with ltspice it seems, that i got it - the big capacitor parallel to the motor was just a stupid idea - testing the possibilities of the mosfet... ;( The needed piece was just a free wheeling diode for the motor (normaly in place by the other mosfets of the bridge):

bueuvNO.jpg

Leading to "normal" output:

AFCb6wY.jpg

This time the values are:

U Batt average: 72,8V

U Motor Average: 18V

IMotor Max:53A Average: 41,8A Min: 30,6A

IBattery Max: 53A Average: 11,7A

PMotor: 756W

P Batt: 800W

P Mosfet: 20W

P D2: 28W (1)

(One sees here a new Voltage: V(p002) which is the voltage across Uback_emf and R2, since V(n003) the same with additionally L1 delivers an phase shifted part and not real power...)

The values seem to correlate quite good with the excel sheed - just the I Battery Max is not to be calculated easily. This also would depend on the "state" of L1

Edit: (1) ... so hopefully all wheels have active free wheeling implemented...

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1 hour ago, Chriull said:

 

After some more time with ltspice it seems, that i got it - the big capacitor parallel to the motor was just a stupid idea - testing the possibilities of the mosfet... ;( The needed piece was just a free wheeling diode for the motor (normaly in place by the other mosfets of the bridge):

 

Congrats! This a well thought out concept. So how can we test out your theory? 

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1 hour ago, Rehab1 said:

Congrats! This a well thought out concept. So how can we test out your theory? 

The next step could just be empirical measurements (or someone with experience declares it right/wrong). And maybe real data for the inductance and resistance of the coils.

Just some more examples:

-  @EcoDrift measured ~4,6 kW at ~30 km/h with the Msuper V3+. This should lead (according to ltspice with the above model) to about 86A average motor current (95A..79A) and 71A average battery current (with the same 95A peak). The excel sheet comes up with 91A average motor current and 68A average battery current.

- if one take the 1600W continous power advertised with a speed of 10 km/h this would lead to about 70A average motor current (81...58A) and 24A average battery current (again with 81A peak). (1)

- the 1600W at 10 km/h with a motor with 0.3 Ohm resistance (for two coils)(2) instead of 0.1 would need 52A average motor current with ~65A peak, average battery current unchanged. (1)

- changing the inductance leaves the average values about the same, just changes the peaks (more inductance gives less ripples)

Looking forward to seeing the first pics of KS18S motor cables and connectors - how they solved this issue beeing in about the same power range then... Or if they just set more conservative limits in the firmware...

(1) should both lead to molten cables... ;(

(2) imho measured from @zlymex on a 350-500W wheel motor.

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3 hours ago, Chriull said:

Looking forward to seeing the first pics of KS18S motor cables and connectors - how they solved this issue beeing in about the same power range then... Or if they just set more conservative limits in the firmware...

I had this photo of KS18A(?, but rated at 1200Watts) after the burn down. It seems that three motor wires are all black, and the endings are Spade Lugs, which match the screwed terminal on the main board. I don't have any idea on the gauge of the motor wires.
59624ca857f50_webwxgetmsgimg(11p).jpg.19ed622892e95525958dba8e257aee12.jpg
 

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11 minutes ago, zlymex said:

I had this photo of KS18A(?, but rated at 1200Watts) after the burn down. It seems that three motor wires are all black, and the endings are Spade Lugs, which match the screwed terminal on the main board. I don't have any idea on the gauge of the motor wires.
59624ca857f50_webwxgetmsgimg(11p).jpg.19ed622892e95525958dba8e257aee12.jpg
 

Since Version KS18AY -1200W they have used thicker motor cable wires....

As can be seen on the pedals(14c style) this must be a VERY early 18a 1200w version...even my 18months old Ks18a has had the newer, wider pedals(16b style).....

@Chriull hopefully in less than 7-10 days i can measure or take a look myself...how the 18S looks

 

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47 minutes ago, KingSong69 said:

Since Version KS18AY -1200W they have used thicker motor cable wires...

Is it possible to measure the outside diameter of the axle and the diameter of the shaft opening for the motor wires? Are there any visible markings on the wires to indicate the AWG? 

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39 minutes ago, Rehab1 said:

Is it possible to measure the outside diameter of the axle and the diameter of the shaft opening for the motor wires? Are there any visible markings on the wires to indicate the AWG? 

I have sold my Ks18a and been waiting for the delivery of the 18S :-)

will take one or two weeks...than i will take a look!

 

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39 minutes ago, KingSong69 said:

I have sold my Ks18a and been waiting for the delivery of the 18S :-)

will take one or two weeks...than i will take a look!

 

Thanks!

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7 hours ago, zlymex said:

I had this photo of KS18A(?, but rated at 1200Watts) after the burn down. It seems that three motor wires are all black, and the endings are Spade Lugs, which match the screwed terminal on the main board. I don't have any idea on the gauge of the motor wires.
59624ca857f50_webwxgetmsgimg(11p).jpg.19ed622892e95525958dba8e257aee12.jpg
 

Those pedal length extenders look great!  That pedal looks like it could easily hold a large shoe.

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1 hour ago, steve454 said:

Those pedal length extenders look great!  That pedal looks like it could easily hold a large shoe.

Poor wheel. But the motor is good, and someone is trying to build a new wheel out of it by adding a case and a Gotway board.
5962c85043cc7_webwxgetmsgimg(9p).thumb.jpg.9b6bfda2de57dab90790a7c65fc2fb20.jpg

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1 hour ago, zlymex said:

Poor wheel. But the motor is good, and someone is trying to build a new wheel out of it by adding a case and a Gotway board.
5962c85043cc7_webwxgetmsgimg(9p).thumb.jpg.9b6bfda2de57dab90790a7c65fc2fb20.jpg

Is the rider still alive?:confused1:

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