Jason McNeil

Illustrating the Risks of Hard Acceleration

33 posts in this topic

btw: Acceleration is the same as braking! good acceleration-good torque-stands for good braking abilities also!

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11 minutes ago, meepmeepmayer said:

What is "acceleration rate"?

Isn't the standard SI units for acceleration Δmeters/sec? If you're setting out from a standstill & acceleration at 3m/s you're basically going to hit 20MPH in two seconds. Maybe the figures need checking again. In the formula of F=ma, where F=Newtons, m=kg & a = m/s?

The figures seem really low: 3m/s is a pretty quick & in the example of the 90kg rider (total 105kg) would mean only 472W of power are need to blast him to 20MPH in 2 seconds.

 

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Posted (edited)

acceleration is in m/square seconds = m/s times 1/s = you modify your speed by 5 m/s per second = 5 m/s^2  acceleration

Starting to understand your numbers...

Edited by meepmeepmayer
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Posted (edited)

28 minutes ago, meepmeepmayer said:

acceleration is in m/square seconds

Right, will square it horizontal. Thanks for the feedback. Agree it needs some refinement, but wanted to put it there to see how it can be made more effective. I'm a bit troubled by the fact that the power figures seem to low to me! 

Edited by Jason McNeil
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Posted (edited)

37 minutes ago, meepmeepmayer said:

I'd also add a low-temperature warning to the 20 degrees standard sentence.

Yes absolutely, there will be another graph for temperature based on published performance figures of a specific cell type.

Edited by Jason McNeil
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Posted (edited)

I don't get what the 1/2m v^2 is. If it's kinetic energy, that only tells you how much energy the moving heavy wheel+rider has (should it hit something), but has nothing to do with acceleration or motor stress - you could rush downhill with barely any motor usage and still have huge kinetic energy.

Maybe this formula helps you:

F=ma is the force needed to accelerate the wheel (m = mass wheel+rider, a = acceleration = change of speed)

You have

P = Fd/t

where P is power, d is the distance moved and t is the time needed for that. So simply

P=ma times speed

So if you know the current speed, and the weight rider+wheel, you can solve for maximum acceleration given a certain fixed motor maximum power (assuming the motor max power is fixed, it also depends on battery level, temperature, and real world godknowswhat)

a = P/mv

You also see that for growing speed and rider weight, the maximum allowed acceleration sinks (aka a small acceleration will overpower the wheel if you're already at high speed) in a 1/x manner.

Not sure what kind of graph you could best build from that.

Edited by meepmeepmayer
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@Jason McNeil Thanks for doing this.  It's a good start.

A couple of thoughts (intended to be helpful, not critical):

1. The average person doesn't have a good reference point for quantities like "4m/sec2" so it still doesn't have a lot of practical implication.  Could you find "real world" examples of what 2m/sec, 3 m/sec, etc. feel like?  That might make the chart more understandable and effective.

2. A lot of the failures I read about in this forum are from sustained high speeds rather than acceleration ("overlean faceplant").  That's easier for the average person to understand: there's a speed limit built into every wheel and it's directly related to motor power, battery power, rider weight, temperature and ascent/descent angle.  The question is, what's that limit?  There's a lot of anecdotal stories here that suggest tiltback and audio warnings aren't good enough to keep us safe.

An infographic that shows how the top safe speed of any given wheel can be affected adversely (and invisibly) by variables like battery age, temperature, rider weight and angle might be very useful in educating riders and preventing more of the "my wheel tried to kill me today" threads.

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Posted (edited)

I tried this, but I think the problem is trying to get too much information and dependencies into one simple graph. It's not very intuitive. This has not even any numbers (purely qualitative), or multiple wheels.

You should think what exactly you want to say, and do a veeeery simple graph without many (or any?) numbers. People have a hard enough time with the concept of acceleration, who in the world can interpret specific numbers.

graph_20170404_163022.jpg

Edited by meepmeepmayer
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Posted (edited)

@Jason McNeil, what seems to be entirely missing in your considerations is the current speed. The torque available from the motor goes linearly down (i.e. right down to zero) with increasing speed. At low speed, the battery might be the limiting factor for maximal acceleration or outleaning, but at high speeds, due to the back EMF, the motor becomes the limiting factor and the wheel not only becomes easier to outlean but outleaning is also (much) more dangerous when it happens. AFAICS, contemplating maximal acceleration without considering the current speed doesn't make a whole lot of sense.

Edited by Mono
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35 minutes ago, Mono said:

@Jason McNeil, what seems to be entirely missing in your considerations is the current speed

Assumption is that it's linear acceleration from 0. Impossible to capture everything on one chart. 

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4 hours ago, meepmeepmayer said:

I think @Mono has a good point with speed, it's hard to ignore it. Also, acceleration is difficult to understand and quantify for people (I certainly don't "know" if 3m/s^2 is little or much acceleration), but speed is easy.

Trouble is a constant speed (any speed) on the flat takes very little power it is just overcoming friction and windage so speed is, in itself, meaningless in @Jason McNeil context here. Acceleration is all power just think of drag racing for example, however unlike a drag racing car, the EUC has to still have the power to balance at the same time.

The only way I can think of expressing acceleration meaningfully would be by expressing it in terms of g-force I.e. 3m/s/s is around 1/3 G (9.81m/s/s ) However it would be quite an EUC if you could really feel the g-force under acceleration;-) I guess expressing it, or thinking of it, in terms of how steep a hill it can climb at constant speed is one way as that is directly working against the acceleration due to gravity.

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On 4/4/2017 at 3:40 PM, KingSong69 said:

btw: Acceleration is the same as braking! good acceleration-good torque-stands for good braking abilities also!

I think they might be different in the sense that @Jason McNeilis trying to communicate.  I know that my V8 regeneratively recharges, and I can see the battery indicator change from "red/yellow" to "blue-3bars" when I decelerate downhill, whereas, it will go to "red" on the uphill.  The uphill cutout will be due to battery drain; a downhill cutout would be due to overloading the circuitry (recharging and motor drive).  

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Posted (edited)

On 4/4/2017 at 3:03 PM, Jason McNeil said:

 

uc?export=view&id=0B-WCZQc2gfJjeS05Y0c2Z

 

Very useful @Jason McNeil ... I would do some conversions which might be more comfortable for us brought up on US metrics:  

1 m/s^2 = 0.102 g's

1 kg = 0.454 lbs

So me+wheel ~ 90kg, and 4.5m/s^2 is about 1/2 g  ==> wheel torque would have to resist about 90/2 kg force vector pointing straight ahead

Computing torque requires knowledge of the lever arm vector, and T = r x F is a vector cross product, and the F vector in this case will be acceleration plus gravitational pull if you are not exactly vertical.  

Bottom line ... accelerate too much and you are "F"-ed

 

 

 

Edited by Chris Westland
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19 minutes ago, Chris Westland said:

I think they might be different in the sense that @Jason McNeilis trying to communicate.  I know that my V8 regeneratively recharges, and I can see the battery indicator change from "red/yellow" to "blue-3bars" when I decelerate downhill, whereas, it will go to "red" on the uphill.  The uphill cutout will be due to battery drain; a downhill cutout would be due to overloading the circuitry (recharging and motor drive).  

thats electrical...i meant that a wheel capable of good torque on acceleration, has this also on braking...

my two 18" wheels have a totally different torque...and they have this difference not only one direction. the smooth has a smooth braking also, the torque monster has a torque braking...

 

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Posted (edited)

A related question, is "overlean" on braking/deceleration (aka braking too hard) as dangerous as overlean from too much (sudden) acceleration, or does the regenerative recharging make sure there's always enough power to brake as hard as you want (assuming the electronics can keep up, which may be a problem)?

Edited by meepmeepmayer
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Posted (edited)

4 hours ago, Keith said:

Trouble is a constant speed (any speed) on the flat takes very little power it is just overcoming friction and windage so speed is, in itself, meaningless in @Jason McNeil context here.

The point was not that speed needs power. The point was that speed is a determining factor in how much torque the wheel can produce, that is, how strongly a wheel can accelerate without to fold. The faster the wheel is going, the less torque it can produce.

motorcurve.gif

Plus, the wheel certainly does need some of this torque to sustain the speed. This sustaining torque grows faster than linear with speed, due to the drag, and the wheel needs at least around 200W to keep up 20km/h speed (which means 10Wh/km).

Edited by Mono
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8 hours ago, Jason McNeil said:

Assumption is that it's linear acceleration from 0. Impossible to capture everything on one chart. 

This however seems to be the least important setting. It is the setting where the wheel has the highest torque and it is the setting where the wheel folding has the least impact on the health of the rider.

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1 hour ago, meepmeepmayer said:

does the regenerative recharging make sure there's always enough power to brake as hard as you want

Definitely not, from my experience. I can easily out-brake my IPS, not so much my Gotway or InMotion, in particular never in the first try (so far).

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On 4.11.2016 at 1:37 AM, RenaissanceMan said:

Here is another graph. It shows the theoretically admissable maximum safe speed when entering a climb of 10% for a given weight (wheel plus rider) with a yield of  70% (i.e. assuming of the 100% energy that comes out of the battery, 70% are converted into mechanical and kinetic energy. (Is this yield realistic? Can somebody pls confirm/deny?)

z -axis and color code denote max speed in km/h.

Clipboard12.png

Maxima code:

plot3d(velocity(P*0.7,m),[P, 500, 2000], [m, 50, 130], [grid, 15, 15], [legend, false], [elevation, 0],
    color_bar, [xtics, 200], [ytics, 5], [ztics, 5], [color_bar_tics, 2], [mesh_lines_color, true],
    [title, "Safe speed for running into a 10% climb at given weight and power available (70% yield)"],
    [xlabel, "Power [Watt]                                                   "], [ylabel, "Weight [kg]   "],
    [palette, get_plot_option(palette, 3)]);

There is another thread with considerations on safe speed, see above.

One aspect that has not been considered in this thread is the yield of the electric motor, i.e. how much of the electric energy used actually contributes to forward motion (less than 100%). This would increase the power values given by @Jason McNeil.

 

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Posted (edited)

This topic is either really simple or really hard and you'd need to make real world experiments to get some kind of curve. I don't know.

@RenaissanceMan Are these experimental velocity values or is this some kind of formula (which I'd like to see then - it looks almost linear, is it the one from the thread you linked to, with only air and incline resistance)?

--

EUCs really should have some kind of audible signal for overstressing the motor. Let's say the wheel starts making a definitive artificial "motor sound" at 70% utilization which increases in pitch and loudness until it becomes the 80% (or whatever) warning. Then Jason could just say "when it gets loud, be careful" and that's it. Not sure how hard it would be on a technical level to do such a thing.

Edited by meepmeepmayer
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4 hours ago, meepmeepmayer said:

This topic is either really simple or really hard and you'd need to make real world experiments to get some kind of curve. I don't know.

@RenaissanceMan Are these experimental velocity values or is this some kind of formula (which I'd like to see then - it looks almost linear, is it the one from the thread you linked to, with only air and incline resistance)?

--

EUCs really should have some kind of audible signal for overstressing the motor. Let's say the wheel starts making a definitive artificial "motor sound" at 70% utilization which increases in pitch and loudness until it becomes the 80% (or whatever) warning. Then Jason could just say "when it gets loud, be careful" and that's it. Not sure how hard it would be on a technical level to do such a thing.

@meepmeepmayer My graphs are all based on physical formulae show in that thread:

@Chris Westland Just amazing, these graphs:smartass:! Opens an entirely different view of the world and EUCs in particular:wacko:!  Could you just elaborate a little more on how exaclty the zone of pain relates to biomass :P as well as on EUCs' relation to the dating zone:wub:?! I desperately need to know:rolleyes:!

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