Jump to content

Illustrating the Risks of Hard Acceleration


Jason McNeil

Recommended Posts

I'm working on a one page safety infographic that will attempt to outline all the important information that is somewhat more helpful than the 'please exercise restraint' message—how is one supposed to know what restraint means, it's not very precise... 

One of the most important aspects of safety is an understanding that the Wheel's powers are finite; push beyond these, & you put your safety at peril. Hard acceleration is definitely at the top of this list & probably the culprit of most Wheel failures.

The object of the illustration is to try to instill the relationship between the load & acceleration. The standard K = 1/2 mv2 F=ma formula makes certain assumptions which are not encountered in the Wheel world, like perfectly smooth acceleration & continuous power delivery. You'll probably want a decent safety margin over the rated power output & a formularized required power measurement, I think it's safe to say that the values here are pretty conservative. 

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

 

Link to comment
Share on other sites

Interesting idea. Not sure if you're there yet.

What is "acceleration rate"? The name makes it sound like change of acceleration, your formula looks like kinetic energy (which has nothing to do with acceleration), your numbers are in m/s (speed?), and I'm not sure how much 3m/s (for example) is qualitatively - much, little, in between? How will people know what the numbers mean for their driving in reality? So either make a mostly qualitative diagram, or you have to clarify that. And what exactly are these W numbers?

The more I look at it, the less I get what you want to communicate. Could you state that (the result) in a short sentence? (Which should be the title of the diagram then)

I'm also thinking you can modify your diagram so people can look up their weight, wheel's nominal power (or just the wheel itself), and look at the accompanying curve - if that's your goal.

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

It's a question what you want to convey, and how much information you want to pack in there. And make "how to read" part of the diagram.

Anyways, appreciating very much your efforts to clarify such things. Right now, wheels give no warning how much they are strained before cut out, so anything helping people understand the limits without experimentally experiencing them (that's an euphemism for cut out/crash;)) is great!

 

Link to comment
Share on other sites

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.

 

Link to comment
Share on other sites

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! 

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

@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.

Link to comment
Share on other sites

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

Link to comment
Share on other sites

@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.

Link to comment
Share on other sites

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.

@Jason McNeil I think the message could be 2 points, basically:

  • be careful with hard acceleration, it may overpower your wheel and then you fall (especially when it's cold, you are heavy, etc). This is essentially what the original chart details.
  • the faster you are going, the less reserves your wheel has, and the easier it is to overpower the wheel with sudden (intended or unintended) acceleration - so you have to be extra careful not overpowering your wheel when going fast vs going slow. This is what my chart shows, how safe acceleration falls with speed.

Maybe make 2 charts like yours (in 1 picture): 1st like your original "safety vs acceleration" (showing weight and motor power dependency etc) and the second "low speeds vs high speeds" with 2 curves, one low speed curve and one high speed curve (same curve, but high speed is steeper, so it hits the unsafe zone at lower acceleration values) so people get the comparison and see that "at high speeds, it gets unsafe faster".

It's complicated, as you said, can't get everything into a simple chart.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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).  

Link to comment
Share on other sites

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

 

 

 

Link to comment
Share on other sites

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...

 

Link to comment
Share on other sites

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)?

Link to comment
Share on other sites

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).

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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).

Link to comment
Share on other sites

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.

 

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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:!

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...