The limitations of an EUC - going downhill.

[brjohnso]

When I go up a steep hill, my EUC slows down based on the steepness of the grade. This is fairly obvious, since the power required goes up substantially. However, I've always deliberately slowed down and maintained a fairly low speed when going down any significant grade. My concern is that since there is no friction brake on an EUC, there is probably a limit to the rate at which power can be absorbed by the control board and batteries. Is this a valid concern? Is there a risk that if I descend a steep hill quickly, I won't be able to stop. Can the EUC lose control, or can the electronics be damaged from excessive reverse EMF?

[OUNCESMUSIC]

A reasonable concern I too have, being cautious in that scenario. 

If your battery is full or close to being full this can happen.
It seems difficult/unlikely to go quickly down a hill without one of the safety precautions preventing this.
You can treat it like a bike, it's fun to cruise down a hill but think about if the front tire hit something at decent speeds.
A decent wreck and probably something visual similar in the case of an overheat downhill.
But at slow speeds (and not nearing max battery cap.), if something should happen it's a step off the vehicle.
The vehicle is more than likely repairable, and your safety is further ensured.

Now, about those unprofessionally operated death battering machines...

[meepmeepmayer]

In my experience, current is the number to watch out for.

If the batteries can produce the current, they can take a percentage of that current as well. So any hill you did go up should be safe to go down, including automatically being safe from overcharging (unless you charged in between).

Also, monitor the current (e.g. Wheellog) - it's really low going down, unless it's a really steep hill. Compare to currents going up, usually they are higher. And to going in flat terrain.

Just beware of high currents for a long time, going uphills or downhills. Cable melting may ensue (at least on GWs, KS seems to be fine, no problems reported).

So current is the thing to watch out for, but if you have a KingSong, I don't think anything can happen unless you ignore warning beeps.

[Smoother]

12 hours ago, brjohnso said:

When I go up a steep hill, my EUC slows down based on the steepness of the grade. This is fairly obvious, since the power required goes up substantially. However, I've always deliberately slowed down and maintained a fairly low speed when going down any significant grade. My concern is that since there is no friction brake on an EUC, there is probably a limit to the rate at which power can be absorbed by the control board and batteries. Is this a valid concern? Is there a risk that if I descend a steep hill quickly, I won't be able to stop. Can the EUC lose control, or can the electronics be damaged from excessive reverse EMF?

1

You've had two good answers already, but I'd like to address one issue you refer to above in bold.  If the hill is steep you will find yourself rocked back hard on your heels just trying to maintain your safe speed.  Any additional braking needed will be severely hampered by the fact that you will have difficulty applying any more pressure with your heels. So YES you might not be able to stop is the distance you need to stop in.  Add to this equation, the fact that average adult male western feet hang off the back of the pedals, and you will find any further leaning simply results in your foot pivoting around the pedal's trailing edge; and not producing any more braking force.  So, yes keep it slow and controlled.  Falling off going downhill at speed adds a whole 'nother level of potential pain and injury, to say nothing of where the wheel might end up.

I have come down more than a few steep hills with my soles flapping lightly on the pedals.  It's not a nice feeling.  But today I have solved that and other pedal issues with my "Big Foot Pedals by Smoother".  Watch for the thread, coming soon.

[Smoother]

14 hours ago, brjohnso said:

When I go up a steep hill, my EUC slows down based on the steepness of the grade. This is fairly obvious, since the power required goes up substantially. However, I've always deliberately slowed down and maintained a fairly low speed when going down any significant grade. My concern is that since there is no friction brake on an EUC, there is probably a limit to the rate at which power can be absorbed by the control board and batteries. Is this a valid concern? Is there a risk that if I descend a steep hill quickly, I won't be able to stop. Can the EUC lose control, or can the electronics be damaged from excessive reverse EMF?

 

Actually, I don't think it's that obvious.  Potentially with a 1200W wheel and a well-stocked battery, there is no power requirement problem that will cause the wheel to slow down on a moderate hill.  I believe the reason (And I'm happy to be corrected on this) the wheel slows down is the contact patch of the tire on a hill, moves ahead of where it is on horizontal surfaces.  This puts it further forward of where your center of gravity is for flat riding.  As a result, your center of gravity is not in the same place, relatively, to your center of contact with the road.  So you are demanding less power that the hill requires, simply by doing nothing. (EDIT: and in fact it's a double whammy, because not only has your contact patch moved forward relatively your center of gravity, which weakens your power input, but the hill is simultaneously demanding MORE power (because it's a hill), for the same speed.  Because we are not delivering the demanded power, the speed reduces to match the power available with your current center of gravity/contact patch relationship,  Which is what it does all the time).  END EDIT  With unlimited power and battery reserve, one could hit a steep grade and lean hard into it to maintain the same speed.  Prudence, however, says let's ease into this to prevent a voltage sag/current spike that might face plant me (been there, done that, didn't like it).  The biggest problem to climbing steeply for western adult males is short pedals. The leading edges of my pedals fall 1/2 way across the balls of my feet.  If I lean forwards too much I just roll off the front, without providing extra power.  I've climbed a few hills balanced on the leading edge of the pedals, furiously "winding down the windows" to keep upright. Sometimes I didn't wind fast enough.  But I've fixed that, with my "Big Foot Pedals by Smoother" mod, which I might write up today.

[brjohnso]

Modest hills don't seem to be a problem up or down, but there are definitely some hills around here that the ks-16s can't maintain top speed when going up. Most are fairly short, so it's not a big problem. I simply make it a rule to go down those hills no faster than I can go up them.

[meepmeepmayer]

29 minutes ago, brjohnso said:

Modest hills don't seem to be a problem up or down, but there are definitely some hills around here that the ks-16s can't maintain top speed when going up.

I don't think you can expect any vehicle maintaining its top speed when going up hills. Seems unphysical

I wouldn't go by speed, going uphills I'd go by how hard you have to "push" the wheel to accelerate or keep the speed (I guess that's the available torque). It's something you feel, so trust your instincts. Simply don't overdo it.

And downhills, if you really want an assurance, look at the current numbers. Low current = no problem ("low" being about the same as you'd have going in flat terrain at whatever speed you enjoy there), no matter how fast you go down a hill. Because the batteries are the limiting part.

But really, it's hard for me to imagine any reasonably normal downhill situation that can be a problem. Uphills maybe, but with the KingSong S-models, no problems reported, they just seem to overheat warn in the worst case.

"Speed" can be deceiving, e.g. at low battery you go full speed, hit a tiny bump/pothole, and suddenly it turns out the wheel can't do any more and you fall forwards.

I guess "torque reserves" is better (because that's literally what's keeping you balancing and upright, and falling down if you run out of), but you can't see that easily. But you can feel it (at least gpoing uphills) in how nimble vs. strained your wheel reacts.

And since you can't feel a low battery, don't push it too much at low percentages. That's all - common sense, especially at low battery, and you should be good (+ wrist guards and other protective clothing).

@Keith may or may not be posting a diagram soon

[Smoother]

49 minutes ago, brjohnso said:

Modest hills don't seem to be a problem up or down, but there are definitely some hills around here that the ks-16s can't maintain top speed when going up. Most are fairly short, so it's not a big problem. I simply make it a rule to go down those hills no faster than I can go up them.

1

Firstly, if you try to maintain "top speed" on hills steeper than modest, you're well on your way to a face plant ( I know, I've seen me do it).  Secondly, a steeper hill moves the contact patch even more forward, so, of course, you're going to slow down more, unless you lean forward more, to put your center of gravity back where it belongs. The problem, as I see it, is those pesky short pedals, prevent one from putting down the power that is required, without simply rocking off the leading edge (I've seen me do that as well).  If you're having trouble visualizing this concept, Imagine your wheel (without its shell but still functioning) pressed up against a vertical wall.  Can you see how far forward the contact patch has moved?  90 degrees forwards, and impossible to compensate for (although, obviously you're not going anywhere).

A while back @Rehab1 did some hill climbing tests on various machines, one being a 16s.  He stated that the 16s struggled with a certain slope.  I thought at the time that 1200 Watts should be plenty and suspected the limits of the pedals, as I have been stumped on a steep hill with 800 Watts; not for lack of power but for lack of leverage.  The wheel wanted to climb, but my heels were literally 2" above the pedals as I tried to get enough force onto the front.  It felt dangerous as hell on that concrete hill, so I gave up and walked it.

I'll try and find a steep hill like @Rehab1 and try a climb with my new Big Foot pedals.

[Rehab1]

3 hours ago, Smoother said:

A while back @Rehab1 did some hill climbing tests on various machines, one being a 16s.  He stated that the 16s struggled with a certain slope. 

Here is the video That was fun! I need to take  Mten 3 and Tesla to the hill when I’m healed.

 

[Keith]

4 hours ago, meepmeepmayer said:

 @Keith  may or may not be posting a diagram soon

Something like this one perhaps:

As far as powering UPHILL is concerned this graph shows why there can be problems.

Firstly, it is important to remember that electric motors have a kV value  ( also known as RPM/Volt) so if the kV is (say) 10 RPM/V and 60V is applied  then 100% RPM is 600 RPM (approx 28MPH on a 16” wheel.), This is also known as the no load speed - and can never be reached as power and torque hit zero at that speed as can be seen in the above graph at 100% RPM.

Now if you are going up a steep hill, leaning as hard forward as you can (such that you are applying full voltage) and the wheel is only capable of travelling at, say, 14MPH (50% of max RPM) you are actually now putting out maximum output power. Efficiency will not be great but the wheel should cope OK (assuming good quality wiring and MOSFET’s of course!)

What happens if the hill gets even steeper? The wheel will slow down further however hard you lean. Let us say it is reduced to 7MPH, which is only 25% of max. As the graph above shows Current has increased a lot further BUT POWER OUTPUT IS NOW LOWER. Where is the energy going  if it isn’t giving you power? As heat of course and you are now in the danger zone.

If the hill gets so steep that leaning hard forward simply holds you in place then current will get as high as it can - 100% - and power will be zero (you are not moving) that may well be massively in excess of that which the wiring, and/or MOSFETS can take - indeed that point may have been reached well before this.

Don’t get confused in the above with (say) deliberately riding up a gentle hill at only 7MPH. If you are leaning forward only slightly, the 60V from the battery is being pulsed so the effective equivalent D.C. voltage is much lower. Let us say the voltage being applied is actually such that the no load speed at that voltage is just 10MPH. in that case the wheel is running at 70% of no load speed. This is well below maximum power and close to maximum efficiency - nothing will be getting hot in this scenario ?.

A wheel with a no load (absolute maximum) speed of 28 MPH is pretty slow. Suppose we now have a wheel with a kV of 20 RPM/V instead, that will have a no load speed at 60V of 1200RPM or more like 56MPH - much better - but in the same physical size motor this HAS TO mean less turns of thicker wire. The good news is thicker wire means the wheel can handle  more current (MOSFET’s allowing) but less turns means less torque. Now supposing we again go, at full lean forward, up a hill steep enough to slow us to 14MPH. For this faster motor this is back to 25% of no load speed, but due to the thicker wiring it is pulling way more current than our first example and heating is going to be a lot worst I.e. the faster the top speed of the wheel the more sensitive to steep hills it is going to be.

Downhill.

As far as power is concerned, any hill the wheel is capable of going up should be OK to come down. Regenerative braking is not as efficient so heating should not be a problem.

However, regeneration can generate quite high voltages (even if not that much power). If the battery is reasonably close to full then the voltage can get too high. Note this does mean that starting full charged and going up a short hill may still result in over voltage on the way back down even though less power will be put back than was taken out. Providing the wheel handles this correctly the result should just be that the wheel will beep and tilt back if it sees too high a regen voltage. Going down the hill more slowly will reduce the voltage generated - or try slaloming to scrub speed off without generating so much power.

 

[Smoother]

@Keith I'm going to have to read that a few more times before I get it, If I get it.

[The Fat Unicyclist]

19 hours ago, Keith said:

Something like this one perhaps:

As far as powering UPHILL is concerned this graph shows why there can be problems.

Firstly...

<awesome technical content clipped>

 - or try slaloming to scrub speed off without generating so much power.