Hub motor in bike trailer to aid steep downhill braking?

Mike_Kelly

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I have a single wheel burley coho and am concerned about steep downhills in high mountains.
Anybody ever use a hub motor to regen brake the trailer?

Thanks
 

amberwolf

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It's been done but how well it works depends on the controller used, the motor used, specific situation at the time, road conditions, tire type, loading of the trailer, trailer hitch design, if the bike has suspension or not (which affects where loading goes because of pivoting actions over terrain), what kind of braking the controller can do, etc. It also depends on the battery's ability to accept whatever regen current is generated, continuously for as long as the worst case conditions would occur...

Unless you have a high enough load in the trailer to force traction on the trailer wheel during braking, it may just lock up the wheel and drag the tire across the road surface. If the trailer is just a single wheel and the forces from it are not completely aligned with the bike, and the trailer can pivot sideways at all, it may just jackknife with the load sliding to the side and around to yank the back of the bike sideways. If the trailer can only pivot up and down, then that won't happen, but traction can still be lost and you end up dragging the tire, wearing flat spots in it.

If all you have is the typical on/off one-level regen braking most controllers have, then the force may only vary with speed, higher speed giving more force and making it more likely to break traction. Usually these also cease trying to brake (or being able to) below some not all that slow speed.

If you use an FOC controller then they usually have proportional / variable braking response that you can adjust in the controller setup for both how much response and how fast it works up to and how slow it will go down to.

So with the typical controller with on/off regen, you might not get enough braking to do anything useful, or it might always be so hard that it tends to break traction, and you just get what you get depending on the situation.

FOC or anything else with variable regen (controlled by throttle or brake lever or whatever, by the rider) gives you the chance to control the braking amount so it stays below the traction threshold. You may still not get enough braking, but at least you can keep traction all the time.

Braking also doesn't regenerate all the energy into the battery--a significant part is lost in the motor itself, and the phase wires, and the controller. In normal city braking this doesnt' matter usually, it just spikes the heat momentarily and then that cools off over the ride. But continuous downhill braking could overheat the motor, wires, and/or controller, and damage them.

Another consideration for using regen braking for long durations and under some other conditions is whether there is any chance of refilling the battery beyond full charge. If this could ever be a problem, some special circumstances that can be very bad can happen. Controllers often (usually) have an HVC for regen setup so that if battery is above that point it won't even allow regen, but if it's a lower voltage battery design than the controller was built or setup for, it wont' kick in. Common controllers are not adjustable for this...FOC should be adjustable, and some others may be.

First, if the battery or controller doesn't have a way to know this is happening and stop it, the cells can be overcharged, which can damage them, leading to future risk of fire.

If the controller HVC works, it prevents this, but if not...then if the battery does have a way to protect itself (common port BMS so it can shut off regen current charging), and it does do this, the generated voltage from the motor without any load on it now will suddenly shoot way up, and can be beyond what the controller parts can take, so it blows up...most commonly taking out the phase FETs which then locks up the wheel (or at least makes it very draggy and hard to turn). Sometimes it also takes out the LVPS in the controller, or other parts; in this event if the failure is of the rare wrong kind it can also take out the motor hall sensors and throttle/pas sensors, etc.
 

Mike_Kelly

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Thanks for the detailed reply. I was not sure if regen braking was effective enough but you can't get anymore effective than locking up the wheel. I am not really concerned about regen for charging so one possibility would be to just put on a resistive load to solve the charging problems you note. The route has 200,000ft of climbing and that means a lot of long downhills. I am just concerned about the brakes on the bike being enough to stop the bike and 90lbs trailer on a long downhill.
 

amberwolf

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If you never want to use the motor as a motor but just as a drag brake, there are two "easy" ways to do it with a brushless direct drive hubmotor:

--Three phase rectifier (like that in alternators, etc), phases wired to the three inputs, and a resistive load wired across the two outputs. That resistor can be a big coil of magnet wire wrapped around the trailer framework. This gives you only on/off braking, with no control over braking force, just max possible with that particular resistive load. May not be appropriate for all situations, as noted before. This requires a big switch (or contactor controlled by a little switch) to connect the resistor to the output of the rectifier; the switch has to handle the full voltage this system might generate (on the order of the typical battery voltage for that motor to get the speed it would be braking from, and the full current the resistive load will draw via the bridge).

A variation of this uses a second big switch (or contactor controlled by a little switch) to disconnect one of the phase/bridge connections, which decreases braking current by a significant amount when less braking is needed.


--Three phase rectifier (like that in alternators, etc), phases wired to the three inputs, and a simple cheap brushed controller's battery inputs wired across the two rectifier outputs, and a resistive load wired across the controller's two motor outputs. That resistor can be a big coil of magnet wire wrapped around the trailer framework. The throttle input is used to vary the braking level. This gives you nearly complete control over braking force for as long as the controller is powered by the motor (or if you use a tiny auxiliary battery of normal controller voltage to run the controller brain so it can turn the FETs on and off as needed, with a diode in the battery supply line so it cannot be charged by the regen to prevent battery damage). The controller's design and operational limits (HVC, LVC, current limit, etc) limit usage scenarios, so you'd need to choose the controller based on how you want the system to work....and this would be experimental, possibly requiring other parts I haven't thought of.
 

amberwolf

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Regarding regular mechanical brakes, you can look into tandem bike brakes for touring; I've seen discussions around the internet about those solutions on various cycling forums.

One option is multiple sets of brakes on each wheel. For instance, you could use dual discs in front, largest possible rotors with as much metal on the rotor as possible, for the most thermal mass (so not the typical bicycle barely-there rotor, but more like a full disc from center out to edge, and probably thicker). Largest pads possible, largest caliper mass, etc.

You could also use rim brakes in addition to the disc for a bit of load spreading...but you don't want to keep running those hard because if you heat the rims enough the air in the tires may expand enough to exceed the pressure they can take.

There are also drum brake hubs...you might be able to modify those to *also* take a disc brake rotor, so you could have three mechanical brake systems per wheel.
 

calab

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I've seen brake levers with dual double cable mounts, with two barrels for adjustment of each. Sunlite or Wuxing. I've had one that was a Wuxing dual cable.
 

Chalo

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You could also use rim brakes in addition to the disc for a bit of load spreading...but you don't want to keep running those hard because if you heat the rims enough the air in the tires may expand enough to exceed the pressure they can take.

Fluctuation of tire pressure with temperature is a common thing to be concerned about, but it's really not much of a concern. Remember that pressure is proportional to absolute temperature, which doesn't change as much as most people think. If braking raises the internal temperature of your tire by 100 degrees F from ambient, your tire pressure change will be from (say) 535 degrees Rankine to 635 degrees Rankine. That's less than a 19% increase in pressure for a hundred degrees temperature rise.

A more relevant effect is that of temperature on the material properties of the tire. If the tire gets stretchier, slipperier, or weaker from elevated temperatures, that's probably more significant than the increase in pressure.
 

Mike_Kelly

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Tires getting heated up is a real problem. We have bike toured in Italy in the Dolomite mtns many times and had tubes over heat and burst. Disc brakes were the worst brakes by far. Rim V brakes with rest stops to let them cool worked and my wifes bike we had rim brakes and a drum.
The problem with mechanical brakes on the trailer is running the cables and having them tether all the time. I need something like the car trailer brakes that actuate when there is pressure on the hitch. With hub motor brakes I could rig up a wireless actuation from the bars.
I guess I need to see how the bikes brakes work alone and then decide. I am usiing TRP Spykes with 200mm rotors. My previous experience says it won't be enough.
Thanks for the thoughts everyone.
 

calab

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Wireless remote control braking system would be easier to fabricate/construct then a pressure sensor on the hitch.
 

sysrq

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Tires getting heated up is a real problem. We have bike toured in Italy in the Dolomite mtns many times and had tubes over heat and burst. Disc brakes were the worst brakes by far. Rim V brakes with rest stops to let them cool worked and my wifes bike we had rim brakes and a drum.
The problem with mechanical brakes on the trailer is running the cables and having them tether all the time. I need something like the car trailer brakes that actuate when there is pressure on the hitch. With hub motor brakes I could rig up a wireless actuation from the bars.
I guess I need to see how the bikes brakes work alone and then decide. I am usiing TRP Spykes with 200mm rotors. My previous experience says it won't be enough.
Thanks for the thoughts everyone.
How do you use 200mm rotors with TRP Spykes if all large rotors are for narrow type pads and TRP Spykes are using wide type pads? This creates accelerated pad wear according to Shimano.
 
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