Have you ever had your brakes fail?
- Never
- Once
- A few times
0 voters
Do you use a rheostatic braking resistor?
- Yes
- No
- Wtf is it?
0 voters
Please share your thinking or experience below. Trying to understand how common is this problem.
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I owned a rheo brake and never used it aside from forcing it in testing. If I lived on a huge hill and was planning on starting my ride off braking, it’s for you.
Otherwise, you’ll likely never need it.
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The brake failing problem is mostly theoretical. Under extremely narrow circumstances it could be a massive problem.
If you wire your BMS bypass you won’t ever have any problems.
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I am curious because last weekend I experienced brake failure about mid way through my ride while descending a long(about half mile) hill. The battery not being close to full, I thought I should have no problem. Yet half way down the hill the brakes suddenly released, causing a sketchy ass dismount (luckily I was able to run it off). The brakes would engage for only split second immediately after that before releasing again. Yet later in the ride the brakes seemed to be back to normal. I was able to recreate the behavior the next day under similar circumstance.
If I understand it correctly, the vesc splits the current generated from the motor braking. Feeding the max bat amperage (set to 6A) to my 2p battery, and dissipating any excess as heat in the controller. This begs the question of what happens when the VESC overheats on a long hill? Is this possibly what happened in my case? In that case would rheostatic brakes help?
Sounds like overcurrent faults
(disclaimer: not an expert) Yup that seems like a mostly accurate summary, and the short answer to “what happens when it overheats” is what happened to you: can’t brake any more.
Most VESCs have cooling designed to deal with heat from inefficiencies in the circuit. This comes from conduction losses (MOSFETS that are turned on and passing current act as a tiny resistor) and switching losses (every time a MOSFET switches on or off it very briefly passes through a middle point between conducting and not conducting, ie it is a less-tiny resistor).
For a good design, efficiency can be very high (90%+) so the energy lost as heat is low relative to the total power passed through, and the heatsink is sized to accommodate this with a bit of margin. When you’re trying to break, you’ve gotta convert the entire kinetic energy (your momentum) to either chemical energy in the battery or heat if you just want to dissipate it. These are super loose figures but for illustration this could mean having to sink heat approaching ~100% of total power instead of ~10%, so the heatsink reeeallly doesn’t like that once it’s been running for more than a few seconds.
Three basic solutions: brake for only a short period of time so the heatsink has a chance to get rid of all the heat and doesn’t burn up your MOSFETs, brake more slowly (power is energy per second, slowing down the rate of deceleration means dissipating less power), or find somewhere to dump all this energy like a braking resistor. Usually one of the top two applies because you only have to brake for a few seconds or you don’t have to brake super hard, but for extended downhill rides or with a heavy rider (more mass, more energy to get them to stop) or whatever, but if you’re having the problem then grab a braking resistor.
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