How do brakes work?

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How do brakes work?

Postby Qvickster » 08 Aug 2022, 06:52

What would happen if I was going 6 MPH and somehow I lost all power? Would the brakes lock up and I go flying (no seatbelt) or would it roll to a stop? Never wear a seatbelt so I was just wondering what would happen.
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Re: How do brakes work?

Postby shirley_hkg » 08 Aug 2022, 07:02

Qvickster wrote:What would happen if I was going 6 MPH and somehow I lost all power? Would the brakes lock up and I go flying (no seatbelt)
Definitely an instant lock up.

or would it roll to a stop?
You still need power to Roll-a-bit .

Never wear a seatbelt so I was just wondering what would happen.
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Re: How do brakes work?

Postby Burgerman » 08 Aug 2022, 10:35

Go along slowly, and while rolling, just turn it off. Then you will know quite how that feels.

Its not so much the brakes that will stop you but the reverse EMF from the motor which will probably bring you to a very rapid stop. This can be programmed to be faster or slower. There are legal certification reasons that mean it MUST be a rapid deceleration. But from testing on many chairs I can tell you it will stop *very* fast!

Depending on your seat angle, (dump) and if you are on a hill/ramp/slope you could well fall out of the front. Its quite sudden.

But ideally when your seat is set to its lowered (non tilted) position there should still be around 5 degrees (or preset "dump") angle. This is for 3 reasons.
1. It prevents the sheer forces from the weight of your torso pushing your backside forwards against the cushion. Which helps prevent pressure sores.
2. Its just more comfortable to be tilted back a little for the same reason, plus your body doesent want to slump forwards.
3. It actively prevents you sliding out of the front if you stop unexpectedly!

Dont do this by activatng "some" tilt and driving. It puts excessive load on the tilt mechanism, and actuator and will sooner or later break something and in any case the seating feels less rigid/solid. Get someone to physically adjust the seating mounting to give this "dump" instead. Most chairs allow for some adjustment. If not use spacers under the seat mount on top of the seat mounting surface. Or whatever it takes. All part of properly configuring the chair. And also you should use a lapbelt but nobody does...
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Re: How do brakes work?

Postby foghornleghorn » 08 Aug 2022, 11:25

Burgerman wrote:Go along slowly, and while rolling, just turn it off. Then you will know quite how that feels.

Nope. That is "Soft Stop" and is Yes/No programmable. Complete electrical failure removes any programming from the equation as the controller is now dead.


No power - for example if the battery has somehow become disconnected - parking brakes will instantly lock on. Any chair will be trying to throw you out if you have no seatbelt on.

Rear wheel drive chair will skid to a stop.

Front wheel drive chair will stop dead, standing on its nose / lifting the rear wheels.
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Re: How do brakes work?

Postby LROBBINS » 08 Aug 2022, 12:06

you should use a lapbelt but nobody does...

Not quite nobody. Rachi actually uses a four-point sports car harness needed to keep her positioned despite distonic movements. It's also saved her bacon a couple times in the last 30 years. For example, when one wheel of her FWD chair hit an immovable obstacle while moving quickly on a too-narrow sidewalk (I guess that's "footpath" for you Brits) and flipped her sideways off the curb and into the roadway. No injuries, and she thought it very funny. I didn't.
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Re: How do brakes work?

Postby Burgerman » 08 Aug 2022, 19:14

Well we all should! But OK most of us dont...
Unless near a keyside, harbour, river...
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Re: How do brakes work?

Postby biscuit » 08 Aug 2022, 19:42

Burgerman wrote:Well we all should! But OK most of us dont...
Unless near a keyside, harbour, river...

I rather think I can float free and let the chair sink alone. Don't bodies float naturally? I do not think I could find the restraint releases quickly enough if I fell into water.
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Re: How do brakes work?

Postby Burgerman » 08 Aug 2022, 20:01

You got me backwards. I mean you should wear one. UNLESS near water. You dont want to be tied to an anchor! :fencing
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Re: How do brakes work?

Postby ex-Gooserider » 09 Aug 2022, 01:09

If you use a belt regularly, you should have enough 'muscle memory' developed to be able to find the release w/o needing to think about it... At least I did when I had that Caddyshack moment a few years back... :oops:

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Re: How do brakes work?

Postby Burgerman » 09 Aug 2022, 08:37

True, but theres lots here with quad arms, that dont work well, and other movement dibilitating deseases like MS etc.

I mean you would likely drown anyway then though! :problem:

Best bet in a chair may be to stay well away from drops, water, edges, train platforms, traffic...
Because I have personally been in a chair that just took off on its own. Theres a few interference, magnetic effects, and failure modes that can make that happen. Fortunately for me it was a super powerul magnet in my hand. And I realised that was making the joystick respond magnetically, and drive me backwards/left into my bench hard. So easy fix, move it away!

Just a warning. But know how to turn it off fast! Where to pull out a bus cable, a battery link or something just in case, because shit happens.
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Re: How do brakes work?

Postby Fusiongoat » 09 Aug 2022, 18:49

LROBBINS wrote:
you should use a lapbelt but nobody does...

Not quite nobody. Rachi actually uses a four-point sports car harness needed to keep her positioned despite distonic movements. It's also saved her bacon a couple times in the last 30 years. For example, when one wheel of her FWD chair hit an immovable obstacle while moving quickly on a too-narrow sidewalk (I guess that's "footpath" for you Brits) and flipped her sideways off the curb and into the roadway. No injuries, and she thought it very funny. I didn't.


This is a good idea. Lap belts really won't keep you in the chair if the it's more like an accident or even lurching forward violently. I also have cp, but until I was 25 I never really wore my belt. I fell out of my chair a few times and then decided to grow up ;) I also get nervous near water and drops, but really I think that one won't be able to save himself after falling from a ledge etc., so best to focus on driving technique and not convince oneself that he can parachute out like James Bond. It ain't gonna work out as we imagine hanged :clap
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Re: How do brakes work?

Postby Burgerman » 09 Aug 2022, 23:58

What really does help is a sensible seat dump base angle. Then you will not slide forwards regardless. And better still it prevents the sheer forces caused by your spine loading the pelvis and pushing it forwards so you are "forced" forwards in the seat over time. That forward sliding force you get on a flat seat base causes pressure ulcers via the sheer forces that "push" your ass forwards as you sit. So think about some adjustment! :hammer
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Re: How do brakes work?

Postby Burgerman » 10 Aug 2022, 00:07

Nope. That is "Soft Stop" and is Yes/No programmable. Complete electrical failure removes any programming from the equation as the controller is now dead.


Yes thats correct. Just tested it.
Well sort of correct. The power module is still powered by the regeneration it gives as you decelerate. To much so in that if a battery is disconnected the voltage spikes as the power has nowhere to go and can damage the mosfets. Theres a large warning about thisin the roboteq manual which works with mosfets in the output stages in the same way.

I pulled the charge 12/24V "loop" out as I was rolling in my kitchen. VERY fast sliding stop. The brakes are applied. But alone they are not strong enough to lock the wheels. I can push a chair with the brake on when its deactivated. And soft stop isnt. Its "softer" but feels pretty much the same to me. The only difference is that the brake comes on only when it is stopped. It decelerates pretty much as fast as far as I can tell on a tiled floor. So soft seems to be as fast as the motors EMF shorted out can stop it.

Either way with some seat dump, I dont fall out! So brake or no brake I dont need a belt. I am T4 and no trunk control. The dump/reclice sorts that out well.
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Re: How do brakes work?

Postby swalker » 10 Aug 2022, 00:19

Qvickster wrote:What would happen if I was going 6 MPH and somehow I lost all power?


Oddly enough, this happened to me recently. It was with a Permobil C500, which has a maximum speed of only 5 MPH rather than the 6 MPH you asked about.

I was going downhill at full speed and my wheelchair threw a code for the voltage being too high. When that happened, the wheelchair shut down.

When it did, the brakes were instantaneously applied at full force (as you would expect given how wheelchair brakes work). This caused the wheelchair to pitch forward with the foot plates making contact with the ground. The foot plates folded upwards, flexing both my ankles.

I wound up with strains in both ankles as a result of this incident.

I always wear a "posture support belt" (that is, a seat belt). I am glad the wheelchair did not tip over, as I had no way to eject out of the chair!

It was certainly exciting!

Steve
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Permobil C500s VS
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Permobil C350 Corpus 3G
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Re: How do brakes work?

Postby Burgerman » 10 Aug 2022, 00:33

In the same way my chairs batteries have a bad cell, and the chair stopped lke it ran into a brick wall as the voltage DROPPED to the point the system switched off, yours did the same thing. Just in the opposite direction. Your voltage spiked due to regeneration current. Why would it do so? Battery impedance too high. Likely either cheap batteries or old/knackered batteries. Thats the only way that the voltage can spike that high. Unless you have a bad battery connection or battery cable somewhere. So take a very careful look.
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Re: How do brakes work?

Postby Frank » 28 Aug 2022, 14:49

I refer to Soft Stop Rate and mine is set at 70 and forward deceleration at 8. Last time I hit the switch off lever while driving and the chair stopped abruptly. If I set this value to 0 will the chair stop smother?
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Re: How do brakes work?

Postby Burgerman » 28 Aug 2022, 17:03

It should. Thats what it claims to do. Test it. I never have. It might run a long way with no way to steer or stop though. Into traffic?
So 0 might be a little extreme. Only way to find out is to try it. Same with every setting.
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