Interesting night...

[Burgerman]

Went to the pub as usual, 5 pints and a bag of peanuts later arrived home. I always let my somewhat angry dog have a run to the end of the street to pee on the tree at the roundabout as by then everyone else here is well asleep.

So as usual I let him out the front door, and he shot off looking for the fox he had been watching from the front door.

I went after him to try and stop him barking and waking up neibours. A car came around the corner, and started down the street. I accelerated too hard while trying to grab his collar on a bit of the road that was not level laterally.

The BM2 chair is very "lively" and wheelies violently if you have no feel for these things. Which for the 1000th time that day it did. I do have a feel, and good control, but like it to wheelie! But being a bit worse for wear I didnt take into account the narrower track, (fat tyres inboard motors) and narrow anti tips that allow great manoeveing in tight spaces indoors. Remember we are driving along an uneven and varying lateral slope here at the edge of the road.

So up the front came, as usual... And then it hit the inboard narrowly spaced anti tips. And the wobbly fat tyres at low pressure allowed the chair to lean further left... And further... And I landed on my side half on the curb (pavement) and half on the road.

Mmm. Now its raining. I am laid there for about 1 minute and I rang my carer who is in my house where the dog came from. Obviously he came in a panic, and I told him he couldnt lift me, and my powerchair alone. So I called the fire brigade for some strong arms to right me! How embarasing. Anyway I thanked them, and no damage done.

So those that want a BM2 that is short, with fat low pressure tyres, and narrow anti tips, and programmed to "go" be warned! I expect this every few months, and accept the consequence. But it may just surprise a few of you! Everything is a compromise.

[LROBBINS]

John,
It sounds like you could use my accelerometer-microcomputer gadget to reduce power when the chair approaches tip over. It lets one go full out, and drops power only if things are getting out of hand.
Ciao,
Lenny

[Burgerman]

I have done it to drag bikes and road bikes at high speed, (flipped them when accelerating hard and hit the http://www.nitrous.info button). And at least 4 times in a powerchair, and dozens in a manual chair. Never hurt once. Bikes looked a bit worse for wear.

But it didnt flip over the rear, it went over to the left side due to the angle of the road, and narrow anti tips. Chair rotates to about 30 degrees or so, back, but the tyres are soft and inboard. When the weight lands on the anti tips, it literally fell over sideways. I normally run 30 to 60 feet on those anti tips for fun. But on level ground.

[LROBBINS]

Tipping Rachi over when the high-torque motors climbed a natural gas pipe sticking up out of the sidewalk was the reason I started experimenting with the 3-axis accelerometer. As it responds to both dynamic (shock) and inclination (gravity) acceleration, and combines all 3 axes to reach a "decision", it should slow the chair before a sideways tilt gets out of hand. Ciao, Lenny

[Burgerman]

Trouble is I often get to those angles on purpose. How will it know I am going to keep the gas on 1/10th second after I should have lket go. It has to wait for it to go too far before it reacts. Which I did myself. If the road camber wasnt there it would have been fine. Not sure it can be made to understand the difference, or if shutting the throttle would have helped ince it reached that point? If I wasnt leaning the wrong way to hold the dog it would also not have happened...

[LROBBINS]

It sums inclination (the effect of road camber) with dynamic acceleration (inertia of the cg), responds in a very few milliseconds, and you can program the trigger points as close to disaster as you wish. Ciao, Lenny

[Burgerman]

What about combinations of roll and pitch?

What I am getting at is I often wheelie some distance where I am the thing holding the chair on its balance point with the joystick. Normally this is no problem, quite the opposite, its fun and often needed to get the wheels up a curb. So I aproach curb, tip back (accelerate) to lift front wheels as high as possible and bang the anti tip wheels into the road, and the chair is at 30 degrees. As high as it can go. So the last thing I need is something taking over and shutting the power off as I head for a 4 inch curb! The idea of hitting it at speed with the caster wheels is not good! Sometimes this is on a downward slope, sometimes the opposite.

How can setting this device help? Unless its also looking for ROLL acceleration. But even then, one wheel may hit before the other. But powering off would be bad...

These things scare me. Once dropped a Honda Blackbird belonhging to a magazine. Some idiot at the factory decided that linked brakes was a good idea for the masses. So rolling down a wet steel plate, off a ramp out of a truck, I applied some rear brake. So that the front wheel didnt klock up. Guess what happened! And I can repeatedly out brake anti lock brakes in wet or dry conditions - tested on bikes and my ford car, at bruntingthorpe by performance bikes magazine from 100 to zero. By an absolute mile. So I have a natural distrust for such things as ABS etc!

[LROBBINS]

If you're reflexes can beat an ABS, I guess that this won't do for you, but yes, it does read roll acceleration as well as tilt (or rather, the sum of the two as its Y axis). Hence, you can tilt to say 20o slowly without it activating, but if you go to 20o quickly it will. Ciao, Lenny

[Burgerman]

ABS is dismal. Even today. You can beat it too. Especially in a car. Try it. Set out some cones on a runway as we did in PB magazine. Do a few 100 to zero stops. mark end point. Then pull off an ABS sensor plug to disable the ABS. Re test. You will be surprised how easy it is to beat even on a wet runway. Its really quite easy especially if you ever tortured cars or bikes on a track. Its at its best when on a flat dry road. And still beatable.

On most cars its slow to respond and does it in lazy innacurate pulses. But worse it senses each wheel for speed. And releases all four brakes at the same time when it senses one is locking.

Worst case scenario then is where you are entering a bend at high speed say. You brake about 70 percent of the max retardation possible as you turn in towards apex and the inside wheel locks up as it is more lightly loaded. Just because that wheel locks for a second does not stop it slowing you down, it just isnt as good. But now you can ignore that one and add another 30 percent to the others... You slow faster, make a bit of smoke from one inboard wheel into a bend.

But you cant do that when the stupid dumb abs keeps letting go of all the brakes to release one intermittently locking wheel... And you run wide or worse.

Same happened to me while braking only about 30 percent aproaching a que of cars in the wet in winter. When the left side wheels went over some remaining snow at the edge of the road. It refused to let me continue to brake with the two wheels on the dry side, as it was trying to not let the left ones lock... I couldnt care less about those! I almost hit the back of the que. Without ABS I would just brake harder and the dry side would still grip/slow me. I dont like ABS... Safer for my mum, but a liability if you drive like I used to do. This was in 97 in a Granada Scorpio, but the way car ABS works hasnt really changed.

[LROBBINS]

Agreed, ABS isn't very smart at all, and I'm sure you, and perhaps even I, can beat it when making a deliberate maneuver or whenever you're paying absolute attention to what you are doing. Where ABS does work is in dealing with the unexpected - where its reaction time is quicker than the less-than-totally-attentive driver's.

I'm afraid my device is also rather dumb in that sense, because it works through the DX controller rather than independently of it. Thus, if a problem is sensed, it sends its signal very quickly (microseconds), but the DX with its slow CAN bus responds when and as it will. Moreover, it might be better to be able to differentially control the two motors, as you would like for ABS, but there's no way to do that. But again, the accelerometer is there to deal with the unexpected or unseen situation. Had I seen the gas pipe coming up from the too-narrow sidewalk, I would have just slowed down a bit and steered around it. Instead, I was to the right and behind Rachi's chair and the pipe was on the left completely hidden from view. I hit it going almost full out, with the chair hitting an obstacle and my body and hand continuing forward the ACU joystick was perforce pushed hard forward, and the chair just "decided" to climb over the pipe. I would not only have needed super-quick reflexes, but I would have had to repeal one of Newton's laws, to get my hand back fast enough to keep the chair from flipping. A microcomputer doesn't suffer the effects of inertia.
Ciao,
Lenny

[Step]

Not that this would help in any way but....

It's such a shame nobody filmed that!
You could've been the next 'Fenton' going viral!

http://www.youtube.com/watch?v=3GRSbr0EYYU

'Arnie.. Jesuus Christ... Arnie!!!!''
lol

[Burgerman]

I dont want to be famous for that...

[Lord Chatterley]

You ought to be more careful - you might end up a wheelchair.

LC

[Burgerman]

Well I did end up in one. The same one I started off in. Thanks to the fire service!

[Lord Chatterley]

I hate the camber I encounter on paths - it is that precise issue which caused me to ask this recently-

Re: Hegar wheels and groove motor adapter
by Lord Chatterley on Tue Apr 17, 2012 6:51 pm

A standard F55 has hard tyres that in theory should prevent the chair from rolling sideways excessively.
The off road tyres are softer, so I imagine they allow greater roll.
The Kshield turf tyres with Hegar wheels make the footprint wider and seem more rigid so allow less roll than the off-road tyre but more roll than the standard F55?

LC
Lord Chatterley

Re: Hegar wheels and groove motor adapter
by Burgerman on Tue Apr 17, 2012 7:27 pm

I see...

OK here goes...

Stock tyres are hard. But they have suspension that makes up for this on big level changes etc but does nothing for the sort of vibration that causes my spasm to kick off... Like cobbles etc. And they sink in soft stuff. Overall though about the same amount of total movement as the all terrain (good tyres not cheap) or turf tyres.

So there should be no difference right? Wrong...

The overall track of a vehicle is determined - as far as roll is concerned - by the centre of the tyres contact patch. Or its centre of pressure. In order to preserve the actual same width as the 3 inch tyres, but use 6 inch tyres we reduce the effective track by some 3 inches each side.

So the TRACK on a stock powerchair is 26inches approx less 3 inches so 23 inches.
The effective track of the fat tyre version is 6 inches less than this. so 17 inches. So it rolls much more.

Its not as bad as it seems, because as it rolls, more of the outside edge of the tyre supports the load. So after a few degrees lean the tyre centre of pressure moves towards the outside edge. It makes the chair feel more tippy in a sideways direction. But in reality it isnt any easier to tip right over. But it does allow caster problems as to not flutter these need equal weight. Or more weight. So the casters must run at low pressure too. So as to maintain ground contact.

Now if the c of g is TOO rearwards casters will flutter. If the c of g is too high (car seat?) the casters will flutter due to more roll of the chassis. So tyre pressures MUST be equal and everything built dead straight too. All of these things have to be conbsidered when building a chair! The reason I added extra weight to the BM3 Centre section is to lower the C of G to prevent flutter that I am actually expecting to be a problem. And I will move the seat forwards 1 inch over BM2s as well. Get any of this wrong and you make an unusable chair.

The Hegar wheels are the same size as thew cheap all terrain quad bike wheels, just better quality.
Burgerman
Site Admin

Re: Hegar wheels and groove motor adapter
by Lord Chatterley on Tue Apr 17, 2012 9:17 pm

Excellent - that is exactly what I wanted to know.
Is there any noticeable difference in the amount of roll with the Kshield turf tyres and the fat off-road tyres or are they more or less the same?

LC
Lord Chatterley

Re: Hegar wheels and groove motor adapter
by Burgerman on Tue Apr 17, 2012 10:14 pm

Not a massive difference, but more stable with the squarer turf tyres.

It is dangerous and often so unnecessary - the landscape is often as flat as a pancake any yet the council workers still manage to create very hazardous camber - as you lean over the steering becomes light and the weight of your arm moves your hand and the next thing you know you are in serious trouble - especially alongside fast road traffic.

LC

[Burgerman]

Its also why I keep trying to explain to people not to fit car type heavy seats, that also sit you up higher on the BM2/3 chairs.

[ex-Gooserider]

Note that the camber is often necessary to encourage drainage in the correct direction, though it's often overdone... Lack of camber leads to puddles at best, and can cause serious erosion issues if not carefully laid out...

When I was a kid we had a summer cottage that had a gravel drive on a fairly steep hill, and we had to pay a LOT of attention to maintaining it, with carefully laid out swales to guide the water of the drive and slow it down. If people weren't used to driving gently on it, they would spin and cut a small groove - which would turn into a major canyon after the next rain if not promptly filled...

If you look at any text on how to build trails and such, you will find they put a lot of emphasis on designing for controlled water flow in order to keep from getting major gullies in the trail or surrounding landscape areas.

ex-Gooserider

[Burgerman]

Yes but 1 degree is plenty steep enough for drainage. In some places where they re dress roads instead of ripping it up and starting again, this can be so steep its undrivable on in a powerchair. And same with pavements that have been badly laid or subsided. And some curb cuts here have a 2 inch step just to get on them!

[Phil Esgate]

I've given up on pavements - I use the roads all the time, unless it's the East Lancs dual carriageway. Not only are the pavements so badly tarmaced and cambered, in coucil house areas that I pass through to get to the shops, they have all had concrete drives put in (for free), which involves them having a retro fit dropped kerb for each drive. As most of the council houses are terraced this means every few yards! - up down up down camber up down camber up down camber. At 6.2mph that's a recipe for further spine damage, ostomy bags being ripped off or worse being tipped out. - I was lucky the other day, when that happened to me an ambulance was passing and they managed to scrape me up off the floor and deliver me home.

[Burgerman]

I was on the road!