BM3 copy wheel wobble

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BM3 copy wheel wobble

Postby funkykeyboard » 22 Oct 2016, 13:07

I have copied, engineering wise so far, the BM three. When I put the larger wheels on there is wheel wobble on the caster when I get to about 7 mph,, with the smaller wheels it is about 4 mph.

Slight differences, I moved the rear wheel and motor mounting 5 cm backwards.

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Any suggestions?
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Re: BM3 copy wheel wobble

Postby shirley_hkg » 22 Oct 2016, 13:28

:D It needs more damper . Put a nylon washer between the nut and bearing . Tighten it to an extend that the caster won't turn more than 3 quarters of a turn, when you slam it hard .
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Re: BM3 copy wheel wobble

Postby hank » 22 Oct 2016, 13:34

Not had any problems with my build at 9mph with phone gps speed.
Try a wave washer under fork nut. make forks just turn on their own weight
and not too loose when tighten locknut.
Try different pressures in fronts.
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Re: BM3 copy wheel wobble

Postby funkykeyboard » 22 Oct 2016, 13:58

sorry, are you both talking about tightening the notes at the top and the bottom of the caster, the one under the cap on the end of the frame?

Is there nothing I need to do with, the spindle that runs through the fork and the wheel? Can you see the picture, I have a countersunk screw and washer in the end of the spindle, which comes through the four hole, then inside between the Fat and the bearing of the wheel, I have a spacer and one washer. Same on the other side of the wheel.

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Re: BM3 copy wheel wobble

Postby funkykeyboard » 22 Oct 2016, 13:59

I think that is what you are saying. What size is the nylon washer on the Castor bearing nut?
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Re: BM3 copy wheel wobble

Postby woodygb » 22 Oct 2016, 14:00

TYRE wobble or wheel RIM wobble.?
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Re: BM3 copy wheel wobble

Postby shirley_hkg » 22 Oct 2016, 14:37

funkykeyboard wrote:I think that is what you are saying. What size is the nylon washer on the Castor bearing nut?


2mm thick and same size as bearing.
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Re: BM3 copy wheel wobble

Postby Burgerman » 22 Oct 2016, 16:39

Looks to me like you have the caster barrel tilted at an angle because the rear is too high.
Also those all terrain style tyres are hopeless quality and will fall apart. And probably shake.

I don't get it with slightly lowered tyre pressures even at 15 or 16 mph. And the casters are free to turn. http://www.wheelchairdriver.com/BM3-con ... /15mph.mp4
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Re: BM3 copy wheel wobble

Postby HotColors » 22 Oct 2016, 19:48

Burgerman wrote: I don't get it with slightly lowered tyre pressures even at 15 or 16 mph. And the casters are free to turn. http://www.wheelchairdriver.com/BM3-con ... /15mph.mp4


Wow, 15mph looks fast from that video. And you are chin pressing a camera and holding a phone. Are you sure you are not a stuntman? Looks really smooth though.

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Re: BM3 copy wheel wobble

Postby Burgerman » 22 Oct 2016, 21:41

Had to. The go pro in my mouth caused my jaw to cramp and it really hurt and wouldn't go away for about 20 secs! Not doing that again :shock:
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Re: BM3 copy wheel wobble

Postby shirley_hkg » 23 Oct 2016, 06:25

Burgerman wrote:Looks to me like you have the caster barrel tilted at an angle because the rear is too high.


I'd say it helps to reduce the problem , because the perpendicular distance from the stem to axle is increased a bit . It would have shaken even more vigorously if it were not the case .

The arms of the fork are too vertical , and should have been extended to the rear further .
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Re: BM3 copy wheel wobble

Postby Burgerman » 23 Oct 2016, 07:12

Apparently not! :P

The higher CG also causes this, which is why I say no heavy seating, and low tyre pressures and much care with assembly, balance and geometry. And those tyres are crap!
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Re: BM3 copy wheel wobble

Postby Burgerman » 23 Oct 2016, 10:07

Wow, 15mph looks fast from that video.


It allows you to take to the roads here and there, and overtake a few cyclists. And where the roads are empty the footpaths make little sense. It gets out of hand if its bumpy very fast though, and it feels a lot faster than that vid makes it look. At this speed fat tyres are not adequate alone, really you need suspension too. And a little extra length. At 6 to 8 mph its fine. At twice that it starts to feel like 4x as fast! And takes a lot of slowing down. I tend to go 8 to 12mph mostly.
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Re: BM3 copy wheel wobble

Postby Williamclark77 » 23 Oct 2016, 17:04

Funkey, I don't know if you will ever get those knobby casters not to wobble. There's so many causes for the caster shake that it's hard to narrow down. The first thing I would do, besides swapping them for smooth tires, is what Shirley said and add a wavy washer under the stem nut to add preload to the caster fork.

HotColors wrote:
Burgerman wrote: I don't get it with slightly lowered tyre pressures even at 15 or 16 mph. And the casters are free to turn. http://www.wheelchairdriver.com/BM3-con ... /15mph.mp4


Wow, 15mph looks fast from that video. And you are chin pressing a camera and holding a phone. Are you sure you are not a stuntman? Looks really smooth though.

Mike

15mph doesn't sound fast until you're doing it in a motorized recliner.

I believe this is after I geared mine down to 12ish mph.

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Re: BM3 copy wheel wobble

Postby popschief » 23 Oct 2016, 17:31

In my experience 12mph is plenty and only gets used when your traveling alone.
I've used that speed when with AB's for crossing high traffic areas because
drivers can't/don't see us low riders then you sit and wait while everyone
catches up. When at 10 to 12 mph in places unfamiliar you run the risk of
hitting uneven spots that big low pressure tires don't smooth out, ouch.
Running at these higher speeds requires more fiddling with tire pressure
to avoid wobble and higher battery drain due to higher gearing.

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Re: BM3 copy wheel wobble

Postby Burgerman » 24 Oct 2016, 19:14

He is getting caster problems at *4* mph! Which can only be the rear of the chair too high, inaccurate or uneven caster loading, caster barrel not vertical, cheap all terrain caster tyres, or some combination of the above. Adding damping, is not the solution. Its something you do once you fix the problem to be sure of no repeat. And the first thing to do is throw away the solid tyres, and the black ones... Both junk.
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Re: BM3 copy wheel wobble

Postby popschief » 25 Oct 2016, 04:31

4 mph ! ! That is serious. :!:


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Re: BM3 copy wheel wobble

Postby funkykeyboard » 25 Oct 2016, 19:50

I think Burger man is right.

I did explain above, the smaller tyres, the smooth solid tyres wobble at about 4 mph. When I go to the Larger circumference tyres, the off-road ones, it doesn't start until about 7 mph. So I deduced, that it is the angle of the Castor that is causing this. This will be my first point of call anyway.

This is just the prototype. Running around in it, to find such problems.

So, on the prototype can I put smaller tyres on the rear? rounded instead of flat like these would give more cushioning?can people recommend a more rounded, cushioning tyre, that has a smaller circumference, thus reducing the height at the rear.
on the front. Is there any adjustment in the two bolts top and bottom of the Castor, that can allow me to drop the Castor at the front, allowing me to raise the front and slightly? Can I put bigger tyres on the front?if so, can anybody recommend such tyres?

I'm looking for workarounds on the prototype. I can adjust measurements on the box when I make a new one.
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Re: BM3 copy wheel wobble

Postby funkykeyboard » 25 Oct 2016, 19:57

what size are the washers at the top of the frame and the bottom of the brain on the Castor? If I can put, to, three, or even four washers on the bottom, it seems this should raise the front end slightly?
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Re: BM3 copy wheel wobble

Postby Burgerman » 25 Oct 2016, 20:33

First lose those horrid front tyres they will likely always shake. The solid ones too heavy. The others are just nasty. Get some quality 3.00 x 4 ones.
Then don't raise the front, the CG is too high already. Lower the rear. Whatever that takes. Then find a really flat floor, and make sure that with about 6psi in the rear, and about 10 to 15 in the front that the load on each front caster is the same.
And make sure they run true. To a mm. And you may not think it matters but balance the caster wheels/tyres.
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Re: BM3 copy wheel wobble

Postby funkykeyboard » 26 Oct 2016, 15:09

these okay? http://www.ebay.co.uk/itm/TYRE-3-00-4-G ... SwOyJX3qt8
which rear wheels tires, would give me more cushioning than the ones I have?
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Re: BM3 copy wheel wobble

Postby funkykeyboard » 26 Oct 2016, 15:10

don't forget, my motors are 5 cm further back than yours, so the centre of gravity is not as far forward as yours.
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Re: BM3 copy wheel wobble

Postby Burgerman » 26 Oct 2016, 15:30

You mean back.

Those should be OK but the quality is variable and there's many makers and they all look the same... Inspect when they arrive. The kenda ones are better. Your CG change may make it better or worse. Hard to know.
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Re: BM3 copy wheel wobble

Postby Williamclark77 » 26 Oct 2016, 20:24

Just fyi, the four Kenda tires I have in that exact same size are roughly 1/2" narrower and taller than the identical looking and sized Chinese ones. Which is correct? Probably neither. :lol:
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Re: BM3 copy wheel wobble

Postby Burgerman » 26 Oct 2016, 21:01

Tyre sizing is "nominal". Meaning that first they make it. Then they measure what comes out afterwards...
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Re: BM3 copy wheel wobble

Postby inglegrump » 27 Oct 2016, 16:35

I had a huge problem with castor shake on my RWD chair at 6mph which, once I realised (prompted by BM) that the cure was not to put more weight onto the castors but less. I moved my seat back as far as it would go and I drive with the seat tilted slightly and the problem was cured. The chair is also actually better to drive slowly indoors as now the drive determines where it goes and not some dubious grip on the castors.

The idea of adding washers to try and 'tighten' a bearing is nonsense. A bearing will only work correctly if the outer is a 'bearing fit' onto say the wheel, ie: has to be drifted in, and the axle, again machined to a bearing fit and has to be drifted into the inner bore of the bearing. If that is the case then the bearing(s) will work as designed. If the shaft is simply a push fit into bearing then all that will happen is the shaft will rotate withing the bearing, at which point you might just as well have a solid lump of metal with a hole through it. Ideally castors should have roller bearings which have a much greater load capacity than a ball bearing and castor forks should have angular contact bearings which will cope with both axial and rotational force. Decent bearings (SKF - NTN ect) have an on load speed rating of about 18,000 rpm + and so should last forever. I was horrified to find on my chair that the castor axle was a bog standard 8mm diam bolt that will just pull out and the fork spindle, which again is just a bolt welded to the fork and that just falls out. I have a well designed and well built chair but they still do stupid things like this...page 1 in engineering school. :shock:
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Re: BM3 copy wheel wobble

Postby funkykeyboard » 30 Oct 2016, 23:03

ookay I'm going to pinch that idea, and move this seatback another 2 cm.

Still got to check the height of the lithium battery pack. If there is room, I will lower the crossbar at the top of the central unit 1 cm.

I will also balance the caster wheels.

And see where that gets me.
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Re: BM3 copy wheel wobble

Postby funkykeyboard » 13 Nov 2016, 16:07

Burgerman wrote:First lose those horrid front tyres they will likely always shake. The solid ones too heavy. The others are just nasty. Get some quality 3.00 x 4 ones.
Then don't raise the front, the CG is too high already. Lower the rear. Whatever that takes. Then find a really flat floor, and make sure that with about 6psi in the rear, and about 10 to 15 in the front that the load on each front caster is the same.
And make sure they run true. To a mm. And you may not think it matters but balance the caster wheels/tyres.

Went to the workshop to get them balanced,, and they said they wouldn't fit on the machine.. Where do you take them?

The only thing I can see different, is that the 2 MK batteries have slid back 4 inches. However,, the wobble has stopped at 8.5 mph with those horrible tyres on. We'll replace them anyway,, as you can feel them really bumpy.
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Re: BM3 copy wheel wobble

Postby Burgerman » 13 Nov 2016, 18:48

I balance them on my bed. Wheels first. Then with tyres. Same way I balance props, or anything. In the case of the wheels, two old bearings with grease washed out, and a touch of thin oil, no rubber shields. Axle and no spacers, in my hands. And the heavy bit stops at the bottom...
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Re: BM3 copy wheel wobble

Postby Williamclark77 » 14 Nov 2016, 14:21

inglegrump wrote:The idea of adding washers to try and 'tighten' a bearing is nonsense. A bearing will only work correctly if the outer is a 'bearing fit' onto say the wheel...


Lol. The wavy washers Shirley mentioned has nothing to do with the bearing fitment. They are to add a slight amount of axial preload to the assembly.

There are too many variables that can cause caster shake. They have to be balanced pretty well and loaded equally. You can't just say "do this" and it be cured. It can be fine one minute and something as slight as shifting your butt in the chair to one side can bring it back.
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