BM3 copy wheel wobble

[funkykeyboard]

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?

[funkykeyboard]

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

[Burgerman]

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.

[Williamclark77]

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.

[Burgerman]

Tyre sizing is "nominal". Meaning that first they make it. Then they measure what comes out afterwards...

[inglegrump]

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.

[funkykeyboard]

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.

[funkykeyboard]

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.

[Burgerman]

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...

[Williamclark77]

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.

[funkykeyboard]

Not sure I said earlier, but for some reason the wobble went away on the front-end. After that, I moved the seatback a further 2 cm, and put the new tyres on, and now the wobble is back again

Can I just double check the fitting of the front wheels? Is it one or 2?
1. It is wheel bearing-washer-spacer
2. It is wheel bearing-washer-spacer-washer

[Burgerman]

It depends what we are talking about.

Wheel or caster bearing?

Wheel
I use 10mm axles, drilled and tapped and this goes through the fork as well.

So its Countersunk stainless Bolt > Flat countersunk washer > fork leg > Spacer > bearing > spacer > bearing > spacer > fork leg, countersunk flat washer > countersunk stainless bolt...

[ex-Gooserider]

Exact details of assembly will vary between chairs... If you are running stock parts, and can get an Illustrated Parts List for your chair, then the IPL will probably show the right parts sequence...

The basic idea is that there should be no side play in the axle setup - which usually means a spacer between the fork and the wheel bearing on each side, and a spacer inside the hub between the bearings... Washers can be part of the spacer stack, but don't have to be. Also note that whatever you are using for spacers must contact ONLY the inner race of the bearing - not the shield or outer race.... Washers are usually big enough in diameter that they are a problem when placed next to a wheel bearing, especially since most are slightly cupped from manufacturing....

Also when putting bearings into a hub, you need to have the outer races pushed in the same amount as the inner races, such that the internal spacer leaves no slack when the entire axle assembly is tightened....

I like to press bearings in using (ideally) a dead flat disk that fits over the outer race on each side and a bolt to draw it all together. Almost as good is a pair of extra bearings (like the old ones you just took out).

ex-Gooserider

Not sure I said earlier, but for some reason the wobble went away on the front-end. After that, I moved the seatback a further 2 cm, and put the new tyres on, and now the wobble is back again

Can I just double check the fitting of the front wheels? Is it one or 2?
1. It is wheel bearing-washer-spacer
2. It is wheel bearing-washer-spacer-washer

[funkykeyboard]

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.

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.

http://www.willsjunk.com/Other/WillChai ... -MNZFRpG/A

what is preload?
What size and what kind of wavy washer?
I'm guessing you mean this nut?
https://gyazo.com/73cce7caea8916764e30f6907f8c5fbd

I have put the smooth tires on the wheels now, and the wobble got worse. They are slightly smaller, so I do think I need to reduce the height of the frame at the back. However if I do, will this not put too much strain on the front mounting bolt that goes through the F 55 into the block on the side of the battery box/central unit?

[Burgerman]

No idea. All depends how you built it.

[funkykeyboard]

Okay, trying to deal with the caster flutter ( http://www.casterconcepts.com/solutions ... r-flutter/ )

Just to reiterate, I have managed to get this running without wheel wobble, but only with the large knobbly tyres (size 4.10 x 4.). I can only presume because they are larger circumference width.

All the below is done with the smooth tyres on (size 3x4 ). And 2 batteries instead of the lithium pack, which I guess is heavier. And my motors set 5 cm further rearward the BM 3.

When I do the forks I have countersunk bolt, countersunk washer, fork, washer, spacer then the bearing. Same on both sides. (Did have the washer against the bearing but I have remedied that.

I then took off the crossmember that secures the frame to the battery box/central unit. Let the frame fall down onto the battery box/central unit, thus reducing the height of the frame at the rear by three quarters of an inch. Same wheel wobble, exactly same speed.

Then we stuck some wood in between the battery frame and the battery box to raise it as high as possible. Still the wheel wobble came in the same speed.

Then my test pilot "he only way 8 1/2 stone", sat right on the front edge, and put us much weight as possible on the from wheels. This reduced the wheel wobble greatly.

I had the seating as far back as possible, in the hole most at the rear of the plate that Berger man designed. I moved the seat to hole 3 in the middle of the 5 holes. This has made it better. The wheel wobble doesn't come in until about 7 mile an hour even with the smaller smooth tyres. To eradicate it I guess I am going to have to move the seat some more forward. I really didn't want to do that, but it doesn't like it is going to work any other way.

I wish I could really work out how to overcome this problem. I would prefer more weight off the front wheels. But seems they were designed on this frame to carry that weight. Wish I could work out how BM 3 did it, because the whole point of the design is to put as much weight as possible on the rear wheels.



I haven't balanced the wheels. I cannot see from your chasing dog in the back garden video where you have fitted the balancing weights. What sort of weights. I am guessing you have also replaced the bearings in the caster and the wheel? What makes do you use? And do you know there sizes? Please.

Actually from the video I can see that your seat is a lot further forward than mine was. I had my seat as far as possible to go without the lighting casing et cetera. But your motors are 5 cm further forward, so I surmised it would be swings and roundabouts. Apparently not.

Wondering which way to go now when building the new central housing. Think I will move it the full 5 cm forward. I am extremely used to a wheelchair that easily tips, as the life stand wheelchair I have will tip up even when I am going up the round into my car. I just hope this does not recreate the wheel flutter.

[funkykeyboard]

No idea. All depends how you built it.

I built it like this https://gyazo.com/25bb5c92c52557f171d106654d76e942

[hank]

Cant understand how you get wheel wobble.
look at pic of my new bm3 clone build ( topic review)
similar spec to burgermans dimensions.
no wobble at all up to 9mph.

[Burgerman]

Or 15...
http://www.wheelchairdriver.com/BM3-con ... /15mph.mp4

[Burgerman]

LOWER all 4 tyre pressures till they look a bit too low. Lower rear. Test...

[funkykeyboard]

I needed a new battery box anyway, so I have built one.


I noticed on the old one that the mounting for the motors with 12 cm at the front, and 9 cm at the back, which was lower than BM suggested.

So with the new one I rectified that. The motor mountings are now 13 cm from the bottom of the battery box, to the underneath of the mounting at the front, and 10 cm at the back from the bottom of the battery box to the underneath of the mounting.

This means now there is a 11 cm clearance from floor to the underneath of the battery box at the front, and 10 cm at the back.


The height from the top of the F 55 frame to the floor is 42 cm. Is this the same as everybody else's this measurement?



Still getting wheel wobble/flutter at about 6 miles an hour at present.. If I move the seat forward putting more weight over front casters it diminishes. But would prefer to have my seat as far back as possible.

Any advice appreciated, Tremulous Tetra. :-)

[hank]

What size and make of tyres are you running on front
Are they running true no wobble when spin by hand.
Lower pressure a little then try.

[funkykeyboard]

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

what is the height from the bottom of your frame to the floor on the BM 3 please?



Mine is 40 cm

Any advice appreciated, Tremulous Tetra. :-)

[funkykeyboard]

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.

yes this is spot on, my experience too.

The garage suggested that I move the weight forward. And though it does lessen it to some degree, it makes this steering round the house absolutely awful. Jerking all over the place when trying to steer. this is an absolute no-no when you are in a small bar and trying to negotiate your way between various people's toes when you are jerking all over the place like a nutter, it makes people really nervous. So having a really positive drive steering experience at slow speeds is absolutely essential.

Originally I move my motors 5 cm backwards. I have now moved them 2.5 cm forwards, but still 2.5 cm or less forward than burger man's. However, I think my seat is now further back than burger man's. I noticed when I tried this for the 1st time the other day, we I did a full acceleration I popped a little wearily, so the centre of gravity must be something similar to burger man's, even though I have the motors 2.5 cm further rearwards than his.

Still having problems with the wheel flutter, but it doesn't come in until about 6 miles an hour at the moment. Going to try something else. (Think I've made a schoolboy error)

Any advice appreciated, Tremulous Tetra. :-)

[funkykeyboard]

It depends what we are talking about.

Wheel or caster bearing?

Wheel
I use 10mm axles, drilled and tapped and this goes through the fork as well.

So its Countersunk stainless Bolt > Flat countersunk washer > fork leg > Spacer > bearing > spacer > bearing > spacer > fork leg, countersunk flat washer > countersunk stainless bolt...

When you make the axle for the caster do you do it exactly the same width as the fork, or do you recess them 2 or 3 mil each side so when you tighten the bolts it allows the fork to flex a little? They had a 10 mm deal axle are they not?

I have forgotten entirely how to make the spacers that cover the 10 mm axle. What do you use?

When I have the axle wheel and spacers on the fork fitted properly as above, there is some movement from side to side of the wheel, because I have made the spacer to short. And all my axles do fit into the forks, they are not flush with the outside of the fork.

Any advice appreciated, Tremulous Tetra. :-)

[Burgerman]

Yes axle 2mm short. Spacers should be one either side and one between bearingsin the wheel. No movement it should be clamped tight, and spin very freely.

[funkykeyboard]

Yes axle 2mm short. Spacers should be one either side and one between bearingsin the wheel. No movement it should be clamped tight, and spin very freely.

What do you make the spacer from I have forgot?
Any advice appreciated, Tremulous Tetra. :-)

[hank]

16 MM Dia round alloy bar
Drilled 10mm hole through each spacer
Make each one to your required lengths.

[funkykeyboard]

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

what is the height from the bottom of your frame to the floor on the BM 3 please?



Mine is 40 cm

Any advice appreciated, Tremulous Tetra. :-)

[Burgerman]

No idea, stuck on bed. Do it the same as a stock F55 chair to keep caster barrel 90 degrees to the ground.