DrJB

I like the power-drive concept. My main argument (initial post) is that the free-rolling wheels on both omni/mecanum are an uncontrollable (troublesome) degree-of-freedom. The simplest experiment I can think of is a car with 4 omni-wheels going on a straight line ... on a bank (road tilt left to right). The car will simply drift down the slope and the free-rolling small wheels on the omni won't prevent it from doing so, as such tires have ZERO cornering stiffness.

Correct, I'm fully aware of the mecanum forklift ... but that works best on flat surfaces. On inclined planes, the lateral friction from such wheels is smaller than that of regular wheels ...

The word holonomic was alien to me ... until I took a course in Graduate Dynamics years ago ... and then fully understood what it meant. In any event, this is not about 2.032 ... I have seen many Lego vehicles use omni-wheels in-place-of regular wheels (e.g. Mahjqa's Grudge), and yet one ponders how come such wheels are not more widely used in real life. Can you think of Pros/Cons for such wheels? (besides the obvious complexity) ... etc I'll start with this: If you push laterally on a regular wheel, it does not move, if you push on an omni-wheel ... it moves freely. In tire-dynamics jargon, Omni-wheels have ZERO 'Cornering Stiffness'. What this really means is, while driving on the highway at 60 km/h, a small gust of wind, or a slight turn would cause the vehicle to drift to the side. The vehicle simply cannot take turns, it will change its heading, but its center of mass will continue on a straight line, no matter what you do with the steering wheel. That's pretty dangerous! Also, if you're driving on a road that has a small bank, the vehicle would roll towards the steepest slope, no matter what you do with the gas pedal and steering ... With all that, it makes sense why we do not see such holonomic wheels in wide-spread applications (besides conveyors) ... it's a nice toy, interesting kinematics, but not much more. You may ask what this has to do with Technic ... Well, I have collected enough parts and now built both omni and mecanum wheels, and now need an application for them. Don't get me wrong, they make nice toys.

Glad we got that resolved. I should have been more specific that we were discussing the old (not oldest) diff all the way, though on the new diff, one cannot add a fourth gear anyway, because of the square obstruction in the way ...

+1 - Definitely an ISA slot.

Very nice Aswin. I've been wanting to build something similar for a while but never got around to ... In fact, I even got the rotacaster wheels and they've been sitting as well. Now you're giving me motivation to bring this project to completion. Some technical aspects: 1. The robot you've built has 3 degrees-of-freedom, 2 translations and one rotation. 2. I'm picturing the control to be done by some 'joystick' that has exactly those DOFs Real question is ... how easy/complicated is it to control the motion? 3. Rotation can be achieved by rotating all 3 motors in the same direction 4. One translation can be done by blocking one wheel and letting other two run in opposite directions 5. Other translation would involve one wheel to move at some speed X, while other two wheels to move at X/cos(60) Are above statements correct? Lastly, would it be possible to share the EV3 code?

My only bad experience was with an overzealous employee who ... well ... I had filled a PAB cup and made sure I had the exact number of specific wheels/tires for a mecanum wheel design ... the cup was 'slightly' over ... the S*B employee took some parts out while I was looking elsewhere ... Of course I did not notice until I got home ... big disappointment after a 45 miles drive ... He no longer works there as his 'people skills' for sure did not match those of TLG.

Not quite sure where the disconnect is here ... both Lipko and I are saying you insert the output gears from OPPOSITE sides of the differential cage, yet at the SAME time ... That is why a good coordination is needed ... or maybe a third hand/fingers if one can afford. What are we missing???

I'm tempted to send you a free differential + 4x 12-teeth bevel gears ... just for you to try it out ...

It is not impossible, but a bit trickier .. though not much so. You can assemble them as per the following sequence: 1. Put the two gears on the spider first 2. slide the 2 gears (that connect to the wheels) in, from each side, symmetrically. You beat me to it... but I agree fully.

Off eBay .. if you 'bargain', you can get them for $12/piece or maybe less. http://www.ebay.com/itm/New-58mm-Plastic-Omni-Wheel-for-LEGO-NXT-and-Servo-Motor-/221370890647?pt=Motors_Car_Truck_Wheels&hash=item338abbb997&vxp=mtr

I agree with BlakBird ... adding a 4th gear will do absolutely nothing. Such 4th gear will always spin (and transmit zero torque) no matter what. If you really want it to help with the torque, you need to add a pin for it to ride on ... but then that makes assembly very tricky (though not impossible). A simple force/torque balance can explain this. As to the other point that, the 14 teeth gear offer higher/lower reduction than the 12 teeth gear ... this is also incorrect. There is NO gear reduction taking place inside a differential cage, and it does not matter how many teeth you use (as long as all are the same). The teeth if such gears are sized purely based on the maximum torque one needs to transmit to the left/right tires ... nothing else. This is something I'd love to 3D print ...

Found a cheaper/stronger alternative than using the original designs ... The original wheels use these parts: 44809 Those are however rather pricey ... especially that you need 24 per wheel ... The alternative is to replace each with 6553 + 60483 . This gives not only a sturdier build, but also increases a bit the diameter of the wheel, and as such, the friction issue I mentioned in post #72 is avoided.

In the end, it does not matter whether the issue comes from plastic formulation, mold geometry, or process (cooling too fast). What matters most is that the end user (Mikuri or anyone else) is not happy with the quality and that is reason enough for TLG to take note and remedy. When a part is defective, I do not care 'how' it became defective or what the specific root-cause is ... And it really matters not what you call it (cheap plastic or else) as not everyone understands fully (nor is expected to) all that it takes to mold plastic parts. You are somehow 'separating' molding issues from formulation ... and I'd disagree, the formulation for plastic affects how easy it flows in the mold. What may appear as a molding issue might be in fact a process issue i.e., how hot was the plastic when injected into the mold. Formulation, mold geometry (mix of thin and thick walls) and process are all intertwined here. I've spent a great deal of my years in rubber/plastic molding ... and it's way more 'convoluted/complex' than the average Joe/Jane thinks.

I would not do that ... plastic tends to become much more brittle in the cold, you do not want to break more parts. However, what has worked for me, is a soak in soapy hot water. The heat makes the plastic slightly soft, and the soap, if it gets in the small opening, may make it easier to pull things apart. The last resort would be WD-40, but that tends to damage plastic if you leave it on for a long time ... I would separate as much as I can manually, then, for the stubborn connections, some WD-40 ... and be ready to wash it off very quickly in soapy water so the plastic does not degrade. Some of the older friction ins have the friction ridges that extend throughout the whole length of the pin, and the connections are a bit harder to undo.

The 'long wait' is rather frustrating in the US ... I wonder, has anyone (from the US) ordered from TLG in a European country and have it shipped to the US? ... I bet shipping will be 'nominal' as TLG often does that.

I did build all three tires in the original LXF file, and here is 'my' verdict: Smallest: Ok, but shape not stable Medium: very stable, smooth small wheels, though a bit rough when installed on a vehicle (of course) Largest: Does not work, due to neighboring small tires touching/rubbing. I was hoping it to provide a smoother ride than the medium size, but, again, rubber interference. Winner: Medium Size. I like those as well, though the webpage does not say if they're lego compatible. In fact, they appear not as they do not have a replaceable spindle.

That explains why 8275's prices have sky-rocketed in recent weeks ... Before the holidays, you could get one off ebay for a tad over $200 ... Now, starting bids are at $300+

You could try some multi-link suspensions ... a rather fascinating subject

Here ya go 73071

I got a hold of 2 of the old differentials (prior to 8880) and have 2 main observations: 1. The differential cage is actually made up of 2 parts that are glued ... interesting 2. The cage can accommodate both the 12 and 14 bevel gears ... First time I see #1, and it took me a while to 'wrap' my head around #2

Are the Trucks Lego as well? look like a modified 8258 ...

Can you elaborate? I did not mention it (thought it was obvious), but I'm using a standard Lego converter cable to go from 9V to PF, and still, it does not work.

Might not be the right forum but ... of all the construction vehicles out there ... how come no one has attempted a bucket-wheel excavator?

Bring extra, I need 2 planes ...