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mudseason

Independent motors drive-train

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This may sound as a dumb question, and probably it is.

It happens that I have two buwizz 2.0 currently parked and the 42125 set.

I'm now thinking about modding the set following the instructions by the buwizz team and some ideas I have to turn it into a sort of 488 pista.

However I see that the drive train does not make any use of differential.

I wonder if the terrain friction is enough to equalize the rotation of the two independent driven wheels or if the resulting model is going to wander left/right instead of going straight.

I never built an independently driven drive-train so far, I don't like the idea from a mechanical perspective, but if it works it works and I saw more than one model adopting this kind of approach.

 

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Do you mean independent front and rear axle motorization? If they have the same gearing, it is fully valid scheme and was alrady used in even offical LEGO sets, like 9398 Crawler.

Concidering a BuWizz capabilities, you may also play with some curves and make some porfiles with, for insatnce "drifting start" when rear axle gets a bit more power at earlier sarges and then equalizes with the front one, so you may get a little burnout when starting but having precise handling when move.

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Thanks for the reply.

Nope, the mod is for a pure rwd.

However left and right wheels are driven by individual RC motors that are not interconnected.

Actually each wheel is driven by a pair of RC motors, but this Is a detail. 

The point Is that there Is no connection between left and right whatsoever.

Hope this clarifies.

 

 

 

 

Edited by mudseason
Typo

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Theoretically, your vehicle should go in a straight line.
Practically, the speeds of two different motors will never be exactly the same, and your vehicle may drive itself (steer) to one of the sides eventually.

You can test individual motors by putting a differential between them. If the diff stays still while the motors are running, you have a pair of good matching motors. If the diff is rotating, one of the motors is faster than the other.

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The deviation from going in a straight line will be much more influenced by the steering. The wobble and zero point will affect way more than using two motors. Just look a Videos from motorized lego mocs, all will go slightly to the left or right. 

Edited by Jundis

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8 hours ago, mudseason said:

This may sound as a dumb question, and probably it is.

However I see that the drive train does not make any use of differential.

I wonder if the terrain friction is enough to equalize the rotation of the two independent driven wheels or if the resulting model is going to wander left/right instead of going straight.

 

Not at all a dumb question. Quite to the contrary: it shows you are keen on having solid drivetrain engineering.

In theory all your thoughts about this are true. In perfect conditions – and close to extreme situations (high speeds, tight corners) – you would notice a difference between an axle with a differential and an axle with two individual motors.

In reality though you barely have perfect conditions… meaning: whenever you lose traction on one wheel – you lose 100% power on the whole axle when using a differential. And there are plenty situations when this happens… body roll, body flex, bumpy underground, slippery underground, rapid breaking, rapid acceleration… Only due to friction in the drivetrain and mass moment of inertia of the moving car you keep on “rolling” into a better traction situation…

So using a motor on each wheel helps you to reduce the power loss of 100% down to 50% in such not-ideal situations.

In the rare moment when you have constantly 100% traction (with sticky third party tires, perfect smooth and grippy underground) you are going to experience understeer.

To compensate this, you could either do, what ESP does (slow down the wheels in the inner part of the corner via brakes (maybe mechanically linked to your steering axle) – or you do what Torque Vectoring does: program your power unit in a way, that [IF STEERING LEFT] happens [POWER MOTOR LEFT] is reduced to 80%... or you do – what from the looks of it nearly everybody does: assume there will be nearly never perfect traction between the car and the ground – so just do not care about it.  

 

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3 hours ago, Jundis said:

The deviation from going in a straight line will be much more influenced by the steering. The wobble and zero point will affect way more than using two motors.

These were my thoughts.

Slightly off-topic from what the OP was asking, but I tested the turning diameter of some different types of differentials in the video in this thread: 

My conclusion from the not-so-scientific tests was that the turning diameters (on carpet) of vehicles with mechanical differential, electronic differential and synchronised left/right motors (like OP is talking about, NOT hard coupled) were similar (within 5%) whereas the turning diameter with locked differential (hard coupled left/right motors) was much higher (+~20%).

So, in theory your vehicle should still be able to turn pretty well, as long as you don't connect the left and right axles together. EDIT: Then again, different motors could behave differently, as could different apps. 

Edited by ord

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I was bugged by the same question once. Then I build a simple, small vehicle with rwd using nxt motors for driving and steering (ackermann). The steering (radius) really improved when the driving motors adjusted the speed whilst cornering compared each wheel going at the same speed.

If the vehicle has ackermann steering the center point of the steering circle is easy to locate and then some math will give you the different wheel speed due to the different radii the inner and out wheel have to go.

Edited by m2fel

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Thanks a lot, this was very informative.

Furthermore I checked the forum and I failed to find, collected in a single thread, all those details.

I hope this thread will also serve others.

BTW: connected driving axle is not good even on a small footprint car: I tested it on a two L-motors modded Fiat 500, I did the mod for fun and the car was jumping while turning! No jumps when I further modified it using a differential.

 

 

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8 minutes ago, mudseason said:

No jumps when I further modified it using a differential.

For steering on a road vehicle (not offroad), yes, you need to have a differential, for smooth drive.
Alternatively, you could power just one wheel. Then you don't need a diff, and the car will not have troubles steering.

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the vehicle I plan to build is a road vehicle.

Now the point is the definition of "smooth".

If the observations done by "ord" are accurate, that 5% may provide a "smooth-enough" experience.

If that 5% deviation support slides instead of jumps than it may even provide a "fun-enough" experience.

:wink:

 

 

Edited by mudseason

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This thread made me curious, so I recreated the test I mentioned but with wider tyres and on different surfaces (wood, lino, carpet again).

800x800.jpg

The results were similar on all surfaces and compared to the previous test: not much difference in smoothness of drive and turning radius between running the motors at the same speed and running them at different speeds (i.e. electronic differential). Locking them together (as in photo) again made the turning radius far bigger (even more so this time) and also made the car jumpy and not smooth (I suspect due to the wider tyres giving more grip).

This was controlled by the Powered Up app and I suspect that other apps/motors might behave differently, especially seeing that m2fel reported different results using nxt.

I am interested to see how it works in your case @mudseason, because in my case I would say that using two separate motors in place of a differential is not very detrimental at all. :)

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On 11/4/2021 at 8:41 AM, ord said:

This thread made me curious, so I recreated the test I mentioned but with wider tyres and on different surfaces (wood, lino, carpet again).

The results were similar on all surfaces and compared to the previous test: not much difference in smoothness of drive and turning radius between running the motors at the same speed and running them at different speeds (i.e. electronic differential). Locking them together (as in photo) again made the turning radius far bigger (even more so this time) and also made the car jumpy and not smooth (I suspect due to the wider tyres giving more grip).

This scheme remained me a racing cart chassis. I used to be a cart racer in a past and it had literally the same setup: short wheelbase, big steering lock, RWD and totally no differential.

Due to this contriversal setup it should be driven totally different to your everyday cars - instead of slow and gentle breaking before sharp turns it should be braked in the midle of turn by literally kicking the pedal for less than a second. This actions casus so high traversal forces, so the cart inclines out of the turn and raise the inner rear wheel. It loose the traction and the cart makes even the sharpest turn on three wheels, so the rear differential is not requred.

In oppostite, if you corner in a "regular road car" manner, the speed will be too slow to incline the vechicle and the result will be exactly the same you described here. 

Edited by Milan
Do not quote images from the same page, please.

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Small update.

I completed the chassis.

I confirm that it can go straight even if the motors are not perfectly identical.

1638397944207-min.jpg.0ff927d8a23d88112f558e959cf234bc.jpg

it also drift, I hope the link works.

I will now focus on the body work.

Here the link to a short animated GIF of the car spinning in "Normal" speed.

https://imgur.com/a/lGYE5mY

 

In fast you have to watch not to trigger the over protection.

In ludicrous... I have not had the courage to try.

 

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Cool! Are you going to add a positive caster on the front wheels? It will help you a lot with going straight

P.S. Look at the car on my profile pic - it has 2x Buwizz Motors and I made an electronic differential for it. Even with positive caster, camber and zero scrab radius it can not go perfectly straight, and it is a shame because then you go 9-10 km/h the straight position is 100% wanted ;-)

Edited by Daniel-99

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