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mudseason

AWD - independent axles

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Hello everyone.

I would like to modify a RWD lego racing car (1:8) with the addition of a power block (2 motors) driving the front axle. Front axle will be provided with a differential.

Real axle also has its own differential.

I have the following quite basic question.

Would the system work if the rear power block and the front one are not connected at all?

The two power blocks are geared up identical, of course, nevertheless I fear that one axle would push the other one to scratch due to small velocity/torque differences if not connected.

Then I see lots of IWD vehicles which is even a further extremization of the concept.

Thanks a lot for any indication.

Cheers.

 

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Lego motors will spin at different rates. So yes the rear axle could push the front or the front drag the rear. But i think the difference is so miniscule that you probably can't even notice. 
Depends on gearing and weight though. 

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Take for example didumos's Greyhound moc. Each of the 4 wheels is independently powered by an L motor. Two of those are on one buwizz and two on another. Even so with the motors not connected at all, it'll pull straight.

Your idea will work just fine 👍

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16 minutes ago, Mechbuilds said:

Lego motors will spin at different rates. So yes the rear axle could push the front or the front drag the rear. But i think the difference is so miniscule that you probably can't even notice. 
Depends on gearing and weight though. 

The chassis will always roll slower than the motors can do due to roll resistance and friction so both will push the model, with no highlighted "pusher" or "puller" (for sure, with the same gearing).

So, @mudseason, please go ahead with your interesting model! Additionally I would add I don't like to add the connection between the axles if I see possibility to skip it, as you might get just one "weak point" to be broken if some axle has stuck. Not so critical for road models but very dangerous for off-road ones.

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The other positive: you don't need a central differential. With Lego's open differentials, you need all 4 wheels on the ground to move.

With two separate drive trains it'll keep moving if any 3 wheels (or two of the same axle) are touching the ground.

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I've built trucks with 8 separate L motors (1 per wheel).  It's all fine.  No harm to the motors.

Major battery drain with 8 L motors, but eh :)

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Posted (edited)

And this is my attempt.

I expect it to be full of naïve and/or wrong implementations, in case you think anything is wrong or should be differently done, do not hesitate to suggest different solutions.

Some note:

- white parts are from the original car and I preferably do not move them (some cannot be actually moved as support other parts).

- the wheel hub is the part 23801, but I do not have it in my LDCad library.

- check the steering solution; the original car (it is the Mantis model by astyanax) has not the double link solution, I wonder if the added bar would just make it more complex without benefit or it could help

- to save time I just partially completed one side, and skipped the very obvious connections with pins.

 

I do not know how to upload the LDCad file, I'll keep it on my Gdrive for a while: Mant6_front_axle_20200526.ldr

 

Here some pictures:

[picture removed as I could not stand the vision] 

;)

Thanks,

Cheers.

 

Edited by mudseason

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Posted (edited)

Not sure where to begin because of incomplete diagram, but I can say it won't work. Yet.

Cv joints need to be in the same plane as the pivot, both up/down and steering. If you fix that, it's still going to be a lot of power to put through old type CV joints if using them for steering too, and they may not take it. Even if they do, they won't allow much steering angle. Try using new type CV joints instead.

Two steering links isn't a bad idea, it can increase strength and reduce slack. However I can't figure out if what you've done will work due to view angles. The racks need to be driven at the same speed (ie by the same size cog) in opposite directions. Seems like you have the first rack off the servo by an 8 tooth but the further rack on a 12 tooth gear? It would move the second rack quicker and they'll fight each other.

Edited by amorti

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Posted (edited)

I see the issue with the pivot for steering.

I will fix it,it is quite obvious now that you said it.

I will def use the new CV joints, I just miss them in my library.

Furthermore, the second steering link is not "driven" and does not use a gear rack, just a lift arm sliding. The one driven is only the one on the front.

The 8 tooth gears are only used to lower the axle.

Thanks a lot.

Cheers.

 

P.S.

I removed the pictures in previous post, I could not stand their vision... ahaha

Thanks again I will correct it, most probably by using a different wheel hub and including.

Just to be very clear, the drawing is intended have a rough picture and buying the parts and spot big mistakes like you did, I will then experiment with actual parts.

Edited by mudseason

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If the second steering rack isn't driven then I'm not sure it'll serve much purpose.

However, they do use an "idle" steering rack as you've drawn here on Lego set 42099 so maybe it does "something" - if you have space left over you can try it out, but I wouldn't put too much effort into keeping that space free.

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I have the 42099... just copyed the idea from that very model.

Regarding the wheel hubs, it looks like that to have the joint where the steering pivot is I should couple in the following way:

Wheel + Portal Axle then... 92909(Hub)-> 32494(CV Joint M, old) -> connecting axle -> 52731(CV Joint M, new)-> 52730(CV Joint F, new)
 
So I will need to use the old CV male joint to the hub as the only hub that directly supports the new CV male joint is the one with planetary of 42099.
Would you confirm that?

 

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Yes makes sense. It's not a portal hub though, that's the ones with hear reductions inside the wheel "hanger".

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Question,

in the "original" model the rear pivot points, for the suspensions, are one stud off (inward) from the pivot point of the CV joint.

There are two pairs of lift arms two above and two below, symmetric to the CV joint.

This is the picture (for simplicity it reports only one arm, but you can see that the arm connects one stud inward re- the actual CV joint).

http://d9vQ7zS.gif

Is that sub optimal, and the suspension pivot shall be exactly on the same line as the cv mating?

In the 42099 all CV joints and suspension pivots are exactly aligned.

Now, leaving aside the drawings, back to real life, I have the car in my hands, and the rear suspensions... work like a charm.

My conclusion is that one stud offset is sub optimal in terms of freedom of movements, but for a racing car the limited working angle is acceptable.

Under the above assumption, I planned this kind of solution for the front, as you can see the pivot point is offset one stud (outward):

t2O4JJt.gifhttp://t2O4JJt.gif

What do you think?

Thanks a lot for the support you all are providing.

Cheers.

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Posted (edited)

Honestly I'm surprised it works at all. Maybe it's helped by how long the wishbones are? It can give a bit of forgiveness in the tolerances since the wishbones are relatively flexible.

You'd probably get away with it using the old CV joints because they can slide along an axle, but it's definitely not ideal and I don't see it working well with the new joints. Although even then you do have one old one at the bottom... so maybe?

You'll have to test it yourself to see if it works when you also add steering.

Edited by amorti

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53 minutes ago, mudseason said:

Question,

in the "original" model the rear pivot points, for the suspensions, are one stud off (inward) from the pivot point of the CV joint.

There are two pairs of lift arms two above and two below, symmetric to the CV joint.

This is the picture (for simplicity it reports only one arm, but you can see that the arm connects one stud inward re- the actual CV joint).

http://d9vQ7zS.gif

Is that sub optimal, and the suspension pivot shall be exactly on the same line as the cv mating?

 

35 minutes ago, amorti said:

Honestly I'm surprised it works at all. Maybe it's helped by how long the wishbones are? It can give a bit of forgiveness in the tolerances since the wishbones are relatively flexible.

You'd probably get away with it using the old CV joints because they can slide along an axle, but it's definitely not ideal and I don't see it working well with the new joints. Although even then you do have one old one at the bottom... so maybe?

Here's a different angle of the axle described by @mudseason. Despite the offset it works fine because the leftmost CV joint part (in dark gray) can smoothly slide inside the wheel hub.

 

800x625.jpg

 

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Thanks Astyanax for the info, very interesting.

I removed the wheel to check it and, that's exactly how it works. Just less than half a stud total motion for complete excursion.

Beside this, the system has a double offset: on the wheel hub side it is outward on the car body side is inward; I do not know if this is "averaging" so that the sliding effect is minimized.

As 'm trying to mod the front axle I fear I cannot exploit this workaround due to the steering mechanism.

The top arm pivot cannot be pushed to the right (see my picture), but... may be there's a solution: in the drawing I'm using the old female/male on the car body side.

The female seems to be 3 studs long.

However the new CV female joint seems being 4 studs long. If this is true then I would have full alignment on the pivot points.

 

 

 

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