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I have built the following MOC, with details here and here.

It is pretty complete, although I would have tried some servo motor for steering.

But after I've finished it, I've got into trouble:

  1. Wheels on one side of the car seem to work sporadically. Could this be related to the steering joint and Cardan joint below?

Steering joint cardan joint

Or the differentials?

differential (image taken from Is it possible to create a differential if I don't have it as a ready to use element?)

  1. It's too weak.

A wool carpet of 2cm thick is an insurmountable problem for the car to climb on it and go, especially for the last wheel or pair of wheels that have to pass the carpet's border (and difference in level), regardless which ones have to do it (back or front).

Are there too many differentials? Too many tooth gears? Too much elastic parts to deal with the torque? I can't find parts blocked.

The driving ring 2L, driving ring 2L, at the page 28 of the building instructions, should act as differential to switch between 2 x 4 and 4 x 4.

Could it be a defective motor? Is one XL motor too weak? Would be more powerful with two motors (in the back)? Which kind?

Batteries are OK for 9398, so it's not them.

Did anybody tried this MOC?

Sorry to have too much questions, it seems I need more experience with LEGO Technic.

UPDATE

I have changed it consistently, adding a gear down for each wheel (horizontally) and that made it 4L longer.

Also, moved the steering motor in front and the up/down and winch motor in the middle (switched their places), using a servo for steering.

As result, it is a bit more powerful, it climbs the carpet now, but I need to work more on the steering because it is not that smooth when it comes back in the default position. 9398 is better at going back to the default steering position. Also, 9398 is more powerful as it gears down 3 times instead of one.

Overall, the new design feels better, yet too lazy.

  • 1
    This is a beautiful MOC. From a glance at the pdf instructions I get the impression that the front M-motor operates the winch, the middle one is for steering, and the XL motor in the back does the driving. Is that correct? The clutches are not connected with the driving, right? I have a 9398 crawler with two L-motors and a servo, it crawles over almost anything. – Aziraphale Oct 22 '17 at 22:31
  • A beautiful MOC indeed. You are correct regarding the motors. I would have used a servo for steering, though. I don't really understand what should do the clutch at page 28 in the instructions. I see no difference when the position of the driving ring is moved. 9398 has several tooth gears pairs to increase the power (over speed). 8297 does not. Also, 9398 has 2 driving motors, one for each pair of wheels. – mike Oct 23 '17 at 4:30
  • I would guess that one motor, even an XL, has more difficulties to distribute the power to 4 wheels than two motors as in the 9398 crawler. I switched the gears in my 9398 (the first gears at the motors) to make it faster, and it works fine. To conclude: I'd say you built the moc correctly and the parts and motor are also fine, but the one-motor setup is the reason. Instead of powering the winch, I would try to power the front wheels. And use a servo for the steering as you suggested. Again, nice MOC and excellent pdf instructions. – Aziraphale Oct 24 '17 at 9:09
  • Indeed, the MOC is wonderful, but I have no clue from the author if it really works better than my implementation. I think too, two motors are needed, but it loses the central differential. Would this be a solution? – mike Oct 24 '17 at 11:09
  • I doubt that more power solves the issue. You would still have one powered axle distributing force over three differentials to four wheels. This gives too many degrees of freedom to the system. As the comments in your link say, it could work for a racing car but not for all terrain purpose. – Aziraphale Oct 24 '17 at 12:04
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The piece you show must have steering axle 92906.

  • 1
    Please be more specific. This is not in the inventory of any of the variants. – mike Oct 23 '17 at 6:05
  • 1
    You suggest to replace the universal joint 3L (61903) with the steering CV joint (32494)? That corresponds to the steering axle 92906. It already has the steering CV joint (32494) on the wheel side and seems to have issues due to that when the car rises, the wheels seem to go intermittently, jerky. – mike Oct 23 '17 at 7:21

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