Guys,

whatever Technic pickup/jeep I build after a few hours to few days to few weeks the vehicle starts to jerk when moving. It happens specially in the rear axle.

I checked and found couple a factors could contribute to this issue:

• if the engine setup is built too tight
• if the engine pistons are not aligned properly- the infamous ones with 3L brown axles and the half-bushes
• if the shock absorbers are made stiffer
• if the DBG control arms (#64872) are connected too tight or squeezed

When they are all checked I still continue to find this happening.

Finally, since last week as I was building the JB70 pimped up version of 2nd 42110 Defender I found this:

as soon as the differential is built and placed within the LBG frame (#64178) and you are done putting the wheel hubs and the connectors or the connecting setup i.e., the 3L axle with bushes and u-joints all going into the little gears inside the differential, this problem starts to arise.

When this part of a chassis is built, just slowly turn one of the wheel hubs- notably the one that is connecting to the crown gear of the diff. I noticed that once in every rotation the differential faces a resistance. It is mild.

Furthermore, if I keep the build 180 degrees flat on the surface. The wheel hubs rotates/spins/moves the differential just fine but, when the setup is paired with shock absorbers it happens as the absorbers creates an angle necessary for the independent suspension.

Interestingly, when the chassis is complete, I put the wheels on, it runs fine, but only for few hours. And then, when the body is built and mounted, it becomes more visible. I also noticed if the jeep is moved faster with slight pressure on it it does not happen. It happens when it is moved slowly.

Luckily, jeep/trucks with live axle in the rear are mostly free of this. My Jeep Bruiser/Willy's Jeep MOC and 42126 Ford Raptor both run more smoother. The jerkiness is there, but it is so light and can easily be ignored.

From this, I made an assumption that in a solid axle that has steeper shock absorbers positioned at an angle, the u-joints struggle to move the differential smoothly. This causes the whole car to jerk or slight bumps when moving.

But oh boy, if I take out any of my jeep/truck with solid axle setup after a few weeks, the initial bumps it makes is annoying. After couple a run, it is reduced, but just by a bit.

So, Is this a common phenomena or am I doing something wrong? Any solution to this?

I wish I could make a video when I was building that part of the chassis to elaborate it more clearly.

Can you post some photos? It's really hard to respond from plain text.

Edited.

Edited May 22, 20231 yr by thekoRngear

The part about no jerking in a flat configuration but jerking in a final configuration can be related to the fluctuating speed of a cardan configuration with a 90° offset.

See the following video at 0'55'' for more information.

 

On 5/15/2023 at 9:33 PM, LegoTT said:

The part about no jerking in a flat configuration but jerking in a final configuration can be related to the fluctuating speed of a cardan configuration with a 90° offset.

See the following video at 0'55'' for more information.

 

That should be the case I think. In my case, the 42110's rear axle's angle (with full weight), although not 90 degrees, is enough to cause that fluctuation. As I have told it already if I press the car a bit it becomes almost flat/straightline and the move is 100% smooth. I think it is a limitation Lego is well aware of and with most recent cars like 42156 42154 etc. Lego strives to improve the situation.

I will try to recreate the rear axle with angles and then post pics (and vid if I can) here in this thread when it is ready; so that Zero can chime in, hopefully.

Edited May 22, 20231 yr by thekoRngear

So I checked the building instruction of this set  on the net and there is indeed a jerking u-joint problem. Instead of an homokinetic shaft made by two u-joint it seems it's the combinaison of a u-joint and a cv joint . 

The wheel speed rotation must ocillate when the shaft is not in a plane.

But why I don't understand it that you say it's jerking only on one side, all wheels must be affected, so maybe there is another issue.

Edited May 24, 20231 yr by LegoTT

20 hours ago, LegoTT said:

But why I don't understand it that you say it's jerking only on one side, all wheels must be affected, so maybe there is another issue.

yes, it is a combination of U-joint and cv-joints.

Maybe I was being a little less clearer/forgot to tell about it fully. It is there; over all four wheels. The front axle seems to carry that jerking ignorably less and both the wheels in the rear seem to have it more. Perhaps the jeep’s rear weight plays a role here- jeep’s body is heavy with all that accessories and that spare wheel. On the other hand, the front only has engine and the hood.

The rear wheels’ jerking seems uneven. The left wheel seems to bear it more as its hub is connected to the crown side of the differential. On the other hand, the right wheel’s hub is connected to the that part of differential where there is no crown.

Meanwhile, I used the new diff (the one with red gear), the situation seems to improve a little.

Edited May 25, 20231 yr by thekoRngear