Strange SCX10 type drivetrain phenomenon!

[Tommygunn]

I have an SCX10 esque vehicle, think all metal (Aliexpress) Chinesium, which is properly shimmed and buttery smooth. However, today I have received a 55T motor and I've been busy messing.

 

I noticed, after removing the spur gear and the attached clutch/friction pad, that when I rotated the opposing alloy friction disc it was buttery smooth but in the other direction there was resistance. By resistance I don't mean friction but more like magnetic or gravitational, if you get my meaning.
I was using the pad of my finger on the edge of the friction plate and it was not a problem in the one direction but slipped in the other, regardless of pressure.

 

I'll say again, everything is shimmed to perfection but what comes to mind is the differentials and the direction the gears rotate.

 

Is there some sort of mechanical phenomenon that produces this? What is also strange, is that if I lift the vehicle off of its tyres the buttery smooth prefection is the same in both directions of drivetrain rotation.

 

Any ideas?

Tom.

Edited September 25, 2019 by Tommygunn

[Oh How Original]

I would say it's possible due to the quality level that one of the gears might be a bit of a dud and therefore creating some kind of uneven pressure, so turning one way is fine but turning the other way isn't.

Another thing which makes this likely is the fact it only does it when it's under load and lifted off the ground.

Check both ring and pinion gears in the axles for tight spots or uneven wear marks.

[Tommygunn]

I have to say that the quality of the parts is really excellent, only let down by the fact that the whole vehicle had to be stripped, shimmed and threadlocked. The shimming of the diff pinions and rings were, however, perfect from the factory.

Ok, first things first, with the gearbox out of the chassis the gears turn perfectly with no imperfections. This is turning both the input shaft and output shaft in both directions - smooth as.

Same goes for the axles in both directions be that done via the input shafts or the wheels. The only difference is, is that the front locked diff has been replaced with an actual diff so a different component, from another factory, with a slightly different feel. Still, perfect shimming and consistent movement regardless of rotation direction or if done by the input or the wheels. 

 

Regardless of whether the gearbox is factory orientated or switched around, with the spur forward (for some torque twist elimination), whether I have one or both axles connected, all combos basically, the effect is the same. If looking at the spur it turns clockwise perfectly, but counter clockwise it does not.

This to me doesn't make sense because in turning the gearbox a 180 then it seems logical that the problem would manifest itself in the clockwise direction but, no, the problem remains in the counter-clockwise direction.

 

Are there any weird mechanical properties concerning the angles at which universal driveshafts exit and what line they take in relation to the direction of gear rotation. That's about the only thing I can think of at the moment.

 

Tom.

 

 

[Oh How Original]

Your UJs should be matching, the yokes, or whatever you wish to call them, those need to line up with each other, that might be the issue.

[Tommygunn]

That could be a problem. I got some diff oil in the post to replace the current front diff oil. When it arrives I'll strip the front axle down and replace it with the original solid diff and see if that is a problem.

 

Thanks,

Tom.

[Oh How Original]

No need to put oil in when the diff is locked.

But having a live diff shouldn't cause that issue.

The yokes I am talking about are your prop shaft ones.

By the way when the transmission is out, does that feel okay, you said the axles move smoothly, but does the transmission if you disconnect the motor?

[Tommygunn]

No, no, I'm simply going to remove the live diff and replace the 100,000cst oil with some 300,000cst so I thought I might as well put in the factory (locked) diff in so that the front and rear axle are the same and eliminate the live diff as the problem.

All components, be they the gearbox or the front and rear axles are buttery smooth when they are disconnected from one another and that includes rotating the component either by the input shafts or output shafts/wheels.

Still I don't understand why it is that the problem occurs, regardless of whether the GB is orientated one way or the other, but the direction the spur is the same.

As far as the UJ's are concerned everything is mounted correctly, straight, fastened tightly and smooth.

 

Tom.

[Oh How Original]

What about the angle of the UJs?

If they are cheap ones, they might not be able to take too much angle, that could cause an issue.