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Slack in the driveshaft

Old 08-16-2014, 07:50 PM
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Default Slack in the driveshaft

Hey, quick question... I have a redcat (I know what you think, I had to go with my budget hehe) Sandstorm 1/10 Scale Baja Buggy. I just replace the ESC and the motor to get some performance and be able to use lipos I already have.

But, after replacing the motor, I have some slack in the driveshaft. I can turn the motor manually like half a turn before the wheels start turning. I have no idea what I did wrong. It's the first time I change a motor in a rc car, I have been mostly working with drones, so it can be my fault...

When the car takes off, it does a weird clucking sound before it starts to move. Once it moves, it goes really well, and alot faster then my old brushed motor.

But I am really wandering what causes that slack in the drivetrain...

Anyone got an idea?

Old 08-16-2014, 10:07 PM
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There two things you want to check. The first is the mesh between the pinion gear on the motor and the spur gear which is something you could've done wrong. I don't know how the mesh is adjusted on your vehicle but the manual should help you out here. If you hold the pinion still you should be able to move the spur gear back and forth a tiny bit. The mesh should be as tight as possible but still have just a bit of play. There is a common trick of putting a piece of paper between the gears to set the mesh which also gives you an idea of how much play there should be between the gears.

The more likely cause of the slack or play you speak of is the front and rear differentials themselves. A small amount of slack will likely be present between the pinion (which the center driveshaft attaches to) and the ring gear. There really shouldn't be any slack here and shims can be used to either move the ring gear closer or to move the pinion deeper to eliminate slack. The most likely cause though is the spider gears. The spider gears are inside the diff cup located on the ring gear. When the ring gear rotates the gears inside the diff cup must spin and tack up any slack before they will turn the axles for the wheels. Shims can be placed behind the spider gears to tighten up the mesh between them as well. Btw my RedCat Ground Pounder is all stock with low runtime and the diffs have too much slack in the ring and pinion with a lot more in the spider gears. They definitely need to be shimmed everywhere.

Shimming is is a trial and error affair. You assemble, check for slack, add shims, reassemble and check again. Slack anywhere in the drivetrain increases shock loads which breaks parts. Brushless is more powerful and only increases these loads. If you're hearing crunching noises it's likely gears are already skipping and need to be replaced. If you want this vehicle to last, lookup the diameter of the differential pinion (inside diameter of the bearings) and pickup some shims with that inside diameter and an outside diameter 2mm bigger with a thickness of .2mm. I have no idea what size to use for the spider gears but something like 6X4x.2mm should work.

Last edited by Maj_Overdrive; 08-16-2014 at 10:12 PM.
Old 08-17-2014, 04:06 AM
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Thanks a lot for the detailed message. I removed the drive shaft and turned the rear and front differential separately and the slack is coming from the back differential. I'll open the diff and see where to go from there.

Thanks again!

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