OK... so I have a problem
 

Hey guys I was out running my hyper st. Here's my problem up until a couple days ago, every time I give my truck a good amount of throttle(100k center diff oil) it makes a very harsh noise as if gears are grinding. I've since disassembled the center and rear diff and found the gears to be in good shape. Am I missing something here?

Check the front diff ??

Also check your CD and motor mounting screws. They may look ok, but if they get loose you could get gear skipping at high power throttle.

if any gears are skipping it will be quite evident. keep digging till you find the carnage. i suspect the front diff. people always assume fronts will hold up, but 75% of the braking force is up front and that'll eat diffs

Rear diff gets a good majority of the force under accerleation..

CHeck your teeth on the rear diff ring gear

they are probably rounded slightly causing the gears to skip under load.


ANother possibility is that one of the magnets inside your motor is debonded and is grinding on the inner windings of the motor- had that happen this past weekend

also check your diff output cup and wheel hex...

he already checked the rear:rules:

Bearings... in the rear diff usually, if worn will allow the gears to slide past each other. Specifically the pinion bearings.

Ok so after taking everything apart again I think it could be the bearings on the rear diff they didn't look to healthy. But to make sure I wanted to run it again with new motor screws.... Well I made a rookie mistake and put the rear diff in backwards. :( lol. So tonight after class I'll be taking it apart again to flip the diff and to test to see if it was just the motor slipping away from the spur gear.

Thanks everyone for all of the input I greatly appreciate it!

What kind of ring/pinion gears does this diff have? Are they helical cut (angled) or straight cut? Helical cut gears are quieter and tend to be more durable (more gear surface area making contact), but also tends to put a lot of thrust strain on the pinion bearings.

I had to use a small o-ring spacer on my inside diff pinion gear btw the gear and the bearing keeping the slack down to keep my rear diff from making that aweful cracking sound. This was only a temp fix to get me through the season though as the gear is warn now and will have to be replaced.

yep, Iv'e noticed that in my hyper 7. even that the diff is shimmed correctly, I had many problems with the rear diff recently.
- the axle of the outdrive has worn out the hole in the crown wheel of the diff (due to the helical cut?) and
- grease/silicone oil gets onto the bearing of the outdrive. Dirt gets sticky and kills the bearing.
- then, the diff crown gear gets too much play - result was a "raaaaatsch"
- before that, once the diff case got worn out, so the bearing at the outdrive had a little play, with the same results.

I think my torsen center diff causes that wear (power slips no longer to the front wheels).

took me some time to figure that out. I've ordered a new diff on egay uk for little money (hyper st and 7 have the same diffs).

The gears are straight cut... After flipping the diff over and trying to run it the same crunching sound almost made me cry... lol. So I'm ordering a new diff(taking the easy way out) to see if that fixes the problem.

when you have a closer look, you probably see that it's not straight cut, it's easier to see on the pinion. afaik hobao has only these diffs in the st, but I can be wrong.

maybe just the bearings are full of sand etc, because you turned it without load and you still got that scary sound. sand can get into the diff too, when the screws holding the diff on the chassis are a bit loose.

brushless causes a lot of wear, so probably you only need some more shims. that's normal and should be done after a few batteries. after it's "shimmed" (always both, diff and pinion), it must run smooth, but not loose.
I installed a new diff yesterday, as the old one was completely worn out, shims, new bearings didn't help.
good luck
flo

How far is the pinion away from the spur? Simple, but may be the problem.