Any decent options for open diffs?

[kmnss]

So, built the bomber, put the Holmes in and let the horsepower do the talking. Built with HPI gears to have an open front diff.

About five minutes into the maiden voyage:



Ok, whatever.... maybe it's a fluke. I've got two sets of gears anyways, these are the HPI gears. The newer versions of the HPI gears that "seem" stronger.

Here is five more minutes into run two:



Now, this is being run on 4S 70C TP lipo, 3500KV Holmes BLXL with Holmes ESC. I know this is big power, but are the diffs going to continue to implode like this?

I've purchased the Axial branded bevel gears to give them a shot, but are there any other options? Right now I'm on the fence about throwing the FO-XX axles under this pig, or just putting a locker up front.

[Brake Weight]

I bought a complete open diff from an SMT part out off fleaBay. Direct fit and holding strong so far.

[Jboucher]

Isnt 4s 3500kv super high?
Most people running 4s are using the lower kv (2200) motors.

Sent from my Nexus 6 using Tapatalk

[kmnss]

Originally Posted by Jboucher

Isnt 4s 3500kv super high?
Most people running 4s are using the lower kv (2200) motors.

Sent from my Nexus 6 using Tapatalk

Running underdrive ratio gears front and rear and have it geared 65/15. Not too far away from insanity.

[FlatLander72]

You are over rev-ing that motor. The calculated RPM is over 50,000. Your next complaint will be your rotor has exploded. Not sure if axles from an 1/8 scale monster truck will work any better.

Try running at 3s or stepping down to the 2100KV motor. Running a pinion with fewer teeth will help too.

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[Bbknucks]

Who wants open diffs anyway. I thought that's what yetis are for

[kmnss]

Originally Posted by FlatLander72

You are over rev-ing that motor. The calculated RPM is over 50,000. Your next complaint will be your rotor has exploded. Not sure if axles from an 1/8 scale monster truck will work any better.

Try running at 3s or stepping down to the 2100KV motor. Running a pinion with fewer teeth will help too.

Sent from my Moto Z (2) using Tapatalk

Nah. You would seriously have to try for the motor to reach the theoretical RPM of 51,800 RPM with 4S (Yes, I know it's higher with a fully charged pack but we're not going to go into voltage sag). Introduce drag, drivetrain inefficiency, and the fact that you would never see those RPM's while it's actually running anywhere because the thing would end up cartwheeling/barrel rolling/crashing in general.

If I set this up on the bench and tried, yes. It would probably implode, but I'm not going to try that. I bought a 3S pack, but after running 4S I don't really want to go that route.

Anywhooo... I'm not technically complaining, was just wondering if anyone else had any success with high KV/high voltage combos with open diffs.

[durok]

I've been reading these forums since just before the Bomber was released - and i haven't heard of anyone running that powerful of a combo. Closest I've seen is 4S on "standard" can 3500 Puller Pro (vs the XL-size), geared down a fair amount from where you are at. I'll be interested to see if anyone chimes in with a similar setup as yours... Best of luck, for sure!

[Pabust]

Originally Posted by durok

I've been reading these forums since just before the Bomber was released - and i haven't heard of anyone running that powerful of a combo. Closest I've seen is 4S on "standard" can 3500 Puller Pro (vs the XL-size), geared down a fair amount from where you are at. I'll be interested to see if anyone chimes in with a similar setup as yours... Best of luck, for sure!

I have the Castle slate 2850 crawling motor and here are a couple short videos where i tested it with 6s, not on a Bomber but a SCX10-2:


https://www.instagram.com/p/BeGxGjUD...c_crawlerchile

[slickwill16]

I had my bomber set up as a go fast basher for a while. I was running a Castle 1410 3800kv motor with the Mamba X and mostly ran it on 4s. Once even stepped it up to 6s. I had a set of open diff spider gears left over from an SMT10 I used to have, so I put them in the front diff and packed them in with some marine grease. I never had an issue with them. They worked flawlessly and were able to handle all the power I could put to them. Only problem I ever had was constantly stretching out the stock rod ends. As they broke, I just modified Revo ends to replace them. Any way, that's my take on it. Hope this helps.

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[Teng]

Not on the bomber, but on my Wraith, Puller Pro 2200kv with 4s ( 14 pinion/56 Spur ) everything shims properly and so far so good until now. But off cause with all the HD gear and SuperShafty trans. Gonna try out mine on 6s next time.

[JatoTheRipper]

Everybody seems to be avoiding the question. No, there aren't any strong spider gears for the open diff in the AR60 axles. Both the Axial and HPI gears will break just the same. It's a shame. I went to a locked diff in the rear of my SMT10 because of this.

[durok]

Technically I was answering the last statement he made which was where he wondered if anyone else had had luck with a similar combo. I hadn't recalled ever seeing that mentioned... Lolz ;)

[kmnss]

I have some decent news.

In testing these diffs, I have found one that has survived two 4S packs and a couple of 3S runs. Without immediate destruction.

After looking at the gears, while similar, they are not exactly the same. The tooth count is the same, but the HPI gears have a smaller set of center spider gears. The Axials are larger on the outer gears that actually drive the driveshafts. When I tried just the Axial gears, I could not get it to mesh and still fit in the AR60 axle. With the HPI gears, everything was too loose, which I believe was leading to my destruction.

My setup includes extra diff gaskets that I actually cut out of cardstock that was used to ship the Axial gears in. The Axial logo is a perfect template that fits the center ring gear and diff cup. I am using a Hot Racing aluminum diff cup, the HPI small center gears, and the Axial outer drive gears. This, with some shimming until they felt "right" has provided a good setup for me so far. Time will tell, but right now on 4S it will almost clear a standing backflip with a good throttle pull. Wheelies are pretty fun with this little truck and 3S provides those at just about any throttle speed. 4S is still nuts.

Sorry, I didn't take any pics while doing this because my hands were covered in grease, but I'll get part numbers together shortly.

[IntrepidMercians]

Originally Posted by kmnss

I have some decent news.

In testing these diffs, I have found one that has survived two 4S packs and a couple of 3S runs. Without immediate destruction.

After looking at the gears, while similar, they are not exactly the same. The tooth count is the same, but the HPI gears have a smaller set of center spider gears. The Axials are larger on the outer gears that actually drive the driveshafts. When I tried just the Axial gears, I could not get it to mesh and still fit in the AR60 axle. With the HPI gears, everything was too loose, which I believe was leading to my destruction.

My setup includes extra diff gaskets that I actually cut out of cardstock that was used to ship the Axial gears in. The Axial logo is a perfect template that fits the center ring gear and diff cup. I am using a Hot Racing aluminum diff cup, the HPI small center gears, and the Axial outer drive gears. This, with some shimming until they felt "right" has provided a good setup for me so far. Time will tell, but right now on 4S it will almost clear a standing backflip with a good throttle pull. Wheelies are pretty fun with this little truck and 3S provides those at just about any throttle speed. 4S is still nuts.

Sorry, I didn't take any pics while doing this because my hands were covered in grease, but I'll get part numbers together shortly.

Something to be wary of with differentials is that the spider gears are equally loaded. A real-life application failed because the spider gears were not concentric to each other or aligned correctly with the side (halfshaft driving) gears. This resulted in uneven loading and eventual failure of the gears.

[NitroDarwin]

What hpi vehicle was the gears you were using? I know the for the savage, they sell HD gears (machine hardened)

[kmnss]

Originally Posted by NitroDarwin

What hpi vehicle was the gears you were using? I know the for the savage, they sell HD gears (machine hardened)

They are these gears for their smaller 1:10 scale vehicles:

HPI #86014 - https://www.ebay.com/itm/HPI-Gear-Di...4AAOSwaLdadboV

They fit the Wheely King, and practically anything HPI made that was driveshaft driven (not belt). Like, back to the Nitro MT/ RS4 days.