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Limited slip diff for Slash 2wd, stronger!

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  • 2 Post By smirkracing

Limited slip diff for Slash 2wd, stronger!

Old 09-05-2022, 07:20 PM
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Default Limited slip diff for Slash 2wd, stronger!

Can anyone recommend a stronger limited slip (or auto locking, like a sprag) rear diff for a 2wd slash 1/10? I see some on eBay but have no idea if they're any good or not. I upgraded the motor to a Castle brushless and slightly larger tires for bashing around on a grass track in my back yard, and have been breaking driveline parts since. Now that I've got steel axles and tightened the slipper clutch, the next weak link was apparently the differential as a handful of teeth have broken off of a couple of the gears, so I'm looking for something much stronger than the original and limited slip would be a nice bonus.

Last edited by SFL; 09-05-2022 at 07:32 PM.
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Old 09-05-2022, 07:40 PM
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Do you have an original gearbox or a new 272r (which has the a bevel and spider gears instead of planetary gears)?

The new gearbox (which is also available as an upgrade, although a rather expensive one) is sealed and can take a heavy oil (say 100k or even more, depending on your preference).

I struggled for years with the old version and bought many different aftermarket upgrades which were better than stock, but never perfect. I think ultimately the planetary gear design is not as strong as the bevel+spider gear design of the current gearbox.

If you want to try upgrading an older Slash, here is the best stuff I found (still using some of this):
1/10 Scale, E Rustler | FastLane Machine
Again, not cheap at all, but at least this stuff lasts and the product support is pretty good (at least, it was before COVID - now I think they're struggling).
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Old 09-06-2022, 06:34 AM
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Thanks for the reply. I've got the older version with the planetary gears, it was the planetary gears that broke. That Fast Lane stuff looks really nice but I'm hoping to not spend quite that much just yet. Do you know if the new style differential will fit the old gearbox, or would I need the new gearbox too?

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Old 09-06-2022, 08:49 AM
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loosen up the slipper clutch a touch, its there specifically to help absorb drive line shock loading ie: standing start launches to full throttle and landing a jump with the wheels spun way up with thrittle mid flight that could help you avoid breaking stuff as much
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Old 09-06-2022, 09:05 AM
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Originally Posted by Pilotx1 View Post
loosen up the slipper clutch a touch, its there specifically to help absorb drive line shock loading ie: standing start launches to full throttle and landing a jump with the wheels spun way up with thrittle mid flight that could help you avoid breaking stuff as much
Thanks for the reply, I'll definitely try loosening the clutch again. I tightened it originally because it was slipping like crazy and didn't seem able to hold the power, there didn't seem to be an in-between, either locked or slipping a lot. Our current track does not have any jumps, but it's a grass track, mowed very short, and with the slightly larger tires it gets fantastic traction and carries the front wheels down the straits. So I think between the traction, larger wheels and more powerful motor it's just a little more torque than the stock differential was intended to deal with. Maybe the slipper clutch too?
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Old 09-06-2022, 02:16 PM
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Originally Posted by SFL View Post
Thanks for the reply. I've got the older version with the planetary gears, it was the planetary gears that broke. That Fast Lane stuff looks really nice but I'm hoping to not spend quite that much just yet. Do you know if the new style differential will fit the old gearbox, or would I need the new gearbox too?
I am not 100% sure but I think the bearings are different (much larger) in the new gearbox, so you can't re-use the old gearbox with the new diff parts. You'd need to check the manuals though to be sure.

Originally Posted by SFL View Post
Thanks for the reply, I'll definitely try loosening the clutch again. I tightened it originally because it was slipping like crazy and didn't seem able to hold the power, there didn't seem to be an in-between, either locked or slipping a lot. Our current track does not have any jumps, but it's a grass track, mowed very short, and with the slightly larger tires it gets fantastic traction and carries the front wheels down the straits. So I think between the traction, larger wheels and more powerful motor it's just a little more torque than the stock differential was intended to deal with. Maybe the slipper clutch too?
I agree with you - the original traxxas 2wd platform is basically "vintage". It was designed for 200-300 watt motors. Average brushless 540 motors are 1000 watts. I also struggled with the stock Traxxas slipper and moved over to an AE double-sided slipper which was better.

Over the course of about 3 years I broke (and replaced, and upgraded) every part of the OG traxxas gearbox. After I strengthened one part, the next part in line would break. The roll pin in the top gear is weak, so you drill out and replace that with a solid pin, and then the idler teeth break. So you upgrade that to Robinson Racing, and then the teeth on the diff itself will break. It is just problem after problem if you've got brushless + high traction. I even sheared the output shafts while at the beach with sand paddles once. Every part of the original gearbox is sized for a brushed world.

Alternatively, you can of course tame your brushless system (punch control, torque limiting, etc) and try to minimize the stress. You can probably still have good acceleration and great top speed while reducing the chance of breaking the diff gears or other components of the gearbox.

In my opinion, one way or another, you'll be best served to bite the bullet and get the new gearbox (272R). Traxxas rarely makes major changes to their established platforms, so when they do, you can bet there's a very good reason (i.e. brushless + lipo = too much power for original gearbox). I haven't used the new gearbox but I did buy one, and based on looking at it, and from the description online, it seems they made improvements to all the weak points (including increasing the pitch of the major gears - that's a huge change).
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Last edited by smirkracing; 09-06-2022 at 11:20 PM.
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Old 09-07-2022, 06:02 AM
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Thank you much for the tips. The 272R gearbox is available for around $120-ish online, a bit pricy but as you eluded it might actually be the less expensive way to go if things keep breaking. We've got 4 of these things to upgrade so it adds up quick. Do you know if it's a direct drop-in, no mods required?

A new metal diff is on the way, I'm stabbing in the dark that the plastic one flexed a little and contributed to the gears failing, but I really don't think that was it. Really I wanted to try thick oil in it to see if I can make it behave like a limited-slip diff. But looking at the failure carefully, the gears on the output shafts are very thin and only engage about 1/2 the width of the teeth on the planet gears, and that happens to be the 1/2 of the teeth that sheared off (image below). This makes me think that output shafts with wider gears for more engagement might actually be the better thing to try, if I can find such output shafts. Parts on the way, I'll try loosening the slipper and turn down the motor a tad and see where that takes me, but maybe the 272R could be worth a try if things keep breaking.






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Old 09-07-2022, 10:43 PM
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That is the exact same breakage pattern I experienced many times. I don't believe there's any way to get more tooth engagement because of the way the planetary gears are set up. The single sun gear interfaces with the lower half of 2 planet gears, and the upper half of the other 2.

I wondered about flex in the plastic diff case but I had the same failure when using aluminum diff cases as well. Those planet and sun gears are just too small for today's brushless motors.

If you want to try adding something to slow the diff action down, I'd suggest trying diff putty. A small amount of diff putty, while much less precise than a given weight of oil, has the huge benefit that it won't leak out. Any reasonable oil weight (20k, 50k, 100k) will leak out of that diff unless you perform some surgery to seal it up. You can try super heavy oil, like 1M weight, to see if that gives you the feeling you're after, because such a heavy oil won't leak out very easily.

Good luck! Part of the fun is experimentation and seeing if you can solve these problems. Maybe you'll come up with a new solution that will help others.

Last edited by smirkracing; 09-07-2022 at 11:48 PM.
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Old 09-08-2022, 05:43 AM
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The metal diff I ordered is (in theory) sealed, and I ordered some 100k oil to go with it, crossing my fingers on that one. I checked out the 272R gearbox, Traxxas has a nice promotional page about it and it does read like it's a significant improvement over the old one. Visually the diff on that one looks like it should be stronger, and they call it "heavy duty" in their sales material. However I emailed Traxxas support asking about it and they said it's no stronger. I'm still considering trying that gearbox if the adjustments to the slipper and motor control don't stop things from breaking on the old one. Here is a copy/paste of my email with Traxxas support, who is impressively responsive by today's standards:

My mail to Traxxas (they previously suggested loosening the slipper clutch):
Thank you very much for the tip, I will try your suggestion with the slipper clutch. Also I see that Traxxas has an upgraded gearbox part number 272R. Can you tell me if this contains a stronger differential than the original gearbox (stronger gearbox in general), and would it be a direct drop-in replacement?
Traxxas response to me:
Hello,

It is a direct drop in replacement but the differential is no stronger.

Sincerely
Stefano
Traxxas Support

If you need immediate assistance please contact our Customer Service department 7 days a week, 8:30am to 9:00pm Central Time at one of the following numbers:

US Customers 1-888-TRAXXAS (Toll Free)
Outside of US 001-972-549-3000
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Old 09-08-2022, 01:34 PM
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Originally Posted by SFL View Post
Can anyone recommend a stronger limited slip (or auto locking, like a sprag) rear diff for a 2wd slash 1/10? I see some on eBay but have no idea if they're any good or not. I upgraded the motor to a Castle brushless and slightly larger tires for bashing around on a grass track in my back yard, and have been breaking driveline parts since. Now that I've got steel axles and tightened the slipper clutch, the next weak link was apparently the differential as a handful of teeth have broken off of a couple of the gears, so I'm looking for something much stronger than the original and limited slip would be a nice bonus.
On Ebay, are you referring to a 'diff lock spool'? ... 'limited slip' and 'locked' are two terms often used synonymously which is not the case, as the spool creates a 'locked' situation between the two output shafts... the spool is a temporary addition that can be installed/removed for testing purposes, but I have a 2wd Slash diff that has been 'locked' with solvent, but it is not reversible ... a locked rear end can lead to erratic handling, but puts full power to the ground at both wheels all the time if you have the situation for it ...
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Old 09-09-2022, 05:14 AM
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No just the metal diff case. I saw the part for locking the differential but that seemed like it would lead to bad turning so I did not go that route.
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Old 09-13-2022, 06:33 AM
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Just to add more info for the next guy with this problem; that all-metal diff case I bought from ebay is no good, the dimensions are off somewhere and it binds up super right, so I went back to the stock plastic one. There is a pin that keeps the output shafts aligned, be sure that's in there (it was missing on disassembly, maybe part of my problem!?). With the slipper clutch 1/4 turn from all the way tight it slips a lot and the cover was hot after just a couple minutes, but wheelieing out of corners is reduced 90% so it's more controllable. I'll play with tuning the slipper and buy some more slipper parts, with this much slipping I'm guessing it's going to wear out quickly, but that's easier to replace than differential gears. Time will tell!
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