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Bulletproofing the gearbox

Overkill

Well-known member
Messages
269
Location
Birmingham, AL
Tekno RC's
EB48, ET48
Driving Style
Basher
The biggest problem I see in the bashing community happens during big air flat landers which creates a tremendous amount of flex. When the rear of the chassis flexes really hard, the center rear driveshaft slams into the pinion gear end and this can make the set screw lose grip. Once that happens you can destroy the pinion bearings, pinion gear, or gearbox pretty quickly. What I have found that works for extreme bashing is simply using a flanged bearing on the outside of the gearbox which makes it so the driveshaft cannot make contact with the pinion gear. Instead all of that force hits the coupler and the coupler hits the flanged section of the bearing and there’s no possible way to damage anything. I also use shims in between the two pinion bearings for added security, so there’s no pressure on the plastic divider.

A high quality flanged bearing from Boca costs $8 and makes the gearbox bulletproof. For the shims, a total thickness of 2.45mm works perfect.

Parts used:
Boca flanged bearing 5x13x4
SMF695-2RS

Boca regular bearing 5x13x4
MR695-2GS

HPI shim 5x7x1.45
103368

Serpent shim 5x7x1.00
909514

Other parts used:
Arrma center rear 5mm steel driveshaft (this is 3mm shorter, only for ET48.3, AR310470)
Knurled set screw M5x5 (BoltDepot.com)
Red loctite


1985


When you have a lot of chassis flex the driveshaft can slam into the pinion gear, which has potential to cause some damage if the set screw breaks loose.

Contact1.png


Contact2.png
 
Last edited:

2015GTRBE

Active member
Messages
40
The little plastic divider in between the pinion bearings in the gearbox can be damaged from impacts, flex, stress, heat, etc. If the plastic divider gets blown out you can damage a lot of parts and your day is over. I see this problem on a regular basis in the bashing community. I would love to see Tekno update the design and use a steel sleeve inside like Arrma does, as well as larger bearings, and easy access. :) My mission is to make everything as strong as possible.

5x13x4 flanged bearing
If the driveshaft/outdrive slams into the bearing, all the force goes to the outside of the gearbox instead of the plastic divider. As a bonus, the flange part is 1mm thick so now the driveshaft can only make contact with the outdrive and not the pinion gear.

5x7x_ shims
These go in between the bearings to take the pressure off the plastic divider.

Parts used:
Boca flanged bearing 5x13x4
SMF695-2RS

Boca regular bearing 5x13x4
MR695-2GS

HPI shim 5x7x1.45
103368

Serpent shim 5x7x_
909514

View attachment 1985
Will try this, thanks. BTW, I Pm’d You.
 

Overkill

Well-known member
Messages
269
Location
Birmingham, AL
Tekno RC's
EB48, ET48
Driving Style
Basher

smirkracing

Well-known member
Messages
63
Well, at first I didn't think I'd need this, but after a session of back and front flips, it seems clear I do. Thanks for posting these great instructions.

For max strength, do you recommend this flanged bearing mod, or the big bearing mod you posted in a different thread?
 

smirkracing

Well-known member
Messages
63
Also, anyone have a clear idea of why the inner bearing blows out? I lost mine today (after a bunch of backflips which I admit slammed the truck into the ground). I am still trying to figure out why exactly the force gets transferred to the inner bearing.

Here are some pics of the resulting damage (bearing blown out, and it spun its way forward, damaging the inside of the gearbox case).
tekno-gearbox1.jpg
tekno-gearbox2.jpg
tekno-gearbox3.jpg
 

Overkill

Well-known member
Messages
269
Location
Birmingham, AL
Tekno RC's
EB48, ET48
Driving Style
Basher
Well, at first I didn't think I'd need this, but after a session of back and front flips, it seems clear I do. Thanks for posting these great instructions.

For max strength, do you recommend this flanged bearing mod, or the big bearing mod you posted in a different thread?
Glad to help. I would recommend doing what's in this thread.

Also, anyone have a clear idea of why the inner bearing blows out?
The damage is usually caused by impacts. My buggy which sees up to 6,500 watts does not have this problem.
 

Gizmo

Member
Messages
6
Location
Germany
Tekno RC's
EB48, ET48, MT410
Driving Style
Basher
The effect of this 1mm flange is also that the Dogbone has 1mm less longitutional travel, isn‘t it?
No issues with that?
 

Overkill

Well-known member
Messages
269
Location
Birmingham, AL
Tekno RC's
EB48, ET48
Driving Style
Basher
Since the flanged section of the bearing is 1mm thick, you'll lose 1mm of free play in the driveshaft front to rear movement. This is not a problem for ET owners (longer driveshaft) because you can just use the 3mm shorter Arrma driveshaft (AR310470). This will provide a very good amount of free play. For MT owners (shorter driveshaft) this may or may not be a problem losing that 1mm of free play and could lead to a bent driveshaft. The chassis and bracing you're using will play a role in that. At the very least I would use the single HPI shim in between the bearings.
 

Gizmo

Member
Messages
6
Location
Germany
Tekno RC's
EB48, ET48, MT410
Driving Style
Basher
@Overkill the single HPI Shim: will it then be held in place as it only has contact to the flanged bearing but not to the plastic ring ?
I am thinking about replacing this shim with a nylon Spacer I found from Moto Guzzi: 5,5x13x1: Nylon Spacer
And leave the Serpent shim out completely as in my understanding it only replaces the plastic spacer of the diff case
 

NEWTDAWG7

New member
Messages
4
do somebody have experience with this metal diff for ET48? I tried to contact him on FB but no joy.
Go on Ali Express, ALL RACING sales their parts there I know for sure. I have seen multiple Tekno parts they make along with crawlers and other brands
 

Overkill

Well-known member
Messages
269
Location
Birmingham, AL
Tekno RC's
EB48, ET48
Driving Style
Basher
@Overkill the single HPI Shim: will it then be held in place as it only has contact to the flanged bearing but not to the plastic ring ?
I am thinking about replacing this shim with a nylon Spacer I found from Moto Guzzi: 5,5x13x1: Nylon Spacer
And leave the Serpent shim out completely as in my understanding it only replaces the plastic spacer of the diff case
The shim(s) take the pressure off the plastic divider. The 2nd shim is only required when using a flanged bearing. That spacer you linked will not work.

One question, instead of using the hpi and serpent shims, could we use this? https://www.amazon.com/Team-Durango-TD310670-Crunch-5x7x2-5mm/dp/B01DPA5T02
I would use a metal shim. The shim length really needs to be precise. If it's too long you'll have back and forth movement and if it's too short the shim won't be doing anything at all. I tried different shims in .05mm increments and found 2.45mm to be perfect.
 

Billyjoe

Member
Messages
20
Since the flanged section of the bearing is 1mm thick, you'll lose 1mm of free play in the driveshaft front to rear movement. This is not a problem for ET owners (longer driveshaft) because you can just use the 3mm shorter Arrma driveshaft (AR310470). This will provide a very good amount of free play. For MT owners (shorter driveshaft) this may or may not be a problem losing that 1mm of free play and could lead to a bent driveshaft. The chassis and bracing you're using will play a role in that. At the very least I would use the single HPI shim in between the bearings.
in case you bulletproof the Front gearbox too, which driveshaft could we use that is 1-2mm shorter?
 

Overkill

Well-known member
Messages
269
Location
Birmingham, AL
Tekno RC's
EB48, ET48
Driving Style
Basher
I've never had a problem with the front gearbox. I don't think you'll find a driveshaft in that size, without going custom. The center front driveshaft has a little more free play to work with.
 

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