Why don't shafty rock crawlers have center diffs?

[fyrstormer]

This is something I've been wondering for years, but I only recently decided to actually write it down. Hear me out, I haven't lost my mind.

Shafty rock crawlers typically have fully-locked drivetrains, oftentimes with a higher gear ratio in the front axle for better climbing and cornering. Additionally, shafty rock crawlers are sometimes equipped with a "dig" gearbox that allows the rear axle to be locked like a parking brake for extremely sharp turns or completely disengaged so it can freewheel.

Despite the use of overdrive gears in the front axle and/or the use of a "dig" gearbox, it is quite common for a shafty rock crawler to jam its front wheels against the front face of an obstacle while the rear wheels continue to push the vehicle forward into the obstacle. Very careful use of the dig function can minimize this problem by allowing the rear wheels to be locked, so they act as a fulcrum while the front wheels start to climb the front face of the obstacle. However, as the front of the vehicle lifts up, the locked rear wheels roll backwards slightly; as soon as the front of the vehicle lifts up enough to relieve the pressure on the chassis, the front wheels will start to lose grip, potentially causing the vehicle to slip and lose progress.

Additionally, when climbing over the apex of an obstacle, it is quite common for a shafty rock crawler to exhibit significant torque-twist with one front wheel lifted into the air, because both rear wheels and one front wheel have significant traction, preventing driveline torque from being relieved. Typically this is dealt with by steering the wheels side-to-side, allowing them to slip slightly until the driveline torque is relieved and the front wheels settle-down onto the ground again.

Overdrive gears are useful for improving cornering (wherein the front wheels need to rotate faster than the rear wheels), for sometimes reducing torque-twist when apexing an obstacle, and for reducing the "jamming" scenario I described two paragraphs above, but overdrive gears are an incomplete solution. They also cause driveline torque to build-up constantly when a shafty rock crawler is driving in a straight line, causing the chassis to lean to the side, which can cause the vehicle to tip-over when side-hilling. Furthermore, as the existence of "dig" gearboxes shows, there are many scenarios wherein it's necessary for the front wheels to rotate so much faster than the rear wheels that no reasonable overdrive gears could provide the necessary difference in wheel speed without causing unacceptable torque-twist. "Dig" gearboxes themselves are an incomplete solution, as they require split-second manual operation for best results, and they inflict an immense load on the motor, causing dangerous heat buildup in a matter of seconds as the motor is forced to drag the vehicle forward against the drag of the locked rear wheels.

It seems to me that these problems could be significantly reduced if shafty rock crawlers were equipped with center diffs. If a center diff were filled with 500,000-1,000,000wt silicone oil, it would split torque almost 50:50 between the front and rear axles even at very low speeds, while also allowing just enough freedom of movement for excess driveline torque to be relieved, and for the front wheels to rotate significantly faster (albeit only at very low vehicle speeds) when cornering sharply or climbing the front face of an obstacle. In particular, when climbing the front face of an obstacle, the rear wheels would still be able to exert forward force on the chassis, keeping the front wheels pressed against the face of the obstacle, while simultaneously allowing the front wheels to rotate much faster than the rear wheels to actually climb the obstacle.

With a center diff, it may be a concern that the rear axle could spin freely while the front wheels are stuck, such as if a front wheel gets jammed in a hole. However, that could be remedied quite simply by equipping the center diff with a one-way bearing that allows the front axle to rotate faster than the rear axle, but not vice-versa. Alternately, the center diff could be equipped with a locking mechanism similar to the diff-lock mechanisms used in the Traxxas Summit and TRX-4/TRX-6, though the one-way bearing solution would eliminate the need for an extra servo and radio channel, thus being less expensive to implement.

Thoughts?

 

[micro racer]

Huh, I never really thought about this. I assume the biggest challenge is space constraints, because small diffs don't tend to work very well.

Since Axial essentially copied an Associated transmission for the AX10 trans, it might be funny to stuff an SC10 differential into the case, or even fit an entire SC10/B4/T4 transmission into an SCX10 (if the holes line up).

>>works at hobby shop
>>will have to gather spare parts tomorrow

 

[micro racer]

On a more technical note, this actual is an interesting concept. Since a differential usually dumps power to the side with the least traction, the diff-action is often considered wasted energy. However, on a rock crawler the "least traction" would more likely be entirely the opposite situation: a bound up wheel/whole axle. In this case, diff-ing out to the other axle would probably be helpful because more power goes to the free axle and is not wasted in getting the truck more bound up.

Okay now I really want to see this tested out...

 

[Voodoobrew]

Not that I have be around long enough to know but I believe the sporty class was meant to be a more economical class that's made from simpler parts. Because the moa class was too competitive and expensive for new comers.

Not that sporty class is still that way but, shouldn't it be?

As far as your idea goes, it sounds unpredictable. But I would watch it, because who knows unless they try.

Sent from my SM-N950U using Tapatalk

 

[fyrstormer]

I suppose it would be slightly unpredictable provided you use really thick diff oil like I suggested, as opposed to being wildly unpredictable if you use thin diff oil. However, I'm not sure it would be significantly more unpredictable than the already-chaotic interactions between a chassis experiencing torque twist, a set of squishy rubber tires, and the uneven terrain they are climbing over. I can see how having a center diff might not be an improvement for a dedicated competition truck that has already been tuned to-the-nines, but I think it could be an improvement for "recreational" crawlers.

 

[HumboldtEF]

Interesting topic

I wonder if it would actually combat torque twist?

I'm imagining the (torque twist) moment after the front tire has been lifted, with the opposite tire still on the ground I dont think the lifted tire will be relieved and settle unless it had an open diff.
A few times I've taken advantage of this on my TRX4 while descending. A rear tire would lift and I'd unlock the diff for the rear axle the the rear tire immediately settles since the drivetrain has an outlet where it can relieve itself.
With locked axles I'd think you wont have an outlet to relieve the drivetrain so long as both axles have traction. I'm just wondering how it would slip? Another reason I wonder about this is my Bully 2 with no connection between the front and back axles has pretty pronounced torque twist.

I'm only spitballing so please dont take this as me shooting anything down. I could be totally wrong.

I wonder how oil filled diffs for the axles would do? I know its another how the heck would you fit that on a crawler but just for the sake of the discussion. I feel like allowing slip in the axle's could allow a lifted tire to settle. You would also be able to fine tune the the front and rear separately.

 

[Jim85IROC]

I suppose it would be slightly unpredictable provided you use really thick diff oil like I suggested, as opposed to being wildly unpredictable if you use thin diff oil. However, I'm not sure it would be significantly more unpredictable than the already-chaotic interactions between a chassis experiencing torque twist, a set of squishy rubber tires, and the uneven terrain they are climbing over. I can see how having a center diff might not be an improvement for a dedicated competition truck that has already been tuned to-the-nines, but I think it could be an improvement for "recreational" crawlers.

I'm skeptical that it would be an improvement overall. Yes, it would help with torque twist, but more often than not, I could see it being limited in what it can crawl up because the center diff is reducing the effectiveness of half of the drive wheels.

 

[OSRC]

Interesting concept, but it needs to be taken a step further.

Using a "nitro style" center diff with a disc brake on each end would be interesting - or at least on the rear shaft at minimum - would effectively allow you to control power output. I expect the most likely scenario would be for the front wheels to diff out when the weight settles to the rear. Applying some brake to the front would force the rears into action, and vice-versa. It's like mechanical traction control on a 1:1....or Axial's "cutting brake" for the XR10 unicorn mod turned sideways.

Hmm...... interesting.

 

[fyrstormer]

Interesting topic

I wonder if it would actually combat torque twist?

I'm imagining the (torque twist) moment after the front tire has been lifted, with the opposite tire still on the ground I dont think the lifted tire will be relieved and settle unless it had an open diff.
A few times I've taken advantage of this on my TRX4 while descending. A rear tire would lift and I'd unlock the diff for the rear axle the the rear tire immediately settles since the drivetrain has an outlet where it can relieve itself.
With locked axles I'd think you wont have an outlet to relieve the drivetrain so long as both axles have traction. I'm just wondering how it would slip? Another reason I wonder about this is my Bully 2 with no connection between the front and back axles has pretty pronounced torque twist.

I'm only spitballing so please dont take this as me shooting anything down. I could be totally wrong.

I wonder how oil filled diffs for the axles would do? I know its another how the heck would you fit that on a crawler but just for the sake of the discussion. I feel like allowing slip in the axle's could allow a lifted tire to settle. You would also be able to fine tune the the front and rear separately.

Older Axial vehicles have room for diffs in their axles. I briefly ran a front diff in my Wraith but I quickly discovered it wouldn't climb anymore. At the time I didn't have access to the super-thick greases and diff oils that I do now, though. Still, I think even the thickest oil in an axle-mounted diff would allow too much slip; that's why I was thinking about only having a center diff.

 

[fyrstormer]

Interesting concept, but it needs to be taken a step further.

Using a "nitro style" center diff with a disc brake on each end would be interesting - or at least on the rear shaft at minimum - would effectively allow you to control power output. I expect the most likely scenario would be for the front wheels to diff out when the weight settles to the rear. Applying some brake to the front would force the rears into action, and vice-versa. It's like mechanical traction control on a 1:1....or Axial's "cutting brake" for the XR10 unicorn mod turned sideways.

Hmm...... interesting.

A brake attached to each output of the diff would definitely be a step further, but then you'd need two extra servos to control them, and extra electronics to operate them (or an extra brain to think about manually operating them). I think most of your concerns could be addressed with a one-way bearing mounted to the side of the diff case, with one of the diff outputs passing through it, so it would only allow "diffing-out" in one direction and not the other.

In any event, with the super-thick oil I was contemplating, any "diffing-out" would be very limited; applying slightly more throttle would easily compensate for it.