Kingcab/madcap diff research

[Kol__ 2823]

On 3/2/2023 at 1:14 PM, Muso31 said:

A2 brackets are now available, £12 forum cost plus post - message me if you need one:

Wow mate, that looks amazing. Looks like an almost exact replica of the original mate and better than the Xtra Speed offerings from the looks of it👍👍👍

[bavee 954]

On 2/23/2023 at 8:05 AM, bavee said:

Now I think about it some more, tyres aren't glued on the wheels I used. So could have been the wheels spinning just a bit in the tyres also. Will have a look when I run the car in daylight.

Finally found time to test the car in daylight. Simply brilliant. No diff slip, just the rims spinning in the tyres a bit.

Car corners really tight. Great straight line stability. Amazing and thanks again!

[Collin 1936]

@Muso31w

Hi. Got all six sets already last week. Measurings all fine and it seems to be a fit&forget solution.

Thanks for this work.

[Muso31 118]

Great news @bavee and @Collin thanks for the feedback - really pleased you've had great results, it makes the effort worthwhile. I hope it's helped get some vintage cars back on the road!

[bavee 954]

9 minutes ago, Muso31 said:

I hope it's helped get some vintage cars back on the road!

Certainly did. The one on the right hit the road today:

[Muso31 118]

We have a geared diff available:

 

£26 forum cost (plus post) - The gears are not included! You will need to order Differential Gear Bag 9405431/19405431

[Kol__ 2823]

Awesome mate, nice work!!! I was wondering how you were getting on. What material did you go for with this gear diff in the end?

[Muso31 118]

it's Acetal (Delrin). Better than the original diffs I think as it's less prone to absorbing moisture etc, therefore the tolerances can be tighter and it has better properties overall

[Muso31 118]

I need to delay release of these to make a tweak. Will keep you posted....

[Muso31 118]

All sorted. Delighted with these, with the SA version you can easily flex the plastic, these are a lot more solid -The acetal is very strong, if anybody is looking to get their diff upgraded just send me a message.

At some point in future I'll work on other elements of the transmission but it's going to be a while, I'll have a busy year with twins on the way soon. 

[Muso31 118]

does anybody know what lid screws are included with the Tamiya diff? ... https://www.ebay.co.uk/itm/295271869767

As I've tried these and they're much better than the larger ones I used

 

...edit the answer is Tamiya 9804477/19804477

[Muso31 118]

 

Someone with more RC experience can comment on this but I find the geared diff to be extremely smooth and very consistent. There's a constant spin in opposite directions of the wheels which is never interrupted. Seems a lot better than a ball diff to me personally as I've found that ball diffs on the vintage vehicles works well, but isn't this consistent: 

 

 

[Sgt.Speirs 1161]

The intention of a ball diff is to have not this completely free wheelspin in the opposite direction.

Why? If one wheel spins freely, the other wheel can no longer move the car forward. The complete motor power goes to the wheel with the least slip (free spinning one).

If there is a small locking effect (by using a correctly adjusted ball diff) you have also (limited power in drive direction) if one wheel spins freely. But if the ball diff gets loose there is no power in drive direction.

With the common oil filled gear diffs today you can adjust the locking effect by using oils with different viscosity.

PS: My limited technical English vocabulary for this explanation  

[Kol__ 2823]

2 hours ago, Sgt.Speirs said:

With the common oil filled gear diffs today you can adjust the locking effect by using oils with different viscosity.

Interesting 👍

Perhaps a thicker grease would help here @Muso31??

[Muso31 118]

26 minutes ago, Kol__ said:

Interesting 👍

Perhaps a thicker grease would help here @Muso31??

I think the geared diff is working as it ought with molybdenum grease (thanks @Champ85), as it's not based on a modern oil filled sealed design. They're a whole other level with bevelled gears.

It's certainly running extremely well when used with a brushless which is nice - I believe some people completely lock their diffs intentionally, although I couldn't comment as to why. Doesn't sound like a good idea to me although they probably know more than I do

[Sgt.Speirs 1161]

7 hours ago, Kol__ said:

Interesting 👍

Perhaps a thicker grease would help here??

Instead of the standard (ceramic) grease you can also take a (sufficient) amount of anti-wear grease which is normally used in the cardan joints.

[Champ85 581]

@Muso31 has concerns about using thicker grease like AW due to the tolerances of the diff.  It may want to "lock up" under certain conditions.  I tend to agree due to the basic design of the style of gear diff used in the Avante. It is not based on the more typical bevel gear and spider gears like most other Tamiya gear diffs.  Rather, this uses straight gears on integrated hard shafts that ride, without bearings mind you, inside holes in the diff casings which are made out of plastic.  The stress of running AW grease could create additional forces on the tiny internal gears and the shafts and gradually bore out the diff case "bushings" into an oval or slot, causing all sorts of destruction.

Let me illustrate:

In a typical bevel gear diff, the forces push on the smaller internal bevel gears outward, which is spread out across the whole back face of each gear.  The two larger gears connected to the axles are also pushed outward, against the diff case halves, and the forces are spread out across the whole back face of each gear as well.  Due to relatively large surface area of the back face of the gears, plus the use of slippery grease on the back face of each gear, there is no wear.

But in the Avante diff design, the internal gears are straight, not beveled. As a result, all of the forces exerted on the gears are perpendicular to the shafts the gears ride on, and those shafts are held in place by holes in the diff case itself, which you can see from the outside.  There isn't much material in the diff case to act as a bushing for these shafts, plus the contact area of the shafts to the diff case is much, much smaller than back face of the gears in the bevel gear design, making the diff case holes more susceptible to wear.

Now, all of that said, when the car is going straight, the left and right rear axles are going at the same speed, so the internal diff gears are not spinning themselves.  The whole diff is rotating as a single unit, of course, but the gears inside are not spinning.  The internal gears only spin when one rear axle spins faster than the other.  So the only time wear can occur on the diff gears and the bushings they ride on is when turning a corner or when one wheel is off the ground and the motor is running.  The free-wheeling case is not an issue because there is almost no load on the diff.  It's really when cornering and powering out of a turn when the highest diff load occurs. (Not including jumps which are way worse instantaneous load)

I think it is important to ensure grease is applied to the internal gear shafts where they ride inside the diff case holes (marked in green, above).  Those internal diff gears and shafts are machined as a single solid unit (see PD1 in the above picture), so the gear+shaft spins in the diff case holes.  My concern is that if AW grease is used internally in the Avante design, it can creates a lot of extra internal pressure on the gears which results in high focused pressure on each diff case bushing hole and the hole could start to elongate over time.

But I do not have personal experience with the Avante gear diff design myself.  Maybe it isn't as much of an issue as I think.

[Sgt.Speirs 1161]

14 hours ago, Champ85 said:

…
My concern is that if AW grease is used internally in the Avante design, it can creates a lot of extra internal pressure on the gears which results in high focused pressure on each diff case bushing hole and the hole could start to elongate over time.

But I do not have personal experience with the Avante gear diff design myself.  Maybe it isn't as much of an issue as I think.

Did you read in the manual of the Avante which grease you have to use for the gear diff? 😉

 

Regarding your explanation, I can follow it and it also sounds plausible. But I think your last sentence probably sums it up in the end, "Maybe it's not as much of a problem as I think." 😁

[Thommo 1180]

 

My experience is that ultra high viscosity gear diff oil works best in these planetary gear diffs.  It slows the diff action considerably more than AW grease and doesn't leak out a black mess.

 

[Muso31 118]

6 hours ago, Thommo said:

 

My experience is that ultra high viscosity gear diff oil works best in these planetary gear diffs.  It slows the diff action considerably more than AW grease and doesn't leak out a black mess.

 

My experience with testing was similar as far as AW grease goes although I've not tried the oil. So far the molybdenum grease works its own way to the gear shafts nicely and runs well in the diff, would be interested to try a few other types though

[Sgt.Speirs 1161]

@Muso31: If you made a special differential spur gear (with one tooth more than the Avante/Astute 2018), did you think about to do also a ball diff spur gear (based on Egress 2013) with one tooth more?

Altgough I have all cars (Avante, Egress & King Cab) in my collection, I didn’t test it in the past if the ball diff will fit into the King Cab gear case.

[Muso31 118]

11 hours ago, Sgt.Speirs said:

@Muso31: If you made a special differential spur gear (with one tooth more than the Avante/Astute 2018), did you think about to do also a ball diff spur gear (based on Egress 2013) with one tooth more?

Altgough I have all cars (Avante, Egress & King Cab) in my collection, I didn’t test it in the past if the ball diff will fit into the King Cab gear case.

It's doable, but I'd need to know if the drive cups would work with the king cab, and if the internals for that diff are readily available.

I'd be interested to know if there are models outside of the madcap/king cab that require parts also as it would be nice to produce some parts for other popular models where there's a need

[Muso31 118]

idle gear CAD files/drawings are done, first off there's a match to the original but machined in Acetal which would be stronger..

 

 

 

Then there's an extra strength design which would have a detachment in steel, the plastic part being Acetal again:

 

These would have to be joined with a relevant epoxy. 

As with the Diff these are a perfect .80 module and stronger than the originals. The metal one is similar to the thorp gears that were available but I've attempted to improve the design as the thorp gears had two of the teeth extended which would upset the balance of the gear. This one has been designed to have the same balance

 

[Muso31 118]

3D print sorted and looking good. I'm making an assumption that these gears are in short supply also, so I'll get the initial all acetal version machined then look into the steel version: