another blackfoot differential thread

[Needler56 0]

i've just recently picked up an original blackfoot and are having problems with the differential. when i first got the truck, the differential would not spin so i rebuilt it. differential worked fine with the rs 540 motor. 

so i tested and tuned with various motors such as a traxxas titan 550, tower hobbies speed secret 14x4 and a trinity dirtinator 10x1 just to see what the transmission could do. the dirtinator really hauled! when the battery was done with the dirtinator, i believe the differential gave out.

i rebuilt the differential, the gears looked fine and put the rs540 motor back in. the wheels spin when off the ground, but when on the ground, the transmission just spins and the truck barely moves like the differential is not engaging.

how do i fix this? can i place shims somewhere to make the differential tighter? i'm assuming what causes this is the common problem of the gearbox plates splitting, but they feel tight to me. or is the thorp setup the absolute fix? 

[Backlash 481]

I used to get this a lot when I raced a Frog back in the 80's. What happens is exactly like you said,  the 2 side gears on the differential are forced outward against the metal side plates of the gearbox when running. What can and does happen is that the gearbox side plates bend slightly, and this then allows the differential to move around inside the gear case.

This sideplay will increase until the side gears, "skip a tooth" on the 3 small spider gears. Eventually, the skipping will increase to a point, that car won't go, and the gears will be damaged.

You could shim the diff so that there is no sideplay between the differential, and the metal sides of the gearcase. A more simple way is to just "bend" the metal sides of the gear case in a little. The easiest way to do this is to disassemble the gearbox, place the plastic part of the gearbox on its side on a hard surface. Then sit the metal side of the gearcase onto the plastic piece, and lightly tap the sideplate with a hammer, where the bearing for the differential goes.

WARNING- you do not have to hit the sideplate very hard to do this. Turn the plastic piece over, and do the other side plate. Now, reassemble the gearbox, and check for sideplay in the diff. If there is still play, repeat the process until the differential sits snugly between the gearbox plates.

If after doing all this, you still cannot get a smooth diff action, then I would also recommend replacing the 2 side gears in your differential.

Goodluck

PS, as a quick fix, I have seen a large pair of Multigrip plires used to squeeze the sideplates together on the gearbox. Whilst this is not the best way to fix the problem, it may get you going if you have a diff failure in the middle of a bash session.

[mymonsterbeetleisbroken 16]

Backlash has answered just about everything you need to know, the only other thing i can suggest is check the drive cups are fully pushed into the diff gears and that the splined shafts are not worn. Sometimes a drive cup will work loose and cause full loss of the transmission - without any crunching sounds - the gearbox spins freely and the truck goes nowhere. 

A crunching sound upon acceleration/pulling away will mean the diff gear has seperated due to wear and wonkey gearbox side plates.

No crunch genrally points to a drive cup spline from the differential output worn.

 

 

[adam1973 1]

Another thing to check is the motor pinion, is it on the flat of it's shaft? If it doesn't have a flat, then thread-lock the pinion screw in.

[Nefarious 21]

I rebuilt my original Blackfoot and gave it to a good friend for Christmas 2021, after 4 NiMh battery runs he had the slipping issue described. It would seem fine held up in the air but as soon as we placed it on the ground it barely moved. I tried your fix. I did both things you described. I slightly bent in the plates and also added some spare Traxxas teflon shims and now we have 0 issues!

Thank you so much for taking the time to post this fix Backlash!

[Nikko85 3443]

I believe someone has also 3D printed a really good solution to this - a gearbox cover that gives much more rigidity.

[markbt73 5315]

Does nobody do the Oldfrogshot through-bolt mod any more? 10 years later, and not a single issue with my Blackfoot. Why mess around with shims?

[Saito2 6686]

4 hours ago, markbt73 said:

Does nobody do the Oldfrogshot through-bolt mod any more?

I have that mod on a couple of my ORV trucks.

[Kol__ 3377]

6 hours ago, markbt73 said:

Does nobody do the Oldfrogshot through-bolt mod any more?

What exactly is that mod @markbt73? You putting a bolt through the drive shafts in the gearbox to pull it all together?

I've gone with the gearbox brace clamp from @Xeostar on mine and look to work very well. I haven't done too many runs with it since fitting, as been working on a body shell for it.

I might go with the MIP ball diff in the future, but I need to buy extra parts to fit that as I'm running the original dogbones setup etc.

[Frog Jumper 3658]

@Kol__
 

https://www.tamiyaclub.com/showroom_model.asp?cid=24674&sid=3142
 

[Kol__ 3377]

Thanks @Frog Jumper

[Nefarious 21]

@markbt73, i remember seeing the post this winter but didn't remember where i saw it!

Ace Hardware is only 2 miles away & has the countersunk m.2×25mm  in stainless steel. I will be buying tomorrow and doing the mod this weekend. Thank you for replying & reminding!!!

...& thank you @Frog Jumper for the link!