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El Gecko

Vintage Brat diff dilemma

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Would you leave the original locked diff or replace it with an actual differential like the re-re Brat?

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1. Clean
2. Rebuild
3. Deal with it

OG Brats deserve to be OG Brats...

IMO,

Terry

 

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Also, if you don't like it, I am in the market for used OG Brat diff-locked trannies...  Sell it to me...

Terry

 

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To me, the locked diff and is part of what makes a Brat a Brat. The current re-re always seemed more like a Frog with different wheels/body than a Brat in my case.

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7 hours ago, El Gecko said:

Would you leave the original locked diff or replace it with an actual differential like the re-re Brat?

I had a Brat which I converted to a Frog back in the 80s. As much as I love the Brat, having no diff is a pain to drive and keeps fish tailing. Once it fish tails at high speeds, it's prone to flipping. When it flips, the nice hard shell/body becomes damaged. 

So I'd strongly suggest to change to diff if you plan to run/bash this on a regular basis. Drives better too especially when turning/cornering.

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Thanks, that's basically confirming what I had thought--I'll be keeping the original locked diff gear in it.

Now the $64,000 question: Is it acceptable to replace my clapped out original drive cups with some from the Frog/Blackfoot? The originals have no hole for the center rod like the later ones, although I believe the hex driveshafts and splines are the same so they should interchange?

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9 hours ago, Nicadraus said:

I had a Brat which I converted to a Frog back in the 80s. As much as I love the Brat, having no diff is a pain to drive and keeps fish tailing. Once it fish tails at high speeds, it's prone to flipping. When it flips, the nice hard shell/body becomes damaged. 

So I'd strongly suggest to change to diff if you plan to run/bash this on a regular basis. Drives better too especially when turning/cornering.

No concern there as it's only running a 380 motor and 10T pinion. I actually like how it drives on and off tarmac with the existing setup, and the locked diff is perfect for drifting around, but one of the hex driveshafts started slipping in a drive cup. At least that's what I think it is... I haven't actually pulled the rear end apart yet to investigate.

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5 hours ago, El Gecko said:

Is it acceptable to replace my clapped out original drive cups with some from the Frog/Blackfoot? The originals have no hole for the center rod like the later ones, although I believe the hex driveshafts and splines are the same so they should interchange?

Are you referring to installing the re-re Frog dogbone style axles and cups? They look like they should interchange looking at the old manuals. I've put the newer Frog dogbone style driveline in a vintage Blackfoot before. Assuming the spline count is the same for the E4 part in the locked diff gear, they should work.

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1 hour ago, Saito2 said:

Are you referring to installing the re-re Frog dogbone style axles and cups? They look like they should interchange looking at the old manuals. I've put the newer Frog dogbone style driveline in a vintage Blackfoot before. Assuming the spline count is the same for the E4 part in the locked diff gear, they should work.

Not sure about re-res but it's part F3 in the original Brat manual, which is the drive cup I mentioned without the hole in the center--totally solid, with a hex drive configuration.

In the original Frog manual, it's part K9, which appears to have an identical hex drive and diff spline config, but the center has been drilled out for the rod that connects through the center of the diff. These have the same diff splines as the new dogbone setup but with hex drives--I'm not concerned about the re-re dogbone setup yet, except as a general knowledge type thing for when I eventually upgrade the Blackfoot.

For the Brat, I'm thinking "why would Tamiya change the diff splines" so I'd assume Brat F3 = Frog K9, and K9 has a hole for the diff rod. However since the locked diff is different than the bevel gear diff, and the two drive cups have different part numbers, they may be slightly different in other ways (shorter or longer, etc.)

My thought was to swap the hex drive setup from my vintage Blackfoot into the Brat, either with or without the BF diff, since the BF needs a stronger rear end setup. I'll eventually pull the trailing arms off the Brat and Blackfoot to compare splines and hexes, I just figured I'd see if anyone knew for sure before I committed to making a big mess on the workbench.

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12 hours ago, El Gecko said:

For the Brat, I'm thinking "why would Tamiya change the diff splines" so I'd assume Brat F3 = Frog K9, and K9 has a hole for the diff rod. However since the locked diff is different than the bevel gear diff, and the two drive cups have different part numbers, they may be slightly different in other ways (shorter or longer, etc.)

My thought was to swap the hex drive setup from my vintage Blackfoot into the Brat, either with or without the BF diff, since the BF needs a stronger rear end setup. I'll eventually pull the trailing arms off the Brat and Blackfoot to compare splines and hexes, I just figured I'd see if anyone knew for sure before I committed to making a big mess on the workbench.

Ok, now I gotcha. Yeah, I would guess the two (F3 and K9) cups would interchange but I can't tell you for sure as I don't have any real Brat cups in my parts stash. The lack of pin/hole in the Brat drive cups again leads me to revisit my old theory that the ORV chassis was not designed for a diff from the beginning, rather "added-in" later when it was clear diffs were here to stay and necessary if they wanted to move the chassis into a more competition oriented buggy market (Frog).

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I owned a Brat new back in the 80's.  It was my first real RC car.  I would 100% without a doubt keep the diff-less gear box if restoring one today.   That was one amazing fun truck. 

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The joints are the same / interchangeable  , just one set with a hole for the align pin as you said . I've got some new , no holes , joints here , but I'm in the UK

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On 3/10/2021 at 12:36 AM, Saito2 said:

The lack of pin/hole in the Brat drive cups

Lancia Rally had the same fixed drive - no diff

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23 minutes ago, KEV THE REV said:

Lancia Rally had the same fixed drive - no diff

Correct, and furthers my guess that a diff might not have originally been in the cards considering the first two vehicles on the ORV platform went without a diff. Again, strictly a guess on my part. The Wild Willy's diff design seemed far more clearly thought out as an initial design concept while the tiny cast gears in the Frog diff always led me to wonder.

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If I remember correctly, the Brat was a lot lower in price vs the Frog at the time.  It was more a budget starter's car, I think (?).

I also owned a Frog later, but that gear box was not entirely bullet proof with the bevels skipping and all since I drove the car so often and thing just wore out too quickly.

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8 hours ago, Willy iine said:

If I remember correctly, the Brat was a lot lower in price vs the Frog at the time.  It was more a budget starter's car, I think (?).

Also correct. The Brat and the Lancia were on the budget end while the Frog was touted as having most of the hop-ups in the kit. Considering Tamiya's earlier "competition" 2wd entry, the Super Champ, didn't have a diff, I'm curious if they meant the ORV to have one either at first, especially considering how questionably it was executed vs the durable Wild Willy variant. A lot of off road cars lacked diffs early on, even the first Kyosho Scorpions, IIRC. Diffs were available (aftermarket) for the SRBs at the Super Champ's release, yet Tamiya chose not to include one. On the other hand, the SRB's days were numbered at that point so maybe Tamiya didn't want to invest the money in a diff for the outgoing model as the ORV was the way forward. Perhaps the ORVs didn't have diffs in mind when it first hit the drawing boards but the concept was brought in at the end of the design process, as diffs were becoming clearly a desirable feature (particularly in the comp market the Frog was aimed at). The Brat was out in July of '83 and the Frog by December "83, so who knows? Just a little conjecture on my part, with no concrete basis in fact. Sorry for any thread derailment.

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On 3/22/2021 at 4:48 AM, Saito2 said:

Also correct. The Brat and the Lancia were on the budget end while the Frog was touted as having most of the hop-ups in the kit. Considering Tamiya's earlier "competition" 2wd entry, the Super Champ, didn't have a diff, I'm curious if they meant the ORV to have one either at first, especially considering how questionably it was executed vs the durable Wild Willy variant. A lot of off road cars lacked diffs early on, even the first Kyosho Scorpions, IIRC. Diffs were available (aftermarket) for the SRBs at the Super Champ's release, yet Tamiya chose not to include one. On the other hand, the SRB's days were numbered at that point so maybe Tamiya didn't want to invest the money in a diff for the outgoing model as the ORV was the way forward. Perhaps the ORVs didn't have diffs in mind when it first hit the drawing boards but the concept was brought in at the end of the design process, as diffs were becoming clearly a desirable feature (particularly in the comp market the Frog was aimed at). The Brat was out in July of '83 and the Frog by December "83, so who knows? Just a little conjecture on my part, with no concrete basis in fact. Sorry for any thread derailment.

I find the discussion interesting so by all means continue! I would agree with your assessment, and I'd just like to note that it would have been very interesting to be a fly on the wall at Tamiya HQ back in the day!

As for the hex cup dilemma, is it usually the F3/K9 drive cups that fail first, or is it the hex ends on the shafts? This is my first time truly wearing one out, and although I haven't taken it apart yet to fully investigate, I did confirm that the right side driveshaft spins in the drive cup like a loose wrench on a stripped bolt.

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5 hours ago, El Gecko said:

As for the hex cup dilemma, is it usually the F3/K9 drive cups that fail first, or is it the hex ends on the shafts?

BITD, when my only window into the RC world was RC Car Action, they spoke much more about the hex shafts giving out. Now, when I got into ORVs 20 years ago, I found more issues with diff separation than I did with the hex cups/shafts, but that's my personal experience. I do know at one point they started putting an extra pair of hex shafts in some Mud Blaster kits. Now, its wholly possible that MRC, Tamiya US importer at the time, added those spare hex shafts to some of the kits. They even mention it in the RCCA review, IIRC. So, if I had to guess, the shafts would wear more than the cups but I'd carefully examine both for wear.

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Not sure how long it will last, but I just flipped the offending hex shaft the opposite direction, and now both sides engage and propel the wheels. I test drove it outside today... it's still pretty darn slow with the 380 boat motor, but being able to actually drive it is a big improvement!

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If you put small spacers in the rear dampers to hold the rear arms up a little the hex's don't bind on full drop , that is where the problems arise with the excess wear - it is a smoother run . The gearbox side flexing can be corrected with the 3D printed brace available on here from Xeostar

 

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