Trying to make a Tra**as SRT fast !

[RC10th]

This will be the build thread of a stock spec vintage race truck, an attempt to make a Traxxas SRT fast.

With the cost and rarity of Associated parts here I decided to make the switch to the dark side, where parts are considerably cheaper and readily available. A truck that I can push 100% (or beyond) and not worry about breaking something obscure.

I started with Jeff's Poor Mans SRT conversion which came with A&L steering, I must say he has done a great job on these kits as they are well put together.

I dyed the battery box plastics black to better fit in with the car, the white battery boxes look a bit out of place to me.

Since this is a stock spec truck cutting down on any weight that you can is beneficial so I ordered some 7075 aluminum screws to fill all the low stress holes, they were pretty crappy so I had to settle on an RC screws Stainless kit. The RC Screws are high quality and exceeded my expectations, down the track I may try some aluminum screws from a different supplier or bite the bullet and buy some titanium screws.

[juicedcoupe]

I'd probably find one of the aftermarket input shafts that allows you to use Associated slipper clutches. After that, I'd switch to a sealed diff case for easier tuning.

I'm not a fan of either of Traxxas's slipper clutch designs.

[RC10th]

I'd probably find one of the aftermarket input shafts that allows you to use Associated slipper clutches. After that, I'd switch to a sealed diff case for easier tuning.

I'm not a fan of either of Traxxas's slipper clutch designs.

Stay tuned

[1300GT]

I reckon you need some lightweight stub axles or it's going to be massively overweight.

[RC10th]

I reckon you need some lightweight stub axles or it's going to be massively overweight.

A little bit here and a little there adds up, I have a feeling the truck will be fairly light but I won't know until I get further along with it.


Since the top deck and shock tower weren't countersunk I went ahead and countersunk them like the box art truck.

I should have gone down this path to begin with but I put all my eggs in one basket and chose to stick to the stock style gearbox. I may play around to see how hard it would be to find an axle combo that works.

[RC10th]

A tale of two diffs.......

I couldn't find any of this information online when I was trying to research parts so maybe someone will find this info useful.

I bought a #2520 SRT Pro Diff and a #4620 Ball Diff since I started building two SRT trucks. Both diffs have their pro's and con's.

The #2520 SRT Pro Diff looks similar to an RC10 diff in a way. One of the included gears (not pictured) looks exactly like an RC10 diff gear, the rings look a similar size, the balls are 3/32 carbide and it uses two bearings internally. I haven’t tried to see if RC10 rings fit but they are pretty hard to come by now. I like the fact the outdrives are drilled for the output yoke screw pins so the yokes can't spin on the outdrives. The Pro Diff in my opinion is a little on the smallish side for a truck although I'm sure it would work fine, it would require more frequent rebuilds. I'd think this diff would be just as smooth as an AE diff and I'd probably use it if rebuild parts were easily available or found a bulk supply for rings/thrust bearings.

For interest sake the Pro Ball Diff weighs 24 grams

The #4620 diff looks a lot more robust. I like the fact that it has pinned diff rings so they won't slip, the diff ball circle is much larger which puts less stress on the diff, and the thrust bearing is way more robust. I don't really like the fact that it uses a Teflon bushing instead of a bearing and the outdrives aren't drilled for the yoke screw pins. It comes with steel 1/8 diff balls so I ordered some 1/8 carbide balls. For some reason I've never tried ceramic diff balls so next rebuild I might try a set.

Again, out of interest the #4620 weighs 27 grams. Three grams isn't really worth worrying about in my opinion.

[RC10th]

And here's a side by side comparison to show the size difference between the two diffs.

I never liked gear diffs but I was originally going to use a sealed gear diff. It weighs in at 57 grams, a whopping 30 grams heavier than the ball diff. I figured the packaging would be about the same weight as the oil

[TRX-1-3]

Regarding the different parts, I know the smaller gear, un-notched rings, etc. Those are TRX-1/3/TCP diff parts. The gear, rings, and balls are also compatible with the Stealth. I believe the header card for MIP rebuild kits from BITD state something similar to "for associated Stealth and traxxas...' I cannot recall if they share the same diff shaft. But it would make some sense if they included enough parts to build one or the other but not two complete.

I still have a ball diff from the original release of the "272 magnum" blue eagle upgrade, which carried over up to the SRT.

If you would find it helpful, I could take it apart and snap an "exploded" pic for you.

[RC10th]

Since Traxxas isn't my forte and I don't particularly like gear diffs I was trying to figure out what ball diffs fit the 727 and what the differences were between the two Traxxas diffs. I could find manuals online for the 4620 but not the 2520. I was even looking at the Thorp diff but figured rebuild parts would be too hard to find now. The MIP super diff was nowhere to be found either.

Even though the Traxxas 4620 diff probably isn't as good as the 2520 I chose to use it due to the easy parts availability and robustness for modern brushless.

Thank you for the offer. It would be interesting to compare if it's different

[RC10th]

Diff is done and doesn't feel too bad. I used Acer 1/8 carbide balls.

[RC10th]

We'll start by cutting off the gearbox ears since we won't be using the integrated mounts. The bottom of the gearbox has also been sanded flat so the gearbox mounts better to the chassis to reduce bind.

Then we'll put In some reasonably high end ceramic bearings as well as a home made titanium idler gear shaft. The titanium probably isn't any lighter than the stock shaft but it sounds much cooler. I actually wanted a carbon idler shaft but the carbon I ordered ended up being slightly undersized.

I bought one of each of these lightweight top shafts to try. I wasn't happy with the fitment of the AE slipper eliminator, the Traxxas one fits much better. I had to cut down the slipper eliminator to fit the short top shaft.

I decided to give all plastic gears a try, I know the POM gear is designed for the aluminum idler but I don't see why it wouldn't work with nylon gears.

And one finished gearbox. Super light and smooth, as JosephS made mention in another thread I can blow on the spur gear and the gearbox spins. Or if you have only one screw in the spur it finds the bottom all on its own.

[RC10th]

Stock gearbox

Stock spec gearbox with arm mounts to keep things fair

A total savings of 61 grams which is quite a lot. And the reduction in rotational mass should free up some horsepower.

[GoMachV]

Which gearbox is that? It doesn't look like any of the ones I have used before (original, drag slash, aftermarket, etc)

[RC10th]

It's only a Hot Racing gearbox but I forgot one detail above, I redrilled a B3/T3 motor plate since I couldn't find a Traxxas motor plate/guard.

I really wanted to use an RC10 motor plate but it just didn't fit well. I should have stuck to the Stealth idea but time is starting to get away from me to get the truck finished.

[RC10th]

Here's what the gearbox looks like installed in the truck, it's a pretty clean fit. I have a longer titanium camber link ball stud coming as I need to run a spacer to clear the shocks.

Ìt is kind of annoying but Lunsford shut up shop while I was accumulating parts for this truck, so unfortunately the opportunity to buy an SRT/Nitro Rustler style titanium hinge pin kit went out the window. I had to settle on a Slash kit which has different steering knuckle hinge pins. Even with the stainless outer pins I was surprised with the weight savings of 8 grams. The titanium ball studs must save a bit of weight too,