Short course racing has become huge in a flash. With the new cars comes challenges and tuning techniques that some aren't familiar with, and sometime not even the race teams that put them out. Tires are probably the most important tuning option out there, and with many SC trucks coming with hard low traction tires, many of the theories and ideas of tuning change a little. But eliminating the tires from the equations still leaves one new tuning option in 2WD electric - gear diffs. Once thought of as cheap RTR parts, gear diffs have proven themselves in nitro, and with the amount of power we're putting through these cars, they've also shown their value in electric 1/10th-scale. They can handle more power and be adjusted with fluid or grease to change handling characteristics. If you haven't started to adjust the fluid in your SC truck you might just want to start thinking about doing that now. The result is well worth your time and effort.

Words: Derek Buono

Issue 171 (March 2010)

We set up a track day with Erik Shauver, designer of the HPI Blitz at our local indoor track OC/RC Raceway in Huntington Beach, Calif. The track was recently changed and that provided lower grip levels than normal, which in this case would help us see the difference more. We equipped the Blitz with a Tekin Brushless setup with a 10.5 mod motor and Pro-Line Bow-Tie tires. This will give us some increased grip and the power to spotlight handling characteristics that could be overlooked in slower classes.

The Diffs
We had five diff options and each was installed and run to get an overall feel and handling characteristic. We didn't do lap times because the track grip levels aren't super consistent and with a new track the more we drove the better we got, but we did keep an eye on speed on the track. Fastest lap doesn't always mean better, but we're pretty confident with our SC racing that the changes are easily going to equate to faster more consistent laps.

Diff 1: HPI Z164 Heavy Duty "Green" Grease (about the viscosity of full-size automotive bearing grease, gears lightly greased)

Acceleration - 1 Stars
This was clearly the worst of the bunch on many levels. The truck was very darty under acceleration, and almost felt like it never wanted to go straight. Hitting timing jumps was also difficult, and required more rounded corners to maintain speed.

Corner Speed - 4 stars
As long as I was off throttle, I had lots of corner speed and could hold tight lines. But since I was always on the threshold of losing traction it was very inconsistent.

Stability - 0 Stars
This yielded the least stability in the test: going out of corners, braking, and even down the straight at full throttle felt like the chassis never settled; it was dancing all over.

Overall feel: 1 - Stars
Terrible, unless you're required by some spec rule, or have super high traction (maybe the track is made of glue). We can't recommend a time where you'd want just grease in the diff. It creates an inherent instability that might be fun for a few minutes in the street, but not so fun on the track.

Diff 2: 4,000 CST silicone oil (same as a stock Blitz RTR diff)

Acceleration - 1 Stars
Our test vehicle was an HPI Blitz and it was surprising that the light stock diff oil wasn't much better than the grease. The rear end was still dancing out of corners and the ability to clear tight timing jumps was still very low.

Corner Speed - 4 Stars
Like with the grease, if you coasted through the corner you could maintain a tight line, but at any second or sudden input the rear end instability was immediate.

Stability - 1.5 Stars
Slightly improved over grease, but still felt very skittish on the straight and could spin out if you just barely hit the gas.

Overall feel - 1 Stars
Still way too light and unstable. Corner speed is still pretty good off throttle, but with the instability everywhere else, was a waste.

Diff 3: 10,000 CST silicone oil

Acceleration - 2 Stars
This was getting closer to where the truck would start to accelerate out of corners harder and hit the jumps coming off a slightly tighter line. It was still not completely stable or comfortable but better than the previous two tests.

Corner Speed - 4 Stars
You had to start to use the throttle in the corners to maintain speed, which is a good thing since now it was starting to feel like it wanted to have power.

Stability - 2 Stars
This was the first diff that we felt added some stability. Down the straight the rear end wasn't dancing back and forth and you could use the brakes and not spin out on a dime in some corners. There was also a noticeable increase in out-of-the-corner punch that allowed the truck to clear jumps using tighter lines.

Overall feel - 3 Stars
Still not rock solid in stability here, but there was a noticeable difference in stability and acceleration that made you feel more confident. This did require that you start using a steady smooth throttle in the sweeper to go faster, but that's much easier to do than holding on to car on the verge of spinning out all the time.

Diff 4: 30,000 CST silicone oil

Acceleration - 5 stars
It was pretty clear from the first lap that we'd reached a spot where the Blitz felt right. You could now punch the throttle just about anywhere, but still overpower the rear end. Hitting jumps was now easier as the truck launched forward.

Corner Speed - 4 Stars
This is an odd rating, because off power, the truck would be slower than an open diff and in some tight high-speed corners. But with the stability and natural tendency to slow down quicker, drivers just had to use throttle in the turn to get the speed back. The natural coasting speed comes back with some throttle, and you can maintain a higher average corner speed because you can use the brakes to slow the truck down or rotate it.

Stability - 5 Stars
This was the most stable feel of the test. Out of corners the truck squared up. If you left off the throttle or if you got the rear end out of shape, the truck squared up and you could just go full tilt down the straight and ease on the brakes with little drama.

Overall feel - 5 Stars
We've done lots of SC racing and our previous testing showed us that these trucks are way easier to drive with a heavier diff. The gains in traction from this weight were immediate and we feel that for 99.9 percent of drivers the confidence will make for faster lap times.

Diff 5: Prototype ball diff

Acceleration - 3.5 Stars
This had some better acceleration out of corners than the lighter oils, but after driving the heavy oil timing jumps showed decreased forward traction. It did track straight, but the lose in forward movement was pretty obvious.

Corner Speed - 5 Stars
This was a pretty good mix of both the lightweight oil and the heavy oil. You could use some throttle but it coasted a higher, tighter line than the thickest.

Stability - 4 Stars
Out of corners and down the straight the truck was pretty rock solid, only really bothered by abrupt use of the brakes. You could get the rear end to get out of shape pretty quick, but overall it was still pretty good.

Overall feel - 3.5 Stars
Back in the day ball diffs were king. We were low in power and weight meant everything. The prototype ball diff we tested is considerably lighter than the seal diff options but at higher-power motor levels that means nothing. This was shown by the heavier, thicker oil diff accelerating better and harder than the lighter ball diff. If you're racing stock motor you probably can't use this diff anyway, plus the maintenance is going to be tenfold higher, which to us is a negative.

Contrary to popular thought, the ball diff is not the best option. While it did show us that it is a good compromise, the fact that we preferred the heavier (mass) and thicker oil gear diffs is proof that times have changed. Most of us have so much power and low enough grip that having more mass might actually help in feel. We were a little surprised that our steering didn't fade out as much as we thought, but since the truck started with a severe over-steer tendency that reduction brought it into the sweet spot. You will lose some turn-in and response as you go heavier, so you will have to figure out the balance you need to go fast. If you're having trouble with the rear end on your SC truck, going to thicker oil will solve most of that issue. It won't teach you to drive or throttle control but it will help remove some of the natural handling characteristics of a high power 2WD car with low traction, A.K.A. spinning out on power.

Where to Start
We'd love to generalize and say run 30,000 wt. in every SC truck, but that would be dumb. Differences in tolerances and diff volume will make the "sweet spot" different for every truck (even though our Slash setup suggests 30 - 50K oil) and can vary with track conditions. Higher traction will allow you to run some lighter oils to get some steering back, and if you have some terrible setup to start, that can make things worse in every change you make. Our suggestion is to start to go thicker with oil and get a feel for the handling. Our testing has shown us time and time again having a more stable truck makes us faster.