Racing!

Finally started to do some actual RC racing at my local track.  While I’ve driven on the track a lot and did some for-fun races, I never got into the serious end of the hobby.

Well, that’s all over now – I did my first official race running the Rookie class, which is basically a “run what you brung” class.  The only requirement being 2S batteries.  There were 10 racers, about half of which ran SC trucks, and half buggies.

I actually thought a fair bit about what car to run, and with the current layout of a fairly tough rhythm section, I knew I wanted to use a 2WD vehicle (see my center diff post to see why a 4x4 is hard to drive).  While I am probably a little bit quicker with the Associated B5, the TLR 22SCT 2.0 is more forgiving to drive, and more importantly, crashes better!  The Rookie races often devolve into bumper car sessions, and the full-bodied SC truck handles collisions and rollovers a lot better than the buggy, so I went with that.

I TQ’d the two qualifiers and led wire-to-wire in the race, which was nice.  I actually only set the 4th fastest lap in the race, but avoided any real crashes.  Like they say, to finish first you gotta first finish.

There are 4 more races in the series (with 1 throwout), so hopefully I can continue to do well and finish at or near the top at the end.

Center differentials explained

One of the most misunderstood tuning options for the Slash 4x4 is the center differential.  I’ve read a lot of posts on center differentials, and the majority say:  use a slipper clutch for bashing, and a center differential for racing.

I would agree with that statement, but why is that?  To understand why, you have to first understand what the two devices do and the differences between them.

Slipper Clutch

In a Slash 4x4, power is normally distributed equally front to rear, left to right.

50% of the power goes to the front, 50% to the rear.  And at each end there is another differential that allows the left and right wheels to turn at different rates.  When going straight, the left and right wheels will turn at the same speeds.  When turning, the outside wheel needs to turn faster than the inside wheel.  I think most people understand that.

What the slipper clutch does is allow the whole drivetrain (all the power in the system) to slip when necessary.  The tighter you make the slipper clutch, the harder it is to make the drivetrain slip.  The looser, the easier.  When does it slip?  When the motor is trying to turn at too much of a different rate than the wheels.  When you take those giant 20 foot jumps and land with the car going 20 mph and the motor pinned at 40mph, the slipper will slip instead of grenading your drivetrain.  This is a good thing and prevents parts from breaking.

You can also do some tuning for low traction conditions:  when you mash the throttle and the motor tries to turn at a ridiculous speed, a looser slipper will slip rather than instantaneously try to apply full power and spin the wheels.  This is often used in 2WD cars to provide a primitive form of traction control.

Center Differential

Now the center differential is more complicated, and nearly everything I read about it on Slash forums is wrong.  At its most basic, the center differential allows the front and rear wheels to rotate at different rates, much like the front/rear differentials allow the left and right wheels to rotate at different rates.

Why is this helpful?  This link has the best description I’ve seen, and I never see anyone explain this simple concept, even though it’s extremely important:

http://4x4abc.com/4WD101/abc4.html

Most people say “You need a center diff to handle better.  You need one for racing.  It drives better”.  Well OK, but why?  What does “handle better” even mean?  In what way does it “drive better”?

In a turn, the front wheels and rear wheels need to rotate at different speeds because each wheel is not following the same line in a turn!  The outside front wheel has to travel the furthest, and the inside rear wheel the shortest.  So to keep each wheel spinning at its correct rate, the front output shaft from the center diff needs to turn faster than the rear output shaft.

This is what the center diff does – it allows the front wheels to turn faster than the rears.  Or: in a turn, more power goes to the front and less to the rears.  If you lock down the center diff (super thick fluid, or use a locked down slipper), the front and rears have locked down output shafts and cannot spin at different rates.

A slipper clutch can’t produce the same effect – power is always equal front and rear, it only allows the whole system to slip, not individually front and rear.

This is why 4-wheel drive cars “handle better” with a center differential.  The wheels can follow their natural line more easily.  As a result, the car doesn’t push as much and will follow the line you’re pointing the front wheels at.

Center diff:  wheelie control

Another effect of a center differential occurs when only two wheels are on the ground.  With a tight slipper, you can easily wheelie with a powerful motor, because the front will lift (rear will squat).  The rear wheels will still get their normal % of power no matter what you do and will continue to drive the car forward (or upwards!).

With a center diff, as the front wheels rise and start to lose traction, it automatically starts transferring power to the end with less traction.  The rear wheels are stuck to the ground, but the fronts are in the air and able to spin more easily.  Since they can spin more easily, they do, and power will transfer to the wheelying front wheels and away from the rear.  This reduces overall motor power applied to the rear.  You can tune the anti-wheelying effect of the center diff by using thinner/thicker fluid. 

While big wheelies look cool, they usually aren’t the fastest way to accelerate.  By not wheelying, you may be able to accelerate faster!  Advanced race motorcycles usually have anti-wheelie control to accelerate the bike faster, and the same concept can be applied to RC cars.  It’s also easier to steer when the front wheels are on the ground…

Center diff: disadvantages

So we’ve talked about all of the great things about a center diff, what are the downsides?  Well, first, the device is much more complicated and prone to failure than a simple slipper clutch.  It’s also more work to maintain and tune and get right, since you have to remove it, drain it and refill fluid.  With a slipper, you just take a wrench, adjust it, and you’re on your way again in 10 seconds or less.

Another effect I found is that there are sometimes track features where you still want maximum power applied to the (rear) wheels when you only have 2 of the wheels on the ground (2 in the air).

At my local track, the current layout has a set of 3 small doubles that we call the rhythm section.  You basically have very small windows in which to jump the car, land it, and then immediately jump again.  And you have to do this 3 times in a row.

With the very small area in which to jump, land, and then jump again, it’s very difficult to land all 4 wheels at the same time perfectly.  If you don’t, however, when you hit the throttle to go off the next jump, power will be bled away from your (rear) drive wheels to the wheels in the air.  The overall effect is that you don’t get very good drive, and it’s difficult to generate enough momentum to make the next double.

With a slipper, this effect is negligible.  With a tight slipper, even if only the front wheels or the rear wheels land, you can still get very good forward drive (just like you can when wheelying) to accelerate the car and make the next jump.  I actually convinced my friend Alex to put a slipper clutch in his race Slash 4x4, just to make this section easier to do, and he found the results profoundly better.

In a 2WD car, the rhythm section is also fairly easy because you always get full power applied to the drive wheels to help you accelerate for the next double.  Even expert 4x4 short course drivers (using a center diff) have a difficult time making the series of jumps, while it’s fairly simple in a 2WD short course truck (my TLR 22 SCT, for example).

The final disadvantage of a center diff is that it does not protect the drivetrain as well as a slipper clutch.  When you land all 4 wheels at the wrong speed (relative to the motor RPM), there is nothing to slip and bleed power off.  The best it can do is bleed power off to a wheel that has less traction, but this isn’t always possible.  Overall the drivetrain will take more of a beating.

Conclusion

I hope you learned a few things about slipper clutches and center diffs and how they work.  It took me some serious thinking and research to figure out exactly why a center diff was better (and sometimes worse) for racing, and I hope I’ve shared that knowledge with you.

Slash 4x4: race upgrades

So you want to race a Slash.  Well, assuming you have a stock Slash 4x4, the most important upgrades are:

• Low Center Gravity (LCG) chassis
• Proper tires
• New shocks/springs
• Faster servo
• Center differential
• New body with vents
• Good ESC/sensored motor
• Good radio
• Rear swaybars

The LCG chassis is the most important, you can’t even use the traction sticky tires will provide, without it.  Tires are always key, and decent suspension is necessary to get the car to handle at least halfway acceptably.
The stock servo is super slow and the plastic gears will break.  A new servo will amaze you with how much faster you can control the car.  A center diff is necessary to get the car to be more agile.  For those of you new to short course trucks, the first thing you’ll notice is how much a stock body parachutes when jumping, so a vented body is pretty important as well just to get the car to fly OK.
Obviously a new motor will help with power and a decent sensored ESC will help with being precise with that power application.
After that, you start getting to tuning preferences and control preferences.  With some of these basic upgrades, your Slash will at least be tunable and have reasonable performance.  After that, it’s really up to you to set up the car to drive best for your local track.  The stock plastics are fairly tough and stiff, so you don’t really need to get aluminum this or that (wheel hexes are the exception IMO).
Until you get more experienced, getting the correct setup for your car will be more important than any other upgrades.

Slash 4x4: setup 9/15/15

Some people were asking what my current setup is for my local indoor clay track.  This setup is pretty darn close, it might need a little bit of tweaking, but it’s quite good IMO.

Enjoy, and if you have some feedback or suggestions, let me know!

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Slash 4x4: chassis brace

So one of the popular upgrades for the Slash 4x4 is to get a chassis brace.  People think they need one to add stiffness to the chassis (which is not actually a good thing when it comes to traction), or to make the car more durable  in crashes (debatable).

But the real reason to get a chassis brace is this:  on my LCG chassis, the rear spline gear does not line up perfectly straight with the center driveshaft, mainly because of the flex in the chassis.  In fact, because the front has a big heavy weight hanging off the front (the entire front assembly) and the rear has a big heavy weight (the entire rear assembly), the flexible plastic chassis tends to bow in the middle, and what happens is the drivetrain does not line up straight.  You can see it here in this picture:

The black rear spline coming out of the motor mount section is at a slight angle and points a little bit upwards with respect to the blue aluminum driveshaft.  This puts extra stress on the driveshaft, the spline, and most importantly the center motor bearing that is under the most amount of duress.

With my VG Racing center brace installed, the driveline is perfectly in-line, and will always stay in-line despite how the chassis flexes

Slash 4x4 hingepins

Got a new package in the mail to fix one of the annoying things about the Slash.  The outside hinge pins on the arms tend to unscrew themselves after 10 minutes of running, so I got some STRC captured pins to fix this problem.  Part number ST3640-RO.

They’re meant for the Slash 2WD rear arms, but those are the same on the Slash 4x4, so they fit.  Now, on my Slash 4x4, it’s only the left arm hingepins that unscrew themselves, and with some minor modification you can use them on the front arm in addition to the rear.

Drill a hole through the castor block, and it should bolt right on.

The package didn’t come with nuts, but they are standard 5.5mm locknuts
you should be able to pick up at your local hobby store or online.  I had some spares.

Don’t overtighten the nut down on the front, just enough to barely touch the castor block without causing any binding or additional pressure.  You want the block to rotate freely in the arm.

TLR 22 SCT 2.0 review

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It’s been a while, but I decided to start blogging my experiences with the RC cars I own.  I now own a Traxxas Slash 4x4 Platinum, an Associated RC10B5, a TLR 22 SCT 2.0, and a TLR 22T 2.0.

I’ve owned the SCT 2.0 for about 8 months now, and have experienced the highs and lows of owning one and think I have some useful information to share.  Since owning the B5, I decided to try one of the other major brands, Losi, and the SCT 2.0 had just come out and was considered one of the premiere 2WD short course trucks.  A discount at my local hobby shop and $280 later, the SCT 2.0 was mine.

The build: 8/10

The build went well, the instructions are fairly clear, but when building the truck I decided to go with the Chris Wheeler setup which meant doing some research on the changes I needed to make.  Referring to the stock setup sheet helps, as it’s a little unclear on the stock kickup angle, the stock toe-in, anti-squat, etc..  So it took me a while to build.  This being my second kit build, I think the total time took me about 13 hours from opening the box to a fully painted and running car.  What can I say, I’m slow and methodical.  I also built the car mid-motor which delayed my start for a while, because the mid-motor kit was out of stock for about 3 months.  I finally found one on eBay.

About the only part I did not like about the build is the TLR fascination with using a lot of different hex sizes.  You need a 0.50″, 1.3mm, 1.5mm, 2.0mm, and a 2.5mm hex driver.  The car is a little fiddly this way, and it’s not 100% metric – there are a couple of ballstuds and screws/nuts that are standard, which is rather annoying.

Performance:  6/10

In mid-motor configuration, you really need a few parts before the car is reasonable.  One is the mid-motor brass rear pivot block.  MM, this car has very little rear bite/forward traction, and the minimum you need to do to fix this is to add some rear weight.  Rear springs are also too stiff (they’re designed for rear motor).  I recommend the TLR 22 yellow SCT/T springs, which are a 2.0 lbs/in rate.  You also want to drop down to 27.5W rear oil to start with and tune from there.

With those changes, the car is at least acceptable driving.  It jumps VERY well, quite flat and predictable, it also carries pretty good corner speed with the MM placement.  It feels light, almost too light really.  But the weaknesses are absolutely no rear bite, and a tendency to swap ends/spin out under the slightest provocation of braking/turning with anything but perfect driving.

So the main issue with the car is that it cannot generate forward bite at all.  I highly recommend looking at some of the later setup sheets (Dunford/Casper), and you’ll see a trend towards adding rear weight, softening up the rear a lot and doing everything they can towards rear weight transfer under acceleration and reducing front weight transfer.

The other main issue I have is that the stock slipper and stock diff aren’t really up to the task of the amount of power and weight this car has (in mod form).  I run a mild 9.5T motor, and you have to snug the slipper almost all the way down, and you have to do the same to the diff spring to get the diff to not slip.  I actually switched to an AE slipper spring, because the stock spring is quite weak.

So performance-wise, as my friend Alex would say, “this is an evil-handling truck!”.

I will say that the body on this truck is awesome.  The Lexan is
super-thick, it has a nice low profile, and tons of cutouts you can make
to get it to flow air.  I liked it so much, I bought a 1.0 RTR body for
my Slash.

Upgrades/Support: 9/10

Losi’s in general have a strong aftermarket as well as first-party upgrades/option list.  You can do a LOT to this truck to tune it exactly to how you want it to run.  The Dunford/Casper mod is to put 22T rear arms on the back to widen the rear a bit, and I did go through that work and the car does handle better, especially under bad landings and mid-exit cornering in terms of roll and exiting grip.  I’ve spent a lot on TLR tuning parts (aluminum/brass parts for added weight front/rear) which is a pro and a con.  It’s great that you can get these parts to get the car to perform better, but it’s a con that you kind of need them to get the truck to handle decently.

Value: 5/10

Value-wise, out of the box as rear-motor the truck is solid.  You get an aluminum chassis, nice plastics, rear aluminum hexes, a quality body and wheels, all for $339.  It’s pretty comparable to most high end kits, maybe a little on the expensive side.  But the quality of the parts are good. 

You also have the option to go mid-motor with the purchase of a reasonably inexpensive kit.  But the reason I mark value only average is that you really need to invest in a fair amount of parts if you go mid-motor.  You need the kit, the brass rear pivot, the aluminum front hexes, some springs and the AE slipper spring just to get started, and then probably additional brass weights front and rear and maybe the Dunford/Casper rear mod.  All of which can add up to $100+.

Overall: 6/10

When it comes to high-end 2WD short course trucks, you really only have a few to choose from.  I don’t know anything about Durango trucks, but other than that you have the big 3:  TLR, Associated, and Kyosho.

With the release of the SC5M, it’s hard to recommend this truck over that.  From all accounts the SC5M is really the hot new car to beat, and if it drives anything like my B5, it’s probably the one to get.

The Losi is super-tunable, has high quality parts, but is a little fiddly with respect to the upgrades you need to get, and the slipper/diff issue is a bit of a problem too.  Some people are switching to the 22SCT RTR gear diff, and I will probably experiment with that when my ball diff needs a rebuild.  If you’re a Losi 22 fan or you need rear-motor, this truck is fine and you will enjoy it and this is probably the car to get, but if you’re not brand loyal and want mid-motor for high trip tracks, I would probably give the SC5M a try.

Build tips:

If you’re going to go mid-motor, I recommend the following:

• Mid-motor kit (duh)
• Associated RC10B5 slipper spring
• TLR 22 rear yellow springs for the SCT/T (they also make a buggy one that you don’t want).
• Super-sticky rear tires, fronts tires are good, but you definitely need grippy rear tires.
• Brass rear pivot hinge pin.  Probably the additional rear brass camber block as well.
• Use longer screws on the rear body mount that attaches to the rear waterfall.  The stock ones are too short and they will strip out of the waterfall.
• Use shorter screws on the bottom of the front of the transmission case.  I think stock is 20mm, but use 16mm.
• Cut out every freakin’ panel on the body.  With the skinny chassis, the parachute effect is tremendous on this car, and you need to flow as much air as possible.