Rock Crawling 101

Rock Crawling 101
Crawling Basics & Building a Very Basic Wheelie King Crawler
By Tony Arnold – StampedeProject.com
RC Car - January 2008

Rock crawling or “crawling” is that hot growing category of RC’s with mind-bending articulated bodies that test your driving skills to the max while traversing seemingly impossible obstacles – going slow has never been so much fun. Crawlers originated from home made rock crawlers with parts from here and a few parts even fabricated. Slowly a few chassis skeletons become available that allowed for easier builds, but you still needed to salvage axels and transmissions from other RCs. Today manufacturers such as Axial and other manufacturers are scrambling to deliver almost ready-to-run kits, however building your first crawler just become much easier by tweaking the new HPI 4x4 Wheelie King. Let’s review the rules, some crawler designs, and then complete a simple crawler conversion.

Why Rock Crawling?
Fred Medel, Marketing Manager for Tamiya USA answered this question best in my vendor pole - “In a world of ready-to-run out of box perfection and expensive groomed tracks, rock crawling allows people to go back to RC basics. You actually create something yourself and then go out and test it. You don’t need twenty people, it can be just you, the rocks, and your crawler.”

The Rules
Crawling is regulated by USRCCA rules (US Rock Crawlers Competition Association) which defines two classes, “2.2” and “Super Crawler”. Although you can view complete vehicle and scoring rules at USRCCA.com, the basics are that points are assigned for DNF (Did Not Finish), DNS (Did Not Start), reversing, touching, repositioning, rollovers, repairs, and hitting gate markers (usually tennis ball halves) on the course – low score wins with timing also being a factor. Sounds simple, right? Not quite, finessing your crawler to a quick low score finish is extremely challenging, especially with some of the crazy courses that are being set up that can even include tightropes.

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Class Requirement Highlights
2.2 Class
12.5” wheelbase X 12.5” total width max.
2.2” wheels and tires up to a maximum diameter of 6”
2-wheel front steering only
Realistic 1/10^th scale body
4-wheel drive

Super Crawler Class
No limits on wheelbase or width other than clearing gate widths of 20”
2 or 4 wheel steering allowed
Realistic 1/10^th scale body or larger required
4-wheel drive

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Design Flavors
There are a multitude of crawler designs simply because everyone tries to come up with a better design that is lighter, has more articulation & clearance, lower CG (Center of Gravity) …etc. Although a number of more advanced designs are available, the most prevalent chassis in both classes are based on tri-angulated four-link type suspensions with solid front axels.

PIC - HPI Wheelie King Axel

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[Caption – Wheelie Kings 2.2 Class type center driveshaft/transmission Chassis design]

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[Caption – Wheelie Kings center transmission axle assembly]

For reference purposes the chassis of the HPI Wheelie King is a good example of a fairly standard four link 2.2 class crawler design. It features a center-mounted motor and transmission powering front and rear axels via drive shafts and direct linked front steering. The Tamiya TLT1 and Axial’s new Scorpion chassis also carries forward this same design idea. The tuning problem to overcome with this design is the chassis torque twist that occurs from lightly suspended axels/differentials and a center mounted transmission design. Additionally, with a center mounted transmission, motors, and electrics the CG can be pretty high, however creative solutions have made this a competitive chassis design.

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[Caption – Tamiya Clodbuster Super Crawler Class axel units combine and axle, tranny, & motor mount]

The larger Super Crawler class chassis usually features stand-alone front/rear Tamiya Clodbuster style drive units (combo axle/tranny/motor mount) with motors at each end suspended with the same type of four link suspension and steering linkage. This design prevents torque twist from the motors, delivers a lower CG, and ultimately better traction for this larger 1/8^th scale chassis with monster truck sized wheels. Dual transmissions allow tuning for a slightly faster front transmission than the rear to improve overall crawling capabilities. 4-wheel steering allows these massive crawlers to turn on a dime. The downside to this design is the much higher overall build cost of dual motors, servos, and potentially ESCs. Since there is little a well-made Super class crawler will traverse, many people are going back to the 2.2 sized crawlers to increase the challenge.

These crawler styles mix and match quite a bit with plenty of single motor Super Crawlers and dual motor 2.2 crawlers. I have seen more than a few modified 4x4 monster trucks crawling and home-fabricated designs all doing well on rocks. One recent national top placing chassis was based on a graphite pan car, so anything goes for a design.

PIC – Axiom Crawler Motor

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[Caption – Axiom’s 85T crawler motor is a great option for going slow]

Locked In and Going Slow
In order to deliver the torque required for zero speed vertical approach angles and guaranteed traction, rock crawlers need locked differentials and very high torque, high turn, low RPM motors like those used for RC commutator lathes. Locking the differentials is easy by packing them with Silly Putty or 5-Minute Epoxy but that method can ultimately fail. Pros use drop in aluminum “diff lockers” like those for the Wheelie King from RCGUY.com. For the motors, super slow 55T-85T motors are used singly or wired in parallel (+ to +, - to -) in the dual motor chassis. Although many competitors use inexpensive lathe motors, Integy and Axiom offer high quality brushed motors that produce higher torque and smoother output. Novak’s brushless crawler system also offers a similar level of RPM, but much higher torque and longer runtimes thanks to the efficiency of brushless power. Slow spin motors coupled with a tiny pinion and huge spur gear equates top speeds of less than 10MPH for the best rock crawlers. All that power is now delivering a tractor like level of torque which will provide the control needed to inch over obstacles without upsetting the chassis and losing traction. Best of all, these low turn motors will run for a while even on those old 1500Mh packs you shoved to the back shelf.

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[Caption - Pull out the diff gears and drop in RCGuy.com differential lockers assure positive non-slip traction]

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Crawler Design Recommendations

4X4 Design
Front and rear solid fixed axels
Suspension isn’t needed only 60+ degrees of articulation
Locked differentials via JB Weld, Silly Putty, or Diff Lockers
Lots of clearance
Weight as low, wide, and forward as possible
Sticky Pro Line Moab tires have become the standard in both classes
High torque servo(s)
Narrow wheels for better traction
55T-85T lathe motors

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[PIC - Pro Line Moab Tires with narrowed and star cut foams are a standard upgrade]

Tires, Wheels, and Foams…oh my
Ask any rock crawler guy what the most important component in crawling second to the motor, and they will usually say the tires. Just like a dirt track, with the right tire and foam setup you are 85% there from a tuning perspective. Although there are hot debates over what tires are the best, Pro Line’s Moabs consistently deliver top placements. The Moab tires are ideal for rock crawling with overly tall, round narrow tube carcass (vs. flat), tread on the sidewall for added traction, and sticky M2 compound. Axial and Losi have both followed with similar tire designs. Foams are typically narrowed and then cut in a star or wedge/cone pattern to allow the tire to conform over rocks and gain more traction. Wheel weight can be added via stick on wheel weights inside the wheels or via about 8oz. of BB’s or plastic airsoft BBs per tire to further lower CG for these crawlers. Regular wheels will work, however narrower crawler specific wheels like RPM’s make the tire balloon up to increase side tire bite and overall traction. Beadlock wheels are also becoming popular for those that tune tire weights and foams often. A number of manufacturers offer bolt on “rock rings” that prevent a rock lodging itself inside a wheel and hanging you up.

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[Caption - RPM's new Crawler width wheels (left) compared to regular width wheels (right)]

Electrics
Rock crawling presents some interesting challenges on your electrical components. The more torque the better when it comes to servos for this slow moving big wheel’ed segment. Even though single or dual high turn 55T-85T motors are easily within the specs of any beginner level ESC, the loads presented to the ESC are going to be at or near motor stall speeds that adds extra stress on the ESC. A good quality high AMP low turn limit ESC with a full 6V 3Amp BEC output is recommended to deliver uninterrupted power to both motor(s) and servo(s). The new Tekin Rebel 2 ESC fits these requirements and has been a very popular economical choice lately at recent events.

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[Caption – The Wheelie King Flexes after a Phase 1 conversion]

High, Low, and Flexible
Although there are a few modified crawlers with independent suspension, a chassis with fixed axels and over 60 total degrees of articulation are recommended. Articulation allows one end of the vehicle to remain planted while the other end conforms to the impending object. Another set of chassis design goals which sound conflicting is to have very high clearance, but with a very low CG. Typically, the best option is to mount the RX, batteries, ESC, and servos just above the axels, however I have seen hundreds of creative ideas that provide lots of clearance with a low CG.

Let’s Build a Crawler
The easiest crawler build options are either building the Axial Scorpion crawler kit (electrics not included) or modifying an inexpensive HPI 4x4 Wheelie King. The easy Wheelie King Phase 1 crawler conversion can be accomplished in under one hour, for less than $20 in upgrades, and will transform a stock 4X4 Wheelie King into an inexpensive entry level crawler that will get you out on the rocks fast.

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Phase 1 – Wheelie King Crawler Conversion

1. Remove the rear sway bar
2. Reverse the tread pattern on the wheels for better traction
3. Remove the wheelie bar
4. Clean and pack the differentials with Silly Putty

OR install RCGUY.com diff lockers (an extra $25).

5. Install an HPI 96T Spur and 15T pinion.

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As you can see in the pictures, removing the wheelie bar and rear sway brace shows the clearance and articulation potential of the Wheelie King immediately. A simple gear change makes the speed and torque more usable for crawling and reversing the tires improves crawler traction. Since disassembling the axles is time consuming, I would install diff lockers to start with instead of taking the chance that the epoxy or silly putty didn’t hold. You are done. Hit the rocks or in my case the landscaping in my yard.

In next month’s article I will complete a Phase 2 build that will add a longer wheelbase, more power, shifted CG, improved traction and a few other upgrades. The Phase 2 build will take the Wheelie King to a whole new crawling level that can even get you through your first set of competitions.

Have Fun Going Slow
Where every RC hitting the market is getting faster and requiring either a $10,000 groomed track or a half-mile of abandoned road to realize it’s full potential, it’s refreshing to see a new category that allows big fun in a small space. Crawling allows us to have hours of fun on the nearest rock pile or just climbing around the landscaping in the yard. You will never look at a pile of rocks the same way again. Interested in crawling, Stephen has asked me to keep you updated on the new “go slow” crawling segment. Next month we build a Phase 2 Wheelie King Crawler StampedeProject style – See you on the rocks.

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Sources: HPI – www.HPIRacing.com, Axiom Motors – www.AxiomMotors.com, RC Guy –www.RCGUY.com, Tekin – www.TeamTekin.com, RPM Racing – www.RPMRCProducts.com, Pro Line –www.Pro-LineRacing.com, Custom Decals - www.SupersportUSA.com

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