HPI Rock King - Rock Crawler Conversion
Phase 2 No Modification Wheelie King Crawler Conversion
By Tony Arnold – StampedeProject.com
RC Car - February 2008
I have received some comments that the article published in RC Car lacked some detail, it turns out RC Car didn't want to run a ten page article so I had to cut it down...a lot.
See the Full Un-edited Wheelie King Build here.
RCGuy.com Servo Relocation Plate and diff lockers
The overkill FXR-Pro Tekin ESC.
The lightweight A123 packs reduce CG
Top link is extended with un-used lower link brace via a little trimming and a long grub screw]
RPM's narrow crawler wheels vs regular width wheels.
Rock King Crawler Project
Phase 2 Wheelie King Crawler Conversion
By Tony Arnold – StampedeProject.comLast month we explained some rock crawling basics and even completed a Phase 1 Wheelie King crawler conversion that put us out on the rocks in a hurry. Facing the facts, the stock Wheelie King’s wheelbase and high rearward CG (center of gravity) isn’t the best for a crawler, we need a longer & wider chassis, a different CG, more power, more articulation and better traction like real rock crawlers. Since last month, I have spent my time morphing our Phase 1 HPI Wheelie King into a sloth-like competition level rock crawler through a lot of trail and error, but it wasn’t easy. Knowing I would cook up some custom fabricated crawler, Stephen made things rough by mandating a zero fabrication build that anyone could complete – OK, so not a bad idea. Though minor Dremel tool modifications were allowed, I could not so much as drill a hole in the chassis.
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Previously Completed Phase 1 Conversion
Remove the rear sway bar
Reverse the tread pattern on the wheels for better traction
Remove the wheelie bar
Installed RCGUY.com diff lockers
Kimbrough 66T 32P Spur for Associated trucks #249
Robinson Racing 9T 32P pinion #0090.
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Build Research
Attempting to consolidate down all the great crawler ideas into one easy build took some time. I canvassed forums and looked at hundreds of crawler-winning designs, played around with a plethora of shock placements, shock-tuning changes, and nominal weight & CG changes on the Wheelie King. My efforts identified a combination that drastically increased articulation by nearly double of other standard conversions and delivered competition level crawling capabilities. As you can see from the scrapes on my crawler, this process took a lot of trial and error. Without cutting, drilling, or fabricating, the result was an innovative, durable, and easy build that transformed a Wheelie King into a competitive Rock King with almost 90 degrees of un-forced competition level articulation with mind twisting CG that was oh-so-fun to drive.
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ROCK KING - PHASE 2 UPGRADES
REMOVED:
Rear sway bar (Phase 1)
Wheelie bar (Phase 1)
Servo
Servo Mount
All the steering linkage except the front steering carrier links.UPGRADED:
Stock 27T motor upgraded to Axiom 85T V10 Crawler Motor
Stock ESC upgraded to Tekin FX-R-Pro
Stock NiCd battery to a slim A123 Racing 6.6V 2200Mh Lithium-Ion pack
Deans Ultra-Plugs on all connections
Servo upgraded to a high torque servo
Shock oil to 1000wt Silicon diff fluid
Locked differentials with RCGUY.com dif lockers (Phase 1)
Kimbrough 66T 32P Spur for Associated trucks (Phase 1)
Robinson Racing 9T 32P pinion (Phase 1)
Drive shafts to E-Maxx replacements with about 3/8” trimmed all ends
Wheels and tires to RPM Crawler wheels and Pro-Line Moabs
CUSTOM:
Dremeled top battery tray flat
Axiom motor end bell was rotated 180 degrees to reverse the motor
Repositioned receiver to the rear of the battery tray
Mounted Tekin ESC between reciver and motor
Moved the rear steering lockout to the front
Rotated suspension stays 90 degrees
Attached new extended top and bottom links
½” fuel tubing added as soft limiters on all shocks
Rear shocks left sprung and attached as shown
Front shocks inverted & installed springless for a drop front suspension
Trimmed rear left carrier to accept a RCGUY.com servo relocation plate
Stared the rear tire foams and V cut the front tire foams
Added ½ cup airsoft BBs in each front tire
Screwed antenna mount to shock mount for an under body antenna
Body was trimmed for articulation clearance
Body side panels were bent inward for snag free crawling
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Leaks & Tweaks
A picture is worth a thousand words and the inserted pictures hopefully do a better job explaining the build that I would, however one particular point deserves a little explaining. Looking at the pictures you might as “why did I only set up rear steering?” The answer is that in order to flip that painful rear biased CG, I made the front of the truck the back and the back the front to move the battery and motor weight over the front of our crawler. The truck is now running backward, however earlier I rotated the end bell of the motor 180 degrees so that the motor and transmission will now run in reverse – worked great. You may also notice one leaky shock received a Traxxas aluminum shock cap and new seals; the others were not leaking after swapping fluids so I left them alone however a rear shock spring kept popping out of its retainer.--------Call Out -------------------------
Phase 2 – Rock King Parts List
2 sets (4 total) Traxxas T-Maxx Turnbuckles 116-118mm #5139
1 set (2 total) Traxxas Half Shaft Drive Shafts #4951X OR HPI E-Savage Drive Shafts #82008
RCGUY.com WK diff lockers
1 RCGUY.com WK Servo relocation plate
Axiom 85T Crawler Motor
1000wt Silicon Diff Fluid
Tekin FXR-PRO ESC
Trinity Golden No Solder motor leads
Deans Ultra-Plug Connectors
A123 Racing 6.6V 2200Mh Lithium-Ion Pack
High Torque Steering Servo
RPM Narrow Crawler Wheels with ½” offsets
Pro Line Moab Tires
1 Cup Airsoft BBs
StampedeProject.com Decals
Fuel Tubing for shock limiting
12 – 3mm locknuts
4 – 3mm x 15 machine screws
8 – 3mm X 30 machine screws
24 or so – 3mm Flat washers
4+ - Shock limiters or pistons for spacers
2 – Traxxas grub screws #3962
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Final Testing & Conclusion
Granted the upgrade list isn’t short, however you may already have a lot of these parts if you really look through your parts bin. Although a ton of testing lead up to our final testing, it was all worth it. The short and narrow stance of a stock Wheelie King is definitely was way under USRCCA 2.2 Class 12.5” wheelbase and overall width limits and flipped on anything over 45 degrees. By stretching and widening that stance closer to the limits and the other upgrades, we now have a competition level crawler which can ascend some near vertical obstacles with a mind bending CG with almost 90 degrees of free articulation. The Rock King performed very well on the variety of rock piles I tackled and cleared all but the most challenging courses I set up in my rock strew and piled landscaping (no need for a road trip, fun can be had in my front yard.)Although I used widely available bolt on parts in the two conversions, a little homemade part fabrication will deliver a truly custom pro class rig with tons of tuning capability. There were a couple changes I would have liked to make such as swap out the flexible plastic steering links with metal, and add custom shock mounts, but those would have required fabrication. Want to see how to make an easy custom crawler chassis? Bug Stephen with an email and I’ll whip one up in a future article.
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Rock King & Crawler Build Tips
· Moving shock placement inward at the top softens the suspension and increases articulation.
· Heavier weight shock oil or even diff fluid in the shocks improves performance.
· Droop suspension allows suspension to droop and provide traction when high centered.
· Cut tire foams in an eight-pointed start pattern greatly increases traction.
· Cut front tire foams in a V pattern and add ½ cup airsoft BBs per tire improves traction and lowers CG.
· A 55T motor may be a little fast in the WK – the 85T Axiom worked great and allows for faster gearing.
· Adding a large spacer to each shock on the right side to combat torque lean.
· RCGuy.com diff lockers and servo mount plate make things easy.
· RPM’s crawler wheels & Pro-Line Moabs greatly improve traction--------------------------------------
The more experience I have with rock crawling the more I love it. Playing around with tons of different ideas really brought back feelings that I have not had since I re-entered RC. Watching something I created clear an impossible obstacle is a great feeling, a feeling I think is almost lost in today’s spectacular quality RTR RC world. Transforming a humble Wheelie King into a truly competitive Rock King crawler with amazing articulation… well, it was just cool. You really can’t explain what it is like to watch and recover from a perfect vertical balance or picking a line and watching your crawler perform mind-blowing maneuvers. Building a crawler is almost as fun as driving one and I had a great time engineering and completing this build. If you are looking for an inexpensive way to get into competition crawling, the HPI Wheelie King is becoming one of the hottest crawler build platforms and with the exception of the crawler motor and diff lockers. Now I know why, this was one of the most fun builds to date. See you on the rocks…
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Final Rock King Specs
Wheelbase: 12.4”
Overall Width: 12.25”
Weight Bias: Approx 75% Front
Motor: Axiom 85T V10
Gearing: 9/66 32P
Battery: A123 6.6V Lithium-Ion
ESC: Tekin FX-R-Pro
Wheels: RPM Crawler ½” Offset
Tires: Pro-Line Moabs-----------------------------------
Sources: HPI – www.HPIRacing.com, Axiom Motors – www.AxiomMotors.com, RC Guy –www.RCGUY.com, Tekin – www.TeamTekin.com, RPM Racing – www.RPMRCProducts.com, Traxxas - www.Traxxas.com, Pro Line – www.Pro-LineRacing.com, Deans –www.deansplugs.com, A123 Racing - www.A123Racing.com, Custom Decals -SupersportUSA.com
FINAL NOTES - I have received some comments that the article published in RC Car lacked some detail, it turns out RC Car didn't want to run a ten page article so I had to cut it down...a lot.
See the Full Un-edited Wheelie King Build here.
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