Converting the AMP MT to a Circuit ST (to race!)

i_r_beej

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After racing this weekend and discovering that entry in a second class is a fiddling US$5.00 I started wondering in which class I should race the Boost.

Hey! What about the Stadium Trucks? Then I remembered the AMP MT with its uuuuuugly body. I have a truck!

So I just ordered a Circuit body, rear shock tower, some AE T4 Ti turnbuckles, RPM rod ends, and ball studs. Now I need new electronics. I may borrow the Spektrum SR310 Rx from my Boost. I'm going to get a Hobbywing XR10 Justock ESC, but haven't settled on a motor. I'm thinking something less expensive like a "GoolRC" 13.5T sensored. I'm seeing those on Ebay for US$24 shipped. Any motor recommendations?

The next race is Feb 18th so I've got to get moving...

I'll post a few pics of the transformation.
 
Ti turnbuckles have arrived. The rest of the chassis material, and body from HH should get here in the next day or two (hopefully!). The power system likely some time next week. I'll be cutting it close since Feb 18 will be Race Day for the "AMP'd" Circuit.

Last night began removing the RTR electronics and cleaning up the chassis.

Here's the AMP MT as it is currently (with RPM steering bellcranks, Traxxas Big Bore shocks, Turnigy 1258TG servo):
IMG_7335-amp_before.jpg

Here's a look under the hood:
IMG_7336-amp-before-chassis.jpg
Traxxas Big Bore shocks in the front with stock ECX springs. I've found the Traxxas dual-stage springs to be a little too stiff for the AMP chassis. Like the Boost, this chassis design has a SERIOUS rear weight bias and I'm still struggling to get proper sag at the front end. The longer suspension arms have better leverage on the shocks so there IS a tiny bit of sag but tuning for the track will be that much more difficult.

Here's the chassis with the stock power system removed:
IMG_7337-amp-before-stripped.jpg

I removed the upper suspension links and began installing Hot Racing front spindle carriers, and serviced the stock transmission. I briefly considered just buying a set of metal gears for the transmission but in the interest of science I decided to see how long I could go before the stock gears go KABLOOEY on me.

I'll be running a shorty battery pack for a couple of reasons: remove weight from the rear and provide enough juice to the motor. I'm trying to figure out a way to attach a battery stop to the OEM battery strap. This is not a primary consideration due to the time constraint so I may just use a foam block.

Next up will be finishing up the chassis upgrades and suspension tweaks once those parts arrive. I'm waiting for the weather to get above 50-deg F so I can paint the body.
 
The second order of parts arrived from Horizon Hobby last night. Circuit body, RPM rod ends, Circuit shock towers, ECX ball studs, and a Dynamite SS fastener kit.

I set about installing all the links.
IMG_7339-am-new-parts-1.jpg
I'm having flashbacks to 1993 with all the anodized bits.

I eyeballed a rough initial camber adjustment and made links on each end match. When I upgraded the links on the Boost to Associated Ti turnbuckles, I purchased the B4 set and they were a perfect fit. For the AMP/Circuit I purchased the T4 set and the front and rear camber links are all the same length, but it looks like the rear turnbuckles could actually be a couple of millimeters longer for my comfort. I haven't performed a camber setup yet so this could change (for better or worse) but with my eyeball initial setting, the turnbuckles are engaged approximately half the bore depth on the RPM rod ends. Will this be strong enough? I may wind up purchasing the long-shank version RPM rod ends.

The AMP and Boost both use the same rear shock tower. The top of the rear shock mounts BEHIND the tower. The Circuit tower is positioned to the rear of the Boost/AMP model so the top of the shocks mount in FRONT. Interesting. The slight rear-ward tilt of the rear shocks on the Boost and AMP seemed to align them with the travel arc of the rear anti-squat angle of the rear swing arms (about 2-deg to my eye). The Circuit tower positions the rear shocks in a more nearly vertical plane. I'm interested to see if this has any impact on shock performance. (Probably won't be noticeable.)

As I've been removing stock Philips fasteners, I've been replacing them with SS hex-keyed fasteners from the Dynamite kit. The Philips fitting is TERRIBLE to work with. Hex keyed fittings are so much easier to work with.

On a side note, this is my second Dynamite brand SS fastener kit. I've come to really like the kit for its good selection of screw sizes, nuts, washers, etc. The quality is good and the price (US$20) is right. I've only had two screws have their hex fitting round out on me and that was because the threads were tight (a tiny bit of fluo grease fixed that) and the hex bit was worn (replacing it fixed any future troubles).

The power system from Hobbywing is supposed to arrive this weekend and temps are suppose to get to about 55 degrees (time to paint the body!) so that means I could be done with the truck, save for a set of racing tires.

I think the AMP/Circuit chassis will prove to be a much more suitable platform for actual racing. We'll see very soon!
 
Have you found the RPM are super tight on the ECX ball studs....like almost to tight? I put RPM on just as stearing arms and I found it was binding up the servo almost?
 
I've found that the RPM 4-40 rod ends do fit ECX ball studs snugly, but not so tight that they bind. They also free up really quickly as the black oxide coating on the ball studs rubs off. You can add a tiny smear of grease or oil to the ball stud prior to mounting the RPM cup to help "break in."

The ECX ball studs are the same diameter as AE.
 
Over the weekend the Hobbywing power system arrived and I stopped at a RC shop to pick up some tires and springs for tuning the suspension. Wow. I forgot how expensive tires, foams, and wheels are.

Oh well, that's why this is called a "hobby," right?

Since the weather was cooperating with sun, no rain, and temps above 50-deg F, i painted the body. While that dried, I glued up the tires and wheels and ran into a problem: JConcepts the Stadium Truck wheels are quite wide and their offset didn't allow the rim to clear the steering rod end. Uh oh. I installed a set of Traxxas 10mm hexes that I had on hand. No go. Fighting the rising sense of DOOOOOOOM I scoured my parts boxes for something to solve the problem. Eventually I settled on some washers (5mm interior hole) to slide on top of the hexes. It took two 1mm washers to make the wheel clear the steering rod end (1mm breathing room!). I may need to install another spacer after I take a few practice laps.

Once that drama was cleared I moved on to installing the electronics:
IMG_7349-amp-power-system.jpg
Hobbywing XR10 Justock ESC (no motor timing adjustment possible) and 13.5t fixed timing Justock motor. Hobbywing produces such nice stuff...

I planned placement of the ESC and Rx, figured out wire routing, and soldered the connections. Bound the Rx and Tx and performed a quick power-on test (wheels turn and motor goes).

IMG_7350-amp-electronics-in.jpg
Schumacher Stagger Rib tires in the front and Schumacher Mini Pin tires in the rear. I run Mini Pin tires on my TLR 22 3.0 on astroturf/carpet and they work really well. I've never used anything like the Stagger Ribs before and they look bonkers. I have no idea how they'll work on my local track's astro carpet. My guess is that they'll hook up like crazy. Or they'll push like a greased ice cube.

In the photo I've got Losi orange (2.9 rate) springs up front and stock ECX springs in the back. The next couple of days I'll be tuning the shocks with different Losi springs. The front shocks seem okay with 30 wt oil but the rear bounces so I'll be refilling with 37.5 wt and see how that goes. I also need to adjust the camber

I'm going to be using a shorty battery pack (HobbyKing's Turnigy NanoTech Ultimate 4600mAh) so I needed to create a stop to keep the battery from sliding backwards in the tray. So I drilled a hole through a LEGO 2x2 brick and screwed it to the ECX battery plate. Perfect!

Detail photos of the various tweaks to come...
 
I'm really enjoying this thread, and can't wait to see more and hear how you get on at the track!

I built an Amp MT for my son at the weekend and am now very tempted to get one for myself too, it's so much fun to drive even as stock, so no doubt this will be great!!

Have to ask, what front bumper is that? It's completely different to the one that came with my boy's MT and looks like it would do a good job of protecting the front of the body which the current bumper really doesn't!
 
It does indeed, shame they don't seem to be too easy to get hold of in the UK :(, well not without expensive shipping costs!
 
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That is indeed the T-Bone Racing "Thrasher" front bumper. I'm sorry that availability in the U.K. Is problematic because they are quite good bumpers and very tough.
 
I could order one direct on ebay but it's $17.75 (approx £15) + $9.99 postage (approx £8) plus there might be import duty on top which I'd need to look into. Considering I only paid £92 for the MT, it'll be getting on for a third of the price just for a bumper which I can't justify really.
 
Unfortunately the Associate T4 Ti turnbuckles are too long for use as front camber links (and not quite long enough for the rear camber links) and I can't switch with the steering links since those are the two short turnbuckles and just right for that use. So I had to cut off 2mm at each end. No problem but titanium really stinks like burnt matches when you cut it using a cut-off wheel on a Dremel tool!

I continued fine-tuning the Circuit chassis (it ain't an AMP anymore!). I've be chaninging the oil in the rear shocks next.

Here's a shot of the 10mm wide Traxxas hexes along with two 1mm washers on the front spindle. I've got maybe a little less than 1mm of clearance between the rim of the wheel and the steering rod end. Whew. And the spindle nut just fits... there are no threads sticking out of the end!
IMG_7354-hex-shims.jpg

I've added 30 grams of weight to the front. There's plenty of room under the steering bellcranks. I'm already getting a tiny bit of sag with the Losi 2.9 rate (orange) springs with no pre-load so the additional weight will help a little. I'm going to experiment with the softest Losi springs available, the 2.5 rate (red). 30wt oil in the front seems to do okay with the soft springs. Dropping from about 18" the rear of the car actually bounces back up off the surface while the front stays planted.
IMG_7356-front-weights.jpg

How do I keep the shorty pack up front? Add a stop to the battery plate. I have plenty of LEGO so one 2x2 brick won't be a great loss. :)
IMG_7355-lego-stop.jpg
The LEGO brick is the red thing. I've added an adhesive strip of foam to prevent rattles.

Two more days until race day!
 
It's looking really good, I'm excited to hear how it goes on the track!
 
The night before the Circuit's first race I was still struggling with the rear suspension. I suspect that it is indeed the AKA tire inserts that are too firm and they are causing a bounce before the suspension has a chance to properly work.

IMG_7358-rear-adj.jpg
In this photo, I'm using Losi orange (2.9 rate) springs. I switched to yellow (2.0 rate) and this reduced the "hop/bounce" of the rear when the car was dropped onto a bare concrete floor. Since it was getting late I decided that would have to do since I'd really need to spend time at the track to properly tune the suspension... this was all "theory and no practice." Literally.

Then I double-checked the toe and camber using some old wheels.
IMG_7357-alignment.jpg
2-deg negative camber front and rear with 2-deg toe in. A relatively neutral starting point.

Final prep and assembly. (Temporarily switched to Losi red springs in the front and orange in the back.) All components are in place and have checked out.

IMG_7360-final-equip.jpg

IMG_7361-final-glam.jpg

Now can I say that I have the beginnings of a "fleet" of race cars? The TLR 22 3.0 (an excellent buggy) and the ECX Circuit, ready as they'll ever be for race day. (The TLR 22 is such a boring car. Easy to drive, handles like a dream, nigh indestructable.)

IMG_7359-final-fleet.jpg


Unfortunately I didn't arrive at the track early enough to really experiment with suspension setup and really only had time to get a feel for how the truck would drive on the astro track. Initial impression? Yep- It's a rear-motor ECX car. Jumps just like my Boost does; nose up. :) However, the Circuit handles MUCH better than the Boost, primarily due to it's much wider track and slightly longer wheelbase.

The truck liked to push and was taking right turns quite a bit wider than left. After my 2nd heat I was able to tweak the endpoints and get that taken care of, but the truck was still pushing (understeering) a little in the turns. By the third qualifying heat I was getting a good feel for how the truck handled so was crashing into walls less. It didn't help that the track layout had changed since my last race two weeks ago and featured a couple of challenging sections: a single jump just before the entry of a hairpin turn to back straight and a super-tricky double-jump to tabletop to single-jump that gave even the Pro-Stock racers a tough time. And lining up the jump wrong would put you right off the track. We had to have extra marshals by this section and they stayed busy putting cars back on the track!

IMG_7362-race-pit.jpg

Racing two classes kept me busy making sure that I switched profiles on my Spektrum DX4C and move the transponder from car to car.

I discovered* that the Hobbywing 3650 motor has a relatively short shaft (My Reedy M2 did not have this issue when it was mounted in the Boost). Sliding the Robinson pinion onto the motor shaft as far as it could go before running into the motor mount plate only let the set screw engage with the very end of the motor shaft. This caused the pinion to sit askew on the end of the shaft. Not a race-stopper, but not something I want to continue running for too long. Looking for a quick fix in the track shop, I was recommended 175RC's "PolyPro" pinions. They look just like slot-car spur gears and feature an aluminum hub with plastic (polypropylene) teeth. These are VERY low profile and solved my problem while cutting pinion weight by 60% over the steel Robinson pinion. Durability is a question though, but asking around and I've only heard good things and they seem to be quite strong (they are recommended for 17.5 and 13.5 only, but some Mod racers running 10.5 and lower were using them with good results). I'll run the pinion (with a slightly looser slipper) until it self-destructs. I plan on taking the Robinson pinion to a bench grinder to remove some material from the tooth side of the gear.

*In Stadium Truck heat two the Circuit just stopped after landing on the tabletop. Wheels responded to the radio, just no driving. The marshal pointed out that the pinion had fallen off! (Since it was indoor I was racing with the gear cover off.) At the end of the race I retrieved the pinion from its resting place at the side of the tabletop. :(

After racing was done for the day I took third in 2WD Stock Buggy with the 22 and... last place with the Circuit. That's okay. The truck had just undergone a HUGE transformation and it handled significantly different compared to my TLR 22 3.0. Interestingly, the winner in the truck class was driving a Traxxas Slash with a beat up VW Beetle body! The other trucks were AE T5s. He was also a Mod Buggy racer and winning that class which just goes to prove that it's not the car, it's the driver!

I spent some time after the track opened up and got the Circuit's front suspension dialed in. Then I was able to take turns much faster and stay centered on the track. I still have to exercise more care when jumping as the truck likes to point its nose to the air (and the jumps all had very short transitions which had a tendency to kick the cars straight up).

Racing the Circuit was a blast and I look forward to the next race on March 4.

IMG_7363-circuit-track.jpg
 
Been waiting to hear how you got on, great to hear that it drove nicely, even if there were a few teething problems which I'm sure are only to be expected, as you say it's been quite the transformation! I look forward to hearing more about how it gets on over time and with more tweaking etc.
 
Hah! "Teething problems." That's great- the truck experienced those not only figuratively but also literally! :)
 
Glad you like the 'teething problems' it amused me as I wrote it.

Truck is looking great and good to hear that it handles pretty nicely. Wish I had a track like that near me!
 
Cool! 1--What track is it & where? 2--I thought you said the 175RC pinion solved the problem = did you use it? Why dd the pinion fall off?!?
Thx
 

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