First Crawler- Vaterra Twin Hammers rebuild...

[steve_rolfeca]

I just picked up a used RTR Twin Hammers, and thought it would be fun to document my experiences as I turn it from a "shelf queen" into what I hope will be a reliable all-weather crawler/trail truck.

I originally wanted a TH, but after doing my market research, I was tending towards an Axial Deadbolt. In the end, a lucky find on Kijiji brought me back to the Twin Hammers, and landed me a thoroughly blinged-out truck and radio for less $$$ than just the kit version with tax.

My past R/C experience was limited to small scale, with a pair of Kyosho Mini Inferno's and a Losi Mini LST. The main thing those days taught me, was that more speed + more aluminum = more and more expensive breakage.

My 8 year old is just starting into R/C with a little Dromida monster truck, and this build is going to be about getting the most performance for the least cost, and being able to share the build with him.

Because of the budget restraints, there's not going to be any cost-is-no-object brushless madness, and I may do some flirting with off brand gear where I think I can get away with it. If it goes the way I want, this could be the $50 a month alternative to Harley's excellent $50 a week upgrade videos..

[steve_rolfeca]

So here's where I started out: I found a used TH that had been set up monster-truck style by the original owner. He is more of an on-road, gas and helicopter guy, plus he's involved with 1:1 stock cars, so this truck had done little more than crawl over the couch cushions a few times.

Although the setup he chose wasn't the direction I wanted to go, his experience in choosing good upgrades should be very useful as I work on my build. Here are the non-standard parts that I received because of his experiments with the identity he wanted for the truck:

Substituted for the RTR parts, OEM parts no longer available:
- Spectrum DX4C transmitter
- Tekin FX-R ESC with capacitor
- Tekin 45T Heavy Duty brushed motor
- Rear springs (one pair of v. soft black, too short for the Vaterra shocks, one pair very hard Red)
- Open spur gear area, original gear cover lost

Substituted for RTR, original parts also included:
- AMF Racing aluminum rear battery tray and lift kit
- AMF Racing aluminum body panel kit
- Vaterra body panel kit, painted with chrome paint, and decaled out as a police patrol vehicle
- Extended 12mm hexes, with barrel wheel nuts
- Hot Racing aluminum front bumper, with winch guide plate
- Hot Racing lower rear trailing arms
- Adjustable upper rear links, all steel and aluminum, including the rod ends)
- GCM front tower plate
- G-Made XD93 shocks
- 20 tooth RRP steel pinion

Extra bling:
- Red/Blue police light bar on roof, switch mounted in interior plate

[steve_rolfeca]

Here are some photos of the truck as received. Judging from my son's reaction when the owner lit up the Police lights, I'm probably not going to get a vote on how this project looks...

[steve_rolfeca]

The best laid plans of mice and men...

I found myself having to spend more than I had budgeted to get the project up and running, and then some health issues stole a bunch of time.

Here's a quick update:

My existing charger and NiMH batteries were way out of date and not suitable for 1/10 scale, so my first stop was at Hobby Hobby in Mississauga.

I loaded up with a 2000mah Onyx 2S lipo, with the intention of dropping it into my son's Dromida monster truck after using it for initial testing. Charging duties will fall to a Hitec X1+.

I also stocked up on a couple of grades of shock oil, Axial hexes, 500K diff fluid and a few other bits and pieces. Somehow, it all added up to just over $200...

So much for the budget!

[steve_rolfeca]

The car climbed pretty well in high-traction situations, but there were a number of issues.

This was my to-do list after the first few runs:

• Limited front up-travel. The G-made shocks bottomed out with the A-arms parallel to the ground!
• Stiffness and lack of static sag at both ends.
• Bump steer and poor geometry at the back- with the extended hexes off, the tops of the rear tires rubbed on the number plates at full flex.
• High C of G at the rear, caused by the big aluminum battery tray and jacked-up stance.
• Heavy rear weight bias.
• Between the open diff and lack of flex, it was very easy to lose traction when crawling.
• Tires were severely out of round.
• Lots of servo noise.
• Lots of gear noise, and unexpectedly high amount of static friction in the drive train.

I also had some drivetrain geometry issues to look into:

• The dogbone pins were half way out of their slots out front, because of the high ride height and too much droop.
• The rear diff was horizontal to the ground at rest, exposing the CV joint below the trailing arms, and creating a steep angle to the rear driveshaft.

[rcntncrawler]

The GCM Rear Axle Truss will fix the pinion/driveshaft angle at the rear diff, and should greatly reduce torque twist.

According to GCM Racing, the shock length (eye to eye) should be as follows:
Open: 85mm
closed: 60mm
Anything other than those measurements will lead to dogbones popping out of the outdrives.

[steve_rolfeca]

Yup, and the GMade shocks were both a little too long, and too limited in terms of travel.

In the end, I settled on a pair of the Axial aluminum shocks, which I found cheap on Amazon. As per GCM's instructions, I put a travel limiter under the shock piston, and another on top of the lower spring perch. Just used some of the spacers that come in the shock kit.

This gave me perfect length open and compressed, vastly improving the amount of front flex available over the GMade's. The dogbones now sit perfectly in the cups. GCM really know what they're talking about with the front shock plate, as long as you follow their instructions.

The only remaining issue at the front, was that the stock springs in the Axial shocks (red top, green bottom) were still too stiff, and the ratio between the stiffness of the top and bottom springs was too high. The tops were almost completely coil bound before the lowers began to give.

The suspension was still completely topped out at rest, even with the battery moved forward to sit on the GCM plate. Also, the 30 weight oil I used, wasn't slowing suspension movement enough against the stout springs.

I decided to order a set of the red bottom springs, as my first attempt at tuning the shocks.

[steve_rolfeca]

Here are some shots of the geometry issues I was dealing with out back:

[steve_rolfeca]

As you can see, the CV joint angle at the axle was pretty severe, and it wasn't very pretty up at the transmission, either.

I decided to put the rear suspension back to stock ride height. Since I had all the pieces, I decided to use them, installing the stock shock towers, etc.

I also re-installed the stock battery plate, since I didn't like the way the back of the cage flexed without it.

This improved the driveline angles somewhat, but the rear tires still rubbed on the number plates at full flex.

Thankfully, I had already ordered the rear truss from GCM, and it only took a few days to arrive. Mounting it, dropping the front mounting point for the upper links to the lower hole, and closing the upper links back to stock length, fixed all the remaining geometry issues.

The improved triangulation out back reduced the bump steer to the point that the tops of the tires never get anywhere near the number plates, and the rear flex was now a full tire height, even though the stiff front springs were preventing any help from the front end.

[steve_rolfeca]

One thing I particularly appreciate about the GCM truss, is the way the driveline angles are held within such a narrow range, through the full stroke of the rear suspension.

Full flex, otherwise topped out:

[steve_rolfeca]

Topped out at rest, both rear tires on the ground:

[steve_rolfeca]

Unfortunately, the battery plate and rear brace bottomed out on the GCM truss, reducing the rear suspension travel:

[steve_rolfeca]

I tried out a few ideas, eventually deciding to use the spare tire as an active part of the rear cage, suspending it from the braces by the now un-needed Vaterra battery strap.

Pushing it forward to shift the weight bias to the front, still restricted suspension travel, because the truss and upper links hit the tire and rim.


Instead, I decided to move the spare back as far as it would go. Flipping the rear brace that normally supports the spare, and hinging up the battery tray rear brace made this easier.

Because the truss went up into the middle of the spare wheel at full compression, the re-purposed battery strap acted as a "soft landing" point for the rear truss.

It also lowered the CofG, stiffened the rear cage, and made it look rather cool:

[steve_rolfeca]

Now that the truck was closer to stock ride height (but still topped out by the too-stiff springs at both ends), it was looking more like a race buggy, and less prone to flipping over on climbs.

Total parts cost for mods to date: $42 shipped for the GCM brace, and just over $30 for the front shocks.

Here's how it looked:

[steve_rolfeca]

Suspension travel at full compression (as though the truck was pancaked coming off a jump) was much better than in "monster truck mode". Rear flex was now more than 1 tire height, still with no tire rubbing:

[steve_rolfeca]

Driving in this config was better overall. The truck was more stable sidehill, and less prone to doing a backflip on steep climbs.

However with the reduced ground clearance and still over-stiff suspension, the open diff was more noticeable than before. The truck could still handle surprisingly difficult obstacles, but it often took multiple attempts, and I had to blast over it in "Wraith mode".

This is not what I had in mind. While I was waiting for the red springs for up front, I ordered some Vaterra yellow and silver rear springs, and started into the driveline.

First up, was 500K diff fluid and a re-lube out front.

This gave the diff a noticeable limited slip function, but still leaves the front end able to spin up a tire in a bind. It was interesting to realize how much diff action is influenced by tire size. On my sons little Dromida 1/18 truck, 100K fluid feels roughly as stiff at the wheel, as the 500K fluid does on the Twin Hammers. Of course, the tires on the Dromida are less than half the diameter of the ones on TH...

Anyway, 500K definitely helps with crawling, but the next time I place a parts order, I think I'll scoop up some 1M, to get a little more grab.

I was interested to find that the rear diff was not as well-lubed as the front, and the transmission was completely dry. Is this why people refer to RTR's as ARTR? This was disappointing, as the Dromida at least had grease in all of the expected places...

The heavy driveline friction and "cogging" sensation I felt at the wheels out of the box, seemed to be mostly due to the strong static torque of the magnets in the 45T Tekin Motor, because once I took the motor off, the driveline spun much more freely.

However, this proved to be something of an illusion, as once I had greased everything according to the specs in the Vaterra kit instructions and reassembled the driveline, the resistance from the motor was much less noticeable at the wheels. The truck is much easier to push by hand, now that it's lubed.

Notice to all noobs: Tear that RTR down and lube it properly before you do any bashing! Who knows how many stripped or prematurely worn gears might have been saved by a little inspection and lubrication.

Reminder to self- I need to check the diffs to see if they need to be shimmed, the next time I have them opened up...

[steve_rolfeca]

The next thing I tackled, was traction.

Up to this point, all my testing was indoors, with artificial terrain. The stock tires are no great shakes on hardwood or tile floors, but I also noticed that they weren't gripping all that well on transitions on concrete or brick.

They were also fiercely out of round.

One of the advantages of the Twin Hammers, is that it comes with beadlocks, so it was a breeze to pull the stock tires.

Sure enough, the stock foams were all bunched up inside the tires, creating the out-of-round condition.

Following advice I found on the forum, I tried hitting the foams with a hair dryer. It took a couple of attempts to determine that I needed high heat, and to hold the dryer quite close to the foam.

Still, it worked, and brought the foams back to their proper shape. I was a little more careful than a Taiwanese factory worker during re-installation, and sure enough, the tires are now almost perfectly round. This should noticeably improve high speed handling...

Before I re-iinstalled the tires, I also took the time to cut the tires, removing the small blocks from the inside and outside edges.

This didn't seem to make a noticeable difference on slippery surfaces like tile, but it certainly improved the side bite when climbing. Another useful free mod.

[jgrim]

The red short, red long Axial spring combination up front will give you a spring rate of about 1.54 lb/in. If that combination is still too stiff, another option is to try Losi Mini T springs. The Losi Mini T spring kit (LOSB1119) has rear silver springs with a spring rate of 0.83 LB/in. The Mini T springs are smaller in diameter than the Axial springs and will require the JeepinDoug Mini T Spring Seats.
You might also pose an inguiry in the Vaterra Twin Hammers thread. Lots of people are using the axial upright shocks and somebody is bound to have found the right combination by now. Jerry

[steve_rolfeca]

There's also a post in the TH forum that lists a set of direct-fit Losi springs in order of stiffness. Nice, tight incremental progression. I have it bookmarked.

The Reds came in the day I cut the tires, as did the stock-rate Vaterra Yellows for the back.

At least for now, the Axial red/red front combo and stock Vaterra rear setup seems to be working fine with 30-weight oil at both ends. Even with uncut tires, the extra flex up front gave me better grip when attacking large, irregularly-shaped obstacles. The truck now sags about 1/3 of the available travel up front, and a little less than that out back.

The snow is still clearing up here, so it will be a few days before I can do some top gear bashing outside, in order to check out the handling/jumping balance at speed. I don't know for sure how far I want to take the balance between crawling/trail running, and bashing.

That's handy, as I had my first noob incident, toasting the stock shift server while trying to figure out the endpoint adjustments on the Spektrum 4DXc radio...

[steve_rolfeca]

Yesterday, I decided to place my first Hobby King order. Two Turnigy 2200mah 30C compact LiPo's in 2C and 3C, plus some body pins and a pair of XT60 to T adapters. $56, with close to half of that being shipping. Getting LiPo's into Canada isn't cheap or easy these days...

Meanwhile, I made an abortive attempt to create a new shift linkage for the Hitec HS-5245 I picked up to replace the stock shift servo. Drop-in fit, but 24 spline. Pretesting showed that the Hitec had better centring and a lot more torque than the stock servo.

While testing, I also realized why the stock servo had crapped out. The previous owner had the Aux 1 and 2 channels on the 4DXc set for 100% travel. The shift servo was on Aux 1, and due to my noobness with fancy Radios, I had simply assumed that the shift switch was on channel 4 (Aux 2).

Between the amount of time that the original owner spent driving the truck with the servo blasting against the stops in both low and high gear, and the time I spent trying to adjust the endpoints on the Spektrum transmitter, the servo overheated badly enough to start glitching. After letting it cool, I found that it was still glitching. Not only that, but the glitches were strong enough to affect the receiver and/or ESC, causing the whole truck to twitch and jump.

Thankfully, removing the dead servo stopped the interference.

The pile of extra parts I received with the truck, didn't include a Hitec compatible adapter for the stock shift linkage. With all Canadian sources back-ordered on the shift linkage, plus a few American ones including Horizon, I decided to go with plan B, and try to build my own linkage.

I couldn't find the recommended RC10GT throttle linkage in stock anywhere, so I went with the Dynamite DYN2568 Pro Tune Throttle/Brake linkage.

The Dynamite offers three different links, and a nice variety of spacers, setscrew collars and a spring. Unfortunately the sockets on the ball joints are much too small to use with the TH.

I have some long setscrews in my parts bin, so I may try to cut and drill the stock TH link. This would allow me to turn one end of the link 90 degrees, so that I could use a regular ball and servo arm to connect the shift servo to the shift mechanism.

The Hitec came with a blue aluminum shift horn, and the length is correct for this application. A bit of work with some fine drill bits, and I just might be in business. If not, it's off to the LHS, to see what they have in stock...