The Complete T-Maxx Performance Guide
Here we go - Part 2 of my Complete T-Maxx Performance Guide. As you have already noticed, my T-Maxx chassis set-up and layout is basically the same as the SportMaxx from my "Maxx-imum SportMaxx" articles. This, of course, is because they share the same chassis and most of their components with each other. The trucks still require different set-ups due to the different drivetrains (2WD vs. 4WD). The differences are mainly in the Part 1 - suspension tuning, and Part 3 - drivetrain set-up.
This installment, covering the chassis, will basically reiterate the same info as in the SportMaxx article. However, I have added a few things that I did not cover on the 2WD. I'll explain the reasons behind the main modifications here.
The first modification that I made to the chassis was the installation of the Traxxas 75cc fuel tank, part# 4448 . This is the same tank that is found on the N. 4-Tec and the N. Rustler models. The stock 125cc fuel tank works fine, and offers well over 10 minutes of run time. It's a great tank for bashing or racing on a sport level, but when it comes down to getting everything that you can out of your small-block truck for serious racing, I recommend using a lower profile and/or smaller capacity fuel tank for several reasons:
1. The TRX 2.5 Racing Engine, as powerful as it is, does not consume a large amount of fuel very quickly. Even in race conditions, you can expect over 6 to 7 minutes of run time from a 75cc tank ( with a properly tuned engine of course). The extra fuel and weight of a 125cc tank really isn't needed, since most racers pit around the 5-minute mark anyway.
2. Like I mentioned above, the tank is smaller, lighter, and lower to the chassis for improved weight transfer in the corners. This tank is very low profile and positioned out of harm's way, which means that it is very unlikely you'll damage the tank in the event you crash or roll over in a turn.
3. Due to the high profile, and large capacity of the stock 125cc tank, the mixture changes that the engine will go through from a full tank to an empty tank can sometimes cause tuning difficulty when trying to find that perfect needle setting for a race. With the stock tank, it is typical to have to tune your engine to be a little bit sluggish when the tank is full, so that it will not be too lean when it gets to the bottom of the tank. This will leave your truck under-powered for the first few laps of a race, and it is just not feasible to have to make an extra pit-stop just to re-tune the hi-speed needle. When racing, you want to be able to run an optimized needle setting for maximum performance for the duration of the entire tank. A smaller tank gives you that ability without carrying the extra weight.
5-Cell receiver pack
The next mod is the installation of a 5-cell NiMH flat receiver pack to power the electronics. A 5-cell/6volt-receiver pack is the only way to go when getting the most out of your radio equipment. Rechargeable 5-cell receiver packs offer more power to your servos to deliver quicker steering response and more powerful braking. For this project, I am using a 5-cell flat style pack. Check with your local hobby dealer for the correct receiver pack.
The receiver pack is mounted under the chassis for low CG, and is also strapped to the chassis with two Zip ties. The Zip ties are plenty strong enough to hold the pack securely without the need for mounting brackets or extra hardware. Notice the clearance between the bottom side of the battery pack and the lower edge of the transmission skid plate in the photo above (see arrow). The battery pack must not extend below the transmission skid plate, or the battery pack can become damaged when landing on top of another jump or truck. This is why I do not recommend hump style packs for this application.
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Mounting the TRX2.5 throttle bell crank onto the Traxxas 4922X chassis
There will need to be a few holes drilled into the Traxxas 4922X hop-up chassis to mount the throttle bell crank post for the slide carburetor on the TRX 2.5 engine. Use the template on the instructions at this link (throttle bell crank template PDF) to mark the locations for the holes that need to be drilled.
Once complete, the assembly should look like this on the bottom of the chassis.
Make sure that when the carburetor is in its set idle position, the linkage looks like this. Notice how the throttle servo horn and the throttle bell crank (red lines) are aligned with each other nearly perpendicular to the rods (green lines). Also, make sure that when the carburetor is at idle, the brake is not engaged; meaning that there is no pressure being applied by the brake arm to the brake calipers.
When the transmitter trigger is at neutral and the carburetor is at idle, there should be a small gap between the setscrew collar and the linkage eyelet on the servo horn. This will ensure that there is enough pressure against the throttle arm to keep the slide valve closed in the idle position.
Throttle bell crank bearings
Smooth out the action of the throttle linkage by installing two 5x8 ball bearings (Traxxas part #2728 ) into the throttle bell crank.
Replace brake tubing with a coil spring
I use this mod on all of my vehicles. A spring provides much more of a linear feel for your brakes vs. a piece of fuel tubing. With fuel tubing, there is a very fine line between a comfortable amount of progressive pressure against the brake arm, and instantly locking up the brakes. A coil spring delivers much more predictable pressure against the brake arm when entering a corner. This makes setting up for corners easier, and also more consistent.
I've tested different coil springs over the years from various mechanisms and gadgets. I have found that the coil spring from a servo saver works very well for this application. You can experiment with different lengths to get the desired progression and feel. The brake spring on my T-Maxx was pulled from a stock Traxxas T-Maxx/S-Maxx servo saver, and cut down to 14mm in length.
Trim the skid plates
You won't save a lot of weight by trimming your skid plates, but you may save them from snapping completely off. If the truck lands just right on its front or rear end, it can snap the skid plates at the bend. Shortening them will improve their durability while still maintaining the function that they were designed for.
Note that the front skid will need more kick-up than the rear. Trim the rear just past the bend. The front needs to be longer to prevent the front end from digging into the track when landing nose-down from a jump.
Direct mount steering servo
When removing the reverse system from the chassis, the servo mount that secures the reverse and steering servos is no longer needed. This area can be simplified by mounting the steering servo directly to the chassis using four 3x8 machine screws (w/ washers) and four 3.0mm lock nuts.
The Traxxas aluminum transmission skidplate (part #4947x) for the Maxx trucks is designed to strengthen the lower chassis braces and to protect the tranny case from damage. Once bolted on, this component is very instrumental in improving the lateral strength of the entire chassis.
Hi-Performance milled 7075 T-6 aluminum 4mm chassis (part# 4922X) – The 4922X chassis is machined out of 4mm thick 7075 T-6 aluminum, and milled out in specific areas to bring the weight down to 6.0oz. This is 1.4oz. lighter than the stock chassis.
Aluminum throttle servo mount (part #4919X) – The aluminum throttle servo mount adds rigidity, and eliminates flex under heavy braking. This allows the throttle servo do its job more effectively and consistently. You'll notice I didn't include the aluminum steering servo mount in this project. I chose to bypass the steering servo mount to save weight. You see, both the plastic and aluminum steering servo mounts double as a shift servo mount. Since I will not be using a reverse shift servo, I chose to just bolt the steering servo directly to the chassis with four 3x8 machine screws, backing each of them up with a 3.0mm locknut and washer.
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