Since its first year of production in 1999, Honda’s TRX400EX has a reputation for being one of the most durable and fun sport ATVs of all time. The design of the TRX hasn’t changed too much in its lifetime because it never needed to. The air-cooled four-stroke engine is practically bulletproof, the lack of liquid cooling greatly reduces complexity, and you can adjust the valves with hand tools. There are lots of riders who love the “keep it simple” philosophy. The chassis is likewise basic, but it handles well and has plenty of potential. While companies like Yamaha switched to 450-class sport ATVs, Honda decided to keep the TRX400 while adding its own 450 sport model. So why not build up the TRX450R instead of the less-powerful 400? It’s simple; there are far more Honda 400s being ridden today than TRX450Rs; it is still a great machine that is less expensive to buy, and it can easily be modified to improve handling and go faster without a big hit to your wallet.
We picked out high-quality performance parts that are affordable for the weekend warrior, whether they are looking for more sport performance for trail riding or to dig tires into the local racing scene.
Our platform started as a stock Honda TRX400X. Aside from cosmetic changes, the X is still very similar to the older EX versions, so most of these upgrades are transferable to all years of the 400. For starters, the Honda is known for how torquey the engine is, but more power can always be pulled out of it. We bolted on FMF’s Powercore 4 slip-on muffler that increases power through the whole range without touching the intake and carburetor, but we wanted more! K&N makes a stock-replacement high-flow air filter and air box lid that are reusable and easy to clean and maintain. After installing their system, we noticed that the 400’s mid- to top end range power was weak. K&N’s system gives the engine more air, which means it needs a richer fuel mixture. Adding a bigger main jet and shimming the stock needle or replacing it with a new adjustable one was needed to add more fuel. Dynojet sells jet kits that have the ingredients to solve the air-to-fuel mixture issue and boost the mid- and top-end power. With the pipe, intake and carburetor settings, the TRX has more grunt at low rpm—always a strong area for these engines—and pulls harder through the rest of the power range. Adding the power up top and allowing the engine to rev happily was vital, since that is where the stock 400 is the softest.
The stock front suspension on the TRX400X has only one adjustment, which is the spring preload, and with an increase in power, the stock shocks weren’t cutting it. We wanted to keep the price point low on this build yet still increase its handling characteristics. We used a set of Honda TRX450R front shocks that can be found easily on Craigslist or eBay. The rebuildable 450R shocks have adjustable preload, as well as external rebound and compression adjustments. All we had to do was shave off a little bit of metal from the upper mount on the shock to fit in the 400’s stock mounts. The 450R reservoir suspension felt a bit smoother but had to be run on the softest settings.
The Honda TRX400X comes with a 45.5-inch width, which makes it nice for tight trail riding. However, we aimed to ride the Honda faster and needed more stability. If you don’t ride tight trails too often, you can widen the front track. We decided that Team Alba Racing’s wide A-arm kit was the right addition to help obtain our handling goal. It adds 2-plus inches of width over the stock A-arms. Alba’s lower arms are designed with a stock-length shock adapter. That can be removed if you ever install long-travel shocks on your machine.
Once we installed Alba’s A-arm kit and reinstalled the 450R shocks on the Honda, we noticed that the 450R shocks felt much softer, the action smoother and the change improved the handling. The added width of the A-arms adds more leverage on the shocks, so they feel softer. We tightened up the spring preload and adjusted the rebound and compression to settings that felt much more comfortable for harder riding. A re-valve of the TRX450R shocks could offer further improvements, and that is not an option with the stock shocks.
As for the back end of the TRX, we toned down the compression and rebound settings on the rear shock. In stock form the reservoir rear shock handles bumps well, and a bit of adjustment helped smooth the ride out. To add more width to the rear end, we went with a set of Slasher wheel spacers that brought out the rear wheels three inches on each side. A wider aftermarket axle is a better bet if you plan to jump a lot, race or ride hard consistently. A stick-type steering stabilizer made by Labtek was the last crucial piece we installed to help with rough riding.
There are many elements that can be upgraded to increase the feel, control and handling of an ATV. We started with the basics. Torq 1 Racing makes stock replacement clutch and brake levers that have a more comfortable bend and are painted with a black coating that helps your fingers grip the levers. We also picked up a set of their Lock-On grips that are a lot more comfortable than the stock set, don’t twist on the bars and are easy to install.
The stock handlebar felt too short and narrow, so we grabbed a Renthal Fat Bar and a bar clamp from Trail Tech. Together that raised the bars up to suit taller riders, are wider and have less sweep for a more aggressive standing riding position. Along with the Trail Tech bar clamp, we picked up a pair of their 70mm LED lights and a mount to connect the lights to the handlebar clamp. The installation is clean without taking up space on the handlebar. Trail Tech’s clamp repositions the stock ignition switch location as well as the neutral and reverse lights. Each LED light runs at 15 watts on low and 30 watts on high beam. We ride with them on during the day to be seen more easily, and when it gets dark the Trail Tech lighting system turned night into day. They are very bright and a great addition to use with the stock headlight. Next, we went hunting for new rims and tires.
DWT had what we needed for desert and trail riding/ racing. We picked up their XC V1 tires, installing a 21×7-10 set on the front and size 20×11-9 tires on the rear. The front tires are mounted on DWT’s Rok-Out rims that are designed to keep rocks and debris away from your brakes. On the rear we mounted the XC V1’s on its G2 Beadlock rim. Whether you want to race or just ride hard, beadlock rims will keep your tires secured to the rim. If you get a flat, you can make it slowly back to camp or the pits.
Protection and safety are important for every rider. We built the Honda into a go-fast quad that is ready to ride hard and even race on a budget. With that said, nerf bars and pegs were a must. Rath Racing has an affordable set of nerfs called the Pro Peg nerf bar. It comes without heel guards, so we kept the stock 400X’s set on with a bit of modification. Rath’s nerfs are tough and durable, and the footpegs they have provide great foot traction. We decided to spend a bit more to try out the Rath Monster peg. We were not disappointed. It adds more outward length to the peg that reaches to the edge of the nerf bar. In many rough riding situations we always had a solid, grippy platform to place our feet and didn’t have to worry about losing footing. We also installed a Rath front bumper onto the TRX.
There is nothing like the feeling of testing out a machine after installing aftermarket parts and feeling how well they all perform. Instantly, the Honda felt like a whole new machine. The Labtek stabilizer soaks up hits to the front end that would normally jerk the bar in your hands. The wider A-arms and rear axle made the machine feel much more stable and predictable. The 450R suspension with the new width helps the 400 corner harder and faster without feeling so tippy. The ride is smoother through rough terrain, it glides over whoops and soaks up hard landings smoothly. The DWT tires grip well in multiple types of terrain and bite in corners instead of pushing and rolling like the stockers did. The new handlebar setup left us feeling more in control of the 400, and the Rath pegs kept our feet planted where we wanted them. The power increase through the FMF exhaust and K&N filter system made the TRX400X feel more alive and ready to charge harder with increases through the whole power range. You spend money to make your quad more fun, and every dollar spent on this modified 400 totally boosted the fun factor.
We took a stock 400 along for comparison, and there were a few tight spots that gave the stock unit a short-lived edge. However, when the going is more open and rough, there was no contest. The wider chassis is simply more fun. You can relax and enjoy the ride without being so concerned with cambers, rough bits and sudden hits on one front wheel. All of the chassis mods work well together, and we couldn’t think of leaving one facet out. At one photo stop the stock 400 was easier to get into the air off a small rise. Of course, that was because it literally bounced into the air. The modded 400 soaked up the rise, so it stayed planted unless we significantly boosted the speed we hit the rise with. That is control, and control is safe and fun, which is a great combination.
Alba Racing: www.teamalbaracing.com,
Adjustable long-travel A-arm +2: $450
FMF: www.fmfracing.com, 310-900-4363
PowerCore 4 slip-on muffler: $319.99
DWT: www.dwtracing.com, 800-722-3746
Rims: Champion box set: $544.95
Front: 10-inch Rok-Out : $95.95/ea
Rear: G2 beadlock 8×8: $173.95/ea
Front: XC V1: $95.95/ea
Rear: XC V1: $109.95/ea
K&N: www.knfilters.com, 951-826-4000
Air filter: $60.99 (air filter wrap: $22.99)
Power lid/filter wrap included: $108.99
Labtek Off-Road: www.labtekoff-road
F16 ATV steering stabilizer: $165.95
Pro Armor: www.proarmor.com, 951-343-
ATV kill switch: $34.95
Rath: www.rathracing.com, 320-234-7223
Pro Peg nerf bar/Honda 400EX: $279.95
(add $25 Monster Peg)
Honda 400EX bumper: $124.95 (colors for
both: aluminum, flat black, gloss black,
Renthal: www.renthal.com, 877-736-8425
Fat bar: $89.95
Slasher: Contact your nearest MTA dealer
3-inch rear-wheel spacers: $100
2-plus-inch three-line kit w/ T-block: $89.99
Torc 1: www.torc1racing.com, 661-505-
Lock-On grips (blue/black/white/pink/red):
Adjustable front brake lever (black):
ATV clutch lever (black): $9.99
Trail Tech: www.trailtech.net, 360-687-
70mm LED lights: 2500 lumens each.
Wiring included w/ mounts: $319.95
Bar clamp/center clamp light mount sold