After racing on the 400EX for almost 2 seasons, my 400EX was in desperate need of a teardown. While it was down, I decided to strengthen the frame & have it powdercoated.I have noticed that my frame seems to have developed an abormal amount of flex after almost 3 years of abuse now. I decided to analyze it and correct anything I could as well as fix a couple known weak spots. I did a lot of studying and testing and came up with what is probably overkill - but I kinda like it that way.


First, I knew that the front upper shock mounts needed to be reinforced - this area is known for cracking right above the cross support that the front shocks mount to - and mine have already cracked once.

What I did here was the typical triangular reinforcements that go from the cross tube to the round down tubes. This should help more evenly distribute the force of hard landings across a larger area of the downtubes.
I decided to take it a step further. I wanted to really strengthen this area, since this is where most of the hit from the shock absorbers is transmitted even when only absorbing small bumps. I felt my chassis was flexing in this area quite a bit & it was getting transmitted directly to the bars. I decided to put another cross tube above the factory one, and reinforce it with triangular, square tube gussets that will help even the load between both sides of the frame and hopefully lessen the torsional flex I believe was occurring in that area.
Next, I reinforced where the downtubes connect to the lower part of the frame near the control arms with another flat steel triangular gusset. I haven't had any trouble out of this area, but I have seen these two tubes break right at the base which can cause catastrophic frame failure.

After I was pretty happy with what I'd done to the front, it was time to analyze the rest of the frame a bit. It seemed that the rear, where the two upper tubes meet the downtubes that the swingarm attach were in need of strengthening. I verified this by tying the chassis down and applying pressure to the control arms that would simulate going thru a rough area on the trail or track. As I thought I would, I saw that these tubes were able to flex to the left and right rather easily.

To correct this, I decided to add square tube gussets from the bottom side of the upper tubes to the front side of the rear down tubes.

Next, I proceeded to fill in the welds on the factory gussets. There are four gussets that are only welded on the sides from the factory, the welds on the ends were filled in so that there would be no areas of the gussets that were physically unattached to the frame.

Be sure to fill in the holes in the factory gussets so that water and dirt don't get in with nowhere to go.. As you can see, I forgot :)
Another place to watch, but not gusset is the steering stem tower. There are flimsy factory gussets here, but if you try to beef them up, you run the risk of causing your upper frame tubes to crack. I had to grind out one such small crack and weld it back in. Many aftermarket chassis' are built in such a way that there is a down tube directly below the the gussets that will transfer any forces away from the upper tubes. I didn't feel comfortable doing this on my frame for fear of it preventing engine removal without disassembly.
After getting the frame to the point that I was fairly satisfied, I moved on to the subframe.
The factory subframe has a known weak point on the left side due to the way it is made to accomodate the battery setup.

What I did here was to place a gussett from the lower subframe mount point to where the bulk of the weight from the seat is transferred via rubber cushion. I also placed a triangular flat gussett just behind the battery box.

The thing to watch here is that your gussets don't interfere with the battery hardware on the left side of the chassis or the exhaust on the right side of the chassis.
On the other side, I had to do things a tad differently due to the pipe configuration. Luckily this side is known for being stronger so I just added a gusset from the same place on the upper tube, down to the lower tube at an angle - this should provide plenty of strength for the right hand side.
While working in the subframe area, I also found that one of the subframe mounts was cracked where it is welded to the main part of the frame. This area was cleaned and rewelded.