Understanding Wheel Offsets (Compiled by QuadMX18, from data on ATVriders forum area)
There are 3 common front offsets for your 400ex. They are 2:3, 3:2, and 4:1. What these numbers mean is the measurment of the rims. The first number is the inside measure of the rim in inches and the second number is the out side measure in inches. The rim offset will not affect shock valving. It will not affect shock valving because the pivot points of the a arms are not changed and the leverage ratio stays the same. This means it will not bottom out any easier.
2:3 Offset - This offset is 2 inches inside and 3 inches out side. This will give the 400ex the most width out of the others. It adds stability but causes more push in the turns and much more bump steer plus it will put more stress on parts like tie rods, spindles, hub studs, bearings, ball joints, and puts some on the upper a arm. A basic definition of bump steer is anything that changes the direction of steering of a tire other than from the driver/riders input. This bump steer is caused by the extra leverage the rims have on the handlebars. Also the offsets change the steering axis inclination (SAI). Stock rims are designed to work with the spindles to give the proper SAI to rim relationship so when you change offsets you change the SAI. So i would not reccomend 2:3 offsets.
3:2 Offset - This offset is 3 inches inside and 2 inches out side. This is the most common offset and easiest to find. It adds one inch to a 400ex total. It eliminates almost all the bumpsteer from 2:3's but there is still a little. It is also very close to stock offset where the quad was designed to be. I would recomend this offset.
4:1 Offset - This offset is 4 inches inside and 1 inch out side. This is usually only used on 400ex's with +3 a-arms to stay under 50 inches. This is not that common unless you go with the hyper rims but you can find it if you look. This is also close to stock offset but you will lose an inch when running it. 4:1's corner better and handle all around better then other offsets because it gives the right steering axis inclination (SAI). Stock offset is about 4:1 and the spindles are designed to have the right SAI with that offset. What SAI means is that if you drew a line thru the center of the ball joints while looking at them from the front, it should hit the ground at the same place a line drawn straight down thru the center of the rim does. This is why the spindle is made with the top ball joint closer to the frame than the lower joint. If you change the offset of the wheels these two lines no longer intersect at the ground. The more the difference the more the leverage that the tires have against you (the handlebars) when any size bump is hit. This leads to more bump steer. Changing from a 4:1 to 3:2 rim adds one inch of leverage to the SAI. A 2:3 rim which many people use to widen the front adds two inches of leverage and more bump steer. Also a steering dampner is used to help this bump steer. A steering damper trys to compensate for the bump steer by making the leverage push against it. People with wider offset wheels will notice a bigger improvement from a steering damper but it is still a helpful modification to any quad. So a wider quad does not always handle and perform better then a narrower one. So a 4:1 wheel gets the SAI where it was designed to be with the spindle to give you better contol. This better control can easily overide the advantage of a wider front for cornering so wider is not always better. Hopefully i have helped some others. Thanks!