EQUIPMENT
Let's face it... if you have flat field and a shovel, you're not going to have 10-foot high jumps. No one wants to spend that much time shoveling. If that IS the case, take advantage of every bump and dip you can find.
For this article, I'll assume you have access to something that can push or lift a fair amount of dirt... maybe a farm tractor with a front-end loader, a skid steer, or even a quad with a snow clearing blade (just use it to lightly scrape the surface and push the results into a pile). If you have full-fledged excavating equipment, more power to ya!
MATERIAL
As I mentioned in the earlier article, you're going to need dirt to build a motocross track – probably a lot of it.
If you have a big enough piece of property, with the right soil conditions, you can just excavate all your dirt from one corner, or remove it from the side of a hill. Otherwise you need to get a little creative.
Creative option #1: See if there's any clean fill nearby. Sometimes you get lucky and someone needs a place to dump dirt. Check with people digging basements, or contractors digging lots of basements, highway crews, and so on.
Creative option #2: Build a pond. Pick a spot off to the side or in the middle of your track and excavate a hole with sloped sides. Use the excavated dirt to build your obstacles and the hole as a retention pond. The pond can also help you keep the track drained and dry, and provide a source of water for wetting and packing the track later. Bear in mind that once you start digging the hole, you'll need to finish the excavating before the next big rain, or have access to pumps to keep everything dry until you're done.
Creative option #3: Stack and bury. Find what you can to take the place of dirt. I've seen a lot of tracks that have rocks, logs, culverts and even scrapped vehicles under every jump. Just place the item(s) where you want the jump and cover with a layer of dirt. In this case, pay attention to the side of the jumps to make sure there's no sharp object or hard debris poking through. It's bad enough if you crash your dirt bike on a jump... worse if you smack yourself up on some jagged scrap metal or exposed concrete.
A variation of this technique is to dig a hole, fill it with your logs, or scrapped cars, and then use the excavated dirt on top. If the track is being built in a treed area, logs work really well. Cut them in six to eight foot lengths and lay them perpendicular to the track. Then stack the to get the rough height and shape you want. When you cover them with soil, they'll eventually rot and compost, turning to dirt as well (hey, an environmentally-friendly dirt bike track).
BUILDING OBSTACLES
Most of us want hills to climb or jump on our motocross track. Depending on the equipment you have access to, the building technique is going to change a little, but I think you'll get the picture after I describe a few of the things I've tried or seen done.
Let's assume you have something with a bucket. Grab your first load of dirt and head to where you want a jump. DON'T dump the dirt in the middle of the track – that's a good way to end up with terribly narrow jumps. Dump the dirt so that the middle of the pile ends up along the left or right edge of your track. On the second load of dirt, dump it so the middle pile ends up on opposite edge. If you're standing further back on the track, your two piles will have a vee between them, lining up with the centre of the track. No get a third bucket and dump it between the two piles.
Now, take a break and run your bike over the pile. You know you're dying to, so get it out of your system.
Okay, back to work. Add piles of dirt in front of, or behind the existing pile – always placing the left and right piles first, then the middle pile. Keep going until the base of the jump is as long as you want it to be. If you're building a table top, you might be done at this point. If you're going for a regular jump, you'll need to pack the first layer and then repeat the process, starting further back from the ends to get your overall rounded shape.
When it comes to packing you can either run the machinery you're using over the jump, or just have at it with your dirt bikes. If you're riding the jump into shape, you'll have to rake it regularly to fill in the ruts. (Note: you should rake out the ruts at the end of every riding day... that way, subsequent rain falls and sunny days will help pack the surface) The advantage to packing your jumps with your dirt bike is that you won't compress the entire jump as much (using less dirt). It's a good idea to seed some grass on the jump or let it weed up a little in this case. The roots will help hold everything together. You can cut it all down occasionally.
I'm purposely not going to get into specific heights for jumps, or how far apart they should be spaced – too much depends on the size of dirt bike you'll be using and how long you've been riding. If you've been riding for awhile, you'll figure it out as you go along... if not, try and find a riding friends to help you out, or pay attention to jumps you ride elsewhere. If you have to measure them or pace them out and transfer dimensions to your track.
I'm also not going to cover shaping the face of the jumps – it's just too hard for me to explain. If I get some time, I'll try and get some diagrams up. Again, pay attention to jumps you're already riding. Most of the time you'll see the face is not a flat, straight ramp, but has a bit of a curve (or belly) to help get you some air when you crest the lip.
One more tip for jumps. If you have some young or beginner motocross riders as well as more experienced dirt bike fans, you can taper off one side your jump at about a 30-degree angle. This gives you a low short jump on the low end, a full height jump on the other, and something for everyone in between.
My best advice for curves and turns is to start riding them, and then add dirt as you go along. They tend to flow better that way. If you want to build up a berm quickly, you can use old tires along the back side (most auto shops will give them to you for free since they have to pay to have them hauled away). If you have any hills or embankments, you can carve some awesome turns into those.
If you have a small space and a tight track, placing a turn in front of and close to a jump forces riders to keep their speed down, so smaller jumps work out great.
