What is a good method for constructing dirt jumps? Not massive jumps, just something more than a simple mound of dirt for a bit of fun. It's one thing to pile up the dirt, but there seems to be a bit of an art form to getting the surface smooth.
Jumps takes time and effort to do right. If you're going to rush or do a half-arse job then don't even bother starting.
Design is important - you need to consider run up space before and run out space afterwards. Make the jump wide enough to give options - a metre would be a fair width unless you're making it difficult on purpose.
Bailing-out options - riders will ride around if it looks too hard, you may make the jump really wide to reduce this, or make it longer to go around.
Try to avoid digging a moat around your jump. Source the dirt from metres further away, you want to avoid having water collect on your track, because wheels and water means erosion.
You're aiming for a smooth curve on the face, so build up your jump a few inches then stomp or compact it. Damp soil will compact better than dry.
Deeper inside the mound you can use big rocks for bulk, but still pack the soil around them. Trees and wood is not so good, it will rot. Tanalised timber is okay for surfacing.
As your jump rises, keep tamping it down. Use water to dampen it.
Remember it doesn't all have to be built in one sitting.
You can use shingle/stones to help reinforce the surface, but don't put sharp edges facing up. Big flat river stones are okay in moderation.
The top - I'm not so sure on this. You want to curve it back down quicker than you went up, but you must not have a lip or a sharp edge.... it won't last. Jury is still out on timber at the top.... if you were building a jump with a wall on the far side (ie you have to get off the ground) then timber supports will be required.
The danger of timber is wear over time may create a tyre-grabbing rut right before the edge, where it's too late to stop.
Finishing - use a watering can or hose with a shower spray to smooth off the surface. A flat spade or rake may be useful for smoothing, or you could even use a garden roller. Also you might want to use some matting or concrete pavers to minimise erosion and to assist traction.
Final thought - do get permission to do what you want to. Or do it on your own land. Don't go damaging trees or wrecking other things to make your jump.
Edit: I forgot to add maintenance. Do periodic work on your jumps, to fill erosion or ruts, or just generally clean up. Rubbish laying about attracts other rubbish so keep it pleasant.
It’s important to emphasise that the soil MUST be nicely damp all the way through if you want to build an even vaguely steep transition and a good lip at the top. Where I live in Zimbabwe I don’t even bother trying in the dry season. Sprinkling a little water on the top won’t do it − it must be damp the whole way through, but not slushy mud.
The dampness allows you to do two important things:
- build a steep slope that doesn’t just collapse
- compact it nicely so that you don’t sink into it when you ride, and it lasts over time, even once it has dried out. You want it to roll fast, in wet and dry conditions.
Obviously the type of soil you use will determine how well it holds together when wet, and how strong it is when it dries out. Clay soils are good, sand is bad.
Make sure you pack that lip tight. You should be able to stand on the top and jump up and down without damaging it. It’ll mean extra work to get enough soil up there, but it’ll be well worth it in the long run. If you skimp it’ll just collapse, and where I am you’d need to wait until the next rainy season to fix it.
It’s fine to use rocks and solid logs and things to bulk it up, but keep these well buried in the centre. When you’re doing your final shaping it’s often best to build it up a bit more than you would want, and then scrape or carve it away to get a nice smooth curve. If you find yourself unearthing your rocks you’ll be in trouble − you’ll have to pull the whole thing out and fill the hole again. Trying to just cover it over will leave nasty bumps in the surface.
Your tools are important. Use whatever is most convenient for moving your soil around, but you’ll probably find that when it comes to shaping nothing beats a nice, smooth, gently curved shovel with a long handle. Get the surface damp, throw on a thin layer of dryer soil and smack it into the surface with the back of the shovel, smoothing downwards in one motion. A square spade can also be handy for cutting nice clean edges, and don’t be afraid to use your hands to pack the soil where it needs to be steepest. You’ll soon find what works for you.