Steep downhills during mountain biking require you to consider the laws of physics. These principles are simply derived from Newtons Law, F=ma. Force equals mass times acceleration. For our purposes, the acceleration is a negative number, which is the same as deceleration. These principles are generally not considered for road bikes because it's not that steep, there's no objects, depressions or sudden changes in surface angle.
This article considers flip-over. Other types of crashing (like loss of traction) are not considered in this article.
First, when going on a steep downhill, your body's center of gravity moves closer toward the front wheel. If your center of gravity should ever go in front of the front hub, the bike will flip over.
Test the principle:
You can test this principle with nobody on the bike: while squeezing the front brake lever and holding the handle bar so the front wheel won't turn, lift the rear wheel until the bike just starts to flips over. When you are on the bike, the angle needed to flip is not as great, because your upper body will be partly over the hub, tending to flip the bike over much sooner. The angle of the bicycle causing flip-over corresponds to the slope when riding downhill.
When going downhill, keep your weight back by putting your butt back. Experienced riders even put their butt behind the seat on very steep slopes. If you don't have confidence, walk it. If there's enough runout, go for it. (Runout is a section where you can easily stop if you want to after a very steep or difficult section.)
Even when most of your weight is behind the front hub, you can flip if you decelerate too much. Decelerate means slow down. As the momentum tries to keep you going forward, a deceleration force will try to swing you in an arch, with the pivot being the front wheel-ground contact. This phenomena is due to the fact that the braking force is not in line with your center of gravity. The braking force occurs at the tire-ground contact. But your weight, your center of gravity, is much above this.
Keep your body weight lower when decelerating by bending your arms. Do this in addition to solution for the Center-of-Gravity. Avoid decelerating beyond the point where the rear wheel lifts up off the ground. On very steep slopes, do not decelerate except by a small amount, if at all.
Test the principle:
On horizontally flat pavement, see how much you can decelerate before the rear wheel lifts off the ground. You can try this in a parking lot, but be careful so you don't flip. Don't use the rear brake, it would just skid the tire. While riding slowing, gradually squeeze more on the front brake until you feel the rear wheel lifting. Then, immediately let go of the front brake lever. If you would be going downhill, much less deceleration is needed before you would flip.
Objects, like rocks, branches, etc., can flip the bike because hitting it causes a sudden deceleration. This effect is magnified when going downhill.
Most important: do not decelerate when hitting the object with the front wheel. That means don't put either your front brake or rear brake on when going across or on the object. For medium size objects, pull up on the front wheel as you cross the object, for example, a branch across the trail. When the back wheel would hit, you can do a little jump, thus lifting the back wheel.
Test the principle:
Practice first by going up driveways. Go slowly, pull up on the front wheel as it's about to go up the lip, then jump up from the pedals just before the rear wheel would hit. When you feel comfortable, you can try slowly going over a curb to get onto a sidewalk with a parking lot or other runout behind it.
Depressions such as a water (drainage) bar or water erosion across the trail or dirt road can also make you flip. When the front wheel goes into them the bike suddenly decelerates as the wheel starts coming out. If you were on a steep slope, it just makes matters worse.
Only practice will tell you which depressions you can ride through and which not. As with objects, don't decelerate when your front wheel is in the depression. Another method is bunny-hopping over a depression like a water-bar, and experience again is necessary to know which you can hop, if you have the practice. If you are on a steep downhill, then decelerate gradually way in advance, and walk across the depression.
In certain riding scenarios, you encounter a sudden change in slope, from steep downhill to horizontal or to uphill. This occurs especially on slick rock, but not on dirt roads. Depending on the severity of the slope change, it can make the bike flip. This is because of sudden deceleration. The bike itself does not decelerate overall, it changes direction. The change in motion in the direction you were originally moving does decelerate. For example, say you were going 25 degrees down then suddenly 20 degrees up. The change in slope is 45 degrees. The bike velocity in the original direction of travel suddenly becomes 0.7 of the original velocity, or a sudden 30% drop in velocity in a distance equal to the hub-to-hub distance. This is a huge deceleration. (Deceleration is the change in velocity divided by the time to make that change.)
Crossing the sudden change in slope at a shallow angle may make it ridable. If you start off being very steep downhill, then the best choice is gradually decelerate and walk across the section.
Test the principle:
Some residential curbs are the type where it gradually goes from street level to sidewalk level, and is the same at driveways. In these situations, when biking onto a driveway, going at an angle instead of head-on gives a smoother ride. If you are going fast and head-on, you could flip-over. Likewise, hitting a sudden transition at an angle instead of head-on is smoother and can prevent flip-over.
For super-steep downhill riding on a dirt road or slick rock (with no objects or depressions), where there is no run-out, you need to use a technique of jeepers (those using jeeps). Go very slowly, just enough to keep your balance, even momentarily stopping every few inches if need be. Keep your butt back, behind the seat if you can.
Should you crash, first make sure you are not seriously hurt. Wash off the scrapes and cuts using your water bottle. Then recreate the crash in your mind. Examine the ground. Analyze what occurred from the position of your bike. What caused the crash? How are you going to modify your riding style to prevent a similar situation from causing you to crash in the future?
Here's an example from two recent flip-overs I recently had in Montana. Both occurred on single track. The first occurred on horizontal trail with small rocks, I was going slowly, and all of a sudden: I'm down. It took a while to figure out what happened. There was one rock that was only about 3 inches wide, but about 6 inches tall with a vertical face. That was the cause. I assumed, in error, that my full suspension would go over all the nearby rocks (or maybe that the small embedded rock would give way), and was looking further ahead on the trail, instead of looking to avoid the nearby rocks. One factor causing a misreading of the trail was a change from sunlight to shade while wearing sunglasses.
The second flip-over occurred on a downhill as I was crossing a log used for erosion control. I had crossed dozens of the same size logs before, by pulling up the front wheel, so what happened this time?
Not seeing anything on the ground to cause the flip, I examined the brakes, and both had a problem. The front pads, while still lined up correctly, had a lip at the bottom of the pad that had formed from wear which made the brake stick. (Solution was to file off the lip or replace the pads). The rear brake was just not as strong as before. The problem was dirt had got in the cable housing. (This I solved by oiling the cable near the housing.) This shows how important it is to keep brake in good condition.
Another factor that caused the crash was that I wasn't as alert as I should have been, so I couldn't react as well to something unusual. I was 30 minutes into the downhill without a break. It's like playing a video action game: you get worse and worse after awhile due to mental fatigue. I should have stopped for a mental rest.
In summary, three factors contributed to this crash: the bike didn't decelerate as much as I expected (due to rear brake problem), which I didn't notice (due to mental fatigue), which resulted in not pulling up in time and the front brake being "on" at the log (due to brake sticking).
Keep on riding -Akos
About the Author: Akos Szoboszlay has been mountain biking since 1983.
Also see: list of bicycle articles.