I find that when riding across loose ground such as dry mud or sandy patches with my mountain bike that the back wheel can sometimes slip and I have to stick a leg out.
What are the typical reasons for this? The bike is new, do the tyres need some working in perhaps?
Here is an in-depth article on mountain bike tires by Tom Stokes. From the section on tread patterns:
The tread pattern and knob locations can provide insight into how the tire will handle, but these are very difficult to read. Trying a tire out for a few days is the only way to truly know how a tire will handle, but there are a few guidelines you can use to judge a tire:
- Tread patterns with lengthwise-overlapping knobs will have a lower rolling resistance because they maintain a smoother contact between the road and the tire as it rolls
- Knobs with ramped leading edges in the center of the tire have a lower rolling resistance
- Widely spaced knobs will shed dirt and mud better at the expense of higher rolling resistance
- Knobs with channels cut through the center have more edges and therefore grip more, but wear faster than solid knobs
- Patterns with a large gap between the center knobs and side knobs can require more leaning in corners to get a solid bite but grip better under hard cornering
But, at the end of the day, as Rivendell Bicycle says in their "Learn About Bikes" knowledge-base -
You can buy tires for every kind of soil imaginable, but it's better to ride a moderate knobby and learn how much to trust it in a variety of conditions, than to ride a different tire for every type of soil, never learning any tire's capabilities really well, and then blaming a crash or washout on the tread pattern, or your tire choice for that day. It's your job to learn what the tire can't do, and then don't do it.
There's a few reasons you could have a rear wheel sliding around. I'll go through them briefly:
Traction: This sums up several different issues, but basically it means the rear tire is no longer gripping the surface you're riding on.
Tires: Moving a certain speed will cause the tire to break loose, no matter how much grip it has. The tire profile also has a strong impact on how it will react. Some tires have large knobs to assist with cornering, or small knobs to reduce rolling resistance. This goes for center tread as well, with some tires having an aggressive pattern to increase traction. Of course, smaller details like tire material can influence this as well (durometer of the rubber, mixed tread compounds, etc.).
Surface: You know a car can slide around in the snow? The same can happen to a bike and not just on ice/snow. Mud, water, leaves, and sand can all cause the same thing. Tire choice can help to reduce the effects of different surface qualities, but eventually you're going to have one surface that acts differently than another.
Weight: Push down on something enough and it becomes hard to move, even if the bottom is slippery. Similar theory can be applied to biking. This is most noticeable when climbing hills. As you shift your weight forward in the saddle (or further when standing) you take weight off the rear wheel which reduces the ability of the tire to remain in contact with the ground.
Spin-out: If your getting the occasional time when the rear wheel seems to just spin in place on the ground you might be putting down too much power and exceeding the grip of the tires. This can happen on loose climbs, flats, or elsewhere. Try lowering the gear to one you can spin in (but not too low).
After all that, there's also the practice of skidding/drifting by purposefully locking up the rear wheel and letting it break free. Sometimes it looks cool, sometimes it allows you to go faster around a corner, but doing it consistently requires you to know how your bike and tires will handle at different speed and varying terrain.
From what you describe I would suggest that your centre of gravity is too far forward rather than anthing to do with the tires.
Was your bike fitted when you purchased it?
Wheel slipping would occur if the COG was too far forward so I would try one of the following:
Apart from the first option which you can do easily at home the other two would have some cost attached and in order to get the right fit it is best to try some different options. You should be able to go to your LBS and get their advice on fitting and what changes you should make. They will probably let you try out different combinations of stem and seat before purchasing.
It sounds like you just need to get a feel for traction. Although bike geometry and tires will affect the traction threshold, the main thing is that you need to ride within the limits of your bike.
Keep in mind that managing traction on a bike isn't at all like a car. With a car, your body is a tiny part of the mass of the vehicle so doesn't play a major role in vehicle handling. On a bike, your body's mass is much higher than the bike itself, and your feet are directly driving the rear wheel, so your body's position, movements and your pedalling technique are all important.
A few factors affect traction on the rear wheel. Tires and riding surface play a role by increasing or decreasing the threshold at which you lose traction. The amount of your weight pushing down on the wheel also affects traction. This depends on your position on the bike and the slope. If you're going uphill out of the seat with your weight forward, for example, this unweights the rear wheel significantly. Acceleration and braking forces also play a role. If you brake hard with the rear wheel or accelerate hard, you transmit a lot of force through the tire to the ground. When accelerating in poor traction conditions, you want to make sure that your pedal stroke is as smooth as possible to avoid spikes in pedal force. Cornering also plays a role, producing lateral forces.
You really need to get a feel for how much traction your bike gives you on different surfaces: then you know how hard you can corner/accelerate/brake without losing grip. You can improve this with technique: for example by moving your weight back to increase weight on your rear wheel, or by easing up on braking or acceleration through tight corners.
