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    2. Trials Driving Techniques: Throttle Control

    Trials Driving Techniques: Throttle Control

    In order to get traction there are really only two basic techniques, which we call “trickle” and “blast”. There are those who add a third, “floating”, but since this is really just fast trickling, I will restrict this discussion to the two basic methods.

    Leaving aside some special situations, such as adverse cambers, or getting round corners for the moment, if we need to get moving straight up a hill there is usually only one correct choice of technique for a particular circumstance.

    Trickling

    Trickle is where there is no slippage or wheelspin, but this does not imply any lack of power or speed. You can trickle at any speed, and in fact a high speed trickle is often a very effective method, particularly on a grass section which gets steeper towards the top.

    In general, trickle should be used where the passage of the car does not change the surface and at all times where traction is not a problem. The reason for this last observation is the fact that a spinning wheel has no directional stability, in other words it can move in any direction. In contrast a rolling (no spin) wheel can only go in the direction it is pointing, so that a car which is being trickled is much less likely to hit marker poles with its errant rear end.

    Examples of surfaces where trickle is usually the right method are grass, heather or any natural growth and any hard ground, such as dried mud or rock provided there are no loose bits on top. Exposed tree roots are a particular “banana skin” which can stop a car which is spinning its wheels, whilst another car can trickle over them without any problems. We have all seen the example where a car furiously blasting its way up a hill comes to a halt, waits for the scorecard to be marked, and then quietly drives off in the same direction!

    Trickle Blast 1

    An icy or frosty surface (not snow) is another example where trickle should be used since wheelspin will produce heat which momentarily melts the surface, then the water produced lubricates the ice below, making traction impossible. Immediately afterwards the water re- freezes leaving a smooth ice surface with no grip at all. This is a devastating negative technique (if I can’t get up I will make sure no one else can!) making it more than difficult for following drivers.

    The reason that trickle is such a useful technique is the “stiction” effect, where it takes much more force to get an object moving than it takes to keep it moving. Natural surfaces (growing plants) have another trick, a spinning tyre will break the plants and liberate sap, which is a superb tyre lubricant.

    Difficulties of maintaining a trickle stem from the fact that engines can be quite feeble at low revs, but power and torque build up rapidly as the revs rise. This means that it is all too easy to break traction at the critical moment and it takes a lot of courage to immediately reduce power and retrieve the situation but to risk stalling the engine.

    Having the right gearing is very important in this respect. If you watch skilled drivers in action you will see that they trickle almost all the time, but they do not drive slowly, or even use low power most of the time. This proves that trickling is neither a low speed nor a constant speed occupation, often using quite rapid acceleration, whilst still avoiding wheelspin, and accelerating at every opportunity when the driver senses there is enough grip to permit it without spin.

    The other aspect of trickling is to reduce power as soon as the hill gets steeper or more slippery, and this is where most novices come unstuck (literally). The problem is that the correct technique is the opposite of the natural reaction, reducing power just as you reach the steepest part of the section.

    The correct method, ably demonstrated by the typical skilled driver on almost every section, is to add speed when the going is easy and to back off and reduce power on the difficult or slippery parts, so you are not loading the tyre contact patch with torque just where the ground has least grip, but you use the momentum gained from the speed to “carry” you over the problem area. This is easier than you might think, it just requires some forethought about where you can add the speed.

    TRickle Blast 2

    Blasting

    This is the opposite technique and has become more common and useful as we moved to radial tyres from crossplies many years ago (1971?). Blasting involves using the highest wheelspin rate we can muster, a function of power and gearing, and the reason for its use involves moving the surface of the section so we can drive on the (hopefully) drier or grippier ground below. Mud is the usual surface where blasting is indispensable. We need to fling off the mud which is blinding the tyre tread, and try to dig down to a better surface. This is only possible if the wheels are spinning fast.

    Another use of blasting would be where the surface is covered with loose matter, such as autumn leaves. Here we need to shift the leaves or loose stones in order to get at the underlying firm surface. This is often the case for the first few cars on a section in the morning. Finally blasting often works when all else fails, sometimes by the drying effect, where heat generated dries the ground and increases traction enough to allow forward motion to proceed.

    In contrast to previous remarks about ice and frost, blasting is often essential in deep snow, being the only way to clear the tyre treads, and here the highest possible wheel speed is needed, which means the use of as high a gear as possible that your engine can pull.

    If you watch a novice fail to climb a straight blasting section, only to have an expert follow and make it look easy, the usual reason is a lack of “violence” in the novice’s approach. Blasting is a brutal affair, it involves “putting the pedal to the metal” as early as possible, and holding it there until you either make it or stop. You need to abandon all pretence at mechanical sympathy, but this is competition, after all! A favourite failing amongst novices in these circumstances is to floor the throttle, get the car moving, and then immediately ease off. This usually results in an ignominious fail. You should only ease off if there is a very good reason, such as going too fast for the conditions, or you are about to hit a pole.

    In a high gear you may also need to use a “racing” start, by dipping the clutch to allow the revs to rise before resuming drive. Some drivers “blip” the throttle, but this is generally not a good idea, unless you are very skilled and are feeling for the grip. To succeed at blasting you need commitment, a throttle that opens completely, and a good engine coupled to a gear that will allow fast wheelspin. Even a small amount of throttle travel restriction makes a huge difference, and even if you set it up at the beginning of the season, you should check regularly since throttle cables can stretch. At the same time do remember that in competition, when the “red mist” descends, you may mash the pedal with considerable force, so do not rely on the cable to act as a pedal stop, you will probably break or stretch it.

    Obviously you need the engine to rev cleanly, and many cars fail on this simple point. The easiest way to check is to ask a bystander at a blasting section to listen, it is surprisingly difficult to detect from the driver’s seat.

    Getting a high wheel speed involves high engine speed coupled to a high gear, but if you are required to negotiate say a tight steep bend further up the section you may not want to use your highest gear, it may prevent you pulling away from the corner. In addition a lower gear may allow the revs to rise more quickly, which might just catapult you off the start, or a bend, better than a higher gear might.

    Remember that trials gears are all very low and close ratio, and it may well be possible to get an equally high wheel speed by revving higher in a lower gear.

    So, to sum up: trickling or blasting both need commitment and planning. Once you have made up your mind, go for it!

    Failure is sure to follow any halfhearted attempt at either.

    Julian