3pMaster / Guides

What a tripmaster measures

A tripmaster shows two distances: the total since the start of the section, and a partial that you reset as you go. Navigating from a roadbook depends on the partial being right. This guide covers what makes it drift, and how calibration brings it back in line with the roadbook.

Total and partial

The total is the distance since the start of the section. It tells you where you are in the roadbook as a whole.

The partial is the distance since you last reset it. Most crews reset it at every box in the roadbook, so it counts up towards the distance printed on the next box. That is the number you drive on from one instruction to the next.

Your distances and the organiser's

The distances in a roadbook were measured by the organiser, in their own vehicle, on their line and with their tyres. Your readings will be slightly different. A box marked 4.72 km might come up at 4.68 or 4.79 on your display, and over a long stage the gap keeps growing.

So a tripmaster is calibrated to agree with the roadbook, rather than with some absolute measure of distance. When your kilometre matches the organiser's, each box turns up where the roadbook says it will.

What changes the reading

A wheel-sensor tripmaster counts wheel revolutions, so anything that changes how far the wheel travels in one turn changes the distance:

• Tyres. Two tyres with the same size on the sidewall can roll a noticeably different distance per turn, and a worn tyre rolls less than a new one.
• Pressure. Letting the tyres down for sand makes the rolling circumference smaller, so the trip starts to read long.
• Wheelspin. In soft sand the wheel turns more than the car moves, which also makes the trip read long.

A GPS tripmaster adds up the distance between successive position fixes, and its errors pull both ways. Every fix wanders by a few metres, and adding up that wander can make the distance read long, especially at low speed. On a tight, twisting track the straight lines between fixes cut the corners, which makes it read short. Under trees, in narrow valleys or anywhere with a poor view of the sky, the fixes get less precise and both effects grow.

Your driving line matters too, whatever you measure with. Taking the corners wide covers more ground than the organiser's line did, and over a full day that adds up.

How to calibrate

Navigation and regularity rallies usually publish a calibration section before the start: a stretch of road with an exact official length. Drive it at the tyre pressures you will use on the day, compare your reading with the official figure, and set the correction. If your event has no calibration section, use a stretch of road with reliable distance markers, or a route whose length you already know.

FIA and FIM rally-raids work differently. They do not normally give you a calibration section, and the organiser's own navigation unit corrects its distance at each waypoint.

The longer the stretch, the better. A 1% error over 2 km is only 20 metres, which is within the normal wander of a GPS position. Over 20 km the same error is 200 metres, and that you can measure with confidence.

Check it again whenever you change tyres or run different pressures.

Reset at every box

Errors build up on the partial. Resetting it at each box throws away whatever has accumulated since the last one, so a small calibration error never has the chance to grow into a big one. A crew that resets cleanly at every box can cope with a calibration that is slightly off. A crew that lets the partial run on across several boxes carries every error forward.

That is why the reset needs to be something you can hit without looking.