Jan 31, 2007

Negative Trail

Look at a motorcycle spec sheet - not the ones on the back of the bike brochures from showrooms - the genuine ones with all of the little details. Scan down the list until you come a column called trail. That's right, we're going to talk about that inocuous looking number (usually in mm) today. First in general, and then in a specific, rather hazardous situation.

Trail, to define the term is a measured length. The first point of this length is the centre of the contact patch of the front tyre of the bike. The other is the point where the steering axis intersects the ground. Most engineering books usually simplify this by saying that 'for simplicity's sake, we'll assume that the steering axis passes through the axis of the wheel (axle).' Wheels can be mounted offset from the telescopic forks, which will affect the trail figure.

Now, then the front wheel turns, a fairly complex physics-type thing happens. It really is a complex thing and uses lots of long formulae to explain the entire event. But, the upshot is that motorcycles with a larger trail number usually exhibit a stronger self-righting effect. As in, the motorcycle will want to eliminate the deviation and if not corrected, will return the wheel to the centre automatically.

The most obvious example (and unfortunately) the most quoted one is that of a bike where the rider has fallen off. In most cases, the motorcycle will lean sickeningly and then correct it. If it's going fast enough, eventually, it'll actually settle into a straight path before falling over once the speed runs out, or crash into something. The leaning over, and then righting the lean is the effect of trail.

Too much trail is not a good thing either. It makes the steering feel heavy and the motorcycle feel like it does not want to change direction at all. Choppers and most cruisers sport fairly large trail numbers, so it isn't surprising that they don't really corner that well. On the other hand, notice what're called power cruisers (Yamaha Warrior, Harley V-Rod etc). You'll notice the lower trail figures.

Also note that since the front suspension dives and extends with acceleration, and also happens to flex a bit, trail, on the move is a moving target. It increases under acceleration (one of the reasons it is harder to turn on the throttle) and decreases under braking and off the throttle (which is why all riding skills gurus recommend turning into corners off throttle).

Now you have understood the basics of what trail is. We've also established that it's a great friend, always on the look out to help you. But there's a specific situation where it turns into the enemy.

And it's called negative trail. Imagine that the ground wasn't fully horizontal. Suppose you measured trail say halfway up the slope of a fairly steep speedbreaker. The rising surface would be move the contact patch centre ahead of the steering axes and the trail number would turn negative. Which explains the etymology of the name. In feel turns, the righting effect of the trail gets replaced by a destabilizing effect. If you were turning just a bit right (or left) as you hit the slope, the sudden arrival of negative trail would be felt as a sudden tendency of the bike to turn harder right (or left). If you didn't correct the unintended steering movement, the bar would roll all the way to the stop, and you'd either make an uninteded about turn, or more likely, topple into an ungainly heap.

Does this actually happen? Oh yes. I remember that I used to love blasting out of the underground parking lot of this five star hotel. The guard, who knew me (and my RD), would tell me in good time if there was any cross traffic (the exit ramp crossed the hotel entrance at right angles). So I could let it all hang out, so to speak. Once I crossed that, there would be a gentle downslope with a nasty, steep but small speedbreaker at the end. I only ever hit it at an angle once, one the brakes and the slightly turned to the left. And blow me down if the damn handle didn't turn all the way left. I must have been damn quick with the correction, so I didn't topple over, but I came damn close. That was the last time I hit the speedbreaker (and any others) at an angle. Now, the only time I recommend taking a speedbreaker at any angle other than perpendicular is if your bike is too low slung to clear it straight on.

Read more:
Tony Foale | American-V | Motorcycle dynamics - Wikipedia | Trail - msgroup.org

Photos: KTM/H Mitterbauer Ugly but effective graphics: rearset


Glifford said...

Very Nicely Explained.

After a lot of motorcycle riding! Wanna ride something really twitchy! Ride a scooter :D

The tiny wheels add to the fun. No wonder it is more tiring to ride a scooter over a long distance than a bike! Your body needs to do the balancing! And active balancing saps energy!

Saager Mhatre said...

Does this also come inot play at railroad crossings? Especially ones where repeated layering of tarmac has sent the rails another 1/2 inch lower. Or is that just the rail pulling the front wheel. I had a few bad experiences (but no falls) taking railroad corssings at an angle.

Anonymous said...

Being mindful of the widespread opinion that negative trail could be destructive of stability, we did some careful research showing that it can sometimes be OK. Please see the April 15 2011 issue of SCIENCE, or this website

Jim Papadopoulos (an author)