Going off at a tangent

by Simon Hargreaves

Why steering geometry is never a straight ahead subject

Indian’s new Classic, Vintage and Chieftain share engines and frames, differing only in trim and styling. Yet while the Classic and Vintage have identical chassis spec, the Chieftain has different numbers: 61mm less wheelbase, 4° steeper rake angle and 5mm less trail. If you were to judge which Indian was sportiest on spec, you’d say the Chieftain. Which is odd, because...

Why steering geometry is never a straight ahead subject

Indian’s new Classic, Vintage and Chieftain share engines and frames, differing only in trim and styling. Yet while the Classic and Vintage have identical chassis spec, the Chieftain has different numbers: 61mm less wheelbase, 4° steeper rake angle and 5mm less trail. If you were to judge which Indian was sportiest on spec, you’d say the Chieftain. Which is odd, because it’s the tourer.

Tourers tend to have more rake and trail because it aids stability/reduces agility, and less aids agility/reduces stability (the relationships are more complex, but that’s the gist).

Rake is fork angle. 0° is a wheel directly under the steering and 90° is straight out in front of you. If you turn the bars, a wheel with 0° turns sharply and a wheel at 90° tilts over instead. Imagine playing wheelbarrows: with the wheel under your chin it turns quickly, but at arms’ length it leans over.

Trail is trickier. Draw a line down from the front wheel axle to the ground. Mark the point. Draw another line down the steering stem (not the forks!) to the ground and mark that point. It’ll be in front of the first point. The distance between them is the trail.
Trail is so-called because although the front wheel is in front of the steering stem, it makes the wheel behave as if it was behind it, being trailed along. This is a good thing because dragged wheels self-align and go where they’re dragged – which is why when you ride with no hands you don’t instantly go into a tankslapper.

So why has the Chieftain got sportier steering than the Classic and Vintage?
Indian tried using the same steering geometry at first. But the Chieftain’s handlebar-mounted fairing and stereo package, plus the altered weight balance of hard luggage and use of crossply tyres (they wanted whitewalls and only Avon supply Triumph with whitewall radials), meant they needed steeper rake to maintain steering agility at low speed. Yet they wanted trail to maintain stability at speed.

How did they get both? Steepening the headstock angle steepens rake (and it looks better to have the wheel closer to the engine on this style of bike). But now you have insufficient trail for stability. To regain it, you alter fork offset.

Look at your top yoke and you’ll see the fork tops in front of the steering stem. This distance is fork offset, and it’s related to trail. Reduce offset (moving the fork tops closer to the stem) and you increase trail, but keep rake the same.

This is usually done to both top and bottom yoke at the same time, so the forks remain parallel to the steering stem. But they don’t have to be. Using a steeper fork angle than the steering stem angle, where the bottom yoke is shallower than the top yoke, means Indian can get both the trail and rake figures they want for the Chieftain. Negative, unequal offset looks a bit weird, but the fairing means you can’t see it.

And, as the Chieftain is both agile and stable, it works, too.