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Handlebars and Steering torque

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by , 02-20-2014 at 08:22 PM (6880 Views)
I guess I am a trifle late with my inputs about motorcycle geometry but then seeing the kind of confusion prevailing on this thread, I believe this post is justified. Lets take a look at these elements in a way that is understandable to anyone keen enough to learn. I'll use technical terms only where there are no substitute words available.

Motorcycle steering geometry has four principle aspects that have maximum influence over the bike's handling.

They are: refer to the figure below..

Rake: The angle that the steering axis or say of the forks makes with the vertical. The 'less' the rake the steeper or more upright are the forks. The 'more' the rake angle the front wheel seems 'kicked' out.

Trail: This is the distance between a line extended downwards from the steering axis (say the bolt on your triple clamp...the one visible near the ignition key slot) and the contact patch of the tyre with the road.

Offset: This is the distance between the fork axis and the steering axis. (i.e. visually it is the distance between a line connecting the two bolts that hold the forks and the steering head bolt.)

The wheel diameter: something that is self-evident.

The best example of 'trail' working in the real world is watching the small wheels of a shopping trolley 'caster' as you roll it. They always seem to get 'in-line' automatically with the direction of travel of the trolley. Notice that their axis (its vertical) lies ahead of the contact patch.

The same 'castering' effect is responsible for the self-straightening of your bike's steering while in motion. An explanation of why this happens would be a trifle involved and so am omitting it for now. To cut a long story short, large trail has a greater self-righting effect, meaning the bike tends to be more stable in the straight line - which also implies its more resistant to turns - i.e. sluggish steering. Short trail has the opposite effect. So its pretty obvious why sportsbikes carry shorter trail than tourers and cruisers. For race-track use, one needs a bike thats more willing towards quick direction changes. Vice-versa with cruisers where straight line stability is in sync with a relaxed riding style that cruisers are all about. And the 'low slung' design of cruisers limits its ground clearence, thus limiting the 'spiritedness' of the bike through turns anyway. So the trade-off of stability vs quick steering doesn't hurt here. The usual trail figures for sportsbikes are anywhere between 3.5 - 4 inches while cruisers can go to 5 inches and even beyond.

Rake and trail move in proportion as long as the steering offset remains the same. So a larger trail requires a large rake - i.e. a more kicked out front wheel. But there are limits to how large the rake can be i.e. how far out the front wheel can be. Too much rake has a strong and strange disadvantage. On turning the handlebars, the steering head tends to drop with the turning of the handle. This can make the front wheel 'flop' over on its side rather suddenly and so the bike falls. This steering head drop is probably what @kyanmantis was asking about in his earlier posts. Large rake makes the front wheel lean further on cornering than a more upright wheel. This adversely affects a bike's cornering ability by robbing the front off its traction quicker than lesser rake. To limit customisers from making outlandish designs that can be dangerous to use on road, some countries have laws restricting rake and trail values within certain limits.

Over the years motorcycle designers and tuners realized that trail could easily be altered simply by 'lowering' the front ride height. This could be done by sliding the fork tubes further into the triple clamps. And if this didn't give them a bike that steered quickly enough, they could further reduce the trail by raising the rear as that steepened the rake which shortened the trail. This 'dropping the front for track bikes' became incorporated into bike design and allied with low C of G, smaller diameter wheels (for lower gyroscopic forces) and ergonomics like rear-set pegs that made it easy for the rider to tuck in to improve aerodynamics, the 'low' look became a standard design element of sports-bike design.


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