Once again I have searched around on the net for a suitable explanation of these things and while I found quite a lot of info, none of it said what I wanted to say in the way I wanted to say it.
I also found quite a lot of myths and misconceptions mixed in with the info on the net, usually to do with steering. So, here I have offered an explanation in my own way with such suitable diagrams as I could find.
Definitions
- Rake
Rake is the angle, from vertical, of your forks.
Trail
Trail is the distance from the point you are steering on to the centre of the front tires contact patch. (or the vertical line dropped from your axle) It is usual for trail to be around 2 to 4 inches.
Steering Point
The point you are steering on is where an imaginary line taken through your steering head and continued to the ground.
Where the blue line meats the ground is your steering point.
What trail does is ensure that the wheel is always following the point you are steering on. This way your steering is stable. If the wheel were ahead of the point you were steering on, it would always be trying to flip around to follow it. This is called negative trail and is very, very dangerous.
Negative trail can occur if you hit a bump. This is because the contact patch moves forward.
You can see that the (green) bump has brought the contact patch forward to the point of the orange arrow.
Such an event will provide momentary instability in the steering. It can cause a few wobbles and if they are violent enough this may lead to a loss of control. But under normal circumstances the stability will re-establish itself once the bump is passed, especially if you apply a bit of throttle.
The other thing that affects your trail is the compression and extension of your forks.
As the forks compress, say under brakes, the front of the bike drops and this effectively reduces both your rake and trail. Conversely, as the forks extend, say under acceleration, the front of the bike lifts and both rake and trail are increased.
(NB. This is also mentioned in the blurb on windscreens that I wrote. viewtopic.php?f=33&t=103)
(NB2 Wheel base, that is the distance between the two wheels, also alters with fork movement )
In the scenario with the bump above, if temporary loss of trail has led to a violent wobble, the panic reaction is to brake. As you can see, this would be the worst thing you could do under the circumstances, as this reduces trail and there for stability.
It should be noted that I’m talking about telescopic forks here. Some other types of forks, notably girder forks, were very much better at maintaining both rake and trail.
At this point people will be asking “Well, why not make sure there is lots of trail?”
The answer is that the amount of trail dramatically affects the way the bike handles and that you can definitely have too much.
As a rule of the thumb, the more trail you have, the more stable the steering, but the more sluggish and heavy the bike will be to steer. The less trail you have, the faster and lighter the handling you will have, and the less steering stability.
As a result, bikes that are required to change direction quickly and easily, that is sport bikes, or bikes that are good in the twisties, tend to have a steeper rake and less trail. Bikes that are intended more for motorway riding, or travelling in a straight line tend to have more trail, more steering stability, and are as heavy to steer as brick outhouses in the twisties.
Some manufacturers give a bike far more rake than it needs or wants purely for the sake of looks. They then reduce the resultant excessive trail to within sensible limits by having the steering head at a different angle to the forks. This is not an approach I admire, being a ‘form follows function’ sort of guy.
They do this by using offset fork yokes.
You can see the differences between the top and bottom picture. The trail has been reduced to improve handling despite having too much rake on the forks.
I note that in the top picture trail has not been taken from the steering head line as it should be, but you get the idea.
To be as kind as possible, it might be assumed that getting the desired ratio of lean to turn may be the excuse for having a different angle on your forks to the steering head. (see Countersteering. viewtopic.php?f=33&t=104)
Deflection and Correction
The fact that your tires are not wafer thin and in fact stick out each side of the centre line means that road irregularities can (and do) deflect them to one side or the other.
The degree to which this happens depends on the width of the tire. The wider it is, the greater the deflection. It is a simple matter of leverage really. The further from the centre line the deflection force occurs, the greater it will be. The fact that the front wheel is effectively being pulled along behind the steering point though, causes it to self correct and come back into line. This is the result of trail and is the steering stability that we have been talking about.
We are all familiar with a simple version of this. It is the shopping trolley wheel, or caster wheel. (a caster wheel is simply a wheel with trail)
The caster wheel is a simple example of a wheel with trail. The wheel is dragged along behind the point it pivots on so it stays in line or ‘self corrects’ despite deflections.
Too much trail.
As the trail is increased, so is the self correcting force increased. This can reach a point where the self correcting force is greater than the deflection force was in the first place.
What happens then is that when the front wheel is deflected to one side by a road irregularity, the wheel self corrects even more violently and it not only swings back to the centre line but continues on out the other side. It then swings back even harder and goes even further out in the direction of the original deflection and so on.
The net result from a riders point of view, is called a “tank slapper” where the front end swings violently from one lock to the other repeatedly and is more often than not a prelude to a somewhat precipitous departure of the rider from the bike.
Again the shopping trolley wheel often gives us an example of this very effect when it flaps violently from side to side. The famed, but often miss-understood, shopping trolley wobble.
This is also why bikes that have a lot of trail also usually also have very skinny front tires. The skinny tire does not extend out as far from the center line so the deflection forces are less. This means the force of the corrections is less also.
Too little trail.
The problem with too little trail is that there are far more moments of momentary negative trail, and therefore steering instability. There is also the danger that negative trail could be produced for more than a moment. This could happen under brakes, due to the downward thrust of a fairing or windshield, or even in a hard corner.
If negative trail is not corrected immediately, then the rider is off the bike immediately. This is because as soon as you have negative trail the wheel tries to reverse itself, or spin round 180 degrees. (added to this nasty event are of course the effects of gyroscopic forces. Things get completely out of control right quickly! ) The only possible way out of such a situation that I know of, should you get time to react at all, is a fist full of throttle.
Normally this effect is not seen with our shopping trolleys as their caster wheels have tons of trail, but you could set it up by trying to push one with the wheels the wrong way round, just to see the effect. It’s fairly immediate.
Negative Trail
Sometimes it happens that a bike is built with negative trail. These are invariably "choppers" or Custom bikes. They have either been built for show ONLY or they have been built by someone who simply does not know what they are doing. In either case, they are absolutely unridable. Any attempt to ride such a bike at any speed faster than that which the rider can hold the front wheel straight by main force (5, maybe 10 mph) is a quick way to find oneself in the emergency ward of the nearest hospital.
