Originally Posted by
Ixion
But you are assuming that lean angle is the be all and end all. It is more complex than that.
Consider a bike going through a corner. Let us assume for simplicity that it is a simple corner, no bumps, unlimited ground clearance, dry road, no gravel, sight lines right through the corner, and that corner speed is not limited by engine power etc. In reality of course, such a perfect state of affairs would be unlikely.
Now, in such a case the limits to the speed the bike can corner at will be determined by two things. (a) the adhesion of the tyres. As the bike goes through the corner, an outward force (I am SO not getting into the centripetal/centrifugal thing - an outward force) will try to push the bike sideways to the outside of the corner. This has to be resisted by the grip of the tyres on the road. At some point, the outward force will be greater than the tyres can resist.
The result we know - the bike will lowside (I know, a skilled rider can control a gentle slide. Let us keep this simple). Or possibly the tyres may slip, then grip again (perhaps they found a bit better patch of road, perhaps the rider panice and backed off, or whatever). The result is a highside. Nasty.
The other limit, (b) is when the bike is leaned over so much that the tyre reaches the edge of its tread. The tread on a sprots bike does not go right round to the rim. There has to be a sidewall. Once the edge of the tread is reache, either the contact patch becomes much reduced, thus reducing the amount of grip resisting the outward force - see (a); or the bike will quite suddenly "fall over" onto the side of the tyre, causing a nasty dumping lowside.
Obviously, we need to delay either event as long as possible.
Now let us consider the effect of leaning, and of shifting centres of gravity. This is the bit where we talk about conic sections. You will have to draw little sketches in your head. I am not going to draw them, because I suck at drawing things. You will have to use your imagination.
A bike, leaned over and going through a corner describes a path determined by the path of travel of a conic section having its base the plane of the centre of gravity of the moving mass. Note that, it is important, it is the moving mass which matters, not the bike itself. The vertical axis of the cone is drawn at a right angle to the mass centre intersecting a line drawn along the ground from the contact centre of the wheel.
Now clearly we see (I said, use your imagination) that if the plane of the centre of mass be moved inward toward the apex (of the conic section, not the corner), then the circumference of the path of travel will be less. In other words the same amount of road (the corner) wil be traversed in less time. Which is another way of saying that for the same angle of lean the speed will be greater . Or conversely for the same speed you need to lean less.
But. there is no such thing as a free lunch. There are two prices you must pay for this (two in terms of physics, others perhaps in terms of practicality). The movement inward of the mass centre increases the speed of traverse of the nominal conic path. That in turn will clearly mean that the outward force will be greater.(think of swinging a bucket on a rope slowly then rapidly - which will be harder to hang on to?). Which is to say, you will be putting more force on your tyres for the same lean angle (but going faster). This does not matter, UNLESS you are near the limits of tyre adhesion.
The second complication is that in practical reality, hanging off will almost always mean that the CoM is not only brought inward, but lowered. Now this is undesireable, since, other things being equal, a higher CoM will mean a steeper cone face, which once again means a higher speed.
In other words, a bike with a high CoM will be able to go through a corner at a certain speed with less lean that a bike with a lower CoM
So the effects of the movement inward of the CoM will conflict with the movement downward of the coM. Ideally we would like to move mass inward, and stay at the same height. That is difficult on a road bike.
And, our inward movement means more stress on the tyre grip.
But, we will lessen the risk of our tyres reaching the edge of the tread, since for a given speed we will be more upright.
Back in the classic days, Mike Hailwood and Joey Dunlop et al did not hang off. This was, in no small part, because the tradeoffs would have been negative to them. Tyres then were narrow (4.5" on a Manx Norton) and did not have today's sticky compounds. The narrow tyres had a more nearly hemispherical shape. Modern tyres are wide section , and low profile. They cannot be as "curved" as the old ones. Mike was not really worried about rolling over the edge of the tread, any more than a present day 50cc racer is. But he was very worried about low siding when his not-very-sticky-at-all tyres ran out of grip. For him, that was the important thing, how long would the tyres hang on. He didn't need to hang off, he could easily lean the bike until he reached the limits of adhesion. Hanging off would just have meant he had to slow down, or lowside.
Modern day racers have super sticky tyres. But they have a wide and relatively (I said, relatively) flat profile. You can reach the edge of the tread, leaned over, before you run out of stick. So it makes sense for a modern day racer to hang off. He increases his speed through the turn, and also delays the point at which he reaches the tread edge.
So, does hanging off also make sense on the road? It depends I guess on how near the conditions are to racing. If you have super sports tyres and you are cornering at the maximum possible speed you will increase your speed by hanging off. The bigger the bike (and thus the bigger the tyres) the more this will be so
But if you are not maxxed out, or if you have more "normal" tyres , you may actualy get into the same negative payback territory as Mike. Especially if you have those "dual compound" tyres with a hard centre section for long wear, and soft outer tread for grip. Because if you hang off when not flat tit round the corner, the resultant lesser lean angle may be such as to put the tyre on the hard not-so-grippy bit. At the same time as the increased outward force (from hanging off) is trying to break that grip.
And there may be practical downsides to hanging off. Doing so on a right hander puts your head further over toward the centre line. Racers do not have to worry about traffic coming the other way, but we do. The cage coming toward you may see your bike and think to allow room. He may not expect your head or leg to be several feet further over the centre line. And if you have to move back from the line, then you probably lose any benefit of the hanging out. When hanging off, your arms should be almost straight, head shouldnt sticking out that far if you take a road line.
On a left hander , hanging off you markedly reduce your visibility , especially to the all important other-traffic-coming right - you can't see through the bike. Overall being lower will further reduce visibility.
And on the road the unexpected happens. I will argue that you have a much better chance of making a sudden change of direction or braking hard if you are reasonably upright in the seat. Switching from hanging off to the left, to veering right is a slow business.
Not if you know how to ride. Even at full lean its possible to change direction quickly without changing body position all that much. its control.
There is nothing wrong with hanging off (the knee down bit I think largely irrelevant) . And if you have race type tyres and are cornering at the maximum your tyres can go over to, it will certainly increase your speed through the corner. Should you be cornering at those speeds on the public road? That is another question.
But if you do not have wide section race type super sticky tyres, you will probably gain nothing , as compared with simply leaning a bit more. And may even be worse off.
And for all riders, the hanging off position involves dangers on the raod which the racer does not need to take into account.
I Agree. I hit a 2 inch pothole on a roundabout with knee down..
I do not think that the vast majority of riders who hang off and try to get their knee down do so because they need to in order to get round the corner. I suspect that in the majority of cases it is because they think it looks "cool". Which does not seem a very good reason to do anything let alone somehting that increases risk
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