| #13  18-Jun-2004, 15:28 | ||||
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 Chassis Geometry - Fork Angle & Trail Stability and Steering Angle The central issue with making this change is stability. A motorcycle is designed to return to its straight-ahead condition after hitting an object or bump in the road that causes the front wheel to deflect slightly to the right or left. In other words, it has to remain stabile for a variety of road conditions, and motorcycle stability is foremost a safety issue. The way dynamic stability is assured is to design a bike with enough distance between the point where the front wheel touches the road and the intersection point between the steering axis and the road. This is called the trail dimension. A longer trail dimension increases the motorcycle's stability on straights, but will also negatively affect the motorcycle's turning effort, i.e. more rider's strength is required in corners and transitions. However, the more trail the greater the ability of the bike to self-correct it's steering. It's a longer trail dimension, for example, that allows you to easily ride with no hands on some bicycles but not others. A shorter trail dimension, on the other hand, produces a lower opposing force to steering inputs. It's kind of like power steering. So the steering requires less rider strength, but larger handlebar displacements from bumps in the road and corners are fed back to the rider. Said another way, the shorter the trail, the more rider input that is needed to hold a line and the more responsive the bike feels since it is more sensitive to steering inputs. The two superbike steering angle positions, 23°30' and 24°30' produce trail dimensions of 91mm and 97mm respectively. The wheelbase, also an important factor in stability, remains unaffected when you change it. As a comparison, the Monster steering angle is fixed at 24° and the trail dimension is 94mm. Adjusting the trail dimension on most manufacturer's bikes is not an option. As an aside, when you change to the steeper 23°30' position you loose a significant amount of steering lock making low speed U-turns more difficult. Also, the ignition steering head lock doesn’t engage in the steeper position. Now, here's Ducati's warning: "Trail should only be altered after all the other (geometry and suspension) changes have been made and you are comfortable on the bike. If the bike displays any instability problems they need to be sorted out first, as this steering head angle change will magnify these characteristics." (One reason, for example, is that part of its effect mimics changing the rear ride height.) The Haynes Service manual goes on to say "Warning: The steering head angle must be set to the road position (longer trail) whenever the bike is used on the road. If the steering angle is set to the race position (shorter trail) ... the handling of the machine could become unpredictable on uneven road surfaces." So, shortening the trail is considered unwise for street riding (unlike tracks) where bumps in corners, potholes and other road hazards repeatedly challenge your bikes steering stability. Here's a case where inexperienced or uninformed riders who set-up their street bike chassis geometry as racebikes are just looking for trouble. Trying to mimic factory race bike set-ups can get you into trouble. It's central to racing that race bikes often need to sacrifice high-speed stability to handling. Riders may initially run the steeper steering head angle but often, as they get faster, they realize they want more stability, not less. To get more stability there are two things that Ducati typically changes on their racebikes: the triple clamps and the swingarm. They use triple clamps with less offset, typically 27mm instead of the stock 36mm, and use a 25mm longer swingarm to increase the wheelbase. These changes to the triple clamps or the swingarm have the effect of moving the center of gravity forward which is the typical starting geometry of the Corsa race bikes. Remember, changing the steering head angle does not, by itself, change the wheelbase or alter the center of gravity. Some here have suggested that you can get the same effect (reduction in trail) with a finer adjustment by increasing the rear ride height instead. However, you'll need to raise rear ride height 16mm to get an equal amount of trail reduction, and in doing so you'll also end up increasing the height of the bike's CG by about 12mm that (among other things) will increase loading to the front tire, so when hard on the brakes, the rear tire gets/feels very loose. One improvement with the steeper angle that’s been observed is in trailbraking - The bike stands up less on the brakes, which can be a benefit on backroads where you never quite know what might be coming up around the next bend. Also, some feel that the steering is more neutral at large lean angles. The area of major concern is tankslappers. Reducing trail by reducing the force that centers the front wheel will give you more headshake, especially when accelerating (less weight on the front wheel) out of bumpy corners. You won't get a tankslapper out of most corners if you change to the steeper steering head angle, but you will make them more likely, and more violent, when they do occur. Some will say to crank-up an adjustable steering damper to settle the steering, but dampers will only resist changes in steering direction and don't provide a restoring force to re-center the wheel like trail does. The higher damping rates also spoil your quick steering and cause weave instability problems when cranked-up too high. For those of you who haven’t experienced this phenomena, see one here: http://www.randtclub.com/Video/cedwa...pper_tt99.mpeg I’ll choose more stability over quicker steering any day. [Edited on 6-18-2004 by Shazaam!]  |
| #17 18-Jun-2004, 17:49 | ||||
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| #19 18-Jun-2004, 19:18 | ||||
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| What I'm suggesting here is that after you've initially pushed the bar to initiate the bikes change of direction, the rotated position of the bars does not change during the turn, therefor during the transition from full left to full right lean the bars are pretty much static, and unaffected by the damper. Also I don't think the damper has any significant resistance compared to the force fed back from the road - Lift your front wheel clear of the floor and see how little effort it takes to turn the bars, even with the damper wound up, this doesn't change when the bike is in motion does it? What I'm trying to achieve is less effort in turning the bike from side to side, which proved to be hard work at Silverstone through those 5 consecutive changes starting at Maggots, and I'm still convinced that the stiffness of the damper has very little effect on turning speed. Yes, the handlebar input has more resistance, but the duration of rotation is so brief that I consider it irrelevant. Also, any resistance given by the damper is only a small percentage of that given by the bikes own natural desire to straighten itself - especially at speed. So when you consider the reduced effort you need as a result of the geometry change it's LESS effort overall. Although I'm nowhere near your level of expertise shazaam, I too studied aerodynamics, and now teach it at grass root level to Air Cadets, so I don't need any convincing that caution needs to be exercised here! I never meant to suggest otherwise, only that it's not noticable to me with the way I ride on the road. I do look forward to taking it to the track where the change should be much more noticable. Roll on Cadwell/Rockingham/Lydd/etc....  |
Shazaam!
could be another argument for countersteering???
Linear Mode
