faffi Posted June 7, 2013 Report Share Posted June 7, 2013 I'm currently reading The Suspension Bible and have found it to provide a lot more information than just suspension stuff. Like geometry. Many of us have heard that if the rear axle sits above the swing arm pivot, we get pro-squat under acceleration, and that if it sits below we get anti-squat. Well, turns out it is far more involved than that. First off, virtually every streetbike have a built-in anti-squat effect. The amount of the force, however, can be manipulated. You can get so much that it tops out and lock the suspension solid, or you can get so little that the rear end squats under power. But it squats from weight transfer being greater than the anti-squat effect, not because there is any pro-squat forces at work. This was news for me. If you have a bike/rider combination with a 50/50 weight distribution you can theoretically accelerate at a rate of 1G before you'll start to wheelie. If the anti-squat forces run directly through the CoG, you will have a 200% anti-squat effect, locking the suspension from movement as it will support the whole weight of the machine. What a chassis maker will want, is about 100%, that is where acceleration does not affect the suspension in either direction. That is not easy to achieve on a regular motorcycle since the anti-squat effect will vary with suspension travel - hit a bump and it is reduced, hit a dip and it is increased - but they can get reasonably close. If the output shaft and swingarm pivot sits on the same axis, suspension travel will not effect anti-squat and explain why these two are so close together on a modern sportsbike; to minimize the effect. A longer swingarm will also influence the anti-squat less for any given amount of wheel travel than a shorter arm. Other things that will influence the anti-squat effect are sprocket sizes, wheel placement, wheel diameter, ride height and more. Another thing that influence the behaviour of our bikes far more than I imagined is trail. Yes, more trail equal more stability and heavier steering. But according to the book, more trail, at least to a point, also increase traction. Improper trail is apparently also why so many motard riders are backing their bikes into corners. However, what I've never thought of is that trail can become negative quite easily. If you hit a bump, the tyre's contact patch moves ahead. On a big bump, it can get far enough forward to create negative trail. Imagine what that will do to the stability of your bike, especially if you hit several big bumps in succession. The same happen if you ride slowly in deep sand (or snow); you push a load of it front of the tyre, in effect moving the contact patch ahead and quickly end up with negative trail. So the lack of control is not just lack of skill, it has a physical explanation. The more I learn about motorcycling, the less I know. Motorcycles are incredibly complex, yet usually also highly forgiving. When I think about all the quite drastic changes I've made over the years to suspension, both in length and in springing/damping, it's amazing handling haven't been more messed up. Usually, the changes have been a success. As such, I'm glad I didn't read this book sooner as it probably would have prevented me from making changes in the first place in fear of making a good bike into a poor one Quote Link to comment Share on other sites More sharing options...
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