Tuesday, September 9, 2008

What is anti-squat and why is it good?!!

Anti-squat in suspensions has become a focal point for cycling suspension designers recently. The term is beginning to become more "en-vogue" for cycling marketing, and I imagine that in a few years, it will be one of the main points discussed in marketing brochures. A good deal of this is partly due to the success of my dw-link suspension, which is a position sensitive anti-squat design, the first of its kind.

Over the years I've been asked "what is anti-squat" hundreds of times. A lot of times I've been asked that question by people with their own preconception on what it is or how it works. This makes explanations challenging at times, but not impossible, just longer winded. I'm not the best at oversimplifying, I tend to think that every detail in necessary in a discussion, and this has not helped me to publish simplified accounts of how the dw-link works. After all, it's not simple at all! I have been working on breaking it down.

One of the incorrect preconceptions that I frequently hear is that more anti-squat somehow equals less traction. Actually, the opposite is the truth. The closer a suspension is to being balanced, the more traction it can deliver at the wheel.

Anti-squat and the concept of squat in general can be difficult to grasp sometimes. As its core, squat is a suspension's reaction to mass transfer that happens during acceleration. Anti-squat is a term for a force that balances the suspension's reaction to mass transfer.

100% anti-squat is the exact amount that a suspension would need to develop to completely counteract the effects of mass transfer. One way to simplify this relationship is to think of it like an old style weight balance. More than 100% anti-squat would overbalance the suspension, and the suspension would extend under power. Less than 100% anti-squat would underbalance the suspension, and it would be allowed to compress under power. pro-squat (negative anti-squat) (and yes, some well advertised bikes actually feature this) not only allows the suspension to compress under power, it forcibly compresses the suspension while accelerating. Any amount of pro-squat is about the worst case scenario from an efficiency and traction standpoint.

On one side you have mass transfer loading up the suspension. On the other side, you have a balancing force. There are three approaches used in cycling suspensions today that one could take to balance (or not balance) out this mass transfer.

1) you could do nothing. The suspension would compress with every acceleration and subsequent mass transfer, and with every compression stroke and rebound stroke of the shock, you will lose energy. Your wheel rate rises as the suspension compresses, and your suspension is now stiffer with less compressive travel. Your suspension is at a disadvantage to absorb bumps. Traction decreases.

2) you could use a shock with a great deal extra low speed compression damping. The suspension would compress less than the first case with every acceleration and subsequent mass transfer, but still some, every time your shock compresses or rebounds you lose energy.

-a little bit of basic damper theory-

Keep in mind, that in order to support the mass that has transferred to the rear wheel under acceleration, the shock needs to develop force at the damper shaft. A damper develops force by pushing oil through a small orifice. As the oil in the damper is pushed at high pressure through the small orifice, the shearing force in the oil causes friction and energy is converted to heat and dissipated. The more resistance in the damper, the higher the shearing force, and the more energy is converted to heat and dissipated.

Here is the worst part. Now your suspension is unbalanced when you are cornering or not accelerating under power.

Think about it, your compression damping is raised to deal with the additional forces of mass transfer due to acceleration. When you are not accelerating, that mass transfer does not exit, and there is less force that the shock needs to deal with. Your shock is now overdamped when you are coasting. Most of your cornering happens when you are coasting, so effectively you have unbalanced your suspension for cornering. Traction decreases in all cases.

Note: Some people take this to mean that an ideal is absolutely zero low speed compression damping. This is not the case. Low speed compression damping is of paramount importance to a properly set up suspension, but like many things, too much is not good for you..

3) you could use anti-squat. This would allow the suspension to react to mass transfer only during acceleration. The closer a suspension is to operating at 100% anti-squat, the closer to being perfectly balanced the suspension is. The closer to balanced the suspension is, the MORE TRACTION the suspension has in all conditions.

In short, a position sensitive anti squat, with a higher level of anti-squat gives MORE TRACTION than any other approach, and especially more traction than approaches using less anti squat. Today dw-link is the only suspension system that features approach 3.

More to come soon.

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