Thursday, September 11, 2008

Suspension Terminology 1: What is Wheel Rate etc..?

Question from Interested Rider
Thanks, this is still far from clear:

Originally Posted by _dw
Designing specifically for wheel rate basically allows for a wider range of tunability with the stock shock settings, and it has the additional benefit that damping does not need to be specifically adjusted to make up for uncontrollable issues that arise when leverage rate is not matched with the air spring curve. All in all the method provides more shock tuning options with less tinkering.

Originally Posted by _dw
Through experience, I've learned how to very precisely tune wheel rates, and for air shocked bikes, this is really important. Each bike's leverage rate is specifically engineered to work with a specific air shock and even air can volume to hit a desired wheel rate or wheel rate options. From there we develop damper tunes to meet the frame requirements.


Some terminology will probably help..

Wheel rate:

Wheel rate is defined as the change of wheel load at the center of tire contact patch per unit vertical displacement of the sprung mass relative to the wheel, at a specified load. - Maurice Olley, Chassis Design Principles and Analysis; ISBN 0-7680-0826-3

(this does not factor in tire and wheel deflection, there is a different term and calculation for that)

So in layman's terms, wheel rate is the force change at the tire to ground contact patch as the suspension cycles through its travel. It can be graphed as force at the tire versus displacement of the suspension. Wheel rate is the force increase that you feel in your feet and your seat as you sit on your bike and compress the suspension.

I like to use the term "feel" to describe the human perception of wheel rate when riding a suspension bike or motorcycle. Heck, why not define it.. just to be clear.

Suspension "Feel": The human perception of wheel rate and other suspension characteristics through a vehicle's interfaces. On a bicycle these interfaces include the seat, pedals, and hand grips.

Air Spring Curve:

An "air spring curve" is the graphical representation of spring rate for an air spring. Spring rate is understood by most people who have taken a high school level physics class. It can be measured as spring force output versus spring displacement. (I use displacement as the term for length change because it is shorter and non-denominational). Where metal coil springs typically feature a constant rate, an air spring typically has a non-constant rate. This non-constant rate provides challenges for a suspension designer, particularly one like me who is trying to engineer a specific wheel rate, or "feel" into suspensions.

Leverage Rate:

This is one of those terms that rolls easily off the tongues of many bike people, yet few actually understand the nuances of how it can- and why it should be manipulated to benefit a suspension bike.

Leverage rate (or more accurately Instantaneous Leverage Rate) is the measured ratio of wheel displacement versus spring displacement at any instantaneous point in a suspension's travel.

This is not to be confused with Average Leverage Rate, which is just a straight division of total wheel displacement versus total spring displacement.

In order to achieve a specific wheel rate (remember, this is the suspension "feel" that I defined earlier), I need to make sure that my leverage rate is designed to specifically account for the shape of the air spring curve. This is just one of several parameters that must be considered. Additionally I spend a great deal of time looking at damper force outputs and anti-squat / braking squat. Those are discussions for another day.

As interested readers of this blog delve into this subject deeper on your own, you will notice the amazing variety of leverage and corresponding wheel rates used in cycling today. From there perhaps you will ask, "why is one more useful than another"?

I'm on my way to California for some spring damper testing, so I've got to cut this short. Thanks for reading and I hope this is somewhat educational!

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