Proportional
Response

Here’s a news flash…Bike riders come in all different sizes! And here at Cannondale, we just don’t think that’s right! Err, no. Wait. That came out wrong. What we meant to say was: Bike riders come in all different sizes, but often are forced to choose bikes designed with someone else in mind, and THAT’s what we don’t think is right. Which is why we came up with Proportional Response, a size-specific theory of engineering that tailors the key elements of a bike’s design according to the average heights, weights, and centers of gravity of different sizes of riders. So everyone gets a bike that feels like it was designed just for them. Because, it was.

The key areas where we apply Proportional Response engineering are: Suspension Kinematics, Geometry, and Frame Construction.

You Deserve Suspension Designed For YOU

We believe that every rider deserves great performance. With Proportional Response Suspension, we go beyond the one-size-fits-all approach of most bike companies and custom engineer specific suspension layouts for each size of frame. We research the average center of gravity for riders for each frame size, then we customize the suspension kinematic around each one, altering pivot locations, shock position, and leverage ratios to ensure that every rider gets the same, great, suspension performance regardless of their size. It’s a lot of work, but it makes for a better ride for everyone.

Suspension design is a complex thing. There are many different ways to design a system that allows the rear wheel to move independently from the main frame to absorb bumps. But no matter which system you choose, one thing remains constant: the size of the rider, and the resulting combined center of gravity of the bike and the rider, is one of the main drivers of how that system responds to the pedaling, braking, and bump forces it sees over the course of a ride. Yet most companies design just one suspension layout, and use that exact same kinematic across all frame sizes, resulting in sub-optimal suspension performance for any rider outside of the one reference size. That didn’t seem right, so we created Proportional Response to bring great suspension to riders from small to tall.

Of these, the suspension characteristics that have the greatest impact on the bicycle’s dynamic performance are factors known as anti-squat, anti-rise, leverage ratio, and pedal kickback. Normalizing these factors across the size-range is what Proportional Response kinematics is all about.

Learn more about suspension characteristics.

Anti-Squat
Squat refers to how the suspension, and thus the bike, behaves under acceleration. Picture hitting the gas in a muscle car. Sending power to the rear wheels causes the mass to shift, compressing the rear suspension and unweighting the front, causing the vehicle to squat. Anti-squat is the amount the design of the suspension is able to resist or counteract that that force, which has implications for how efficient a bike feels, and how much traction it can provide under pedaling.

Anti-Rise
Rise describes how the suspension behaves under braking, kinda the opposite of squat. Hit the brakes and the mass shift causes the front suspension the compress and the rear suspension to extend, or “rise”. Anti-Rise describes the suspension’s ability to counteract that tendency when using the rear brake. Dialing in the right amount of anti-rise can prevent the suspension extending or compressing too much under braking, keeping the rider in a neutral position and allowing the suspension to remain active.

Leverage Ratio
In bicycle suspension, the term Leverage Ratio refers to the mechanical relationship between how much the rear wheel moves and how much the shock compresses. Leverage ratio determines the spring rate at the rear wheel, the wheel rate, and the change in wheel rate as the suspension compresses. There are three general classifications of leverage ratio extensively used in mountain bike suspension: linear, progressive, and digressive rates, which can be manipulated by varying the length and position of the shock links and pivot locations.

A linear rate means that the leverage ratio remains constant through the suspension travel. A digressive rate means that the suspension stiffness decreases through the travel, and a progressive rate means that the suspension becomes stiffer as travel increases. We have found that for most MTBs a progressive leverage curve performs better on the trail; the softer stiffness in early travel provides good sensitivity to small bumps, with improved support through the middle and ramped up stiffness to resist bottom out at the end of the travel.

Pedal Kickback
Also referred to as Chain Growth, pedal kickback is driven by the movement of the rear axle relative to the bottom bracket as the rear suspension moves through its travel. If the rear axle path moves away from the bottom bracket, it increases distance between the chainring and rear cog. This causes the chain to pull on the crankset, causing a counter-rotation that is felt at the pedals. Minimizing pedal kickback across the gear range improves bike control, braking performance, and ride comfort.

Most companies chose a median center of gravity, design their suspension kinematic around that CoG, and use that one kinematic across all frame sizes. This means that anyone who falls outside of that median CoG is getting suspension performance that is fundamentally designed for someone else. In our experience, this has meant that larger frames see reduced pedaling efficiency, while smaller frames see decreased braking performance.

When we are crafting a Proportional Response kinematic, we do extensive research to determine realistic average centers of gravity for riders of each of the intended frame sizes. We then custom-tune the kinematic for each size by varying the pivot locations to achieve the same anti-rise and anti-squat values across the size run. Recognizing that taller riders are also usually heavier than shorter ones, we also adjust the leverage ratio by adjusting links and shock angle to compensate. This size-specific leverage ratio provides riders with the optimal wheel rate and keeps the setup air spring pressures in the ideal range.

The result of all this careful engineering are bikes that feel as though they were made just for you. Bikes that deliver an uncanny blend of pedaling efficiency, supple suspension, and active braking, no matter how big, or small, you are.

Linear Stack and Reach
Stack and reach are two of the most important values in a bike’s geometry when it comes to how a bike fits and feels.

Stack is the vertical measurement from the center of the bottom bracket to the center of the top of the headtube.

Reach is the horizontal measurement from the center of the bottom bracket to the center of the top of the headtube.

Together, they influence how upright or forward leaning your position is, and how your weight is distributed between the wheels.

With Proportional Response Geometry, we strive to deliver a relatively linear relationship between stack and reach as the frame sizes grow. This keeps the feel consistent when moving between sizes, and aids greatly in bike fitting.

Consistent Trail
The Trail measurement is the distance between where a vertical line drawn from the center of the front axle, and a line through the center of the headtube both meet the ground, and it has a big influence over how the bike steers. The more trail a bike has, the more self-centering effect the wheel has, leading to greater stability, but slower handling. A bike with less trail presents the opposite effect – quicker handling, with less stability at speed.

With Proportional Response, we keep our trail values within a consistent range across the sizes, either by keeping the Head Tube Angle and Fork Rake the same, or by varying the Head Tube Angle and Fork Rake proportionally. This ensures consistent handling for riders of all sizes.

Size-Specific Rear Centers
Rear Center is the distance from the center of the bottom bracket to the center of the rear axle, also commonly called Chainstay Length. With Proportional Response Geometry, as our frames get bigger and the front of the bike gets longer, our chainstays grow proportionally longer as well. This ensures that the rider’s weight stays centered between the wheels for better traction and more predictable handling.

In mountain bikes, there is an interesting interplay between Proportional Response Kinematics, and Proportional Response Geometry. The Geometry influences the weight distribution on the bike. Weight distribution influences how the suspension behaves. Customizing them both, simultaneously, allows us to deliver the ultimate “made-just-for-you” performance.

Larger riders are usually heavier and more powerful than smaller riders, so we engineer larger frames to have greater overall stiffness, while maintaining a smooth ride feel. Smaller frames are engineered for smaller riders – less stiff and more compliant, so that each size yields the same performance for the intended rider.

This is achieved through careful manipulation of frame shape and materials.

With our alloy frames we can vary many factors, including tube dimensions, internal butting, and wall thickness, across the sizes to control stiffness and flex.

With carbon, we have even more options. We can vary the tube dimensions – larger cross sections for more stiffness, smaller for less. We can alter the carbon layup – the precise combinations of fiber type, quantity, and orientation – to deliver a near-infinite range of performance attributes. Or we can do both. However we approach it, the end result is frames that feel and ride the same for riders of all sizes.

Conclusion
Proportional Response is certainly not the easiest way to make a bike. Tailoring all these individual elements according to size adds a ton of time and complexity to the creation process. But the result is a better ride for everyone; a bike that fits better, feels better, and performs better. We think it’s worth it. And after a ride or two, we’re pretty sure you will too.