Progressive vs Linear MTB Suspension: Falling/Rising/Mixed Rate
We had a lot of great questions today with the launch of the Yeti SB150 chassis. Riders want to know the difference of Progressive vs Linear MTB Suspension. So let’s take a look at mechanical falling, rising and mixed rate suspension design.
A picture is worth a thousand words – and we have a few of those – but let’s take a 2 1/2 minute quick reference on mechanical suspension design. You can read the details shown in the video below.
Got the concept? It’s ok if you don’t, we’re going to go through the basics. Come back to the video after a quick read…
For this blog let’s look at the suspension rate as a factor solely of the mechanical manipulation of the shock through a linkage.
Essentially in modern MTB we have an oversimplification of the terms “Linear” and “Progressive”.
By definition Linear suspension is a non-progressive coil. Each inch of travel requires a specific addition of pressure. A 250lb spring requires 250lb PER INCH of compression. A linear (or non progressive) spring requires 250 for the first inch, 500 for the second, 750 for the third and so on.
The thing about it is, because of mechanical advantage on a swing arm, there isn’t a “linear” way to apply a load to the spring. As the input angle into the spring varies so does the percentage of effective resistance.
Rising Rate Suspension
Building coil suspended cars, with non-progressive springs, rising rate IS the way to design. For single pivot motorcycles it is a similar thought process. This allows for the best small bump compliance and mechanically challenges bottom out forces. If we were to apply it to MTB it would look like this:
At the full extension of stroke the input angle between the link arm and the shock eye to eye (pink) is an acute angle. As the shock compresses it finally stops at a 90 degree, right, angle. Spring resistance effectiveness is modified depending on the angle between the link arm and the shock. The cosine factor is calculable. For each degree closer to 90 a higher percentage of the spring rate is effectively applied to the suspension.
Falling Rate Suspension
With non-progressive sprints falling rate suspension really can be a problem to tune. Because of the mechanical “disadvantage” of the design standard springs will bottom easily while providing poor small bump compliance. That said, this is how the mechanical layout works.
At full extension the suspension will be at 90 degrees, or a larger obtuse angle. As the suspension cycles the obtuse angle becomes greater lowering the effective spring rate due to the cosine ratio. Without a progressive spring this is problem in almost any type of vehicle.
Mixed Rate Suspension
If you design a car with mixed rate suspension be ready to have one that is nearly impossible to tune with a standard coil. They shift personality as the suspension rolls from one rate and crosses over into the other.
That said, mixed rate, particularly rising then falling, suspension is the modern norm for MTB mechanical setups. This is because almost all modern bikes are designed around air shocks, which are by nature very progressive. Designers work to balance the increasing air spring progression with a decreasing mechanical efficiency to produce confident, fluid, non-“notchy” bikes.
Between the orange and red lines the suspension crosses a cosine efficiency of 1.0. After that right angle it continues obtuse decreasing the mechanical advantage on the spring. This of course correlates with the air spring’s progression pushing back harder and harder per inch of travel.
Designers work to balance shock tune with a variety of rider weights, which effect static PSI – which effects ramp rate percentage, with the mechanical advantage/disadvantage as needed.
Ready to go even deeper? Multi-pivot bikes can control the linkage swing by moving the lower pivot point!
In this image the lower pivot of the Switch Infinity suspension has the capacity to move vertically (an eccentric bearing would work similarly) which is used to control rear wheel track as well as the swing of the shock mount linkage. This can modify how much of a shock’s stroke is used at which point in suspension travel. Lots going on here huh?
Dialing in mechanical advantage, a range of air suspension progression for riders of various sizes as well as controlling the swing rate of the linkage through lower pivot is truly mastering the design.
Makers or Fakers
Dialing in all of the factors is the reason why there are so few MTB suspension systems that truly function as advertised. We work with the best in the business. Ibis, Yeti, Alchemy and Evil all have cutting edge suspension, tuneable for a variety of rider sizes, styles, and ground speeds.
Looking for the best trail personality from Switch Infinity, DW, or Sine? At BikeCo.com we have you covered. Chat with our expert staff about the advantage of working with the acknowledged industry leaders in MTB – from stock setups to personalized Fox Pro Tune Suspension The Bike Company has the expertise to dial you in. BikeCo offers the best spec, setup, tune and pricing in MTB.
Chat online, email CustomerService@BikeCo.com or call 949-470-1099 for more info!