“The primary design consideration for suspension systems is to design
the system so that it responds to bumps but does not respond to rider
induced forces. If the suspension responds to the rider's forces, it may
be absorbing valuable energy which could otherwise be helping to
propel the rider and bike faster.”
Since the conception of rear suspension there have been may different designs to provide just that, the best ride quality.
In order for the rear suspension to be as active as possible without effecting ride quality the rear axel during its travel needs to move at a right angle to the chain force acting upon it. If that requirement isn’t fulfilled there are three different things that can happen: pedal bob, bio-pacing, and brake jack. Pedal bob is when pedaling activates the suspension, causing the bike to bounce up and down due to lengthening and shortening the drive train. Bio-pacing is a feeling of inconsistent cadence while going over bumps, caused by the activation of the suspension. Brake jack is when the suspension locks up due to braking forces.
There are six common suspension designs that bike companies use to try and solve the aforementioned suspension problems and they are: the soft tail, single/mono pivot, unified rear triangle (URT), 4-bar, Future Shock Rear (FSR), and Virtual Pivot Point (VPP).
The soft tail suspension design relies on the elasticity of the ear triangle and a rear shock or elastomer that is in line with the seat stays. Soft tails are technically a 3 bar link system but have no moving parts aside from the shock or elastomer. The ride feels very similar to a hardtail however the limitation of the soft tail design is that it only allows for about 25mm of travel.
The single/mono pivot design uses a single swing-arm/solid chainstay that connects the rear axel to the bottom bracket area, which allows for a perfect arc axel path around the pivot point. It’s a simple design which handles small bumps well however the design is subject to pedal bob, brake jack and chain growth.
The URT design is just that, the rear triangle of the bike is one solid piece and the pivot it is placed between the front and rear triangle which allows the rear axel and bottom bracket to move in relation to the front triangle. The URT design is related to the floating drivetrain design in which the bottom bracket is mounted to the swingarm. The URT design is now considered outdated however the floating drivetrain design is still in used today. Both designs have trouble with pedal bob, but on the bright side do not suffer chain growth.
4-bar linkage systems are one of the two most common linkage designs. The 4-bar design has the rear axel mounted on a floating rear link and uses several other link points to activate the shock. The 4-bar system has a pivot behind the bottom bracket, infront of the rear wheel and one at the top of the seat stays. The 4-bar design allows for large amounts of travel and has a similar axel path to the single pivot design. Most importantly the 4-bar system almost entirely eliminates pedal bob as well as brake jack.
There is one variation on the 4-bar linkage, which was developed by Specialized Bicycle Company in partnership with Horst Leitherm, the FSR. The design is very similar to the 4-bar design the only difference is the “Horst link” which mounts the shock to the seat stays. The addition of this link adds stiffness to the rear triangle while also improving the axel path keeping it more concentric with the bottom bracket.
The next common linkage design is VPP, designed by Santa Cruz Bikes to activate the suspension based on the input it get from the pedals and brakes. The VPP like FSR is a type of 4-bar suspension and behaves the same with minimal pedal bob and brake jack.
Now that I’ve introduced the different suspension designs I’m going to finish up by talking about setting up suspension on a bicycle. There are three factors on any suspension setup that affect the shock itself: spring and sag, rebound damping, and compression damping.
Shocks are “sprung” using either air or coil springs, the latter comes in different spring rates and materials. A heavier rider, when setting up their suspension, should use a coil spring with a higher spring rate or, if they are using an air spring should use more air in their air shock. Selecting a spring rate or volume of air can be daunting however there is a simple way to test which to get. Depending on the style of riding you intend on doing when sitting on the bike the suspension should activate, or sag, 10-33% (10-25% for cross-country applications 33% for downhill). Too much sag makes the ride lose travel needed for bump control, too little sag give the ride feel jerky, and similarly makes the bike lose traction.
Rebound dampening, which controls the rate at which the suspension system relaxes to its “natural” position, is done in one of two ways in suspension systems, air dampening and oil dampening systems. Air dampening systems thought lighter, are more difficult to dial in correctly because the air tends to expand and contract within the shock, thus increasing or decreasing the dampening force on the shock. Oil dampening systems work the same way as air dampening systems but instead of pushing air through a piston push oil though a piston. The added benefit of oil dampening systems is that oil comes in different viscosities and lubricates so one can really carefully dial in their rebound dampening. To set rebound dampening ride over a bump that sends a lot of kick-back though the bars and increase dampening until the kick-back feels manageable. Too much dampening will make the suspension feel inactive, while too much dampening will make the bike buck and kick back when hitting rough terrain.
Compression dampening works the same way as rebound dampening, with both air and oil dampening systems, however does the opposite. Compression dampening stops the suspension system from bottoming out. Too much compression dampening makes the bike feel harsh on the trail and does not allow the bike to use its full travel while too little compression dampening makes the suspension feel soft and can damage the suspension by bottoming out. To set compression dampening ride and adjust the dampening system so the suspension is fully active without bottoming out.
No comments:
Post a Comment