Ultimate Guide: Metric Shock Sizing

The Advent of Metric-Based Shock Absorption Systems

Recently, several bicycle component manufacturers including Cane Creek, DVO, Manitou, RockShox, SR Suntour, and X-Fusion announced that for 2017, they would be introducing shocks and frames in lengths based on the metric system. In addition, Fox stated that it would continue to produce both Imperial and metric-based shocks dependent on market demands. Currently, most shocks are based on the Imperial system.

Currently, USA trails the rest of the world in terms of fully implementing the metric system. A system that is very logical as it’s based on multiples of ten, which makes many calculations possible without use of a calculator. This system will allow frame builders and shock designers to manufacture products using a straightforward measurement system that is easy to comprehend and implement.

Historically, shock designers created products in response to rider demands, CAD modeling, and test riding on trails. Sounds logical, yet this reactive process quickly deteriorated into a free-for-all as diverse shocks were created in response to many disparate riders’ inputs, which generated a wealth of products without a logical basis for progression in stroke length or shock dimensions. Shock engineers designed their products around the frames. Consequently, there was little standardization within the industry. This problem was magnified by the end-result wherein the imperial-based system yielded a bewildering number of shock options, 80+ options, versus the new metric system that offers 18 choices with stroke based on 5mm increments and eye-to-eye measurements in 20mm increments; a very simple and logical system.

*Eye-to-Eye refers to rear shock measurement and is the distance, center-to-center, between the two bolts used to connect the shock to the frame.

^Stroke (or stroke length) refers to the distance the shock compresses under load. A lever ratio is often stated for a shock and represents rear wheel compression: shock compression (stroke) for a given length. For example, if the ratio is 2:1, then for every 1mm in shock compression, the rear wheel compression is 2mm. But having a greater stroke is not always desirable for a specific frame because rear wheel rub against the seat tube could occur or the shock’s connecting linkages to the frame might bottom-out. Shock and frames must be designed to function in unison.

^^Trunnion mounts: can be comprised of two separate metal protrusions that are used to mount something, which in turn will be limited to one plane of movement, perpendicular to the axis of attachment for the trunnion. Additionally, trunnions can be modified to limit their range of motion. Since they are generally integrated into the design of whatever they are supporting, they can increase the supported object’s strength and stability.

Looking Ahead at Metric Shock Sizing:

With an easily accessible metric-based system, designers will be able to create eye to eye, shock stroke, part tolerances, and frame geometries from XS to XXL and for small thru large ranges in bike travel in a progressive, logical framework that will render the conjoint efforts from all related design fields easier to implement, thus avoiding confusion resultant from a free-for-all approach to bike building and using the fractions or decimal-containing numbers used to convey feet and inches in the Imperial system.

What’s Happening Now:

RockShox announced the release of two new air shocks named Super Deluxe and Deluxe, which they claim address numerous issues whose solutions result in better shock performance. Several key points arose from their statements:

• The new metric shocks allow for increased bushing overlap (Bushings are situated between the shaft and the external housing and could be viewed as tubes surrounding the shaft, which keeps the shaft centered as the shaft moves through it). This overlap indicates the distance separating two bushings and the larger the gap, the less binding occurs under lateral frame flexion, which improves rear tire traction and durability. This results in reduced wear-and-tear on the components and reduced oil leakage.
• Larger shocks can contain increased air volumes resulting in an increase in suppleness from smooth air flow within the chamber.
• Having fewer choices in sizing means greater ease in locating replacement parts or in making upgrades to existing components.
• Trunnion incorporation in the eyelets will allow engineers to design reduced eye-to-eye lengths in relation to the shock’s stroke length, which will reduce standover height
• Simplified adjustments of air flow via red tokens (volume spacers)
• Reduced friction due to placement of bearings in the shock mounts

However, the following issues arise:

• Lack of compatibility with most existing frames
• Increased costs, initially, to manufacturers that must re-design expensive carbon molds
• Larger sizes and increased weight
• It won’t improve every shock or type of suspension system
• Supply and demand rules in a small market, so if you buy a frame with this new system, it could become difficult to obtain replacement parts down the road if this system becomes obsolete

The Dust Settles:

What’s clear is that it’s not metric standards that produce better shocks. It’s full usage of existing technologies and how they are integrated to the purpose at hand. Also, please note that new hype is unceasingly generated by marketing gurus every year, with each company’s product(s) superior to the competition. Foreign-sounding words/phrases from engineering jargon are thrown around without adequate understanding on both sides of the table; designer versus consumer. If you’ve got a great working shock, it doesn’t suddenly convert to crap just because some marketer says otherwise. Remember, in America “Marketing is King”, and like mantras or political slogans; if you repeat it frequently, people will believe it.

However, a metric-based system is far more logical and easy to work with versus the outdated Imperial system that grips common folk in the USA. -AND- developing suspension systems and frames incrementally and progressively, versus haphazardly, will make life easier for all contributors to the finished mountain bike. So, cry “YES” to the embrace of metrics, let the imperial measurements rest-in-peace, and cork the bottles filled with hype for another day.