This next section describes RMB’s SMOOTHLINK™ Suspension, a design based on RMB’s ETS patent. Useful links are referenced below. I am not, and have never pretended to be an expert on suspension design so this next section is almost entirely based on materials provided by RMB and from discussions with RMB’s engineers. I’ve decided to include this lengthy, technical exposition because there appears to be interest about the Altitude’s design and some speculation about whether or not RMB has successfully produced a design that does not infringe the Horst Link and ICT patents.
- Horst Link Patent (Horst Leitner – US 5,509,679 – now owned by Specialized Bicycles)
- ICT patent (Tony Ellsworth – US 6,378,885 – owned by Ellsworth)
- ETS patent (Duhane Lam – US 6,843,494 – owned by RMB)
- Some speculation by Feed the Habit is contained in the article (“Horst Link or Not“).
I asked RMB’s engineer to put some thoughts together on this topic and asked for, and received permission to put some technical notes in this article. The engineer’s thoughts are as follows:
The Patent number is US6,843,494. This is the patent for the ETSX bicycle, issued in Jan 2005. In short, the ETSX patent places the lower pivot above the rear axle at all points of travel, whereas the Horst Patent places the lower pivot below the rear axle at all points of travel. It is my assumption that the original Horst patent was written as such because Mr.Horst did not think that a pivot point could be placed above the axle, since the chain is in this position when in Cog 1. With our placement of the lower link, it is now parallel to the tension side of the chain, and thus follows the Instantaneous Center of Rotation (ICR – VPP, whatever you call it) more closely, than the Horst design (which is at an acute angle to the tension side of the chain).
All pictures below are excerpted from RMB’s presentation entitled “Altitude and Vertex Media – NA” and reproduced with permission. Italicized text is from the RMB presentation. My subjective comments about the suspension’s performance will be contained in the section of this review where I discuss the performance of the bike.
Extension of Energy Transfer System (ETS™) patent.
This new suspension configuration is defined by the ETSX patent (US 6,843,494). As well, there are additional patents pending which pertain to the manipulation of the rearward pivot, which utilizes a novel step-down drive-side clevis: it allows the lower drive-side stay member to avoid contact with the chain at all times. The ETS patent stipulates that a line drawn through the main pivot and the rearward clevis pivot is always above the rear axle at all points of travel. See Figure 1.
Tuned linkage follows chain force vector, eliminating pedal bob.
The lower linkage member is almost parallel to the Average Chain Torque Line (ACTL), at all points of travel. This allows the Instantaneous Center of Rotation (ICR) to follow very closely to the ACTL, thus practically eliminating pedal induced suspension bob. See Figure 2. NOTE: the ICR never crosses the ACTL at any point in travel, as this would be considered “tracking” as defined by a competing patent. (US6,378,885)
Instantaneous Centre of Rotation (“ICR”) and Average Chain Torque Line (“ACTL”)
Minimal chaingrowth creates fully active system.
The chaingrowth has been reduced by half (now only 8mm through 140mm of travel) in order to create a more active suspension feel, while still maintaining the traction and forward drive created by the ETS™ effect. It is ideal to have the rear wheel trajectory shape be as closeto a circle as possible, as any deviation from a perfectly circular path will introduce an irregular rate of change between the bottom bracket and the rear axle as the rear wheel compresses. Should the Chain Stay Length (CSL) grow at such an irregular rate, the bike would be more difficult to control when the rear wheel is weighted. See Figure 3.
Flatter suspension rate creates bottomless feel, low air pressure system.
The Altitude has a more level Instantaneous Suspension Rate Curve than its predecessor. This flatter curve, in combination with a 6.5mm longer stroke shock, will allow the suspension a greater degree of movement through the travel than the ETS-X, thus producing a more “bottomless” feel. Also, less air pressure will be required in the Altitude shock absorber than the ETS-X because of the higher initial rate value. See Figure 4.
Caliper mounting position reduces Brake Jack.
By removing the braking forces from the lower linkage, they are isolated and do not act on the suspension system, thereby greatly reducing “brake jack”, or the stiffening of the suspension under braking. The slight change in the angle between the caliper and the lower link creates a “floating disk brake” effect.