More Trek Session 88 Details of its Evolution

Jun 9, 2008

Prior to Sea Otter this year Trek announced that they would be doing a media launch of the Session 88 in California. We were there and you all enjoyed the coverage from it.

Here are some more evolutionary break downs of how the Session 88 came to be and what all the technical facts mean.

“Our goal with developing a new Freeride and DH bike is in line with Trek’s guiding principle to have the best in class product for every category we pursue,” said Joe Vadeboncouer, Product Manager for Trek. “The new Session products are my personal favorites of all the full suspension bikes we build at Trek.”

The development focus of the Session design team was three-fold: create a new platform for gravity application that capitalizes on all of Trek’s recent suspension technology improvements, optimize geometry and reduce overall bike weight.

The testing for the new Session platform took place over the past three years, as a staff of engineers, designers and test riders traveled the globe to find the most diverse conditions under which to test the platform. “The bike’s development reaches far into Trek’s history. In a sense, we’ve been waiting for this moment for a very long time,” said Senior R&D Suspension Engineer Dylan Howes.

[PI=2110007 size=m align=c]First Prototype[/PI]

Considered the innovator of Trek’s ground breaking new patent-pending technology, Active Braking Pivot (ABP), Dylan Howes found himself and his development team in a precarious spot when an 8” version of the Session was approved for production two years ago. “We really had something with the original Session 8. The geometry was dialed, we were spellbound with the handling, and the weight was acceptable. But we knew from the Fuel EX and Remedy development that new technology would surpass what we were developing. So we stopped production and went back to a clean sheet of paper.”

[PI=2110006 size=m align=c]Second Prototype[/PI]

The team went back to the drawing board. Inspired by the “best in class” principle, Trek engineers and designers evaluated promising new technologies and embarked on a journey that would take two years to complete. The new and improved version of the Session platform would come to be known as the Session 88.

The Session 88 platform features all of Trek’s recent full suspension development efforts including the one piece of technology responsible for creating the most active suspension system under braking, ABP. The Session 88 platform also combines Trek’s Full Floater shock mount with a custom-tuned Fox DHX 5 coil shock, delivering the small bump compliance, good mid-stroke control and bottomless feel associated with all of Trek’s full suspension offerings.

[PI=2110011 size=m align=c]Third Prototype[/PI]

Stiffness and strength, balanced with market-leading frame weight, were a major concern for the Trek team. To that extent, Trek's industrial designers employed hydroforming and extensive frame shaping to achieve project goals. Beyond its structural elements, designers styled the Session frame to exude a visual aggressiveness intended to inspire confidence in the rider before he or she even sits on the bike. Also of note in the frame construction is the one-piece EVO rocker link and E2 tapered steerer/head tube technology, both of which make a powerful visual statement.

The two distinct bikes feature category-specific parts builds for both Freeride and Downhill, as well as graphics inspired by the environments in which the bike will be used. “Our engineers did a great job creating a bike that’s inspiring to ride. On the styling side, our goal was to use every component to create something that’s equally inspiring to look at,” said Trek’s Graphic Designer Chad Bailey, who orchestrated everything from the gold anodized frame hardware to the custom Session-branded chainstay guard. “I think we pulled it off, but we’ll let the riders be the judge of that.”

[PI=2110009 size=m align=c]Fourth and Final Prototype[/PI]

With the frame construction and aesthetics dialed, the performance of the Session 88 platform is remarkable as well. “The most exciting thing about the Session 88 is that there is not a single bike that can match its overall performance,” said Jose Gonzalez, Director of Full Suspension Technology. “Another platform might offer a similar weight, or geometry, or braking performance, but nothing else out there can match the overall package. This truly is a bike that leapfrogs all existing bikes.”

A key member of the product development team is Trek sponsored athlete and Freeride legend Andrew Shandro. “There’s a trail at Whistler called Dirt Merchant. I know that trail front and back, but on the Session, I completely overshot the jump,” he said, citing his first test ride on the final version. “The suspension technology, the platform’s efficiency, and the bike’s overall weight allowed me to carry so much more speed than I ever thought possible. It has all the qualities you want out of a performance gravity mountain bike.”


Regarding suspension performance, Trek takes considerable time and effort to fine tune its suspension componentry. “In addition to the bike’s features, the amount of testing and development we did with Fox on the suspension parts insures that no bike available will work anywhere near as well as ours, even if they also use Fox parts,” said Vadeboncouer, noting the extensive development work performed with Trek’s primary suspension partner, Fox Racing Shox.

The Watsonville-based suspension manufacturer supplies its popular DHX 5 coil rear shock, but with a special custom tune specific to the Session platform. “During development, we had prepared several different shock tunes to perform back to back blind taste tests on repeatable terrain across a broad cross section of riders. The resulting feedback on shock tunes and bike geometry ultimately led to the production version of the Session 88,” said Fox’s Mario Galasso. “The Trek/FOX relationship has developed into a mutually beneficial one and we look forward to being included at this level of the product development process in the future.”

When riders finally get on the new Session 88, they’ll find that it’s a bike by which all others will be judged.

Views:9573    Faves:6    Comments:2

The Session 88 DH and Session 88 FR will be available in three sizes: small, medium and large, and are slated for June production. They will be released to U.S. and International markets mid-summer. Final pricing has yet to be established.

---

Read on for information on the Session Shock tuning and Leverage Ratio's

---

Session 88 Rear Suspension Tuning
Foreword
The new Session 88 platform was designed with a completely different philosophy than the Session 10. One of the primary differences was basing the design around a shim valve shock that offers a position sensitive damper effect (Fox DHX shock with Boost Valve). We also took into consideration that we were working with 8” of travel instead of 10”, yet wanted to maintain a similar overall feel and performance as the Session 10 – more with less. Since the platform was developed around a shim valve, any other shim valve style shock can be used. However, the Fox DHX is the only shock that offers the combination of shim valve for small to medium bump performance with a position sensitive Boost Valve for large bump performance. As you will appreciate after reviewing the following graphs and information, any other shock than the custom tune Session 88 spec DHX 5.0 will force the rider to lose the optimally tuned Trek settings, which were derived from countless hours of exhaustive testing to perfectly blend shock performance with the Session 88 Full Floater system.

Although the standard FOX DHX 5.0 Coil is a high performance, highly sophisticated shock that works extremely well in standard form, it’s only possible to achieve the top levels of performance by tuning the shock to a very specific leverage ratio and rate. This requires a certain level of co-development with very clear goals, knowledge and suspension expertise from both parties and exhaustive field testing to accomplish.

The FOX DHX 5.0 Coil found on the Session 88 uses Fox’s latest main piston and coupler bolt designs which offer a superior compression and rebound feel over previous DHX Coils. Additionally, the shock is tuned with a custom dual stage compression shim stack that increases velocity sensitivity after the valves opens a specific amount. Trek has taken great lengths to tune the rebound stack in order to liven up high frequency response, while maintaining great chassis control and end stroke control (extension from the last 1/3 of travel). We have also preset the Boost Valve pressure and volume to offer the ideal blend of position sensitivity and bottom out control with no influence prior to mid-stroke.

All the following graphs and comments depicts the sole effects of the damper and do not show the effect that the coil spring adds to the system. The coil spring will add a linear increase in compression resistance that is also position sensitive. Due to the altering leverage rate of the rear suspension system, the spring force at the rear wheel (wheel rate) varies depending on where the bike is in the travel stroke. This allows for decreasing & increasing wheel rate effects that are solely based on where the bike is in the rear wheel travel. However, this changing wheel rate also affects damping, as it cycles the shock with varying amplitude and velocities depending on where it is in the travel. The quicker and greater the compression of the shock damper, the more damping effect that’s produced.

[PI=2110065 size=m align=c]Boost Valve Volume[/PI]

The Boost Valve found in the DHX shock is a pressure and position sensitive valve that controls the fluid transfer from the main damper body to the reservoir. It characteristics are completely dependent on the internal pressures created by the gas charge in the reservoir. Since an air spring is progressive as it compresses, the pressures within the damper increase according to how much the shock is compressed. Due to the Boost Valve functioning off this internal pressure, the compression damping becomes progressive as well.

The above graph shows the effect of changing the volume of the Boost Valve cavity. Since it is a position sensitive feature, the Boost Valve volume has virtually no effect in the initial 40% of shock travel. We can also see that the changes in actual force from one extreme to the other are substantial enough to be noticeable, offering nearly 60lbs of difference in force between the largest and smallest volumes. We found the best setting is with Boost Volume set in the middle, which gives us the bottom out effect we were looking for when combined with Boost Valve pressure, the two-stage shim stack, and the leverage rate of the Session 88. Ideally, we want to slow the shock gradually without harshness. Not relying heavily on a bottom out feature will give better use of the overall travel.

As a bonus, having some external adjustment also allows for individual tuning of bottom out feature to accommodate varying trail or course conditions without altering the overall compression characteristics. This is an extremely valuable feature for freeride and downhill bikes.

[PI=2110066 size=m align=c]Boost Valve Pressure[/PI]

The graph shows the position sensitive Boost Valve feature at a relatively low shaft velocity of 5 in/sec. This low shaft velocity means that the damping forces are primarily a result of the Boost valve; these shafts speeds are too low to get much of an effect from the two-stage shim stack. The graph reflects that pressures above 170-175psi start to cause compression damping effects before the 40-45% mark of the shock stroke. The 170-175psi Boost Valve pressure optimizes the balance between bottom out control and minimizing influence on the initial 40% of the rear wheel travel. This becomes even more important when we introduce higher shafts speeds as we will see in the next graph.

[PI=2110064 size=m align=c]Combined effect of Boost Valve Pressure with Dual Stage shim stack[/PI]

This graph shows the combined effect of velocity sensitivity from the dual stage shim stack and position sensitivity from by the Boost Valve feature. Running the shock at higher shaft speeds of 25in/sec produces substantially higher compression resistance (force produced) due to the influence of the Dual Stage shim stack. The Dual Stage shim stack comes into play once the shock is able to accelerate and increase velocity, which requires certain amount of shock movement to accelerate and see a noticeable increase in force. Focusing on the 170-175psi Boost valve pressure, the combined effect of position and velocity can be better appreciated with the following comparison:


Because of this combined effect of Boost Valve and Dual Stage shim stack, we get a highly responsive and controlled rear suspension that offers a very supple ride on smaller bumps such as braking bumps and roots, but at the same time handles big drops and jumps with a smooth bottomless feel.


Field testing with a standard DHX 5 revealed that we needed a greater level of control on compression at higher shaft velocities found in big hit situations. In order to achieve this we moved to a dual stage shim stack on the compression side of the piston. As shown in the above diagram, the two stacks of shims are separated. This allows the first set of shims to give a supple response in lower shaft velocity terrain or in small shock amplitudes. When shaft velocities increase due to larger hits, the relatively pliable first stage of shims contacts the second stage of shims causing the system to firm up giving the rider more control in the middle to end of the shock stroke.


The graph shows the effects of shock shaft velocity on the damper. It reflects the increased damper effect as velocity increase due to the dual stage shim stack. The enhanced speed sensitivity of the dual stage shim stack doesn’t come into play until a certain size bump or obstacle, meaning that we can achieve enhanced control without affecting suppleness. The first stage shim stack must be activated and deflect a specific amount prior to coming in contact with the second stage and increasing the damping rate.

We can see this clearly on the force vs. velocity graph above when comparing the standard single stage shim stack to the Session 88 dual stage shim stage. At 5 and 15 in/sec, the two act very similarly and produce similar force. But at 25 in/sec there’s a noticeable difference between the two. At 30 in/sec the difference is substantial. This combined with the Boost Valve feature create a very sophisticated and dynamic damper that is ideal for aggressive descending on highly technical terrain.

[PI=2110061 size=m align=c]Rebound Tuning[/PI]

This graph reflects the rebound performance enhancements we achieved by combining the new piston coupler and new main piston with careful tuning of the rebound valve. The objectives of rebound tuning were to:
1) Reduce mid to high speed rebound effect of the standard shock, which had limited our response time to successive hits.
2) Achieve this reduced mid-high speed rebound effect while maintaining similar low speed rebound characteristics as the standard shock, which controls rider input and enhances chassis stability well.

Achieving these would enhance bump responses, allowing the shock to better track the ground, particularly in high frequency response situations, such as in rock gardens, roots and braking bumps. At the same time, we balance this quicker response time with good end stroke control to control shock extension from full bottom out (maximum spring force pushing back).

The standard rebound tune we tested required us to run the rebound adjustment knob close to full open on the scale of adjustment in order to get the quick end stroke response we were after. This offered the mid velocity frequency response we were looking for but made the low velocity response too quick and still did not lighten the high velocity response enough. In other words, the shock rebounded too quick at low velocity and not fast enough at high velocity. After testing numerous tunes and versions, we locked in on the setting shown in the graph.

The graph above reflects rebound forces in the same rebound adjustment knob setting of 8 clicks out from full close (12 clicks out is full open). The Session 88 rebound tune produces similar low velocity response as the standard tune shock at 5 in/sec, and has just slightly less rebound force at 10 in/sec than the standard shock. However, as the velocity increases, the custom tune shock has significantly less rebound effect, producing 200lbs less rebound force at 20 in/sec, and where the standard shock maxed out our dyno load meter when cycling at 30 in/sec (due to going well over 1,200lbs of rebound force) the custom tune produced 1,000lbs of rebound force. This amount of high velocity rebound force still offers enough end stroke control, when the spring is extending the shock with maximum force, but offers improved terrain tracking at mid velocities due to its quicker response.

[PI=2110067 size=m align=c]Blending Shock Tuning with Instantaneous Leverage Ratio (ILR)
[/PI]

Although this graph over-simplifies the complex and dynamic function of the damper, it gives a good picture of what is taking place, and offers a better understanding and appreciation for what Trek’s engineers were after. The graph reflects how the different features, technologies and tuning blend to achieve a very sophisticated feel and the highest performance levels. Additionally, it gives some insight to the relation of the shock tuning to the ILR.

By looking at this graph, we can get an appreciation for the influence of the compression damping features through the entire rear wheel motion. It’s clear that the two most influential elements are the Dual Stage shim stack and the Boost Valve pressure. These two technologies control over 80% of the shock compression damping characteristics and is the reason why Trek’s suspension R&D group and FOX focused on dialing in the combination of these two.

[PI=2110062 size=m align=c]Leverage Ratio & Rate[/PI]

Trek focuses on the Instantaneous Leverage Ratio (Or ILR) when developing rear suspension geometry. This gives an excellent and very precise view of what the rider feels - the combination of wheel rate and damper effect.

To truly appreciate the Full Floater ILR found on the Session 88, we first need a little history lesson…The Session 10 was designed around a SPV (Stable Platform Valve) shock. Along with the platform effect, this shock technology is a position sensitive damper that progressively creates more compression damping as the shock is compressed. Because of that, the Session 10 was a completely linear rate design that relied on the SPV damper’s position sensitive progression for a rising rate effect. Due to the amount of travel on the Session 10, we also wanted to keep the spring on a linear rate to allow for better use of travel. We relied solely on the coil spring’s natural linear spring rate increase for best feel and use of 10” of travel.

When comparing the Session 88 to the Session 77, we flattened the rate and reduced the progression, which offers improved “push through” the mid-stroke with velocity sensitive dampers. Additionally, we added a slight regressive effect after mid-stroke in consideration of the compression damping enhancements offered by the 2nd stage of the Dual Stage shim stack and the Boots Valve feature. Again, this allows for better “push through” after mid-stroke and allows for consistent use of the available travel. This offers that bottomless feel and sensation of more than 8” of travel.

The advent of Full Floater technology has created more tuning options for Trek’s suspension engineers to create a moderately progressive (or rising rate) rate up to mid-stroke, which transitions into a slight falling rate in the last 1/3 of the travel. This rate is specifically designed to take advantage of the velocity sensitive shim stack and the position sensitive Boost Valve found in the DHX Coil, yielding a ride that has enhanced small bump compliance, great control through the mid-stroke and a predictable and stable feel at speed. But at the same time, it offers a ride with a seemingly bottomless feel and the perception that there’s more travel than the available 8”. When coupled with the Dual Stage shim stack and Boost Valve technology, the slight regressive rate after mid-stroke allows the bike to effectively use the 8” of travel in a very controlled and bottomless manner.

www.trekbikes.com