Review: Trek Session 9.9 29


bigquotes The Session 29 is an ultra-light, ultra-stiff machine that's unforgiving at times and best suited to strong or heavy pilots who are precise and highly skilled. Paul Aston


Construction and Features
The Session 9.9's lightweight OCLV carbon frame (claimed 13.6 lb frame and fork) has exceptional finish, with simple, clean lines. It uses a tapered head tube that leads into a huge downtube which arrives at a BB83 threaded bottom bracket. Trek's ABP 'Active Braking Pivot' rotates around the rear axle and uses Trek's own bolt-thru axle to secure the wheel, but also protrudes a ways out from the frame.As with all carbon downhill bikes, especially one this light, downtube protection is a must, along with chainslap guards and fork bump stops, and the Session 9.9 has all of these zones covered.Cable routing is internal through the front triangle and is clamped at the exit port on top of the lower portion of the downtube. From here the brake hose stays on the outside all the way to the caliper, where the gear cable housing runs through the chainstay, which protects it from chainslap. Riders who frequently ride in muddy conditions will be glad to see that the carbon frame is almost entirely free of places for dirt or water to collect, which makes it fast and simple to clean and wipe down.

Geometry & Sizing
Trek offers the Session in four sizes from S-XL in the 29" chassis, with reach numbers spanning 396mm to 461mm. This makes the small-sized frame one of the smallest out there, great for riders who need a small bike, although there might not be much clearance between backsides and that big rear wheel in the steeps. It's on the short side compared to some other new DH bikes out there like the GT Fury (size L is 470mm) and Commencal Supreme DH 29 (XL is 495mm), but similar to the Aurum HSP (L/LX is 461mm) and the current Specialized Demo 8 (XL is 460mm).With the Mino-Link in the Low position the bottom bracket drop is low at -23mm, and the head angle is raked out at 62.1º on paper, and with the front end set at the height I liked, it was under 61º with my angle reader. Flipping the Mino-Link will raise the BB by 10mm, and steepen the head angle by 0.5º, which Trek say has no effect of the suspension setup. The chainstay is fairly long at 453mm, and the headtube length is the same across all sizes at 115mm.
Suspension Design Trek have well and truly applied the 'looks like a Session' concept to the four-bar link Session 9.9. The main pivot is slightly below the top of the 36t chainring, and slightly behind the bottom bracket. The ABP layout rotates around the rear axle and connects the chain and seatstays. The seat stays drive a huge one piece, magnesium, 'EVO Link' rocker that engages the shock. The upper shock mount is now a trunnion mounted metric shock, and bearings are used in the rocker link here, which should remove any problems with friction caused from upper shock bushing rotation often found on this type of design.Trek would only provide a graph of the leverage ratio which they describe as "smoothly progressive leverage rate" which means the progression increases, but in a near-straight line. They stated that they don't take those graphs as gospel and explained why in the following way:

From Trek:

Anti-rise is something we look at, but really we tune that through the ABP “rotation factor.” In short, we’re looking to minimize the amount of the rotation the tire contact patch would have with the ground as the suspension is cycled from full extension to full compression. This can be graphed as Anti-Rise, but the amount of force that can actually be created due to friction between a tire and varying trail surfaces and conditions actually becomes quite small. So we simply look at minimizing that rotation to create the active feel under braking our bikes are known for.

Pedal kick-back is also not something we really look at anymore. That was something we looked at in the early days of MTB full suspension as a way to compare axle paths and judge how much one bike would pedal compared to another. In those early days, pivot placement was all over the place, and really what we were feeling with “high kickback” bikes were generally bikes with really high pivot placements. These high pivot placement bikes had such Anti-Squat numbers that the bike would rise up / extend the suspension with each pedal stroke, then sag back down as our human power delivery systems (ie, legs) varied between high and low output cycles. If you compare all the really good performing bikes today in terms of axle path, chainstay length growth, and anti-squat numbers, they’re all going to be really close. Whether a single pivot or multi-link system, they’re going to move and behave very similarly.

Anti-Squat is also not something we look at in terms of providing XX% of Anti Squat. This simply isn’t something you can really design for on a mountain bike. Yes, it can be calculated for a given bike size with a presumed center of gravity. But on a bicycle, this center of gravity moves around all over the place. It’s different for seated pedaling vs standing pedaling vs pedaling up-hill vs on the flats. A-Mar and I are about the same height, but have way different seat post heights. Chris J and I weigh about the same, but because he always skips leg day, our mass is in different spots. What we do is simply look at anti-squat as a scalar number and compare one bike to another. We then move around the main pivot to create more or less anti-squat or a steeper or shallower AS curve based on how we want to change a bikes performance from one year to the next or one usage case (XC Race vs Enduro for example).

Leverage Ratio Graphs. Ah yes! This is really where the magic happens that can be quantified and is the most important factor in determining a bikes performance. Again, comparisons to previous bikes and what sort of changes are wanted are important and need to be compared. But this also has to be viewed in context of how the shock is tuned in both damper and spring qualities. These are tied together and are the first things we work on when working on new bikes. They can get a bit messy… as can be seen on this one!


At the time of ordering, Trek only had a frame with Fox shock and fork option, but they provided a complete build to test which was covered with Saint drivetrain, Bontrager components, and DT Swiss wheels wrapped in Maxxis rubber. Recently, they announced the 2019 complete bike which has a similar build with Bontrager wheels and drops in at $8,399 USD.

Riding The bike is well proportioned, and the XL size was a comfortable fit on steeper, tighter tracks, although it felt slightly short for me in some places. The 461mm reach isn't huge, but being a 29er it still sizes up more spaciously than smaller wheeled downhill bikes due to the higher stack height. The 453mm long chainstay had a good balance with the 461mm reach, and as long as the trail had a decent pitch to it I felt centered and stable in the middle of the bike. When doing back to back runs with the other 29er DH bikes I've been testing on the same trail, the Session felt less planted at higher speeds, especially on straight lines when you just want to let go of the brakes, hold on, and charge through. In these sections it was also harder to keep my feet in place on flat pedals. The reach may have been a factor here, but I believe that being the lightest 29" downhill bike I have tested this year also contributed to feeling less planted.As with any bike that has a good balance front and rear, and equally adjusted suspension, the Trek got airborne easily with no surprises. Coming in to really heavy landings the bike took the big hits without question. If you went too far though, the Session 9.9 did have a fairly harsh bottom out—the bike now comes with the 2019 version of the Float X2 shock, which has a much bigger bottom out bumper and should help avoid that. The combination of the bike's light weight, suspension design, and massive stiffness makes the Session pedal and accelerate very well. It also rails fast, smooth berms exceptionally well, and it's easy to see why some riders have had success on the World Cup circuit with this bike under them.That said, heading into the rough stuff I found the Session noticeably, well, harsh. Tracking across off-camber sections and carving flat corners didn't inspire confidence - the rear wheel had a tendency to lose traction when the bike was leaned over. Despite the 'Active Braking Pivot' design keeping the rear triangle active under braking, I found the braking traction lacking, with the bike skittering into rough corners when I was most looking for grip. The harshness I experienced may have been affected by settling on slightly less pressure than recommended by Trek, with the theory being that I could have been running deeper in the travel than necessary. That said, for the terrain I tested on I generally prefer a softer setup and the Session was set up similarly to the other bikes I tested. Alternatively, a coil shock could be the ticket here, which could improve the sensitivity and tracking of the back end of the bike.

Similar to what I experienced with the Norco Aurum, there was also some vibration from the fork in the lower setting when riding mellow bike park trails with small stutter bumps. I'd put this down to the slack head angle and the 29" fork being long and riding high in the travel on flat terrain. I generally set the bike to tackle steeper terrain where this issue didn't appear, but flipping the Mino-Link to the high position helped, and if I was riding less steep trails more often I would be lower the front end of the bike through the crowns and raise the stem or handlebar to keep a similar riding position. I tried this in the Whistler Bike Park, where the trails aren't quite as steep, and it did end the vibration issue, but then I was in need of a higher rise bar or direct mount stem spacers to maintain my preferred bar height.

What about compared to the Session 27.5?Here's what Mike Kazimer had to say: "I tested the two bikes back to back over 2 days in California with LITPro GPS timing data. The 29er felt calmer and smoother than the 27.5, and the straight line speed of the was impressive. The data showed that the Session 29 was faster than the 27.5” bike – to the tune of nearly 5 seconds on a 2:20 course. On both bikes, my times improved on each lap as I got more familiar with the course, but when I switched back to the 27.5” bike from the 29er my times slowed down again, illustrating that the bigger wheels made a significant difference. There was also less need to make little micro-corrections to the steering. Looking at the data confirmed this sensation — the 29er allowed me to brake less, which in turn led to faster cornering speeds.

After two solid days of riding, I was firmly convinced that 29" DH bikes have massive potential, and not just for elite riders." (Read Mike's full 27.5 vs 29 story here)

Technical Report

Shimano Saint groupset: Not having an update or refresh for six years hasn't held the Saint products back. They are still solid, and the brakes are still some of the most powerful and dependable out there. My only gripe with any of the parts is the slight rattle from the finned IceTec brake pads on small bumps.

190mm travel: 200mm of travel or more is 'standard' for downhill bikes (the 27.5" Session has 210mm), and there seems to be the notion that 29ers don't need to have as much travel as their smaller wheeled counterparts. I'm not convinced - I want to have big wheels and all the travel, especially on a downhill bike which spends the majority of its life heading straight into the rough. It's fairly easy to bump up the travel of the Fox 49 with a change of air spring, but more difficult to get any more from the rear.

Hub spacing: I'm convinced that the maximum width of the back of a downhill bike should be as narrow as possible; after all, they are often threading the needle through rocks, stumps and ruts at high speeds. The Session with its ABP pivot and axle is one of the widest on the market.

Bontrager 35mm LinePro handlebar and grips: I rarely get sore hands from riding, but this combination of a super stiff bar and slim lock-on grips beat me up. A quick change to some alloy Renthal FatBar's and Push On grips helped.

Chainslap: Even with the chainslap guards the chain still made a racket, and the front triangle of the bike seemed to echo sounds. Extra textured rubber tape or Velcro is recommended to dampen the noise.

Is this the bike for you? If you are looking for an exceptionally finished, lightweight bike that has an impeccable racing pedigree, the Session might be for you. It rails bike park corners, feels great in the air, and pedals like mad. But, if you want to challenge the roughest and gnarliest downhill tracks, you'd better be strong, accurate, and on point.
Pinkbike's Take
bigquotes The Session 9.9 29 is an ultra-light, ultra-stiff machine that's unforgiving at times and best suited to strong or heavy pilots who are precise and highly skilled. Yes, it's a big slack downhill machine with quality damping, grippy tires, and powerful components, and it's clearly capable of winning races at the highest level. But, when taken head to head against some of the other fantastic 29" downhill bikes that have been released this year, I found it to be more challenging to handle through the rough and tumble. Paul Aston

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