Yamaha FZ-09: Improving the suspension

Stoltec-suspension-top

We recently reviewed the 2014 Yamaha FZ-09 after spending a few months with it. I was very excited about the bike at its launch, and continue to think it’s one of the most important bikes released in the last year. Unfortunately, I also felt it had a number of problems that rendered it basically unrideable — the biggest being the suspension.

Enter Nick Stolten, CEO of Stoltec Moto out of Pennsylvania. I originally came across his work on a great forum called fz09.org, and I reached out to see if he would be interested in helping me get our FZ-09's suspension sorted. The FZ-09 has an unbelievable amount of potential, but I was going to need help to figure it all out. Nick was kind enough to respond to my e-mail and, as it turns out, is based not too far from the RevZilla headquarters, where I planned to visit the week after we connected.

Long story short, I got to ride his modified FZ-09... and it’s awesome. Like, really awesome. It’s both everything this bike could have been and should have been and I want to buy one now, but that’s a story for another day.

Here’s what Nick had to say about getting the suspension sorted.

CT: What is wrong with the current suspension?

NS: There are a few issues with the suspension as delivered from Yamaha. Up front, the fork springs are very soft — 0.75 kg/mm linear rate. With the stock springs, that means 1.5 kilograms of force is required to compress the fork by 1 mm (constant throughout the spring's travel). For metrically challenged Americans, this equates to about 84 pounds per inch of travel. Although the FZ-09 is lightweight, the average FZ-09 owner weighs about 200 pounds in gear and requires 0.90 kg/mm springs to obtain proper suspension sag without bottoming on large bumps. With the 0.90 kg/mm springs, the bike can support approximately 100 pounds per inch of travel. This may not sound like a substantial change, but over the full fork travel, the heavier springs can support an additional 86 pounds of load. That's like strapping a bag of dry-mix concrete to your handlebars! If set up properly, correct spring selection will improve ride quality and resist brake dive.

By the same token, the fork's damping is inadequate. Put simply, damping is the ability to dissipate energy. Springs compress to store energy and damping is used to control motion while compressing and extending the suspension. Without proper compression damping, there would be little resistance to prevent the suspension from fully compressing. This would do so very quickly, resulting in a rather harsh impact. Conversely, the same thing happens during rebound, when the suspension extends after being compressed. Without proper rebound damping, the energy stored in the spring is unleashed very quickly with little control. Ever find yourself being bucked off the seat, like riding a pogo stick? This is because there wasn't enough rebound damping.

Out back, it's more of the same. The shock spring's rate is 550 lb/in and rebound damping is minimal, even when set to maximum. Our 200-pound theoretical rider would benefit from a 600 lb/in spring for typical street riding (provides an additional 105 lbs of load capacity after accounting for the 2.37:1 linkage ratio). Of course, neither end of the stock bike provides compression damping adjustment, a feature found on most modern sportbikes and all aftermarket replacement shocks.

The bottom line here is that proper springs are required to support the bike and rider during typical riding and proper damping is required to maintain the tires' contact with the road. If ridden slowly and smoothly on well-maintained roads, the rider may appreciate the softer suspension. If taken out to the canyons or the racetrack, though, things unwind quickly. When we elevate our pace, we start asking more of the suspension system. It's critical that the system is tuned for the rider and the use. There is no one-size-fits-all approach to suspension tuning.

CT: What is your fix?

NS: Proper springs are the first place to start. As mentioned above, the right spring will take the rider's weight, bike setup, and intended usage into consideration. Rider sag (the amount the suspension compresses under weight of the bike with rider aboard) should be optimized to provide adequate travel in both directions; compression under braking or upon impact is important, but so is the ability for the wheel to 'drop' as the road falls away from the bike.

The fork springs can be easily swapped for the correct rate, but the rear requires more consideration. Be aware that adding a heavier spring will only increase the aforementioned pogo effect unless damping is adjusted to suit. Remember, a spring stores energy. So, under full compression, a heavier spring will store more energy. More damping is required to slow the rebound stroke. Heavier fork oil will restore rebound damping to the front end (though at the expense of more compression damping), but the shock requires disassembly. Unfortunately, the stock FZ-09 shock is not serviceable.

Recall from earlier that damping is the dissipation of energy. Damping is developed by forcing oil through orifices and is tuned by adjusting the oil's viscosity, orifice geometry, and the valving that regulates flow through the orifices. Stoltec Moto worked with GP Suspension to develop a suitable fork revalve kit. This kit replaced the stock compression valve, rebound piston, metering needle, and valving. Although the kit doesn't provide compression adjustment, the rebound performance improved drastically and eliminated the pogo effect. Low-speed compression (controls brake dive) and high-speed compression (controls bump compliance) were both optimized to deliver more control and feedback during spirited riding, without introducing discomfort typically associated with a performance suspension.

Once the revalve kit was road-tested and finalized, we moved on to a set of GP Suspension 25 mm fork cartridges to obtain adjustable compression damping. This kit replaced the stock internals with complete AMA-grade internals that reduce stiction and unsprung weight while providing the compression adjustment we required. The 25 mm cartridge kit is much stiffer than the stock setup (both in compression and bending), so our work was cut out for us. Multiple spring and valving configurations were road- and track-tested over several months to provide the desired balance of comfort and grip that we were searching for.

On the rear of the bike, the stock shock was sent off to Penske Racing Shocks in Reading, Penn., for dyno testing to determine the damping baseline. While the shock was in expert hands, the rear suspension geometry was measured to ensure a proper spring selection could be made with a complementary valving approach. One at at time, prototypes were built, dyno'd, and road-tested. In the end, we developed an 8900E emulsion shock, an 8983 'double clicker', and an 8987 'triple clicker' to address various levels of performance at different price points.

The 8900E emulsion shock was designed to serve as an entry-level shock that provides combined compression/rebound adjustment. The 8983 "double clicker" utilizes a remote reservoir to increase oil volume and separate the nitrogen gas from the oil while also separating damping adjustments. The 8987 "triple clicker" shares the 8983's architecture but adds high-speed compression adjustment. All models are custom-sprung and valved for the rider's weight and riding style.

CT: How does it feel different?

NS: In base form, the 8900E fixes the rear end's woes; reduced squat under acceleration and increased damping without detrimentally affecting ride quality. Gone are the days of mid-corner bumps upsetting the chassis and disturbing the rider's intended trajectory. Adjustability and feel increase as you move up the food chain to the 8983. Although the 8983 was originally conceived for racers and track-day junkies, the on-road performance is fantastic. The ability to 'feel' what the rear wheel is doing through the right wrist transcends most riders' experiences. It needs to be felt to be understood. The 8987 takes the 8983 to the next level, and allows the rider to tune squat and bump compliance separately — beneficial for rough roads and track days alike.

Overall, the whole bike feels better and more controlled. Braking hard to the apex and rolling on the throttle used to cause the chassis to pitch forward and stand on its toes, followed by an abrupt weight transfer to the rear. As exciting as it was to wheelie out of turns, the feel throughout the whole event was limited. All of this is changed for better. Trail braking now delivers good feedback to the rider, mid-corner bumps upset the chassis less, and drive out of the corner rivals most sportbikes. Most importantly, the bike responds predictably to rider input. As the odometer counts off the miles, the bike's actions become transparent and maybe, telepathic. The rider is left to focus on more important matters: body positioning, their line, the scenery, or the minivan swerving into their lane up ahead.

CT: How can people fix the suspension on their FZ?

NS: Become educated. There are a lot of great print and web resources. Learn the terminology and basic suspension operation so you can have an intelligent conversation with your suspension tuner regarding your expectations and budget. If the factory settings haven't been altered, take the time to adjust spring preload and rebound damping while you and your tuner work on a strategy. If strapped for cash, you can look into retrofitting a shock from another model to hold you over. Several ZX shocks can be used with minimal levels of modification. Of course, if you're looking for a well-engineered setup, we’re the guys to see. If you aren’t looking to spend a lot and just want to make the bike more predictable and less squirrely, the GP Fork Suspension Piston Kit ($250) and Penske 8900E rear shock ($650) will drastically improve your ride. For those those of you looking to really turn your FZ into something special, or who want the ability to adjust the settings for different conditions, the GP Suspension 25mm Cartridge kit ($1,149) and Penske 8987 Triple Clicker ($1,275) simply can’t be beat.

Editor’s note: After spending some time riding the Stoltec Moto FZ-09, I can attest to the fact that this really is a beautifully handling motorcycle. Brake dive and acceleration squat are non-existent, and what was once a terrifying bike to hang off of, is now completely confidence-inspiring in the corners.

Stay tuned for our next installment in this FZ-09 series when we address the fueling issues.

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