During the last few years we've seen an almost exponential rise in the number of options available for modifying your bike's suspension. A variety of companies are offering more parts and services, ranging from tweaking your stock boingers trackside to completely replacing major components. One relatively new development is the availability of reasonably priced, complete fork cartridges, which are said to perform better than revalved stock parts for many reasons and are legal for supersport racing. Just as we were contemplating testing some aftermarket cartridges ourselves (and we've already got more in the swag cube to try), GP Suspension rang up and offered a day at the track to test several of the company's options for our Yamaha YZF-R6. How could I refuse?
We spent a day at Willow Springs...
We spent a day at Willow Springs with GP Suspension and two Yamaha YZF-R6s to experiment with a slew of suspension options.
I reserved a spot with the friendly people at MTC for one of their track days at Willow Springs, and as a benchmark I took along our stock Yamaha R6 test bike. The GP Suspension folks brought along their own R6, also stock except for suspension modifications, and we fit both bikes with fresh Dunlop D208 GP JLB tires.
While GP Suspension is a relatively new company, its owner, Dave Hodges, is well respected and has years of experience in the field. Before we started, I spoke with Hodges about the R6's stock suspension. "The stock components in the new R6 fork-in my opinion, they have good stuff to work with, but they don't give you enough adjustability. You have to replace the compression needles to get a broader range of adjustability out of it. They're trying to make a valving setup that works for every Joe Blow who buys one out of the showroom, [but] it's not going to work for the guy who's 260 pounds. He's going to have to make changes to make it right for him."
This dyno graph of the stock...
This dyno graph of the stock Yamaha fork (below) shows the compression damping adjuster needle (left) has no effect after just a couple of clicks out. The GP Suspension needle (right) has a subtler taper to provide more adjustment range.
The new inverted fork on the R6 does offer some improvements over last year's conventional unit and previous inverted Showas as well. "Most of your stock cartridges are aluminum-based rods with a coating on them, and you can't polish the coating off because it wears the aluminum out," says Hodges. "The R6 has a steel rod. You can polish it, but they come from the factory with a pretty finely polished rod to try and reduce stiction. They've changed the upper tubes where they put both bushings fixed in the tube [rather than one bushing in each tube], so you don't have two bushings changing locations and the rigidity of the fork. Showa just went [to that configuration] last year; Kayaba's been doing it for about five years. Showa finally got on board with the '04 GSX-R600/750 and switched to two fixed bushings to keep the rigidity in the fork."
The first setup I tried on the GP Suspension bike included a stock shock and fork with a mix of stock and the company's own internals. Hodges produced some interesting dyno charts that showed the R6's fork compression adjuster has no effect after just a couple of clicks out. An adjuster with a longer, shallower taper replaces the blunt stock piece. Citing insufficient damping in the stock compression valves, GP replaces the stock piston with its own valve and shim stack. "My valve is different. I've never seen one out there, either OEM or aftermarket, but maybe someone's built one. It's a 20mm piston with a five 2mm-hole port configuration with a 17mm shim base. Ohlins does four 1.6mm holes, [and Honda's] HMAS does something similar." On the rebound side, the stock piston is used with a new shim stack.
GP's replacement fork valves...
GP's replacement fork valves (above, right) have sets of five damping orifices as opposed to the usual three or four (above, left). This forces the shim stacks to open more uniformly, providing better control over flow (and damping).
Hodges says the new inverted fork on the R6 doesn't use its full travel because the inner tube bottoms on the fork cap before anything else, so he trims 5mm off the tube to liberate that last bit of travel. Finally, stock springs were used to accommodate my 140 pounds, and each tube was filled with Motorex 5W oil.
Inside the Soqi shock, Hodges retains the stock piston with his own compression and rebound shim stacks and drills and taps the reservoir cap to accept a Schrader valve. A 4.5mm shim was slipped under the shock's clevis to raise the rear ride height, and the fork tubes were dropped from their stock height by 8mm, placing the tops slightly inside the clip-ons. "You're trying to give [the swingarm] more downslope to get better rear-end traction. Once you go to some of the bigger tires [which the D208 GP JLB is compared to stock] and put too much ride height in the back, you lose the trail, so you have to push the forks down to get the trail back for straight-line stability. Rear grip, front-end feedback and front grip are what we're trying to get out of the bike, and also keep it stable at the same time."
These 25mm replacement fork...
These 25mm replacement fork cartridges are made in-house at GP Suspension; their larger diameter mainly helps prevent cavitation at high damper speeds, but there are other benefits. The rods are titanium-nitrided to reduce friction, a longer top-out spring keeps the front wheel in better contact with the ground on acceleration, and the body is hard-anodized to reduce wear.
After a few laps on our stocker, I tried the GP-equipped bike. Initially, it steered a bit sluggishly compared to our stocker, and fork dive under braking was quite different; this bike seemed to dive through the first portion of its fork travel quickly, then get to a stiff portion without using as much travel as the stock bike. Hodges made a couple of adjustments that quickened steering and helped use more of the available travel, and soon the modified R6 turned more like our test bike.
Immediately the GP Suspension bike felt more attached to the track in Willow's long sweepers, with increased traction and better tire feedback. The bike was also more stable over bumps and exiting turns; some bumps that caused a brief wobble in the stock bike weren't even noticeable on the modified bike.
Next up, Hodges installed a Penske shock, which was fitted with a straight-rate spring (the stock spring is slightly progressive) and set to the same length as stock. "I prefer the Penske shock because they work so much better out of the box than other shocks. Most other shocks, in my opinion, you have to fiddle a lot more with the valving, spring rates and high-speed compression springs [than with the Penske]. With the Penske I can custom order anything I want. They're easy to work with, and they're here in the U.S." Surprisingly, the aftermarket shock did not seem to be a huge performance increase over GP's revalved stock shock, but there still was a definite difference in the bike's behavior. Feedback was better, and I could feel more of the smaller, ripple-type bumps than with the stocker. I suspect the real benefits of the Penske shock compared to a well-revalved stock unit are more in the available adjustments (high-speed compression damping and ride height, which the stock shock lacks), more consistency in the damping and more rider confidence.
The only noticeable change...
The only noticeable change to the revalved stock shock is this Schrader valve. And the sticker, of course.
Finally, the GP crew swapped the modified fork for one with the 25mm cartridges installed. While the stock cartridges are 20mm in diameter, Hodges says, "The big thing with the 25mm cartridge is just the cavitation, trying to keep that down. At race speeds, when you're doing a lot of hard braking, the velocity goes from 5 inches/second to 35 inches/second. You push all the oil out and you have nothing for the rebound stroke. That's the biggest misperception in cartridges, where some people say 25mm [cartridges] don't work and some say they do. That's why fork tubes and shock bodies and cartridge bodies keep getting bigger." Hodges then points out that some Suzuki models and the Kawasaki ZX-12R have 25mm cartridges, that most aftermarket cartridge kits are either 23mm or 25mm, and that most current motocross forks have 32mm cartridges.
GP's cartridges for the R6 are slightly longer than stock, extending tube length and meaning the forks don't have to sit below the clip-ons' tops. After just a few laps on the 25mm-cartridge-equipped bike, I was a big believer. The R6's front end felt like it had much more support on the brakes, diving more consistently and with better control. Additionally, there was even better traction and feedback than with the modified stock fork, and combined with the improved braking characteristics, this gave me a lot more confidence to rail into the faster turns hard on the brakes.
We used Dunlop's D208 GP A...
We used Dunlop's D208 GP A tires on both bikes, the rears in the newly available medium-compound JLB. While we had some stability issues with the Dunlops at the 2005 R6's introduction, they worked fine with some tweaking to the stock suspension. A step below the Dunlop's new Sportmax GP tires, the 208 GPs provided excellent and consistent grip over the course of the day. One benefit specific to the rear JLB tire is that it is not much larger in diameter than the stock tire and requires fewer setup changes to accommodate.
At the end of the day I took our stocker out for a few laps and noticed it still steered a tad quicker than the GP Suspension bike. Taking lap times at a track day is always a hit-or-miss affair, but I needed an open track and fresh tires on the stock bike to match the best time I set on the cartridge-equipped bike on worn tires in traffic. And while I thought more time to fiddle with the modified bike could make it steer as quick as the stocker so I could go faster, I was closer to my personal limits on the stock bike.Interesting stuff, certainly, but what does it all mean? Hodges says you only need suspension upgrades to match your ability and pocketbook. "I get a lot of guys [who] call me and say, 'Hey, I was thinking about your 25mm cartridge; I'm one year out of novice.' I try to sell them the product they need. I say, 'You'd be more than happy right now at your skill level revalving stock stuff.' He isn't going to benefit [from more expensive parts]. I don't try to upsell people even though they want to spend the money. I try to downsell them just for the fact they'll be happier than going, 'Wow, that's a lot of money to spend'."