Article reprinted from International Motorcycle,
July 2001 Vol.11/Iss.3
BONDO ON SUSPENSION
By Steve Bond
Watch an apres-race interview with any top road racer, and I'll betcha 10 bucks that at some point, they'll mention they had a good setup. "Setup" basically, is adjusting the spring preload, compression damping, rebound damping and suspension geometry to optimize the steering and handling characteristics of a motorcycle to that particular racetrack on that particular day.
The Mosport round of the 2000 Parts Canada Superbike Championship was gonna be a fun weekend. Not only was I watching Canada's top racers master the country's premier racing facility, I was riding Phil Unhola's Aprilia Challenge Cup bike in the 250 Amateur GP class. Phil runs the Moto Canada 125 Grand Prix School and uses the Aprilia as a track vehicle to keep an eye on his students. When I rode the bike at an earlier track day, Jordan Szoke said it was probably way too soft for me, so we made it stiffer. It seemed better.
And that's probably what most street riders and beginning road racers do - twiddle with the preload and mess around with the damping. Playing with the adjustments without understanding what they do is much like removing your own appendix. You're pretty sure you did it correctly, but there will probably be unpleasant repercussions further down the road. I wanted an expert to help me with the Aprilia's suspenders and the logical choice was John Sharrard.
Sharrard is generally acknowledged as Canada's premier suspension guru and, at the time, ran consistently near the front in both 600 and Open Pro Sport Bike classes. John retired from racing at the end of last season and is now working on Jeff Williams' bikes for Honda Canada. I plunked the Aprilia in John's pit after Friday practice where random samplings showed me circulating in the 1:47 - 1:48 range.
The first step is to properly adjust the chain because axle placement determines wheelbase, thereby affecting the active length of the swingarm, and how it acts on the rear shock.
Place a zip-tie (or O-ring if you've had the forks apart) on one of the fork sliders to measure suspension travel. When the fork compresses, the zip-tie moves to the point of furthest travel. "Zip ties are the poor man's data acquisition system," says John. "Ideally, there should be 3/4" of suspension over and above where the zip-tie is, and as you go faster, you compensate by adding more spring preload."
The zip tie on the Aprilia's fork showed that it was bottoming out and obviously (to John anyway), we needed more preload. He checked the baseline settings to ensure they were both the same, then added two more turns.
Next, John checked the fork stiction. "Stiction is the result of static forces on the forks while the suspension is inactive." Stiction is partly the fork seals wiping the sliders, but it's also misalignment of the fork legs at the axle. "Support the front of the bike on a stand that doesn't contact the forks, and you should be able to thread the axle all the way in with your fingers." If you can't, the axle is misaligned because the fork lowers are slightly off center. "Excessive stiction can affect handling because the forks actually bind slightly before they begin to function."
John checked the Aprilia and there was noticeable resistance. "Now try my R6." I pushed on the triple clamp and the Yamaha was smooth as butter. "Loosen the pinch bolts at the bottom of the forks and bounce the bike hard with the axle still tight. That should align everything properly and reduce stiction."
Sharrard then moved to compression and rebound damping which is, "a feel that's developed only with practice." Compression damping was good but rebound damping was lacking. "Mosport has a bump about every 18 inches and the fork is compressing adequately but the rebound isn't allowing it to come back up quickly enough. The fork just keeps compressing over the bumps until it's almost bottomed out." John added two turns of rebound damping until the fork was acting to his satisfaction.
With the front baselines set, we moved to the rear and measured static sag. Have someone support the bike at the front and measure from the rear axle to a mark on the seat. The rider in full kit (helmet, leathers and boots can weigh 20 pounds or more) then sits on the bike in a racing tuck and the same measurement is taken. For track use, adjust the rear preload until the difference is as close to 30 millimeters as possible.
We cranked the spring to its limit to get 30 millimeters, indicating that the spring was too soft for someone of my (ahem) weight. A serious racer would install the next stiffer spring, but that option wasn't available.
When cornering, if the back end is too soft the rear will squat, extending the front end and, basically, you're trying to steer a chopper. Then when you get on the gas, the rear is pushed down even further and the bike wants to understeer right off the track. You then have to muscle it around or feather the throttle to get it turned and neither option is conducive to fast lap times. Rear compression damping was good but rebound was slow so John hiked it up to the maximum. The hind end wasn't perfect but there was no more adjustment available.
We now had our baseline settings and I noticed that the rear felt much higher than it did before. "Try it tomorrow in practice, check the zip-tie and see how it feels." John said.
In the next morning's practice session, it felt much better although the zip tie still told its tale of softness. I dialed in two more turns of front preload and dropped another second. One more turn got me into the 1:44s in the heat race but it was still bottoming.
"Try more preload and increase the compression damping by a turn or two and you could even add 10cc of oil to each fork leg." For the final, I cranked the preload as far as it would go, about 1 1/2 turns, and added one turn of damping to the front.
Can an old dog learn new suspension tricks? My race times were consistently in the low 1:43 range, which is pretty good for an old guy on a warmed-over 250 street bike. More importantly, I took over four seconds a lap off my "pre-set up" times on the Aprilia. Further improvement would require stiffer springs front and rear or me dropping 20 pounds. Neither was gonna happen that weekend.
Don't despair if you can't actually "feel" a difference once the suspension is dialed in. John says every racer rides at his own comfort level and, "once the bike is set up properly, you may find yourself inexplicably going a second or two quicker without riding any harder." I immediately noticed that the bike steered much better with the back end stiffened up and my lap times agreed. Plus, it was much easier to ride at that pace than when I was doing 1:48s before. Amazing.
Profound thanks to John Sharrard for allowing me to pester him on his very busy Mosport Superbike weekend. Even though he was racing two bikes, he was very patient while babysitting me through Basic Suspension 101. John has branched into a business called Accelerated Technologies (905-986-0064) where he revalves and reworks shocks and forks. Thanks also to Phil Unhola for allowing me to experiment with his Moto Canada company car.
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