Bondo gets suspended

Bondo takes a Buell 1125CR to the track and gets a 101 course on how to set up the suspension from guru John Sharrard.

Many motorcycles these days feature fully adjustable suspension. Compression damping dials, rebound damping knobs, preload rings — but where adjustment is possible, maladjustment is likely.

Twiddling with preload and fiddling with damping is like juggling chainsaws — impressive when it’s done right, but better left to the pros if you don’t know what you’re doing.

In order to get a helping hand with the ins and outs of suspension set up I convinced John Sharrard of Accelerated Technologies to help me set up a Buell 1125CR that I had scored for a Pro 6 Cycle track day at Mosport, Ontario. Sharrard is a former national-level professional road racer, and currently one of Canada’s top suspension gurus.

The results were quite noticeable, so we thought that we’d share them with the CMG readership by making a new instalment of our Technobabble series — suspension set-up 101.

A SAGGY ARSE

I’d already ridden the CR on the street for a few days and it seemed to be a good handling motorcycle, but my first track session (on factory settings) was an eye-opener.

Even at a moderate pace the CR didn’t want to turn and, once into the corner it wouldn’t hold a line. When power was applied, it wanted to understeer right off the pavement, and when hitting about 220 km/h on Mosport’s back straight, a disconcerting slow weave got my attention.

I told Sharrard the instability was a notch below life-threatening, so he took some measurements to establish baseline suspension settings.

First was rear sag. Sag is simply the amount the suspension compresses from fully extended when the rider sits on the bike. Firmer suspension settings are needed for racing or track use, but less so on the street. Here sag is usually set at about 30 to 35 mm, whereas on the track rear sag should be close to 25 mm.

Adjusting sag is critical because it sets the bike’s ride attitude. Raising or lowering front or rear ride height by setting the sag affects chassis geometry and thus alters the bike’s turning ability. Raising the rear, as was done here, steepens the fork angle and reduces trail, which quickens steering response.

Alternatively, steering will slow if the rear sits low. You can alter this geometry depending on the track you’re riding, but going to extremes in adjustment will have a detrimental effect on handling, so experimentation and note-taking are a must.

The Buell’s rear sag was 54 mm with me aboard, indicating the rear end was sitting way too low — and when the back end squats, the fork naturally extends (to give more chopper-like geometry!), causing the negative handling characteristics I experienced.

Increasing preload will fix this (BTW, preload does not stiffen the spring rate; it merely exerts more of the spring’s pressure on the shock, thus raising the rear end). Sharrard cranked the Buell’s preload, I got back on, we remeasured, and continued until we attained the required 25 mm of rear sag.

PUTTING A DAMPER ON IT

Once sag is set, damping can be addressed. Compression damping sets the speed with which the suspension compresses, while rebound damping controls the speed with which it extends. If compression damping is too soft the bike might bottom too easily or dive excessively on braking, and if it’s too hard, it will bounce off bumps and the wheels will leave the ground.

Rebound damping has an equally dramatic effect on suspension; if it is set too soft the bike will wallow and weave, and if it is too firm the suspension will extend too slowly and a succession of bumps will compress the suspension until there’s almost no travel left.

To keep an eye on how much travel was being soaked up on the fork, Sharrard attached a zip tie to one fork tube.

“Zip ties are the poor man’s data acquisition system,” said Sharrard. “Ideally, there should be 15 to 20 mm of suspension over and above where the zip tie stops after a session.”

With a feel developed by years of experience, Sharrard set the damping baselines and sent me on my way.

As a result of the increased preload, the back end felt higher and I noticed an immediate improvement, as the Buell turned into the corners much easier. It still understeered when getting on the gas exiting a turn, but not as severely.

With the bike working better and my increasing confidence, I rode faster than the previous session. As a result the digital speedometer was now flirting with 235 km/h on the straight; however, I was still weaving at top speed.

STUCK ON STICTION

After returning to the paddock, the zip tie told its tale; we were still too soft up front so we cranked in one turn of preload and a couple more clicks of compression damping. Handling improved once again, and I went faster still.

And that’s the double-edged sword of suspension sorting. Adjustments make the bike behave better, enabling you to push the bike harder, which in turn overwhelms suspension settings requiring further adjustment to bring the bike up to snuff again.

Eventually, we ran out of adjustment and the zip tie was still almost bottoming, indicating that a rider of my (ahem) weight and ability would require a spring with a heavier rate. Sharrard says you can add 5 or 10 cc of fork oil to increase damping without adversely affecting fork function but do not overfill as this will likely blow the fork seals.

Fork stiction can also affect handling negatively by causing the forks to bind slightly before they begin to function.

“Stiction is the result of static forces on the forks while the suspension is inactive,” says Sharrard. It’s partly the fork seals wiping the sliders, but can also be misalignment of the fork legs at the axle, whether from crash damage or poor assembly. “With the front wheel off the ground, you should be able to thread the axle all the way in with your fingers.”

If nothing is bent, loosening the pinch bolts at the bottom of the forks and on the lower tripleclamps and bouncing the bike hard should line things up.

AERODYNAMICALLY CHALLENGED

We got the bike to where it would turn in quite well. Now, what about that weaving?

Sharrard figured the Buell’s mirrors and “lobster claw” side pods were causing the high-speed instability — it was a matter of poor aerodynamics. To reduce this weaving, dirt trackers often take their left hand from the bars and grab the fork tube up by the steering head. Sharrard suggested I do the same.

I wasn’t keen on taking my hand off the bars at 240 km/h but thought I’d give it a try nonetheless. After all, how bad could it be?

During the final session, with the speedo indicating 247 km/h and the bike nosing around like a beagle searching for a pheasant, I casually removed my hand from the left grip and grabbed the fork tube.

In final trim the CR was noticeably quicker around the track.

The weaving stopped and the Buell was rock solid. Since the Buell was just a loaner I refrained from ripping off the mirrors and side pods, but a day at the track had significantly improved my ride and identified the other issues.

Sharrard says that many riders can’t actually feel a difference once they start dialling the bike in, but they suddenly find themselves going a second or two quicker without riding any harder. And suspension adjustment isn’t just for racers — a proper setup will make your track day or street ride a more pleasant experience.

It was a real eye-opener how an initially ill-handling motorcycle could be transformed through simple adjustment of the stock components. A big thank you to John Sharrard and his suspension tuning service, which is available at all Pro 6 Cycle track days and Shannonville regional races.