Workshop: chassis

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Workshop: chassis
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Workshop: chassis

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Chassis: adjust and improve correctly

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“I can’t get the bucket around the bends and I urgently need advice.” PS reader and hobby racer Stefan Tachliski sent this call for help on racing4fun.de. PS drove his Suzuki GSX-R 750 K4 from the nonsense.

Werner Koch

08/21/2009


Workshop: chassis


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Stefan’s GSX-R 750 is no longer fun, but sorrow: it drives how it wants, not how it should.

Hobby racer Stefan is at the end of his nerve and his suspension Latin: “With my GSX-R 750 I drive 4 seconds slower than with my old Honda PC 25 and I am twice as stressed.” The good Ohlins shock absorber has been modified by a presumed specialist, the fork has been renovated, new Metzeler K2 tires stretch over the rims – all for the bin. His Suzuki resists any command, drives strange lines, tips over unpredictably and stands up for no apparent reason.

On this cucumber, the driver is exposed to physical gimmicks that screw up his fun and even more so his lap times. When the expensive rubbers then rub themselves in the blink of an eye and every kilometer, every liter of fuel wasted time and money senselessly, Stefan Tachlinski asks himself the cardinal question: Sell and prefer to ride a bike?

So it’s a good thing that PS reserved a place at the chassis workshop in Boxberg. Organized and advertised by the MOTORRAD action team, motorcycle chassis are explained here, adjustments are worked out and checked on a nice handling course. The meeting is arranged quickly. Stefan carts his lame horse in the transporter to Boxberg and has girlfriend Sandra in tow for emotional support – in case the PS troop should not get the patient on his feet and Stefan needs psychological care.

At the first all-round contact with the Suzuki GSX-R 750 There are no major mistakes: It makes a solid impression, is clean, built without daring tinkering and questionable tuning parts. The GSX-R 750 is also flawless in the second step. Steering head angle, track offset, ride height, swing arm angle – everything is OK. Stefan is allowed to slip on his leather suit and experience the current situation in the truest sense of the word on the handling course. Three or four brisk laps are enough and Stefan rolls to the makeshift workshop. “Just as shit as always. In the alternating curves, the box wobbles and sways like drunk. I don’t hit a line or a vertex.” Well then, get to work.

Check 1: steering head bearings


Workshop: chassis


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If the steering head bearing is not 100 percent in order, the choice of line becomes a lottery.

Some possible sources of error can already be deduced from Stefan’s description: If the motorcycle is unstable and wobbly when driving relatively slowly, the suspicion is that the steering cannot swing quickly and without resistance. This oscillation is important because the steering angle, i.e. the steering angle, adapts automatically depending on the lean angle and cornering speed. If something blocks the steering even slightly, the driver has to correct the steering angle, which he does not succeed one hundred percent even with the best trained fine motor skills. The result: the motorcycle unintentionally tilts over and must be stabilized by turning slightly towards the inside of the curve. In hard cases, this turns a 180-degree curve into a hexagon.

In order to be able to check the freedom of movement of the steering precisely enough, we unhook the steering damper. As Stefan suspected in advance, the steering head bearing is adjusted a little too tight. The rolling elements have already pressed small depressions into the bearing shells, so the steering easily engages in the middle position.

As an emergency measure, we loosen the adjusting nut of the steering head bearing by 1/16 of a turn, which alleviates the problem for a short time; In the medium term, the bearings have to be exchanged. Stefan goes on a test drive in the pylon forest, but breaks off after a few laps. “Drives a little better, but good is different”, is his comment.

Check 2: steering damper


Workshop: chassis


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Corroded bearings on the steering damper can have a massive impact on driving behavior.

First step: Check the freedom of movement of the spherical bearings. The piston rod and damper cylinder must be able to rotate freely and without tension over the longitudinal axis over an angle of around 3 degrees. If only one of the two ball joints is under tension, the side force on the piston rod can generate so much friction and breakaway force that the steering no longer runs freely. This freedom of movement must be guaranteed over the entire steering angle. Experience shows that retrofit dampers, in particular, are often incorrectly installed and under tension.

Everything is fine here with Stefan’s Suzuki. So we tip the motorcycle over the side stand to completely relieve the front wheel. Tender steering movements around the middle position check whether there is too much friction, damping or both. But everything is in balance on these points too.

And now? Remove the steering damper and move the piston rod back and forth with a lot of force. Once, twice – whoops, what’s going on now? A good 5 millimeters of free travel in the damper causes the piston rod to fall through with a jerk. The next time you try it, it slides creamy into the damper housing. A sneaky mistake: a steering damper that sometimes dampens and sometimes doesn’t make cornering unpredictable. If the worst comes to the worst, he cannot intercept the violent beating of the handlebars. So, steering damper off, out on the test track. Stefan circles the course for five minutes, waves confidently and grins: “Now you can ride a motorcycle again. Only handling and turning behavior don’t really fit.”

Workshop: chassis

motorcycles


Suzuki GSX-R 600/750/1000

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Check 3: shock absorber


Workshop: chassis


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A spring that is too hard means too little damping, because the damper oil can only brake movement, i.e. dampen it, where something is moving at all.

What does spring stiffness have to do with steering behavior? A lot. Ultimately, the geometry of the steering and drive / swing arm changes depending on how deep and how synchronously or asynchronously the motorcycle compresses front and rear. The negative and positive spring travel on the fork are within the tolerance range; However, not much moves on the shock absorber: With a driver weighing 100 kilograms (in full gear), the rear sinks just 24 millimeters; without a driver, it is 10 millimeters. Conclusion: the pen is too hard. As a result, the rear wheel does not bounce sufficiently on short waves and bumpy slopes, but lets the tire rumble over the asphalt and bounce off the road like a rubber ball in the event of a bump.

Where there is little springing, there is also little damping because it can only build up on the shock absorber via stroke and speed. What remains? Exactly, change the pen. A spring rate reduced from 100 to 90 N / mm produces useful negative spring travel. 10 millimeters without and 28 millimeters with the driver ensure more feedback and better shock absorption. We adjust the damping in the rebound stage to click 14, in the low-speed compression stage to 10, and high-speed to 2 revolutions.

Test driver Stefan moves out, comes back satisfied and only complains about the stubborn braking behavior with the front wheel jumping slightly. Because the negative spring travel is correct and 9.25 springs fit the 190 kg bike, it can only be the oil level that this brutal spring progression builds up.

Workshop: chassis

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Check 4: fork


Workshop: chassis


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To measure the air cushion with a ruler, the spring must be removed and the piston rod must be completely immersed in the standpipe.

Fork plugs and springs out, the oil level measured – all clear: in the left fork leg, the oil is 75 mm below the upper edge, on the right 135 mm. With only 75 millimeters of air cushion (without spring, measured with the piston rod fully immersed), the compression ratio between the extended and compressed state is extremely high and is about to be completely blocked. The fork blocks on the overfilled hydraulic oil, the wheel jumps and loses its grip. This error often happens because the fork is not fully compressed or the piston rod is not fully immersed during filling and re-measuring.

Another noticeable feature: when converting the fork with a special damper kit, the adjustment range was designed so absurdly narrow that only the first turn of the rebound stage can be used. With correctly filled fork oil (110 millimeter air cushions on both sides), a rear springing synchronously with the fork and creamy, rich damping front and rear, the stubborn Suzuki GSX-R 750 becomes a real cornering sweeper with good stability and sufficient comfort.

Only when accelerating out of the curve does the Gixxer push a little outwards. For that you have to take a little more time on a real racetrack; Only there can the setting options be combined to an optimal compromise, depending on the curve speed and route characteristics. Because the coordination of ride height and steering geometry is a complicated puzzle, PS gets to the bottom of this secret in issue 10/2009.

The most important things in a fast run


Workshop: chassis


fact

All noble accessories do not help if the base has mechanical flaws. So check them first, then spend money.

Before you want to remedy the defects on the chassis of your motorcycle with extremely expensive accessories, basic mechanical functions must be checked. If the strut redirection is worn out, even the best Super Duper TTX leg won’t help.

1. Check all screw connections on the frame and engine mounts, as the engine is the load-bearing element on almost all sports motorcycles.

2. Check freedom of movement and play in the steering head bearing, to do this dismantle the steering damper on one side and tip the motorcycle over the side stand. Check worn bearing shells with slight movements around the center position.

3. Check the fork and shock absorber for oil loss on the stanchions or damper rod.

4. Check the rear suspension and deflection system in the unloaded state (tilt over the side stand) for vertical play. Height play on the rear wheel / reversing lever must not exceed 1 to 1.5 mm; it can trigger chattering.

5. Check swing arm bearings and wheel bearings for play when they are at operating temperature.

6. Check the air pressure of the tires when they are cold, adjust to the manufacturer’s instructions if necessary. Check the hot air pressure immediately after driving, compare it with cold pressure and make a note.

7. Check that the rear wheel is correctly aligned. Chain tensioners must have the same markings on the left and right. For checking purposes, two straight measuring rods can be placed on the left and right parallel to the rear wheel.

8. Spring preload and damping should initially be set according to the manufacturer’s instructions.

9. The fork and shock absorber must respond sensitively and without great resistance to load. ATTENTION: After installing the front wheel, relax the fork legs when the clamps are open.

10. The driver’s seating position must be absolutely relaxed. Switches and levers must be positioned within easy reach, and shift and brake levers must be operated without twisting.

You can find out the next workshop date at www.actionteam.de or by calling 0711-182-1977.

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