Tuesday, May 31, 2011

Honda VF700S head shakes on deceleration?

First check steering bearings -- you know, by jacking up the front end and grabbing the fork to feel for play.
Sooner or later, all the larger late model Hondas and Yamahas develop a problem with the steering bearings that results in a head shake when decelerating from 40 mph and less. For want of a better term, we'll call it a "deceleration wobble."

The cause
There is a rule in motorcycle suspension technology which says that problems in handling that occur under 40 mph are due to defects in front of the steering head, while those occuring at higher speeds are found in causes aft of the steering. It's a tried and true rule of thumb, and a decel wobble obeys the pattern. Many things can cause weaves and wobbles, whether on acceleration or deceleration -- tire wear is especially critical. But decel wobbles have their own special causes, and if the front tire isn't excessively worn or the wheel badly out of balance, the cause is almost always the steering bearings. But we're not talking looseness. We're talking about something that is not addressed in any service manuals, factory or aftermarket. Whether because the frame is made of softer material or what, the steering bearing races "walk" in the frame. That is, they shift in their recesses and become out of square with the steering stem, and out of parallel with each other.
see fig below:---














Modern motorcycle steering bearings tend to "worry" -- shift back and forth -- in their frames, especially on the heavier bikes. The resulting non-parallelness sets up torque forces in the steering which manifest themselves as attempts by the fork to correct itself, with the result: shimmy, shimmy. Again, the problem isn't looseness. Mere tightening fails to correct the problem.

The following procedure is one circulated by Honda's District Service Representatives. It is based on the above premise, as well as a procedure found in Honda Service Letter #126. Three tools are needed: a torque wrench, the special factory steering bearing nut socket (Honda's is part # 07916-3710100), and a good quality tubular 0-10 lb. spring scale. A floor jack or something similar to jack the front of the motorcycle off the floor will be handy, too.


Follow your manual's instructions for removing the top triple clamp (Honda calls this a "bridge"), so that the pair of special castlelated nuts becomes accessible. The upper one is just a locknut. Remove it and set it aside, along with the special washer. Jack the front end up off the floor, and feel the bearings as you turn the bars each side from center. If the bearings are notchy or the front end has a self-centering action, the bearings need to be replaced, no second-guessing here. After replacing them if necessary, continue. Get the front end off the floor. Turn the fork to full right lock, and with the torque wrench and special socket, tighten the bearings to 40-50 ft-lbs. The fork will be very stiff. Don't panic. It's only temporary. Now turn the fork lock-to-lock, repeatedly, at least twenty times. You will probably notice something interesting: that ridiculously high tension will loosen up; the bearings will get looser, indicating that they have squared up and settled into the frame. In some cases, you won't be able to tell, but even if you don't notice the bearings loosening up, proceed. Turn the fork to full left lock now and loosen the nut until it's just finger tight, then turn the fork to the right lock again and tighten it to 7-10 ft/lbs.

see fig below:---




Attach your spring scale onto one fork tube, using a piece of shoestring or something similarly soft so as not to scratch the tube. With the fork assembly pointed straight ahead and the tire off the floor, slowly pull the spring scale straight ahead until the tip of the fender arcs about one inch. Note the poundage. You're looking for a 5-7 lb. pull. Five for motorcycles under 600 lbs., more for heavier machines and those with fairings. Tighten the tensioning nut as needed, a little at a time, and check with the spring scale.











After adjusting, drop the special washer back into place, and screw on the locknut, but don't tighten it. Though you probably found the locknut jammed against the tensioning nut, that's not the correct way to install it. It should beclose to the tensioning nut, but not jammed against it. Leave a little space -- about 0.020". Then bend the locktabs into the locknut to keep the two interlocked. The locknut's job is to isolate the torque of the bridge nut from the steering bearings. Reassemble the rest of the fork per the manual. If a test ride reveals that there is still a decel wobble, or the bike sways side to side like a rowboat (the bearings are too tight), readjust to higher or lower spec as needed. Ride safely.


If the steering head bearings need servicing, consider replacing ball bearings with roller bearings if your bike doesn't already have them. One list member reports that the bearings cost about $50. Follow the manual to remove the forks, take apart the stem, drive off the old bearings, and tap in the new bearings. This can take some patience, as they can be stuck tight. You will need a drift to remove the lower race from the stem (it has to go around a bend).

You can install a grease fitting while you have the stem apart. Find a spot near the top of the stem and below the top bearing which does not interfere with cables, wires, etc. and can be reached with the drill and grease gun. Drill and tap the hole and install the zerk fitting. Grease the bearings, assemble the stem, adjust the bearings, and use the grease gun to fill the stem with grease. Stop when grease starts oozing out of the bearings. Wipe off the excess and reassemble.

Some VF700S appear to suffer from a wobble or weave under certain high-demand conditions, especially high-speed (>=90 mph) sweepers. This appears to be inherent and probably is due to flexing of forks, frame, or both. Wobble under other conditions indicates that repairs or adjustments are needed somewhere.

Diagnosis is as follows:

  1. Check that the steering head bearings aren't loose. This is probably the most common cause of front end wobble.
  2. Check that the tire pressure, especially the front, is at least up to the recommended level. If you are heavier than average or carrying a load, the pressure may need to be higher. Some tires also seem to perform a little better with slightly higher than recommended pressures. UNDER NO CIRCUMSTANCES EXCEED THE MAXIMUM PRESSURE STATED ON THE TIRE SIDEWALL!
  3. Check the condition of the tires, especially the front, for "scalloping" or other uneven wear. Make sure they are installed in the correct direction of rotation.
  4. Loosen all the triple clamp bolts and make sure there is absolutely no "stress" on either fork tube. Check that the fork tubes are straight while you're at it. Also check that the handlebars are straight, then re-tighten the triple clamp bolts. If the fork tubes are not even with the top of the triple clamp, make that adjustment prior to tightening.
  5. Once you're sure it is not any of the above, you're down to other, less likely items. Systematically check everything that affects damping or could develop excessive tolerances or become misaligned in both the front and rear suspension: wheel bearings, fork bushings, fork oil (you do have fresh fork oil, don't you?), swingarm bushings, front/rear wheel alignment, and wheel runout.
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It has been my experience that torquing the steering head bearings to specs doesn't always work.
I like to put the bike on it's center stand (on my motorcycle lift) and strap the back end down so that the front end is free to rotate left and right.
Then I torque the steering head bolt to the specs and then rotate the front and tighten more until there is a slight drag.
Take it for a test ride up to around 60mph and take both hands off the handlebars and see if it does a head shake.
If yes, back to the garage for another tightning until the head shake is eliminated.

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Your shake is being caused by suspension not being dialed in properly for your weight. Those front springs are probably too light. Deceleration drops the front end slightly, laying the front forks more verticle, and also more unstable (i.e. shake). As a simple test, take the preload adjustment all the way out of your rear shock. This will lower the rear, and put more rake & trail into the front. Test ride the bike and your shake will be gone but it will also resist turning in the sweepers so be careful. Once that simple test proves your problem is suspension/geometry related, find another mechanic who understand suspension and get your bike adjusted properly for you.------------

A lesson I learned recently that may help explain why the front tire replacement was unsuccesful was that the use of Dynabeads will cause a headshake in roughly 30%-40% of 6 gen VFR's. So if your installer is using them, ask him to do a more traditional static balance using wheel weights instead. If he his in fact using dynabeads, this will more than likely solve the problem.--------------

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One incident with our client about head shake:---


A buddy of mine had a similar problem with a high speed headshake on his honda vf. He initially brought the bike over to my garage complaining of looseness in the front end. An inspection revealed that his fork bushings were badly worn and his head bearings were toast. His front wheel bearings had a "gravelly" feel to them as well.

We replaced the steering head bearings,wheel bearings,and rebuilt his forks. When we were through doing that my buddy took his rims and polished them. When he was done with that I installed the wheels and took the bike out for a test ride where I discovered that when I relaxed my grip on the bars it now had a fairly good headshake at high speed.

I removed the front wheel,unbolted the rotors and discovered that when my buddy bolted the rotors back onto the rim he somehow got a big honkin' dollop of polishing compound trapped between the right side brake rotor and the rim. I cleaned that out and retested the bike. Much better but it still had a tiny bit of a headshake at high speed which got worse when you used the front brakes.

I got out the dial micrometer and checked the runout on those front brake rotors and sure enough the left side rotor was out of spec. A closer inspection of that rotor revealed some damage. It looked to me as though the front wheel might have been leaning up against something and had fallen over and the rotor had hit the ground hard enough to cause that damage. My buddy swears up and down that never happened but the evidence on the rotor said otherwise.

We picked up a good used rotor off Ebay from a salvage yard and installed it,then took the bike out for another test ride. Problem solved.

Dunno if that's what's causing your headshake but since you checked everything else of note it couldn't hurt to have your mechanic check the runout on your front brake rotors too. It's possible they might have been damaged .---------



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