I have a Kawasaki ZX-14. In an effort to quicken/tighten the cornering abilities, I changed my rear tire to a 190/55-17 from a stock 190/50-17. Since the tire change I experience noticeable headshake from 45 to 55 mph. The dealership said the front tire balanced perfectly, and they are reluctant to do anything. The tires are smooth at all other speeds, and the bike handles well. Any comments concerning the tire swap and cause of the headshake would be appreciated.
A high-frequency headshake is indicative of a front-end problem, but by installing a taller rear tire you've steepened your bike's rake, reducing stability and worsening the situation. Make sure your tire pressures are correct, and check that the front tire is round and true by raising the front end and spinning the tire; more than a few millimeters of runout in any direction will cause problems. Headshake at the speeds you are encountering can be the result of a wheel that is not dynamically balanced (as opposed to being statically balanced). Static balancing is performed by spinning the wheel on an axle. The wheel will stop with the heavy spot on the bottom, and weights are applied opposite for balance. Manufacturers generally use weights that clip on to the center ridge of the wheel, but many service shops use stick-on weights placed closer to the edge of the rim. If a lot of weight is required and it is all positioned on one side of the wheel, this can lead to a pendulum effect and cause a wobble. Remember trying to ride your bicycle home from the store with a shopping bag slung over one side of the handlebar? It's the same idea here, with an offset weight swinging back and forth around the steering axis.
Check your wheel and see if the dealer used stick-on weights all on one side of the wheel. The solution may be as simple as splitting the weight and placing half on each side-we've experimented with this and found that as little as a couple of ounces applied unequally can make a difference. You can also find a shop that will dynamically balance your wheel. This requires a machine that spins the wheel and electronically detects where on the rim-and on what side-to place the weight.
On the ZX-14 it's difficult to make adjustments to account for the taller rear tire because the fork tubes-which would normally be lowered in the triple clamps with such a swap-are already flush with the clip-ons. If the headshake persists after you've addressed the issues up front, you can try increasing front preload or decreasing rear preload to bring the geometry closer to stock, but that may cause other problems. As a last resort you may have to switch back to a 50-series rear tire.
An Oily Dilemma
I bought a 2003 GSX-R1000 with about 9000 miles from a private owner who had been using Motul 100 percent synthetic 10W-40 engine oil. He's only owned the bike for about one year, and before him the original owner used regular 10W-40 mineral oil. My question is, can I switch back to mineral oil safely? I just spent almost $60 on the synthetic oil, and I don't think synthetic oil is necessary for the type of riding I do (street/commuting). I called two different dealerships, and the mechanics gave me two different answers: One said to just stay synthetic, and the other said I can switch to semisynthetic blend for about two oil changes, then back to regular mineral oil.
In the early days of synthetics there was justification for not changing between mineral- and synthetic-based oils, but modern technology-in both engines and oils-means you can safely go back to using a mineral oil. Motul's 300V and 7100 synthetic oils are ester-based, and many synthetic blends use a mix of mineral oil and esters. If you're concerned about going directly to mineral oil, do as the one dealer recommends and use semisynthetic oil with esters (such as Motul 5100) in between. To be absolutely safe, stick with the same brand and viscosity of oil in each iteration-Motul 5000 would be the mineral oil to eventually switch to. Finally, always be sure to use oils that meet the API (American Petroleum Institute) or JASO (Japanese Automotive Standards Organization) standards listed in your manual. Suzuki recommends API SF or SG oil for your GSX-R; Motul's 5100 semisynthetic and 5000 mineral oil both meet the SG designation.
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