Service
SERVICE
How low can you go?
PAUL DEAN
Q I'm 62 years old and ride a 2005 Suzuki SV650 that I love dearly. I’m 5-foot-9 with a 29-inch inseam, and it is getting harder for me to swing my leg over the seat. I’ve been reading about suspension lowering links and would like to know if there is a danger in using them. Is stability at high speed threatened? I like my bike’s ground clearance even though I don’t drag my knees. Should I drop the front end to match the rear? Is there a way to calculate the link length versus the suspension drop? Please don’t recommend that I get a cruiser; I like the weight of my bike and don’t believe any cruiser can match the performance of my little SV650. I don’t want a hip replacement just so I can get on and off my bike, and I don’t walk well in elevator shoes. Marcel Thomas
Hobart, Indiana
Lowering links are model specific, and most of the companies that manufacture or sell them state exactly how far each set lowers the bike it is designed to fit; so you don’t have to do any calculations to figure out the length of the drop. There are quite a few different brands of links on the market that lower the rear of the bike anywhere from 1 inch to 4 inches, depending upon which one you choose. A couple of companies even make adjustable links that are essentially sturdy tumbuckles that allow the rear height to be adjusted anywhere within a given range.
Not all motorcycles react in the same way to changes in chassis attitude, but in most instances, installing a lowering link does not have any adverse effects on a bike's high-speed stability. Dropping the rear end increases a bike's steering-head angle and front-wheel trail, and both of those changes would tend to increase rather than decrease stability. But those same steering geometry increases also slow the bike's handling, and the lower ride height reduces cornering clearance; obviously, the lower the bike, the less lean angle there is available. Whether or not you can live with that reduction is up to you and your riding style.
If you intend to lower the rear more than an inch or so, I recommend that you also lower the front an equivalent amount, thereby allowing the bike to retain its original steering geometry. When you combine that fact with the lower center of gravity such changes will provide, the handling should become even easier and lighter-the tradeoff, of course, being even slightly less available lean angle. But if you only lower the bike an inch or two, it should still have reasonable cornering capabilities.
By the way, that should be elevator boots, not shoes.
Tapping tappet
QI have a 2003 Kawasaki 1500 Vulcan Classic that I bought used, and it now has about 6000 miles on it. One of the lifters in the engine had been tapping for about 500 miles; so I removed the engine, took out all of the lifters, bled them down and, according to the procedure in the factory shop manual, submerged them in kerosene and flushed them about 10 times each. I then filled them up with 10w40 Kawasaki motor oil, put them back in place, cleaned the oil filters and reinstalled the engine. It now runs well with no lifter tapping. Could the cause of the problem been that I had been using 20w40 motor oil of another brand instead of 10w40 Kawasaki oil? Is there any additive I should put in the oil to prevent this problem from happening again? I would have replaced all 8 lifters, but the Kawasaki dealer wanted $55 apiece, which seemed extremely high for such a small part.
Ken Alexander Ft. Loramie, Ohio
A Chances are that the tapping was caused by a tiny piece of contamination that got lodged in the check valve of the affected lifter, not as the result of the 20w40 oil you had been using. The contamination prevented the little ball in the check valve from fully sealing, causing an audible tapping. Even if you had been using automotive oil, which in many cases is less than ideal for motorcycle engines, it would not have caused the lifter to hang up.
What’s more, putting an additive in the oil would not preclude such an occurrence in the future. Modern oils, including those sold under the Kawasaki brand, already contain additive packages intended to prevent these kinds of problems; dumping even more additives into the system would likely just be a waste of money.
In fact, because you bought your Kawasaki used, the source of the problem may have originated before you took ownership of the bike. The previous owner may not have been fussy about oiland filter-change intervals or keeping the oil topped up, all of which can lead to higher levels of contamination in the oil. You dealt with the problem in a very thorough, admirable manner and achieved positive results. If you continue to be that exacting when it comes to changing the oil and filter at prescribed intervals, you should not experience any more problems with sticking lifters.
Shock value
Q Could you please explain what is meant by “gas” shock absorbers and why some shocks have reservoirs that are either part of the shock housing or connected to a hose? I’ve wondered about this ever since I started riding three years ago but have not yet gotten what I consider a sensible explanation.
Michael Wayne Carter Annapolis, Maryland
A Those two design features are intended to eliminate a damping problem called "cavitation."
A basic shock absorber works by forcing oil through small orifices on the inner end of the shock shaft. The orifices are calibrated to provide the desired level of damping, but cavitation occurs when the wheel hits something that makes the shock compress so rapidly that the oil cannot move through those orifices quickly enough. That resistance causes the damping to momentarily become much stiffer; but as compression continues, air bubbles quickly form in the oil. And since oil with air in it is less dense than oil without air, the damping values are greatly reduced.
If, however, there is no air in the shock in the first place and the body is completely filled with oil, air bubbles cannot form in the oil. Problem is, as a shock goes from full extension to full compression, the shock shaft moves down inside the shock body, displacing some of the oil. You can’t compress a liquid, however.
so if the body is already completely filled
with oil, either the shock would not be able to compress or, if the force was great enough, the compression would rupture the body.
A man named Christian Bourcier de Carbon in 1953 came up with the idea of putting a separate floating piston inside the shock body, with pressurized gas on one
side and air-free damping oil on the other, the two sides sealed from one another with O-rings. Then, as the shock shaft would enter the body, the effective oil level was able to rise by pushing the floating piston against the pressure of the gas, which can be compressed. With no air in the oil, cavitation was eliminated.
This is where the reservoir comes in. The de Carbon floating piston had to be located down near the very bottom of the body’s interior, where its presence significantly reduced the available room for shock travel. But if that piston were moved to a separate reservoir, one either attached to the body or at the end of a hydraulic line, the entire interior of the
body could be used for shock travel without any adverse impact on the anticavitation effect. Plus, the use of a reservoir increased the total volume of damping oil, allowing the shock to run at a lower temperature-an unexpected but welcome bonus.
One bump-stick or two?
QI would really appreciate it if you could explain, in terms a nonengineer might understand, the comparative advantages and disadvantages of single overhead cams and dual overhead cams. Both types are currently in popular use, and I’d like to know how and why one type is chosen over another when a company is deciding which to use.
Paul McKelvey Provo, Utah
A A manufacturer’s decision to use one engine design or another is dictated by many factors. But generally speaking, if the bike is intended for allout performance, such as today’s megapower sportbikes, it will most likely be powered by a dohc engine; if it is being designed for less-demanding use, it will probably-but not necessarily-have a sohc engine.
Candid Cameron
I would like to know the reason that sportbikes use longer exhaust-valve timing and greater exhaust lift than the intakes. This is opposite of any car engine I have seen. Why are bike engines designed this way?
Kurt Russman Posted on America Online
I should have been able to figure this one out myself, but occasionally it’s nice to be rescued instead of having to swim the Atlantic. The reason for longer exhaust timing is that exhaust-port design is not something the Japanese have paid a lot of attention to-the idea being that the intake has only atmospheric pressure to drive it, so it needs all the help we can give it. But the exhaust more or less has 100 psi of late-stroke residual pressure to push its flow, so it will take care of itself.
That is simply untrue. On most engines, exhaust flow is turned inside the head so it exits at 90 degrees to the cylinder, or even slightly downward. Because the flow doesn’t want to make this turn, it is flung up to the roof of the port, while hardly anything happens in the bottom third of the port (and flow may even reverse there). Because of this poor flow, extra time is
necessary to get the exhaust out of the cylinder. Contributing to this is the fact that exhaust valve lift is generally a millimeter or two less than intake lift. Everyone likes to give exhaust valves-which run very hot-something of a break.
The late Jim Feuling made a good living showing Detroit and others that they didn’t need exhaust ports and valves as large as they were using. With smaller ports and valves, valve durability was improved and less heat was transferred to the reduced area of port walls. Also, there was more room in the combustion chamber for larger intake valves. The 350-cubic-inch LT-1 in my wife’s old Buick wagon has amazingly small, flat-bottomed exhaust ports, so I think I see some of Jim’s work there.
When a few changes increased exhaust flow 30 percent in the Honda CB450based vintage racer of Todd Henning, he could no longer use the race exhaust cam, and even the stock exhaust cam was too much.
There is a lot of work to be done here, and if you’d like to see an example of a pretty good exhaust setup, have a look at a Ducati Testastretta head. The exhausts turn through considerably less than 90 degrees in the head, and both they and their valves are quite small.
Kevin Cameron
One of the critical aspects of ultrahigh-performance engines is maximum rpm; the faster an engine can spin, the more power it can make. And one of the key factors that limit rpm is valvetrain weight. The more mass the valvetrain involves, the harder it is for those parts to follow the profile of the cam lobes. At high rpm, the valve is subject to exceptionally rapid acceleration as the cam forces it open; and as the valve reaches maximum lift and the cam lobe begins its closing phase, inertia wants the valvetrain to continue moving in the same direction, opening the valve farther than intended. This is called “valve float,” and it can destroy engines by allowing valves to make contact with pistons, causing all manner of disastrous results-valve heads snapping off, pistons being destroyed, connecting rods bending or breaking.
Aside from Ducati’s Desmodromic system, there are two basic cures for valve float: stiffer valve springs and lighter valvetrains. Stiffer springs also increase frictional losses, though, which are not conducive to increased power, and they accelerate wear on valvetrain contact surfaces. A better solution is to make the valvetrain lighter, thereby reducing inertia.
Modern motorcycle engines use combustion chambers with semihemispherical shapes requiring valves that are splayed at shallow included angles. As a result, sohc designs require some kind of finger-type lobe followers so that the single cam can operate all of each cylinder’s valves. But not only do those followers add to valvetrain mass, they have to pivot on shafts and have two separate wear surfaces (between lobe and follower, and between follower and valve stem), adding to overall frictional loss.
Dual overhead cams, however, can be placed directly above their respective valves, usually operating them via a lightweight shim-and-bucket arrangement. Valve action is direct, there is only one major wear area (between lobe and bucket) and valvetrain mass is comparatively small. There are, of course, highperformance engines of sohc design, just as there are lower-performance examples that use dual overhead cams. But in general, the factors outlined here are what designers use to decide which type of valvetrain to use on any given engine. □
Feedback Loop
Q In regard to your response to Davey Simone's question about moisture in braking systems ("Not the liquid of choice," January, 2008), it is important to note that brake fluid absorbs moisture from the atmosphere very easily. To keep water from affecting a braking system, you should not use an old container of brake fluid when topping off, even if the cap has been kept on tightly. You also should flush a braking system periodically, especially on a motorcycle, where moisture can aid growth of algae, etc. in the system. You should have seen the green garbage growing in the front caliper of my old KTM. Bryan Bailey
District Manager NAPA of Memphis, Tennessee
A ExceUent point, Bryan, one that I forgot to mention in my response. Thank you for keeping me on my toes.
Got a mechanical or technical problem with your beloved ride? Can’t seem to find workable solutions in your area? Or are you eager to learn about a certain aspect of motorcycle design and technology? Maybe we can help. If you think we can, either: 1) Mail a written inquiry, along with your full name, address and phone number, to Cycle World Service, 1499 Monrovia Ave.,
Newport Beach, CA 92663; 2) fax it to Paul Dean at 949/631 -0651 ; 3) e-mail it to CW1 Dean@aol.com; or 4) log onto www.cycleworld.com, click on the “Contact Us” button, select “CW Service” and enter your question.
Don’t write a 10-page essay, but if you’re looking for help in solving a problem, do include enough information to permit a reasonable diagnosis. And please understand that due to the enormous volume of inquiries we receive, we cannot guarantee a reply to every question.
