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Suspension Adjustment
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April 8, 2011 at 7:57 pm #4367TrialsRiderParticipant
Let’s crank open a fresh can of worms to explore
…not for the timid!Maybe you’ve been riding for a while and finally got around to actually inspecting your bike, or you read the owners manual for the first time and discover; Damn! my suspension components have knobs, valves and adjusting collars, but what can I do with these thingies, I don’t dare touch them, surely the dealer put them right where they belong. …Of course everyone knows the true reason suspension adjustment knobs exist is so some kid can mess with your motorcycle when it’s parked unattended. (sarcasim intended)
So where to start:
On most front fork stanchions or rear shock damper rods, you can usually see faint dirt or oil track marks after some spirited riding, these marks suggest the amount of suspension travel you are actually using. This simple telltale can give you insight into how strong your spring pre-load is relative to your load weight and how aggressive your riding style and terrain is. If you are barely using any suspension travel, chances are your bike is over sprung for optimal performance and you likely have room for adjustment. If you are using most or all your suspension travel (bottoming out on occasion), your spring rates are possibly too low, parts may require replacing, or you’re simply too heavy for the bike in it’s current configuration.Some general considerations:
Overall lowering of a motorcycle generally improves handling, largely a result of a lower center of gravity, lower center of mass or whatever Owlie says;) The low limit is determined by your need for ground and cornering clearance. Lowering the front or raising the rear has an effect of increasing steering sensitivity. This would be a desireable change if your current setup is prone to ‘understeer’ (slow to respond to input going into and exiting corners) Too much forward bias can result in highly sensitve steering, steep fork rake, smaller front wheels and shortened wheelbase, do tend to get around corners great but inspire less confidence at high speed straight runs, the bike has a lighter feel and responds quickly to subtle input at the controls. (much like a Trials bike at high speed, which BTW is a very scary concept) Lowering the rear, raising the front forks or increasing the front wheel diameter imparts slower steering response, transfers weight rearward and tends to improved high speed straight line stability.Lowering the triple trees on the front fork stanchions is a popular mod with Cafe Racer aficionados, it usually accompanies lowering of the rear suspension, because the desired effect is to achieve an overall lower center of gravity, center of mass or whatever Owlie says;) …looks way cool too. The limit for lowering your triple trees on the forks is determined by clearance, and not just with the ground but also between the front wheel travel and other bike parts.
Lowering or raising the rear is much easier to accomodate on ‘linked’ mono shock suspensions, a pair of small aluminum plates can be exchanged to alter the ride height, without affecting spring rates or preload. Most non-linkage rear suspensions can only be altered by replacing the shock and spring unit with a longer or shorter version, or by repositioning the shock mount on the frame or swingarm.
Fork oil viscosity:
The easiest way to alter your front fork dampening beyond the range of any built in adjustments, is to replace the fork oil with thinner or thicker oil. Typical fork oil weights are 5, 10 and 15 weight. My personal experience has indicated 5W is best for Trials and Motocross where lots of suspension travel is required, 10W for street use and my results with 15W or thicker fork oil was a waste of money. It is far more difficult to determine the best shock rebound dampening setup, the change through adjustment is barely noticable, right up to the point where you need it to work perfect. If in doubt; set the adjusters to the middle position and when you are testing, make significant setting changes in either direction to experience the difference.Tire pressure:
Don’t overlook the effect that tire pressure has on handling, the manufacturer numbers are a good start point but inappropriate tire pressure can result in a variety of what feels like suspension issues. Both on & off road Racers usually run lower pressures to improve traction. Not sure what the land speed record guys do ?:i but I’m guessing they just run em’ hard all the time.Things to watch for:
Stand next to or straddle your bike (not on the stand, no rider weight on the bike) and lift up on the handle bars (easier with the front brake applied). Does your suspension come up an inch or more and then ‘clunk’ as it tops out? …chances are it did not do that when it was new and it’s not a good thing! This indicates the fork springs are fatigued or there is inadaquate spring preload, which you might be able to adjust on some forks. Similarly lift on the rear suspension, on a mono shock with linkage, freeplay can indicate worn parts that warrent closer inspection and possibly replecement. On a related subject, some front forks have air valves in the top, it might be indication of a shock with nitrogen gas cartridges, but more often they are just simple valve stems to increase spring pressure and adjustment. 2 or 3 pounds of air is typical so do NOT visit your local gas station and pump them up to 40 PSI.Danger: Nitrogen Gas Shocks are commonplace on modern motorcycles, the reservoirs and shock bodies contain nitrogen gas under extreme pressure and you should NOT drill, weld or attempt to dismantle things that can blow up. They work great because they are far less affected by the internal heat generated in normal operation and normally require very little maintenance.
…hope this helps some:)
April 9, 2011 at 12:25 am #29459TrialsRiderParticipantFull Floating Brake System:
…not to be confused with what some now market as a ‘full floating disk brake rotor’.Decades ago dirt bikes featured an innovation called a “full floating brake system”, it was a super simple design feature where the brake drum backing plate that is normally anchored directly to the swingarm by a post or short rod, is instead anchored to the frame by a long rod with near equal length to the swingarm. The advantage was under heavy braking on rough or washboard road surfaces, the wheel is far less prone to ‘wheel hop’ and chatter while the swingarm is moving through it’s travel on the bumps. This is such a simple design change it is puzzling that it didn’t find it’s way onto all motorcycle rear brake systems that followed, except for the fact it is less visually attractive and most public roads are considered smooth. Look at circa 1978 Suzuki RM400 or Yamaha motocrosser and enduro pictures for examples of the full floating brake arm system.
April 9, 2011 at 3:17 pm #29466Jeff in KentuckyParticipantoriginally printed @ beginnerbikes.com, August 2003
from a longer article by Richard Rose, with 7 Honda Shadow VLX bikes, some stock and some customized.Here we shall look at a topic many of us slept through in school: geometry. Specifically front end geometry concerning motorcycles.
First off we’ll look at rake. Rake (also known as Castor Angle) is the angle of the steering head — and consequently the forks as well as on most bikes they are parallel, in relation to a perpendicular line going from the ground through the steering head. A steeper angle yields a shorter wheelbase, a larger angle yields the chopper-esque longer wheelbase. Of course the frame/swingarm design also has a lot to do with the wheelbase, but the angle of the front end will ultimately seal the final figure.
Wheelbase will ultimately determine how much lean a motorcycle requires to handle a turn at a given speed. Take two different bikes regardless of weight and power into the same curve at the same speed, the one with the longer wheelbase will have to lean at a greater angle than the shorter wheelbased model in the curve. This means a 250 Rebel with its diminuative 57.1″ wheelbase will be able to take “Deadman’s Curve” at a speed of 45mph with a 25º lean, a Vulcan Mean Streak with it’s massive 67.1″ wheelbase would take the same curve at the same speed but with a sharper lean of 30º (these figures are simply for reference concerning this imaginary situation) This means that the faster both bikes go, the Vulcan will be dragging parts sooner than the Rebel as it will be leaning more to maintain the same speed as the Rebel in the same curves (further proof of the falacy concerning “outgrowing a beginner bike”). This is why sport bikes have shorter wheelbases by design than cruisers, it enables them to take curves at higher speeds.
Next up is trail. Trail is a somewhat confusing concept. The measurement itself is taken by drawing an imaginary line through the steering axis to the ground. Next drop a perpendicular line from the front axle to the ground, which in any production motorcycle should fall some distance behind the point where the steering axis line touches down. The distance between these two points is your trail dimension. Trail ultimately defines the handling characteristics of a motorcycle, how stable it will be at higher speeds, how easy it will flick through S-curves, and how easy it will be to control at sub idle parking lot maneuvers. Bikes with a short trail will be real easy to handle at slow speeds and quite responsive when the road gets occupied with esses, but at higher speeds these bikes will respond more to the road conditions and feel a bit twitchy. On the other side of the coin, bikes with longer trail dimensions may handle like a wheelbarrow in a parking lot, require a bit more encouragement to tackle an S curve, but track straight and true like an arrow at freeway speeds, offering little response to the road until the handlebars are activated by the rider. The trick for the motorcycle designer is to find some happy medium for the bike he/she is designing, something that will match the role the bike is intended for.
Let’s look at two different motorcycles that are compatible in terms of size and weight, Kawasaki’s EN500 Vulcan and Honda’s 600cc Shadow VLX. The Vulcan tips the scales at 439 pounds (dry weight), has a 62.7″ wheelbase, and sports a 33º rake and 5.9″ trail dimension. The Shadow weighs in at 439 pounds dry (or 445 with the chrome Deluxe package), stretches out to 63.2″ for wheelbase, and has a 35º rake/6.5″ trail dimension.
Without riding either one, we can presume the following handling characteristics based on what we now know about rake and trail: The Vulcan should be able to negotiate slow speed maneuvers with a bit more ease compared to the VLX thanks to a shorter trail, and at the same time it should be able to handle curves and corners slightly faster than the VLX as it won’t lean over as far at any given speed due to its half inch shorter wheelbase. The Vulcan also shouldn’t require as much force to negotiate an S-curve. On the freeway leading out of town, take both bikes up to the posted 75mph speed limit and the VLX will handle better, because its longer trail dimension offers better stability at higher speeds than the Vulcan. I’ve ridden both machines and can attest that these are in fact true.
If you don’t do the research and just start throwing things together from aftermarket catalogs, you could end up with a Franken-bike sporting an unholy handling pact that can bite back.
Bikes with longer rake and trail dimensions can be expected to be quite stable on the freeway but will feel more sluggish in curves and slow speeds, those with shorter rake and trail will be easier to control at slow speeds and in the curves but will feel more twitchy on the freeway.
(Shadow Shack answers email at an addy that can be found in his forum profile
Chop 2 It!)One word of caution on rigid/hardtail rides like stock Harleys built before the 1960s (Indians had back shocks much earlier); forget what you know about proficient cornering. Curves and corners will need to be taken slowly, as any bumps in the road can send your rear skipping and skating along causing a loss of traction. If you habitually try to beat every yellow light in right turns, this isn’t the mod for you. Bumps and potholes can send your tail skywards too, and rough roads can rattle your fillings loose. All in all this is a barhopper’s mod, but if you’re willing to sacrifice good handling in the name of style, nothing looks cooler than a slammed rear end and a fat tire stuffed up into the fender. Rear lowering kits can be had from numerous manufacturers including Highway Hawk, Scootworks, and Cobra. One good source for modifying OEM shocks is Sons of Thunder Metric Cycles, and SOTMC along with Scootworks carry hardtail struts for some models.
The most extreme choppers out there have frames that are modified for more rake, raked trees are added for looks and to make the bike more manageable, and the rear is often lowered.
Check out Seeger Cycle Accessories for info on chopper kits, they have them available for both Harleys and Metrics.
Check out the various HR3 kit bikes in the Custom Chrome catalog, you can get a complete kit that works right and looks good starting at $13,000 and up, and oftentimes that’s less than the asking price of a used stock Hog. Getting a title on a kit bike is easier than a custom build, but not as quick and painless as it would be with a factory production model.
In closing there are a few things I’d like to point out about the chopper mod. Do the research before buying your first part(s). Lots of it. Especially concerning rake and trail. Make sure you have everything you’ll need for the transformation before turning that first wrench, after all you don’t want to have the major stuff done and then find out you need a longer brake and speedo line. This means you’ll be forced to drool all over an unrideable bike while those parts are awaiting delivery somewhere in Podunkville. Having a second bike to ride is always beneficial, I have no qualms about the “one to ride and one for show” principle. Also of noteworthy mention, aftermarket parts designed to spec doesn’t neccessarily translate into quick and easy bolt-on simplicity. Expect to run into snags along the way, one company I went with (Rizoma) was so far off in every respect with my Shadow I think I would have had an easier time adapting Harley parts. Aftermarket parts compatibility isn’t the only issue to contend with, in some cases your kickstand ends up being too long or too short and the bike will be parked in a near upright or waaaaay leaned over stance, so be prepared to modifiy or exchange that too. If you can, purchase the most expensive parts first as they won’t get any cheaper in the next year or two thanks to inflation.
Finally, while this kind of modification can be easily performed on a beginner bike, it is not a beginner mod. A chopper is to the new cruiser rider as a 600+cc supersport is to the new sport rider. They handle much differently. They brake much differently. They corner much differently. In other words, get some good saddle time in on your first bike before considering a chopper. Besides, with a long wheelbase you’ll never pass the DMV riding portion of the license test when it comes time for the slalom through the cones. And leave early for work, because while the front wheel may be on time you’ll be five minutes late.
Keep those raked knees in the breeze.
April 9, 2011 at 5:27 pm #29468TrialsRiderParticipantBut on the statement: “…with a long wheelbase you’ll never pass the DMV riding portion of the license test when it comes time for the slalom through the cones” Not necessarily true, when I took my rider test in 1971, the Biker, Biker riding just before it was my turn aced the maneuverability test on a Triumph chopper stretched nearly as much as Captain America, it was truly awe inspiring !
…unfortunately he passed the test under an assumed name, because his original license was suspended for DUI.
April 12, 2011 at 12:00 am #29488WeaponZeroParticipantThe tip about fork oil is especially helpful as most beginner-friendly bikes have non-adjustable suspension and it’s really the only thing you can do to help it at all without upgrading the suspension. And let’s face it, most of us will end up replacing our first bikes before we replace the shocks and upgrade the forks on our beginner bikes
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