Shock absorbers & springs, steering
dampers (dampeners), suspensions, ....
for motorcycles and sidecar rigs.
article 54, section 8A
First ....a basic discussion of shock absorbers, and some things about springs:
Shock absorbers do a number of things, including what the name implies, 'absorbing shock' (bump) energy. In almost all modern motorcycles, the shock absorber is both oil and air operated, and is almost always combined with the suspension spring, typically the spring is located surrounding the shock absorber parts. The energy captured by the spring in either direction is moderated by the shock absorber, otherwise the spring would allow the parts of the suspension called 'un-sprung weight' to 'pogo' ...or oscillate up and down ...and any oscillation means less time the tire is on the ground. The total weight of the motorcycle is vastly greater than the 'unsprung weight', so the suspension moves the most. Shock absorbers are quite often, if not nearly always, designed such that their effect in one direction is different than in the other direction; and some shocks are adjustable in one or both directions. Often these adjustments are called Bound (compression) and Rebound (extension or lowering of compression). With the proper shock absorber valving, and correct amount and type of oil, and correct amount of air above the oil, the desired characteristics of the shock absorber can be had reasonably closely. Sophistication of a shock absorber....and spring....are highly variable.
The spring length at rest is usually adjustable on rear suspensions by a lever or special wrench, sometimes by hand rotating ring. The spring length compression adjustment on front suspensions is usually adjusted by adding or removing spacers. In some types of forks there are more than one spring. Some types of front suspension units have adjustable spring perches, that set height (front suspension length, etc.).
For the front suspension on Airheads, the shock and spring parts are inside the fork tubes and fork tube lowers. The spacing of the fork tubes and the point where clamps are installed (and, handlebar attachment and wheel axle) sets parameters, as opposed to most all rear shock absorbers with their springs, that are rubber mounted and do not set anything but height of the rear suspension. Spacers are used inside the front forks to set SAG. Sag measurements are of TWO types. (1)Static sag is the amount that the front fork settles, from the the position of front wheel and tire being OFF the ground; compared/measured .... to the position when front wheel and tire is ON the ground. This type of sag is measured with the motorcycle NOT on the center-stand, not loaded with luggage NOR rider NOR any passenger. (2) For the second measurement, the motorcycle is loaded, tires on ground, ready to ride...note that the tire off the ground, but the second the rider is on the motorcycle; plus any other loading with luggage and rider/passenger, and the motorcycle is being balanced by a helper, and NOT by the riders toes/boots. Both types of sag are important, and there is even a third type, a variation on the two. Sag is needed to ensure that the suspension works in both directions under normal conditions, without bottoming out nor topping out. Springs are selected for the expected weight, etc., but the final adjustment of sag is usually adjusted by installing spacers of appropriate length at the top, sometimes bottom, of the fork springs. Sag is typically a few inches.
Misconceptions are common about how springs, and adjustable spring perches actually work. Just one such is that spacer (front forks) or perch adjustment (rear fork, some front forks) sets the spring "strength or stiffness". The spring RATE (force per movement amount) stays the same (until the spring binds-up from extreme movement, and there is no further compression of the spring possible. There is a type of spring that is progressively wound. That type of spring begins movement softer, and then that portion of the spring becomes bound-up, and the rest of the spring is then functional, which has the effect of progressively stiffening the suspension. This can be good ....or not, ....depending on MANY factors. The spring coils THEMSELVES almost always have a fixed thickness, as changing the thickness someplace along the spring length may bring weaker places. The mathematics for calculating spring performance are not easy to understand, and while I have the formula(s) on this site, I am not putting them in this article.
If nerdy enough, go here:
https://bmwmotorcycletech.info/nerdy-stuff.htm ....scan down to one of the spring formulas, at item 13.
My attempt at a simple SAG explanation:
Imagine a spring that is quite long, and place it vertically. It will be a measureable amount tall. Now place a substantial weight on the top. It will cause the spring to compress a measureable amount. If you add another such weight it will compress the same amount, again. You can keep adding weights until the spring acts solid, as all the space between the spring coils is used-up. If you went back to where you used just the first weight, and put a light weight spacer between the spring and the large weight, the large weight would still compress the spring the same amount, but the height of the top of the weight would be higher. This is how suspension sag is adjusted, by spacers, or by adjustable perch that raises motorcycle height.
The common shock absorber and spring units made by Koni (now IKON) and installed at the rear of the twin-shock Airheads, most of which used the 7610-1298 model, have an adjustment at the SPRING perch for ride height. The idea was that as you increase loading, such as adding a passenger, you could easily adjust the spring perch to compensate for the suspension sagging. Many folks do not know that these shock absorbers may have an adjustment via a black toothed wheel under the rubber cup at the top, that is an adjustment for REBOUND valving. Some types of Ikon and Koni shocks do not have that wheel, but are adjustable by rotating the upper part, which is kind of a PIA, compared to the wheel type which is very quickly adjustable. Some models have NO adjustment.
Springs can be made in progressive or linear types, but a general feature, while the spring is operating in primary mode, is that the pressure to make the spring collapse, or release pressure, is ~ the same per unit of travel. If the spring unit is not in a good operating range, it is best to change the spring to stronger or weaker.
Suspensions of motorcycles tend to be compromises, more or less.
There are both stock and aftermarket springs, cartridges, and other 'goodies' available. For the Airheads, one popular modification long ago was the addition of a separate short spring at the bottom of the inside of the front forks. The results were mixed. There are some much more modern modifications that work better, over-all.
Besides absorbing spring energy; shock units, properly designed and set-up, will greatly help keep the associated tire & wheel in contact with the ground, particularly on quite bumpy roads; and most particularly, at least to the typical rider, during bumpy downhill turns. Generally, better-quality shocks are quite good at this. A complicating component is the speed of the shock absorber action and reaction. Extremely numerous short-spaced road irregularities or bumps, such as from track-driven road grading machines, are very difficult to accommodate, compared to large long smoother bumps.
Another factor is stiction. Stiction is a term usually used to mean the difference in force needed by a bump or other force STARTING movement, and the lesser force needed to continue movement. Stiction can also be used to describe excessive continuing force. Stiction should be minimized, particularly on front forks, otherwise small road irregularities are not going to activate the suspension, and you will get a harsh ride and tire skipping on irregularities. It is common for slightly twisted or misaligned front forks to exhibit excessively large amounts of stiction; and very common for such as poor fender mounts/braces to put sideways forces on the fork tubes, causing excessive stiction. You can eliminate a lot of stiction by careful adjustment. On some forks, excessively stiff seals will cause excessive stiction. Stiction is seen now and then on the rear suspension too, but is mostly overlooked and not dealt with. While sometimes seen on nearly frozen-up suspension bearings (poor swing arm bearings can mask good suspension operation), it is also seen from such as bent shock absorber rods (it is also seen from kinked/bent front fork tubes).
A complex matter, since it affects nearly everything, is adding and removing weight to the unsprung parts. Lesser so is changing weights of the sprung weight.
I have seen quite a few instances where an owner has increased the thickness (viscosity) of the front forks oil, to try to 'compensate'; or, somehow 'improve' the ride........when the REAL problem is improper fork alignment and excessive stiction.
High quality high performance rear shock units are expensive, but will give better control over bumps and other road irregularities ...no matter the size or frequency of such, and will not 'fade' or change characteristics very much, as the absorbed energy is converted to heat that is dissipated better than lesser stock units. Rear shock(s) have a very beneficial or detrimental effect on how the front suspension works. Beware of purchasing shock absorbers that have excessively stiff springs.Whether talking about front forks springs and shock absorber parts or rear spring/shock units, the principals are the same. A poor shock unit at the rear will have the suspension bottoming out, might top out, and usually the worst part is that the tire will skip-around a lot ...and transfers its bad effects to the front end! I have even seen rocking-horse operation! Premium shock units are much better at controlling the bike under such as grading machinery tracks, often just called Cat Tracks, on such as hard-packed roads. They are called Cat Tracks, from the company name Caterpillar, that makes heavy duty road maintenance vehicles with steel treads (as on military tanks). Rear shock absorbers are sometimes seen on motorcycles that have much too stiff springs. At one time, Ohlins were commonly sold like that, and I don't know why. At the front suspension, comfort, but especially handling, under almost any imaginable road conditions, or no roads! ....is important. BMW has compromises in its suspension designs, changes are possible for an individual's desires. Alignment and stiction are primary concerns that must be dealt with first.
A tell-tale of a very improperly modified or mal-adjusted front fork on a BMW Airhead is finding a overly thick oil in the forks, because often an oil considerably thicker than stock viscosity is wrongly installed due to high stiction ...which leads to the next paragraph. ...leading us back to stiction again.
A good suspension, which includes the shock absorber(s) and spring(s), should have as little stiction as possible. Stiction is exceptionaly annoying when it's value is high, and you will especially notice when you ride over tar strips or other small to a bit more moderate irregularities in the road. It feels like the suspension is not moving, not absorbing the shock of the irregularity; and, that is exactly what is going on. It is very common to see high stiction in front ends of telescopic fork motorcycles. It is usually fixable with modest effort and often with NO parts cost!
As a somewhat general rule, the cheapest shock absorbers have characteristics that may be adequate but are not really all that good, and ride quality and handling suffers ...sometimes quite substantially. Except on very high-end motorcycles, the factory shock units are generally a compromise between cost, performance, and average rider acceptability ...and abilities. While this applies mostly to the front forks, it actually also applies to the rear shock absorber ....which often surprises owner/riders. Lower cost factory rear suspension units tend to not last very long, and tend to not be rebuildable, or not easily.
Although the front fork suspension shock/spring unit and the rear shock spring unit may look totally different, they both have to do a similar job, and the performance of one will affect the other ....in particular the rear affects the front considerably. IMO, the front suspension, due to its long travel, is much more critical.
Many have no idea that an assortment of springs are available for just about any shock absorber unit, here is the list of them for the common Koni shocks (Koni is now owned by IKON of Australia):
STEERING DAMPERS (also called Dampeners):
Steering dampers are a shock absorber without a spring and with equal characteristics in both directions. There are various types of 'dampers' ....and just the word 'damper' and the two words 'shock absorber', all have somewhat different meaning in various Countries ....typically though, the difference is between suspension shock absorbers and steering shock absorbers; where 'damper' is more often used with steering.
Some steering dampers are hydraulic, oil operated, may have air or nitrogen damping. Most steering dampeners have equal damping in both directions, and are not very sophisticated; but some hydraulic types are quite sophisticated, and these are generally not original motorcycle equipment. Early Airheads had friction discs dampers. The characteristics of simple hydraulic types versus friction types is quite different. A friction damper is nearly ideal for a sidecar rig with its shortened trail and propensity to go into low speed wobbles without the damper. Friction dampers work very well on very rough roads. Friction dampers may cost more to manufacture than simple hydraulic cylinders in much more common use. Some BMW motorcycles have no dampers. A popular conversion when adding a damper for sidecar rigs is to use a common and cheap VW steering damper; which can work well-enough with careful attention to stroke amount, otherwise the damping may be excessive and the steering not return to center by itself, which can be very tiring.
Steering dampers can have excessive stiction.
Some have installed; or, turned on their dampers (steering shock absorbers) to mask out effects from other problems ...such as excessively loose wheel bearings or loosely adjusted steering head bearings. Some few still seem to think that dampers are the answer to supposed tendencies of some bikes to have tank slapper's. Airheads with properly set up bearings, & proper use of accessories, including fork-mounted windshields, don't have problems. There is an article on this website about tank slappers, and other such, here:
The short wheel base versions of the /5 models (prior to ~mid-1973 change to the longer wheelbase) can have substantial problems with larger FORK-mounted fairings or FORK-mounted windshields, and the stock damper can help with that. Any Airhead with large fork-mounted windshields or fairings can have instabilities. The R65 with the short wheelbase (early models of R65) is less susceptible than the SWB /5.
Motorcycles can get increasingly close to, and even get into serious instabilities with such fork mounted windshields or fairings, but problems tend to be increased by use of rear trunks, side luggage, backrests, rear weight ...AND, VERY ESPECIALLY, with flat-worn (that is, squared-off) rear tires! In some instances the use of a bit of damper can allow 'more comfortable' higher speed cruising, than without the damper. As more and more damper action is being used, the bike will start to, more and more, exhibit WEAVING; or, this might be described as heavy handling, being more pronounced at quite slow speeds. The damper is then beginning to inhibit self-centering of the steering, which is why the weaving. Excessive damping can get to the point that the steering must be manually moved back to center, which can be quite tiring.
SO: There are obvious limits to steering damper 'help' ....and that limit might well be very suddenly reached. That is why a PROPERLY set up bike is VASTLY better than using any damper, at least for paved roads use ....except in a few instances, such as using a bit of damper action on rain groove roads with ribbed front tires, and a few other situations. Just because the bike is wiggling some does not necessarily mean you need some damper turned-on. For some riders, any wiggle is frightening, however. More on this below.
Use of steering dampers always makes steering effort a bit ...or more than a bit ...higher. They can, whether friction disc or hydraulic-type, do a good job at masking certain types of road irregularities, but at the cost of heavy handling ...more or less. They were often used by folks that had mostly unfounded fears of loosing control of their bikes on the popular rain-grooves being increasingly added from quite some years ago to paved roads (especially concrete freeways). This was, and still is, particularly so with straight-ribbed front tires ...the old design series Continental's are a prime example. Straight-ribbed tires are still made, ...most very considerably follow rain-grooves ...and when the rain-grooves were made by tipsy-road-workers, they can really wake you up from your 'only one cuppa coffee' on your morning commute.
Dampers, not used to excess, CAN be of substantial help in very soft off-road work, in particular they are nice on irregular cobblestones, rocky surfaces, and so on ...and in any situation where some larger irregularities might try to, seemingly, not necessarily ...wrench the bars out of your grip. Use on such roads; and, compensating for tiring rain grooves on freeways, is a prime use for dampers. I prefer hydraulic dampers for off-road riding in areas where the surface has SUBSTANTIAL sizeable rocks, stones, & irregularities. In a few instances of quite soft-going, they can be of help too.
Many riders use dampers when such use is NOT appropriate.
BMW has had friction or hydraulic dampers on various models at various times. In general, the pre-Airheads and into early Airheads, have flat plate friction dampers. BMW added three-position adjustable steering dampers of the hydraulic type to such as the Airhead RT models ...but managed to eliminate dampers entirely on the last RT models.
Dampers & 2-wheelers and/or sidecar rigs:
Unless your motorcycle has a sidecar attached and has wild oscillations problems with proper fork steering head adjustment (adjustment is tighter for sidecar rigs); or, you have a front fork mounted windshield or fairing on a 2-wheeler and are having instabilities; or, have a big dislike for the 'feel' of the bike on soft or very irregular surfaces; or, insist on a ribbed front tire but dislike rain grooves ...I see no good reason for a damper.
The use of steering dampers has a drawback, barely mentioned earlier. They WILL reduce the 're-centering' forces, thereby making you work harder to steer your bike. This is true for 2-wheelers as well as sidecar rigs. I have seen sidecar rigs in which the steering was just plain UGLY, due to the wrong use of dampers. It is FAR better to properly adjust the steering head bearings, and POSSIBLY NOT use any steering damper, whether it is a friction or hydraulic type, and this is for 2 wheel bikes and 3 wheel sidecar rigs... and trikes. If you have to use a damper, something is WRONG if it needs to be so stiff that weaving occurs, and re-centering is nil.
Dampers & Sidecars:On many sidecar rigs a damper is an absolute necessity, some rigs are uncontrollable in the range of 10-25 mph without a damper. Use of flat tread sidecar-type tires on the FRONT will lessen the amount of damper needed. Excessive dampening will result in heavy steering and sore shoulders, etc. MUCH of the time front end oscillation that requires a damper is due to the sidecar rig forks having much shorter trail than stock forks. The effect is VERY similar to oscillating small wheels on a supermarket cart, with, perhaps, a different rate and amount. When I find I have to install a damper on a sidecar rig, I ALWAYS make sure the wheel bearings and steering head bearings are properly set up FIRST, since if these things were not set up correctly (example: the steering head bearings are almost always, or should be, set up TIGHTER for a sidecar rig than for a 2 wheeler), there MAY be NO need for a dampener to be added. Since the type of front tire has a moderate effect, that is taken into consideration too (the flatter and wider the tread, the LESS dampening needed, since the tire itself acts as a dampener, due to friction with the road; same for higher weight). A small amount of oscillation, in the ~10-25 mph speed range, particularly when just starting off from a stop, is often not a good reason for a damper. In many instances, where a small oscillation occurs, keeping the hands on the bars, and not tightly gripped either, is enough to dampen any tendency for oscillation on a sidecar rig. If oscillation is much larger, first check that the steering adjustment is properly tight enough ...that will ALWAYS be tighter than if riding a 2-wheeler.
Because sidecarists may use large handlebar inputs at times, some prefer friction dampeners, but not all prefer them over hydraulic types, and many times it is not so easy to install a friction dampener. Installing a friction damper on a motorcycle or sidecar rig not equipped with one already, can be quite an effort. It also can be very worthwhile, over using a hydraulic type. Friction dampers also usually last forever, not so the hydraulic types.
In many instances it is possible to install a hydraulic damper in a way that you can select the amount of STROKE. This is a nice improvement. BMW's hydraulic steering dampers are adjustable for stroke per unit of travel, by the large knob in front of you, in the bars area, but often are not effective enough. You can use the minimum amount of stroke that works OK, without adding the need for too much steering effort. Unfortunately, stock dampers, particularly the hydraulic types, are often not effective enough for sidecar use, and many times one can modify them with perhaps another steel plate and one more friction pad. Most install hydraulic dampers, and some of us make them adjustable for stroke, which is usually easy to do.
For the hydraulic type the best adjustment MIGHT BE, on SOME types, to modify the internals of the damper for the correct dampening action, but only a few nerdies like me do that, as it can be quite a job, and some educated guesses need to be made about valving changes ...or oil viscosity changes ...OR BOTH. I do both, and often also adjust the stroke amount by means of selectable mounting points.
04/08/2009: Add section at top on shocks in general.
01/23/2010: Clarify and add more on damper effects.
05/12/2012: Add link to the springs pdf, inadvertently left out previously.
10/14/2012: Add QR code, add language button, update Google Ad-Sense code.
09/26/2014: Slight updating, and fix for smaller screens ABP.
10/24/2015: Typos fixed.
03/27/2016: Update metacodes. Clean up article some, clarify some details. Change formatting method to be better with smaller screens.
11/08/2016: Metacodes, scripts, H.L., better explanations in some areas. Fix HTML problems.
04/19/2018: Reduce use of HTML, colors, fonts. Clean up layout. Add 10 pxl margins. Minor explanation improvements.
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Monday, December 07, 2020
Last check/edit: Monday, December 07, 2020