Steering dampers, suspensions,
shock absorbers, springs....
© Copyright, 2012, R. Fleischer
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 hydraulic and air operated, and is almost always combined with the suspension spring, typically the spring is wrapped around the shock absorber cartridge. The energy captured by the spring in any one 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. 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).
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 adjustment on front suspensions is usually adjusted by adding or removing spacers.
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) is the amount that the front fork settles, from the the position of front wheel and tire being OFF the ground .... to the position when front wheel and tire is ON the ground; (2) 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, 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. Sag is often 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) sets the spring "strength". 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 has 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:
http://bmwmotorcycletech.info/nerdy-stuff.htm ....scan down to one of the spring formulas, at item 13.
My attempt at a simple SAG explanation:
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, adjust the spring perch for a higher setting. 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. Other things are involved in shock absorbers, such as air cushion distance and amount, shock action, etc., but, adjustment of a spring perch, done for adjusting riding height such as for a very heavy rider, or two-up, etc.
The truth is that the suspensions of motorcycles tend to be compromises, and if the rider or rider with passenger, loading, etc., is such that the spring unit is not in a good operating range, it is BEST, THEN, to change the spring to stronger or weaker.
There are both stock and aftermarket springs, cartridges, and other 'goodies' available. For the Airheads, one popular modification was the addition of a separate short spring at the bottom of the inside of the front forks. MANY aftermarket modifications will actually make handling WORSE.
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 is to be minimized 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 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. 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).
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.
The best REAR shock units are usually 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 cheaper stock units. Even 'Just Good' REAR shock(s) have a very beneficial 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). At the front suspension, comfort, but especially handling, under almost any imaginable road conditions, or no roads! ....is very important. BMW has compromises in its suspension designs, changes are possible for an individual's desires. Alignment and stiction are PRIMARY CONCERNS TO 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 MUCH heavier 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 VERY annoying when it's value is high, and you will especially notice it 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.
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.
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):
The short wheel base versions of the /5 models (prior to ~mid-1973) CAN have particularly nasty problems with larger FORK mounted fairings or windshields, and the stock damper can help with that. In fact, any Airhead with large fork-mounted windshields 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 rear trunks, side luggage, backrests, rear weight ...AND 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.
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 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.
BUT: MANY riders use dampers when use is NOT appropriate ...this is especially so for paved highways in good condition with few irregularities, maybe only some rain grooves now and then.
BMW has had friction and 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, etc. models.
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. You can use the minimum amount of stroke that works OK, without adding the need for too much steering effort. In the case of the stock friction dampers on some motorcycles, they 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.
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.
2-wheelers AND/OR sidecar rigs:
Unless your motorcycle has a sidecar attached and has wild oscillations problems with proper fork steering head adjustment; 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.
Unless your motorcycle has a sidecar attached and has wild oscillations problems with proper fork steering head adjustment; 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.
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.
© Copyright, 2012, R.Fleischer
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