Church of Choppers Blog

SUSPENSION 101 FOR THE SUSPENSION CHALLENGED BIKER

Most of us are pretty much in the dark about how shocks can, should and do work.  They don't generally teach this stuff at school unless you are majoring in hydraulic engineering! So bear with me as I attempt to give you some basics on shocks and setup!

Shock absorbers (dampers) are there to damp (slow down) the up and down movement of the bike and load, ideally as gently as possible to avoid transmitting harsh impacts to the rider(s).  Basically, the shocks should keep you floating between bottomed out and topped out as the vehicle runs over various surface conditions from smooth to choppy to undulating and everything in between.  There are several components that work together in a shock to achieve this end. 

1) The first is the spring(s).  The correct spring(s) will support the weight of the vehicle and load at correct ride height and return the bike to the correct position after each compression or extension of the shocks to position them correctly to absorb the next bump.  Ride height is commonly referred to as sag.  This is the measured vertical distance the bike squats down under the weight of rider/passenger/baggage load (Ride Sag) when the load is placed on it. Ride sag with load aboard is what you need to know and set correctly.  Sag is the difference, measured vertically at the wheel, between fully extended suspension with no load and loaded. If you have shocks with air springs, the adjustment for sag still applies. 

Correct sag is approximately 30% of total wheel travel.  If you are riding a modern HD FL model with stock length (13.0 inch) shocks, you have close to 4 inches of wheel travel.  Therefore, 4 X 30% = 1.20 inches of sag.  To check it, stand the bike up vertically and have an assistant lift up on the rear fender to make sure the shocks are fully extended.  Then measure from the center of the rear axel to any arbitrary point vertically above the axel (bottom edge of fender, bottom of fender rail etc.) (Note** If you have saddle bags, the measurements can also be made from the ground to any arbitrary point on the bag, as long as your measurements intersect the center of the wheel/axel as closely as you can).  Record this dimension.  Next, get on the bike and bounce it up and down a couple of times to fully settle the suspension.  With your full weight(s) on the bike, have your assistant make the same measurement to the same point.  The difference between the two measurements is the ride sag.  Sag should be the same whether it is just you on the bike or you and a passenger and or baggage.

 This is where your shock preload adjustment comes in to play.  Many shocks have a ramp type adjustment on them with 3 or more positions.  Better quality shocks have a threaded adjustment which allows a more accurate adjustment.
 Preload is the amount of pressure on the spring.  Let’s say, for example that the spring on the shock has a 120 lb. per inch rate.  This means that it takes 120 pounds of force to compress the spring one inch, 240 pounds @ 2 inches etc.)  With one inch of preload, the spring will resist (support) 120 pounds of weight. 
You need to increase the amount of preload to raise the bike up if it is more than a 30 % sag dimension or decrease it to lower the bike down if the sag dimension is too little.  Springs that are rated correctly will allow a correct sag adjustment without excessive preload on the spring.  Excessive preload or the inability to raise the bike to the correct sag setting indicates a spring rate that is too soft to support the load correctly.  The opposite effect, when the ride sag is not enough, and the bike will not squat down to the correct level, indicates a spring that is too heavy for the load. 
Most motorcycles sold in the USA have shocks that are sprung for “ average “ rider weight of appx. 180 pounds + or -.  If you weigh 120 pounds, chances are that you will not be able to get a correct sag setting and conversely, if you weigh 250 pounds, the same may be true.  For the heavier rider this is, of course, made worse if you also pack a passenger and a baggage load.  The light rider often benefits by having more weight on the bike.

There is more to say on springs (dual rate vs. single rate,  etc. maybe at a later date.  I'm trying to give you sufficient info to understand and set up your bike without boring you to death!!

2) The second part of the functioning of your shocks is piston valving.  Internally, there is a piston on the end of a shaft that plunges through oil whenever the suspension moves up or down.  The configuration and design of this piston is what controls the up and down movement of the bike through its range of wheel travel by controlling the rate of oil flow through the piston. 

If oil flows at too low a volume relative to an impact it results in a harsh ride quality.  You can probably identify with this effect in relation to expansion joints or pot holes. When you feel a harsh spike under these situations, it can be as a result of oil passing through the piston at too slow a rate.  Oil is non compressible and this creates a hydraulic lock and the suspension goes solid for an instant, transferring that sharp impact to the rider and passenger.  The impact is more pronounced for the passenger who sits directly over the rear wheel.  This is why your sweetie tenses up when she sees a bump coming!  This effect can be mistaken for bottoming out the suspension, which occurs when oil flows at too fast a rate and inadequately slows the downward movement of the bike.  There are a few different piston designs that are more or less effective.  More on that at a later time. 

At his point I'll address the common quote of " I want shocks that won't bottom out!"  You do NOT want shocks that will not bottom out.  What you DO want is shocks that bottom out under adverse conditions and not every time you go over a normal irregularity or small bump in the road. Your shocks should use all the travel available to effectively damp the vehicle's movement.  Shocks with valving that would be aggressive enough to never bottom out would be most unpleasant to ride on.  You cannot expect to hit a 6 inch deep pothole on a bike with 4 inches of travel and not experience bottoming.  On the other hand, if you're riding a modern dirt bike with 12 inches of suspension travel, you can cruise straight through such an impact because of the added distance the suspension has to absorb and dissipate the energy of the hit.  The more distance you have to damp the weight of the vehicle in, the gentler the impact will be.

This is a good time to address the issue of lowering a bike.  There are only two "good" reasons to lower a bike. Firstly, to get your feet on the ground; and secondly, for ”the look".  In either case, the effect on the ride quality of the bike will be adversely impacted.  Again, the more suspension you have, the better the ride will be.  If you take a bike that has 4 inches of suspension and reduce it by 50% the results will probably be less than pleasant.  You can't expect a cruiser (approaching 1000 pounds w/rider aboard) and only 2 inches of travel available to damp that weight in to ride like a '57 Caddy!  Ain't gonna happen!!

Sooooo........... before you get to overload, that’s it in a nutshell and I'll stop here.

Some things to consider when you shop for shocks: 

Your choices are either generic, off the shelf or custom built shocks.  Surprisingly, there are custom built shocks available out there at very similar costs when compared to the generic brands.

Generic or "off the shelf" brands have common piston valving,  This means the piston valving that controls the movement of the bike through it’s range of travel is the same  whether you weigh 300 pounds or 120 pounds.  Some offer a spring option, standard or heavy. Generic shocks generally do not come with a performance guarantee, for obvious reasons.  They may work ok for some and not for others.   If you weigh 250, carry a 200# passenger and 70 pounds of baggage and tow a trailer too, don’t expect any generic to work well for you.  Some generic brands may be rebuildable and most are not.  Generic shocks that are  shorter than stock are usually pretty stiff to ride on.

Custom built shocks by companies like Works Performance  (www.worksperformance.com) build your shocks for you based on your actual weight requirements, riding style and skill level.  They offer a ride quality warranty and a parts and workmanship warranty and all their products are fully rebuildable and American made.  They do sell through Drag Specialties and the shocks they supply to them are built for the listed weight parameters and still come with the afore mentioned warranties.  You can get specific product info for your bike or order directly from Works at sales@worksperformance.com.  The Only hitch with custom shocks is that you have to wait for them to be built, as they make them for you to your specs.

Ride on and be careful out there!!
Thank You,

CoC will have a permanent side bar link to this. 
Thank you Tom for the info.