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The Rec.Bicycles Front Suspension FAQ

The Rec.Bicycles Front Suspension FAQ
by
Jim Gourgoutis [skoop@slackers.net]
August 16, 1996
Revised Version

TABLE OF CONTENTS:

  1. Suspension Basics

  2. Description of Common Spring Types
    • Steel Springs
    • Elastomer Springs
      • Solid Elastomers
      • Microcellular Urethane Elastomers (MCUs)
    • Air Springs

  3. Description of Common Damper Types
    • Friction Damping
    • Elastomer Damping
    • Oil Damping
    • Air Damping

  4. Springs & Dampers?--Putting It All Together!

  5. Suspension-Fork Mechanism Types
    • Telescoping Forks
      • Design
      • Inverted-Slider
    • Other Types of Forks
      • Linkage-Type Fork Designs (AMP, GIRVIN VECTOR, LAWHILL LEADER)
      • Internal-Headtube Designs (CANNONDALE)

  6. Aftermarket Accessories for Suspension Forks
    • Suspension-Specific Hubs
    • Brake Arches
    • Replacement Springs and Dampers

  7. Suspension Stems
    • Theory
    • Single-Pivot Designs (GIRVIN)
    • Parallelogram Designs (ALLSOP FRANKENSTEM)

  8. Comments Gleaned from the Net

1. Suspension Basics

Any suspension system will consist of two parts: a spring and a damper. The spring is what supports the load, in this case, the weight of the bicycle and rider. The damper is, essentially, what controls the velocity of the movement of the spring. A shock absorber in a car is an example of a damper. Any type of damper works by converting friction into heat. If you have a weak damper, the spring will be able to compress/expand very rapidly, and often will bounce several times after the initial impact (e.g., when a car has "bad shocks" it continues to bounce after hitting a pothole). Alternatively, if you have a strong damper, the spring will compress/expand slowly, and the spring's motion will be limited to one compression/rebound cycle.

The damper in a suspension system can take many forms. The simplest form is a friction damper, which just basically relies on the internal sliding friction inherent in the suspension mechanism. Next is what I'll call elastomer damping, which employs the rebound characteristics of certain rubber or polyurethane materials (more on this later). Finally, you can also find oil dampers, which are similar to an automotive shock absorber. This type of damper relies on forcing a fluid through a plunger with small orifices.

Some suspension mechanisms divide the damping action into two discreet stages, namely compression damping and rebound damping. Compression damping applies to the suspension while the springs are being compressed, while rebound damping applies to the suspension when the compressed spring tries to recoil back to its original shape. It is possible to apply different damping characteristics to each stage of the spring's travel to fine-tune the performance of the suspension.

Another important concept to understand in suspension forks is the notion of preload. Preload is a way to adjust the initial sag of the suspension, caused by the rider's weight on the stationary bicycle. Too little preload will cause the fork to sag excessively, using up much of the available travel (range of motion of the fork), while too much preload will make the suspension too stiff and harsh. Generally, manufacturers provide a way to adjust the amount of preload to suit any weight and type of rider.

2. Description of Common Spring Types

Steel Springs

The most basic type of spring mechanism is the steel coil spring, which most people are familiar with. Some forks simply employ a steel spring in each leg, which can be changed to accomodate different weights, and has adjustable preload to fine-tune the ride.

Elastomer Springs

Next in complexity is the elastomer-type fork. Rather than using a metal spring, this type of fork uses urethane rubber bumpers (typically from 1 to six), as the spring mechanism. Typically, the elastomers slide onto a skewer, or are stacked together with snap-in plastic spacers, and in this way, a composite stack can be assembled with different durometers (hardnesses) of bumpers, to give a wider range of performance (i.e., a couple of soft bumpers for small stutter bumps, a couple of medium bumpers to handle medium bumps, and then a couple of stiff bumpers for large bumps). The number and/or type of bumpers can be changed to accomodate differing rider weights, and often, the preload can be adjusted to fine-tune the ride.

Note that there are currently two distinct types of elastomer fork bumpers: solid and MCU. Solid bumpers are just that: solid rubber material. Micro-Cellular Urethane bumpers, on the other hand, consist of the same rubber elastomer material, interspersed with thousands of tiny air bubbles, similar to a really stiff sponge. This type of elastomer is supposed to offer better rebound characteristics and less sensitivity to cold temperatures.

Air Springs

The last type of mechanism is the air spring. Here, a chamber of compressed air acts as the spring. The stiffness of the air spring can be adjusted by pumping more/less air into the shock, usually using a special pump, or at least a special adaptor for a regular bicycle floor pump.

Usually, air springs must be topped off with air periodically (just like your tires). The air-tight seals between the stanchions and the sliders are very delicate, and must be kept clean to prevent damage. (It's a good idea to install shock boots on ANY sus-fork).

3. Description of Common Damper Types

Friction Damping

The most basic type of damping found in suspension forks is friction damping, which simply relies on the inherent sliding or rotating friction of the suspension mechanism to minimize excessive bounce and spring compression/rebound velocity.

Elastomer Damping

Next is what I'll call elastomer damping, which employs the inherent rebound characteristics of urethane or MCU bumpers. Elastomers do not "snap back" after compression like a steel spring, rather, they exhibit a certain amount of controlled rebound due to their material properties.

The degree of damping is determined by the hardness of the elastomers that you choose to install in your fork...the harder or stiffer the elastomer, the quicker it will want to rebound into...unfortunately, the harder the elastomer, the stiffer the "spring" in the fork becomes. In an elastomer fork, the two functions (spring and damper) are interrelated.

Oil Damping

Oil damping consists of a chamber filled with oil, through which a plunger passes during the fork's movement. The damping rate can be adjusted in a variety of ways...The oil can be replaced with an oil of higher or lower viscosity, which will either slow down the damping rate (with thicker oil) or speed it up (with thinner oil). The amount of oil, called the _level_, can be changed. Also, some forks have built-in adjustable damping, usually achieved by some type of knob. This knob is linked to the plunger in the fork leg, and controls the size of an opening in the plunger. When the opening is larger, more oil can flow through it, therefore the damping rate is sped up. In contrast, when the opening is smaller, less oil is allowed to flow through the opening, so the damping rate slows down. Finally, the size of the opening can be permanently changed by machining the plungers or installing new ones--this is called revalving the shock.

Air Damping

Air damping is very similar to oil damping, in that a plunger is forced through an air-filled chamber. Generally, air damping is not as efficient as oil damping, since air is much more compressible than oil is, and therefore may actually compress somewhat during the damping process. However, the benefits of air damping include lighter weight and less maintenance, since air is lighter than oil, and air damping requires less-intensive seals than oil dampers.

See KMR Cycles Online's Technical Tips & Instructions/Damping Modification Page for an interesting article on adding air damping to telescoping-type steel spring and elastomer forks.

4. Springs & Dampers?--Putting It All Together!

Now that we've talked about all the different kinds of springs and dampers that comprise modern bicycle suspension technology, you're probably wondering how these things work together. Basically, it seems that, with a few exceptions, it's a mix-and-match free-for-all. When bicycle suspensions were first introduced on the market, there were only two popular types: air springs with oil dampers from Rockshox, and elastomer springs (and therefore with elastomer damping) from Manitou. Now, the current trend seems to be to mix oil damping with MCU springs, or oil damping with steel springs, mixing MCU's and steel springs to maximize large and small bump performance, etc.

5. Suspension Fork Types

Telescoping Forks

Design

The most common type of bicycle front-end suspension in existence today is based on the telescoping slider-tube design. Basically, there are two tubes on each side of the front wheel. One tube slides inside the other, with the spring/damping system housed inside the tubes. The outer tube is called the slider, and the inner tube is called the stanchion. Usually, the slider is connected to the front wheel, so the front wheel and the two sliders move as a unit. The stanchion tubes are connected to the rest of the bicycle by the fork crown and the steerer tube, which runs up inside the head tube of the frame of the bicycle.

Perhaps the main disadvantage of this type of fork is what is known as "fork flex" or "wheel slop". The slider-type fork is essentially made up of two independent suspension units (each slider/stanchion set and the corresponding internal spring and damper). The two legs of the fork are connected by two elements: The front wheel axle, which clamps into the dropouts at the bottom end of the fork, and the brake bridge, which is an arch that loops over the wheel near the fork crown. The brake bridge serves two purposes--it serves as a mount for the front brake, and it also serves as a link between the fork's two sliders.

If the two connecting elements (the brake bridge and the front wheel axle) are not designed properly, they will allow the two legs of the fork to move somewhat independently, causing the front wheel of the bicycle to twist out of line (i.e., the front wheel will no longer be parallel to the steerer tube). When this happens, the front wheel may rub on either of the front brake pads, hence the name wheel slop. Steering precision is reduced.

Because of this, a number of companies manufacture aftermarket brake bridges and suspension-specific front wheel hubs with oversize front axles. These devices are reputed to increase the "stiffness" of the front suspension fork, and reduce wheel flop.

Inverted-Slider Types

Presently, there are a small number of forks available that use the inverted-slider design. Basically, rather than having the slider as the outer tube, the telescoping mechanism is turned upside-down, so that the outer tube becomes the stanchion and the inner tube becomes the slider. The outer tube is then fixed at the fork crown and the inner tube is attached to the front wheel. There are two theoretical benefits from this design: First, since the lateral stiffness of a round tube increases with its diameter, the larger-diameter stanchion tube (which is now the upper tube) should be stiffer than on other forks. This fact is supposed to reduce and/or elimite "wheel flop", and generally improve steering precision. Second, because the slider tubes are now smaller diameter, the unsprung weight of the sliders and front wheel should be less, which is generally touted as being a good thing in suspension systems.

One of the big problems of using this type of design in a suspension fork is that mounting a front brake can be a problem, since the standard cantilever brake bosses, if mounted in the usual location, would be mounted on the fixed portion of the shock, and therefore would not move with the front wheel. Generally, a disk brake is used to get around this problem. However, one of the more popular forks on the market which uses the inverted design is the Halson Inversion fork...standard cantilever brakes can be mounted on the Halson, because long slots have been cut out of the front surfaces of the upper (outer) stanchions, allowing the cantilever bosses to be attached directly to the movable slider tube inside. Therefore, the brakes are able to move along with the front wheel.

Other Types of Forks

Linkage-Type Forks Designs

A few front suspension forks on the market shy away from the telescoping slider-tube design altogether. Manufacturers of these forks claim that telescoping forks suffer from stiction, or static friction. Stiction may prevent telescoping forks from operating smoothly, in that the friction between the fork's sliders and internal bushings must be overcome before the fork begins to compress. Linkage-type forks, since they do away with sliders and rely solely on pivots, do away with sliding friction altogether.

Another advantage of linkage-type forks is that they are more rigid than telescoping forks. What is meant by this is that since the entire fork moves as a unit, wheel flop is totally eliminated.

One of the most popular linkage-type forks is the AMP fork. This fork looks like a regular rigid fork that has been cut in half at the fork crown, and a dual parallelogram linkage (this looks like a rear derailleur) attached between the fork legs and the steerer tube. The linkage allows the fork legs to move relative to the steerer tube, but remain roughly parallel to it. A spring and a small oil damper are included in the linkage to complete the suspension.

Another model of linkage fork is the Girvin Vector fork. The idea behind this fork is similar to the AMP, in that both forks use a dual-parallelogram linkage to define the fork's movement. However, while the AMP design places this linkage *underneath* the headtube of the bike frame, the Girvin Vector puts the linkage in *front* of the headtube. Needless to say, the Vector is an odd looking fork...the fork legs extend up past the fork crown, all the way up to the handlebars, and the legs are actually in front of the front axle! The dual-parallelogram linkage is made up of the upper portions of each fork leg, attached by four swingarms: two upper and two lower. The upper arms link the top of the fork legs to the steerer tube just below the handlebar stem, while the lower arms link the middle of the fork legs to the fork crown. An MCU elastomer-type spring/damper runs diagonally across the linkage, to complete the unit.

Internal-Headtube Designs

Yet another method of suspending the front end of a bicycle is the internal-headtube method. This is perhaps the cleanest-looking design, since all of the mechanism is housed inside the headtube of the bicycle, and the fork looks pretty much standard, except for a flexible rubber boot that covers the exposed area of the suspension. The idea behind this design is pretty straightforward: instead of having two telescoping suspension units, one on each side of the fork, this design uses a single unit to suspend the entire fork, and places it inside the headtube. This design yields an advantage similar to the linkage-type fork, in that wheel flop is eliminated, since the entire fork moves as a unit.

Probably the most significant drawback to this type of suspension is that it must be purchased as a unit with a specificly-designed frame; in other words, you can't retrofit a rigid frame for this type of suspension (at least in most cases that I know of).

I know of three manufacturers of this particular suspension design: Cannondale (perhaps the most well-known, with their "HeadShok(tm)"), Hannebrink, and Action-Tec(?-I'm not sure if they're still around, or even if this is the right name for the company).

6. Aftermarket Accessories for Suspension Forks

Suspension-Specific Hubs

Brake Arches

Replacement Springs and Dampers

7. Suspension Stems

Theory

The basic theory behind using a suspension stem, rather than a suspension fork, is to "suspend the rider, not the bike." Needless to say, this is a hotly debated topic, since there are staunch supporters of suspension forks and suspension stems. The idea is that since the bicycle is much lighter than the rider, and therefore only a fraction of the total weight of the bike/rider system, it is a better idea to suspend the rider, not the entire bicycle. In contrast to this is obviously a motorcycle, where the motorcycle constitutes the major portion of the bike/rider system...here it makes clear sense to suspend the whole system, rather than just the motorcycle.

Single-Pivot Designs

The single maker of single-pivot suspension stems is Girvin, Inc. Their stem is essentially just a regular stem with a pivot placed between the quill and the extention. The movement of the pivot is governed by an elastomer bumper, and different hardness bumpers are available for different riders. The advantages of this design are that it is light and relatively cheap. The drawbacks are that it provides very little travel, and that the handlebars rotate in relation to the rest of the bike, since the extention rotates through an angle at the pivot. Girvin stems are, I believe, available in 1", 1-1/8", and 1-1/4" quill sizes.

Parallelogram Designs

Softride, Inc. manufactures a few varieties of dual-pivot, or parallelogram, suspension stems. The main types offered are an aluminum and a chromoly stem, and also an aluminum Aheadset-compatible model. These stems all have a parallelogram design, similar to a rear derailleur, so that during movement, the quill end and the handlebars do not rotate in relation to each other. This feature is advantageous since the handlebars do not rotate, as they do with the Girvin stem. The movement of the Softride stem is governed by a metal spring with adjustable preload. There is no true provision for damping adjustment, although some users report that you can tighten/loosen the pivot bolts to adjust damping somewhat. Softride now also offers an add-on oil damper that attatches to the side of the stem. Since the Softride stem has more parts than the Girvin, it obviously weighs a bit more and is a little more expensive. Originally, the standard stems were available only with a 1" quill diameter, with shims available to adapt it to fit 1-1/8" and 1-1/4" sized steerer tubes. Softride now produces multiple quill diameters, with shims used on some models. The aheadset-compatible model is available to fit 1" and 1-1/8" steerer tubes.

8. Comments Gleaned from the Net

III. COMMENTS GLEANED FROM REC.BICYCLES.TECH AND .MARKETPLACE
From: michael@Ingres.COM (Michael S. Wong)
Subject: Re: M3 vs Mag21
I haven't ridden the new Mag 21 (SL, SL-Ti, MR2, 320i, MTV) but I've read 2 tests that said the plusher valving really doesn't offer that much more small bump sensitivity than the '93 Mag 21 (which I've ridden a lot and I find lacking this area).

I have ridden the M3. If you thought the M2 was active, then wait'll you ride the M3. Like MBA put it, "This thing will activate just by breathing on it". I currently own an FSX and I think I've found I prefer the more active forks. There was that article by some guy in Bicycling or BG who said he rides his road bike off-road and regularly dusts suspension forked MTBs, and that such paraphenalia is good for only 5% of the trail (not my trail! everyone replied). While he's probably full of it, it did somewhat remind me of my fork, with which I have to be moving at fairly high speed on fairly large bumps to get much effect. Otherwise, it's basically a very heavy rigid fork. The Manitou3, on the other hand, will absorb when you're just jammin' on a flat fire road or even when climbing.

I don't know about maintenance. I've always thought elastomer forks to be much lower (and cheaper) maintenance than air/oil. I *am* concerned about the teflon impregnated coated legs (the main reason for its sensitivity). What happens when they get scratched? I guess it just reverts to Manitou 2 sensitivity (not bad).

As for big hits (your main question), I can't offer much help there (maybe I'll go test ride one this weekend and let you know). Manitou took big steps to address this with the 3, and I'd imagine the Manitou reacts to big hits like a Quadra, with more damping and less bottoming.

From: godogs@dogpower.Corp.Sun.COM (Rick Brusuelas)
Subject: Re: control tech (lawill leader) fork
In article bit@news.u.washington.edu, topper@cac.washington.edu (Eric Schurman ) writes:
> Does anyone have any experience with these forks? The concept of a leading > link fork seems good. Does anyone have statistics on the fork > (weight, travel, etc)?

The magazines reviewed it within the last few months. Seems to have regained interest since Control Tech (a local company?) now markets it and now that Klein (another local company?) offers it as a fork option on some of their bikes (like the new Pulse).

As I recall, the magazine reviews were quite positive. The new Leader is a little lighter than the original ML Leader, but it still weighs in at over 3.5 pounds. They all like the travel provided (over 2.5 inches, I believe). And the leading-link design is supposed to virtually eliminate the problem of stiction.

It would seem to add too much weight for my taste, and I would also wonder about steering precision it... but it does offer an interesting alternative to sliding-tube suspension.

From: mbuk@cix.compulink.co.uk (John Stevenson)
Subject: RE: Suspension forks
Rick Brusuelas writes
>Couple of question regarding the Cannondale though... is the
>DV frame design (with the extra bar reinforcing the top tube/
>head tube junction) required to support the extra forces
>caused by the monoshock? Are the other two designs (specifically
>the Hanesbrink and Action Tec) susceptible to breakage, since
>they appear less reinforced in this area?

AFAIK the Delta-V design is intended solely to achieve a sensible standover. However, the guy who designed it, Mark Farriss, told me that they found after the fact that it was much, much better than a diamond frame in terms of fatigue life. Since the whole front end is braced more than a diamond, this doesn't seem too surprising. This is why Dale are now producing rigid bikes with this design, the Killer-V series.

>And a last question, specific to the 'dale... have they solved
>the seal problems that plagued the early shocks? Even in the last
>MBA review (which was very complimentary), the test bike suffered
>a seal problem.

I've not heard of seal problems on recent Dales. I believe Marzocchi now makes the suspension component for them.

From: godogs@dogpower.Corp.Sun.COM (Rick Brusuelas)
Subject: Re: 1993 Quadra vs. 1994 Quadra???
In article 14207@blue.cis.pitt.edu, skoop+@pitt.edu (James W Gourgoutis) writes:
> Anyone care to comment, agree, or disagree? What's the deal with the
> Quadra 21? What does it have that the 1993 Quadra doesn't?

The Quadra21 has the fork bridge of the Mag21. While that may make the bridge stronger and lighter (???), the main benefit is that you can now replace the steerer tube (important if you ever change bikes). There may have also been a change in the way you are able to adjust the preload.

I am not clear what elastomer difference, if any, exists between the Quadra10 and Quadra21. There have been concern about the performance of the Quadra10, which seems surprising if they share elastomers.

> Also, someone recently commented that the Manitou 2 doesn't take big hits
> well. They continued to say that the Manitou 3 took hits better, "more
> like the Quadra". Do they mean the 1993 Quadra, or the 1994 Quadra?
> ARGH!

The Quadra has been praised for its elastomer "height" It uses a very long (single) elastomer, which allows it to offer longer travel without coming close to the "compression" limits of the elastomer. This is supposed to improve its rebound characteristics. Another elastomer-based fork that uses a long single elastomer is Halson.

I have yet seen a Halson (either in store or on a trail). But the 1993 Quadras I have tried all suffered from so much stiction that I found it difficult to plop $250+ and *hope* it improved (I do not believe any had been "pre-greased"). But many do praise it.

I do remember that even though MBA praised the original Quadra, they offered the curious suggestion to have a firmer bumper in one fork leg and a softer one in the other (supposedly achieving better performance). I also recall Rock Shox stromgly recommending *against* this.

The Manitou3 also offers a "tall" elastomer "height". But instead of using a single elastomer, the Manitou3 uses a number of short ones stack up. This allows the user to fine-tune the fork by mixing elastomer durometer (hardness). The Manitou3's travel is supposed to meet or exceed that of air/oil forks.

Its cost ($360-380) puts it a little above the 1994 Mag21.

From: brant@cix.compulink.co.uk ("Graham Brant Richards")
Subject: Rock Shox Quadra 10
I know something was buzzing on the 'Net about these a bit ago, so...

After inspection of the drawings of the forks, I can tell you that last years Quadra is far closer to this years Quadra 21 than the Quadra 10. Although the Quadra (93) didn't feature a bolt-up crown, the internals in the Quadra 21 are very close to those in the Quadra (93). What the Quadra 10 is lacking in all this is a "friction ring", Rock Shox friction damper which tames some of the bounce of elastomer forks. Both the Quadra (93) and Quadra 21 have a friction ring.

From: slagaly@midway.ecn.uoknor.edu (Scott Lagaly)
Subject: Re: mounta bike shock ... opinions

The Answer Manitou threes are great forks. I also have a set of Marzocci XC-400's. The Manitous perform well for ALL bumps. From little stutter bumps to the big hits, they absorb everything. HOWEVER, the Zokes might not handle the little bumps very well, they do a better job at taking the larger ones. To sum up: if you like the feel of rigid forks, but those big hits are impeding your fun, buy some air/oils. If you want your front end to float like it's on water, buy the elastomer. I still haven't decided which ones i like best. But I do think the new XC-500 Zokes will compare better to the Manitous, since it has a four posistion damping adjustment, to allow more or less progressive damping. I.e. more it responds to the little bumps.