the theory page on this website gives a fair bit of insight into
the physics of vehicle dynamics, this next section aims to show how
that knowledge can be used to put theory into practice.
This section summarises most of the common types of suspension
systems available on the market today, and describes how a suspension
designer would go about working with them.
It looks at the pros/cons of each system from the designer’s perspective.
a more complicated suspension configuration doesn't guarantee a better
suspension design. A simple suspension configuration can easily be made
to perform better than a poorly designed more complex configuration.
This section focusses on how using different suspension configurations
affects the designer’s ability to tune the pedalling behaviour
(Anti-Squat or AS), and braking behaviour (Anti-Rise or AR) only, and
doesn’t go into detail about shock actuation.
With each of these
configurations, there are many possible ways of implementing the shock
actuation (between any two moving members). So, for simplicity, we
won’t go into detail of the different shock actuation possibilities.
It’s important to remember that ultimately the pedalling and braking
performance of any suspension system is defined by the AS and AR
curves, regardless of the physical configuration.
is the most basic suspension configuration. The swingarm pivots on the
front triangle in a single location (hence the name). The axle follows
a perfectly circular arc during suspension travel.
- The brake caliper is mounted to the swingarm.
- The location of the pivot affects the AS and AR curves.
- AS can be made to be decreasing, constant (approximately), or increasing, and the AS curve is very linear.
- AR is typically always decreasing, and the curve is also relatively linear.
choosing the location of a single pivot, the designer only needs to
consider what AS and AR values they want at a single positon of travel
- From these values, it’s possible to work backwards through the ‘graphical methods’ for quantifying AS and AR, to determine the required pivot location.
- To achieve a particular AS value at sag, then the pivot must lie somewhere on a specific AS line (Line 1 from the AS Graphical Method).
- Note that the pivot could be anywhere along this line, and it would produce the desired AS value at sag.
- To achieve a particular AR value at sag, then the pivot must lie somewhere on a specific AR line (Line 2 from the AR Graphical Method).
- Note that the pivot could be anywhere along this line, and it would produce the desired AR value at sag.
two pieces of information fully constrain where the pivot must be
located. Altering the AS and AR values at other suspension positions
cannot be achieved without affecting the values at sag.
‘reasonable’ AS and AR values are desired, the pivot location usually
ends up being close to the top of the front chainring. This is pretty
much coincidental, and has much more to do with other dimensions like
wheelbase, CoG location, and BB location than anything to do with the
pivot being at a ‘special point’ on the chainline.
for a bike with ‘normal’ geometry, the slope of the AS curve can be
significantly altered by making relatively small changes in the AR sag
- For example, if you want 100% AS at sag, and 100% AR at sag, then the AS curve will be decreasing.
AR at sag is dropped to around 85%, and the AS remains at 100% at sag,
then the AS curve becomes close to a horizontal line (constant AS).
AR at sag is dropped to around 80%, then the AS remains at 100% at sag,
then the AS curve begins to have a positive slop (increasing AS).
provides a fairly effective tool for the designer to be able to tune
the slope of the AS curve, by making only relatively small changes to
the AR value at sag.
- While it is possible
to create a stable pedalling platform (AS increasing through travel),
due to the AS curve being fairly linear, it means that AS will continue
to increase beyond the pedalling zone (after about 70% travel). This
has quite a negative impact due to excessive pedal kickback on big
- Therefore, as a
compromise between having a stable pedalling platform and reducing
pedal kickback, and AR values, suspension designers usually design AS
to decrease throughout travel (unstable pedalling platform), which
lessens the pedal kickback deeper in travel.
- In summary, any two of the following three characteristics can be tuned independently:
- AS value at sag
- AS curve slope
- AR value at sag.
Axle Concentric Pivot (aka Floating Caliper, Split Pivot, ABP):
Caliper: The simplest form of this is where an additional pair of links
is added to a single pivot configuration. The brake caliper mounts to a
‘brake link’ which pivots concentric to the rear axle (this means that
the brake caliper always stays concentric to the disc rotor). A
‘control link’ is pivotally connected between the ‘brake link’ and the
front triangle. The benefit of this design, is that the AR can be tuned
by changing the dimensions of these additional links. Tuning the AR in
this way does not affect the AS, therefore the design allows for
independent tuning of AS and AR. This is the primary benefit of this
design. A secondary benefit is that AR values can be tuned more
precisely than with a single pivot arrangement. Since the axle path is
still governed by a single pivot, the AS characteristics are exactly
the same as what can be achieved with a single pivot design. Typically,
with a ‘floating caliper’ configuration, the brake link is quite short,
and the control link is quite long.
Pivot and Trek ABP: For the purpose of AS and AR tuning, these two
configurations are identical; despite Dave Weagle and Trek each holding
separate patents for these configurations. The reason this is possible,
is that these patents don’t actually claim the main performance benefit
of this configuration (being able to tune AS and AR independently) but
rather they claim different specific details about how the shock is
mounted and/or actuated (which has nothing to do with AS and AR!). With
these configurations, the brake link and control link have a dual
purpose, in that they control the position of the brake caliper, and
also actuate the shock. Typically, the brake link is quite long, and
the control link is quite short (opposite of floating caliper
configuration) which allows for more interesting tuning of the AR
curve. Specifically, AR can be made to increase throughout suspension
travel. Since the axle path is still governed by a single pivot, the AS
characteristics are exactly the same as what can be achieved with a
single pivot design.
- When designing an axle
concentric pivot system, the designer should start with deciding what
AS value they want at sag, then the main pivot must lie anywhere along
that particular AS line. The shape of the overall AS curve can then be
tuned by moving the pivot to various positions along that line. Closer
to the rear will have a steeper decrease in AS, and closer to the front
of the bike will have a flatter (or increasing) AS curve.
the main pivot location is established, the AR curve can be
independently tuned by altering the dimensions of the brake link and
- A shorter control link will
make the IC move more rapidly, which can be used to generate an
increasing AR curve. While a longer control link will produce a
decreasing AR curve.
4-bar (Horst Link):
Link is a 4-bar linkage where the brake caliper and rear axle are
mounted on the “4th bar”, which is connected to the front triangle by a
pair of links (upper and lower).
- The lower
link is quite long, and connects to the 4th bar quite close to the rear
axle, and the upper link is relatively short.
Horst Link is a true 4-bar linkage, from the point of AS and AR tuning,
Horst Link offers very similar traits to the Split Pivot / Trek ABP
configurations described above.
- Since the
axle is mounted on the 4th bar, its arc is non-circular. However, due
to the closeness of the lower pivot to the rear axle, the axle path is
very close to being circular. This means that AS characteristics are
very similar to what can be achieved by a single pivot. In fact, we
would suggest that there is no significant advantage in AS tunability
with this configuration over a single pivot.
can be tuned by changing the location/length of the upper link. Again,
because of the closeness of the lower pivot to the rear axle, this has
a very small effect on the AS curve. Because of this, the
Horst Link configuration allows semi-independent tuning of AS and AR.
Link has been hugely popular, and has featured on many different bikes.
The most likely reason for this is that it offered the ability to
semi-independently tune AS and AR, with a package that could be
manufactured economically (i.e. without having an axle concentric
- Specialized have used Horst Link on
their bikes for many years as “FSR”. Sometimes the suspension
configuration gets a reputation for poor pedalling performance. This is not a characteristic of the Horst Link configuration,
rather it’s the way that Specialized have chosen to implement it.
process for designing a Horst Link system is very similar to that of a
Split Pivot / ABP configuration. The main difference is that when
tuning the AR, there will be a small effect on the AS, so it may be
necessary to go back and adjust the position of the main pivot slightly
before returning back to final tuning of the AR.
4-bar (Short Links):
link 4-bar configurations offer a significant advantage over
single-pivot systems, particularly for the tuning of AS and AR.
are various embodiments of this configuration, such as DW-Link, KS
Link, LA Link, Switch Link, Maestro, VPP, and probably many others.
one or both links can be replaced by eccentric pivots, which are
kinematically identical to pivoting links, but it just allows them to
be shorter than the diameter of a pivot bearing.
systems have both links rotating in the same direction, others have
them rotating in opposite directions, and others change direction
through travel. None of this is kinematically significant, but this
feature has been used to characterise the different systems as ‘novel’,
thereby allowing patents to be granted to some of them.
each configuration, the axle follows a non-circular path, which is
determined by the location of the ‘instant centre’ of the wheel carrier
link relative to the front triangle.
- Due to
the shortness of the upper and lower links (and the longer distance
between them), the instant centre can be quite ‘volatile’ and can be
made to rapidly change direction throughout suspension travel, if
- The ‘volatility’ of the instant
centre can be roughly gauged by looking at the lengths of the upper and
lower links, and the distance between the upper and lower pivots:
- A design with short links separated by a large distance will be very volatile.
- A design with short links separated by a small distance will be less volatile.
- A design with long links separated by a large distance will be less volatile.
design with long links separated by a short distance will have very low
volatility (and will approach what can be achieved with a single pivot
a design with ‘low volatility’ doesn’t really make full use of the
capabilities of a 4-bar configuration. However, it may be the
designer’s goal to have single-pivot like AS and AR characteristics,
without having to include a physical pivot at a particular location. So
it’s not always a designer’s goal to make the most of a ‘volatile’
- A good suspension designer will be
able to take advantage of a more volatile system, by carefully tuning
the migration of the instant centre (IC) so that it is a the ‘right’
location at every point in travel.
example, AS can be tuned to rapidly change values as suspension travel
passes from the ‘pedalling zone’ to the ‘non-pedalling zone’. AS can be
made to rapidly decrease after about 70% travel, to reduce pedal
kickback on bigger suspension compressions. Note, this
overcomes/reduces the compromise to AS tuning described in the
single-pivot section. Also note, in order to make the AS decrease
rapidly after about 70% travel, the axle path must arc forwards
significantly in the last part of travel. This introduces other
compromises with regard to suspension compliance on big hits.
- When designing this type of system, the designer needs to know what they want their overall AS and AR curves to look like.
path of the instant centre can only be fully constrained if the
designer knows what AS and AR values they want at ALL points in travel.
- Knowing the AS and AR curves will allow the
IC to be located at every point in travel, and the IC migration path
can be determined.
- The challenging part is now working out what positioning of pivots/links will generate this IC path.
process is called linkage synthesis, and can be done by trial and error
(very time-consuming) or more likely can be done using sophisticated
- In either
case, it’s likely that the 4-bar linkage will not be able to produce
the exact IC path desired, and so AS and AR must be compromised
- Also there may be further
compromises required if the calculated location of the pivots/links is
not spacially convenient, and so AS and AR must be compromised
while it’s theoretically possible to independently tune the AS and AR
curves independently via the IC migration, the reality is that one or
both is usually compromised to make it work with a 4-bar linkage within
the spacial constraints of a bike frame.
- There are a few other 4-bar configurations that fall somewhere between Horst Link and Short Link configurations:
- Ellsworth ICT, Inverted Horst Link, Breezer MLink.
on the discussions above, it can be estimated how useful these systems
are, and how a designer might approach working with these systems.
6-bar (e.g. Felt Equilink):
are numerous possible configurations that fall under the banner of a
6-bar linkage, so we won’t go into detail about the different types.
the same way that short link 4-bar configuration offers kinematic
advantages over a single pivot, a 6-bar configuration offers further
advantages over a 4-bar.
- The reason for
this, is that with a 6-bar linkage, the instant centre (IC) can be made
to be even more volatile, and make even more abrupt changes in
- The design process is the same
as with a short link 4-bar system, in that the designer needs to know
what they want their overall AS and AR curves to look like.
both the AS and AR curves allows the IC to be located at every point in
travel, and the IC migration path can therefore be determined.
there, synthesis of a suitable 6-bar linkage is much more complicated
than with a 4-bar linkage, and will certainly require computational
- It’s possible that a 6-bar
linkage can achieve (or get closer to) a desired IC path, where a 4-bar
linkage was not able produce the desired IC path. This would be a good
justification for using a 6-bar linkage.
due to the additional links and pivots, and the ‘spacial’ limitations
of a bike frame, the 6-bar linkage is unlikely to be a more convenient
solution than a 4-bar, so it’s likely that the AS and AR curves are
compromised because of this.
- If considering
purchasing a bike with a 6-bar linkage, we'd want to see that the use
of the 6-bar configuration has produced an improvement in the AS and AR
curves that is not possible with a 4-bar system.
sliding link is kinematically equivalent to an infinitely long pivoting
link, so that it travels in a straight line rather than a circular arc.
- Recently, Yeti has changed design from an
eccentric lower link on the 4-bar system, to a sliding lower link.
Kinematically, there is very little improvement with this change, so we
can only assume that the change has been made to avoid patent
infringement that their earlier ‘Switch Link’ design was suffering.
design process is the same as with a short link 4-bar system, in that
the designer needs to know what they want their overall AS and AR
curves to look like.
- Knowing the AS and AR
curves, will allow the IC to be located at every point in travel, and
the IC migration path can be determined.
- It should be noted that the instant centre of a sliding link is infinitely far away, and perpendicular to the slider.
Fixed Idler (idler mounted to front triangle):
- All the suspension configurations discussed above use a ‘conventional’ drivetrain.
using a conventional drivetrain, there is really only a fairly narrow
window of possible axle paths that will produce reasonable pedalling
- With all these systems, the axle path is approximately perpendicular to the chain line.
the designer wants the axle path to be more rearward, then orientation
of the drivetrain needs to be altered so that it will still produce
reasonable pedalling performance.
possible option is to raise the BB, so that the chainline is again
approximately perpendicular to the axle path. However, this is a fairly
impractical solution, as it drastically affects the ergonomics of the
- Another option is to include an idler
pulley, fixed to the front triangle, so that the chainline is again
approximately perpendicular to the axle path.
of the configurations described above could be used to generate a more
rearward axle path, therefore a ‘fixed idler’ configuration is possible
with each of them. The design process and the pros/cons of each
configuration remain the same as if used with a ‘conventional
drivetrain’, with the exception that a more rearward axle path allows
for higher amounts of anti-squat, with less pedal kickback.
Unified Rear Triangle (URT):
- Trek, Klein
of the systems described above have the BB mounted on the front
triangle. This is kinematically significant, because the rider’s input
forces are all acting on areas that are part of the one body (handlebar
(acting on head tube), cranks (acting on BB), and saddle).
a URT system, the bottom bracket is mounted on the same structure as
the rear wheel. This means that there is no ‘distancing’ between the
rear axle and the BB, eliminating the drivetrain’s influence over
suspension behaviour. However, it introduces a new influence on the
suspension, in that the BB (where the rider’s feet are connected) now
moves relative to the front triangle (where the rider’s hands/backside
are connected). Now, when a rider exerts pedalling forces between two
movable elements of the system (action at the BB, reaction at the
bars/saddle), it affects the suspension behaviour. Ultimately, this
behaviour can still be described by an AS curve, however it requires a
different method for calculating AS, which needs to account for details
of the rider position, rider dimensions, and whether they are standing
or sitting. At best, this results in an approximation of AS.
enables designers to raise the pivot height (hence create a more
rearward axle path) without causing any distancing in the drivetrain.
The higher pivot produces a more rearward axle path, but at what cost?
With the BB now attached to the rear triangle, it means that the BB
also moves rearward and upward as the suspension compresses. Since much
of the rider’s weight is carried at the BB, it reduces the
effectiveness of the suspension if the BB has to move upwards/rearwards
each time the rear wheel encounters a bump. The irony of this system is
that it’s quite good at absorbing bumps when seated (with feet off the
pedals), however most riders shift to the standing position when riding
over rough terrain!
systems are similar to the URT system, in that the BB is not mounted on
the front triangle. Instead, it is mounted on a link that moves
relative to the front triangle and the wheel carrier link.
are two main configurations: Single Pivot axle path (GT I-Drive, GT
Independent Drivetrain, GT AOS, Mongoose Freedrive), 4-bar axle path
- For the Single Pivot
Axle Path version, the swingarm is mounted to the front axle via a
single pivot, and the BB is mounted on one of a pair of links connected
between the swingarm and front triangle.
- For the 4-bar version, the swingarm is the 4th bar of a linkage, and the BB is mounted within the lower of the two links.
- Theoretically, 6-bar versions are also possible.
- This configuration can been implemented in a few different ways, for example:
approach: Some designs (GT AOS, Mongoose) include a significantly
rearward axle path, and then design the BB to move significantly
rearward/upward to control the distancing between the rear axle and the
BB. This approach results in similar behaviour to the URT system
- Another approach:
Other designs have a more conventional axle path, and have the BB make
only subtle movement. Perhaps the BB can be made to be fairly stable
through the pedalling zone of travel, then begin to accelerate towards
the rear axle around 70% travel (thereby causing a downturn in the AS
curve, reducing the amount of pedal kickback during big suspension
possible implementation, would be to have the BB move forward/down as
the suspension compresses (and the rear axle moves up/rearwards). Of
course, this would introduce large amounts of distancing between the
rear axle and BB, which would need to be overcome with the introduction
of an idler on the front triangle (or one of the other links). Here at
i-track, we extensively explored this configuration back in 2012. Our
conclusion was the moving BB does not offer any significant geometric
advantages (over a basic rearward axle path design), and it still
couldn’t generate ‘sophisticated’ AS curves.
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