In this free lesson, you'll learn all about the important job of bump stops in motorsport
suspension.
Bump stops are one of those suspension components that are largely ignored
by many people however, as we'll see in this lesson, they are not only critical to the
safe operation, they are also a useful tuning tool in their own right.
In road cars, the primary purpose of a bump stop is to prevent damage to components
when the suspension is fully compressed.
Bump stops can be arranged in a few different ways but the most common is when they're mounted
concentrically with the damper as a part of a coilover unit.
Without bump stops, the suspension travel will be limited by the mechanical travel of the
suspension which would usually be metal to metal contact.
The problem with this is that it puts a large shock loading through the suspension and is
likely to damage part of the suspension or even the chassis itself.
If full suspension compression is reached with metal to metal contact, not only are we likely
to cause damage, but because the effective
spring rate of that corner of the car becomes almost infinite, we're also likely to lose grip on
that corner of the car which can cause some nasty handling behaviours and even a complete
loss of control.
By using bump stops, we can lessen the shock loading at full compression by providing a
progressive force as the suspension is fully compressed.
In motorsport, we use bump stops for the same reasons as in road cars but we can also
make use of them as a tuning tool in their own right.
In a similar way to fitting different springs or antiroll bars.
When the suspension is operating and the bump stops aren't engaged, the loads on
each corner of the car are reacted through the springs dampers and antiroll bars as
the car negotiates the course.
If the suspension compresses enough that the bump stops begin compressing, then they act
as another spring in parallel, adding to the total effective spring rate at that corner
of the car.
Bump stops are commonly made from a polymer based material and are available in
all sorts of different materials, lengths and geometries.
All of these properties can be modified in order to achieve a different stiffness of
a bump stop.
Here we see some typical examples of bump stops and the kind of force response we might
expect to see as they're compressed.
In particular, the external shape can have a big influence on the rate of force buildup.
It's not uncommon for people to modify these by changing the heights, cutting out sections
and stacking them in different combinations to get the stiffness response we're looking for.
The different ways in which we build up bump stops is about tuning how progressive the rate
increase is.
In many cases we don't want the initial engagement to add a large step in stiffness.
So the initial rate is low.
Then as it becomes more compressed the stiffness can progressively ramp up.
One of the interesting properties of a polymer based bump stop is that they are visco elastic.
This simply means that some energy is lost as heat as they're cycled.
If we look at the force response curve as we cycle a polymer bump stop, we can see that
the force changes depending on wheter we're compressing versus extending
the bump stop.
The area between these curves represents the amount of energy lost to heat.
Another term for this is hysteresis.
This is essentially a similar effect to what's happening inside a damper during operation
which we'll discuss in the next section of the course.
But for now, just understand that we're adding some damping to the coilover when
we engage a polymer bump stop.
A downside of using a polymer is that over time, their behaviour changes as
they break down and deteriorate.
This causes changes both in terms of their compression stiffness and their damping
behaviour.
Which means that polymer bump stops are considered a consumable item.
Temperature also affects the way polymer responds which is something that needs
to be considered when we're tuning them precisely.
Another type of bump stop is actually more correctly termed a bump spring.
These aren't as common but act in a similar way to bump stops.
They're essentially a small coil spring that replaces the bump stop.
This allows them to be precisely tuned because being made from steel, they can be manufactured
to closer tolerances and their response can be more consistent, particularly in relation to
temperature.
Steel also has very low damping properties, meaning that the energy required to compress
a spring, is largely recovered as it extends.
Little energy is lost as heat.
We can see this by comparing the force response of a polymer bump stop versus
a bump spring in a plot.
While some energy is lost in hysteresis of the polymer bump stop, the force of
the bump spring is the same on compression versus rebound.
The amount of energy lost to heat is represented by the area within the
compression and rebound curves of the polymer bump stop.
The larger this area, the more energy is converted to heat, meaning the larger the
hysteresis.
As the bump spring has essentially zero area between the compression and rebound, this
indicates almost no energy loss.
In some cases, metal matrix type bump stops are used.
Here, the bump stop is made of a matrix of very small diameter wire.
One of the main properties of this style of bump stop is that their race can increase very
aggressively compared to polymer which is helpful when we're looking to limit travel
very precisely.
Something that's very useful for aerodynamic reasons.
Another big advantage of this style of bump stop is that they tend not to break down like
a polymer does, giving more consistent performance over time.
When we talk about using bump stops to tune the mechanical behaviour of the car, this is
usually to reduce the amount of suspension travel under a certain condition.
For example we might want to limit rear suspension travel under acceleration of
a rear wheel drive car in order to prevent the rear wheels gaining too much rear camber.
Which would otherwise reduce our available drive force from the tyres.
By using a bump stop that engages at a precise point, we can also progressively
increase the spring rate of the rear suspension as it compresses and that allows
us to run a softer main spring than we would otherwise be able to.
The same would be true of limiting the amount the front drops during braking.
For example, there may be a minimum ride height that the front splitter needs to stay
above the ground during braking in order to keep it working effectively.
We could use bump stops in this situation to control the minimum front ride height without
having to stiffen up the front springs.
Probably the most common reason for tuning bump stops is to control the ride height
of a car that has a lot of downforce.
Because the aerodynamic forces acting on the car increase with the square of road speed,
at high speeds the right height will quickly reduce.
For cars with powerful underfloors, the proximity and angle of the floor with respect
to the road surface has extreme effects on both the amount of downforce and how it's
distributed.
Again, we can use bump stops to tune the height and angle of the chassis at high speeds, making
sure we keep it within bounds that we require.
Using bump stops in this way, gives us an opportunity to run a setup more conducive
to good mechanical grip while also controlling our aerodynamic platform.
As part of tuning our bump stops we can also vary the gap between them and the damper body
which is generally referred to as tuning the packer gaps.
Once we're happy with the stiffness of the bump stop, this allows us to fine tune when it engages
by adding and removing solid packers.
These packers can be easily slid in and clicked over the damper shaft which means we don't
have to disassemble the entire damper unit in the pits every time we want to make an adjustment.
One thing we do need to be careful of when tuning the gaps is engaging the bump stops mid corner.
If this happens too aggressively, it can upset the car by making the effective spring rate increase
quickly.
Making use of data acquisition by using damper pots is the best way to monitor this.
As a side note, although data analysis is outside the scope of this suspension course, we do offer
dedicated motorsport data courses at both the fundamental and advanced levels.
I'd highly recommend checking them out if you want to get the most out of your racecar and
your time behind the wheel.
In summary, bump stops primarily exist to prevent damage to the suspension when it
becomes fully compressed.
But they also offer tuning opportunities through different materials and designs that can provide
extremely different behaviours that are going to affect how the car behaves under heavy
suspension compression.
Bump stops are commonly used to control high suspension deflection which we may want to do
for mechanical or aerodynamic reasons.
That was just one of the 37 modules taken from our motorsport suspension course.
If you want faster, more consistent lap times, along with an understanding of the concepts
and tuning tools used by the pros, then you'll be interested in this course.
You'll start with the basics of motorsport suspension and build on the knowledge in
this course, covering choosing springs, tuning your dampers and adjusting roll centre heights.
To be able to understand and tune your suspension for yourself.
If you're interested in advancing your skills, this is the course for you.
For more information and to purchase this course, click the link to enrol now.