- Bump stops don't always get the attention they deserve in a suspension system and
when setting up your suspension, paying attention to what's happening with the
bump stops is actually pretty important.
So here in front of me I've got an array of different bump stop types, all different
shapes and sizes which is pretty typical when dealing with bump stops.
The normal function of a bump stop in say an OEM or road car application is really
just to limit the suspension travel safely to stop suspension components hitting each
other when you've got large bumps in the road, say pot holes or something like that,
it's really just to stop the suspension crashing into itself, damaging suspension arms,
dampers, things like that or even the tyre hitting the inside guard.
The way a bump stop is typically packaged in the suspension is normally sitting on a
damper such as this in a coilover application.
It'll sit inside there underneath the spring.
So for the bump stop to become engaged, you've normally got a top hat sitting on here
on top of the damper, the entire spring travel would have to be used up until the top hat
comes in contact with the bump stop.
At that point the spring and the bump stop are working together to provide a resistive
force inside the suspension.
The rate of the bump stop in an OEM application is generally quite a lot higher than the spring.
That's because you're really just trying to resist that last bit of force to stop the
suspension components causing any damage when you're at or very close to maximum travel.
In a road car application the amount of suspension travel you have is usually
quite large compared to the bump stops.
So it's not normal in road cars for the bump stops to be active much of the time.
It's really much more of a case of a highly transient thing like a bump in the road
or a pothole or something like that or maybe even a really heavy braking event.
Typically the rate of the suspension is controlled by the springs and the anti
roll bars and bump stops aren't usually that active.
In racecar suspension setup you'll often hear the term bump stop gap or packer gap.
These are the terms people often use to describe how they've got their bump stop
set up.
So what that's referring to is let's imagine there's a top hat sitting on the top of this
suspension here so that's essentially that top platform that will bolt to the car.
This gap between the top hat and the top of the bump stop when the car's at ride
height is normally what people are referring to when they're talking about
bump stop gap or packer gap.
The reason they often use the word packer gap is because to tune the position of the
bump stop or to tune that height, you often insert packers underneath the bump stop
so you'll have packers at different increment sizes, say one, two, three, five millimetres
and such that you can slide over the damper shaft with the whole setup, with the whole
suspension, it's already assembled, to modify that bump stop gap.
One of the simplest things you're usually using the bump stops to control is to just
stop mechanical component clashes.
So in a motorsport application where you may have large tyres or you're running particularly
low ride height, one of the things you're often trying to stop is contact between
the tyres and the fenders or the inner guard in the wheel well.
So the tuning of the bump stop becomes really critical to get that right.
Sometimes it means choosing a different bump stop or it can mean tuning the packer
gap, so adding those spacers I was talking about underneath the bump stop so that
when the suspension compresses we make sure that that bump stop is
engaged really hard before the tyre comes into contact with anything.
We don't want the tyre to come into contact with anything so we use the bump stops
to control that.
The rate of the bump stop is a function of things like material, shape length.
So you see here in this range of bump stops there's some different shaft sizes
and that's really just a function of which damper they're designed to fit.
So you don't really want the bump stops to be super tight on here, you want
them to be a close sliding fit, you don't want to have to force one of these on.
It should be able to slide relatively freely on the shaft.
So the material, obviously you've got stiffer and softer materials.
That certainly has an implication, the material choice also has an implication on how they
react to the temperature so some bump stops will be, as the temperature changes,
their stiffness behaviour changes quite a lot and that's definitely something that
you need to consider if they are in quite a hot area of the car.
Say particularly at the front of the car or if you've got a front engined car,
the bump stop characteristics can change quite a lot with temperature.
The shape is also really important to how the bump stops behave.
So as the bump stop's compressed, different sections will compress at different times and
that generally results in a progressive stiffness increase so the more the bump
stop is compressed, the higher the rate ends up being.
That's really just to control, as you get closer and closer to your mechanical limit of your
suspension, the higher the rate is to stop your bottoming out anything and causing
damage in your suspension.
Apart from using a bump stop to control things going wrong or having damage from
tyres hitting the inner guards or suspension components going beyond their travel,
another important thing with the bump stop gap is tuning the bump stop gap such
that the bump stops engage at a time where you do or don't want them to.
For example when you're in a corner and your suspension compresses a certain
amount, what you generally don't want is to start engaging the bump stops while
you're in a corner.
It's not necessarily a bad thing for the bump stops to be engaged during the
corner but you don't want to be coming on and off the bump stop while you're in a
corner.
What's happening when you're transitioning from being off the bump stop to having the bump
stop engaged is that suspension rate on that corner of the car is changing quite drastically,
it's quite a big step in stiffness and that can definitely have a big effect on your chassis
balance 'cause essentially the corner that is beginning to engage the bump stop within
the corner is having a large increase in suspension rate transiently just when
the bump stop's being engaged.
And again it's not necessarily a problem to be on the bump stop, you just don't want
to be coming on and off, on and off on and off within the corner.
So that's often another reason you'll be using packers and different bump stop heights
to try and make sure that when you're in a corner, you are either fully on or fully off
the bump stop to avoid problems with rate changes mid corner.
Probably the most common way to understand the bump stop gap or where you are in your
suspension travel and if and when you're engaging your bump stops is to use something
like a damper potentiometer.
So this is a linear sensor that can slide in and out and you can use that signal to tell
you which corner of your suspension is in which position.
So there's all sorts of ways they can be mounted, they can be mounted to the suspension,
they're often mounted in parallel with the damper itself so with a bracket from
somewhere on the shaft or somewhere attached to the shaft and somewhere on the body.
And using that you can tell where your suspension is at any point in time when
you're on track which is not just useful for bump stop gaps, it's useful for ride
heights and damper analysis and all sorts of things.
Another reason we make use of bump stops is in high downforce applications.
In a car with a really effective floor, say with front and rear diffusers and
maybe even a flat centre section, the ride height is really critical to the
performance, it affects the performance and drag a lot.
We're using bump stop gap and material to tune the ride height both on the straights
and in the corners and that often comes, once you've got your bump stop rate in
the range that you need it, that often means changing the packer gap underneath the
bump stops to get the floor and the ride height window that you're looking for.
In high downforce applications such as GTs, prototypes and things like Time
Attack cars, this bump stop gap is really critical because it controls the ride, it has
a large bearing on the ride height.
A difference in ride height of plus or minus a millimetre makes a measurable
difference to the downforce, it really is that important, particularly at the front of the
car so what you'll often see is very high suspension rates, very high spring rates,
you'll see very high bump stop rates and quite small gaps to the bump stops such that the
cars can even be running on the bump stops even when they're sitting in the pit lane.
That's just because they're trying to keep that front floor, that proximity to the ground in
a very very tight window.
As I said, a millimetre one way or the other will make a measurable difference in downforce
so that's how critical it becomes.
You will often see packer sizes of one millimetre being changed in the damper.
It's actually an area where race engineers in high downforce cars spend quite a lot
of their time both pre event for simulation work and during an event looking at
the data to understand how engaged the bump stops are, where they're being
engaged, what the resultant ride height is and whether you need to make any changes
to either stiffness of the bump stop or the amount of packer you've got to control the
ride height at a certain position on track.
The most common type of bump stop is a polymer type like everything we see here.
One of the things to be aware of with polymer bump stops is that they are very sensitive
to both temperature and how many cycles they've done so their characteristics will change
over time so their rate and their internal damping changes quite a lot as they're used
and as their temperature changes on track their characteristics change a lot as well.
Because of this, in professional motorsport we often make use of a metal matrix type bump
stop instead which is essentially a whole lot of small diameter stainless steel wires
wrapped up into a little, almost looks like a large washer and their performance generally
doesn't change a lot over time.
A bump stop like this in a high downforce application, it really has to be considered
a consumable item.
So their performance can change quite a bit, certainly over the length of a season
and as a result you've got to be quite on top of checking the stiffness of them,
checking the condition of them,
If it's something that you're using to control ride height in a really critical way.
One tool you do have at your disposal to tune bump stop performance is to actually
cut down the bump stops.
That's a perfectly legitimate way of changing their rates particularly in high
downforce cars where you don't have a lot of suspension travel, the bump stops in
OEM applications are typically much too large to be actually packaged on the shaft.
You may only be talking about 50 or so millimetres of usable suspension travel
and that's greater than the entire length of some of these bump stops here so it's
perfectly acceptable to cut them down and affect their stiffness properties.
So that was a quick rundown on the basics of bump stops.
Certainly a little bit more than meets the eye and definitely a part worthy of your attention
when you're getting serious with your suspension tuning.
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