Shocks (dampers) and the AE86 (Part 3)

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jondee86
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Shocks (dampers) and the AE86 (Part 3)

Postby jondee86 » Thu Aug 28, 2014 5:36 pm

SO, HOW DOES A SHOCK ABSORBER WORK ?
Regardless of what colour it is or what brand is on the outside, all shock absorbers
work by forcing oil to pass thru a small orifice (hole). The Laws of Hydrodynamics
say that the faster you try and push the oil thru the hole, the greater the resistance
will be. To allow the shock absorber to handle both small and big hits, there is a
spring (shim) stack that essentially allows the hole to open up on a big hit when
there is a lot of oil trying to get thru the hole in a hurry. There are separate orifices
for extending and compressing the shock, allowing different damping rates for
bump and rebound.

Image
A simple gas charged monotube shock absorber

Image
Typical shock absorber shim stack.

Conventional shocks are biased in favour of rebound, and will give greater resistance
(damping) when the shock is being extended than when it is being compressed. This
stops the car bouncing after a big hit. Now, LISTEN TO THIS… the stiffer the spring,
the faster the wheel/axle will try and rebound after a hit, and the more damping you
need to prevent the rebound of the spring causing the car to bounce (pogo effect).

This is why your shocks have to be matched to your springs. Not enough rebound
damping and the car will bounce. Too much rebound damping and the wheel may not
have fully returned by the time you hit the next bump, and the car will “pack down”
reducing available bump travel. Packing down is only really a problem if you are
travelling fast and hit several bumps in quick succession.

USING RANDOM SHOCKS JUST BECAUSE THEY FIT…
Now we start to get into the guts of how to make your car handle really badly :?
When you go into your local autoparts store, you will see a shelf of shocks in different
lengths and with different end attachments. And the prices will be way cheap. So let's
grab that pair there that look just like my old AE86 shocks, but are 3 inches shorter…
SCORE :) Come off a pickup truck, so will be nice and stiff… how can you go wrong ?

Take them home and bolt them on… now go for a drive. First thing you notice is that
your cars seems to rattle a bit more than it did yesterday ? And when you get to the
place where there is gravel because they have been fixing the road, your wheels are
spinning when you hardly touch the gas pedal… WTF ? And then when it rains, you
have to slow down because on every bend the back is stepping out ?

And the reason is that those bargain shocks have way too much damping in both bump
and rebound. They were made for a heavier vehicle that is designed for carrying heavy
loads. Point being, if you want to use random shocks that were not designed for your
car, at least find someone who has used the same shocks, and ask how they worked.
Don’t be fooled into thinking that a stiff and bouncy ride is “sporting”. It’s not. You
will have less grip than with the factory suspension. But if you want to go drifting with
a SR5, then it might be just what you need to get sideways :P

SPRING PRELOAD
The factory springs are preloaded… that is, they are compressed when they are fitted
to the fully extended strut. Why is that I hear you ask, so let’s do some math.There is
around 300kg sitting on each front wheel, and the factory spring rate is about 2kg/mm
so just the weight of the car sitting on the spring will compress it by 300/2 = 150mm.
The strut has 190mm of travel. So if the factory was to use a 190mm long spring, there
would be 190-150 = 40mm of bump stroke available.

Image
Using spring compressors to relieve the preload before dismantling a strut.

A good rule of thumb is that 2/3 of the stroke should be available for bump, and 1/3
for rebound, but in this example we have 150mm for rebound and only 40mm available
for bump.

To get around this problem, the factory fits a spring that has a 380mm free length,
and compresses or preloads the spring by 80mm when it is fitted to the strut. Now
when the weight of the car is applied to the spring, it compresses by 150mm minus
the preload of 80 = 70mm. This means that there is now 40+80 = 120 mm of bump
travel and 70mm for rebound. This is in line with the rule of thumb, and means the
factory can use a long and relatively soft spring without needing a shock with a
300mm stroke.

OK… SO WHY NO PRELOAD ON G-REDDY COILOVERS ?
The AE86 G-Reddy and similar store bought coilovers typically have a shock stroke
of around 120mm and come fitted with 8kg/mm springs. Drop 300 kg on that spring
and the shock compresses by 300/8 = 37.5mm leaving 82.5mm of bump travel which
is in line with our rule of thumb for bump vs rebound travel. So there is no need to
preload the spring. Doing so will result in your wheel spending more time out of
contact with the road surface, meaning less available grip.

And this pretty much brings me to the end of this article. I'm not going to try and tell
you which shock is better and why... because I don't know what you want to achieve
with your car, and how you plan to use it. If you just want "the look" then budget mods
are probably all you need. But if you plan at being competitive on the autoX course,
you will want all the grip you can get, and you will want quality components carefully
matched to your car.

The interweb is full of information on shocks and suspension mods and car setup tips.
Do your research before you start changing things, and plan your changes with a
definite end result in mind. Here are a couple of links to more detailed and technical
suspension articles....
http://club4ag.com/tech-data/tires-and-handling-yokohama-tires/
http://club4ag.com/tech-data/dampers-choosing-shocks-with-you-in-mind/

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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phanist
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Re: Shocks (dampers) for the AE86 (Part 3)

Postby phanist » Fri Aug 29, 2014 8:01 am

Just beautiful!!!!

Second sample picture seem to be simalar type of shock that
ShockWork use?

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jondee86
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Re: Shocks (dampers) for the AE86 (Part 3)

Postby jondee86 » Fri Aug 29, 2014 3:31 pm

There are as many shock valving variations as there are shock absorber makers.
Here is a nice big picture showing the three main designs of shocks...

Image

Right click and view image to see how the construction varies from one to the other.
One interesting aspect of these designs, is in how they handle the difference in oil
volume on either side of the piston. It is not possible to completely fill a shock with
oil and have the piston travel end to end. On one side of the piston there is a rod
which occupies some of the cylinder volume.

As the piston moves down the cylinder, more and more of the rod is inside the cylinder,
taking up space that should be filled with oil moving thru the piston orifices. The extra
(displaced) oil has to go somewhere, otherwise the cylinder pressure will increase to
bursting point, or the piston stops. So each design has an air space into which the
displaced oil can move.

When the shock is working hard, oil is passing thru the piston at high speed and it
becomes aerated, forming foam. Oil is not compressible, but foam is, so aerated oil
is not a good thing inside a shock. The first technique for minimising the problem
was to try and separate the air from the oil by using two chambers, and keeping the
valves submerged in oil at all times. Further improvement was achieved by replacing
the air with a pressurised inert gas such as nitrogen. Pressurising the gas means that
the temperature at which the oil begins to foam is raised.

However, a better system for preventing foaming was to keep the oil and gas quite
separate with a floating piston. This is the system favoured by Bilstein and several
other performance shock makers...

Image

Now, despite all this technology, none of it is worth a knob of goat **** unless the
shocks are matched and valved to suit your car and driving style. That is the one
thing Shockworks have over most other store bought coilovers. They have road tested
and tuned the shocks specifically for the AE86 on real roads... just like the roads most
AE86 owners will be driving every day.

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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