----- Original Message -----
Sent: Friday, March 14, 2003 2:00
PM
Subject: RE: hey guys....LONG
Ben,
Here
goes,
Spring rates relate directly to corner weight and leverage ratio on
control arms. To make any sense of spring rates you need to work out the
effective spring rate at the wheel. On the
stanza/sunny with stock lower arms the ratio is 375mm (pivot to tyre centre)
divided by 320mm pivot to ball joint centre =0.853.
On this car for example a 360lb/inch spring has an
effective rate of 307lb (140kg) at the wheel. If the arms have been
lengthened for more camber this rate will vary as the rate become closer to
unity.
A
good guide for a club race car with good quality race rate shockers, is the
wheel spring rate in kg's/cm should be in the range of 16 - 20% of
the static corner weight of the car. Eg - at
16% wheel rate 260 kg/0.16=41.6kg/cm or 232lb/in this means a 272lb/in
spring; at 18% wheel rate 260
kg/0.18=46.8kg/cm or 261lb/in this means a 306lb/in spring;
at 20% wheel rate 260 kg/0.2 =52kg/cm or 290lb/in
this means a 339lb/in spring
The
heavier the spring, the faster the wheel movement is
slowed during bump. This also means the wheel is pushed back
faster towards the road surface during rebound.
Race
cars use heavier springs to ensure that the wheel is able to follow the
road surface at race speeds where these events take milli
seconds. To stop the spring energy being transferred to the body
of the vehicle the shock has to control the transfer of energy to the spring
during bump and as the energy is release from the spring during rebound.
It gets rid of this energy as heat.
As
you get above 15% sprung wheel to corner weight rate, you need much
higher quality damper control as the suspension has lower
distances to move due to the higher spring rates.
Once you have the spring rate at the wheel, you design your shock for
damping at a range of piston velocities typically encountered in the
application. BILSTEIN for example use 32.5mm/sec, 65mm/sec, 130mm/sec,
260mm/sec, 520mm/sec as their test velocities.
Usually the shock rate on bump is much less than on rebound. This
allows the energy to be absorbed by the spring without lifting the body
excessively and the energy is dissipated at a slower rate as the spring
returns to normal ride height during rebound. This is why
cars with stiff shocks for the spring rate usually sit a bit after a bump
if the rebound rates are a bit high.
A
good shock should provide consistent damping over a wide range of
velocities without being harsh at any part of its range.
Any
comments or flames?
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