Originally Posted by Bill Verburg
shocks fall into 2 main design categories
twin tube - Koni, KW, Boge, some Bilstein's
monotube - Bilstein, JRZ, MCS
twin tube are cheaper, offer more travel, are slower to react, are generally used where comfort is the main goal, some have low gas pressure bags that help reduce cavitation in herent in twin tube design
monotube are more expensive and are more designed for ultimate performance, they are generally stiffer, react quicker and add ~40# to 100# to spring rates due to the high gas pressure used to prevent cavitation, generally ~200psi
no matter the base construction each shock uses different types of valving, some are adjustable, some are inverted, some use mono-ball connections at one or both ends, some have coil over springs some don't
which to use depends on what your goals and use are, for street use you don't want mono-ball connections at both ends, monoball top alone is ok, inverted has many advantages over non inverted, mostly this is a track oriented advantage, coil over are only necessary when you run out of t-bar spring rates, this isn't a street issue at all
our 911s are getting pretty long in the tooth and will require complete suspension rebuilds
for a street car all the rubber bushes f/r need to be replaced w/ stock or sport rubber
back in the day base shocks were twin tubes, w/ Bilstein hd as a performance option, over the years many have found hd front and sport rear Bilstein to be a good way to go.
on a lowered car the biggest concern is reduced shock bump travel, stock front ride height is 107+/-5mm, lower the car to 132mm and you lose an inch of bump travel where there were only a couple of inches to begin w/
a second effect of lowering is worse roll steer
bump travel is addressed by raising the spindle height, roll steer is addressed w/ steering rack spacers and adjustable height steering knuckles
another big issue w/ 911s is the stock roll characteristics, there is too much roll for aggressive street or track use, so, t-bars and sways are upgraded to reduce roll, raised spindles help this by keeping the roll center higher reducing the roll lever arm length.Tthe downside is reduced mechanical grip and comfort so care must be taken to not take it too far, 20/26mm seems to be a good limit for 99% of street use. The upside of bigger t-bars is less roll and geometry stabilization during corning which more than offsets the mechanical grip loss.
in back the issues are much less severe, just make sure bushes and shocks are nominal
when choosing shocks digressive valving is very desirable for grip, control and comfort it's a win win choice for street or track
lastly don't forget wheels and tires, they are your only connection to the road and are very influential wrt grip and comfort. w/ 16 7 & 8 or 7 & 9 is the usual choice for an SC/Carrera w/ 205/55 & 225/50 or 205/50 & 222/45 tires on the 7 & 8 or 205/55 & 245/45 or 225/50 & 245/45 on 7 & 9
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