This is the standard rear smart fortwo 451 suspension.
Similar to the 450 you have a separate shock absorber and spring.
Jack the car up and secure on axle stands.
Get yourself a pair of spring compressors. Place them over the coils and wind them in.
This compresses the springs allowing you a bit of movement and slack.
Remove the plastic cover at the base of the shock absorber (inset) by pulling out the plastic pin.
Using an E14 socket, remove the lower shock absorber bolt fully.
With the shock absorber bolt removed you will be able to push the suspension arm
down by the wheel hub enough you take the compressed spring out of the cups.
You can see the cups top and bottom with the spring removed.
I am fitting S-Mann lowering springs. You can see they are shorter and progressively wound.
Take the rubber ends off the old spring and place onto the new spring. Put your foot on
the hub and push the suspension swing arm down. You should have enough room to
install a lowering (shorter) spring without the use of the spring compressors.
Now remove the arch liner by removing the 3x 10mm plastic nuts.
Finally there is a plastic pin that slides out to release a plastic insert.
Pull the arch liner down and out of the arch.
You will now be able to see the top fixing of the shock absorber.
Grip the top of the shock absorber stem with some pliers and undo the nut just below it.
With that nut removed, the shock absorber will come free from the car.
Get your new shock absorber, slide on the bump stop and then the large plastic cover.
Slide on the metal collet and the top cap.
You should now have this as shown in the picture below.
Slide the rubber washer onto the shock absorber stem, flat side down.
Place the stem back into the bodywork then fit the washer and nut to secure.
This top nut should be tightened to 20Nm
Line the lower part of the shock absorber up so the lower bolt can be pushed in and tightened.
It is made easier if you use a jack under the swing arm to lift it to line up the holes.
Tighten this lower bolt to 65Nm using your E14 socket. Refit the small plastic cover that covered
the lower shock absorber bolt, refit the arch liner including its plastic rivet and 3 plastic nuts.
Replace the wheel, repeat on the other side.
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