Microcellular jounce bumpers- are the correct term for "bump stops" on ours (and most modern vehicles). These are designed specifically for the vehicle along w/ the dampers and springs.
Technical || H&R Special Springs, LP
What do they have to do w/ the price of tea in China? plenty
First a couple of things about suspension travel, bump travel is the upward travel of a suspension, droop is the downward travel. Having adequate suspension travel is paramount for maintaining grip- especially w/ uneven/bumpy surfaces.
I've done some measurements of late and the results may suprise some. First the WRX in oe form does have pretty good travel (you'd probably expect that out of a rally inspired car). The first thing to do is measure ride height- the best way is to measure from the center of the hub straight up to the wheel arch. A WRX sedan will measure (give or take) 396mm in front/376mm in the rear. We can easily measure droop by simply jacking one end of the car into the air until the tires come off the ground. Know take that same measurement (hub to wheel arch)- this full droop (the farthest you suspension will travel downward). To measure droop simply subtract the ride height from full droop. A oe WRX front full droop will be ~ 496mm- yeilding droop travel at 100mm.
What we want to know next is bump travel- this is a little more difficult, but not overly.
First we need to know the damper stroke- this is the full range the damper piston will move- with a disassembled strut- simply push the piston as far as it will go and mark the spot on the piston, now fully extend it and measure from the top of the housing to your mark. The oe front WRX strut has 165mm of stroke. The next measurement we need is the top mount deflection- how much (if any) does the mount deflect under normal loads- fortunately Whiteline has tested this and it is ~ 5mm, half that for Grp N tops and essentially 0 for metal plates. The last measurement needed is Motion Ratio- for a Mac P strut is for practical purposes 1 (closer to .96) but 1 will work
The formula for BT (bump travel) is BT = DS (damper stroke) - (FD full droop - TMC top mount compression- RH ride height) x MR motion ratio
just plugging in the numbers BT= 165-(496 -5 - 396) X 1; BT = 165-(95)x1, BT = 70mm
now what the sam heII does this have to micro whatchma call its? Well the oe "bump stop" is 60mm in length, our total bump travel is 70mm- this means that when the oe springs are only compressed 10mm they are into the "bump stops". Think of how easy it is in almost any turn w/ the soft oe springs to ompress 10mm, any braking for that matter as well. Now we know these are really "bump stops", but infact jounce bumpers. These are very progressive in nature- the top nubs are soft, you can squish them w/ your hand, they get progresively stiffer very quickly however. They are in fact "springs" and play an integral role in your suspension.
now lets lower the car
how about 1.5" - nice modest drop by many accounts. 1.5" = ~ 38mm. now we know we have 70mm of bump travel at oe ride height- guess how much w/ 38mm of lowering... yup 70-38= 32mm, wait we have 60mm microcellular jounce bumpers
they are 60mm in length- that means at rest (ride height) we are into the stops 28mm already. Think this might have some effect on handling?
How about cutting the stops- yes they can be cut, but how much and what end. I think most folks simply cut some of the top portion off. remember though this is the softest section, so now as you get into the stops it's going to be less progressive and more harsh. A better route IMO is cutting from the lower section to keep the interaction of the bumper more smooth. You don't want to cut too much then you jeopradize the life of your struts.
Is there a good solution, well yes and no. First for other reasons (read the sticky on lowering above) we know it's not wise to overly lower an Impreza- this would be another
The more mild the lowering the less worry about stops. Also if we lower AND firm up the rates appropriately then it also becomes less of concern- w/ firmer springs you resist bump more- so you can get by w less. Traditional camber plates can exasberate this problem- look at two cars with same ride height- one w/ thinner paltes, one w/ thicker
the thicker plate has "eaten" up some precious bump travel.
There are aftemarket stops on the market that vary in length and rate- these would be worth looking into if your lowering (I'm doing so as we speak
That's probably enough to digest at one sitting :lol: