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Big Sky 03-23-2007 09:44 PM

Microcellular jounce bumpers- what the heII is he talking about?
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.

[url=]Technical || H&R Special Springs, LP[/url]

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:

BithellWgn06 03-23-2007 10:59 PM

Nice right up. Never really thought much about those whatchamathingies til now. Thanks

Big Sky 03-24-2007 06:32 PM

Here's some measurements I made w/ my setup (Koni's/STi Tarmac springs/RCE front plates)

First I found out the Koni's have a little longer stroke than the oe struts (albit minor)- 5mm up front and 10mm in the rear. Secondly the RCE plates lower my front an additional 8mm w/o penalty to bump stroke- added to the 22mm the springs lower- it gives me 30mm of lowering

In the front, front droop is 104mm (oe WRX is 93mm), bump travel is 66mm (oe WRX is 70mm)- not too bad considering the ride height is a full 30mm lower than stock

Thaks to ericd for these pics- which really illustrate exactly what I'm getting at. He assembled a strut w/o the springs, but included the bump stop. He then moved the suspension through several ride heights, oe then 20mm lower, then 30mm lower. Mind you these are Koni's, so the oe strut would be slightly worse off yet. You can clearly see that at 35mm lower it's well into the stop already- at rest.

oe height


20mm lower


35mm lower


I've got a few bump stops from Ground Control that I'm going to play w/- both are slightly shorter than the oe stops.

wagonracer 03-24-2007 07:20 PM

You are always a wealth of suspension info. Thanks for taking the time to share with us mortals !

thebean123 03-24-2007 07:40 PM

thanks i feel a little smarter now

Big Sky 04-01-2007 09:48 PM

Here's the GC stops

left one is the firmest and shortest 47mm, middle one is firm and 53mm the right one is the oe one 59mm and the softest

I'll give the middle one a try


mattxander12 04-15-2007 08:59 AM

[QUOTE=Big Sky;145137]Here's the GC stops

left one is the firmest and shortest 47mm, middle one is firm and 53mm the right one is the oe one 59mm and the softest

I'll give the middle one a try


updates ? I desperately await your sharing of knowledge

Big Sky 04-15-2007 09:40 AM

I've been waiting a little in hopes the Whiteline rollcenter adjuster kit comes out soon- keep the alignments to a minimum :)

Big Sky 05-23-2007 10:53 AM

well I went ahead and swapped stops- went w/ the 52mm one- so about 7mm of additional bump travel before getting into the "stops"- added a 3mm washer to the domed washer as well (my front springs have settled ~ 3mm since I put them on about a year ago)- this will get the ride height up a smidge and add to the bump travel as well.

craig@neo 01-30-2012 01:48 PM

nice! like!

its good to see quality tech like this on the forums.

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