LETS Talk about Bump stops/jounce bumpers. >:0

phil136

phil136

Well-Known Member
363
122
Los Angeles CA
Vehicle Model
Civic LX
Body Style
Sedan
Figured I should start this discussion since there isn't much on this that i could find in the forum.​
So I have been reading we are all riding on bump stops. And when you lower your car its even worse. This is complete ^&%^&%&, if you are not getting any travel on the damper. And if you cut the bump stops/jounce bumpers your car will be lower and not gaining any travel anyway since your car will now be off the bump stops will be close to the damper anyway. So in other words, H&R purposely build the springs to sit on the bumps as they do no recommend cutting them. :banghead:

Im kind of infuriated by this. since now Im looking at about a 2 inch drop (from 1.6) in the front and very little travel gain if I cut the stops about an inch. :rantsmiley: Its seems to get any effective bump travel you need to cut more than an inch. GRRR


Any input on this? Have you guys cut your bump stops? ​
I have discussed this in these threads as well:​
 
phil136 said:
"Yes, the measurements in this thread 100% prove that we're just barely off the bump stops until a driver gets in the car. With a 100lb+ driver, an STI is sitting on its bump stops. And, of course, all lowering springs put the car well onto its bump stops.​
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Here's the thing about lowering springs. If a spring lowers the car 1", it's lowering the car approximately 1" onto the bump stops. However, here's why trimming the bump stop won't actuallyhelp: if you were to trim your bump stops that full inch, you'd end up lowering your car.​
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Think about it. Your lowering springs are designed to lower the car on untrimmed bump stops. By cutting the bump stops, you'll end up putting 100-150lbs more weight on the spring. On most springs, that translates to about half an inch of travel. Trimming your stops will thus lower your car another half inch- not just putting you on your bump stopsagain, but doing so with almostzero bump travel left in the strut. There's no way to win.​
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I have to say, speaking strictly from this data, that lowering springs on stock and all stock-length struts are a bad idea. All of them- even the "good" ones that people like. Lowering shouldalways be done up front via RCE's lowering camber plates because they do not compromise suspension travel.​
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Since I've done this modification, I've noticed the tendency of the car to understeer on corner entry is gone. IMO that was always the fatal flaw of the STI's handling, and it seems the fix is free!" ​
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---subaru forum​
Click to expand...



This is from that extensive thread.
 
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Nix said:
I wouldn't think that H&R, Eibach prokit, or other 1" to 1.5" drop springs would cause you to sit directly on the bump stop. Pics would definitely be helpful if you could. Its possible that the bump stop in the front has simply slipped to the bottom of the strut "rod" and you can see it but you should have a black accordion dust boot covering that part.
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aaronMusngi said:
Well mine is , ive checked it a lot of times , i did jump on my front door to see how it moves ( movement is not gonna reach .2 inch ) ,i tried to put my finger and feel the gap between the rubber and the struts and i can't feel any gap between two stuff . And when i cut the rubber the car sits lower ,almost half an inch .
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Oh duh. You are fine. Don't trim them. They are MCU elastomers. It's a very cheap type of foam spring. They use them in bicycle suspension forks all the time.

It is an inexpensive, lightweight, zero maintenance, long lasting spring. They are an active part of the suspension. Nothing to worry about unless they start to bind up on the strut from wear. That takes a long time.


I was a little confused on what part you were talking about in the other thread.


As far as the koni bump-rubber replacements... you probably don't want to start messing with those unless you really really understand spring rates and corner weighting in your car and have a very very purpose built and tuned suspension where those minor changes will have a measurable effect.
 
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Don't trim them or you will possibly allow the strut to have metal/metal contact and that could mean serious damage. Riding on the MCU is fine. It's not really a bumpstop the way they used to be. Sorry about my confusion which delayed my answer... ugh I feel like I just had trouble trying a pair of velcro shoes.

in any case... First up fun link:

http://fatcatmotorsports.com/FCMstops_comp.htm

info...

Nix said:
Here is a pic of some different durometer (hard/stiffness) and length MCU "bumpstops" people swap into their suspension to fine tune it.

FCM_vs_NB_bump_stops_4.jpg
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Yah. They are called jounce bumpers. Aka cell foam. I understand that its okay to ride on them(some have said otherwise). Since the car is unstable on those things. Which is what i have been feeling on the roads.
I attribute the floaty/bouncy feel of my car to these in theory for now. However aaronMusngi got good results from cutting only one inch (see 3rd post). Even though the travel gained was minimal. Having better shocks also helps as you have said in another thread.

These were previously made to do exactly what you said: to prevent that metal to metal when shock /spring is pushed to the limit; and they still do. However, as much as i would like to think they are not made to be part of the dampening process, they actual are as you have said. At least the most modern cars. They are cheap and make the ride more comfortable, so manufacturers are using them as part of the suspension(as a progressive spring).

So in the performance sense though, you really don't want that foam used all the time as the damper/spring(which is unfortunately the case, especially with lowering springs ). You want the actual damper to be dampening.

Yes, the best way to go about this is to buy new shorter ones. Cutting will compromise the integrity and durability.

However, the gains might be minimal, as i have noted prior , and as stated in the quote above. As you will essentially drop the car more and lose travel once more. Unless you buy a really short bump or really cut one half or more. Or, buy shocks that have more travel/longe; or not lower your car...... And if you cut a whole lot or buy a really short one; when you bottom out or coil bind you will be in for some discomfort. Bottom line is the need for measuring what you need to do.


Funny how some spring manufacturers with similar spring rates and amount of drop tell you to cut and some don't. Wouldnt it apply all around? At least Eibach has sense enough to supply new "bump stops" with their sport lines. :banghead:
 
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