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Old 05-23-2006, 03:53 PM   #1
drees
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Default Bumpstops, rideheight and bump travel, oh my!

A quick bit of history: A few months ago, I installed Eibach wagon springs on my wagon w/o cutting the bumpstops on stock struts. This put the ride height at appx 14" front, 13.5" rear. A bit lower than I wanted or expected but the ride and handling was OK except on big bumps. On bumps which which would compress the suspension a good deal, it felt like I was definitely getting into the bumpstops by how quick the car would rebound. (stock shocks don't help, either!)

So last night I jacked up the car so that I could trim off a nub of each bumpstop. While I was up there, I decided to do some rough measurements to get an idea of how much travel I have before hitting the bumpstops and how much travel would be available without bumpstops.

For reference, on the 02 the front bumps stops are about 2 1/4" long, the rears are almost 5" long. I'm guessing the rears are very long to let one carry heavy loads.

So at the front, I jacked up a wheel and measured the hub to fender arch at 18". Then I pulled the bumpstop/dustcover down and measured from the top-hat down to the dustcover and got a bit more than 4". 18"-14" = 4" which means that I am just about on the bumpstops and only have 2 1/4" bump travel before bottoming completely! OK, because the motion ratio is slightly less than one I have a bit more, but not much.

In the rear, it was a similar story. No wonder the car responds so harshly over bumps which compress the suspension!

I ended up cutting off 1-nub off the front bumpstops and 2-nubs off the rear bumpstops. The ride is improved over big bumps, but it still feels like I need more bump travel in the front. The smallest nub is very soft so doesn't do much, I think I need to get into the 2nd nub to make a significant difference. But I'm wary of cutting off another full nub off the front since the front bumpstops only have 3 nubs total. Maybe another 1/2 nub in the front would be good, but the long term plan is to get some different springs which bump the ride-height up at least 1/2" (SPT wagon springs!).

My measurements were pretty rough given that I was working with the suspension on the car, but I'd be curious to see what others have come up with and if their findings were similar. I would have expected to have found more bump travel in the front after seeing pictures of how compressed the front wheels get during hard cornering.

I was surprised to see how close to the bumpstops I was given the relatively normal rideheights I'm running, and how low others seem to run. Any lower and you'd be sitting on the bumpstops! Do coilovers typically increase the amount of bump travel?
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Old 05-23-2006, 06:50 PM   #2
duncangrant
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You seem to have the mindset that the bumpstops are rock hard but they are not. Modern bumpstops are made of foam and will compress to about 1/4 of their free length. They are sometimes refered to as spring assisters or even Microcellular Jounce Bumpers in an effort to get folk out of the habit of thinking that its bad to 'hit the bumpstops'. They are an active part of the suspension.

If you feel a hit then that is not you hitting the stop but rather fully compressing it - quite a different story. If you feel a really hard hit then check that the bumpstops are long enough that you cannot reach coil-bind.

I like to do a test install of the suspension without springs then lower it onto its wheels (with wheels on blocks so I can underneath!) in order to get a reasonable idea of the fully compressed length of the stops and ensure that the distance between the spring perches is greater than the spring bound length. That lets me check for wheel to body interference also. Alternatively I might just compress the stop in a vise or press to determine its compressed length.

If you shorten a foam stop you are shortening its compressed length by a relatively small amount and gaining that amount of travel but you are making the hit more severe when you run out of bumpstop travel.
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Old 05-23-2006, 07:04 PM   #3
Arnie
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ideally, shorter, stiffer bumpstops would be a better bandaid to lack of travel, but in lieu of that, most have to get away with the cutting and "hope" the leftover is enough.
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Old 05-23-2006, 07:38 PM   #4
drees
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Yep, I completely understand that the bumpstops are very compressable. The OEM stops on the Impreza are very progressive! The first nub on the OEM stops are soft enough that you can compress it to 1/4 of the original length by hand! The 2nd stop is soft enough that you can compress it to 1/2 of the original length. They get progressively stiffer as you work your way to the top.

Quote:
Originally Posted by duncangrant
If you shorten a foam stop you are shortening its compressed length by a relatively small amount and gaining that amount of travel but you are making the hit more severe when you run out of bumpstop travel.
That is my goal exactly! To remove the most compressible bits is to give me a bit more bump travel without completely removing the function of the bumpstop, which is to provide a progressive stop to prevent harsh bottoming.

That said, I still feel that encounters with the bumpstops seem to be too frequent on a lowered Impreza, especially in the front where there seems to be hardly any bump travel at all (spring assistor/microcellular jounce bumper or not!). The rear seems to be OK which has a very long bump stop.

If anyone has measured the amount of bump travel on an Impreza, I want to hear about it to see if those measurements correspond with mine.
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Old 05-23-2006, 07:56 PM   #5
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Both ground control and powerflex offer a variety of bumpstops- lengths/firmness- I think the better answer is what you've already stated- firmer springs, less drop.

I remember somebody posting up travel #'s- it may have been a couple of years back though- I'll see if I can dig them up.
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Old 05-23-2006, 08:45 PM   #6
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Drees, I run custom made springs (made here in UK) because the Eibachs were too low for my needs. I run about 70mm bump travel in the front with springs and dampers about 60% stiffer than stock. I have no trouble (now) with bumps.

One of Arnie's posts on the Group4s has info and discussion on bump travel.
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Old 05-23-2006, 08:52 PM   #7
drees
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Quote:
Originally Posted by duncangrant
Drees, I run custom made springs (made here in UK) because the Eibachs were too low for my needs. I run about 70mm bump travel in the front with springs and dampers about 60% stiffer than stock. I have no trouble (now) with bumps.
70mm is a bit more than I have now on the Eibach springs if my measurements were accurate. But I doubt the springs are 60% stiffer than stock.
Quote:
Originally Posted by duncangrant
Maybe you should try running longer stops to help the inadequate springs some. Don't worry about running on the stops all the time.
Not a bad idea as a workaround, running longer softer bumpstops to compensate for a spring that that is too soft/short as a helper spring. Though I would rather just fix the problem the right way by getting the right springs in the first place and forget about it if I have to pull the struts apart again.
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Old 05-23-2006, 09:07 PM   #8
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The extra bump damping makes a big difference also. I ran AST coilovers for a while and even with stiffer springs optimised for extra bump travel I still had issues with bottoming due to the weak bump damping.

BTW I've noticed that KYB shocks tend to fail first on bump, maybe yours are already on the way out.
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Old 05-23-2006, 09:17 PM   #9
drees
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Quote:
Originally Posted by duncangrant
BTW I've noticed that KYB shocks tend to fail first on bump, maybe yours are already on the way out.
Could be, 64k+ mostly freeway miles on the stockers, though only about 5k on the Eibachs.

Anyone have actual bump travel or shock travel measurements? My search for actual numbers has been futile, just lots of references of "I need more bump travel!".
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Old 05-23-2006, 10:40 PM   #10
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I thought someone posted up #'s for the stock setup front/rear, but not haivng any luck finding it. Found references of 8" of total travel in the rear (droop+bump) but nothing breaking it out and no reference for the front .
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Old 05-24-2006, 04:21 AM   #11
stealth04
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I did some measuring a while back and I think I posted it into this forum- maybe try a search under my name.

From memory at a ride height of 350mm at the front there was only about 10mm of bump travel left with stock length bump stops
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Old 05-24-2006, 10:20 AM   #12
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Not completely on topic (but close ), most camber plates will additionally decrease travel. The RCE plates by theri design actiually net you some additional bump travel.

Also I measured up some WRX bumpstops- it appears front stops are the same 02-06 at ~ 60mm, the rears changed sometime- 02's are 120mm- 04's only 90mm
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Old 05-25-2006, 12:44 PM   #13
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FWIW - http://forums.nasioc.com/forums/showthread.php?t=998125

Compared to the Prodrives, the SPT Pinks put the fun back in driving my '06 WRX sedan. It's getting aligned today.
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Old 07-27-2006, 03:22 AM   #14
drees
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BigSky bumped an old thread which mentioned bump travel, which had a link to another post from WL which contained actual bump travel measurements. So now I bump this one.

Some general bump travel discussion:
Having Koni 8610/Yellows Shortened

Specific post w/stock travel measurements:
Whiteline Group 4 Coilovers

WL measured the sedan front bump travel at stock ride height at 71mm including 60mm long bump stop. Frankly, I have a hard time believing that number since my wagon sitting at 14" front would have been compressing the bump stop 10mm, where I found it to be just nearly touching.

Either way, it does highlight the need for stiffer springs/shocks when lowering the car. Now with SPT Pinks on the car instead of the Eibach Prokit, I am much happier with the ride quality and now I can concentrate on driving instead of dodging bumps and potholes.
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Old 07-27-2006, 10:06 AM   #15
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I too was a little questioning on their measurements, but not having done it I can't really say. I wish I would have measured things out when I first installed each of my setups, would have been some extra work, but it would be nice info to have.
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Old 02-13-2011, 01:38 PM   #16
brandubrx
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I just received my swift springs and kyb gr2's and am installing today. Unfortunately there were no Instructions regarding the bump stops with the swifts. Could someone please confirm the amount to trim (front or back) as directed by swift? I have an 02 wrx sedan. Thanks so much!
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