Does anyone else's stock RX8 bottom out in rear on the mildest of speed bumbs?
10-17-2019 | 09:22 PM
#51
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Joined: Oct 2019
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I found out the solution
I have a 2004 RX8 Touring Edition that feels like it bottoms out when I go over any bumps at all. Speed bumps at more than a crawls pace hit hard in the back... like teeth jarring hard. Even road construction where the pavement level jumps up an inch or so causes it to bottom. I've had a few shops look at it and they tell me the shocks seem fine & there are no visible leaks. The car only 70k miles on it, so they all say springs last a long time so they can't be the problem. 
I push down on the back end & it doesn't bounce whatsoever. It goes down, then immediately back to the starting position.
Does anybody know of stock coils getting soft extremely early or does what I described sound like it could actually be the shocks needing replacing? Could this be another problem?
I have parked on my uphill driveway for 6+ years. I'm wondering if the hill has put more weight on the back end & over time it's compressed the rear springs.
BTW - how can I measure how much sag there is in the back (such as wheel well to center of wheel measurement)?
I already bought Tein S springs to put on it but would like to find out beforehand if I should be replacing shocks while I'm at it too.
Sorry for the book... I'm just getting frustrated with people telling me they don't see a problem when I'm sure as hell feeling a problem.
I push down on the back end & it doesn't bounce whatsoever. It goes down, then immediately back to the starting position.
Does anybody know of stock coils getting soft extremely early or does what I described sound like it could actually be the shocks needing replacing? Could this be another problem?
I have parked on my uphill driveway for 6+ years. I'm wondering if the hill has put more weight on the back end & over time it's compressed the rear springs.
BTW - how can I measure how much sag there is in the back (such as wheel well to center of wheel measurement)?
I already bought Tein S springs to put on it but would like to find out beforehand if I should be replacing shocks while I'm at it too.
Sorry for the book... I'm just getting frustrated with people telling me they don't see a problem when I'm sure as hell feeling a problem.
10-18-2019 | 06:24 AM
#52
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Joined: Jun 2007
Posts: 715
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RX8 guys have chased this problem for years.
The rear shocks are gas filled and do part of the lifting, so when they get old it can't lift anymore.
I put a Bilstein B12 kit on with shocks and Eibach springs on all fours, and it lifted the rears from 13.5 to 14.25 inches.
I also already had the rear camber pulled up to -1.65, the minimum I could get on both sides.
Then, I took off rear yellow Gt sway bar, and put 32 psi in front tires, and 34 in rears.
This car is a dream , and can haul *** over any bump in the parking lots.
The rear shocks are gas filled and do part of the lifting, so when they get old it can't lift anymore.
I put a Bilstein B12 kit on with shocks and Eibach springs on all fours, and it lifted the rears from 13.5 to 14.25 inches.
I also already had the rear camber pulled up to -1.65, the minimum I could get on both sides.
Then, I took off rear yellow Gt sway bar, and put 32 psi in front tires, and 34 in rears.
This car is a dream , and can haul *** over any bump in the parking lots.
10-18-2019 | 01:42 PM
#53
I chased a similar problem for months, finally bought a new suspension, replaced it, when I tore the old ones out the rear bump stops were gone. Look there first. You may be able to peek up from the bottom. The dust that was left on mine, and the little bit trapped in a washer, was pink. The rear springs, even he stiffer S, are still under 150 lbs per inch. That is soft. The bump stop is a working part of the suspension.
DING DING DING !!!!!!
It only took ten years to get the correct answer to this issue but people still go and replace the shocks when all they need do is replace the stops . Almost every 8 I've ever worked on with more than 50000 miles on it has no rear bump stops left.
Hate to think how many perfectly good shocks have been replace because of this !
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04Green (10-19-2019)
10-18-2019 | 09:37 PM
#54
Water Foul
Joined: May 2013
Posts: 2,521
Likes: 262
From: Republic of Texas
DING DING DING !!!!!!
It only took ten years to get the correct answer to this issue but people still go and replace the shocks when all they need do is replace the stops . Almost every 8 I've ever worked on with more than 50000 miles on it has no rear bump stops left.
Hate to think how many perfectly good shocks have been replace because of this !
It only took ten years to get the correct answer to this issue but people still go and replace the shocks when all they need do is replace the stops . Almost every 8 I've ever worked on with more than 50000 miles on it has no rear bump stops left.
Hate to think how many perfectly good shocks have been replace because of this !
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04Green (10-19-2019)
10-19-2019 | 05:48 AM
#56
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My new 07 had 4 different ride heights within a year.
Rears were less at 13.5 and 13.75 inches while fronts were 14 and 14.25.
Clearly the rears were shot while sitting and not 10k miles on it.
Took me several years to get that the rears are riding on air and a spring.
So, I got close to worlds best air shock.....Bilstein.
Bump stops should rarely be touched in any shock.
Now car sits at 14.25 all the time on all 4.
Rears were less at 13.5 and 13.75 inches while fronts were 14 and 14.25.
Clearly the rears were shot while sitting and not 10k miles on it.
Took me several years to get that the rears are riding on air and a spring.
So, I got close to worlds best air shock.....Bilstein.
Bump stops should rarely be touched in any shock.
Now car sits at 14.25 all the time on all 4.
10-19-2019 | 10:24 AM
#57
Brettus nailed it. The 8. like the miata, corners on the bump stop. Think of it as an extra layer of progressive spring. And, the material for the rear one is on par with the sponge you use to wash the car. Thing is, it completely disappears, so when you change shocks you do not even know it is missing. I found a sliver wedged between the washer and shaft on mine. Added stops, problem when away.
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Brettus (10-19-2019)
10-20-2019 | 11:58 AM
#59
Cutting the bumpstop applies if you're lowering the car.
On many cars the bumpstop adds spring rate toward the end of compression, and isn't touched in normal driving. Not on ours.
On many cars the bumpstop adds spring rate toward the end of compression, and isn't touched in normal driving. Not on ours.
Last edited by Loki; 10-21-2019 at 08:50 AM.
10-21-2019 | 03:46 AM
#60
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I think some of those specs are just thrown around to give people a measurement like the others do.
Told the Mazda service guy to maybe cut bump stop if necessary, so don't know what bump stop is like now.
Fronts stayed about the same at 14.25, and the rears raised from 13.5 to 14.25, so 8 is 14.25 all around, and feels right.
No more track days, but the ones I had were good to test and tune.
10-21-2019 | 11:24 PM
#61
The bumpstops on the RX8 are very long and designed to work as a secondary spring rate during heavy cornering and big bumps. That’s why the main OE spring rate is so low. When you go to race spring rates and lowering you don’t even need to run a shorter rear shock. Just removing the bumpstop gains a lot of extra compression clearance. If you get rid of the rubber bushing mounts at the shaft top and weld on a spherical bearing cup instead you get even more clearance for racing.
10-22-2019 | 07:01 AM
#62
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So what you say is use no bump stock, and others say it's integral to system.
I think I understand your idea better, and for years guys would talk about touching bump stops on curves, and spring rate going to infinity.
In my street world I might accept feeling bump stock on a 20 mph bump, but my new Bilsteins are stiff as ****, and I can't tell if they are hitting or not.
On a side note, I'm the guy that shortened the rear sway bar link years ago, and it did make the car start to lose rear when pavement slick.
Now with Bilstein B12, I took off the rear yellow bar, and car drives like dream over bumps in corners without that drama.
Of course that threw it into understeer which is fine with all the straight roads in West Texas.
I compensated by putting 32 psi f, and 34 psi rear. also, rear camber is minimal at -1.65 deg.
Now it feels almost the same understeer that my X1 has ...hard to spin, but corners the way I like it.
I don't ever want a hang out the rear drift car....I want traction and have Michelin PSS.
I think I understand your idea better, and for years guys would talk about touching bump stops on curves, and spring rate going to infinity.
In my street world I might accept feeling bump stock on a 20 mph bump, but my new Bilsteins are stiff as ****, and I can't tell if they are hitting or not.
On a side note, I'm the guy that shortened the rear sway bar link years ago, and it did make the car start to lose rear when pavement slick.
Now with Bilstein B12, I took off the rear yellow bar, and car drives like dream over bumps in corners without that drama.
Of course that threw it into understeer which is fine with all the straight roads in West Texas.
I compensated by putting 32 psi f, and 34 psi rear. also, rear camber is minimal at -1.65 deg.
Now it feels almost the same understeer that my X1 has ...hard to spin, but corners the way I like it.
I don't ever want a hang out the rear drift car....I want traction and have Michelin PSS.
Last edited by REDRX3RX8; 10-22-2019 at 07:08 AM.
10-22-2019 | 10:38 AM
#63
Read carefully what Team wrote: if you're modifying the suspension with race-spec spring rates, you can choose to remove the bumpstop and let the springs do all the work.
All a bumpstop ever does is increase spring rate. If you're on stiff racing springs, you probably don't need that.
In the RX8, on a stock suspension, it smoothly increases effective spring rate with compression to achieve the designed ride and handling.
On some other cars hitting bumpstops can definitely be a jarring experience, but that's other cars' problem.
Last edited by Loki; 10-22-2019 at 10:40 AM.
10-22-2019 | 11:38 PM
#64
It’s not going to infinity until fully squashed. It’s intentionally designed as a secondary spring. I want to say they’re about 2” or 3” long in the rear with a specific lb/in crush rate just like a spring. In circle track they now have tiny high-rate bump springs with soft main springs such that it only rides on the bump spring in the turns. So the aero compresses the main springs to get the car lower than the allowed minimum static height and then the bumpspring kicks in to handle the cornering load. Kind of been studying them for a while now
Bump Springs | Swift Springs USA
This is not a coilover shock (main spring is separate between the suspension arm and chassis) so you can see it in action
Bump Springs | Swift Springs USA
This is not a coilover shock (main spring is separate between the suspension arm and chassis) so you can see it in action
Last edited by TeamRX8; 10-22-2019 at 11:48 PM.
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