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Rear End Shocks or Springs to fix bottoming out

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Rear End Shocks or Springs to fix bottoming out

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Old 04-12-2018, 02:40 PM
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Rear End Shocks or Springs to fix bottoming out

So, I've read several of the threads about rear end bottoming out over bumps and am still unsure what to do about it. The underside doesn't hit, but it fully compresses the shock and it hits the bump-stop pretty hard.
When I first bought the car, I noticed that it would bottom out over a certain bump when I took it at speed(a bump that my other cars have no issue with).
So, I replaced the rear shocks with kyb's.
After replacing, no more issues with just me in the car or with me and the wife in the car.
But if I put the kid in the back seat or any luggage in the trunk-still bottoms out.

I've read that a set of Bilstein HD's in the rear should fix it. I've also read that some people do springs and stiffer shocks, but I don't want to lower the car-so I don't want to replace the shocks if I can avoid it.

What do you suggest?
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Old 04-12-2018, 02:54 PM
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Springs determine the ride height and the amount of compression. I would check that your bumpstop is not destroyed and replace it if it is. It could be that simple. The bumpstop isn't like a door stopper that stops hard impacts, its supposed to compress smoothly even when used, just like a spring. On a regular car it's activated more often than you realize. Its when it's broken apart and only the last few inches of it are activated that you get a crash.
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Old 04-12-2018, 04:15 PM
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Shocks and springs on the RX-8 determine ride height and sagging stock rear RX-8 suspensions has been an issue for a long time.

I have a thread here with a similar issue that I had ten years ago or so. I had brand new Tein S techs and with my then young daughter in the car, the rear tires would tuck.

So in listening to the experts here tell me that shocks did not play a roll in ride height, I sent the springs back to Tein and they tested them for free and told me they were fine.

So I reinstalled another new set that I had (just in case Tein was wrong) and installed new KYB's and never had an issue again. Since that I have fixed this issue on a couple of RX-8's and all were bad rear shocks, I have never seen stock springs fail but I imagine it is possible now with age. The springs I had tested went in another RX-8 and the owner never had an issue with them so Tein was right.

Last edited by 9krpmrx8; 04-12-2018 at 04:21 PM.
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Old 04-12-2018, 04:22 PM
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Typically blown shocks are obvious and in some RX-8's since I have seen ones that were gone completely but even on sagging cars I have seen shocks that did seem to compress and rebound normally by hand so I can see how some would think they are good.
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Old 04-12-2018, 08:18 PM
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You're right, shocks play a role (had a whole argument with Koni about this.. ) but since the shocks have already been replaced with reasonable replacements, I'm thinking there is something else to this one.
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Old 04-13-2018, 06:16 AM
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Originally Posted by Loki View Post
You're right, shocks play a role (had a whole argument with Koni about this.. ) but since the shocks have already been replaced with reasonable replacements, I'm thinking there is something else to this one.
Y, I doubt the shocks are the issue as the kyb's have been bottoming out with a load in the rear from the time I first had them installed (but anything's possible).

The springs are stock. I'm unsure the condition of the bump-stops.
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Old 04-13-2018, 07:37 AM
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It's rare but it happens, two SARX guys that had Koni yellows fail in months and two other locals had the same issues with Tokico D specs. I have never heard anything with KYB's and after I ran them several others did too and no one had issues, a few have a ton of mileage on them now.
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Old 04-13-2018, 07:54 AM
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Check the bump stops first, they are much cheaper than shocks or springs.

This is what was left of mine.



And what they are supposed to look like.



Have not "bottomed out" since I installed them.
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Old 04-13-2018, 08:11 AM
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I had similar issues about a year ago, so I went out and bought some Koni yellows, progress tech springs, and new bump stops to see if that took care of the issue.

The aftermarket springs lowered the rear more than the front for whatever reason (the front didn't seem to change), so any large rut or bump in the road still affects the rear more than the front. But the car doesn't bottom out near as much as it used to. I'm 95% sure this is from replacing the stock shocks and turning the Koni's to a tighter setting. The bump stops help when you reach the end of the shock travel, and makes the "bottoming out" hit less severe. Mine had deteriorated pretty far when I replaced them last year.

I'll be digging into the rear suspension again this summer, and in all honesty, I'll probably just reinstall the stock rear springs again and see if the ride height increases to what I'd prefer. It always sucks when you hear the undercarriage scraping on a speed bump with more than one person.
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Old 04-13-2018, 08:45 AM
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These cars and Miatas (maybe most cars these days?) are designed to engage the bump stops when cornering, to increase the overall spring rate progressively. So it's not just a stopper. That's why they wear out. If your shocks were blown at any point, the undamped vibration when riding on the bumpstop causes it to crumble sooner.
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Old 04-13-2018, 08:53 AM
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Originally Posted by Loki View Post
These cars and Miatas (maybe most cars these days?) are designed to engage the bump stops when cornering, to increase the overall spring rate progressively. So it's not just a stopper. That's why they wear out. If your shocks were blown at any point, the undamped vibration when riding on the bumpstop causes it to crumble sooner.
So if I understand correctly, if an RX-8 has excessive body roll, it could be the lack of bump stops?
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Old 04-13-2018, 08:53 AM
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Y, I'm starting to think that to get this seutp right, I may be looking at multiple rear suspension rebuilds.
I'll start with the bump stocks. Then if that doesn't fix it, I'll try again with stiffer rear shocks. I've heard the Bilstein HD's sb good. Then I guess worst case scenario if that doesn't fix it, I'll then have to find new non-lowered springs--but I tend to think the springs are good. I've never had a car where the springs wore out. Broken, yes, but never worn out.

Originally Posted by 0-TO-100_Real_Quick View Post
I had similar issues about a year ago, so I went out and bought some Koni yellows, progress tech springs, and new bump stops to see if that took care of the issue.

The aftermarket springs lowered the rear more than the front for whatever reason (the front didn't seem to change), so any large rut or bump in the road still affects the rear more than the front. But the car doesn't bottom out near as much as it used to. I'm 95% sure this is from replacing the stock shocks and turning the Koni's to a tighter setting. The bump stops help when you reach the end of the shock travel, and makes the "bottoming out" hit less severe. Mine had deteriorated pretty far when I replaced them last year.

I'll be digging into the rear suspension again this summer, and in all honesty, I'll probably just reinstall the stock rear springs again and see if the ride height increases to what I'd prefer. It always sucks when you hear the undercarriage scraping on a speed bump with more than one person.
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Old 04-13-2018, 01:29 PM
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Well how about before replacing parts, just inspect thr bump stops. I'm pretty sure you can just lift up the dust boot and look at it. If it has crumbled, you have your answer.
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Old 04-13-2018, 08:18 PM
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Most reputable aftermarket springs come with million mile warranties these days. OE springs are built to similar levels of quality. It most likely is not the springs.

Bump stops are known to wear out. Rear shocks are known to fail before the fronts do, and Mazda shocks are typically gas charged to support some of the weight of each corner. Those are the things you should be considering.
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Old 04-14-2018, 01:54 PM
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I'm too tired to put on shocks, so I just bought the Bilstein B12 kit and paid Mazda dealer $550 on a discount coupon to do it.

The ride height when new on my GT 07 was anywhere from 14.25 to 13.75.

Now all corners are 14.25 and drives great.

I had any idea when they came back from dealer.

All were around 14.25, but the front driver was 14.50, so I took wheel off, put bottle jaCK UNDER arm, and loosened and tightened it to get 14.25.
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