Bouncy Rear End after Suspension Mods

[Mustangcol]

I just did a lot of suspension mods to my 2017 Mustang GT non-PP and the rear bounces a lot on rough roads. The idea was to have the car close to stock ride height so I did GT350 front springs 211lbs (10mm drop) and PP rear springs 728lbs. The plan was to raise the front 1/4" with steeda spacers to OEM height, but after driving a week with the GT350 front springs I think the ride height will be fine here in Colombia so I'm thinking of doing GT350 857lbs rear springs (10mm drop) or steeda min drop 800lbs with 1/4" spacer.

Front:
GT350 Springs 211lbs
Ford Track Struts
Performance Pack Control Arms
36mm GT350 Sway Bar

Rear:
PP Springs 728lbs
BMR BK081 bearings in control arms
Bilstein B6 Shocks
BMR Rear Sway bar (softest setting)
BMR Level 2 Subframe Lock Out CB005

I asked the shop if they indexed the bushings and they said no. So he did it with the rear on jack stands and put the jack under the control arm to put tension, but I'm reading that indexing should be done on a alignment lift with all wheels on the ground. I will take it to another shop to properly index the front and rear, but any thoughts as to what would cause this? It seems odd considering the rear has stock ride height springs and shocks.

Could it be the ride frequency between front and rear? The current spring rates gives a ride frequency higher in the front than the rear. 1.51 and 1.46 respectively. Getting the GT350 rear springs would get me to 1.51 and 1.59, 4.82% difference.

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[Bluemustang]

It could be a mismatch in damping front to rear. But I think it's the rear shock mounts. When you stiffen the suspension, and especially, after you've done the RLCA bearing, the deflection in the rubber bushing in the OEM mount. You need to swap to a spherical bearing shock mount.

You might not think such a thing could make a difference but it does. I modified almost the entire suspension in my 2015, but it wasn't really until I went to the RLCA bearing that I noticed the bounce. As you stiffen the suspension and reduce deflection in the system, the forces will move to the least common denominator and in this case it's the OEM rear shock mounts. Since you removed the binding RLCA bushing, the RLCA can now move freely and articulate. It makes vehicle motion more reactive. That's a huge change.

IMO, what you are noticing is the rear damper is having a tougher time doing it's job now. It needs a better mount which will allow it to do it's job and damp the road forces rather than trying to fight itself in the process.

I would not have necessarily gone with two different dampers front and rear but honestly I don't think the difference in damping curves would be enough to do what you're describing and you should look to the shock mounts. Steeda and J&M make a very good mount. I've used both and now run the J&M mount for my Ohlins coilovers. My suspension is well sorted and I have more mods than you, and it made a big difference over the OEM shock mount. My 2 cents.

 

[Mustangcol]

It could be a mismatch in damping front to rear. But I think it's the rear shock mounts. When you stiffen the suspension, and especially, after you've done the RLCA bearing, the deflection in the rubber bushing in the OEM mount. You need to swap to a spherical bearing shock mount.

You might not think such a thing could make a difference but it does. I modified almost the entire suspension in my 2015, but it wasn't really until I went to the RLCA bearing that I noticed the bounce. As you stiffen the suspension and reduce deflection in the system, the forces will move to the least common denominator and in this case it's the OEM rear shock mounts. Since you removed the binding RLCA bushing, the RLCA can now move freely and articulate. It makes vehicle motion more reactive. That's a huge change.

IMO, what you are noticing is the rear damper is having a tougher time doing it's job now. It needs a better mount which will allow it to do it's job and damp the road forces rather than trying to fight itself in the process.

I would not have necessarily gone with two different dampers front and rear but honestly I don't think the difference in damping curves would be enough to do what you're describing and you should look to the shock mounts. Steeda and J&M make a very good mount. I've used both and now run the J&M mount for my Ohlins coilovers. My suspension is well sorted and I have more mods than you, and it made a big difference over the OEM shock mount. My 2 cents.

I was just reading that long thread about the shock mount. I don't see a steeda shock mount available for the bilstein b6 shocks. I'm thinking to go with the ford shocks to match the front and the steeda billet mount. I found a used shock mounts at a good price so it won't hurt so bad. If that solves it then I can order the GT350 rear springs and I'll have basically the GT350 set up

 

[Bluemustang]

Also, the RLCA does not need indexing because it's a bearing. And considering you did not change the ride height being that you are running PP rear springs, that doesn't seem like it would be a problem.

You are now running a higher ride frequency in the front, which is a departure from the "flat ride" (self-settling) suspension theory but your frequencies probably aren't far off enough that porpoising should be an issue. The ride frequencies front to rear on my Ohlins are MUCH more a mismatch than what you are running and it doesn't porpoise. I know what that feels like. The shocks take care of most of that. It's different than the flat ride that Ford uses but it isn't a problem.

 

[Bluemustang]

I was just reading that long thread about the shock mount. I don't see a steeda shock mount available for the bilstein b6 shocks. I'm thinking to go with the ford shocks to match the front and the steeda billet mount. I found a used shock mounts at a good price so it won't hurt so bad. If that solves it then I can order the GT350 rear springs and I'll have basically the GT350 set up

Again I don't think that's your biggest problem, but adjustable shocks would be nice. If I were you and I were building it, I'd match it with the GT350 rear springs and Ford Performance track dampers. AND the spherical bearing shock mounts. You'd love it. Of that I have no doubt.

With a stock GT Mustang, no one would notice the shock mounts. Once you stiffen things up and that RLCA bearing... it rears its ugly head lol. But that's solvable! You will feel the rear damping like it's supposed to and it'll feel right.

 

[Mustangcol]

I just ordered on ebay Steeda 555-8151 for $125 that fits the ford shocks. Says he bought it from Steeda in july, his shock broke, and he bought bilstein b6 that didn't fit. Lethal racing lists the ford shocks for $176 each but when i go to checkout it raises the price. I'll give them a call tomorrow.

 

[Mustangcol]

Again I don't think that's your biggest problem, but adjustable shocks would be nice. If I were you and I were building it, I'd match it with the GT350 rear springs and Ford Performance track dampers. AND the spherical bearing shock mounts. You'd love it. Of that I have no doubt.

With a stock GT Mustang, no one would notice the shock mounts. Once you stiffen things up and that RLCA bearing... it rears its ugly head lol. But that's solvable! You will feel the rear damping like it's supposed to and it'll feel right.

I was reading that thread and it seems like everyone who did BK081 and CB005 has this bouncing problem. It's exactly what I changed on mine. I thought the sway bar was impacting it bc I had it on the middle hole but I adjusted it to the softest and I didn't feel any difference. You can imagine how miserable this is with the roads here in Colombia. It's so bad that my girlfriend noticed it

 

[Bluemustang]

BTW I love those Ford Performance shocks. I ran them for years and they're good.

 

[Mustangcol]

BTW I love those Ford Performance shocks. I ran them for years and they're good.

Awesome. I tried to save money and do a weird set up for stock ride height and handling. It didn't work out. End up spending more in the end haha Thank you for the help!

 

[Bluemustang]

I was reading that thread and it seems like everyone who did BK081 and CB005 has this bouncing problem. It's exactly what I changed on mine. I thought the sway bar was impacting it bc I had it on the middle hole but I adjusted it to the softest and I didn't feel any difference. You can imagine how miserable this is with the roads here in Colombia. It's so bad that my girlfriend noticed it

Yes! It's likely the BK081 that is the biggest contribution. That RLCA is basically bound up with the stock bushing. I've seen people hang on it with car lifted. Now it is free to move, and it's putting more stress on the shock. With the OEM mount, essentially it's fighting itself and trying to damp the road forces at the same time which is causing the bouncy feeling. Once that's under control, it can do its job to damp properly.

 

[Bluemustang]

Awesome. I tried to save money and do a weird set up for stock ride height and handling. It didn't work out. End up spending more in the end haha Thank you for the help!

Been there! LOL. Suspension tuning is a dark art in some ways. Once you modify, the next least common denominator will reveal itself. But you are on the right track! I've been around with this car since 2015 and so I had to learn from trial and error, and spent more than I care to mention here. But, I got an education in the process. You can always PM me if you want to chat and I'll do my best to help.

 

[Mustangcol]

Been there! LOL. Suspension tuning is a dark art in some ways. Once you modify, the next least common denominator will reveal itself. But you are on the right track! I've been around with this car since 2015 and so I had to learn from trial and error, and spent more than I care to mention here. But, I got an education in the process. You can always PM me if you want to chat and I'll do my best to help.

I ordered everything. FR3 shocks, GT350 rear springs, and Steeda billet mount. I figured not to wait on the springs because now I address 3 problems, the ride frequency between front/rear, the sticky shock mount, and difference in dampening. I hope the additional $856 + shipping from miami to colombia will sort this out.

 

[Twi5ter.0range]

I’m having the same bouncy issue with my car with Bilstein B6’s. Except I have steeda min drop springs. Everyone on here has said good things about the Bilstein B6s but I think the rear shock rebound is too hard. It could also be that the 800 spring rate is too high for B6’s. Also tried to look at the Steeda shock mounts but of course they don’t fit the B6’s. Everything else is stock on the rear besides the springs, shocks, and IRS brace. Yes the bump stops are trimmed properly. I don’t think the oem mounts will cause that much of a bounce that i’m experiencing. If there’s a small dip on the road that covers the whole lane and I go over it about 40-50mph, it slightly jumps me out of my seat. I have sorta given up on what could be causing it to be that bouncy. I might try putting the stock shocks back on just to see if I can eliminate it being the bilstein shocks.

 

[Mustangcol]

I’m having the same bouncy issue with my car with Bilstein B6’s. Except I have steeda min drop springs. Everyone on here has said good things about the Bilstein B6s but I think the rear shock rebound is too hard. It could also be that the 800 spring rate is too high for B6’s. Also tried to look at the Steeda shock mounts but of course they don’t fit the B6’s. Everything else is stock on the rear besides the springs, shocks, and IRS brace. Yes the bump stops are trimmed properly. I don’t think the oem mounts will cause that much of a bounce that i’m experiencing. If there’s a small dip on the road that covers the whole lane and I go over it about 40-50mph, it slightly jumps me out of my seat. I have sorta given up on what could be causing it to be that bouncy. I might try putting the stock shocks back on just to see if I can eliminate it being the bilstein shocks.

It sounds like your issue is different than mine. I have 728lbs stock ride height PP springs with Bilstein B6. I actually have little issue with how it damps larger bumps. I feel like it is slightly faster rebound than the front with 211lbs and FR3 struts, but I suppose that could be caused by the differences in ride frequencies (I currently have a weird mix of dampening and springs) But don't take my word for it, I'm new to car suspensions.

My issue is small bumps and it's like the shock stops dampening. There's a long tunnel here with small bumps like that for a long stretch. Like braking bumps, and it will bounce up and down for the entire stretch like it stops dampening

I searched for images of the OEM upper shock mounts and it's only a large bushing. My rear control arm moved A LOT because the bushings were shot and the rear subframe bushings were OEM. I think those bushings would allow sufficient movement during rear articulation and the shock would center on the shock mount when compressing the spring and not causing issues. These cars by design are tourers with a lot of compliance in these bushings. Take out that compliance with IRS lockouts and control arm bearings and then the upper shock mount introduces lateral load to the shock and dampening is impacted.

I'm waiting for the replacement FR3 shock, steeda shock mounts, and GT350 springs. I had read somewhere that the FR3 strut/shocks have GT350 valving, and I am impressed with the front FR3 strut + GT350 springs so I'm hoping I will feel the same in the rear.

Some thoughts: I wasn't sure about the Bilstein B6 because they are listed for "stock ride height". If you look at the spring rates available for non-PP and PP, there is a 10% difference in spring rate. What spring rate did Bilstein design the dampening? Then I read that people are running minimum drop springs with 800lbs and saying they are great. How can that be? In mountain biking when I changed spring rate it always meant that I had to put a few clicks different on rebound to match the rate. A few clicks too fast rebound and the tire starts to loose traction on small bumps. I really think the better choice for 800lbs min drop is the Steeda adjustable shocks, drive down some bad roads, and adjust the rebound for your spring rate. I considered replacing my Bilstein shocks with those, but ultimately decided that GT350 rear springs with ford's FR3 shock would be a set it and go solution. Maybe you can call Steeda and ask them because they sold you the springs.

 

[Bluemustang]

It sounds like your issue is different than mine. I have 728lbs stock ride height PP springs with Bilstein B6. I actually have little issue with how it damps larger bumps. I feel like it is slightly faster rebound than the front with 211lbs and FR3 struts, but I suppose that could be caused by the differences in ride frequencies (I currently have a weird mix of dampening and springs) But don't take my word for it, I'm new to car suspensions.

My issue is small bumps and it's like the shock stops dampening. There's a long tunnel here with small bumps like that for a long stretch. Like braking bumps, and it will bounce up and down for the entire stretch like it stops dampening

I searched for images of the OEM upper shock mounts and it's only a large bushing. My rear control arm moved A LOT because the bushings were shot and the rear subframe bushings were OEM. I think those bushings would allow sufficient movement during rear articulation and the shock would center on the shock mount when compressing the spring and not causing issues. These cars by design are tourers with a lot of compliance in these bushings. Take out that compliance with IRS lockouts and control arm bearings and then the upper shock mount introduces lateral load to the shock and dampening is impacted.

I'm waiting for the replacement FR3 shock, steeda shock mounts, and GT350 springs. I had read somewhere that the FR3 strut/shocks have GT350 valving, and I am impressed with the front FR3 strut + GT350 springs so I'm hoping I will feel the same in the rear.

Some thoughts: I wasn't sure about the Bilstein B6 because they are listed for "stock ride height". If you look at the spring rates available for non-PP and PP, there is a 10% difference in spring rate. What spring rate did Bilstein design the dampening? Then I read that people are running minimum drop springs with 800lbs and saying they are great. How can that be? In mountain biking when I changed spring rate it always meant that I had to put a few clicks different on rebound to match the rate. A few clicks too fast rebound and the tire starts to loose traction on small bumps. I really think the better choice for 800lbs min drop is the Steeda adjustable shocks, drive down some bad roads, and adjust the rebound for your spring rate. I considered replacing my Bilstein shocks with those, but ultimately decided that GT350 rear springs with ford's FR3 shock would be a set it and go solution. Maybe you can call Steeda and ask them because they sold you the springs.

I am disappointed. A lot of folks whose opinions I tend to trust have vouched for the Bilstein B6s. But recently I have seen a few people have less than good reviews of them.

Suspension modifications are somewhat of a dark art to get it right. Essentially redesigning the suspension that Ford spent a lot of money in R&D. It can be done better, but it takes some knowledge or usually trial and error lol. It's easier to do wrong, much harder to do right.

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