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20mm rear sway bar installed - Too Stiff!

1261 Views 32 Replies 15 Participants Last post by  DougNuts
My previous RSB experience was w/ my '13 Legacy, I'd installed a 19mm and it just didn't seem to make a noticeable improvement so I went to a 21mm whiteline and was happy w/ it.

So I figured the 20mm WRX rear bar for my '17 Limited would be the right move. I've had it for a week and found when the OB is packed w/ my musical equipment (gigging musician), the OB is jumpy over uneven surfaces. The 20mm has made the ride too rough for everyday ATL interstate and surface road driving.

The choices are go back to the 16mm or buy a 19mm. I've read on these forum that folks report the 19mm upgrade pretty mild in terms of feel but made the improvements that were expected.

Just looking for direction and advice here,.. 19mm or back to 16mm. I understand the 20mm is 50% stiffer than the 19mm,..
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This was part of the discussion in the 19mm Gen5 and then later the 20mm gen5 sway bar threads. Those are long threads to read through.

since the 20mm is a bit stiff for your driving area (and that’s what‘s really important) - the 19 might be a good choice. There is also an 18 available. I think I found an OE 17 that fits.

For percentages, you should look at how much more stiff it is vs your OE 16mm, I wouldn’t compare the 19 vs the 20 difference only. Different perspective when you look at the numbers. All that stuff was also in those long threads back when.

It might be cheaper to get the 19mm perrin adjustable, which could be the equivalent of the 20 you tried, the 19, and an 18. It’s not terribly expensive. If you can find a good deal on a 19, I’d say try it out. I’d like to try a 19. Or even an 18. It’s not urgent though.

for some of the driving/loads I carry, the 20 was annoying enough that it wasn’t worth it over when you’re just driving around on smooth roads. Reminded me of a solid rear axle, like my old pickup truck in the same situations. I took a chance on the 20 because of the price at the time, and that didn’t work out. It’s still hanging on the garage wall - I really should sell it.
 

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2016 3.6 Limited with ES
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I'm a pavement driver 95% of the time and I changed out my RSB to the 19mm. Everyone suggests something similar but I also added 85d urethane bushings. That's nirvana.

The 19 was fine and I had no complaints but with the 85d bushings, it's just firm enough to stay planted on most, but not all, pavement surfaces. The time it's not planted are for uneven surfaces but I'm very happy. It's to the point where the wife says, "Don't go so fast into the corner!" That's when I know it's just right.

You can also go with 75d bushings and that is also a good set-up. I had both for a bit and went with the 85d.

Our own member member @traildogck makes and markets bushings. He's a good people and his pours are first rate.
 

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2018 Outback Limited 2.5
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The 20mm has made the ride too rough for everyday ATL interstate and surface road driving.
Just tell GDOT to put some more aesthetically pleasing asphalt around those pot holes on 285, then you won't have any issues. LOL

Former driver of said potholes, err. roads in GA.
 

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2017 Outback
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Discussion Starter · #7 ·
Thanks for the feedback and suggestions. I went ahead and ordered the 19mm kit. Put the tire pressure down to OEM specs for now and that took the edge off. I’ll report back once the deed is done.
 

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2016 3.6 Limited with ES
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I have the 20mm on. I honestly couldn't tell a heck of a difference, but it was a couple years ago. I've since gotten used to it and don't even think about it. You think 1mm is going to be a game changer? IDK.
Yes, it makes a huge difference.

It's not the 1mm increased diameter that is important but rather how much increase in tortional stiffness there is between bars. If you laid a penny on top of a nickel there would only be little bit of a difference in height but look how much bigger overall the nickel is compared to the penny. That's where the diameter on a RSB comes into focus.

Assuming that different sway bars are made the same with the only difference being the diameter.

A 19mm RSB has about a 99% torsional stiffness increase over 16mm and a 20mm has a 144% increase over a 16mm. That 1mm does make a difference.

If you want to look at RSBs less than 19mm, an 18mm has a 60% increase and a 17mm has a 27% increase.

There's a math formula for the calculation and some brainiac here probably can post it but those are the overall numbers.

18 and 19mm seem to be sweet spots.
 

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2022 Subaru Outback Wilderness Dark Grey. Metallic Grey.
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I run the Whiteline 20MM on my Wilderness in the Atlanta area. Having had no issues so far, I do tend to avoid the Grand Canyon-sized monster pavement distortions on the north side of I-285 which has been in perpetual construction.

I like the stiffer unit verse the OEM. Makes the Wilderness handle and track better with little wandering.
 

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Discussion Starter · #12 ·
I have the 20mm on. I honestly couldn't tell a heck of a difference, but it was a couple years ago. I've since gotten used to it and don't even think about it. You think 1mm is going to be a game changer? IDK.
We won’t know til we do it. But I wouldn’t waste the bucks if I didn’t think it would. The 20mm is said to be 50% stiffer than the 19mm. Seems significant “on the internet”.
 

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The Whiteline 20mm for the Gen 6 is roughly equivalent to 19.5mm in the soft setting and 21mm in the hard setting because the lever arm length of the Whiteline bar is either slightly longer (soft setting) or slightly shorter (hard setting) than a stock bar.

The torsional stiffness of a rod is proportional to the 4th power of its diameter.

The Gen6 stock sway bar is a HOLLOW 19mm and the Whiteline is a SOLID bar. The Wilderness rear sway bar is a different part number so I'm not sure what that bar is.

To relate the stiffness of a hollow sway bar to a solid sway bar, you calculate the stiffness of the solid bar and then subtract the stiffness of the hollowed section as if it were a solid bar.

So a 19mm hollow bar with a 16mm hollow (1.5mm wall thickness) is equal to the stiffness of a 19mm solid bar minus the stiffness of a 16mm solid bar.

19^4 = 130321 and 16mm^4 = 65536 so 130321-65536 = 79696, somewhere between the stiffness of a 16mm and 17mm solid bar.

Font Material property Number Circle Screenshot
 

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2016 3.6 Limited with ES
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The Whiteline 20mm for the Gen 6 is roughly equivalent to 19.5mm in the soft setting and 21mm in the hard setting because the lever arm length of the Whiteline bar is either slightly longer (soft setting) or slightly shorter (hard setting) than a stock bar.

The torsional stiffness of a rod is proportional to the 4th power of its diameter.

The Gen6 stock sway bar is a HOLLOW 19mm and the Whiteline is a SOLID bar. The Wilderness rear sway bar is a different part number so I'm not sure what that bar is.

To relate the stiffness of a hollow sway bar to a solid sway bar, you calculate the stiffness of the solid bar and then subtract the stiffness of the hollowed section as if it were a solid bar.

So a 19mm hollow bar with a 16mm hollow (1.5mm wall thickness) is equal to the stiffness of a 19mm solid bar minus the stiffness of a 16mm solid bar.

19^4 = 130321 and 16mm^4 = 65536 so 130321-65536 = 79696, somewhere between the stiffness of a 16mm and 17mm solid bar.

View attachment 558320

This right here folks is the brainiac I had in mind.
 

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2018 2.5i Limited; "Wanderlust II"
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Here's my take: I opted for the 20mm WRX bar over the 19mm, but my car is also lifted. If your car doesn't have a lift kit, the 19mm bar will probably do you right. But that extra millimeter makes a world of difference with a 2" body lift.
 

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Here's my take: I opted for the 20mm WRX bar over the 19mm, but my car is also lifted. If your car doesn't have a lift kit, the 19mm bar will probably do you right. But that extra millimeter makes a world of difference with a 2" body lift.
I have similar thoughts.

Here's my take: I opted for the 20mm WRX bar over the 19mm, but my car is also lowered. If your car doesn't have other suspension mods, the 19mm bar will probably do you fine. But that extra millimeter makes a world of difference with a significantly updated on-road suspension.
 

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2019 Crystal White Subaru Outback 3.6R Limited
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yep. That's the one. Whiteline has an 18mm that is adjustable. That would cover roughly what a 17,18, and 19mm would cover. It's marketed for the scion/brz crowd. I'd thought about trying that one out for kicks, but it's really low on the list of things to do. Plus, I'd rather have an OE in there, I don't need adjustable stuff.
 

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2019 Crystal White Subaru Outback 3.6R Limited
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yep. That's the one. Whiteline has an 18mm that is adjustable. That would cover roughly what a 17,18, and 19mm would cover. It's marketed for the scion/brz crowd. I'd thought about trying that one out for kicks, but it's really low on the list of things to do. Plus, I'd rather have an OE in there, I don't need adjustable stuff.
If we're (probably) certain it will fit, I'd be willing to take the chance. Ever since I removed my SSD tower brace, the OB feels weird with the 19mm and no other suspension modes. I'd like to meet somewhere in the middle between the 16mm OE bar and the 19mm choice.

Based off the percentages listed above, I did not realize the 19mm was almost a 99% increase over stock - even though have had it on my car almost a year now
 
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