What's inside an inverted strut? Are they better?

RaceComp Engineering · 05-24-2023, 03:00 PM

A lot of Subarus use inverted struts, and a lot of people get some of the facts about them wrong. The big one is that they don't actually have huge shafts...they're usually smaller than a regular non-inverted shaft.

We put together a video showing what's inside and some of the basic advantages of inverted struts.

YouTube

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REX_WGN · 05-25-2023, 12:42 PM

thank you!

RaceComp Engineering · 05-26-2023, 09:49 AM

No problem! We plan on a few more tech videos in addition to the podcast series we have going. There's also one on coilover spring basics.

GoFrogs · 05-29-2023, 12:29 PM

Quote:

Originally Posted by RaceComp Engineering

No problem! We plan on a few more tech videos in addition to the podcast series we have going. There's also one on coilover spring basics.

I love a good/great spring/shock combo and this video helps out a bunch.

Curious if the base trim WRX can run inverted shocks/struts like the Premium and Limited trims. Should have never bought the base trim.

TitoBandito · 05-31-2023, 09:37 AM

Can't watch the video at the moment as I'm listening in on a work call (but distracting myself on NASIOC), so apologies if this is answered in the video.

I was surprised to read that the shaft on the inverted struts are not larger, and can be smaller, than non-inverted struts. Is the primary reason/benefit then, that the sliding housing that the shaft is upside-down inside of is providing a larger surface area/diameter to absorb the side loads or angular loads, if the shaft itself isn't bigger?

If I remember after work, I'll watch the video. I was wanting more options that weren't coilovers when looking to replace the OEM struts on an 2004 STi a few years ago. Aside from other well-worn used 04 STi struts, all I could find was the FEAL fixed perch struts, which while tempting, were getting into coilover price territory like your RCE Tarmac Zero's or SS1's. Wound up going with Koni shock inserts inside cut WRX strut housings. I don't rallycross my STi, so I guess I'm fine with non-inverted struts for this car.

Blitzcrank · 05-31-2023, 01:27 PM

Quote:

Originally Posted by GoFrogs

I love a good/great spring/shock combo and this video helps out a bunch.

Curious if the base trim WRX can run inverted shocks/struts like the Premium and Limited trims. Should have never bought the base trim.

Any WRX can run inverted struts.

RaceComp Engineering · 06-07-2023, 11:18 PM

Quote:

Originally Posted by Blitzcrank

Any WRX can run inverted struts.

Yup, as long as they are the right fitment (which can get complicated for pre-2015 WRX).

Quote:

Originally Posted by TitoBandito

Can't watch the video at the moment as I'm listening in on a work call (but distracting myself on NASIOC), so apologies if this is answered in the video.

I was surprised to read that the shaft on the inverted struts are not larger, and can be smaller, than non-inverted struts. Is the primary reason/benefit then, that the sliding housing that the shaft is upside-down inside of is providing a larger surface area/diameter to absorb the side loads or angular loads, if the shaft itself isn't bigger?

If I remember after work, I'll watch the video. I was wanting more options that weren't coilovers when looking to replace the OEM struts on an 2004 STi a few years ago. Aside from other well-worn used 04 STi struts, all I could find was the FEAL fixed perch struts, which while tempting, were getting into coilover price territory like your RCE Tarmac Zero's or SS1's. Wound up going with Koni shock inserts inside cut WRX strut housings. I don't rallycross my STi, so I guess I'm fine with non-inverted struts for this car.

Right, basically you don't need thicker shafts with an inverted strut since the housing is taking the side loads, and thicker shafts have some drawbacks.

Inverted struts for the rear of 2002-2007 Impreza sometimes have issues staying lubricated which can then lead to clunks if not periodically re-greased. Konis would be a find solution, though a pain to install since they're an insert. I think there may be some non-inverted options out there (Tein Endurapro Plus) but I can't speak to how good they are. Double check 2004 vs 2005-2007 STI fitment since they should be different.

- Andrew

caterpill · 06-11-2023, 11:21 PM

Nice Video.

Might or might not be the right thread for this, but hey.

Question on Inverted Dampers - The wear sleeves, some manufacturers' products are replaceable, some are not.

Care to shine some light on these wear items? I have a set of aftermarket inverted dampers that have been seriously neglected and am sure need some grease. I've read that they are not serviceable and would likely need to be replaced entirely.

What would the craziest scenario be? Would the sleeves be worn to the point that it grooves the inverted damper shaft, or starts squealing? or just clunk itself to death / leak due to increased clearances?

2pot · 06-13-2023, 06:24 AM

If gc/gd then you'd need to renew the a 44mm od, 39mm id bushes. Good luck finding that size. Assuming the chrome rod hasn't been damaged by the worn bushes?

Wash out the old grease and use a synthetic polyurea grease.

OEM KYB front inverted insert:

I replace with 44mm od and 40mm id bushes and use Bilstein inserts. Machine some top caps to take the top seal. Then either weld a thread on to the base of the housing or machine a threaded locating adapter.
Replace the oem bump stops - which are too soft at the rear and too long/stiff at the front.

KYB to Bilstein insert conversion:

RaceComp Engineering · 06-14-2023, 11:19 AM

2pot, do you have an alternative source for internal Bilstein bumpstops? They don't seem to have anything listed in the motorsports catalog and I couldn't find anything else that easily fit (other than the what we used in some of our kits). They do have a nice external bumpstop I was using on the Bilsteins I was using on my own car (BRZ) for the rear shock, rather than reusing the really crap OEM rear that they ask you to use.

- Andrew

caterpill · 06-17-2023, 11:10 PM

Quote:

Originally Posted by 2pot

OEM KYB front inverted insert:

I replace with 44mm od and 40mm id bushes and use Bilstein inserts. Machine some top caps to take the top seal. Then either weld a thread on to the base of the housing or machine a threaded locating adapter.
Replace the oem bump stops - which are too soft at the rear and too long/stiff at the front.

KYB to Bilstein insert conversion:

I do appreciate your knowledge with this.

Could you share some Bilstein P/N's for the inserts?

This would be a game changer. I would rather rebuild my STi bits when they wear out, and being that it'll be replaced with Bilstein products, you can't go wrong. I feel that the OEM bumpstops seem to disintegrate due to age, or being exposed to the grease that is needed in the inverted strut cavity; is there some kind of Urethane equivalent?

Granted, you would want a progressive bumpstop but the last time I re-greased my 04' STi takeoffs, the bumpstops were crumbly and mushy as if they weren't supposed to be exposed to oil. Go figure.

2pot · 07-06-2023, 04:46 PM

Bilstein make the inserts to my specs.

Bump stops are a wear item.

It's usually a damper oil leak, that breaks down the bump stops, or constant heat cycling, due to low front ride heights.
If an sti, and you're running under 14" front ride height, then you're constantly engaging the oem 60mm front bump stops, within the sti's inverted strut housings.

2pot · 07-06-2023, 04:51 PM

Quote:

Originally Posted by RaceComp Engineering

2pot, do you have an alternative source for internal Bilstein bumpstops? They don't seem to have anything listed in the motorsports catalog and I couldn't find anything else that easily fit (other than the what we used in some of our kits). They do have a nice external bumpstop I was using on the Bilsteins I was using on my own car (BRZ) for the rear shock, rather than reusing the really crap OEM rear that they ask you to use.

- Andrew

Hi Andrew
What stops are you using at the moment? Any pics?
What were you thinking of trying? Length/stiffer/softer?
Mark

RaceComp Engineering · 07-17-2023, 10:38 AM

Quote:

Originally Posted by 2pot

Hi Andrew
What stops are you using at the moment? Any pics?
What were you thinking of trying? Length/stiffer/softer?
Mark

That's me in the video, you can see the standard Bilstein B6 for BRZ bumpstop at 49 to 52 seconds if you pause it and again later in the video...sorry I can get a good pic later this week. It's about 65mm long. This is ~5 mm longer than the OEM (external) front bumpstop. The OEM is quite a bit firmer, though the Bilstein still isn't very soft.

I'm using these with TRD Japan "rally" springs, which are too firm for rally but +5mm in ride height at the front. They are 4.6 kg/mm which is about 1.85 Hz. Stock is around 2.7 kg/mm.

So I'm +5mm in ride height but +5mm in bumpstop, and with a much stiffer spring. It's not very compliant.

Looking for something a little bit shorter (maybe 45 mm) and much softer. This isn't a hardcore rally car, just occasional rally-x and light gravel/snow fun. No jumps.

I could cut the standard Bilstein stop but there isn't a softer/stiffer end so I'd be making the overall stop stiffer...unless I put some holes in it. I do have 1 extra bumpstop to screw around with (the Bilstein in that video is busted) and could even dyno it.

Another alternative is to just use stock springs which I'm leaning towards more and more. My coilovers with 5k/6k springs ride better but I'm 30mm lower.

- Andrew

2pot · 07-17-2023, 02:31 PM

The softest 40mm internal stop is twin node, rubber:
E4-B46-243
It's very soft, so won't help with pitch control - that would have to be controlled by the compression damping.
There is also a 60mm rubber stop, of the same durometer - as above, but with 3 nodes.

There are others, but they're all stiffer.

RaceComp Engineering · 07-17-2023, 04:03 PM

Quote:

Originally Posted by 2pot

The softest 40mm internal stop is twin node, rubber:
E4-B46-243
It's very soft, so won't help with pitch control - that would have to be controlled by the compression damping.
There is also a 60mm rubber stop, of the same durometer - as above, but with 3 nodes.

There are others, but they're all stiffer.

thanks!

06-13-2023, 06:24 AM	#9
2pot Scooby Newbie Member#: 301042 Join Date: Nov 2011 Location: UK	If gc/gd then you'd need to renew the a 44mm od, 39mm id bushes. Good luck finding that size. Assuming the chrome rod hasn't been damaged by the worn bushes? Wash out the old grease and use a synthetic polyurea grease. OEM KYB front inverted insert: I replace with 44mm od and 40mm id bushes and use Bilstein inserts. Machine some top caps to take the top seal. Then either weld a thread on to the base of the housing or machine a threaded locating adapter. Replace the oem bump stops - which are too soft at the rear and too long/stiff at the front. KYB to Bilstein insert conversion: Last edited by 2pot; 06-13-2023 at 08:49 AM.

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05-25-2023, 12:42 PM	#2
REX_WGN Scooby Guru Member#: 107618 Join Date: Feb 2006 Chapter/Region: Tri-State Location: NY Vehicle: 06 GG WRX / 09 FXT @florencedestinedfxt	thank you!

05-26-2023, 09:49 AM	#3
RaceComp Engineering NASIOC Vendor Member#: 54202 Join Date: Feb 2004 Chapter/Region: MAIC Location: Elkridge, Maryland Vehicle: ASK ABOUT NEW RCE SWAY BARS FOR STI	No problem! We plan on a few more tech videos in addition to the podcast series we have going. There's also one on coilover spring basics.

05-31-2023, 09:37 AM	#5
TitoBandito Scooby Newbie Member#: 318302 Join Date: Apr 2012 Location: CT Vehicle: 2004 Sti WRB	Can't watch the video at the moment as I'm listening in on a work call (but distracting myself on NASIOC), so apologies if this is answered in the video. I was surprised to read that the shaft on the inverted struts are not larger, and can be smaller, than non-inverted struts. Is the primary reason/benefit then, that the sliding housing that the shaft is upside-down inside of is providing a larger surface area/diameter to absorb the side loads or angular loads, if the shaft itself isn't bigger? If I remember after work, I'll watch the video. I was wanting more options that weren't coilovers when looking to replace the OEM struts on an 2004 STi a few years ago. Aside from other well-worn used 04 STi struts, all I could find was the FEAL fixed perch struts, which while tempting, were getting into coilover price territory like your RCE Tarmac Zero's or SS1's. Wound up going with Koni shock inserts inside cut WRX strut housings. I don't rallycross my STi, so I guess I'm fine with non-inverted struts for this car.

06-11-2023, 11:21 PM	#8
caterpill Scooby Specialist Member#: 27136 Join Date: Oct 2002	Nice Video. Might or might not be the right thread for this, but hey. Question on Inverted Dampers - The wear sleeves, some manufacturers' products are replaceable, some are not. Care to shine some light on these wear items? I have a set of aftermarket inverted dampers that have been seriously neglected and am sure need some grease. I've read that they are not serviceable and would likely need to be replaced entirely. What would the craziest scenario be? Would the sleeves be worn to the point that it grooves the inverted damper shaft, or starts squealing? or just clunk itself to death / leak due to increased clearances?

06-14-2023, 11:19 AM	#10
RaceComp Engineering NASIOC Vendor Member#: 54202 Join Date: Feb 2004 Chapter/Region: MAIC Location: Elkridge, Maryland Vehicle: ASK ABOUT NEW RCE SWAY BARS FOR STI	2pot, do you have an alternative source for internal Bilstein bumpstops? They don't seem to have anything listed in the motorsports catalog and I couldn't find anything else that easily fit (other than the what we used in some of our kits). They do have a nice external bumpstop I was using on the Bilsteins I was using on my own car (BRZ) for the rear shock, rather than reusing the really crap OEM rear that they ask you to use. - Andrew

07-06-2023, 04:46 PM	#12
2pot Scooby Newbie Member#: 301042 Join Date: Nov 2011 Location: UK	Bilstein make the inserts to my specs. Bump stops are a wear item. It's usually a damper oil leak, that breaks down the bump stops, or constant heat cycling, due to low front ride heights. If an sti, and you're running under 14" front ride height, then you're constantly engaging the oem 60mm front bump stops, within the sti's inverted strut housings.

07-17-2023, 02:31 PM	#15
2pot Scooby Newbie Member#: 301042 Join Date: Nov 2011 Location: UK	The softest 40mm internal stop is twin node, rubber: E4-B46-243 It's very soft, so won't help with pitch control - that would have to be controlled by the compression damping. There is also a 60mm rubber stop, of the same durometer - as above, but with 3 nodes. There are others, but they're all stiffer.