rear inverted coilover options; similar to bc racing

in preparation for a future project, i started doing the google image search phase today and came across a design i would like to incorporate into my car. i like the idea that bc racing does which has its strut maintain full travel and the height is adjusted within a additional threaded body(probably not describing this correctly). are there any other versions of this? my goal is to have a couple options and pick the one i feel i couple duplicate successfully. here are some images of what are there any commonly available threaded bodies that would allow someone to make a similar setup as image below? here is an option i found that may get the job done. if i can find an off the self solution that works for a reasonable price i will purchase; from what i understand i should not hold my breath. this will be for a e36 and i plan on mounting the lower part of the strut in double shear; this is what makes an off the self solution more difficult. any options for a mazda, nissan, honda etc. that may work? thanks for your time.

Same design as the JIC Cross coil overs sitting in the trunk of my e46 M3 waiting for me to install them. I can get you better pics if you want. Instead of adjusting height by adjusting the length between the upper and lower spring perches (and preloading the spring) this design changes the length between the lower spring perch and the lower strut mounting point.

That sort of shock allows the manufacturer to use a generic shock cartridge and just screw on a base to suit a particular car. Good for cost containment. The downside is that you have less overall shaft travel. A properly specified shock will be the right length for the application, the "feature" of being able to adjust the body length is just marketing in my opinion. Unless you're way off on shock body length, spring preload is not an evil. I'll take increased shaft travel instead.

That said, you should be able to find a good range of off-the-shelf strut inserts. I recall the Dirta shocks for the 323 GTX were built like this, a standard strut insert into a housing. A quick Google indicates that this is pretty common on an E36 too.

How do you plan on mounting the base? A strut has to be pretty firmly anchored at the bottom, as it's actually structural.

The arguments I've heard in favor of the threaded shock body is this:

1. reduced/eliminated need for helper/tender springs
2. true preload adjustment
3. and similar to #2, preload is not effected when corner weighting
4. changes in ride height do not effect shock travel

You need helper/tender springs when you want more travel than your springs can give you. Say your main springs only compress 2" at rest because of the weight of your car and the rate of the springs, but you have 3" of droop. You need a helper - and preferably one with a non-trivial spring rate so it can extend the shock. Shorter shaft travel will get around that by limiting your droop to 2". It's nothing to brag about.

Other than the boundary case where your preload is greater than your cornerweight, the amount of preload is not critical. Your spring (in the above example) is going to compress 2" at rest regardless of how much preload you have.

I like to chose my wheel movement limits by what is possible. Either grounding out (chassis on the pavement) or suspension binding. If you do that, then you'll get the maximum amount of compression travel available regardless of the ride height. Changing the ride height will affect the amount of compression travel (basically a straight tradeoff between compression and droop) but you'll always have as much as is physically possible. Adjusting the shock body length means you will not have as much as is physically possible.

It's how you adjust your marketing to make a bug into a feature.

Back to the original question...

Inverted struts - you're going to have to make sure you can run whatever you choose inverted. Typically, that means no twin-tube shocks unless they have an internal gas bag. So no Koni. Bilstein, however, should be invertable from what I understand. I'm still curious about how you plan to mount it in an inverted manner, as a strut has to support the upright. AFCO might be able to build what you need although it all depends on how you're mounting the end of the strut shaft on the upright. They've got a number of options, and I know you can invert them.

some bilstein sports (e30 fronts for example) are inverted internally off the shelf, so flipping them "right side up" shouldn't be an issue

thank you for the replies.

i was looking for threaded struts last night to no avail.

the mounting of the strut housing to the strut tower is not what i am concerned with, at this time. if i find a strut that can get it done i will do what it takes to mount it. the issue i still need to spend more time looking at is making a dual shear connection on the rear trailing arm with the shaft side of the strut. i recall looking at it a couple months ago and it seemed like it would work. i recall seeing some images of people needing to invert the rear for clearance with either and/or the tires and springs. people have stayed using the stock mounting point but if i can double shear i will. i also need to confirm where the interference comes from; if it is the springs by mounting it how i want it will increase clearance slightly.

this idea is still very green and with time i may dismiss the notion. if i do end up fabricating some i should be able to alleviate any possible issues with less shaft travel when compared to off the shelf options.

What is double shear?

here is an explanation i found from this link, http://wiki.answers.com/Q/Difference_between_single_shear_and_double_shear

Single shear carries all load on one face whule double shear carries it on two faces, so stress is lowere by a factor of 2 for a given load.

For example shear stress on a cantilever pin is V/A ( load/area, single shear) but on a pin between two supports it is V/2A

I wasn't sure if you were using the term in the normal sense or not I'm having trouble figuring out how you'll locate that upright properly with only one attachment point, even if it is double shear. On a normal strut, there's no change of any movement between the strut body and the upright, either due to multiple attachment points or something similar. But if you've only got the end of the strut shaft and thus a single attachment point, I don't see how you can keep it from rotating.

Anyone got a pic of this done?

my initial thoughts are the struts will have a hole in them and i will make a round bracket that fit up into the strut tower location i will then weld some ears that are spaced to allow the strut to be slide in-between. there may be an easier solution, that with time will present its self but this idea should work. here is an image of the two common mounting surfaces on struts; that i have seen. the one with the two ears being much less common. i located some more threaded struts here is a image link. i also found some diy style threaded housing i may be able to use; seem kinda pricey at 260 dollars. i should be able to draw something up on solidworks and get them done for less if i need to.

those jic cross coils look to have complete threaded housing and i may contact the manufacturer to see if i can get a price on some threaded struts, housing and coils.

anyone recommend some threaded struts i may be able to use?

edit: i found threaded struts for days; pitstopusa.com this project is many months out but when it draws closer i will bump for critique of my plans.

edit 2x: calling it a night but still looking for 2" i.d. and 1 5/8" i.d. threaded housings; e.g. like the last image linked in this post. i will send some emails in the coming days.