Turning Lowering Springs + Bilstein B6 into coilovers
12-16-20, 01:46 PM
#1
Turning Lowering Springs + Bilstein B6 into coilovers
So this may be a completely misguided notion, but I wanted to throw this out there.
I currently have a pretty fresh set of Bilstein B6 struts all around combined with some Vogtland lowing springs. I didn't want to go coilovers because I'm old and this isn't a track car. Maybe that decision is coming back to bite me. The car sits okay as is. It's not perfect, as rear is about 1/2" higher than the front. These struts have 2 positions (about 1/2-3/4 inch apart) for which you can set the ride height with a clip that attached around the shock body. I have the front in the highest position and the rear in the lowest position, and yet I have this slight discrepancy.
The plot thickens because I've been thinking about upgrading my stopping power since getting some wheel and rubber that can handle more negative acceleration. I'd prefer to go the route of the LS400 calipers due to weight and they look slightly better than the Supra. As we all know, RCAs will be required (I'm not considering hacking at my tie rod nut/bolt) to accommodate the larger calipers. This will drop my front end an additional 0.7 - 1+ inches, giving me a net 1.5" rake in the rear. I can't handle that.
So I've been weighing options. At first I was thinking about a spacer in the front that would take up this amount of space. I still think this might be the easiest method, but I haven't found anything specifically that I can confirm would work. Then I caught on to the idea of a coilover sleeves. There's no specific offering for these struts, but there are offerings for "2 inch steel body struts/shocks" .
My idea is to grab a set of those sleeves, put the snap rings on the lower perch of the strut body and assemble the sleeve onto the strut. Now with any luck, I will be able to retain the lowering springs I have. Luck meaning if the bottom coil section of the lowering spring is close enough to the 2.5" spring perch that comes with these coilover sleeves. If this works well, then I think I have a very good solution for keeping the car level in spite of the RCAs and natural rake I was left with for this combination.
However, if the Vogtland springs do not have the right radius at the bottom to fit these perches, I will be looking at some aftermarket racing / coilover style springs. This is where I'm REALLY in uncertain territory. I've owned good coilovers in the past, but I'm not knowledgeable on the topic - of specific interest to me now is how spring height relates to the spring rate. Based on my reading here, I'm going to shoot for a spring rate in the 10-12kg/mm. What I'm more nervous about (and very unsure of) is what the right height of the spring should be? The current springs are sitting at about 12 inches (and that's with the preload) tall.
The coilover springs I've found don't come in lengths greater than about 10 or 11 total inches, so I'll not be finding any springs as tall as the one I have if I must replace my current springs. But that's okay, because I expect their spring rate is much greater than what I have on these, and I can move the collar/perch as high as I need on the sleeve to set the ride height. So does it really matter if I pick a spring that's say 7" long vs. 6" long? Would it be best to go for the longest spring I can fit (keeping in mind some preload) and my height goals? Any recommendations on preload?
I'd certainly appreciate any input here.
For rough estimate of cost, I figure if I get lucky and can use my own springs, I'm out about $80-100 for the sleeves and perches. If I have to invest in springs and top retainers, etc., probably an additional $150-200. For comparison, I doubt I can find a set of coilovers I'd be happy with for under $1,500.
I currently have a pretty fresh set of Bilstein B6 struts all around combined with some Vogtland lowing springs. I didn't want to go coilovers because I'm old and this isn't a track car. Maybe that decision is coming back to bite me. The car sits okay as is. It's not perfect, as rear is about 1/2" higher than the front. These struts have 2 positions (about 1/2-3/4 inch apart) for which you can set the ride height with a clip that attached around the shock body. I have the front in the highest position and the rear in the lowest position, and yet I have this slight discrepancy.
The plot thickens because I've been thinking about upgrading my stopping power since getting some wheel and rubber that can handle more negative acceleration. I'd prefer to go the route of the LS400 calipers due to weight and they look slightly better than the Supra. As we all know, RCAs will be required (I'm not considering hacking at my tie rod nut/bolt) to accommodate the larger calipers. This will drop my front end an additional 0.7 - 1+ inches, giving me a net 1.5" rake in the rear. I can't handle that.
So I've been weighing options. At first I was thinking about a spacer in the front that would take up this amount of space. I still think this might be the easiest method, but I haven't found anything specifically that I can confirm would work. Then I caught on to the idea of a coilover sleeves. There's no specific offering for these struts, but there are offerings for "2 inch steel body struts/shocks" .
My idea is to grab a set of those sleeves, put the snap rings on the lower perch of the strut body and assemble the sleeve onto the strut. Now with any luck, I will be able to retain the lowering springs I have. Luck meaning if the bottom coil section of the lowering spring is close enough to the 2.5" spring perch that comes with these coilover sleeves. If this works well, then I think I have a very good solution for keeping the car level in spite of the RCAs and natural rake I was left with for this combination.
However, if the Vogtland springs do not have the right radius at the bottom to fit these perches, I will be looking at some aftermarket racing / coilover style springs. This is where I'm REALLY in uncertain territory. I've owned good coilovers in the past, but I'm not knowledgeable on the topic - of specific interest to me now is how spring height relates to the spring rate. Based on my reading here, I'm going to shoot for a spring rate in the 10-12kg/mm. What I'm more nervous about (and very unsure of) is what the right height of the spring should be? The current springs are sitting at about 12 inches (and that's with the preload) tall.
The coilover springs I've found don't come in lengths greater than about 10 or 11 total inches, so I'll not be finding any springs as tall as the one I have if I must replace my current springs. But that's okay, because I expect their spring rate is much greater than what I have on these, and I can move the collar/perch as high as I need on the sleeve to set the ride height. So does it really matter if I pick a spring that's say 7" long vs. 6" long? Would it be best to go for the longest spring I can fit (keeping in mind some preload) and my height goals? Any recommendations on preload?
I'd certainly appreciate any input here.
For rough estimate of cost, I figure if I get lucky and can use my own springs, I'm out about $80-100 for the sleeves and perches. If I have to invest in springs and top retainers, etc., probably an additional $150-200. For comparison, I doubt I can find a set of coilovers I'd be happy with for under $1,500.
Last edited by KyleH; 12-16-20 at 03:26 PM.
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KyleH (12-17-20)
12-16-20, 07:34 PM
#3
Yeah, what you're describing is basically what pretty much everyone was doing before cheap fully assembled coilover kits became commonplace. In the 90s and early 00s, Ground Control coilover sleeves were the go-to, paired with Eibach springs. There were other popular budget options too, like Skunk2 and Arospeed. And people used them exactly as you describe, on a variety on struts. The solutions for each car were slightly different depending on the strut design, but the overall idea has been around for a long time. You should get in touch with Eibach or Swift about selecting the right spring specifications for what you're trying to do. As an aside, springs are measured by their free length, not their loaded length. You will need to know how much travel you want, how much the strut itself can give you, your desired ride height, and your desired spring rate. Or you could just select something equivalent to your current springs.
https://eibach.com/us/c-75-products-...cessories.html
https://www.swiftsprings.com/
More importantly, can the Bilsteins handle 10-12k spring rates?
https://eibach.com/us/c-75-products-...cessories.html
https://www.swiftsprings.com/
More importantly, can the Bilsteins handle 10-12k spring rates?
Last edited by firelizard; 12-16-20 at 07:43 PM.
The following users liked this post:
KyleH (12-17-20)
12-17-20, 06:24 AM
#4
You should get in touch with Eibach or Swift about selecting the right spring specifications for what you're trying to do.
As an aside, springs are measured by their free length, not their loaded length. You will need to know how much travel you want, how much the strut itself can give you, your desired ride height, and your desired spring rate.
As an aside, springs are measured by their free length, not their loaded length. You will need to know how much travel you want, how much the strut itself can give you, your desired ride height, and your desired spring rate.
Or you could just select something equivalent to your current springs.
https://eibach.com/us/c-75-products-...cessories.html
https://www.swiftsprings.com/
https://eibach.com/us/c-75-products-...cessories.html
https://www.swiftsprings.com/
My call to Bilstein didn't turn up any useful information, unfortunately. Since they don't pair any springs (of theirs) with these struts (anymore), their representatives don't have any data to share as it relates to what spring rate the struts can handle. I might do some searching on the innerwebs to see if there's some prevailing wisdom on the topic.
12-17-20, 07:05 AM
#5
If you go to the sites I linked, you will see that both companies make springs for just about every application. A passenger car custom coilover is not out of the ordinary, and selecting an appropriate spring length will not be an issue. Also, with adjustable perch height you don't need to worry about spring length for that reason, it will be for the purpose of having the correct amount of travel.
Last edited by firelizard; 12-17-20 at 07:08 AM.
12-19-20, 09:14 PM
#6
What I've found through calling multiple spring and strut manufacturers as well as retailers, is that there's not a lot of information they can share about what works best with this chassis in an a-la-carte sort of setup - which is my predicament trying to adapt their parts to combine with these Bilsteins. Whether because they don't know, don't want the liability, or feel that they need to protect their information, I'm not too sure.
For length I'm thinking at least an 8" spring. I could be wrong about that - like so many other variables, I'm not sure what the stroke length is of the Bilstein struts I have - and guess what, neither can they tell me. I can disassemble the springs and fully compress the strut to determine this length, as I'll prefer to rely on the bump-stops on the strut assembly, not a fully coiled spring, should the scenario play out. Therefore, finding a spring that has a block height less than the bottom-out value is a near-must, from what I've gathered. So that's the short-end of the spectrum. High end is a little less obvious. I suspect it to be the distance between the length-maximized spring perch/collar and upper perch / top-hat with the spring preloaded. If it's too long, I won't be able to lower the car to the effective level.
I tend to overthink things from time to time, but here's where I'm thinking (caveat I don't know what the Bilsteins can handle in terms of spring rate):
Based on some assumptions, I'm going to say the front corner of the car will have a sprung weight of 1006lbs. This includes the exclusion of the wheel, tire, rotor, caliper, spring/strut assembly, and the knuckle - all of which is unsprung - by my guesstimate, about 125lbs. And I threw in 100lbs for the front seat occupant.
A 600 lb/in spring is equivalent to 10.7kg/mm. This seems to be softer end of a lot of the sportier setups out there, but maybe by doing so I allow my struts to live.
Because math, putting the corner weight of the 1000lbs on that spring would immediately shorten it 1.7". Add another 0.16" for the preload. Depending on the height of the spring, the block varies a bit (taller spring - more block height). What all this results in, is in a total movement as low as 1.7" for a 6" spring, up to 3.6" for a 10" spring. But, going back to the prior premise that the strut needs to be the bump-stop, not the spring (block height), the effective range of motion is even less than the movement ability of each respective spring length. Thinking about soaking up pot-holes, dips, etc. at highway and above speeds with well under 2" of travel such as with a 6" spring seems intense. The "game" I'm trying to play here is to give up some spring rate in exchange for spring length, enabling for a longer living strut, and a smoother ride. For example, a 600lb/in (11kg/mm) 6" spring would have less ability than a 400lb/in (7kg/mm) 10" spring to resist/avoid bottoming. And that 10" spring would ride a lot smoother.
I'm still thinking about how / if strut length is relevant / restrictive. The B6 I have is sized for a near-stock height springs. The B8 version is for shorter springs. But with the adjustable coilover spring perch, I can keep the top of the strut rod in the same position it is now, relative to the strut body. In fact, if I'm thinking correctly, in order to accommodate for the ride-height drop from the RCA, I'd actually have to raise the position of the lower perch, in turn elevating the body and top of the strut rod relative to current day position. It may be something that once I have the parts in my hands it will make more sense... it's just a variable that I'm keeping in mind throughout this process.
I'd like to hear from anyone that has coilovers that can share their points of view on the following:
For length I'm thinking at least an 8" spring. I could be wrong about that - like so many other variables, I'm not sure what the stroke length is of the Bilstein struts I have - and guess what, neither can they tell me. I can disassemble the springs and fully compress the strut to determine this length, as I'll prefer to rely on the bump-stops on the strut assembly, not a fully coiled spring, should the scenario play out. Therefore, finding a spring that has a block height less than the bottom-out value is a near-must, from what I've gathered. So that's the short-end of the spectrum. High end is a little less obvious. I suspect it to be the distance between the length-maximized spring perch/collar and upper perch / top-hat with the spring preloaded. If it's too long, I won't be able to lower the car to the effective level.
I tend to overthink things from time to time, but here's where I'm thinking (caveat I don't know what the Bilsteins can handle in terms of spring rate):
Based on some assumptions, I'm going to say the front corner of the car will have a sprung weight of 1006lbs. This includes the exclusion of the wheel, tire, rotor, caliper, spring/strut assembly, and the knuckle - all of which is unsprung - by my guesstimate, about 125lbs. And I threw in 100lbs for the front seat occupant.
A 600 lb/in spring is equivalent to 10.7kg/mm. This seems to be softer end of a lot of the sportier setups out there, but maybe by doing so I allow my struts to live.
Because math, putting the corner weight of the 1000lbs on that spring would immediately shorten it 1.7". Add another 0.16" for the preload. Depending on the height of the spring, the block varies a bit (taller spring - more block height). What all this results in, is in a total movement as low as 1.7" for a 6" spring, up to 3.6" for a 10" spring. But, going back to the prior premise that the strut needs to be the bump-stop, not the spring (block height), the effective range of motion is even less than the movement ability of each respective spring length. Thinking about soaking up pot-holes, dips, etc. at highway and above speeds with well under 2" of travel such as with a 6" spring seems intense. The "game" I'm trying to play here is to give up some spring rate in exchange for spring length, enabling for a longer living strut, and a smoother ride. For example, a 600lb/in (11kg/mm) 6" spring would have less ability than a 400lb/in (7kg/mm) 10" spring to resist/avoid bottoming. And that 10" spring would ride a lot smoother.
I'm still thinking about how / if strut length is relevant / restrictive. The B6 I have is sized for a near-stock height springs. The B8 version is for shorter springs. But with the adjustable coilover spring perch, I can keep the top of the strut rod in the same position it is now, relative to the strut body. In fact, if I'm thinking correctly, in order to accommodate for the ride-height drop from the RCA, I'd actually have to raise the position of the lower perch, in turn elevating the body and top of the strut rod relative to current day position. It may be something that once I have the parts in my hands it will make more sense... it's just a variable that I'm keeping in mind throughout this process.
I'd like to hear from anyone that has coilovers that can share their points of view on the following:
- Length of your coil - please confirm if the measurement is preloaded or free length
- What your spring rate is
- How you'd rate your ride - comfortable, sporty, firm, bouncy, rough, etc.
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