Modifying Bilstein Shocks

[JoeHK]

Originally Posted by strat61caster

If you're leaving the damping untouched then switching springs is going to be a gamble if it gives you anything useful other than being pretty. Without any knowledge you might get some benefit going to springs rates within the progressive rates provided by Bilstein but if you step outside of those the dampers may not keep up.

Alright! That's good to know actually, i'll definitely stay into the recommended range of spring rates then if i decide to switch them! as not to kill my dampers prematurely lol!
Now to look what's available & what mods are necessary when switching to a 60mm Inner Diameter and 8" tall spring
Things that i've understood:
- upper spring perch
- upper mounts
- helper springs to keep the new ones in place?
- those eibach thingies you linked earlier

Greetz



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[Ro_Ja]

Originally Posted by JoeHK

Alright! That's good to know actually, i'll definitely stay into the recommended range of spring rates then if i decide to switch them! as not to kill my dampers prematurely lol!
Now to look what's available & what mods are necessary when switching to a 60mm Inner Diameter and 8" tall spring
Things that i've understood:
- upper spring perch
- upper mounts
- helper springs to keep the new ones in place?
- those eibach thingies you linked earlier

Greetz

- upper spring perch
- upper mounts

I understood this as upper perch and camber plate. Camber plate is optional as you could just use camber bolts. It's up to you. You will need an upper spring perch though to accept the smaller diameter springs. You will also need a spring divider if you want to run helpers.

[JoeHK]

I already have camber bolts, so if that means i won't need camber plates that would be quite a cost less, which is more interesting for my budget.

I've read about those spring dividers, but what's the exact use of those helper springs, since the Bilsteinds don't have these out of the box right?
Or isn't it necessary to use helpers?

When you say Koni, you mean they have to spring adapters?
Wouldn't it be safer though swapping the entire upper spring perch, or is this quite a big job?

Again, sorry guys for all the, maybe noob-ish, questions, but i'm interested in actually "customizing" my Bilstein coilovers, but i don't want to do anything wrong, thus i need the right info :p

Thanks a lot already! I'm definitely learning more by the minute


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[Ro_Ja]

Originally Posted by JoeHK

I already have camber bolts, so if that means i won't need camber plates that would be quite a cost less, which is more interesting for my budget.

I've read about those spring dividers, but what's the exact use of those helper springs, since the Bilsteinds don't have these out of the box right?
Or isn't it necessary to use helpers?

When you say Koni, you mean they have to spring adapters?
Wouldn't it be safer though swapping the entire upper spring perch, or is this quite a big job?

Again, sorry guys for all the, maybe noob-ish, questions, but i'm interested in actually "customizing" my Bilstein coilovers, but i don't want to do anything wrong, thus i need the right info :p

Thanks a lot already! I'm definitely learning more by the minute

Helper springs are only there to keep the main springs captive on the perches when the suspension is in full droop. You don't need them, but it also doesn't hurt. When you do the measurements for springs you want, just ensure you get a proper length spring (I haven't gotten that far yet, so I don't have an answer to that yet). But when I convert my B14s to Hyperco springs, I'm going to use them, but that's just me.

Ignore what I said about the Koni adapter pieces. I don't know what I was thinking - the Eibach adapter threw me off, lol, and it appears they are the exact same thing. They would not be necessary because you want to go from 2.5 springs to 2.25 ones, not the other way around. Sorry for the confusion.

To recap, you will need:

Upper perch (can be purchased from HVT or Raceseng (if using either brand camber plate), Racecomp (if running OEM mounts), or probably made at local machine shop)

Main springs: Eibach, Swift, or Hyperco. Rates and lengths are up to you.

Helpers: Optional depending on spring lengths and/or preference.

Spring dividers: Only if running helper springs.

*Lower perches/Height adjusters*: I have not confirmed this, but probably isn't necessary since the spring tapers down. If needed, you can buy them from Bilstein retailers. Just ask for the RSR pieces.

Finally, the biggest piece of the puzzle is valving. I definitely would get the shocks dyno'd or try to get the valving info out of Bilstein (I haven't called them and asked yet - it could be worth a shot as they gave us the info for the B6 and B8s) as a baseline of figuring out the rates if you can use without revalving. If you are intending to revalve, you obviously can use any rate you want for the given use of the car.

[JoeHK]

Thanks man! It seems the coilovers haven't really been customized yet for our platform, or at least i haven't found anyone yet who did.
From what you're telling me we're even still missing some crucial info to pick the right springs & rates...

Is the revalving a necessity if you replace the springs with approximately same-spec'd, but thinner, springs?

Or might this not be a smart idea since there aren't any springs which have the exact same lenghts as the provided ones from Bilstein? Since this was mentionned here earlier.

Btw, the upper perches, all i can find right now is the camber plates, but those are all 400+ $$$, isn't there a more budget-friendly option? I even checked the racecomp site... Maybe i'm just looking in the wrong place though haha! Or looking for the wrong piece, since all we need is the top perch to be able to "accept" a smaller diameter spring up top..

Also, apparently the bilstein spring rates are:
Front: 2,5 - 4,5 kg/mm
Rear: 3,0 - 7,0 kg/mm

Lengths (stated earlier by strat61caster):
Front: 8,6"
Rear: 8,9"

So would either of these fit for example?
http://store.resuspension.com/produc...cat=369&page=1

http://store.resuspension.com/produc...cat=378&page=1

This combined with the right upper perch, which i still haven't found, SHOULD be all we need if we don't mess with the rates too much so valving could stay the way it is

Greetz
And once again: thanks again! This is actually fun to research


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[Ro_Ja]

Originally Posted by JoeHK

Thanks man! It seems the coilovers haven't really been customized yet for our platform, or at least i haven't found anyone yet who did.
From what you're telling me we're even still missing some crucial info to pick the right springs & rates...

Is the revalving a necessity if you replace the springs with approximately same-spec'd, but thinner, springs?

Or might this not be a smart idea since there aren't any springs which have the exact same lenghts as the provided ones from Bilstein? Since this was mentionned here earlier.

Btw, the upper perches, all i can find right now is the camber plates, but those are all 400+ $$$, isn't there a more budget-friendly option? I even checked the racecomp site... Maybe i'm just looking in the wrong place though haha! Or looking for the wrong piece, since all we need is the top perch to be able to "accept" a smaller diameter spring up top..

Also, apparently the bilstein spring rates are:
Front: 2,5 - 4,5 kg/mm
Rear: 3,0 - 7,0 kg/mm

Lengths (stated earlier by strat61caster):
Front: 8,6"
Rear: 8,9"

So would eather of these fit for example?
http://store.resuspension.com/produc...cat=369&page=1

http://store.resuspension.com/produc...cat=378&page=1

This combined with the right upper perch, which i still haven't found, SHOULD be all we need if we don't mess with the rates too much so valving could stay the way it is

Greetz
And once again: thanks again! This is actually fun to research

For the upper perches, it's usually something included with the camber plates and often not sold separately. You just have to make a special request for the piece. Since it sounds like you'd like to keep your OEM upper mounts, Racecomp would probably be your source and I would expect them to get you exactly what you need for a reasonable price.

Revalving is not a necessity, but it's important to have all the information to make the best choice. Racecomp has stated that 350 lb/in springs is the upper limit of the stock valving for the B14.

I am not a suspension expert, but I know that spring rate is the amount of weight required to compress the spring 1 inch, but I do not know and cannot find any information on the relationship between rate and length (if there is one).

For reference, if we look at the pinned thread in this forum, we see the rates for the OEM springs.

GT86 (Asia & EU)
Front: 2.3 kg/mm = 128 lb/in
Rear: 3.3 kg/mm = 184 lb/in

FRS
Front: 2.3 kg/mm = 128 lb/in
Rear: 3.8 kg/mm = 212 lb/in

BRZ
Front: 2.7 kg/mm = 151 lb/in
Rear: 3.5 kg/mm = 195 lb/in


So, I did my own "calculations" (read: BS math lol) for figuring the rates (for a street car) and I came out with something like:
Front: 200 lb/in
Rear: 275 lb/in

How I got those rates are as follows:

Bilstein B14
Front: 2,5 - 4,5 kg/mm = ~140-252 lb/in
Rear: 3,0 - 7,0 kg/mm = ~167-392 lb/in

I figured the midpoint of both rates seemed like a good average for street driving. Doing that would result in:
Front: 3.5 kg/mm = ~200 lb/in
Rear: 5 kg/mm = ~280 lb/in

Unfortunately, no one has the valving info for the B14s (yet) and even if we did have that info, we don't know what the stock Showa (or now, Sachs) valving is either for a comparison. However, Bilstein has stated that the B6 are valved to a similar curve to the stock units (and we know the B8s have the same valving too), so if we can get the valving for the B14s, we can see how they relate to B6/B8 which would give us a clue to how the factory dampers are valved. This would verify what I find below...

So, for comparison of the rates I found above, I analyzed the rates for Swift lowering springs for stock dampers. It's definitely NOT an apples to apples comparison, but I was looking for just a ballpark estimate of what seemed to work since the Swifts were highly regarded for the stock dampers and balance of the car.

Swift Sport (FRS rates) (Swift claims it's the optimal street spring for performance vs damper life)
Front: 3 kg/mm = 168 lb/in
Rear: 4.5 kg/mm = 251 lb/in

Swift Spec R (same rate for FRS and BRZ) (Swift claims the rates to be optimal for track duty and in the upper limits of stock dampers)
Front: 4.4 kg/mm = 245 lb/in
Rear: 5.3 kg/mm = 296 lb/in

Again if you take the midpoint of both their offerings (what I would think would be in-between the Sport and Spec R), you end up with roughly:
Front: 3.7 kg/mm = ~206 lb/in
Rear: 4.9 kg/mm = ~274 lb/in

This is all pure speculation though, but it matches up to the 200/275 rates I found above and falls well within what Racecomp said in post #19.

After looking at a dyno plot of the standard valving I would put the limit at 350 lbs/in springs.

I have absolutely no clue how the car will handle and what kind of balance you get with those 200/275 rates, but I figured with progressive rates kinda all over the place, it can't be too bad if I were to pick something within that range. The difference in rate is also close to the difference in rate found in the OEM FRS rates.

Again, I'm farrrr from a suspension expert and this only gives me a number (I/we still need information on spring length and if/how it affects rate) there are a shit ton of other variables to consider (shock valving, motion ratio, suspension geometry, travel, and frequency, tires, road conditions, etc.), so I'm not sure if my line of thinking is at least on the right track, but it seems to make some sense and I would be appreciative if someone else could clear it all up for me/us.

[JoeHK]

Well, i can follow you completely in your calculations and it does seem logical indeed cutos to you for actually digging in so deep and detailed, and thank you!!

Maybe if @Racecomp Engineering could pitch in here and confirm that we're thinking correctly or tell us what should be done differently, that wouldn't be too bad

Greetz


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[JoeHK]

Or can anyone else, who might also have some knowledge on customising Bilstein shocks, pitch in? @ FT86-Speedfactory ?

[Racecomp Engineering]

Originally Posted by JoeHK

Well, i can follow you completely in your calculations and it does seem logical indeed cutos to you for actually digging in so deep and detailed, and thank you!!

Maybe if @Racecomp Engineering could pitch in here and confirm that we're thinking correctly or tell us what should be done differently, that wouldn't be too bad

Greetz


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If you guys are able to wait just a couple of weeks, we may have something interesting for you.

- Andrew

[JoeHK]

Originally Posted by Racecomp Engineering

If you guys are able to wait just a couple of weeks, we may have something interesting for you.



- Andrew

Hmmm... Kind of secretive there guys lol
For me there's no hurry, by all means, take the time and build us a nice product, but i am curious though if we're thinking/analyzing the situation the right way and if we're on the right track lol :p



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[Racecomp Engineering]

Yes on the right track...but I don't recommend comparing to stock spring rates very much. Many other variables. We do have something coming and I'll hopefully have more details for you all soon.

- Andrew

[JoeHK]

Originally Posted by Racecomp Engineering

Yes on the right track...but I don't recommend comparing to stock spring rates very much. Many other variables. We do have something coming and I'll hopefully have more details for you all soon.

- Andrew

Alright, i'll bite! You've got my attention and my curiousity. Don't wait too long though lol


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[strat61caster]

@JoeHK IMO 60mm springs, 8" length, 3k-4k front 6k-7k rear, maybe a little softer if you like (although given that you seem to be focused on going low I'd be wary about being too soft) and get some cups made at a local machine shop to keep the 60mm springs centered in the oe top hats (easiest if you can take the parts there and work with someone to come up with a design) slap it all together, set the ride heights, get it aligned and call it a day.

[JoeHK]

Originally Posted by strat61caster

@JoeHK IMO 60mm springs, 8" length, 3k-4k front 6k-7k rear, maybe a little softer if you like (although given that you seem to be focused on going low I'd be wary about being too soft) and get some cups made at a local machine shop to keep the 60mm springs centered in the oe top hats (easiest if you can take the parts there and work with someone to come up with a design) slap it all together, set the ride heights, get it aligned and call it a day.

I'm definitely going to look into those cups with a local suspension shop
See if they can fab something or have custom parts laying around, they build full custom coilovers too, so i bet they can work something out
If they can't help me out i'm curious though as to what Racecomp has planned..
Thanks for the tips and info! Now to see what i'll be putting on there, Swift or Hyperco, although there don't seem to be many differences lol


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