Thucydides

Bingo! Stiff springs require helpers, soft springs require preload.

acedeuce802

iTrader: (1)

There's a lot written here, didn't follow all of it. I'll just add my 2 cents.

With adjustable spring perches (assuming perch height is infinitely adjustable, ie: able to cut new circlip grooves lower than stock perch), the only thing that matters about spring length is stack height. A 6" spring of 300 lb/in will be the exact same as a 7" spring of 300 lb/in with the spring perch moved 1" lower. Your long calculations on page 66 are valid if spring perches are fixed, but if you can move the lower perch, just get a spring that's the right rate and has enough travel, then adjust perch for ride height.

Preload doesn't directly matter for ride height. With equal perch heights (distance from lower perch, to lower eye), two springs of same length and same rate will have the same ride height, regardless of pre-load (disregarding slight ride height changes from nitrogen pre-charge and different piston locations at ride height). As you set the car down, the spring with preload will not move until preload is overcome (preload distance times spring rate) and then once full vehicle load is applied, it'll be at the same height as the non-preloaded spring. But, if these two examples had the same top hat design, then pre-load would directly correlate to perch position and ride height would be affected.

My theory for choosing perch heights/preload/etc on my car: run basic calcs or use baselines (stock, other peoples setups, etc) to get desired spring length and rate (make sure spring has enough travel for shock travel, you for sure want bump rubber before coil bind). Install and set ride heights with adjustable perch. Measure lower eye to perch length, perch to top hat (spring seat) length, and top hat to shock mount (the raised part of the top hat). Decide on a bump/droop ratio, perform heavy calculus, take the raw data and analyze it, make sure bump travel seems reasonable (because it's sometimes advantageous to have more droop than bump, unless it causes you to bottom out, bottom out equals unpredictable handling). These calculations will lead you to the "ideal" top hat height to get your bump/droop ratio, and you'll also be able to recalculate (or just assemble and test) if you need helper springs. If you do, lower spring perch for helper spring stack height. Now your ride heights may be a bit different, because your shock piston is in a new location which can affect total spring rate, so reset ride height and corner balance.

Thucydides

I'll be compressing them a good deal more than that!
Not correct. Spring preload is a byproduct of low spring rates, just like helper springs are a byproduct of high spring rates.

Once the spring has taken the weight of the vehicle, any adjustment up or down will move the vehicle up or down. Keep this in mind - the preload is less than the vehicle weight, so even a preloaded spring will compress further to take whatever vehicle weight is not accounted for by preload. From that point on, like I said earlier, any adjustment up or down will move the vehicle up or down.

Droop is a function of the shock and the shock shaft limitations, and is not dictated by the spring. You need bump stops to avoid jamming the tire into the body of the car because the shock isn't designed, in this case, to limit bump travel to prevent putting your tire into fender. That's your bump stop's job. Preload, preload, preload....

Spring length and shock shaft travel are not related and shock travel is not at all required to answer to spring travel.

Leafy

iTrader: (1)

Wait are we really discussing using higher rate and longer than stock springs to achieve a ride height that doesnt result in basically no droop travel? Carry on.

vteckiller2000

iTrader: (4)

You are missing a very important part of this equation. Motion ratio. Your 702 lbs is acting more like 525 lbs. 525/97 = 5.4". Your theoretical spring is now 6.3"

Your words: Not sure how to make this any simpler. 11.7-6.3 = 5.4. Now you are also not accounting for gas charge in the shock, which will resist compression nearly exponentially as it compresses as well as bushing stiction and/or torsional resistance of the bushing itself (depending on the bushing style). You would be surprised how hard bushings resist twisting motion.

What can I say. There's always the iterative approach.

Nope. I have 6" 800/500 lb springs and don't use helpers.

Again, no. Spring preload is a requirement for your intended ride height with those rates. Helper springs are not required with high spring rates.

How does what you just wrote explain why anyone would need or want preload? I have no preload and I don't have any issue with droop or bump travel.

They more certainly are in fact related. A proper spring length is required to effectively utilize shock droop and compression travel. Too short of a spring may induce coil bind, reducing your effective travel.

concealer404

iTrader: (3)

The issue here is that in your theoretical setup, and my real world setup, there is/will be near as makes no difference the same amount of total travel. Your scenario is going to put your car at static height, at near top of its range. Especially at the higher height that you're talking. This will allow you next to no, if not zero droop travel. This is muchly bigly bad for the type of driving that you're talking about doing. As you succinctly said prior, you want to aim for the middle. You're doing everything in your power to not do that.

Spring travel is not measured in a vacuum. Shock travel has to be taken into play. When your shock runs out of downward travel, that's it. You're done, no matter what spring you have packed onto that thing. I think you've drastically overestimated the amount of adjustability you're going to have with these springs. As in you're not likely to be able to hit your lower ride height goals. And that's bad.

Another member of the 800lb (actually 850, thank you very much) and not using helper springs club checking in.

I hope this works out for you. I really do. Just in my experiences (and i've built a fair number of these setups, and developed these setups), you're going at this soooo far from how anyone else who has ever built one of these setups would, it makes my head spin. Bringing spring rates that a 1st gen rally Escort used into the discussion is completely irrelevant and has no bearing on these cars. These cars are absolutely horrendous on low rates like that. They generate way too much grip for low rates like that. And will even do that on crap roads like you're talking about. Absolutely not suggesting you run anything like the track day rates we all run, but if it were my car, i'd be looking at 350-400 in the front, 250-275 in the rear. And probably 7-8" springs at most, because those will give you your desired ride heights, not dislodge, and still have enough coil travel to match the shock.

The math is great. I don't understand it, nor do i really care to. There's hundreds of us that have done this "our way." And have logged thousands and thousands and thousands of miles on them. Expect to get some flak if you come in here (and my extensive write-up on the other forum) and start telling us we're all wrong because a piece of paper says so, with a setup you have no hands-on experience with.

Thucydides

First off, thanks for the response. I know you're writing to help me avoid what you believe to be major traps and pitfalls, and I'm sincerely grateful. I'm not going to go through your list of concerns to refute them, because we've done that already. Nor am I going to tell you what I do, my education, or my experience to puff myself up (ok, I'm a retired farrier), but I'll simply state that I'm not who you think I am. Let's leave that aside.

The car you drove home in, the airplane you flew in to visit Hawaii, the computer or iPad or cell phone you're reading this on, not to mention the Falcon Heavy that launched the Tesla into interplanetary orbit, were all designed using mathematical models. None of these things, nor any other complicated device we take for granted, would be possible without mathematical models. Let's think how a mathematical model works for a moment. Someone observes something, let's take a spring for example, and discovers, or think he discovers, that when the load on the spring is doubled it compresses twice as far. y=Cx and Bob's your uncle, spring rate. Now some nube has a problem with a hydraulic system and ask's some office type engineers how to spec a gauge to monitor the hydraulic system, and because they don't understand the full range of parameters of that particular hydraulic system, **** things up by an order of magnitude. Happened yesterday afternoon and everyone involved enjoyed roasting the nube responsible. The point is a that a mathematical model is a virtual representation of reality, and is only as useful if it accurately represents that reality. You have to have confidence in your model, and confidence comes from experience. I'm confident in my very simple model, and I believe I have an adequate understanding of its limitations. If I'm wrong, and everything goes to hell, I'll do what every good engineer does and ferret out the flaws in the model, readjust, and reapply. Landing rocket crashes onto floating barge, fix model, try again.

Anyway, all's I'm trying to do is put modify a completely unsuitable car to take me where I have no business going, and doing it in a manner that everyone says can't possible work. What could possibly go wrong?

I'm moving on to GRM because I think I've gotten all of the help I can possibly take over here , and because folks over there rally their cars (not that I'm putting together a rally car because that would imply I aspire to rally driving skills, which I most certainly do not nor ever will possess in any measurable way) and I think they'll have empirical experience with spring rates for those kinds of things. So thanks one last time to one and all, I learned quite a bit from all of you.

Jim

Caverly

You should check back in here and give us an update when you're done. I think you over-complicated it, but I'm still interested in the outcome.

Junkwhale

Finally got round to looking at the amount of rear travel. With no bumpstop in, the shock bottoms out on the OEM NB tophats with what looks to be about 25mm (or ~1") wheel travel left.


Had to stop for the night but will measure with extended tophats (have 1" and 1.5" to try) to see how much compression travel is available for the shock before the wheel bottoms on something.

Bronson M

iTrader: (1)

With a narrow tire it's pretty rediculous how much bump travel is available. A wider tire will of course find something to hit sooner. In the front it's usually the top hat/shock tower.

As a data point I found with a 245 mounted on a 9" 6ul I ended up with a 1" front bump stop. On the rear I had the same 1" bump stop but with a 1" extended top hat.

Junkwhale

Will try a 225/45/15 (which is what I'll be using and was going to do once got round to checking bumpstop length). It's surprisingly hard to see where contact is being made with the wheel stuffed up at full compression (at least in a small garage with the car on jack stands).

JoeTheZoe

iTrader: (1)

EDIT: Full disclosure, my calcs are all correct for the spring and shock parameter example, but I have not accounted for the proportionally higher force at the coil-over assembly vs the approx. corner weight measured at the tire. That force must be increased by the motion ratio of the suspension, the shock and spring see the corner weight (lets say 600 lbf) x the motion ratio (1.52, the proportion of moment arm length; think LEVER) = 912 lbf...so the spring length and preload requirements for the same "weight" should be higher. But again, provided measured loads at the shock, the math below checks out, but would only technically apply to a much lighter Miata...like a Lotus 7 replica or something, lol. Leave a negcat if you want...I deserve it.


Not to beat this horse too much more, just breaking this down a bit, even though you may have permanently left this thread - so this could be for the benefit of others.

I ran some of your proposed numbers (14.5"-15" ride height, 200 lb/in front spring, 150 lb/in rear springs, 60:40 bump ratio) through my spreadsheet and found that an 8" or 9" front spring and 10" rear spring will easily accomplish the task on OEM NB Bilstein shock geometry, advanced autosports/allstar sleeves (at stock circlip height), and stock NB top hats in front and MAYBE a slightly deeper top hat at back.

Caveats:
1. My spreadsheet is still in development and I am verifying the measurements against my NA, but should be close enough.
2. You ordered the B6 shocks, which I think have slightly different body length than the OEM NB bilsteins, but likely almost the same stroke length? Close enough for quick verification and I just assumed a simple 5" stroke for the example below.
3. None of this considers the very real effect of control arm bushings on the spring loads and settled ride height.

You said this in reference to the suggestion that you would need to preload long, low-rate springs at least 3"-4" for some spring lengths you proposed. However, since you clearly defined desired spring rates of ~200 lb/in front and ~150 lb/in rear, and considering even conservative estimates of corner weights as 600 lbf front and 500 lbf rear, just 3" (600 lbf) front preload and 3.33" (500 lbf) rear preload would result in complete negation of the corner weight...keeping your shocks in full droop (fully extended) at rest under the weight of the car. As you surely agree that this would be BAD, and you defined a desired bump ratio of 60:40, you simply will not preload springs at these rates by even 3", let alone "a good deal more than that!"

Because you defined your desired bump ratio and we know the shock stroke length, we can immediately calculate the amount of preload you would want. I will pull up my spreadsheet once home tonight to verify numbers, but even assuming just 5" (actually, I think the OEM NB bilstein fronts are only 4.5") of total available stroke length on many of these shocks, and assuming bump travel WITHOUT a bump stop, your prescribed aim (not that I am advocating for or against) is 3" bump, 2" droop (60:40 ratio). Meaning, after preload and install, your coil-over assembly should compress 2 more inches from the corner weight when you lower it off the jack. As we already defined your desired spring rates as 200f/150r, the amount of spring load NOT due to preload should be 400 lbf front and 300 lbf rear (200lb/in x 2in and 150lb/in x 2in). Considering the assumed, conservative 600 lbf front corner weight and 500 lbf rear, the springs should be preloaded 200 lbf front (600-400) and 200 lbf rear (500-300), or 1" and 1.33" of SPRING preload compression, respectively. If each shock has 5" of stroke, the minimum required spring travel (for use when sourcing spring lengths) is the preload (1"f/1.33"r) plus the stroke, or 6" front and 6.33" rear total spring travel. Now...this should be more than enough, because it is not even accounting for the bump stop block (compressed) height. When I calculated this in my spreadsheet I actually subtracted the bump stop block height from the total available stroke, then determined the bump:droop travel from that...in the above case, assuming a 1" bump stop block height (for simplicity), the available stroke becomes 4" total, or 2.4" bump, 1.6" droop for a 60:40 ratio. If you want to look at it that way, then the "resting" spring load, after preload, in the above example, should be 320f/240r, increasing the spring preloads to 280f/260r, or 1.4" and 1.73" at their respective rates.

This is completely true IF you plan to use all of the available stroke with spring preload. But that truth is what limits YOU to a very specific, quickly-calculable suspension setup, because your droop travel is already set by the shock geometry and your desire for a roughly 60:40 bump ratio. For most of the setups in this thread, using much stiffer springs and desiring lower ride heights than you are, we end up with zero preload, reducing shock stroke by the gap between the spring and mount when the shock is fully extended. The usable droop travel is then only corner weight divided by spring rate, but with much-reduced bump travel, due to suspension geometry and the low ride height. Extending the top hats by the length of the aforementioned gap would allow for utilizing the same droop travel, but increased bump travel - still requiring even longer bumpstops to prevent tire-chassis contact. You can make a case for helper springs, but they are not going to do much more unless they are a high enough rate to overcome the combination of unsprung weight inertia and CA bushing preload. I would rather extend the top hats for longer, more-progressive bumpstop engagement, but there is a limit to that game.

They are not related, but this is why you have adjustment collars for height. However, adjusting the spring perch will vary your bump ratio, unless you change out the bumpstops (which is basically scaling the overall available travel) or adjust the top hat depth...maybe a competent welder can produce adjustable top hats out of some spare adjustment sleeves?? haha, that would be ideal, but slightly crazy.

I hope to hear back about your setup and wish you success with it, but I still think you could pull this off with mostly-stock components and slight adjustments, such as spring spacers, though you would lose the ability to adjust height.

mrmonk7663

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JoeTheZoe

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mrmonk7663

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JoeTheZoe

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I've seen a few figures thrown around in this thread, maybe like 1" or something, but don't quote me. I think there is more focus on maintaining at least a minimum bumpstop thickness, to control how harsh their engagement is. But someone with more experience and expertise should chime in on that. I know that controlling which bumpstop engages first (front vs rear) while cornering or hitting a bump mid-corner will greatly affect handling balance, such as if you engage the rear bumpstops well before the fronts from lateral acceleration, you are rapidly increasing effective rear spring rate and, therefore, reducing the FRC toward oversteer. Likewise, if you hit a bump mid-corner and the rear shock slams up into a short, stiff bumpstop, the potential to snap into a spin increases. But your spring rates, sway bars, and alignment all affect that, so it gets way more complex.

acedeuce802

iTrader: (1)

I have a very similar setup, MSM bilsteins on an NB and I had 550/400 springs with 36mm bumpstops. I think I was around 12.25" rear, 12.0" front (you want the rear slightly higher). Extended top hats were a night and day difference as the bumpstops were basically engaged at ride height. I forgot the actual height of the top hats I made, but I have it in a spreadsheet somewhere.

mrmonk7663

iTrader: (9)

Junkwhale

Junkwhale

And since this aimed at the best budget setup for most of us, I think the general recommendation should be: OEM NB fronts, Extended 1" to 1.5" rears depending on the bumpstop lengths you run.