Rear bang over bumps/railroad tracks

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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Nov 07, 2017 9:28 am

So when going over dips/railroad tracks while going at a swiftly speed(35,40+). The fronts go over pretty nicely, lands nicely. The rear ends up going up over the tracks and then slams down hard. Is this just expected with coilovers and just having to learn to live with it/approach bumps/dips/railroad tracks more cautiously? It has Simultaneous Rebound and Compression and I've tried adjusting that from changing the fronts, rears, both, having fronts a lot higher than the rear and vice versa(I have no idea what I'm doing, just trying all the different settings to see if it made a difference over railroad tracks). Didn't seem to help much.
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Nov 07, 2017 11:07 am

Less rear shock stiffness should help. But, if your springs are too stiff...

What rally cars do to soften their stiff springs is basically spring stacking. They have different names, like tender springs (usually a super soft spring that expands at full droop to keep the spring stack from falling out of its perch) or helper springs.

What you want for this is a helper spring that is fairly stiff, but short and with coils pretty close together. When you're at full droop (flying over a RR track), the helper is in play. It effectively cover your spring more coils... Which the magic of spring rate math says makes the spring softer... Which cushions your landing. And then, as the weight of the car settles onto it, the helper collapses. (It may not go into full coil bind until you enter a turn, normally) when the helper is fully collapsed, it behaves like a spacer.

This is exactly how "progressive springs" should work. But, it's what rally cars, or any race car that needs to get over a bump without becoming unsettled do.

Or... You could just slow down. :grin:
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Nov 07, 2017 12:57 pm

Loren wrote:Or... You could just slow down. :grin:
Image

So I'm assuming adding helper springs to the coilover springs will increase the ride height thus throwing off the corner balance? If that's the case I could just lower the mounting perch to get the same height it's currently at so I wouldn't throw it off then?
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Nov 07, 2017 1:26 pm

You have to "engineer" your spring stack to be the correct overall length to work with your application. Maybe you've got enough adjustment in your spring perch to allow you to fit a taller spring stack? Maybe you don't. If not, you'd have to make your main spring shorter to compensate.

I looked into this a lot once, but never acted on it. It gets expensive. You have to buy main springs, tender springs, little doo-dads to fit between the springs (unless you want to really commit and weld them together), etc.

Eibach probably has the best selection of "oddball" springs to choose from.
https://eibach.com/de/sites/default/fil ... global.pdf

The strange thing about spring rates is that if you take two springs, say a 7" 500# spring and a 3" 500# spring and stack them... the resultant spring rate is significantly LESS than the rate of each spring. You end up with a new spring with a rate that is related to the total number of coils (and the thickness of the wire of each spring), and number of coils is a significant factor in the equation.

There's a spring rate calculator out there somewhere that will calculate the rate of a stack for you. At least, I'm pretty sure there is. It's been a while. I might have just plugged all the formulae into a spreadsheet.

Apparently, I've got "tender" and "helper" spring names reversed, at least according to Eibach. Whatever, you get the spring that has the rate, length and coil spacing that you need... whatever they want to call it.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Nov 07, 2017 1:56 pm

Hmmm I'll have to check how much room I have left for the springs but if I remember correctly I should have a nice chunk of space left. If I did the rear, would there be any reason to add them in the fronts also even though I'm not having any issue there?
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Nov 07, 2017 2:00 pm

Unless you're planning on doing some more severe driving that would have the front also getting air, you could probably get by with just doing the rear.

You're opening a big ol' can of worms that takes a lot of thought and calculation, though.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Nov 07, 2017 2:30 pm

Just daily drivng and autocross so no crazy jumps, except railroads crossings :grin: .
Loren wrote:You're opening a big ol' can of worms that takes a lot of thought and calculation, though.
So it's not as easy as putting helper springs in(as long as there is enough room), putting on the perch's/seaters, then putting regular springs over them? Ugh, I need to go to one of the Fast Monday nights and get details on all this shenanigans.
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Nov 07, 2017 4:08 pm

I guess in theory, if you pick just the right spring, it's going to be the perfect rate, length and coil spacing that it will be out of the equation under normal driving conditions. Hit a bump, it does nothing. Initial turn-in, it does nothing. But, when you go over a hump and bounce the car up, it will come into play an soften the recovery. What I don't know is... when you dive into a corner and get into significant body roll... now that spring is back in play again. I don't know if there would be any negative affects from that. I mean... if you get enough body roll that you're lifting a tire, it would soften the "landing", and might be a good thing. I just can't quite wrap my head around whether there could be any potential negatives to consider.

Obviously, Eibach is marketing this concept, though. Which means people are doing it. Which means there should be information out there about it. You're going to want to do some research and find that information.

Spring rate calculators might help. I seem to recall that you actually want your helper spring (tender spring, whatever) to be a REALLY high rate. Coil spacing will be so short that it will go into coil bind as soon as you set the weight of the car on it... but it might be something like 1,000# per inch. The reason for that (based on the math that I did years ago) is that if it's NOT a really high rate, then your resultant "soft" full-droop rate ends up being really low, and the difference between that rate and your "normal" rate would be a pretty harsh transition. (like 150# to 400# instantly the moment the short spring binds)

I was able to find stuff like 800-1500# springs in lengths like 2" back then... so, I'm pretty sure I was on the right track. (no other reason for such oddball springs to exist) I just never wanted to spend the money.

(My project at the time was a Miata that I was trying to tune the suspension around having NO swaybars... in an effort to save weight. You have to go REALLY stiff on the front springs to get away with that. I already had 450# springs on the front... was thinking about maybe going stiffer, but with a softer progression. This was probably 10 years ago.)
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Rear bang over bumps/railroad tracks

Postby twistedwankel » Tue Nov 07, 2017 7:43 pm

May not be your issue but take a good look at your rear suspension sway bar endlinks.

One pix of endlink I put on new straight as an arrow and the other two of both endlinks bent at 90 degrees. Ultimate cause was a blown Koni shock on the right side? Or was it a result of the bent endlinks killing the shock? This could have been this way before I even bought the car?? Or put on the koni's. Anyhow the koni that failed according to the company was a smashed adjuster in the bottom of the shock. Got a new set. Sold them. Put on new Bilsteins. End of problem with straight endlinks.

Dunno. They were bent so perfectly that I thought they came that way. Got new ones and thought they were the wrong ones. Nope. Noise drove me nuts over 1" deviations clunk and even a slow drive apron bump or sometimes at speed tar strips. "Clunk". Then quiet. Andrew and I drove full tilt at a test and tune and neither one of us could figure out why he spun out on new tires once? This was the cause of that spin at max speed but the car handled well otherwise for him.

Sounds like you're doing more of a "General lee" leap and bottoming out? If you yell: "Heehaw." You will not hear the noise.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Wed Nov 08, 2017 9:17 am

Loren wrote:tune the suspension around having NO swaybars... in an effort to save weight.
Damn, what people do for weight saving.
Loren wrote:You're going to want to do some research and find that information.
Yea I'm doing that and a lot of forums people try to talk about it and it seems most of them mix up the helper and tender springs. Also there is a contradiction on the springs as well on how they work... Internet is usually a good place to find solid info but this one is iffy finding the right info.

So I can safely assume that the rear is in full droop and then it slams back down? So at the moment until I figure out definitely what the correct term for the springs are we'll say the tender spring is the one that has a very very low spring rate and is always compressed unless the suspension is in full droop. Then the helper spring being the one that isn't fully compressed but compresses when there is load(from cornering and what not).

So would the tender spring then keep the rates the same overall(besides the 20 or so lbs it adds) and make the rears from not going full droop unless I from what the Mayor says I "General Lee" it and then no matter what probably all 4 wheels will be in full droop even with tender springs.

But with helper springs(from what people are saying) it would act like a different set of springs initially whatever rate they're. When the threshold for that rate is fully used(compressed) then the main spring would take over. So if I got a 200lb helper spring, then when the rates exceed what it can handle it will compress fully and then the main spring takes over and does the rest. My fronts are 560lbs and rear 616. So the helper springs would make the car feel like it ride on whatever lower rate it's until I exceed those rates then? Also this would give the suspension more droop travel as well?

Also since the spring is compressed 1/8th of an inch from the locking perches on the coilover it should never come unseated then right?

Whatever I just wrote is my understanding so far on the springs from what I've gathered so far from the internets :headscratch: ...
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Rear bang over bumps/railroad tracks

Postby Loren » Wed Nov 08, 2017 11:21 am

My logic for nomenclature was "tender spring" "tends" to the main spring to be sure it doesn't fall out of its perch at full droop. It's a "nothing" spring rate, just takes up some space to keep things from falling apart. Eibach calls the stiffer spring a "tender". Think of their logic as "it's there to make the suspension more "tender" when you recover from being at full droop.

So, whichever nomenclature you use, you know what you're talking about.

Now, spring rates. I'll throw some examples at you to explain what I was saying last night.

The formula for springs in series is not as simple as it seems. They are NOT additive. It's not A+B=C, it's (A*B)/(A+B)=C

So, if you simply take a 600# spring, and stack another 600# spring on top of it, the rate is LESS. Much less. It's actually half because the spring rates are the same.

600x600 = 360,000
600+600 = 1,200

360,000 / 1,200 = 300.

So, this is what I was talking about last night. If you're coming off of being airborne, with this stack, you'd have 300#... but as soon as the spring with shorter coil spacing binds, you instantly go up to 600#. Big transition. In my mind, that seems like too much of an instantaneous transition. (maybe I'm wrong)

So, if your main spring is 600#, and you want LESS of a transition from being at full droop, you could make the second spring STIFFER than the main spring. Say, we went up to 1200# with it?

600x1200 = 720,000
600+1200 = 1,800

720,000 / 1,800 = 400

Okay, lets say we're satisfied with that. 400# to 600# isn't too extreme of a change. Will cushion the landing a bit. It'll work.

Now, how long do the springs need to be???

You need that 1200# spring to be compressed at static ride height. (or at least very close to it... you could allow maybe a half-inch of body roll or suspension compression before it bottoms out... and that would improve your ride quality, while taking away a tiny bit of steering response... which you could bring back with a stiffer swaybar) So, let's say you've got a 3600# car with 60/40 weight, and we're talking about the rear suspension. That's 40% of 3600 divided by two on one rear corner. 720#. You need that 1200# spring to be fully bound in no more than an inch!

And what does Eibach have for us? A winner!

That spring is 2" tall, compresses to about 1.25". That's 3/4" of travel. And best of all, it's PROGRESSIVE. So, your rate will ramp up from 400-600, not do it all at once. Pretty cool.

Not cheap. But, an interesting solution.

So, if it's progressive, 600-1300#... it's probably going ot compress about half-way under 720#. Leaving you another 3/8" taller static height. To fit this and retain your current ride height, you'd need to lower the spring perch by 1-5/8".

Of course, that's probably not right, because I completely neglected the main spring in the calculations. The main spring is ALSO going to deflect under the load of the car... the two springs are working together. In reality, until the 2nd spring collapses, the whole stack will be compressing at the total calculated rate. So, whatever your full stack height is... less... 750/400... call it 2 inches.

Did I mention that this gets complicated? If you do the math just right, you can get really close. But, odds are that once you install, you'll have to make a final adjustment to get it right where you want it.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Wed Nov 08, 2017 11:42 am

^

freaking gold
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Wed Nov 08, 2017 2:40 pm

Loren wrote:So, if you simply take a 600# spring, and stack another 600# spring on top of it, the rate is LESS. Much less. It's actually half because the spring rates are the same.
I'm not really understanding this part. I can't wrap my head around having two 600# springs on top of each other would make the spring rate half :read: .

Also, with the formula, whatever it comes out to will be the result of the helper springs spring rate then, and not the main springs/overall.?
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Rear bang over bumps/railroad tracks

Postby Loren » Wed Nov 08, 2017 4:47 pm

Physics. You can look up the exact formula for a spring rate, but it's a factor of the material, the thickness of the material, the diameter of the spring, and the number of coils. More coils = less spring rate.

So, you take two identical 600# springs and stack them, the only variable you're changing is the number of coils.

Most of us non-engineers just cheat and use a spring rate calculator, but here's the formula:
https://www.engineersedge.com/spring_comp_calc_k.htm

Now, if you put those two springs NEXT to each other, then they behave as a parallel system, and the spring rate would be double. But, end to end... nope.
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Rear bang over bumps/railroad tracks

Postby twistedwankel » Wed Nov 08, 2017 10:14 pm

Loren loves Rally and Dan loves driving an Evo into the air...and beyond.

I think I know where this is all going. Cheaper than a Mirage. Able to leap tall RR crossings in a single bound. Stronger than a locomotive. It's an Evo, it's an Sti, it's the men of steel.

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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Thu Nov 09, 2017 12:23 pm

I can live with it if it only happens on railroad tracks but some minor humps/dips on highway I can't really react quick enough to avoid it. Also would rather not try to do anything sudden like manuvering/braking and freak other drivers out.
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Rear bang over bumps/railroad tracks

Postby Tim_M » Fri Nov 10, 2017 3:54 pm

Daniel, I'm sure your car picked up much roll stiffness and agility, but think you are seeing the compromises of firmly suspended auto. I prefer OEM replacement like struts for my cars-I know I'm giving up ultimate performance, but the nut behind the wheel is the biggest detraction for me. Note: my OEM replacement Ohlins struts have 150,000 miles on them with one refresh. Better ride than OEM's that they replaced.

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Rear bang over bumps/railroad tracks

Postby Tim_M » Sat Nov 11, 2017 7:11 am

I should add when I installed the yellow 'sport' springs that lowered the car .75 inch...a noticeable drop in ride comfort came along with it. These aren't that stiff, but still the ride suffered as I expected-the performance increase outweighed the disadvantages though.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Mon Nov 13, 2017 10:08 am

Honestly the coils feel more smooth than the oem struts on the streets and highway when they're at a softer settings. Just those one off things I listed that make it bang on certain scenarios. I'm still trying to figure out exactly what to do. I have some ideas on what to try but honestly whatever I try, I won't be sure on what the outcome will be beforehand so feels like I'll be going in blind.
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Rear bang over bumps/railroad tracks

Postby Tim_M » Mon Nov 13, 2017 4:50 pm

I'm not a suspension guru, but I'm sure there will be a solution.

I would assume you could simply purchase a lighter poundage spring from the coil over manufacturer? Nonetheless-good luck!

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