Rear bang over bumps/railroad tracks

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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 03, 2018 11:33 am

So I was looking at Swift Assist springs, the 4-6k spring rate one. Link below...

So lets say I got the 6k(336lbs) 2.56" assist spring. My rears are roughly around 650lbs per corner. Per description of assist springs it says it'll be fully compressed with 573 lbs but I'm assuming that's only sprung weight it's counting and not unsprung. Now the rear of the EVO has a multi-link rear suspension and is mounted inwards on the arm a little(screenshot down below). I'm reading the rear of the suspensions motion ratio is somewhere between .6 to .9. Makes me wonder how that will play into what spring rate rate should be chosen.

Another question is now that the assist spring is fully compressed while the car is static, will it only get the benefits of extra droop travel or will there be any comfort added just for normal street driving. I'm guessing if it's compressed fully at static then it would right off the bat be riding at the 11k main rate then which the comfort benefit wouldn't be there then besides the added droop.

Also here is link about the evo motion ration on the first post to get a better sense of it..
https://www.evolutionm.net/forums/evo-t ... r-evo.html


Image
http://www.swiftsprings.com/products/as ... r-springs/
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Apr 03, 2018 12:41 pm

Getting into motion ratios gets complicated. I mean, it's simple in concept. It's just a lot to keep track of.

If your "assist spring", which is really what everyone else is calling a "tender spring" is fully compressed at static, it's not going to do much for ride quality. It will soften a "landing" any time the suspension goes into droop, though. Which can be significant depending on the situation.

It's not going to ADD droop travel, though. You adjust that with your shock height adjustments.

Say you've got the ride height that you want, and your spring perch is near the top of its adjustment... and the shock length adjustment is near full short. That would give you a big tall gap between the top of the shock body and the upper spring perch. The area that your bump stop works in would be significantly tall. Lots of COMPRESSION travel. Not a lot (potentially none if you set it up just wrong) of DROOP travel.

If you took that situation, and lowered the spring perch by 2". Now the car is 2" lower... extend the shock body by 2" to get the ride height back. Now you've moved your shock's range of motion by 2". Instead of having max compression travel and almost no droop, you've got 2" less compression travel... and 2" MORE droop travel.

Have a good look at that. It might be part of your problem. With stiff springs, you're not going to use... and thus don't NEED a lot of compression travel. But, droop travel is what's going to smooth out the "crash" that you're talking about when the rear gets light and/or lifts a wheel.

You can't add in a second spring without either adding some droop travel, or raising your ride height, or making your main spring shorter. Gotta get some space to work in.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 03, 2018 12:57 pm

Attached a picture of rear coilover. The spring perch (which I think you're talking about where you preload the spring) is compressing the 6 inch spring 1/8 of an inch. The height adjustment has over 5 inches of free space. What would you recommend to alter to lower compression and add droop?
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Apr 03, 2018 1:04 pm

Look under your dust boot. How much space is between the top of the shock and the upper perch?

That's your total working stroke.

Now, install that on the car. Set the car down until the tire just touches the ground. Measure ride height on that corner for reference. That's your full droop height.

Set the car down and go drive it to settle everything. Measure actual static ride height of that corner. The difference between the measurement above and this one is your droop travel.

The difference between your droop travel and the full space available from the first paragraph... that's your compression travel. (less your bump stop height)

Figure out what's going on with your travel. Just looking at what's in that photo doesn't provide enough info.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 03, 2018 2:37 pm

Thanks, I'll do some measurements when I get a good free hour.

I sent an email to fortune auto(coilover company) about this about a month ago and spoke about droop. I said...

"So since my rear springs are preloaded 1/8th of an inch, would it be best to try to basically have them be at 0 preload(0 meaning the spring is not compressed at all but still touching the top of the perch and the locking nut it sits on). Think the 1/8th would make a difference? Since the spring rate 11k is around low 600 pounds and my rear corner weight 650 on one corner and 680 on the other. Does that mean the springs on the rear is barely compressing thus if I go over something I won't have much droop at all?"

They replied back with..
"That's correct if you reduce the amount of preload you will increase the amount of static droop. Although it will only be a little bit but it will be some."
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Apr 03, 2018 5:34 pm

CaptainSquirts wrote:I sent an email to fortune auto(coilover company) about this about a month ago and spoke about droop. I said...

"So since my rear springs are preloaded 1/8th of an inch, would it be best to try to basically have them be at 0 preload(0 meaning the spring is not compressed at all but still touching the top of the perch and the locking nut it sits on). Think the 1/8th would make a difference? Since the spring rate 11k is around low 600 pounds and my rear corner weight 650 on one corner and 680 on the other. Does that mean the springs on the rear is barely compressing thus if I go over something I won't have much droop at all?"

They replied back with..
"That's correct if you reduce the amount of preload you will increase the amount of static droop. Although it will only be a little bit but it will be some."
You really have to wrap your head around what's going on. I'm not sure they gave you a good answer. I hesitate to call it "wrong", though I sort of think that it is. But, I think it's less that they're wrong, and more that that they just aren't explaining it well.

Let's put your car on jackstands. Full droop. The coilover is just hanging there. Lower control arm or whatever is dangling from the end of it.

What happens if you adjust the lower spring perch down? Do you get more droop? If you raise the lower spring perch up and introduce preload... do you get less droop? Not really. The spring in no way LIMITS our droop. It can't. It resists compression. That's all it does.

What's determining our droop travel? What is LIMITING our droop travel? Unless you've installed some kind of a droop limiting device (like a cable, strap or chain that physically limits how far the suspension can droop), or there is some physical limitation in the suspension itself that keeps it from drooping past a certain point... the only thing left to limit DROOP travel is the shock itself. When the shock is fully extended, it can't droop any more. The spring and the spring preload is not a factor there, right?

You want more droop travel... put the car on stands, let it droop... and then adjust the shock body LENGTH. Make the shock body longer, now you have more droop travel.

And then when you set the car back down, you'll see that you've also raised the ride height, which you can bring back down by lowering the spring perch.

You'll balance those two things. The perch height and the shock length to get the travel where you want it. Maybe you want 50% compression and 50% droop? Maybe because you're stiff and low, and you have 5" of travel, you only want 2" of compression and 3" of droop. Whatever you want... you adjust those two things to get it.

Preload is kind of secondary to that. You can set everything mentioned above with NO preload. (which is how I normally do it... just enough preload to keep the spring from falling out of the perch at full droop) You can add preload from there... and it WILL raise the height of the car some. But, in addition to that... any body roll or bump that you hit will have to overcome the amount of preload before the suspension will move. I learned that you can definitely go to far with that and lose a lot of steering feel... so, preload is kind of a subtle thing that you can play with if you want to. But, up until now, I never put in more than 1/8" or so of preload... and I'm likely to continue doing it that way, at least for street cars.

So, back to the answer you got from the coilover guy, and what's "right" about what he's saying.

"That's correct if you reduce the amount of preload you will increase the amount of static droop. Although it will only be a little bit but it will be some."

If you reduce the amount of preload... you increase the amount of SPRUNG droop. Is that a better way to say it? If you drop the spring perch so far that you've got an inch of air for the spring to flop around in, you now have unsprung droop. You don't have more droop. The length of the shock is still limiting the droop, and always was. But, you're either raising the lower spring perch up into it, or you're not.

If you go the other way and add a bunch of preload. Say you add an inch of preload by raising the spring perch up. Droop is the same. But, you've actually raised the ride height a bit.

It can be hard to wrap your head around, I won't lie. But, you really need to understand "how much droop travel do I have" (from static height to wheel coming off the ground) and "how much compression travel do I have" (from static height to fully compressed bump stop... and if you're tuning bump stop and spring rates, you also want to know at what point the bump stop starts to engage).

Once you know that, you can start thinking about and playing "what if" and experimenting. If you determine that you don't have enough droop travel, then you need to lengthen the shock body and lower the spring perch. That gives you more droop... less compression.

The thing a lot of people don't realize is that you can have things set up COMPLETELY fucking wrong, and still get the ride height and "preload" that you want. Easy to do. If you go too tall with the shock body, and then lower the spring perch way down to compensate... boom, there you are. No compression travel... gonna ride like shit, and as soon as you turn the wheel, something's gonna load up on the bump stop, effective spring rate goes through the roof instantly, and you lose traction.

I think you may be in the opposite situation. You might have your shock body too short, which made you raise the spring perch too high to capture the spring and get preload. So, you probably have LOTS of compression travel, and not enough droop travel. Such is my theory... which you can quickly test with a jack and a tape measure.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 03, 2018 7:26 pm

Here are measurements.

Wheel touching ground, static weight. From middle of rim (hub) to top of fender.
13 3/16 inches.

Wheel just touching ground from jacking up car. From middle of rim (hub) to top of fender.
14 5/8 inches.


Droop = 1 7/16
14 5/8 - 13 3/16 = 1 7/16

Total working stroke
4 3/8, not minusing the bumpstop.

Bumpstop
1 inch.

Compression travel.
4 3/8(total working stroke) - 1 7/16(droop) = 2 15/16 compression. Minus 1 inch from bump stop.

Total compression - bump stop = 1 15/16 compression before I hit bumpstop.
Last edited by CaptainSquirts on Tue Apr 03, 2018 7:58 pm, edited 2 times in total.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 03, 2018 7:44 pm

CaptainSquirts wrote:Here are measurements.


Wheel touching ground, static weight. From middle of rim (hub) to top of fender.
13 3/16 inches.

Wheel just touching ground from jacking up car. From middle of rim (hub) to top of fender.
14 5/8 inches.

Droop = 1 7/16
14 5/8 - 13 3/16 = 1 7/16

Total working stroke
4 3/8, not minus in good bumpstop.

Bumpstop
1 inch.

Compression travel.
4 3/8(total working stroke) - 1 7/16(droop) = 2 15/16 compression. Minus 1 inch from bump stop.
Total compression - bump stop = 1 15/16 compression before I hit bumpstop.
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Apr 03, 2018 9:00 pm

I'm a simple kind of guy. This ain't no F1 car, let's lose the 16ths!

So, about 1.5" of droop travel and 3" of compression travel.

With an 11k spring. That's 616 lbs. Rear motion ratio turned up 0.64 in a Google search. Seems reasonable. So, your actual rear wheel rate is about 394.

And the math works. If you've got spring contact at full droop... as you set your 600ish pounds of corner weight onto the suspension, it's going to compress that 394# by about 1.5". (I like it when math makes sense)

So, you've got 3" of remaining compression travel... 2" to the bump stop.

If you're in a 1G turn, in theory, you've transferred all of the weight from the inside to the outside. That's an additional 600# or 1.5". That tells me that at full-tilt... you're very likely not even touching the bump stop! WAAAAAY too much compression travel. Conversely, only have 1.5" of droop travel is why you're actually lifting an inside tire.

My theory was correct. You need to adjust the travel so that it's more correct for the spring that you're working with. Will give you more droop travel, probably take care of your banging over RR tracks, AND probably keep your inside rear wheel planted in a turn. (loading and unloading a drive wheel like that has GOT to be hard on your rear axles)

Where do you want to be? Well, assuming you've got nice modern progressive bump stops, you actually DO want to use them. So, let's say our 1" bump stop will compress to somewhere between 1/4 and 1/2". That means at full bump stop compression, we're "wasting" a half inch. Add that to our desired compression travel of 1.5". Bingo, we want 2" of total compression travel, and the rest will be droop.

How do we get there? Well, you've got 3" of compression now. So, we want to reduce that by 1". Raise the spring perch by 1". Shorten the shock body by 1". That should put you in the ballpark.

This is all oversimplified. Even though we did consider motion ratio and all that... we didn't consider the swaybar (and it's motion ratio) and what it's adding to the wheel rate in a turn. With that, you're actually getting even less compression under cornering load. This will still get you close, though. Maybe take away another 1/4" from your compression travel if you want to really maximize the droop travel.

Go turn some wrenches!
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 03, 2018 9:20 pm

So you're saying preload the spring 1 inch and then raise the shock body 1 inch?
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Postby ImpostorDan » Tue Apr 03, 2018 9:27 pm

Just accelerate over the railroad tracks already.
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Postby CaptainSquirts » Tue Apr 03, 2018 9:32 pm

ImpostorDan wrote:Just accelerate over the railroad tracks already.
Yoo I did that last night. It freaking felt so much better! :lol:
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Rear bang over bumps/railroad tracks

Postby ImpostorDan » Tue Apr 03, 2018 9:34 pm

CaptainSquirts wrote:
ImpostorDan wrote:Just accelerate over the railroad tracks already.
Yoo I did that last night. It freaking felt so much better! :lol:
The sequence really isn't different than apexing a turn.
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Postby Loren » Tue Apr 03, 2018 9:53 pm

CaptainSquirts wrote:So you're saying preload the spring 1 inch and then raise the shock body 1 inch?
That sorta is what I said.

But, it's not what I meant, dammit!

You actually want to LOWER the spring perch. That effectively moves the top end of the shock further up into the spring and reduces your compression travel. It also lowers the car. You'll need to extend the shock body to get the ride height back.

I may have stated it backwards. That's why this is so much easier as soon as YOU understand what the hell you're doing. :thumbwink:
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Postby Loren » Tue Apr 03, 2018 10:01 pm

And, yes, that means your spring is too short.

But... your spring IS too short. Looks like a 7" spring. And you can count 6 thicknesses of wire that is about 1/2". So, you're losing 3". That means the max your spring can compress is 4". And you've got 4.5" of shock travel.

You either need a longer spring, or a helper spring (zero rate just to keep the spring seated), or a tender spring (that will lower the overall rate when in droop... which we've discussed).

Looks like you've got plenty of room you could lower that perch 3 or 4 inches if you wanted to, and make room for a proper helper or tender.

I'd go ahead and experiment with just making the adjustment as-is. Just be careful when you lower the car down so that you know the springs are seating correctly. Check the theory on droop travel, see if it all makes sense. Drive it around and see what it feels like. Maybe don't hit the RR tracks at mach 3 and get air. Maybe don't autocross it that way because you don't want to unload a wheel and unseat a spring. But, you could experiement with it, and then put it back before Saturday.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 03, 2018 10:06 pm

Ok I think I understand... so basically loosen the spring perch so the coil isnt't preloaded at all, it'll basically give the spring 1 inch of floaty free space if the car flew off a cliff :-D . But since it has a nice chunk of extra droop it shouldn't be in a free fall state since the wheel should always be on the ground, unless like above, flew off a cliff.

Ok just just your post. So basically tender/helper spring is the way to go it looks like :thumbwink: . Most likely it'll be tender springs for the extra street comfort. I'll have to figure out exactly what tender springs to get with all that math and arithmetics next.

Learned a lot of good stuff, thanks Loren! :salute:
Last edited by CaptainSquirts on Tue Apr 03, 2018 10:15 pm, edited 1 time in total.
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Rear bang over bumps/railroad tracks

Postby Loren » Tue Apr 03, 2018 10:10 pm

More or less.

Just be aware that your spring CAN unseat if you hit a yump, or maybe possibly in a hard autocross-type turn.
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Postby Loren » Tue Apr 03, 2018 10:15 pm

Another thing to check. Since you'll be extending the shock body... you want to be sure you're not extending it too far. If you can't get inside the tube from the bottom to measure, you'll have to unthread it to see how many threads you have engaged.

I have no science behind this, but I like to be sure I have at least an inch of thread engagement. Probably isn't all that critical as it's not side-loaded. All the force is straight down.
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Postby CaptainSquirts » Wed Apr 04, 2018 10:24 am

At this point I emailed fortune auto about their bumpstops for specifications on it since they don't list any information on it on their website. I just want to see if the bumpstop is usable first before I lose an inch of compression.

After that, I'll have to see if I raise the shock body by an inch if I'll have a good amount of threads left still in the lowering perch. There is a hole about a good inch down(if I remember correctly) that will show if the body is still threaded or not. So I most likely won't have to unthread the hole body.

Then at last, I will need to look into a tender spring that is short enough to compress all the way before I all the weight goes from the inside to the outside tire so it'll then use the main spring at that point(does that sound like the right? should the tender be fully compressed before all the weight is transferred from the inside to outside or should I shoot for some other way?).
Last edited by CaptainSquirts on Wed Apr 04, 2018 11:13 am, edited 1 time in total.
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Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Wed Apr 04, 2018 10:57 am

Alright alright I got a response back from fortune auto about the bumpstops. They said the following...

"I went out and measured a swift helper spring and at full extension it was 2,3/4 inches, and at full compression it was about 7/8 of and inch so there is about an 1,7/8 if static droop added if running the helper. Our bump stop is basically a rubber spacer. Really just designed to keep parts from hitting each other. So there isn't anything to factor in like when running a progressive stop. I would run the helper over drooping the spring. I don't like having an un captive spring so I would lean towards the helper route. If you have anymore questions please feel free to ask."

So sounds like it's probably not the best option to actually use the bumpstop when at full tilt and have compression at most be a little before it.

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