Rear bang over bumps/railroad tracks

[Loren]

Yup.

[CaptainSquirts]

From there, what do we do? If we lower the spring perch, we're lowering the car and effectively moving the top of the shock up into the bump area... making our compression travel less. So, if we go down with the perch by 2", it puts us back where we were. 2.5" compression + bump stop and 1" droop. FROM THERE, you need to do more.

Ok so the next part..

So at this point with the 9" spring, it's compressed by 2" and is sitting at 7" due to original preload. Stroke and everything is the same. Now we lower the spring perch 2". The spring is now 9" tall with 0 preload.

Now this part I don't understand. We just removed the 2" of preload, spring is 9" tall. Wouldn't everything (stroke, comp, rebound, ride height(if car was static)) be the same?

[Loren]

Now you're getting too complicated for words, which is why I suggested a scale drawing.

Okay, I just drew partial scale drawing, and you're right! Even with the 9" spring, if we adjust for 2" compression travel and 2.5" droop, we're falling out of the spring perch.

You either need a complicated dual-rate spring stack, or simply a helper spring to capture the main spring under droop.

Going with a helper spring would also solve your main spring's problem of not being long enough to prevent coil bind at full compression. Instead of compressing that spring a full 4.5" (which is could barely do... or maybe not quite do, I remember it was close), it would compress the helper spring 1" or 1.5" and the rest of the compression would be on the main spring, which it could handle without going into coil bind.

So, a helper spring, or any short spring that's going to be fully compressed at static load.

[CaptainSquirts]

Okay, I just drew partial scale drawing, and you're right! Even with the 9" spring, if we adjust for 2" compression travel and 2.5" droop, we're falling out of the spring perch.

Me second guessing you means you taught me well . I feel like a novice instead of a beginner about this stuff now . I'm going to go for a tender spring(one that compresses at static) route most likely then. But, one person is selling a helper spring and if I get i can get it at the right price the next I'll prob go that route.

My only concern is that hopefully it doesn't all of a sudden change how the rear reacts now since it feels great (autocross wise).

[Loren]

Me second guessing you means you taught me well .

Yes, I'm quite proud of us!

Since you're not changing spring rate, the only thing that could have a significant effect on how it feels at autocross would be if you reduced the compression travel "too far" and getting into the bump stop too early or too hard didn't feel right. I think the benefits of keeping a driven tire on the ground would outweigh any potential negatives.

But, it would be easy to fix. Just raise the spring perch to get a little more off the bump stop, and extend the shock length.

Do you have a Torsen diff in the rear?

[CaptainSquirts]

Do you have a Torsen diff in the rear?

My rear diff is a lsd clutch type, whatever you would call that type. Torsen looks like it uses bunch of gears instead of clutches(thanks google). So no not a Torsen.

[Loren]

I don't know much about clutch diffs. I know Torsens. If it was a Torsen, they require a little bit of resistance on the "slipping" side in order to transfer torque. If there is NO resistance at all (the wheel is in the air), then the Torsen just spins that free wheel. Give it just a little bit of resistance, and it works like it's supposed to.

If that was your situation... going from having a wheel that lifts to a wheel that doesn't could REALLY change the feel of the car. Having that wheel down would provide the resistance needed to make the diff work and put torque on the loaded wheel... which could help you dig out of a turn... or in certain low grip situations (like a wet road), could cause that loaded tire to go into power oversteer.

No idea if your clutch diff works that way or not.

I just know that if you put a Torsen in place of an open diff on a Miata... the car will OVERSTEER wildly in a low-grip situation where it would have just done a "one wheel peel" before. And if you have a Torsen and manage to lift a wheel, it behaves like an open diff.

[CaptainSquirts]

Well maybe it can give you an idea how it acts when the tire is lifted. A couple times at the turn around where I was full throttle early, before the car was fully straightened. The inside rear was lifted and as it straightened out, the inside tire landed, and had a pretty major jerking motion and chirped the tires good.
https://youtu.be/t6SnyOkEqvc?t=20

[CaptainSquirts]

Was able to get a pair of swift helper springs for 50 bucks , this one to be exact https://frsport.com/swift-h60-060-030-c ... 3-0-kgf-mm. It will fully compress with 265 lbs so a little less than half the corner weight.

Comes with shims and a coupler/adapter. I don't think it will create much noise issues since the rear shouldn't really have any turning?

Current setup.
4.5" total stroke.
3.5" compression(2.5" usable due to bumpstops)
1" droop
1" bumpstops.

New setup plan
Preload the helper spring by 0.25" (should be about 42lbs worth of preload on helper) also the main spring will compress by 0.07" which is basically nothing.
4.5" total stroke
2.25" compression
2.25" droop (3.5" at the wheel due to MR)

Can't wait to fly over railroad tracks . Oh and I guess the other benefit of inside wheel no longer lifting unless I somehow get more than 3.5" of wheel droop which I can't ever see happening at autocross, unless brooksville start getting some huge holes leading to China.

[Loren]

Perfect! Well played, sir!

[Native]

Gotta love it when a plan comes together.

[CaptainSquirts]

Installed the helper springs. So I miscalculated. I didn't account for the helper spring compressing which would lower the ride height by however much it compresses. It normally was 2.4 inches tall, I preloaded it to 2" due to total damper stroke was 4.25" instead of 4.5". A lot easier to measure that when it's out of the car.

So the helper spring would compress from 2" to 0.9"(full coil bind) at static. So that 1.1 of compression made my ride height 1.1" lower. I need to raise the car through the lowering perch but from my previous post I could only get about 3/4" on one side and 1/2" on the other since I didn't have enough shock body. Lowering perch had about 3/4" threaded after trying to raise it.

Wish the shock body was longer on this coilover setup but wondering if I raise the shock body but only had 1/2" of threading still in the lowering perch would be fine since it should only be an up and down movement on the rears.

Attached some pics. Pic of the car is where it sits with the helpers.

[Loren]

Dude, your car is upside down!

So, how much droop travel do you have at this height? (which will also tell you how much compression travel you have)

Seems to me that you should be able to raise the spring perch to get the ride height you want. You'll lose some droop travel, but it's physically impossible for you to not have more droop travel than what you started with because you made the spring stack taller.

Improvise, adapt, overcome.

[CaptainSquirts]

Dude, your car is upside down!

So, how much droop travel do you have at this height? (which will also tell you how much compression travel you have)

Well since the helper spring is preloaded to 2" and it'll be at 0.9" at full compression so I should've added 1.1" droop at the shock.

[Loren]

Right. So, if you have to take away a half inch of that to get your ride height back... you've still got .6" more than you had.

Or, if it feels right and you don't have ground clearance problems, you could leave it like that. Maybe lower the front a touch to match?

[CaptainSquirts]

Right. So, if you have to take away a half inch of that to get your ride height back... you've still got .6" more than you had.

Or, if it feels right and you don't have ground clearance problems, you could leave it like that. Maybe lower the front a touch to match?

I actually will get tire rub if car leans a little. So I would need the fenders pulled. So I'm going to try to get as much ride height as I can through the lowering perch and hopefully I won't need a pull of the fenders. I can also lower the front if need be as well.

[CaptainSquirts]

So I was able to finish messing with the ride height, preload and all that shazz this morning. I raised the ride height like 3/4", leaving me with 3/4" threaded I'm lowering perch. Preloaded spring some to get less compression from them at static so it wouldn't lower the car so much.

Just from measuring from the wheel at static from center of wheel to top of fender is 13 1/2". Droop from wheel when it just touched the ground when jacked up is 16".

So 2 1/2" wheel droop with new setup and old was 1 1/2". So I gained 1" from the wheel and like 0.6ish" from the shock.

Took car for drive, didn't get any ture rubbing anymore. Cornering felt a little different at the rear, maybe placebo effect. Going over railroad tracks seemed better, only went over a couple so gonna see how it feels throughout the week.

Here are pics from car at static and pic of wheel touching the ground when raised.

P.s I dunno why images show upside down on this site...

[Loren]

Looks good.

[CaptainSquirts]

Thanks.

I might get the rears pulled so I can not have so much preload on the helpers to get more droop. Maybe....

But then again not sure if rears getting lowered sk mich would jack up suspension geometry and what not and make it worse.

[Loren]

Autocross has transpired. What's the verdict?