Rear bang over bumps/railroad tracks

Post your questions or tips about wheels, tires, alignment, or anything related to preparing an autocross car here.

Moderators: Native, Jamie

Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Wed Apr 04, 2018 4:49 pm

So I am trying some calculations. I guess this is just more of a place holder for information since there so many numbers and crap which is hard to keep track of if not written down.

Corner Weights when the car was corner balanced some odd months ago. There was alterations mostly me raising the fronts by 3/4 an inch, rears are the same. So the rear weight might be a little more now since the front was raised a little.

LF 1008lb RF 1031
LR 684 RR 645

Main Spring Rate = 11k 616#'s.
Motion Ratio = 0.64
Wheel rates(if I understand it correctly is spring lb multiplied by 0.64) = 394

For shizz and giggles I tried mathing stuff for a tender spring, this one in particular https://eibach.com/us/i-1433-tender-spr ... ssive.html.
Spring is 1300 lbs, 2.0" has 0.79" travel when fully compressed.

So for the dual spring rate for my 616# and 1300# is 421
FORMULA FOR SPRINGS (RATE 1 x RATE 2) / (RATE 1 + RATE 2)
(616x1300) / (616+1300) = 421 x .64 (motion ratio) = 270

So with that number, my Left rear spring should compress by 2.53" at static weight.
my right rear spring should compress by 2.39" at static weight.

Questions trying to figure out.
1. Should I look for linear or progressive spring for the tender spring. I believe my swift springs are linear from the looks, but may be wrong.

p.s gotta leave work, gonna look more into it later....
P.s.s dual spring rate compression I calculate is incorrect. I didn't calculate motion ratio in it.
Last edited by CaptainSquirts on Wed Apr 04, 2018 6:05 pm, edited 1 time in total.
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Wed Apr 04, 2018 5:12 pm

You're better to think of your spring heights separately. Overall spring rate and overall compression at static height is good... but, it doesn't tell you how much your TENDER spring is compressing. Is it in play or not? Kinda important.

So, yes, as long as the tender is not fully compressed, you're working with your 421# spring rate. Good. It's less than 616, should be a little more comfy. But, it's not insanely soft, either. Good there.

But, what's the status at static height? Given the we don't know exactly what your unsprung weight is, and for simplicity, let's just say that your sprung corner weight is 600.

So, you've got 600 on your main 616# spring, compressing it a little over an inch. Cool.

And you've got the same 600 on your 1300# tender, compressing it about a half inch. You said it compresses to .79. So, at static height, you have .29 x 1300 = 377 pounds of load before coil bind. That's roughly 60% of your corner weight, so maybe about .6G would be your transition point where the spring rate goes from the combined 421# to the main spring's 616#.

AT THE SPRING, you're compressing a total of 1.5" at static load. With zero preload, that means you have 1.5" of droop travel. That's not bad, but remember we've got 4.5" of travel. I'd aim for at least a half in more on the droop side. That means a softer tender spring.

AT THE WHEEL, we need to divide that by .64. So, at the wheel, you're seeing about 2-3/8" of droop travel.

If we go to a 600# tender spring, your combined rate will be a lot lower. But, it will also be fully compressed at static. So, it would only come into play when you're coming out of droop. And I'd guess it would be adding about .3" of droop travel. That would put you closer to 2" of droop.

You mentioned that the bump stop is claimed to be "just a bump stop". I'd consider fitting a progressive bump stop in there. It sounds like you're set up to mostly rely on your spring rate and not really get much into the bump stop. But, if you fit a taller bump stop that's nice and progressive... you'll NEVER, EVER feel yourself hitting the bump stop. When you start getting to full tilt, the bump stop will start engaging and raising the effective spring rate gradually. So, if you end up with 2.5" of compression travel. Get a super-progressive bump stop (something that's a segmented cone shape off of a stock car that has a similarly sized shock shaft, you can usually find something cheap) that's maybe 2-2.25" total length.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Steve --
Forum Admin
Drives: whatever I can get my hands on
User avatar
Location:
St. Pete
Joined: November 2006
Posts: 8330
First Name: Steve
Last Name: --
Autox Car: whatever I can get my hands on
Location: St. Pete

Rear bang over bumps/railroad tracks

Postby Native » Wed Apr 04, 2018 7:01 pm

My head hurts, but this is great stuff.
Thanks for typing it all!
Steven Frank
Class M3 Miata
Proud disciple of the "Push Harder, Suck Less" School of Autocross
______________
I'll get to it. Eventually...
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Thu Apr 05, 2018 10:23 am

Loren wrote:But, what's the status at static height? Given the we don't know exactly what your unsprung weight is, and for simplicity, let's just say that your sprung corner weight is 600.
1. So for getting the unsprung weight, I know I need to minus the wheel and tire and I read you need to count part of suspension components like the control arms, axle, brake assembly. But I wonder if there is a way to calculate it on how much the car compresses at the corner to get the unsprung weight. Tires themselves per tirerack weight 27lbs each. Wheels approx 20lbs each.

Also from eibach, theres a small handful of different tender springs. First, they have linear and progressive springs. For linear the highest rated one it is 300#. It's 4 inches tall and can compress up to 2.25". Block load(im assuming is how much weight for the spring to fully compress) is 675#. So with some math just under static weight it'll fully compress. 300/.64 = 192. Just guessing unsprung weight is 600 so it'll compress a little over 3" at static so the linears tender selections go out the window since I would like something that would add a little comfort. Link for the progressive springs https://eibach.com/us/c-54-ers-tender-s ... ssive.html. Also let me know if I'm mathing right. Last thing I need to do is not math right and screw everything up :bangwall: .

So I think the option is to look into the progressive springs. From the list, I did the calculations for the 1300 spring listed in previous post above and using the "guess" unsprung weight of 600. The 1300# spring would act like a 832# spring. So that spring itself would compress itself at 0.72 inches. The max that spring could compress is 0.79 and it just needs 606lbs for it to compress fully. I don't think this spring would work since if there was any leaning surfaces like streets normally have it'll lean the car in a tad and uneven surfaces would probably shift the weight around and I think I would be on the verge of going in and out of the combined rate and into the main spring rate which probably would feel odd.

So the next spring to look at is this one https://eibach.com/us/i-1432-tender-spr ... ssive.html. Only thing about this is it has two spring rates. 600 / 1300. So I know initially it'll use the 600 lbs but I have no idea the transition of it from when it goes from 600-1300. It's 3.5" tall but I def have enough room for it so I should be fine there.

P.S getting tenders/bumptops adds up to a lot of monies. Did you have to put something like a coupler or something to stack your springs together?
Native wrote:My head hurts, but this is great stuff.
Thanks for typing it all!
My head still hurts from yesterday just trying to make sense of everything. It's just getting worse and worse the more I dig into the rabbit hole. But I did get some pretty damn good sleep last night. So maybe the mental exhaustion helped me there.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Thu Apr 05, 2018 3:34 pm

Loren wrote:But, what's the status at static height? Given the we don't know exactly what your unsprung weight is, and for simplicity, let's just say that your sprung corner weight is 600.
Are you asking about the static height about the 1300 spring? It should be 1.28" since it should compress 0.72" and its originally 2.0". Unless you're asking about something else.
Loren wrote:And you've got the same 600 on your 1300# tender, compressing it about a half inch. You said it compresses to .79. So, at static height, you have .29 x 1300 = 377 pounds of load before coil bind.
1. I'm kinda confused on the .29 x 1300 part. Where does the .29 come from.

2. Does the wheel compress the same amount as the spring or are they 2 different things?
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Thu Apr 05, 2018 4:00 pm

To the first part, I'm just playing hypothetical games, not really asking a question.

1. You said the spring has .79 of compression. We calculated that it's compressed "about" a half inch at static height. That means that it has another .29 of compression left before coil bind.

2. That's where your motion ratio comes in. If your MR is .5, that means that for every inch the suspension moves, the shock moves .5".

So, wheel movement x MR = shock movement.

Or... shock movement / MR = wheel movement.

You got an inch at the shock, in your case that's 1.0 / 0.64 = 1.56" at the wheel.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Thu Apr 05, 2018 4:41 pm

Thanks Loren, I owe you some booze(or iced tea, whatever you prefer...) next time I come out on monday.

Ok now #2 about the wheel movement and shock movement is making a lot more sense now. But I'm now confused of my original measurements. On my post for my measurements, I measured the static height (from the center of wheel/where the axle pokes through to the top of the fender) of 13 3/16". Then I did with wheel just touching the ground from jacking car up. Measuring from the same area it measured 14 5/8".

Then we got measurement of the droop to come out to approx 1.5"(14 5/8 - 13 3/16 = approx 1.5") We then got the measurement of the compression travel(from the shock stroke body) of 4 3/8". Then I minus the droop (which I got from the wheel) 1.5" and subtracted it from the workable stroke (approx) 4.5" which gave 3" compression.

Now the confusing part for me is wouldn't that be incorrect now since the droop was calculated from the wheel movement and the compression was done on the shock movement? If so then I could just math it which the droop for the shock end should be 0.94" by following the wheel movement x MR = shock movement (1.5 x 0.64 = 0.94")?
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Thu Apr 05, 2018 4:54 pm

Yeah, you need to convert all of your measurements to "at the shock" to be accurate. We may not have done that. I wasn't paying enough attention... and I'm so used to working with a McPherson strut front suspension that has a motion ratio of something like .97... I've just not been thinking about it. Easy to overlook.

But, yeah. Take any measurement you took at the wheel. Multiply by your motion ratio to get it at the shock. Redo all of your math with the terms relative to the shock.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Fri Apr 06, 2018 12:15 pm

So with the updated calculations my droop is approx 0.94"(approx 1 inch) and compression now is approx 3.5". So I lost about .5" of droop. Gives me more room to add more droop but not sure if needing more than 2" of droop is needed. But if a corner never compresses past 2.5" then maybe I should add more droop on top of the extra 1", not sure of the benefits.

I want to actually get the rear motion ratio of my car so everything can be more accurate. So I'll need to get protractor or something later to do that. Not sure if lowering the car and adding negative camber affects motion ratio so that's why I want to check myself. I will get more wheel travel now if I get a tender spring so I'll have to get my rear fender pulled since they're already rolled, unless I add height to the rear which I don't want to do.

So if I did use the 1300 tender, 2.0" length, 0.79" max compression, it would compress by 0.461" under static. The main spring compresses by 0.974". The spring combined will give it a new rate of 417.95lbs. Total compression of the 9" spring(using the 418lbs)) is 1.435" when the vehicle is static. Yay the math is all working out now! This was just with the guesstimate of minusing 50 pounds from the corner unsprung weight.

Formula for combined rate calculation:
(A spring * B spring) / (sum of A and B spring) = combined rate.

Motion ratio, shock and wheel travel make so much more sense now.

My main questions
1. Since the tender is compressing by .46" and has about .33" of compression left. Like how much cornering will I need to transfer the 350lbs to get the tender to fully compress. The tender spring needs 947 lbs to compress fully. So about half of the corner weight from the other side needs to shift in order for it to fully compress, about 350 lbs give or take some. Will it be hard cornering will it to transfer that amount. Or will like street driving turning at normalish speeds cause enough of the weight to transfer over.

2. What is the best amount of droop/compression travel. Should I try to get the compression to get close to the bump stop(not sure if I need to hit the bump stops since I have so much damn compression already and I can spare a nice chunk to droop) and have the rest of the travel for droop?
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Fri Apr 06, 2018 2:38 pm

Glad you're making sense of all this!

Your motion ratio is a factor of suspension design and geometry. There's probably a bit of a "curve", but for the most part, it doesn't matter that you lowered the car. Your motion ratio is still the same. (you may be in a slightly different part of the curve, but I'd bet it's fairly linear)

You can measure that without anything fancy. Support the car, pull the wheel. Measure the height of the wheel mating surface from the ground, and measure the installed length of your spring. Now put a jack under the hub and raise it up an inch at the wheel. (or 2 or 3" if you can... will give a more accurate result) Take the same measurements. That gives you the distance that the wheel moved and the distance that the spring moved. Do maths = motion ratio.

Answers:
1. My rough logic is that your G force is related to your weight transfer. If you're cornering at .5G, you've transferred half of the weight off of the inside wheel. So, if your corner weight is 600, then you'll transfer "about" 300 at .5G in a steady-state turn.

2. More droop travel = less inside wheel lift. On the back of a FWD car, that's not a big deal. But, all 4 of your wheels are driven, so I think you'd want to try your best to keep all 4 wheels on the ground as much as possible. So, as much droop travel as you can get without sacrificing anything else.

So, if you've got 1" of droop travel and 3" of compression travel... but, your springs are so stiff that you never see more than 2" of compression... you're "wasting" an inch of travel on compression that you could actually be making use of in droop.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Sun Apr 08, 2018 1:11 pm

Well I wanted to try and give the rear an extra inch of droop but couldn't. I didn't have enough room to extend the shock body by an inch. At most I was able to get 3/4th an inch and that's with only having about 3/4th still threaded in the height adjustment perch. The other side of the car is originally 1/4" higher due to corner balance. So I would only have been able to raise the shock body by 1/2" with 3/4th" still threaded into height adjustment perch. For me that means if I go over the extra 1.17"+ the original 1.5" wheel movement on one side of the rear, the coil will then get air born and then crash. The other end that got even less droop will only have an extra 0.78" before it flops around.

If I were able to have enough body of the shock in the lowering perch I would've been a lot more comfortable trying this. So I think my next step is actually getting a tender. The tender itself would compress about almost 1/2" both ends.

!!!!! Now that I think of it, the 1300 tenders only compressing by 1/2inch means I would only get an additional 1/2" of droop instead of the extra 1" I was at least looking for if I am thinking this through right... So I think I need to find a taller spring that'll compress more at static but still have a good dual rate of 400ish lbs.
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Sun Apr 08, 2018 2:32 pm

Taller spring. Yup.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Mon Apr 09, 2018 9:09 am

Welp, I'm having issues trying to find springs that are 2.5" to 3" tall that would have a high enough spring rate to try to keep the dual spring rate around high 300's to 400's. This spring https://eibach.com/us/i-1433-tender-spr ... ssive.html seemed perfect but it lacked the length. I don't see many short springs that have a high spring rate. I'm not used to looking at a bunch of springs so there probably are springs out there but I probably don't know all the ins and outs on what to google search. I could stack a helper spring onto that tender which would give me that additional droop since the helper would just collapse at static height. Problem is that'll just cost me a nice chunk more doing so.....

Edit, actually my springs may not be 2.5" in diameter, not sure what they're(just assumed it). Gonna contact the vendor on that. I don't feel like doing more measurements since I won't really have time to.

P.S Spoke to fortune auto and they said they use the 60mm springs.
Last edited by CaptainSquirts on Mon Apr 09, 2018 2:17 pm, edited 1 time in total.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Mon Apr 09, 2018 12:15 pm

Now that I think about it, I never really had complaints about daily driving except when I ran out of droop. I like the instant feedback it gives and if I did dual rate it I wouldn't have that instantaneous feedback until I get a little aggressive on cornering. I'm now thinking of going towards tender spring route that will compress fully at static but give me that additional droop I am looking for. The tender i am looking at will compress by 1.6"(fullly compressed) at a little over half the static weight. It will also cushion the landing whenever my main spring is fully extended. I don't need the extra 1.6" additional droop so I would probably preload it by about 1/3" which will give me approx 1 1/4" extra droop. A lot more simple going this route plus fixes my original issue.
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Mon Apr 09, 2018 12:20 pm

You could just fit a taller fixed rate spring. Your spring is just too short for the ride height and droop travel that you want.

When you say 600# springs to a light car guy, it sounds like a lot. But, given your motion ratio, you're only talking about wheel rates in the 300's. That's not crazy stiff at all. Linear rate spring should be fine.

I'd be looking for a spring of about the same rate that you have that's about 2" longer. Use that 2" for droop travel.

Fairly cheap solution (especially if you can find used springs) because it's only one pair of springs. And you're not adding complexity or points of additional noise or other problems.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Mon Apr 09, 2018 3:27 pm

Well can't really find 600#ish pound springs on ebay. So at the moment its either get a new pair of 9" springs(might go to 12k springs, seems like the hot setup is 10kfront which I have and 12k rear) or get the assist springs which come with shims and coupler.
The assist spring if I went that route. http://www.evasivemotorsports.com/mm5/m ... ST_SPRINGS

So options are atm.
1. Go with 9" 12k springs(should fix droop issues, little less street comfort but is supposed to be the better setup).
2. Go with the Assist springs. Might be adding additional noise but I dont think it'll be as bad as your fronts since the rear don't necessarily turn.
3. Go with 9" 11k springs.

***I just thought about something. Will getting a taller spring even make a difference in droop? Sure the spring is taller, it will still only compress the same amount as before since the same rate. So I think I would still at least need to run a helper spring if that's the case. Right?
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Tue Apr 10, 2018 12:52 am

Sometimes drawing a picture helps. I'm not gonna do it for you, but sometimes it helps. :lol:

I'd go with the 9" 11 or 12k springs. You probably won't notice much difference betwen the two. It's like an 8% change. Not gonna be night and day with regard to either handling or comfort.

Right now, you can't set your lower spring perch any lower because you don't have enough spring height. Taller spring means you can set the lower perch lower... and thus the suspension can droop that much more.

JUST lowering the perch right now would also lower the static ride height. So, imagine that you lowered the spring perch... how can you get the ride height back? TALLER SPRING!

Draw yourself a picture if it doesn't make sense. FIgure out how much compression and droop travel you'll have for each spring combination with proper height adjustments.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 10, 2018 8:56 am

I drew a picture.... bask in it's glory

So the spring on the left is one with a 9 inch spring, right is 7". I would have to lower the spring locking ring by 2" to fit new spring 9" with 0 preload. From doing this, I can't see how I would be able to get additional droop unless I am running negative preload like the last idea that was tried(but didnt have enough shock body to fit in lowering perch). Man, its just flying over my head :bangwall: .

p.s when I say droop at static I mean since I have 1" compression at static then at most I can get 1" droop.
Attachments
Spring.png
Spring.png (31.47 KiB) Viewed 2748 times
Loren Williams
Forum Admin
Drives: A Mirage
User avatar
Location:
Safety Harbor
Joined: December 2006
Posts: 16891
First Name: Loren
Last Name: Williams
Autox Car: A Mirage
Location: Safety Harbor

Rear bang over bumps/railroad tracks

Postby Loren » Tue Apr 10, 2018 11:24 am

Start with the known. That's your sketch on the right.

If you add 2" more spring with no other change, that raises the car up and increases compression travel. If you did that, you'd actually run out of shock shaft extension, you'd have 4.5" of compression travel and ZERO droop.

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.

If you lower the spring perch another 1.5". Now you're getting closer to the bump stop, which is fine. You've got WAY too much compression travel as it is, and you'll never use all of it. What we take from one end, we give to the other. So, we've added 1.5" of droop travel. And we have enough spring to keep the spring seated. (I think)

BUT... we've now lowered the car 1.5", which we didn't want. How do we get that back? Extend the shock body!

What your drawing needs is to be on graph paper with scaled marks so you can see exactly where you're placing all of your adjustments, where your bump stop is, etc, etc. Scale drawing would be much more helpful here than the rough sketch.

But, I think I maybe explained it well enough above. Set the spring perch and spring length to get the shock travel and preload that you want. Use the shock length to set ride height.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Daniel Dejon
FAST Elite
Drives: 2006 Lancer Evolution
User avatar
Joined: November 2015
Posts: 1022
First Name: Daniel
Last Name: Dejon
Autox Car: 2006 Lancer Evolution

Rear bang over bumps/railroad tracks

Postby CaptainSquirts » Tue Apr 10, 2018 12:32 pm

Loren wrote:If you add 2" more spring with no other change, that raises the car up and increases compression travel.
I'm going to take this one step at a time to understand it :lol:

So with no change I'm going to assume you mean the spring perch is where it was when it had the 7" spring. So the 9 inch spring will be compressed by 2"(this is with the wheel off and that end lifted off the ground). The 7" spring had the 4.5" compression and 0 droop as it was. Now the 9" spring still has 4.5" compression and 0 droop since 4.5" is the max compression I could have since total stroke travel is 4.5".

Does that sound right so far?

Return to “Car Set Up”

Who is online

Users browsing this forum: No registered users and 12 guests