Coilover setup question

[CaptainSquirts]

So coilover has 1/8th of preload. Normally the springs are 7 inchs tall and preloaded so they're now 6 7/8 inchs(thats when the wheel isn't on and not on the ground). Now when it's on the ground it compresses to 6inches. Does that mean I will have 7/8 of an inch of maximum droop?

[Loren]

Yep, that's what it sounds like.

Well, disregarding the motion ratio of the suspension. But, strut suspension usually has close to 1:1 motion ratio.

Would be more accurate to measure something like a side pinch weld or front spoiler height. Get a static height, and then jack the car up until the tire just loses contact. The difference in height is your droop travel.

[CaptainSquirts]

If i was trying to measure the rear from the pinch weld, would it matter where I jack the car up from?

[Loren]

Doesn't matter where you jack from. Just where you're measuring from.

I mean, if you jack the right front and you're trying to measure the left rear... it might not be the most efficient operation.

[CaptainSquirts]

I mean, if you jack the right front and you're trying to measure the left rear... it might not be the most efficient operation.

How about compression, what's a good way to measure that?

[Loren]

You pretty much have to remove the spring to measure compression travel.

If you've got enough adjustment to lower your spring perch beyond the amount of compression travel that you have, you can do it that way. Otherwise, getting the spring out of the picture (and swaybar, too) is going to be required to be able to fully compress the suspension.

Alternately... if you know what the stroke of your shock is (I had the foresight to measure this before I installed mine)... if you know you've got 1" of droop and a 4" stroke... well, then you've got 3" of compression travel. Minus whatever your compressed bump stop is.

[CaptainSquirts]

Is it the spring itself that holds all the weight? For example if there was a spring that was 200#'s compared to 500. Would the 200 compress more than the 500 when just sitting on the ground?

[Loren]

Is it the spring itself that holds all the weight? For example if there was a spring that was 200#'s compared to 500. Would the 200 compress more than the 500 when just sitting on the ground?

Yes, that's what the rate is telling you.

Let's say your car weighs 3000 pounds and has a 60/40 weight distribution. You have 300# springs in the front. Your spring is pre-loaded, and you're just setting the weight down on the left front corner.

As soon as the tire touches down, the spring starts to compress. How much? 300 pounds per inch. So, as we put the full weight of the car onto the suspension (and the garage floor is perfectly level, and we've bounced the suspension to settle it), we've put 60% of 3000# onto the front suspension. Divide by two for one corner. That's 1800/2. 900# on the left front corner. 900 divided by our 300 spring rate means that we've compressed the spring by 3 inches. Science!

You can take that further. Let's say we're in a hard autocross sweeper pulling 1.0G. We've transferred "all" of the weight of the car onto the outside tires. So, (disregarding swaybars, and whether we're accelerating or braking, and everything else) we've now got an additional 900# on that front corner, and it's compressed another 3" from static.

So, if you started with a 7" tall spring that coil binds at 5"... your pre-load took up 1/4", and the weight of the car put down another 3"... 5" available spring travel less 3.25 = 1.75. And we're asking the body to roll another 3". That puts us solidly into coil bind.

As you can see... this is the kind of math we use to determine our minimum spring rates for the ride height we choose to run.

If you've lowered the car such that you only have 2" of compression travel, you really ought to have spring rates and lengths that will take that full body roll weight without going into coil bind. And if you're relying on progressive bump stops to limit travel, you need to be sure they're good enough to handle what you're asking of them.

2" of compression travel. 900# added to the front corner at 1G... you'd need spring rates of at least 450#. And your spring travel would need to be at least 4". So, a 6" spring (compressed length probably about 4", but you can look that stuff up, or calculate it from the thickness of the spring wire and the number of coils) probably wouldn't cut it. You'd want a 7 or 8" long spring of at least 450#.

And, of course, swaybar rates figure into that. If you've got a beefy front swaybar that adds 150# of rate, then you could run a softer spring.

And motion ratio, as mentioned before, can also come into play. With a typical sports car double-wishbone suspension, motion ratio might be on the order of .7 or .75. That means that the leverage on the spring makes its effective rate 70% of the actual rate. So, a 400# spring on a Miata might only be a 300# wheel rate... whereas on a McStrut suspension, 400# is 400#.

Further and further down the rabbit hole we go.

[CaptainSquirts]

Feels like more I ask about suspension stuff, I get an understanding of bits and piece but still not the "whole whole" picture yet. Its like a freaking 10000 piece puzzle and I'm putting piece here and there and is forming group and can start to make out more and more of the big picture . Any who, so that's why people say get higher spring rate springs if you get more grippy tires, makes sense.

[CaptainSquirts]

So what happens with people running stock suspension rocking re-71's. Would the vehicle roll enough for the springs to fully bind then since the spring rates are usually pretty low or is there just that much stroke available on struts?

[Loren]

Unlikely that a stock suspension would be designed such that it allowed coil bind to happen. But, stock cars regularly DO run out of suspension travel and get into their bump stops. In fact, I'd say that most modern cars are designed to corner ON the bump stops.

Modern bump stops have become sort of "progressive springs". As you get into the progressive bump stop in the last 2-4 inches of compression, it gets stiffer and stiffer as it compresses until it's like 1000 or 1500 lbs/in or more. Which is a lot! But, being progressive, it's way better than just abruptly running out of travel and instantly going from 300# to "infinity".

If you run out of travel in the front before the rear, you get severe instant understeer. If it happens in the rear first, snap oversteer. But, easing into a progressive bump stop mitigates that somewhat.

[CaptainSquirts]

A little change in topic but still has to do with coilovers and the spring so I'll just ask here.

So I currently have an 8 inch 10k spring on my fronts. They will fully coil bind at 2856 lbs(on that corner). I have a set of 11k springs but are 7 inches tall and they will fully compress at 2648 pounds. Can I run the 11k springs without running into issues with the spring being at full coilbind?

My corner weights(me in it) are...
Front Driver - 1069
Front passenger - 1030
Rear Driver - 683
Rear Passenger - 646

[Loren]

Total weight of the car is 3428 (useless observation: You have more weight on your front tires than the weight of my whole car).

Obviously, you'd be okay in a steady-state turn. 1069+1030 = 2099. Even if you pegged like 1.2G, it would still only be about 2520.

And it's impossible to have the tires fully loaded both laterally and longitudinally at the same time. (performance driving 101) But, let's say you somehow could do .8G laterally while also braking at .8G. (this would be like for a brief moment while trail-braking, if at all... unless you're Philip, of course)

Total front weight of 2099 x .8 = 1680
Total rear weight of 1329 x .8 = 1063

Total weight on the loaded front corner would be 2743

I'm just throwing numbers around. I really have never researched how much a tire can handle braking WHILE turning. But, I'd say that if you're not bottoming out now, you'll probably be okay. That's what bump stops are for, right?

And that's something you need to consider. If your setup is correct, the last inch or so of travel should be into a progressive bump stop and the effective spring rate is going to ramp up. With that, you should definitely be okay.

[CaptainSquirts]

I really have never researched how much a tire can handle braking WHILE turning.

In those scenarios where I screw up and end up turning the wheel and braking at the same time (I know, a no no), I understeer , HARD. Trail braking seems fine whenever I do it(mostly on the turn around), feels really composed. Doesn't feel like I have a ton of body roll from the side since I'm braking hard, turning the steering wheel very little by little and release the braking slowly as the more I turn.

Sucks about my "bumpstop" is it's just a piece of hard rubber just so the shock itself doesn't bottom out.

Regardless, sounds like it should be fine.
Thanks

[Loren]

Bear in mind that the symptom of bottoming out the front suspension is essentially "snap understeer".

You did video on the rear suspension. Maybe do the same on the front to see what your spring is doing?

[CaptainSquirts]

Posting here maybe due to coilover setup, prob not but anywho.

So new rear diff installed which has more locking. My rear sway bar is disconnected currently due to stripped threads so the rear sway isn't connected. But when accelerating out of a corner I noticed the rear corner that had the load was pogoing stupid hard, like riding a mechanical bull or something. I did have a little during autocross and was told to lower rebound a little which did seem to eliminate it. But would more locking of the rear diff cause this or the disconnected rear sway bar. I'll have the sway bar connected back in a couple of days but just wanting to see some opinions on this.