why should i set pre load with shocks off car ?
 

why should i set pre load with shocks off car ? why cant i set it with them on ?

bushing and sway bar bind.

You set initial preload before install.

Then you can change it after its installed by counting thread turns.

Disconnected bottom shock mount when I put larger spring on rears and let damper centralize to preload a equal amount ,then bolted up to adjust final preload.
Worked out ok also.

I set mine on the car, but I install my coils with zero preload (note the position of the spring perch). Then I turn the perch until it rises whatever distance I need my preload to be.

Yes. Initial preload when NOT installed on the car.

Then adjust further while its installed.

But the initial setting (0mm - or whatever you choose) has to be done before install.

After it is installed, using a measurement to modify preload is not reliable.

Don't measure.
Use perch turns.

Tein moves 2mm per turn.
Ohlins or Bilstein is 1.5mm per turn.
etc.

So if you turn 1 full turn, you affected the spring by that amount.

Setting preload off of the car is honestly easier to do. You can work on a bench top or your garage floor. Plus it is easier to move the spring perch, count turns or measure with calipers. Check your instructions, my guess is they say to set before you mount to the car.

B serious and noodels helped me with my install and my coilovers are dialed in great.

Correct, which is what I did before putting the coils onto the car.

I guess I don’t see the issue with measuring. If I move my upper spring perch until it moves 15mm from where it started, how is it any worse than eyeballing 10 turns on my Ohlins to get 15mm of preload? In the end, the perch moved 15mm, no?

because you're only working on 1 side at a time.

Your bushings and swaybar are compressing the springs slightly...which is why we agreed that setting initial preload OFF the car is a must.

Example:
If you set your preload to 0 and then install the shocks...you find that the springs will "mysteriously" be compressed because of bushing bind. It will measure out shorter...as if there was additional preload on it.

Think of how hard it is to push down the rear suspension to get your stock shocks out. You have to like..sit on a pry bar. That's how strong the bushing bind is. Its going to compress the spring a little (assuming factory bushings and assuming you clocked them properly).

And also... even if you throw out the concept of bushing bind...if you affect just 1 side by adjusting preload or total shock length...the swaybar will affect the other side. Again, giving you a false reading.

You'll get mixed up pretty quickly.

As far as accuracy via perch turns goes...put a dot on the uppermost perch with a paint stick. Everytime that dot comes around...that's 1 turn.

There's ways around this so you can use measurment...but why? Perch turns is so COT dang easy, and it translates into a very reliable measurement that's independent of outside factors.

Its also faster and more efficient.

With measurements, you have to turn the perches to what you think is close to 15mm...then measure...then turn the perches again. And repeat.

With perch turns, you count AS you turn. No guessing on how much to turn to get 15mm is. Its 10 turns (with a 1.5mm pitch).

I think you're under the assumption that I'm measuring the spring length, I'm not. I'm measuring the distance between the upper perch and the locking ring beneath it so it wouldn't matter what the spring is doing up above. Let's say I wanted 17mm of preload. I can measure out and see 17mm in a jiffy, or turn it 11.333 turns. Either way works fine, but not everyone knows the thread pitch of their coils (I certainly didn't!). In fact, I don't even know what it is for my other car's HKS coils, but I can certainly measure the preload with a ruler.

Well...whichever way works for you. Measuring hard threads is more accurate than measuring spring lengths.

Thread pitch is often listed in the manual, though. In case you need to find it.

Measuring is error prone. There is parallax. There is angle you hold the tape at. There is miscounting hash marks. There is squinting...

Counting turns has none of these faults. And precision is less important than consistency. 11.33333 turns aren't necessary. 11 will do fine. But getting exactly 11 on each side is what counting turns does that measuring is less likely to achieve.

^Yes. Resolution is another advantage of turn-counting.

Any dummy can remember and obtain dot position accuracy within let's say 1/8 turn. Which translates into a measurment accuracy of +/-0.19 mm...or 0.007" on a 1.5mm thread pitch.

Even if you're within a 1/4 turn of dot position, that's pretty COT dang accurate.

This is outside the scope of this thread, but shouldn't the preload be adjusted from the base installed preload once installed to level out sag individually? I guess I'm curious why installed preload measurements are the commonly traded thing when a sag number (and thereby position in the damper travel / distance to bumpstop) is actually what you're aiming for and will vary based on each corner and individual car based on mods (changing corner weights). Unless this stuff somehow works differently than motorcycles or the installed preload specified by companies / people is just to put the average car 'in the ballpark', I guess.

Sure. For cars, a spring will sag a tiny bit after its been on the car a while. But not much...unless its a complete POS spring.

You still cannot set initial preload while the shock is installed.

A car has 800 bushings that all bind up and hold the shock/spring from fully extending. As well as sway bars. A motorcycle doesn't.

Removing the shocks from the chassis is still the best way to set initial (base) preload. Regardless if the spring is pre-sagged or not. You can ACCOUNT for sag by putting a few mm more preload on a brand new spring. But its a guess.

Cars don't settle much because the spring takes a set. Bushings not being clocked properly will cause a HUGE ride height delta, however. So when people talk about how their car took time to "settle" after a spring install...most of that settling comes from their bushings slowly tearing from being non-clocked.

Some springs (rare cases) take like a 1/4" set. But...honestly, my used Ohlins Eibachs probably went from 200mm to like 199mm after they took a set. The set is negligible AF in that case. Teins come pre-set. So do Swifts and some other brands.

There is a small issue with setting 0 preload as your initial. And that is that the rubber mount on top of the spring takes like a 1/8" (~2-3mm) set. I think that's why Ohlins and lots of other companies recommend a ~2mm preload.

Oh, yeah, there with you on the initial preload. The clocking on the bushings makes sense too. Sway bar is why you set your ride heights with the swaybar disconnected then use your adjustable endlinks.

When I say sag I mean extended length of the damper versus installed on the car with car weight, not actual spring sag.

Not sure I follow. What's the advantage/purpose of setting preload with the weight of the car on the ground?

Theoretically, it sets absolute position. The car weighs different weights at different corners, etc.

But...its hard to do (its easy on a motorcycle). And you can always adjust first and then measure free travel with the car on the ground later...and just touch it up.

Adjusting the preload is how you change where the damper sits in its stroke at rest, so you'd set your set your installed preload specified by the manufacturer / tuner before you install them, install them, get the car settled, then measure where you are in the stroke from fully extended (this is your sag number) to see if you need to add or remove preload to change where you are in the stroke.

edit: Your edit got what I was getting at

Yep. Determining the right amount of preload requires knowing where the bumpstop sits at static height. Its only one factor...but its necessary.

I hope nobody is blindly dialing in preload...lol

Setting preload based on 'sag' is a motorcycle specific thing, simply because final vehicle weight is so dependent on the rider. Yo fat a$$ as a huge impact on how much weight is applied to each 'shock'. Much more so than with a car.

With cars, my 750 lb corner weight isn't going to have that much different sag from your 800, or the other guys 700. So its much easier to just say with this coilover using this spring, start with this much oreload.

For all you non bikers out there, you set sag by putting a tie wrap on the fork tube, so that when you carefully sit on the bike, you have a high mark of how much the suspension compressed. You keep setting preload until the amount of sag is where you want it (there are rules of thumb that for this type of riding, use this much sag, etc). The goal of sag is to have appropriate range of bump travel at normal ride height. To balance compression travel vs rebound travel.

We do the same with cars, its just that stating a specific preload amount is a sufficient starting point.

I kinda figured it was due to not being that much different, but if you think about it, these things are measured in small mm increments. I guess after mathing it out a bit 50-60lbs on a 10kg/mm / 560lb/in spring is only about 3mm so a non-issue unless you're comparing a full weight 2900lb street car to something like a 2200lb track car.

Well...the S2000 has a pretty big MR.

Sprung weight is like 550ish LB per corner.

So a small amount of weight goes a long way.

And yeah, the best way to corner weight and preload is with driver ballast.

But...these are race car problems. For a street car whose weights will vary depending on passenger and luggage load...or for casual track cars...just keeping it simple works.

If you use a large tape measure, I can see it being troublesome. I have a tiny 6" machinist ruler that works great for tight spots. No need to squint either! Different stroke for different folks I suppose.

Unfortunately that wasn't the case for my HKS coils. The support at the time from HKS was poor and it didn't help that my coils were not popular at all. It's like trying to find info on a vintage typewriter.