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.