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Old 12-10-2018, 02:52 AM   #547
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Quote:
Originally Posted by remhex View Post
I ended up with 30 mm preload in the rear and went down to 24mm in the front. So far, I have mixed feelings about them compared to stock. They solved 2 problems and made another problem very sligthly worse. Once I'm not on greased marshmellow winter tires I'm sure I'll notice more of an improvement.
3cm preload on a 50Kgf/cm spring as yours will give you 150kg static force on the spring right?
So for the first 150Kg of load suspension remains uncompressed (ride height remains the same as corner weight is more than 150Kg), I thought preload keeps wheels on the road during extension not compression, during compression it adds load on outside wheels but I may be wrong though...
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Old 12-10-2018, 11:36 AM   #548
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Correct me if I am wrong:
with 5Kgf/cm rear spring and 3cm preload you have actual spring 5Kgf/cm X 20cm / 17cm = 5.88Kgf/cm X 0.75 rear motion ratio = 4.41Kgf/cm!
with 5Kgf/cm front spring and 2.4cm preload you have actual spring 5Kgf/cm X 20cm / 17.6cm = 5.68KGf/cm X 0.95 front motion ratio = 5.4Kgf/cm!
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Old 12-10-2018, 11:56 AM   #549
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Preload doesn't change the spring rate or wheel rate.

(Also you multiply spring rate times the square of the motion ratio to get wheel rate).

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Old 12-10-2018, 12:00 PM   #550
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I don't have any math for you. Preload increased and my ride height increased without changing shock mounting position. Bump travel increased. I'm not sure I could explain it better.
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Old 12-10-2018, 12:51 PM   #551
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I don't have any math for you. Preload increased and my ride height increased without changing shock mounting position. Bump travel increased. I'm not sure I could explain it better.
Springs are not perfect, the first little bit of compression is not as stiff as the rating of the spring.

By 'pre-loading' the spring you've changed the effective rate as the suspension becomes loaded. Say the first half inch of spring is actually an average of about 150# instead of the rated 300# when you first set the car down on that spring with 'zero pre-load' the spring will compress through that range to it's static position. By 'pre-loading' the spring when you first set the car down on the suspension the spring will be closer to it's working range from the moment it touches, it will 'act stiffer' as the car is set down meaning that there will be less compression on the spring as you've loaded the suspension, the car will sit higher and you will gain a touch of bump travel when the car begins working dynamically.

On any decent linear spring with reasonable amount of pre-load you're not changing the effectivity of the spring, by setting the car down you should be near the middle of the springs effective working area allowing the suspension to compress and extend in a predictable, controlled manner and you shouldn't significantly affect it's performance. There will likely be a difference but there's too many variables to tell if it will significantly affect performance, experimentation is key.

This is how single perch adjustment systems work, there's nothing wrong with it, it's been used effectively for decades at all levels of commercial consumer, amateur and professional motorsports.

Eibach, for example, rates their springs based on 20% compression to 70% compression. So for a spring with 4" of travel they do not expect the first 0.8" of travel to be up to spec, nor the last 1.2" giving you a nice working range of ~2"
https://eibach.com/us/p-101-suspension-worksheet.html
Quote:
Determining Spring Rate
All Eibach motorsport springs are tested between 20% and 70% of the spring’s total travel. This spring rate can be measured easily using the following steps:

30mm of preload sounds like a lot, but sometimes you have to work with what you have to get what you want although I'd be looking for alternate solutions.
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Old 12-10-2018, 02:02 PM   #552
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You got it right with experimentation being the key. I run the roads that I'm tuning for and make adjustments to fix problems. So far, I've got it handling shudder bumps and mid corner holes and dips with more stability. Now I'm working on making the frost heaves buck the rear a little less. I'm just getting the settings mostly close so I have the pre load right before I get it aligned. Once I go back to normal tires, I'll re-assess my settings.
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Old 12-10-2018, 02:06 PM   #553
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I also wanted to add that my ride height is pretty close to stock, maybe a smidge lower. As long as I'm not scraping anything or having negative handling effects, I don't really care about ride height as long as the car drives to my liking.
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Old 12-10-2018, 02:47 PM   #554
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That's why I rarely change preset coilover preload except by a tiny amount, what I have as preload in my mind is that makes a softer spring to act as a harder one without been harsh and when you change preload you should correct ride height if possible.
Too many factors contribute to final wheel rate to be sure if what you do is to the desired direction, at least for me!
For example Ohlins for MX5 has ony 2mm preload compared with the 20mm of ours but stiffer springs 7-4 although it's much lighter car (yes and has different motion ratio, higher rear!). May be is the reason Ohlins preset such high preload for us!


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I also wanted to add that my ride height is pretty close to stock, maybe a smidge lower. As long as I'm not scraping anything or having negative handling effects, I don't really care about ride height as long as the car drives to my liking.
I contacted Ohlins dealer and told me R&T still comes with 4F-3R Kgf/mm springs (you have to pay extra for the 5Kgf/mm springs except in cases with distributors as RCE) and initial spring height is 230mm NOT 200mm as with the old 6Kgf/mm kit, obviously due to softer spring rates!
What to believe, dealer, internet manual, or the manual it comes with?
I don't care also about ride height as long as the car drives fine especially with BRZ that has minimal weight transfer even stock!
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Old 12-10-2018, 05:42 PM   #555
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@remhex found this on another forum, it may help your calculations:

The force to compress a spring increases the more you compress it, check
out the link below and put some spring numbers in to see the difference
adding 30mm or so preload makes to force/rate at fitted length compared
to free length.

http://www.reliablespring.co.uk/calculator.htm

Many
coilover kits only have minimal preload (just enough to hold spring
from being loose) so the rate when fitted onto damper is pretty much as
the springs base/free length rating, there is then of course the
leverage ratio depending from where spring is mounted on suspension arms
etc to take into account to get static spring rates with car sat on its
wheels but that another story.

The MP20 kit uses a longer and lower base/free length
rate spring but with quite a bit of preload when fitted onto damper
which has the effect of bumping up the static spring rate from the
base/free length rate of spring, preloading doesnt change the free
length/base rate of a spring just puts it further into its travel which
means more force is required to compress it further, as the Ohlins
coilovers have seperate height adjustment for damper length this is used
to set ride height.

As you say preload is often just used to
raise/lower a car by lifting up the spring platform and car with it but all depends on
spring rate as to what it does, in the case of the MP20 kit it used to get firmer setup closer to the older 6kg spring kit but with improved ride of the softer springs.
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Old 12-10-2018, 08:01 PM   #556
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Thanks for the link. Interesting stuff. To be honest, I don't math or calculate anything. I write down my settings, test on the exact same portion of road, and adjust from there. It takes a while but thats how I do it.
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Old 12-11-2018, 02:11 AM   #557
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Preload doesn't change the spring rate. The only thing it does is to adjust the car height, because the spring compress less and the car sits higher. If you remove preload then the car sits lower. Ohlins has a separate way to adjust height, so the amount of preload should be fixed and what is recommended by the manufacturer. They just added enough preload to earn internal stroke length given the soft rate of the new springs.
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Old 12-11-2018, 04:11 AM   #558
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Does preload really adjusts car height, if under car mass load springs still get compressed past preload point? Imho preload just limited droop travel, no?
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Old 12-11-2018, 04:18 AM   #559
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Spring rate doesn't change, what changes is the initial force needed for the spring to compress making it behave like harder spring, when preload is added to the spring, it effectively shifts it's curve (for linear springs) to the left proportional to the amount of the preload force as in the following example:



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Old 12-11-2018, 04:34 AM   #560
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Both my front and rear preload settings are within manufacturer guidelines. I have the preload increased for bump travel, ride height is just kinda an afterthought. I'm going to try lower control arms to get a little more travel out of the rear since I need em for camber adjustment anyways. Eventually I might end up with stiffer springs as well. If none of that works, I'll start looking into different coilovers at least in the rear.
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