Lets say I have 8" 900 lbs springs on the front and I'm going to replace them with 6" 800 lbs springs, how much do I move the perch to keep the ride height exactly the same? I know its not 2" because we have to add in the 100 pounds softer on spring into the equation.

We can just say the car weights 2600 lbs and is even all the way around. Granted we all know it's not even.

Thanks!


Thinking 800F/600R or maybe 800F/550R on my STR car, input? Current is 900F/800R but the car seems like it's slightly over sprung for what surfaces I normally run on and I'd like to settle down the rear some more.


also how much should I move up the rear perch if going from a 6" 800 lbs spring to a 6" 600 lbs spring?

 

We just did the opposite on the Bsp car and went up 200 lbs all around and had to lower the perches an additional.. 5-.75 in order to get approx the same height. I would say in your case probably adjust the perch about 1.75 and see where you land. BTW our car was almost perfect cross weights with driver and almost perfect front/rear split

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2.13" should theoretically get you the same.

2" for spring length. 0.13" for the spring rate based on 650# corner weight and .7 motion ratio.

 

^I might regret asking this but how did you calculate the change based on MR and the extra compression of the softer spring?

 

Lol no problem, it's pretty simple.

650# corner weight divided by .7 motion ratio puts 928# on the spring.

928/900 = old spring compressed 1.03"

928/800 = new spring compressed 1.16"

Difference of .13"

 

How is the .7 motion ratio value determined? Is the motion ratio the same on the front and rear suspension on the S2000?

Answered my own question: https://www.s2ki.com/s2000/topic/380...tios-of-the-s/

Thanks for your explanation.

 

Damn this thread. Now I am curious how one factors tire size into the equation? I understand experimentally why one would choose higher spring rate (and therefore higher wheel rate) in the front when moving to 255 tires all around, but can someone explain in theoretically? I guess in the end it sort of comes down to both methods anyway huh? Engineers suggest a spring rate they have determined to be suitable, it is tested and changed accordingly.

 

Ah, of course. I forgot the whole metric of spring rate is determining how much the spring compresses with weight. Thanks.

 

Tire size is not factored in - the motion ratio is to the center of the contact patch. No matter how wide your tires are the centerline is in the same place, assuming the same wheel width/offset.

Unfortunately because of how offset is measured if you have a different width wheel/tire with the same offset, it actually moves the centerline of the wheel slightly. so I guess technically width does matter. but not actually because of the width change itself, just because the way offset is measured it's dependent on the width.

Anyway this is really just mental masturbation - in practice offset and width only change the motion ratio by a very small amount so most people ignore it. The reason you put a higher spring rate up front if you have wider tires up front is not because of motion ratios, it's because of something completely different (the fact that you can now load the front tires more).

 

+1. Move it up 1/8" and see where that puts you.