Question about my suspension

[skyshack]

I really don't get this 8/6 Kg/mm stuff. I especially don't get why the front always so hard.

The stock spring is front 1.6 Kg/mm and rear 2 Kg/mm, the front is softer than the rear.

(assuming 300Kg at each corner)

Front spring free length 360mm, installed length 230mm, stroke 160mm.
Preload = (360 -230)*1.6 = 208 Kg
Static compression = 300/1.6 = 187.5mm.
Drop/rebound stroke = installed length - (free length - static compression) = 230 - (360 - 187.5) = 57.5mm.
Bump stroke = stroke - static stroke = 160 - 57.5mm = 102.5mm
Fully compressed length = 230-160 = 70mm.
Full bump load = (360 -121)*1.6 = 464 Kg.
Due to spring wire diameter and number of coils springs become coil bound at 63.5mm so it has 6.5mm clearance..

Rear spring free length 367.5mm, installed length 305mm, stroke 155mm.
Preload = (367.5 -305)*2 = 125 Kg
Static compression = 300/2 = 150mm.
Drop/rebound stroke = 305 - (367.5 - 150) = 87.5mm.
Bump stroke = 67.5mm
Fully compressed length = 305-155 = 150mm.
Full bump load = (367.5 -150)*2 = 435 Kg.

In 200K miles I have never known it top out except for deep pot holes and possibly when I took off on a switchback road and blew a shock. This means those fully compressed loads are about as much as you can expect on rough back lanes. Lowered and harder suspension limits use of car to nice smooth tracks where bump loads will be lower.

So you drop the car 25mm and now you have bump stroke of 77.5mm front and 42.5mm rear.
Front 8 Kg/mm full bump load = 300 kg static + 77.5 * 8 = 920 Kg.
Rear 6 Kg/mm full bump load = 300 Kg static + 42.5 * 6 = 555 Kg.

Front 6 Kg/mm full bump load = 300 kg static + 77.5 * 6 = 765 Kg.
Rear 4 Kg/mm full bump load = 300 Kg static + 42.5 * 4 = 470 Kg.

Front 4 Kg/mm full bump load = 300 kg static + 77.5 * 6 = 610 Kg.

To obtain the same full bump load with 25mmm drop it needs front 2.1 Kg/mm and rear 3.2 Kg/mm.

But with 25mm less ground clearance you may need a bit more than that.

If you want to control roll fit a bigger ARB!

[Jonny Wilkinson]

OEM for an S13, the front is 2.2kg and the rear is 1.8 kg.

I always assumed it was because the front is heavier than the rear as it has an engine sitting over it.

You're right to a certain extent about controlling roll using an ARB but there is only so much to be gained by shifting the weight to the opposite side.

As standard, the S13 is underdamped for a sports car. It was aimed at the Exec Coupe market though so that's not a criticism of Nissan.

I've found 4/3 springs with uprated ARBs and more chassis stiffness through triangulation using strut braces gives a superb seat of the pants drive that is all-day-long comfy and won't ping your granny against the roof lining over high-speed yumps.

[skyshack]

OEM for an S13, the front is 2.2kg and the rear is 1.8 kg.

You must be reading a different FSM to mine. JDM RPS13 180SX? 240SX?

UK FSM
Front


Rear

[zeppelin101]

The rear is stiffer than the front for OEM to control ride frequency between the two, i.e. when you hit a bump the rear "catches up" with the front so it produces a more comfortable ride.

It's more desirable to have it the other way round for driving on track because it makes it weight transitions under acceleration and braking easier to manage where controlling ride comfort is not important.

Correction - if you want to control roll then re-jig the suspension geometry for better roll centre control.

The stuff on the aftermarket imo is not designed with road bias in mind when it comes to adjustable coilovers.

[Carta]

6/5 or 5/4 then for road you think guys?

[piman2k]

You'd never notice the difference to be honest. But if you're road going, then go lower on the spring and set the dampers to soft.

[autosri]

Id go 5/4 for road