[TheTallJS]

So I got an email back from a dude who sells swifts saying the linear ones won't fit on the oem shock because the oem springs are progressive, I'm thinking because one end of the spring is wider than the other end and that smaller end also sits on a smaller spring seat on the shock and the wider end sits on a wide spring perch up top.

SO, I'm wondering if it's possible to put a different spring perch up top that matches the same width as the shock's bottom spring seat, and also fits with the oem top strut mount.

That's for the front struts. For the rear, I'm hoping that both the up and bottom spring seats/perches/whatevers are the same size (though I doubt it). Surely its not as difficult because other people have installed linear springs at the back no probs with different perches maybe.

Don't want new coilovers (yet), don't care how horrible the ride will be (yet), just want to swap to linear springs and keep as many oem parts as possible while making everything fit.

It really can't be that horribly difficult...

[justpete]

You sure that's a good idea with the existing shocks? I'm assuming you're not changing ride height?

[TheTallJS]

It might well be a bad idea, but I don't care. Yes would like to keep ride height about the same, prolly rear a little bit lower to level the car as someone said the oem rear sits a bit higher than front.

[justpete]

Originally Posted by TheTallJS

It might well be a bad idea, but I don't care. Yes would like to keep ride height about the same, prolly rear a little bit lower to level the car as someone said the oem rear sits a bit higher than front.

There's an adjustable perch installed on my car for the rear springs along with zero rate springs (helper springs I think they're called) to keep the springs from getting loose when totally unloaded. Not sure who makes them but I'm sure they can't be hard to find.

How are you going to choose the linear spring rates for the front and back? They'll need to be close to the linear range of the progressive springs if the shocks/struts are going to provide the correct damping. Could end up with a sucky ride and soon dead dampers otherwise.

[TheTallJS]

Orb has some interesting info in some older threads, and using maths came up with ~350 front ~750 rear, of course exact numbers need to be recalculated to get a better setup but I think an estimated good number is close enough for my purpose. Spring lengths would be about the same length compressed as oem compressed, which I don't know the numbers to and will need to measure. As long as the linear springs don't compress nor expand beyond oem, I don't see the reason why the oem dampers will die any sooner. If the linear springs are going to be more stiff, they shouldn't go beyond oem limits. And since these are old oem dampers, replacing them was expected anyway, only time being the difference. So that is why I want to do this, because there really is no downside aside for harsh ride and maybe figuring out how to do it.

[justpete]

Originally Posted by TheTallJS

Orb has some interesting info in some older threads, and using maths came up with ~350 front ~750 rear, of course exact numbers need to be recalculated to get a better setup but I think an estimated good number is close enough for my purpose. Spring lengths would be about the same length compressed as oem compressed, which I don't know the numbers to and will need to measure. As long as the linear springs don't compress nor expand beyond oem, I don't see the reason why the oem dampers will die any sooner. If the linear springs are going to be more stiff, they shouldn't go beyond oem limits. And since these are old oem dampers, replacing them was expected anyway, only time being the difference. So that is why I want to do this, because there really is no downside aside for harsh ride and maybe figuring out how to do it.

Damper life won't be affected all that much if the operating length is the same, no problem there. Get what you're trying to do, makes sense to me. Are you taking into account front and rear roll rate and roll axis height when looking at spring rates?

[TheTallJS]

True no maths was done to account for roll, and it is likely you do increase chance of rollover, but surely it won't be an instantaneous 50% higher chance of rollover with stiffer springs. The car was designed to handle fairly well and surely bmw did the maths and left some fudge safety factor into everything so idiots like me won't flip the car after new springs. And of course people should always be cautious when fiddling with suspension because it is very if not the most important part of the car and it is very easy to make the car dangerous. One argument might be that you could roll basically any car as you might have seen the crazies in the middle east driving on 2 wheels.

Also I've thought about m3 arms or even wheel spacers, but those would be after the springs.

Speaking of roll, I just found this neato little free program that can help calculate for all the maths if you want to.
http://www.suspensioncalculator.com/ (i'm not affiliated nor do I promote this thing, just saw it and think it's neato)