Ohlins road & track vs Bilstein B12.

Bimmer-Bob · 12-29-2014, 12:49 AM

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Originally Posted by fe1rx

Yes, I do. There is a short answer, and this isn't it.

The damnable thing about these units is the difference between mass and force (aka weight), which we use interchangeably, usually without appreciating the difference.

Mass:

1 kg = 2.204 lbm (where to avoid confusion, lbm means pounds mass)

Force = Mass x Acceleration. In this context Acceleration is acceleration due to gravity, i.e. "1 g"

A = 1 g = 9.81 m/s^2 = 32.2 ft/s^2

In common practice 1 lbm "weighs" 1 lbf (pounds force), which is to say 1 lbm exerts a downward force on a scale of 1 lbf.

In metric, the unit for Force is Newton (N) and,

F = M x A = 1 kg x 9.81 m/s^2 = 9.81 kg.m/s^2 = 9.81 N

Now 1 kg is 2.204 lbm, so:

1 lbf = (9.81 kg.m/s^2)/(2.204) = 4.451 kg.m/s^2 = 4.451 N

There are 25.4 mm in an inch so:

1 lbf/in = 4.451 N/in = (4.451 N/in) / (25.4 mm/in) = 0.1752 N/mm

Therefore,

60 N/mm = (60 / 0.1752) lbf/in = 342 lbf/in

To add to the confusion, sometimes metric units are bastardized and springs are expressed in terms of kg/mm. Because 9.81 is very close to 10, a 60 N/mm spring is sometimes called a 6 kg/mm spring (but it isn't really).

Sorry for the long answer, but a 400 lbf/in spring is actually a 70 N/mm spring, so going to 60 N/mm would actually be a bit softer than your current 400 lbf/in spring.

Thanks again.

I was getting my conversions from an online calculator, of course. Anyway, long story short: it sounds like my #392 7" springs would work. Is it worth it to use helper springs in order to get the perches lower and minimize preload?

fe1rx · 12-29-2014, 07:46 AM

Quote:

Originally Posted by Bimmer-Bob

Thanks again.

I was getting my conversions from an online calculator, of course. Anyway, long story short: it sounds like my #392 7" springs would work. Is it worth it to use helper springs in order to get the perches lower and minimize preload?

Helper springs use up some of the strut's stroke allowing you to move the bump stop engagement point to a lower point in the main spring compression, which could solve any coil bind issue on a short main spring. They also give you essentially zero main spring preload as you have noted. I have discounted them as an option for our cars because we are challenged for wheel to spring clearance, which is made worse with a taller spring, or a stack which includes a helper spring. I am trying to get my lower spring perch as high as possible to maximize tire clearance, while still maintaining adequate travel and proper bump stop engagement. If you have the clearance though, a helper spring is not a bad way to go. The helper spring "preload" should not be set arbitrarily though. Any extension set into the helper spring at full droop reduces the effective suspension travel because when the helper spring is extended in use, it is functionally the same as picking that tire off the ground. So set your helper springs as short as possible (i.e. maximize helper spring "preload") while ensuring that your bump stop will bottom out before your main spring.

Of course reducing overall suspension travel with a helper spring could solve a rubbing issue potentially, but from a performance point of view maximum possible suspension travel while staying off the bump stops at maximum steady state cornering conditions is optimum.

PeteA · 12-29-2014, 05:33 PM

Should be plenty of clearance on the 7" spring.

Wheels are ET45 8.5" and a 235/40/18 RS3 tyre on the front with M3 control arms and vorshlags. No spacers.

Bullitt · 01-12-2015, 03:30 PM

^ PeteA, is that with zero preload in the front? With the 7" spring, there still needs to be some preload in order for the spring perch to clear the tires. Harold installed my Ohlins and said there was a lot of preload needed in there for the fronts.

Quote:

Originally Posted by fe1rx

...

2) The Ohlins front struts have internal bump stops that you can't see; however they have a stroke of 89 mm to the bump stop and 109 mm until the bump stop is fully compressed. Swift 7" x 60 N/mm (p/n 65-178-060) springs have a maximum usable stroke of 106 mm and an absolute maximum stroke of 118 mm before they go coil bound. That means that these springs should be installed with minimal preload to prevent them being over-compressed when hard on the bump stop. With 12 mm preload on the springs, they will just go coil bound when the bump stop bottoms out. This is the absolute maximum preload you should consider, but really is too much. Better would be somewhere between 0 and 6 mm. This means that you have very little ability to adjust the front ride height with these springs if you want to keep them properly in the working range. They are the absolute shortest 60 N/mm spring you can use with this strut. The Ohlins 200 mm 60 N/mm spring, or a Swift 8" spring will give you more adjustability, but at the expense of tire to spring clearance. This issue is exactly why it is better to have a strut that adjusts preload and ride height separately (as the 1M/M3 Ohlins front strut does). Unfortunately they don't work with the 135i steering knuckle.

There seems to be a lot more than 12mm preload in my install. If I remember correctly, there's about 0.5" to 0.75" of preload when I eyed the installed spring length. I'll have to go back in there and measure it more carefully with a ruler. Any less preload with 7" springs will result in a slammed car look in the front. As is, the spring perch barely clears my 255/35 tires.

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