https://bmw.spoolstreet.com/threads...racing-adjustable-rear-camber-arm.3744/unread
From this thread
Now that I have my suspension set as high as it can go my springs have 0 pre load but are seated at full droop.
Another important question is how much travel from static height is recommended on our platform? I have front m3 arms I don't think that alters much besides caster and camber though.
So at full droop your spring is seated on both the upper and lower perch but not "preloaded" any? If so that is good. (springs being seated on both perches at full droop is actually a DOT/TUV requirement for OEMs)
How much recommended travel depends on the design of the suspension kit more than the chassis platform. The problem is there is no way to answer the question of how much travel you need. There are too many factors at play.
Person X could tell you they never bottom their 6" spring. Person Y could tell you they bottom their 6" spring all the time. It depends on damper valving, spring design, bump stop design, the stroke position of the damper at ride height, what kinds of bumps you are hitting on the road, weight of the car, list goes on.
I will say a general rule of thumb that I like to use is 1.5" to 2" of bump travel on the street. Again, it depends on so many factors. For example, if you live in a city or where roads are bad you would want 2"+ travel. If you live where roads are nice and not torn up you likely could get away with 1.5" of travel. If you have adjustable performance dampers set to "stiff" you could get away with 1.5" of travel. If you have softer dampers you might want to allow for 2"+ of travel.
In general try to fit as tall of a spring as possible. If you can't, set it up to that a bump stop is actuated before the spring binds. You can trim and stack bump stops to accomplish this.
I'll toss out some random things to note...
Obviously you never want the spring to coil bind or the damper to bottom out (assuming no bump stops). That means your wheel rate goes infinite and you loose traction instantly.
A cylindrical "race" style linear spring is not linear from free height to coil bind. Its linear from free height to roughly 80% of its travel then the rate starts to ramp up quickly the last 20% or so. (differs from spring to spring of course)
The BMW MacPherson strut has close to a 1:1 wheel rate vs spring rate (depends on KPI). Because of this a cylindrical "race" spring in the 250 lb/in to 500 lb/in spring rate range is difficult to setup. They are soft enough to allow lots of suspension movement over bumps but they don't have a lot of travel to coil bind. This is why you see a lot of aftermarket suppliers moving to custom wound barrel springs or tapered springs. Basically they allow a lot of spring travel.
The stock suspension actually relies on the bump stop as part of the dynamics of the suspension. The OEM bump stop has multiple rates (slowly progressive for lack of a better term). It is very soft for the first inch or even two inches, then the force ramps up quickly to prevent bottoming the shock. This way the manufacturer can run a soft spring for ride comfort then use the bump stop for roll resistance. They use tall bump stops so they are actuated as soon as the suspension compresses.
An aftermarket suspension kit typically does not rely on the bump stop as part of the dynamics. They typically use a very small and very stiff bump stop. They are usually progressive as well but much more stiff than a stock bump stop (quickly progressive) and are not used in the same way. They are mostly to prevent an instantaneous spring rate and to prevent damper damage but not to assist in roll resistance.
