Quote:
Originally Posted by XutvJet
Are people actually testing the springs on a bench or simply running calcs on the coil diameter and number?
Reason I ask is 185 in/lb for the fronts and 415 in/lb for the rears seems off for a 3,400-3,500 lb. RWD performance car. The front rate seems too low and the rear too high.
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The thing everyone is missing in this thread is motion ratio. The effectiveness of the spring is completely dependent on how far from the wheel it sits.
In the front, BMW uses a mcpherson strut with a true coilover design. The spring and strut travel about 93% the distance that the wheel does. Meaning, for every inch the wheel travel up or downthe spring only moves about .93". Meaning, a 130lb spring is actually only as effective as a 120lb spring.
In the rear, BMW is using a multi-link "HA-5" suspension. BMW has been using this design since the E8x/E9x generation (2006 or 2007+?). Small revisions have been made to the arms and geometry over the years (F series and now G series) but not much has changed whereas the E36 and E46 have significantly different rear-ends.
The rear uses a divorced spring. Meaning, the spring is mounted separately from the strut. The rear strut sits further from the spindle than the McPherson front strut. In the front, the strut is literally mounted IN the spindle which is why the motion ratio is close to 1:1 up front. In the rear, it sits at about .72? Meaning, for every inch the wheel travel the strut travels .72". Further, the spring sits even more in-board. The motion ratio of the spring is somewhere between .3-.4. Meaning the rear springs are effectively 1/3 as effective as their rating. A 400lb rear spring provides pretty much the same effective wheel rate as a 130lb spring up front. And like being pointed out by others, the general rule of thumb is to keep the rear spring rate 10% higher than the front. This maintains a "flat" ride as teh rear needs to react quicker to bumps than the front in order to keep the car from pitching and bouncing. It's a lot more complicated than "10%" but you can do your own research on flat ride.
This is all conceptual information I am providing based on fairly close numbers to reality. Just based off what I know about the E8x. The new F and G series rear multi-link will have very similar spring rates and motion ratios as I noted above. Street models will come in around 100/300 and performance variants usually run closer to 200/600 with stiffer sway bars. Effective spring frequency is like 1.1hz for a base model and 1.5-1.7hz for the more performance oriented models. There are countless part numbers for springs because of small package variations. The biggest differences though will be coupes vs sedans vs verts. Verts carry more weight over the rear wheels due to the roof so the rear spring rates will be higher than a coupe.
Like others are mentioning, BMW uses bump stops as "spring supporters." In the front, you should expect to find a 3" bump stop. In the rear, you will have less struts travel and therefore a slightly smaller bump stop closer to 2". Again, this is because of the rear struts lower motion ratio. The rear strut doesn't need as much travel to achieve the same rear wheel travel as the front. The bump stops themselves have effective spring rates close to 800lb+/in. Sway bars add another 100-300lb/in depending on the part number/package. Even the sway bar bushings themselves contribute to effective wheels rate and performance package cars do differ from base model cars in this regard. Reality is that most cars on the road have effective roll rates of over 1500lb/in+ regardless of the 100lb/in spring rate you are finding up front.
Lowering a car should generally be accompanied by cutting or replacing the bump stops. This is something Dinan is a good example to follow as they usually always do this with their lowering springs. Since stock suspension already has very little bump travel before touching the bump stop you can create a very jarring ride by reducing bump travel by an inch without cutting the bump stops back a bit to regain some of the lost bump travel.
hopefully this information helps basic understanding of why the stock rates are what they are and some of the other considerations to be made when messing suspension.