Spring rates / ride height, and 50/50 weight distribution

This happened to me once before with my RX-7 and how it's happening again with my E46.

If the advertised weight distribution is 50/50 by the manufacturer, why are the spring rates different.  Years ago, with the RX-7, someone said "ah, but that big heavy engine is in the front" and I said "ah!  that it is"

But now I'm wondering.  Why does the spring care?  Weight is weight. 

Second question:  Regardless of spring rates, if the weight distribution is 50/50 why do aftermarket spring manufacturers change the ride height of the vehicle differently from front to rear?

For my E46, each aftermarket "sport" or "race" spring options seems to lower the front more than the rear.  Wouldn't this upset the 50/50 weight distribution (putting more weight on the front)?  Is "rake" just another item that's compromised for fuel efficiency vs performance, vs handling vs comfort vs it's just a passenger car dont overthink it?

Your thoughts appreciated.

Thanks guys
 

1) Are you asking why the spring rates aren't equal front/rear?

B)  Rake won't have a large effect on weight distribution.  Lowering the front more than the rear or vice versa will shift the CG fore/aft a very very very tiny amount.

Begin your research with "motion ratio" and go from there. 

You can have different spring rates but the same ride height front to back.  If spring A is twice as stiff as spring B, but they have the same mass sitting on top of them, and they are both sitting on the same level surface, AND spring B is twice as long (free length, i.e. unloaded) as Spring A, then with the same mass sitting on top of them, the masses will be at the same height.

That covers the statics.  Dynamically speaking, spring rate does not necessarily have to relate to weight distribution.  Stiffer front rates relative to the rear will, all things being equal, induce more understeer (desireable from most manufacturers' point of view).  Stiffening the rear rates will cause the handling to tend more towards oversteer.  Equal spring rates F/R may not necessarily give "neutral" handling, either.  There's a lot of other forces at play- roll center, alignment specs, how the anti-roll bars are set up, etc.  

As mentioned earlier, spring rate has nothing to do with ride height.  A manufacturer may have a stock rear ride height slightly higher than front, even with 50-50 weight distribution and equal spring rates (or not).  So aftermarket springs may adjust ride height to make the car ride more level.  If the car doesn't have a load-leveling suspension, manufacturers will often make a car ride higher in the back, to compensate for any trunk loading the consumer might induce.  

EDIT: I re-read and notice you said the aftermarket springs lower the front more- increasing rake.  This is different than what I'm accustomed to- usually people lower the car more in the rear to level it out.  I personally am not a fan of the "raked" look.  But again, if one plans on carrying any cargo in the trunk, there needs to be some suspension travel there to accommodate the added mass.  

The spring rates are different for two reasons: motion ratio and ride quality.  

Motion ratio is basically the leverage that the suspension imparts to the spring as it compresses.  The front of your car is a mac-strut, so there's not much lever arm there, but if you look at the rear suspension, there's several inches outboard of the spring to act as a lever arm to compress the spring.  Therefore, you need a stiffer spring.  The ratio of what the wheel sees vs what the actual rate is is called the motion ratio.  If you have separate springs and shocks, you can have different motion ratios for your shock and your spring.  

The second factor is related to ride quality and pitch.  In general, manufacturers aim to have the ride frequency about 10% higher in the rear than the front so that the car doesn't pitch back and forth when you hit a bump.  You hit a bump, the front goes up, then the rear goes up.  If the ride frequencies were the same, the front would drop down, then the rear would drop down and you could potentially get an oscillation back and forth if your shocks were a bit worn or underdamped.  However, if the rear drops faster, then your whole car will drop down vertically, and the bump will feel smoother.  Google "flat ride miata".  There's a Miata guy that talks about it like he invented it, but car oems have been using it for 50 years.  It doesn't mean he's wrong though, and I think its part of the reason that some coilovers ride so poorly--they've ignored the "flat ride" of the car.  

volvoclearinghouse said:

EDIT: I re-read and notice you said the aftermarket springs lower the front more- increasing rake.  This is different than what I'm accustomed to- usually people lower the car more in the rear to level it out.  I personally am not a fan of the "raked" look.    

Most cars I've seen on aftermarket springs, especially RWD cars, are raked like that. Hell, my BMWs are raked from the factory, not just in stance, but in bodylines. My E38 is raked pretty heavily in stock form and I love that look:

In reply to Chris_V :

Reminds me of this bullE36 M3:

Rothwem got it - motion ratio and ride frequency related to maintaining flat and not pitched ride.

volvoclearinghouse said:

In reply to Chris_V :

Reminds me of this bullE36 M3:

Lol! Well, it's not that, so get over it and let your intellect overcome your biases.

Another note - while rake won't affect front/rear weight distribution, it will affect weight transfer in a corner and so it will have an effect on handling. It also has an aerodynamic effect.

Chris_V said:

volvoclearinghouse said:

In reply to Chris_V :

Reminds me of this bullE36 M3:

Lol! Well, it's not that, so get over it and let your intellect overcome your biases.

You mean, my bias towards good handling cars that don't look like they were modified by a 17-year-old?  

Rake will increase effective area, increasing drag (though I will admit it may improve rear downforce, sort-of like a spoiler I think).  It'll also raise the rear CG, which will affect the CG axis of the car.  I think it might affect roll center/ roll axis, too.  

volvoclearinghouse said:.  I think it might affect roll center/ roll axis, too.  

Depends on the suspension design.

In reply to volvoclearinghouse :

Yeah. Your bias that says you're a closed minded **** that would rather be insulting than anything else.

I grew up on cars like this and my BMW that say a bit of rake is just fine.

the BMW BODY is raked by itself (look on my pictures for how the bodyline rises front to rear and the wheelwells sit compared ot the body), as well as the suspension, from the factory, If you want to call it badly modified by a teenager, so be it, but you'd be wrong, as usual.

rothwem said:

The spring rates are different for two reasons: motion ratio and ride quality.  

Motion ratio is basically the leverage that the suspension imparts to the spring as it compresses.  The front of your car is a mac-strut, so there's not much lever arm there, but if you look at the rear suspension, there's several inches outboard of the spring to act as a lever arm to compress the spring.  Therefore, you need a stiffer spring.  The ratio of what the wheel sees vs what the actual rate is is called the motion ratio.  If you have separate springs and shocks, you can have different motion ratios for your shock and your spring.  

The second factor is related to ride quality and pitch.  In general, manufacturers aim to have the ride frequency about 10% higher in the rear than the front so that the car doesn't pitch back and forth when you hit a bump.  You hit a bump, the front goes up, then the rear goes up.  If the ride frequencies were the same, the front would drop down, then the rear would drop down and you could potentially get an oscillation back and forth if your shocks were a bit worn or underdamped.  However, if the rear drops faster, then your whole car will drop down vertically, and the bump will feel smoother.  Google "flat ride miata".  There's a Miata guy that talks about it like he invented it, but car oems have been using it for 50 years.  It doesn't mean he's wrong though, and I think its part of the reason that some coilovers ride so poorly--they've ignored the "flat ride" of the car.  

That's awesome, thanks!

for giggles I googled the spring rates for H&R "Race springs".  Spot on 10% difference in rate.  330/360 f/r

I'll check out the flat ride miata

You'll also see that rake from the factory as they are designed to carry weight in the back of the car outside the wheelbase. Because of that, the OEM will typically give a little rake to the unloaded ride so that when you load it up with all of your crap in the trunk it levels out. 

I remember from the GRM Mustang spoiler episode they mentioned something similar to what volvoclearinghouse said.  i'm afraid I dont know enough about how rake will effect MY vehicle to know the benefits/drawbacks or if factory rake to compensate for load is a compromise or not.

I got out the measuring tape.  in stock (worn out?) form, my car's ride height measures 60cm from the bottom lip of the rim to 12-o-clock on the wheel arch (both front and rear).  Interesting...

A lot of BMW guys started running more rake in the E36 chassis in autocross, I'm not sure the science behind it, but it was a popular move when the car was in STX, so I imagine it's "good" for autocross handling dynamics. I sold my E36 before that happened so I don't have first person experience with it however.

As an aside regarding H&R race springs, the consensus (by people smarter than I) about ten years ago was that they were too short for their stiffness. Not sure if that's changed, but just wanted to pass along that tidbit. 

The effect of rake on handling will vary from one chassis to another.  All depends on what the suspension does at different ride heights, etc. 

rslifkin said:

The effect of rake on handling will vary from one chassis to another.  All depends on what the suspension does at different ride heights, etc. 

Looks like it's coilovers and skid pad time 

In reply to CyberEric :

They were just the first springs that popped up in my search.  I saw the ISC N1 coilovers on Bimmerworld had a pretty favorable following.  i was going between that and the Eibach pro kit.

Small adjustments in rake seem to make a pretty big difference on an E36 track car.  Too much or too little rake and they won't plant well in the brake zone.  That's a symptom of other things like the designed brake balance and the differences in the front and rear suspension.  It can wind up being a little fiddly, but it's one of the things that need to be addressed in the whole package of adjustments, or the car will never be right.  People usually set the rear higher by 3/4" measured at the front and rear jack pad inserts on the rocker panel and tweak from there.   If you need more rake, raise the rear instead of dropping the front.  If I get the front too low, the camber goes positive in compression.

rothwem said:

Google "flat ride miata".  There's a Miata guy that talks about it like he invented it, but car oems have been using it for 50 years.  It doesn't mean he's wrong though, and I think its part of the reason that some coilovers ride so poorly--they've ignored the "flat ride" of the car.  

I have one of his coilover sets on my car, they work very well.  Nice guy too.