Spring rates...can manufacturers be wrong?

[mattmayhem]

Hey guys. Ill be honest right away. This question is in relation to a 2003 nissan sentra spec-v and not a BMW. For the record I did have an E30!
Anyways I'm turning to you guys because the average spec-v owner is more concerned about "slamming" their car or trying to fit 20 inch rims so I can't get a respectable answer or discussion going on the forums dedicated to this car.
Anyways my dilemma is in regards to the "proper" front/rear spring rate ratio on my car. I know the more cultured automotive guys here can probably help me.
Basic sentra info:
Weight: 62% front 38% rear at about ~ 2900 LBS
Suspension design: front is macpherson strut and rear is a solid beam with scott russel linkage which reacts very slowly.
Now the stock spring rates and the aftermarket lowering rates based on those don't make sense to me.
Front is 154 lbs and the rear is 247 lbs
To me it doesnt make sense that a front heavy FWD car should have a stiffer rear than front. Does that make sense?
The "gurus" and autocrossers on our forums tell me that its because the rear needs to be stiffer to counter understeer....that makes sense assuming 50/50 weight distribution but the rear on my car is so light that a "softer" spring rate is still technically stiffer.
To make things worse on my brain the various coilover kits avalible for the car come with rates that are stiffer in the front EX: ground control kit is 375 front and 300 rear and JIC is something like 380/280.
I know some of this has to do with driver preference but at a certain point something has to be "right".
So is nissan and the autocross ricers know what they are doing?
For reference I use the car for daily driving and navigational rallies so I want to do something stiffer than stock but at stock height so I will probably be using a GC setup on my koni shocks. I was thinking around 275 front and 225 rear but maybe Im wrong.

[Turbo_Bimmer]

FWIW,
I don't know how the rear suspension on your car is made, but if the rear springs are not of a coilover design and the springs are more inboard (like your previous E30), the spring rate needs to be higher to compensate for the lever effect of the rear suspension.

Why the coilover kits have a lower settings than stock? May the their kit removes the stock spring from the OEM location, and a coilover setup is used at the rear, so the spring is at the wheel and needs a lower spring rate.
???

[mattmayhem]

The rear suspension is a coilover design. actually it looks sort of like an E30 but with a spring on it. Very very limited travel.
The coilovers aren't softer than stock but they changed the ratio of front/rear stiffness.
Stock springs are front 154 and rear 247
ground controls are front 375 and rear 300 so the ratio is reversed.

Hope that makes more sense...

[mattmayhem]

This is how the rear is setup:
http://i11.photobucket.com/albums/a160/sleeper33/rearsusp.gif

and front while were at it
http://i86.photobucket.com/albums/k100/saint0421/FSU.jpg

[Turbo_Bimmer]

Umm, it's hard to tell from the drawing, but if the rear springs are progressive, like most OEM spings are, you will have to find if the 247 is for the softer part of the spring (coils closer to each other) or for the stiffer part.
If not, may be GC made a mistake in the listing. You should call them to verify.




The rear suspension is a coilover design. actually it looks sort of like an E30 but with a spring on it. Very very limited travel.
The coilovers aren't softer than stock but they changed the ratio of front/rear stiffness.
Stock springs are front 154 and rear 247
ground controls are front 375 and rear 300 so the ratio is reversed.

Hope that makes more sense...

[mattmayhem]

Umm, it's hard to tell from the drawing, but if the rear springs are progressive, like most OEM spings are, you will have to find if the 247 is for the softer part of the spring (coils closer to each other) or for the stiffer part.
If not, may be GC made a mistake in the listing. You should call them to verify.
Michel,
Im not sure but I think the stock rates given are the max AFAIK. I know eibach makes the pro kit springs which are progressive and the max rates are about 180 front and 300 rear.
Ksport coilovers are what I had on the car before and like the GC they were linear springs I believe 391 front and 280 rear.
That setup was fairly good and I did not have problems with understeer....
You can see that the big differences in setup have confused me! Some of the autocrossers are running 500 front and 550 rear on the ground controls to get the rear loose to make it around the tight tracks.


I found this place called coil spring specialties and they said they can make custom springs for around 170$ for each end of the car (could save alot of money...also quebec winters wont hurt spring/shock combos like they will winters). If I use the GC example I can raise my front rate to around 275 to eliminate camber change under cornering then theoretically have a better setup at 275 front and 247 rear but again I think there is alot that I might not be considering like wheel rates and motion ratios....

[Andrei]

The rear suspension is a coilover design. actually it looks sort of like an E30 but with a spring on it. Very very limited travel.

Bingo!

I bet the range of suspension travel in front is much higher. So they keep the engine bobbing up and down more and make the shock absorber do the work of stopping that. Makes it more comfortable. Makes sense for the street.

But if you don't want the car to lean/dive too much under weight transition you have to bump up the spring rates in front much more than you need in the back. This is why the GC setup is like that.

[mattmayhem]

Bingo!

I bet the range of suspension travel in front is much higher. So they keep the engine bobbing up and down more and make the shock absorber do the work of stopping that. Makes it more comfortable. Makes sense for the street.

But if you don't want the car to lean/dive too much under weight transition you have to bump up the spring rates in front much more than you need in the back. This is why the GC setup is like that.


Ah ha! Thats the kind of explanation I was looking for... so basically if you stiffen the front you are increasing the resistance to camber change to hold a better contact patch which will increase handling without increasing understeer?
I did some reading and I think people don't understand how spring rate effects the balance of a car. On street cars the spring rates are so soft that you aren't really in a territory of drastically effecting balance with spring rate change as much as a track car that is very well balanced.

I guess with an econobox any improvement in spring rate should be an advantage.

I think I will have that custom company make me some stiffer front springs to keep stock height and won't increase the rear which is already high.

[Andre]

Keep in mind also, some autocross setups for fwd, will have equal or higher spring rates in the rear to help the car rotate. This can be very helpful in auto-x but you don't necessarily want your daily-driver set up on a knife edge...

This being said, I had 250 lb/in springs front and rear on my Acura EL with Koni adjustables and it was a great setup. Very easy to drive, very forgiving and a lot of fun. In short, the ultimate compromise, it was good on the street and acceptable on the track as well and soo soooo much better than stock.

I'd say, talk to the guys at GC, explain to them what you want, they'll probably point you in the right direction and then you can always fine-tune your balance with a sway-bar.

-a-

[mattmayhem]

Keep in mind also, some autocross setups for fwd, will have equal or higher spring rates in the rear to help the car rotate. This can be very helpful in auto-x but you don't necessarily want your daily-driver set up on a knife edge...

This being said, I had 250 lb/in springs front and rear on my Acura EL with Koni adjustables and it was a great setup. Very easy to drive, very forgiving and a lot of fun. In short, the ultimate compromise, it was good on the street and acceptable on the track as well and soo soooo much better than stock.

I'd say, talk to the guys at GC, explain to them what you want, they'll probably point you in the right direction and then you can always fine-tune your balance with a sway-bar.

-a-

I understand I guess like you said they (autocrossers) will be fine tunring for a specific purpose.
Im glad you have something similar to what I want andre, does that setup work well in the winter aswell? I used to enjoy going low and very stiff but that doesnt make sense for back road navigational rallies nor does it make sense in the winter.
I spoke with the reps at ground control and they seem unwilling/unable to help they said they wouldint provide advice beyong their stock setup which is 375 front and 300 back on a 6-7 inch spring.
I think something like 250 front and 225 rear on a longer 10 inch spring (maybe 12 inch if i can clear it) would work well for me.

[Emre]

Suspension design: front is macpherson strut and rear is a solid beam with scott russel linkage which reacts very slowly.

Front is 154 lbs and the rear is 247 lbs.

To me it doesnt make sense that a front heavy FWD car should have a stiffer rear than front. Does that make sense?Yes it does make perfect sense. Largely for the reason that Andrei mentioned (i.e., uneven weight distribution). Perhaps more importantly, for a reason I'll outline below.


The "gurus" and autocrossers on our forums tell me that its because the rear needs to be stiffer to counter understeer...They're correct. They problem has to do with your McPherson strut front suspension and (to a smaller degree) your beam axle in the rear.

All of the stuff you read about suspension tuning will tell you that if you want to increase grip at one end, you need to soften that end (e.g., lower spring rates, longer torsion arms on your anti-roll bars). So, if your car understeers, soften up the front end through some combination of spring rates, damper settings, and sway bar thickness.

What they don't tell you is that all of that is true ... provided that the car is perfectly balanced and the suspension is already optimized for track use. So, if you have a formula car with 25% of its mass at each corner, ground-skimming ride height, double-wishbones all around, and ideal camber/caster/toe settings at all 4 wheels, then the normal rules apply. If you have a 3000 lbs. street car with insufficient camber, suboptimal suspension, and poor weight distribution, then your theory falls apart.

Your front McPherson struts are the single biggest offender. Because of how they work, you'll never have enough camber in the corners unless you're running some crazy amount of fixed negative camber (like -4 or -5 degrees negative) and the car is a quarter of a millimeter off the ground. With normal spring rates and normal camber angles, your front tires will be rolling over onto their outside edges in the corners, which severely limits your ultimate front-end grip.

The solution is to increase roll resistance as much as possible. That means high spring rates, very tight low-speed damping, and a massive front swaybar. In other words, exactly the opposite of what Carroll Smith tells you to do for a race car! Keeping the front tires from rolling over in the corners using springs/dampers/sways allows you to run less extreme camber angles, which means more grip under braking and acceleration on your FWD platform.

This is far better than just stiffening up the rear (as many amateur auto-crossers do). The car has an extreme amount of understeer built in due to a combination of poor weight distribution, FWD, and McPherson struts. If you tried to balance it by focusing on the rear end, you would need to make the rear ridiculously stiff so that it "lets go" before the front. That means you're throwing cornering grip out the window. Sure, the car will be more balanced/neutral ... but your ultimate cornering speeds will be lower.

So, rather than dumping rear grip, you want to increase front grip. Softening up the front (as the textbook tell you to do) won't do you any good since your whole problem is that the front is too soft to begin with! You need to prevent the front from rolling under lateral loads ... and that means a stiffer front end.

[mattmayhem]

Yes it does make perfect sense. Largely for the reason that Andrei mentioned (i.e., uneven weight distribution). Perhaps more importantly, for a reason I'll outline below.

They're correct. They problem has to do with your McPherson strut front suspension and (to a smaller degree) your beam axle in the rear.

All of the stuff you read about suspension tuning will tell you that if you want to increase grip at one end, you need to soften that end (e.g., lower spring rates, longer torsion arms on your anti-roll bars). So, if your car understeers, soften up the front end through some combination of spring rates, damper settings, and sway bar thickness.

What they don't tell you is that all of that is true ... provided that the car is perfectly balanced and the suspension is already optimized for track use. So, if you have a formula car with 25% of its mass at each corner, ground-skimming ride height, double-wishbones all around, and ideal camber/caster/toe settings at all 4 wheels, then the normal rules apply. If you have a 3000 lbs. street car with insufficient camber, suboptimal suspension, and poor weight distribution, then your theory falls apart.

Your front McPherson struts are the single biggest offender. Because of how they work, you'll never have enough camber in the corners unless you're running some crazy amount of fixed negative camber (like -4 or -5 degrees negative) and the car is a quarter of a millimeter off the ground. With normal spring rates and normal camber angles, your front tires will be rolling over onto their outside edges in the corners, which severely limits your ultimate front-end grip.

The solution is to increase roll resistance as much as possible. That means high spring rates, very tight low-speed damping, and a massive front swaybar. In other words, exactly the opposite of what Carroll Smith tells you to do for a race car! Keeping the front tires from rolling over in the corners using springs/dampers/sways allows you to run less extreme camber angles, which means more grip under braking and acceleration on your FWD platform.

This is far better than just stiffening up the rear (as many amateur auto-crossers do). The car has an extreme amount of understeer built in due to a combination of poor weight distribution, FWD, and McPherson struts. If you tried to balance it by focusing on the rear end, you would need to make the rear ridiculously stiff so that it "lets go" before the front. That means you're throwing cornering grip out the window. Sure, the car will be more balanced/neutral ... but your ultimate cornering speeds will be lower.

So, rather than dumping rear grip, you want to increase front grip. Softening up the front (as the textbook tell you to do) won't do you any good since your whole problem is that the front is too soft to begin with! You need to prevent the front from rolling under lateral loads ... and that means a stiffer front end.

Thanks Emre! You confirmed alot of what I was begining to suspect! Alot of the sentra guys like to just buy the nismo sway bar and run it at the stiffest setting but I always tried to educate them that they were really doing their suspensions the wrong way by weaking the grip on the rear to help the front out...but the rear sway feels more balanced according to them.
I think if I leave the rear rate around 250 and increase the front to around 275 I should get a nice benefit. Its a shame alot of guys are reading performance handling articles but mis-understanding the context that you pointed out.

[Phil]

Let me add that I learned the hard way that suspension tuning and dialing in a car can be a time-consuming and expensive experience. Being penny-wise and dollar foolish is not the way to go. Unless you are a very experienced track junkie, it is better to let someone who has done the testing build a set up for your car and not try and guess about it yourself.
If there's isn't anyone racing a Sentra in FSAQ or CASC-OR try SCCA or NASA to get advice on the parts to get and which vendors to buy from. Stay way from the glossy catalogues if there isn't substance behind the claims.