S197 650

While browsing around, I stumbled upon this spreadsheet done by Vorshlag:
https://spreadsheets.google.com/pub?...3I3mP8cOvc5vZQ

They took a bunch of OEM springs and tested them out for their spring rates.

I noticed some differences between manufacturers:

BMW M3 - The rears were almost 4x stiffer than the fronts.
Aston Martin - The rears were about 1.5x stiffer than the fronts.
Nissan 350Z - The rears were about 0.5x stiffer than the fronts.
Mustang GT - The rears are less than 0.5x stiffer than the fronts.

I know that spring rates don't operate in a vacuum and everything in the suspension is designed to work together, but why is there such a wide spectrum in OEM tuning philosophy?

The BMW rear springs rates particularly jump out at me. BMW is widely renowned as having some of the best handling suspensions in the world, so why do aftermarket spring companies offer springs that are completely the opposite of what BMW does?

For example, every spring kit I've found for the '12 GT (besides Pro-Kit) makes the front spring rates higher than the rear. Yet, every OEM iteration for the new Mustang (GT Brembo, Boss, Boss L/S) has stiffer rears than fronts?

Suspension gurus, what's going on here?

onederful100

i am no suspension guru by any means.
but i will take a stab at the philosophy of it.

OEM springs are made for a balance of performance and comfort
whereas aftermarket ones are made for performance and sacrifice comfort.
the difference between makes depends on the balance of the car between front and back,
and thats where you get the different ratings with different cars.
the stock Mustangs are nose heavy, that would explain the rear stock ratings of only .5 stiffer than the fronts.

thats my take on it, from a guy with stock suspension. lol.

S197 650

That might help explain things. The M3 and Aston Martin are nearly 50/50 weight distrubtion. Both the 350Z and Mustang GT are closer to 55/45. Still, that's only a 10% difference.

JAJ

The spring rate as measured is not the spring rate at the wheel. In the BMW case, the wheel rate is dramatically lower than the spring rate because of the mechanical ratio between the spring and the wheel. The "final" rates are quite different than the measured rates, so a table like the one you read really doesn't tell you anything useful.

The factory chooses springs based on corporate ride and handling standards that the aftermarket ignores. They end up with different results, of course. There is no guarantee that the aftermarket is right, however.

Whiskey11

This, on a MacPhearson Strut, the spring rates are basically going to be your wheel rates, but on a live axle it depends entirely on the location of the spring on the axle. The further out on the axle that the spring sits the higher the motion ratio (meaning you need less spring rate for a given wheel rate). With a Live Axle car, you WANT the axle to articulate for handling purposes. Locking it down is a surefire way to find yourself a concrete barrier to hug with your face. I don't remember quite what the rear motion ratio is but I know it's less than some fully independent cars because the springs sit inboard of the wheels by about 5-6 inches. So do some other vehicles springs though.

If you really want an eye opener, look at the spring rates for an SN95 or a Fox Body. The front spring rates are abnormally high because of the poor motion ratio due to the modified MacPhearson strut set up. The springs sit inboard of the actual front strut (unlike our cars where the spring is over the strut) so they have to jack spring rates up to get a specific wheel rate.

Their rear axle springs are even more interesting. They don't even sit on the axle center line, they sit on the lower control arms just in front of the axle! Talk about interesting!

S197 650

Thank you guys for opening my eyes on this. That also jives with the similar spring rates found on the 350Z (their rear springs sit further inboard just like ours.)

My question still stands on why our Front/Rear spring rate bias changes so dramatically going from OEM to Aftermarket.

For example:
'12 Base GT - 123 / 156
'12 Boss L/S - 137 / 191
Hotchkis - 180 / 180
Steeda Sport - 200 / 175
Steeda Competition - 225 / 185

Does it have to do with the fact that as the front end loses suspension travel with lowering springs, there's more of a concern of fender/tire rubbing under compression? Is there something else going on here?

Whiskey11

Yes there is the worry of bottoming out the front suspension to worry about but a lot has to do with the front roll stiffness which contributes a lot to handling. Increasing the front stiffness keeps the front suspension in a better part of the camber curve to prevent going positive in camber, it also helps the steering responsiveness as well as tire loading. It also helps reduce brake dive on hard braking.

As for why is it so high compared to the rear? Aside from the above mentioned fact that you need a live axle to articulate and conform over bumps to reduce upsetting the chassis from those bumps, the assumption is being made that you will also be tuning rear roll stiffness with a heavier rear swaybar which also helps dial out understeer. You also want weight to transfer to the rear wheels for acceleration purposes.

Softer rear rates also ride better as it is far easier to move the 250lbs of unsprung live axle over bumps and deadens the blows so to speak.

I'm sure there are other reasons that folks more knowledgable than I can explain that I am leaving off.

clowe1965

Don't forget that the aftermarket springs also have a shorter unloaded spring height. The higher rate and the lower CG will give the possibility of better handling, but the springs must support the same weight with less deflected travel than the OEM springs. Assuming a 30" stock ride height and ignoring any mechanical advantage, a 3600 lb car has 990 lbs average on each of the front wheels. With the spring rates above the stock springs have to deflect 8.04" to support this. The steedas have to deflect 4.95" to support the weight, but they also give approximately a 1" drop, making them about 2" shorter than the stock springs. Yes you are correct in that rubbing has an effect on the spring rate selection when lowering springs are used.

Higher rates in the front vs the rear is partially because of the weight distribution and also for stability. If you have a really stiff rear you will oversteer (also accomplished with your left foot). Since most spring sets are designed for handling (vs weight reduction and launch for drag), oversteer is something that they will not try to build into a suspension system. Having a stiff set up in the front tends to exhibit understeer, something the majority of drivers can handle far better than having the rear slide out from underneath them (safety related).
They differ drastically because of the manufacturers goals with the spring set. The performance groups such as Hotchkis, and Eibach cater to many other groups than Mustangs. They provide spring rates that their current equipment is capable and select the spring rates using their combined experience from their R&D group (mainly finance driven, same thing as the 100mm bore spacing of the 4.6 carrying over to the 5.0). Ford most likely outsources their springs and slaps a FRPP sticker on it. Steeda caters solely to Ford cars and specializes in Mustangs. Since they have a (relatively) limited customer base they will put more time and money into making a product that works to create repeat customers. As such, they are slightly higher in price.
All of them will most likely produce a better handling car over OEM, but since specialty groups such as steeda only sell to Ford/mustang owners, their products are made specifically for your car in more than just the "it fits" category.

Just to clarify, I do not own a single Steeda product. Not saying I wouldn't though.
Hope that helps.