Spring rate question

[RedMisted]

Last season was my first in racing and my car wasn't fully modified, suspension-wise, due to the initial high entry costs of going wheel-to-wheel.

Therefore, I raced the whole season using Eibach performance street springs that had a progressive spring rate of approx. 425-530 lbs./front and 200-300 lbs./rear. (I know, really soft for hard core track running.)

Yet, I liked the balance of the car (oversteer vs. understeer) at the end of the year after some changes were made to the rear ride height and front camber.

Question: I had race-specific springs recently installed on the vehicle. These have linear rates of 900lbs./front and 250lbs./rear. Given that the increase in the front rate is way in excess compared to the rear, won't I see the handling balance tilt toward increased understeer? Shouldn't the front spring rates be lower, say 800 lbs?

Thanks for any help.

[rsx858]

I would contact a race shop that has experience preparing mustangs like yours, find out what rates they prefer and have them dial in your car. You can spend alot of money buying 100 sets of springs and testing them every which way. Makes sense to capitalize on the research already done by racers over the years.

It does seem like your new setup is a departure from the old but again find out what other people run. Perhaps you will find this setup is still better than the last. and aside from spring rates, there is alot to be fiddled with in terms of ride height, dampers, corner balancing and alignment.

When I started out I went right to a good shop and did it right the first time and as a result I have had to make only minimal adjustments... has saved me a lot of dough!

[RedMisted]

I've done a bit of research here on the web and it seems that most front race springs for the Mustang have rates that top out around 850 lbs. I assume that these rates are optimized for the heavier V8 cars. (GTs, Cobras.) Being that my car is much lighter, it just seems that 900 lbs. on the fronts is somewhat high.

In case anyone is wondering why I have 900 lbs. springs, it's what the suspension wizards at Ground Control recommended. They were told what springs I used to run on the car and given data on vehicle weight, weight distribution, preferred handling characteristics, etc. on which to base their decision. Only after I saw what the rates were of my old springs, and had a chance to take my car for an informal test at 7/10 on a back road (where I felt some understeer), did I begin to raise some questions...

[frnkhous]

Chris,
Brian(friends with both matts)
Try these springs, don't judge anything by what it felt like on fine ohio roads this time of year. Remember you have a big heavy car with struts up front. the big front springs may not cause the car to push because if it doesn't roll as much you don't loose as much camber. second piece of advice is every time I've ever gone softer on a solid rear axle car(and not bottomed out/coilbound) the driver likes the car better. Remember that where the springs are mounted(not sure if this was changed) will effect spring rate as well. I know this all seems counter productive(more push) but give it an openminded try.

Hey Get some BIG tires on that thing... don't remember what size wheels you were running but get some 8.5" wheels and some 275 or bigger width tires.

my $.02

[lateapex911]

Ground Control, has, in my experience, been "right" and "wrong". And when they were wrong, parts ripped off the car. ugg.

So, I am sure they mentioned shocks. if your springs are 1:1 ratio or even close, you might need some serious shock upgrades. You didn't mention anything one way or the other damper-wise, so I'm just tossing it out there...

[chuck baader]

The spring rate is only half the story. If you liked the balance with the old springs, you will hate the new...will push quite a bit IF the sway bars were not changed to compensate for the change in roll stiffness. What you need to know is the actual wheel rates of both the old and new setups (the combination of actual spring rate plus sway bar rate) and tune accordingly.

Since you were happy with the balance before, I suspect you will want springs very close to those and just increase roll stiffness with bars. Chuck

[cjb25hs]

I would talk with Maximum Motorsports, most of all the front running guys in NASA use there stuff, I am sure that some of the AS Mustang guys do as well. From looking on their site looks like the race valved Bilsteins are good for around 500lb front and say 325 rear. They have a few different options.

Here is a link

http://www.maximummotorsports.com/

[Spinnetti]

Well, my .02 like Chuck said, if you liked it before, you should keep the same differential bias front to rear and maybe just up the both of them. I would slightly disagree that you need to know wheel rates since you already have a baseline with the current setup, but it can't hurt (you need to do some math if its gonna help you any though). Also, what is leading you to go with stiffer springs? Basically under full cornering, how much roll do you have? (you should measure) If at full load your wheels are vertical or slightly negative to the road surface, then you already have enough roll stiffness (assuming you aren't running crazy camber). The whole purpose of this is to keep you tires basically vertical so they can put the most rubber on the road, and thus generate maximum grip. Just following what others do is one way I suppose, but it seems to me everybody is following each other, and these choices may not always be the best practice (different tracks need different things too). There are a number of good books on suspension, and you can learn allot. Coupled with a bit of instrumentation, you can come up with your own rates that will work best.

BTW, (slightly off topic)... What's with the current fad of king kong spring rates anyway? Aren't the squishy bits SUPPOSED to move? What ever happened to Carroll Smiths advice of running just enough spring to keep the bodywork off the wheels under full braking/cornering, and doing the rest with sway bars? I somehow feel I'm out of the loop with the current fads (I run about 1/2 the spring rates of others), yet my car seems to handle just as well (maybe better than some since it doesn't skip over the rough stuff). Seems to me you can go to far, and just beat up your chassis (not to mention shocks unless you matched those to the rates).

[lateapex911]

That's a good point. The rates we see on cars in World Challenge, for example, won't help in IT, where we have limited chassis stiffening options. In IT there's no way to achieve the same chassis stiffness that they do in WC with the cages that tie into the front structure. At some point, the chassis becomes an undamped spring. And this is good, because as chassis stiffness goes up so goes the ability to run super stiff springs, and dampers cost exponentially more.

[JeffYoung]

Red, I've got an IT car that suspension wise is pretty close to yours. Macpherson struts up front, live rear axle.

I run 600 lbs up front and 300 rear, and it's balanced. I'm not sure the chassis can handle much more spring and I may drop the rear rate just a bit.

I have a bigger front bar than stock and am going to try not running the rear bar. Konis are revalved for those rates.

Like Jake said, I'm not sure about super high rate springs on a RWD IT car (I understand the need for them on the rear of a FWD car), nor do I (respectfully) buy the Carroll Smith approach of "just enough spring" and then big bars. That was the way of the world in the 70s, 80s and even 90s (talk to some of the IT Z guys, tha tis what they did), but higher rates -- within limits - seem to be the norm now, the idea being it helps maintain contact patch.

Hope that helps some. Suspension development is still a black art to me.

[chuck baader]

I guess it is just the engineer in me, but I want to know what my actual wheel rates are. That way I can change sway bars and/or springs and keep the front to rear balance the same.

Carrol Smith: I've only know a couple of individuals who could read and comprehend what he has to say. I tried the stiff spring/soft bar approach and finally had the good sense to listen to a friend of mine who can actually understand Smith. I have since gone that route...my friend's comment is that "old school works as long as there is rubber between the car and the road" is just as true today as it was 30 years ago:happy204:

But hey, what do I know..y'all run those stiff springs. please:eclipsee_steering: Chuck

[Streetwise guy]

There are a couple of points I want to address, and have answers to if I am wrong.

1- A Fox body Mustang does have struts, but the springs are not on the struts. They are in the traditional SLA location, being about halfway in on the lower control arm between the pivot and the ball joint. That requires a higher spring rate to provide the same amount of wheel rate. Therefore, asking me, driving a Neon with spring on strut, what my spring rates are requires math to mean anything to you, in a Mustang. Are the rear springs in front of, on top of, or behind the rear axle? The further ahead they are, the stiffer they must be.

2- I am of the opinion the mega (front)spring rates in a FWD strut car come from the lack of camber gain compared to a proper sla suspension. We have to limit travel, because we lose camber after the spring compresses far enough to bring the lower control arm past level. Its also why my Neon doesn't gain performance when it is lowered too much. Roll center goes underground or something along those lines. Monster rear rates are there to get the car to rotate, a problem you are less likely to have in a rwd car.

3-I own my own shop, I have been a journeyman automotive technician since 1985, and I still have a terrible time absorbing information from a book about chassis setup. I learn better by doing than reading, even though I have better than average reading skills. Find a smart guy and crew for him.

[frnkhous]

normally I'd agree with all of you about changing the spring rates to keep the same roll stiffness percentages, but I know the driver, and the shop doing the work and I suspect there is a reason for the really high rate.(or ground control was consulted as well and screwed up) The car really really needs more front spring and I honestly having watched the car think that the front end likely rolls way to much and he looses all his camber. I can only suspect that this is why the front rate is so high. Last I knew I think this car still had the stock front bar on the car.

Yeah with independent 4 wheel suspension I'd agree you move by percentages but there are always exceptions and a car I think has almost 900 lbs on both front corners(maybe a little high 800?) and a stock front bar may be an exception. less roll may equal more grip. When you stiffen the rear of a solid axle car to much my experience has been the backend just slides out under power even if the percentage is the same.(note this isn't my own driving or the expectations the drivers expected which to me helps to validate them on this type of car)

[xr4racer]

As someone said earlier the spring on the Mustang is not a coilover it is in the stock location. The 900 lb spring is only equivalent to approximately a 450 coilover, I do not think that is very high for a 3100 lb car. The springs that were on the rear were more like 135 pounds and in front of the axle, again in the stock location. The shocks are valved to handle the new rates. Some real track time will tell.

matt

[xr4racer]

Brian, what the hell do you know about solid rear axle suspension setup??? Oh yeah years of setting up solid axle Rolex GT cars.

See you at MO this summer

matt

[RedMisted]

Thanks to all who weighed in on this topic. All I can say is that I'm still a virgin when it comes to all this suspension theory/tech!

I agree with Brian that it's possible that increasing the rear spring rates may make it difficult to get the power down on a solid axle car. And a quick check of what is being run on typical V8 Mustang race cars show that their rear rates are way down versus their front rates. Now, I know I'm examining the V8 car here, but that's because I'm the only one I know who is attempting to race the V6 version. I simply have nothing else to compare to but my bigger V8 brethren.

For those who wondered above, the dampers that I used to run on the car BEFORE the spring change were non-adjustable Bilstein HD street shocks/struts. The car used to have -2.5 degrees camber at the front before changing to -3.0 for the final race of 2008--and God, what a difference it made! (In conjunction with a rear ride height change, the car rotated like I couldn't believe. I actually was able to fine-tune the balance between oversteer/understeer by just tweaking the rear tire pressures a bit. I was amazed.)

But now I've got Koni double-adjustable race units on the car and ride height adjustability in the front. Maybe these new dampers and new spring perches will help the 900 lbs. front springs to do their magic. The front anti-roll bar is still stock.

I still don't understand why the typical V8 Mustangs run springs softer than 900 lbs. when those cars weigh much more than my humble six-banger. Perhaps it is because the V8 cars typically run monster-thick aftermarket front anti-roll bars and my car retains its thin, stock bar?

I think I may have just learned something here....

[frnkhous]

hope we've been helpful chris.
I'll add that while I think you need alot of sticky tire and to focus on getting off corners because the strength of your car is the torque of the motor, not the suspension/weight.

I guess I should add that from what I know about your car it likely isn't possible to ever make weight which is why I dismiss the minimum weight of 2670 as being lighter than most of the rwd cars. honestly i'm not sure chris would want this but this car likely should be reclassed to its at a heavier weight it can make.

[RedMisted]

I'll add that while I think you need alot of sticky tire and to focus on getting off corners because the strength of your car is the torque of the motor, not the suspension/weight.

Wow. I've never thought that the strength of my car would be the motor. But with every meaningful suspension/chassis upgrade, things like power and torque can increasingly show themselves

[jimbbski]

I won't comment on the front spring rates but the rear. If the rear suspension is still the stock 4 link with the stock bushing then the low rates in the rear are in the ball park. The stock bushing add quite a bit to the rear wheel rate. That's one reason why most AS racers convert to a torque arm/panhard bar set up in the rear and ditch the upper links and convert the lowers to sphirical bearings or polyurethene bushings. They also have to go up in rear rate to the 350-450 lb/in.

The other thing is the spring position in the rear is on the lower link, about 2/3 of the way between the axel and the forward mount, closer to the axel.

For every inch/pound you raise the spring rate in the rear will gain a larger percentage, about 65 % of the increase vrs the front, which is around 50% gain.

[Spinnetti]

Good discussion all...

I guess my car is basically like the Mustang, just 1000lb lighter :).. I forgot that wheel rates do matter quite a bit where they are not over the wheel center (like a conventional strut which is close to 1:1 spring rate/wheel rate).. Its easy enough to do the math. If the spring is inboard 1/4 of the way to the pivot, its proportionally that much weaker at the wheel since the wheel is working through that lever arm. Impact is that through a big lever arm, you have to make much bigger spring changes to notice anything. Half the front rates in the back seems about right (its about what I run). I'm old enough and been a Carroll Smith fan long enough to start soft, but I've had to slowly up my front spring rates over the years as the tires keep getting grippier... When I first started, 200lb was enough to keep under 3 degrees roll and from scrubbing the tires under braking, but I keep having to go up. Here's my "secret" :) One easy way to check roll is to have somebody take a picture of your car at full cornering... little protractor work and you can see how much max roll you have (Mid-Ohio has a great spot for this). A combo of springs, bars and camber to get you as close to 0 camber (or a bit negative) at full cornering is what I look for, with less than 3 degrees static camber so that you aren't sacrificing braking performance for cornering performance.. Plenty of positive caster helps too as it allows lower static camber and gives more negative camber as you turn the wheel. Asymmetric static camber is sometimes helpful too (I use it since there are more rights than lefts where I race)... This stuff ain't that complicated for most of the "old school" cars (unlike a modern Honda or Audi which are a lot more complicated), but some books are a worthwhile investment.. Physics is still the same, 30 years later ;)

[RedMisted]

I guess I should add that from what I know about your car it likely isn't possible to ever make weight which is why I dismiss the minimum weight of 2670 as being lighter than most of the rwd cars. honestly i'm not sure chris would want this but this car likely should be reclassed to its at a heavier weight it can make.

At first I argued the same thing about the weight. Ron Earp explained to me a long time ago that my car is too powerful for ITS and that those with the power to reclassify would never heed a request to demote the SN95 3.8L Mustang.

But now I really feel that the car has potential for ITR. Maybe not be a world beater, but a respectable car that can take a win or two.