Spring rates, higher rate in the front?

[fateo66]

I was mocking up my coilovers and noticed that the 350lb spring is intended for the front coilovers and the 275lb spring is for the rear. Does this seem backwards to anyone else and perhaps just a misprint? In a conventional front engine car that would make since but with most the weight over the rear axles shouldn't that have the higher rate?

[Mechie3]

The rear is heavy statically (when it's just sitting) but under heavy braking or turning the mass of the car acts about the center of gravity and creates a higher combined dynamic + static load in the front than in the rear.

[D Clary]

I am not sure but the actual rate may be different because of the location of the spring to the wheel. The leverage on the rear could give it a higher effective rate. Or could be a missprint

[Bob Cowan]

You also have to look at the design of the front and rear suspension, and how the shocks/springs are mounted. The angle of the spring, and where it mounts on the arm will determine the wheel rate. The wheel rate needs to be balanced according to the balance of the car.

That's why you can't simply increase or decrease the spring rates at one end of the car and not the other. There must be balance in the force.

[Bob_n_Cincy]

The back springs are almost vertical.
So 1" of wheel travel is 1" of spring travel.

The front spring is at an angle so 1" of wheel travel may only cause 3/4" of spring travel.

I'm just ball parking numbers without doing the geometry, but the effective spring weight to the wheel might be the same.

[fateo66]

Aw, I knew someone smarter then me had the answer. Thanks for the info guys.

[Jeff Kleiner]

What Bob said (actually what both Bobs said!).

Break out your tape measure and protractor then plug the numbers in here to see what your actual wheel rate is:

http://www.leskoracing.com/wheelrate.htm

Jeff

[CHOTIS BILL]

There is more to it than just wheel rates there is also roll couple which compares how much roll resistance there is in the front compared to the rear. This takes into account such things as wheel rate, anti-sway bar rates, and roll center heights. For my DSR which had a weight distribution of 47/53 F/R I found that a roll couple of 61/39 worked the best. The springs were 1700/900 F/R with a wheel rate of 820/638 F/R. Knowing the best roll couple I could change spring rates and then set the anti-sway bar rates to achieve the proper roll couple and the car would be almost spot on when I roll it off the trailer. I do have a spread sheet to do the calculation if anyone is interested.

Bill Lomenick

[Scargo]

Is there anything that can be extrapolated from the 43/57 weight distribution and the 350 lb front/275 lb rear spring rates (and possibly the 17mm sway bar), to give you the roll couple?

[nkw8181]

I'd like a copy if you don't mind :-)

[ehansen007]

Intrigued for sure! I managed to get a full set of 275s and 350s so I have the ability to adjust. Sounds like Erik Treves likes his set up just fine. For the Hot Rod there was a HUGE thread on swapping out springs mainly for comfort reasons.

[StatGSR]

lets not forget that these are still pretty light spring rates (atleast from a track stand point, they are fairly firm for street duty). my back ground is mostly with FWDs and AWDs which will both most commonly have a front weight bias and both of which commonly have heavier front spring rates, but as others have mentioned already, all the weight still goes forward in hard braking and you need stiff springs to help with that.

In anycase, preferred spring rates has a lot to do with the driver and how they like the car to feel, the sway bar you use or don't use comes into play as well. Coming from the honda world, their are several different ways to get to the same end goal of fast last times, but for each way the car will drive a little different. My 2500lb track integra has 850lb front and 750lb rear spring rates, I also run a fairly large rear sway bar and still run on street tires. For me i feel like the back end is loose enough to go quickly, but not so loose that I am worrying about spinning. Others in my position would prefer to use more spring rate (1000lb/in rear) to help the car rotate and maybe run a smaller rear swaybar. Others will choose to run no front sway bar. Options are out their and each depends on the driver and their preference.

moral of the story, if you drive it and think you could use more rear spring, swap them front to rear (if they are the same size) and see how it drives, or buy a pair of 450lbers for the rear. Its one of the great things that comes with using standard size springs, you can pretty much open up the eibach or hypercoil book and buy whatever you want.

[Santiago]

Are the 350/275 spring rates standard for the S-car or are these the R-car? I would've guessed the S-car, but why guess if someone knows? =)

Oh, and Bill, I'd also be mighty obliged to get a copy of that spreadsheet as well.
(psst., my smiley face is nicer than Nolan's...)

-j

[Wayne Presley]

Guys, you really need to figure the motion ratios in to extrapolate what the spring rate front to rear.

The lotus I drive uses 475/650 with the rear springs nearly vertical but mounted further in board on the lower arm than the front. The fronts are leaned in more than the 818 and the lower mount is further in on the arm.

[CHOTIS BILL]

I should have included my e-mail address for anyone wanting a copy of the spreadsheet I mentioned above. Send me an e-mail and I will send it out to you.

BLOMENICK (at) DELTATAU (dot) COM

Bill Lomenick

[Rasmus]

You fellas should look into using Spring Frequency to determine what spring rate you run. Using Spring Frequency factors in the sprung weight of the car per corner into determining what spring rate to run. If you determine you want 2.2 Hz for the front and 2.48 Hz for the rear. Just plug in the weight, motion ratio of the arms, and your Hz and out comes your spring rate. Using spring frequency allows you to transfer and/or compare "ride" between dissimilar cars. So if you want your 818 to ride like a 1968 Cadillac Coupe deVille find the Hz of the Cadillac then recalc and poof out comes spring rate. If you want it to ride like a Radical race car, same deal.

If you want to ride stiffer because the track is smoother up the Hz in small amounts (+0.1-0.2 Hz) If you want to ride softer because the track is rougher lower the the Hz in small amounts (+0.1-0.2 Hz) If you remove weight or shift weight you can recalculate your spring rates just by plugging the new weight with your target Hz into the formula.

To calculate spring rate based on Hz:
( ( MotionRatio * TargetHz / 3.13 ) ^ 2 ) * SprungCornerWeight = SpringRate

When I did this on my Subaru for SCCA Solo STX ( ( .96 * 2.2Hz / 3.13 ) ^ 2 * 897Lbs ) = 408 lbs/in. So I purchased two 400 lb'ers and installed them up front.
If I wanted my WRX to ride like a '68 Cadillac. ( ( .96 * 1.1Hz / 3.13 ) ^ 2 * 897Lbs ) = 102 lbs/in. Install 100 lb'ers and drive on a cloud.

To calculate Hz based on spring rate:
( (SpringRate / SprungCornerWeight) ^ 0.5 ) * 3.13 / MotionRatio = Calc'dHz

[Scargo]

I'm still waiting on my suspension book. This sounds crazy easy if it's so straightforward... Is this consistent and accurate? For example, If I know all the numbers and I like how my STi rides on a smooth track, I can dial that into the 818? What about the fact that the STi is front heavy and I compensate with bigger rear springs to get it to rotate or that the 818 might have more or better aero? Isn't aero another factor to consider? With aero you need more spring, I believe.
Jim Schenck just told me that without aero the car would need closer to 400F/325R spring rates rather than R spring rates [600F/350R]* that may come with the aero package.
*Per Bill Waters who has an R.
EDIT: I apologize. Jim told me the rates on the 818R are 300 front and 500 rear. He also said,"...these are optimized for track use and work best with full aero. For a non aero car the rates would be more similiar to an 818S but probably 50lbs stiffer."
So, in my comment I used the numbers fateo66 gave in his post, above and added 50 pounds to each. These may now be in error.

[Rasmus]

This sounds crazy easy if it's so straightforward... Is this consistent and accurate? For example, If I know all the numbers and I like how my STi rides on a smooth track, I can dial that into the 818?

That's the beauty of using Spring Frequency. You can translate one cars ride to another. You can get a crazy 3:1 bellcrank suspension FSAE racecar to ride just like a track tuned STi on Struts. You just need to know: Motion Ratio of both cars suspensions, Sprung Corner Weights of both cars, and the Spring Rate of the car you have "Dialed in". Run it through the formulas and wham-bam you'll get a similar riding car on the back end.

A lot of the hypothesis and argument comes in with how to pick what frequency to run. Most seem to write that the rear of a sporty/race car frequencies need to be 1.3 times that of the front. So if you're running 2.2 Hz up front you need to run 2.86 Hz in the back. Might need to look that 1.3 up. Not certain I'm correct on that. Consensus seems to be that the rear Hz should always be higher than the front Hz.

Everyone seems to be in agreement that under no circumstance should you have the front and rear frequency's exactly the same. Apparently you end up with a really bad harmonic teeter-tottering effect that's almost impossible to drive.

Aero? Neat thing bout Aero is it acts just like corner weight. So if you know that the front splitter and under tray are producing 200 lb down force at the speed you're dialing your car in for you just add an extra 100 lb (200 lb / 2 corners) to the Sprung Weight. If the rear wing and diffuser add 500 lb, just add 250 lb to the sprung corner weights in the rear. Of course as you slow down your car's spring frequencies will increase giving a harsher ride.


So for my previous Subaru for SCCA Solo STX ( ( .96 * 2.2Hz / 3.13 ) ^ 2 * (897lb Sprung Weight + 100lb Aero Downforce ) = 453 lbs/in. So I'd need run 450 lb'ers instead of 400's to get the same feel.

[Bill Waters]

Santiago, Scargo;

My 818R springs are 600 front and i believe 350 rear.

Bill

[CHOTIS BILL]

There are several ways to compare cars suspension stiffness. One is to calculate frequencies another is to compare wheel weight vs. wheel spring rate. It is easer for me to grasp a ratio of .75:1 vs 1.25:1 than a frequency of 2.5HZ vs 3.0HZ. But both methods come up with number that can be compared one car to another car. The only problem comes when comparing suspensions that are rising spring rates or falling spring rates.

I do however use frequencies to try to keep the front and rear frequencies at least 10% different. I am a firm believer in having the front providing around 60% of the roll couple and therefore the front frequency is the lowest

When it comes to how aero affect spring choices the more the down force the more important it is to keep the chassis as stable as possible and therefore the springs need to be stiffer. With the amount of down force produce by the 818 I would think a wheel weight vs. wheel spring rate in the area of 1:1 would be a good place start. Having a fairly high ride height compared to full ground effect cars a change in ride height shouldn’t change down force much. I have run high down fore cars with a ratio of 4:1 but the track needs to be very smooth to get away with a ratio that high.

One of the great things about running cars like this is that you are able to change things like spring and shock rates and go out and see what difference it makes. It is nice to have theories to use as a guide line but you never know how it will work until you try it. Another thing when changing parts like springs take a big enough swing at it so you know for sure how it affects the handling and if you go to far you can then dial it back. If you change the spring rates by 25# you may not be able to tell the difference.

Bill Lomenick

[Scargo]

It's great to get this kind of information that's directly applicable to our cars and racing. Thanks guys!
Bill Waters: is that your calculated spring rate decision or what you were able to get from/came from FFR?

[Bill Waters]

The latter, Scargo - just the pure, listed spring rates of the springs themselves in an out-of-the-box R as received from FFR.

Bill

[longislandwrx]

Interesting, I got 500# fronts with my R

[Wayne Presley]

300's for the fronts and 500 on the rear on the R

[Bob_n_Cincy]

300's for the fronts and 500 on the rear on the R

Hi Wayne
The 818S rear is 275#/in. The 500 is a BIG increase.
What changes in handling will I see with this change?
Bob

[Santiago]

Wow...we're seeing some strange numbers thrown around here.

Front S: 350 lbs
Front R: 300 lbs (Wayne), 500 lbs (LongIsl), 600 lbs (Bill W.)

Rear S: 275 lbs
Rear R: 350 lbs (Bill W.), 500 lbs (Wayne)

Either we've got some error in reporting going on here or FFR is making significant changes along the way. Hope Jim Schenck chimes in on this one.

I can see doubling spring rates going from S to R, but we're not seeing agreement on where the doubling takes place. If the S figures are accurate (and I'm not saying they are at this point), one would expect that the fronts would be the larger of the two on the R if it were purely a matter of addressing harder use (combating brake dive, excessive chassis sway, etc.).

Given that the R has anticipated aero enhancements, that expectation shouldn't be stubbornly relied upon. But still, on the face of it doubling the front suggests massive aero gains relative to the rear - which doesn't seem to be the case given what was published of the aero results. IIRC, they were pretty consistent in preserving close to 42% front / 58% rear distribution in the aero load they gained.

I want to say Wayne's figures make more sense insofar as the rear springs clearly need to increase, but my sense is that this assumes it is quite different from the S-car's basic balance. If that's wrong and the S-car (a) starts as our baseline for balance and (b) has something like a 42/58 weight distribution to start with, things change. That would make the increased aero loads track the mechanical balance established by the S (which the R might share absent its aero). In this case, maybe it does make more sense to see 500 or 600 lbs in front and 350 lbs in the rear.

Curious...and confused.

Best,
-j

p.s. Bob, as a general rule of thumb, if you were only bumping the rear rate +225 lbs and leaving the front at whatever it was previously, you'd see the rear get looser (i.e. increased tendency towards oversteer). Unless, of course, you were making the change to address a huge increase in rear downforce from some new device you added to an otherwise balanced chassis.

[Scargo]

I'm sorry if I've added to the confusion. Jim Schnech told me the rates on the 818R are 300 front and 500 rear. He also said,"...these are optimized for track use and work best with full aero. For a non aero car the rates would be more similiar to an 818S but probably 50lbs stiffer."
So, in my comments (above) I used the numbers fateo66 gave in his post, above and added 50 pounds to each. The other numbers came from Bill Waters. These may now be in error.

[Canadian818]

Where does this leave us "S" guys running full aero?

[Santiago]

Where does this leave us "S" guys running full aero?

Depends on the specific aero. If it's what the R ran, you're probably looking at modestly lighter rates since the cars are quite similar. My guess is that since the standard S-car's ride height is a bit higher than the R-car, you're not going to gain the same under-car aero advantage (particularly at the front). That suggests you might also want to back off on the rear wing angle (assuming the same wing) for balance. The result would be an overall reduced level of downforce, but hopefully it would be as well balanced as the R-car.

The "hope" is just that. You're looking at testing to zero in on your rate. The upshot is that springs are easy to swap out on a coil-over shock. All this is assuming you are strictly track oriented. If you're trying to come up with a dual purpose street/track set up, you're on your own (since your interest won't be simply performance...that whole "ride quality" worry is highly owner specific).

Best,
-j

[SixStar]

What was the verdict on this?

It seems as if we're 100% sure that the stiffer springs go in the REAR on the 818R but there's not really an answer on the S. Do what the manual says and put the stiffer springs in the front and hope it's not a typo?

[Bill Waters]

I'm sorry if I've added to the confusion. Jim Schnech told me the rates on the 818R are 300 front and 500 rear. He also said,"...these are optimized for track use and work best with full aero. For a non aero car the rates would be more similiar to an 818S but probably 50lbs stiffer."
So, in my comments (above) I used the numbers fateo66 gave in his post, above and added 50 pounds to each. The other numbers came from Bill Waters. These may now be in error.

Amongst my kit packages were front and rear shock/springs. The front ones had springs which were marked (etched or painted on the spring) "600 lbs."

Bill

[RM1SepEx]

Amongst my kit packages were front and rear shock/springs. The front ones had springs which were marked (etched or painted on the spring) "600 lbs."

Bill

It wouldn't be the first time that FFR sent someone the wrong parts.. call and ask, they respond and ship the correct parts quickly by FedX and returns are just a call tag away!

[longislandwrx]

Wow 600lb fronts strange.... I do hope someone from FFR Chimes in. I guess mine are on backwards, I put my 500s up front (that's how they were labeled) and 300s in the back.

[Scargo]

For my R, which I picked up on the third, I received 500# for the front and 300# for the rear. What the heck, Bill?

[FFRSpec72]

Wow 600lb fronts strange.... I do hope someone from FFR Chimes in. I guess mine are on backwards, I put my 500s up front (that's how they were labeled) and 300s in the back.

The challenge car is the same, I run 650 in front and 350 in rear, I can expect the same for the 818 R, higher spring rates in front.

[Bill Waters]

For my R, which I picked up on the third, I received 500# for the front and 300# for the rear. What the heck, Bill?

Good question... I have to call them on Monday about something else. I'll ask. But, if Jim and Wayne have directly told us it's 500 or 600 front and 350 rear, I suppose that's the answer.

If they shed any light on things, I'll post it here.

Thanks,

Bill

[nuisance]

My "R" (build date 3/8/14) came with 300# springs packaged with the rear suspension parts, and 500# springs packaged with the front suspension parts.

John

[Bill Waters]

Good question... I have to call them on Monday about something else. I'll ask. But, if Jim and Wayne have directly told us it's 500 or 600 front and 350 rear, I suppose that's the answer.

If they shed any light on things, I'll post it here.

Thanks,

Bill

Joe at FFR confirmed today that the parts/inventory sheet is wrong; the 500 + springs go in the back and the 350s go in the front.

i have learned to rely on Wayne's assertions.

Bill

[wleehendrick]

This could explain Brandon's front end twitchiness (he just got his R on the road), if he followed the documentation.

How about the S? I built to the manual: 350F, 275R. Don't tell me I need to swap them!

[Xusia]

This could explain Brandon's front end twitchiness (he just got his R on the road), if he followed the documentation.

How about the S? I built to the manual: 350F, 275R. Don't tell me I need to swap them!

Yeah. That would be a pain!