need help figuring spring rates

[Pro-touring towncar]

With the custom springs I am getting for the town car I can have any spring rate. How do I figure it out? what would be better higher or lower for handling? please help

Tim

[baz67]

Wow the regular suspension guru's must be busy. There are two main theories on spring rates for a car built for handling. One of them is soft spring and large anti-roll bar. The other is high spring rates with a smaller bar. I am a follower of the latter. So IMO a higher rate is better for handling and you tune with the anti-roll bar.

What springs are you looking for the rates on? The front coils or rear leafs.
Brian

[Norm Peterson]

With the custom springs I am getting for the town car I can have any spring rate. How do I figure it out? what would be better higher or lower for handling? please help

Somehow I doubt that there's a huge bank of handling-oriented "this will work" quick answers for a Town Car. You're essentially starting from scratch, so pick some realistic values for maximum lateral acceleration, roll angle, and average speed that you drive at. Be honest about how firm of a ride that you are willing to tolerate.

Basically, you can select springs based on ride frequencies (which you can estimate from your ride comfort and driving speed requirements) and see how much roll would develop at your target lateral g. Then you bring the roll angle (which will be quite a bit over your target roll angle with springs only, assuming some sanity in your rate choices) back down with sta-bar selections.

That's the rough outline, as there's quite a bit of other information that's needed in order to arrive at a sane balance of front/rear roll resistance.

I checked my Moog spring catalog, and the TC supposedly comes with either 428 or 446 lb/in front springs and 125's in the rear. I made some really crude guesses as to weight and a few other things and I'm guessing that this results in ride frequencies of 0.9x Hz up front and about 1.2 in the rear (with the car unloaded). So I think you could go quite a bit firmer in terms of spring rates.

But I'll stop short of making any specific rate suggestions at this point as there's way too much information missing. Myself - I've been entirely happy with a 1.4-ish Hz front ride frequency (think 500 lb/in front springs on a 2300 lb Pinto). That's admittedly a bit extreme, as to match that up front on your TC you'd be looking at springs with four digit rates (and 250's or higher in the rear for flat ride considerations @ 65 mph or lower). Maybe consider a 1.4 Hz front an upper bound, shoot for a target front ride frequency that's somewhere in between OE-soft and that, and we can go from there.

Norm

[derekf]

Norm,

Do you have a reference for the book-impaired like myself that explains ride frequency vs spring rate vs how it rides?

[Norm Peterson]

Ride frequency is a real-world application of simple harmonic motion (Physics class). Fred Puhn's "How To Make Your Car Handle" (among others) explains ride frequency as a function of wheel rate and corner weight, and mathematically it looks something like

[Frequency in Hz] = 3.13 * SQRT( [Wheel Rate] / [Sprung Corner Weight] )

Note that you have to go through all the [Motion Ratio]^2 math to get from spring rate to wheel rate. There are at least 3 different MR's to consider, including the position of the spring on the control arm relative to the length of the arm, angle of the spring away from perpendicular to the arm, and a loaded ball joint vs wheel center location correction (FVSA related).

How it rides is by implied definition quite subjective. Everybody's butt-meter is calibrated a little differently for NVH (and this is part of the 'V'). Puhn hints at 1.0 Hz for soft sedans and 2.0 for a race car. And the Millikens' "Race Car Vehicle Dynamics" mentions 70 - 90 cycles per minute (1.17 - 1.5 Hz) for sports cars and 95 - 120 cpm (1.58 - 2.0 Hz) for Indy cars without ground effects. Something to think about is that your perception can be muddied by local resonances around the seat attachment points and pedals, and in the steering wheel.

Norm

[Steve Chryssos]

A good place to start is by borrowing (or renting) a set of corner scales. From there, you can measure wheel travel @ bump, droop and at ride height. Springs are rated in pounds/in. Gotta know the inches too.

[Pro-touring towncar]

I am a bit confused( nothing new) So I would use the corner scales to wieght my car. then measure the ride hight, the wheel travel, droop(?)

Norm, say if I went up to say 500-550 Lbs front and 250-300Lbs rears that would stiffin my handling.
I should pic up that book about handling.

Tim

[chicane67]

I dont think I would go that high of rate in the rear with that small of a rate in the front.

I'll have to look to see if I can find the rates used in the "B4" Police package available for that model year to get an idea of what exactly Ford themselves used. It would be a great starting place and it would also give an idea of rate balance when considering a rate increase, front and rear.

You might want to give Jason at Eibach Spring a call. He is quite "in the know" on matters such as these......

[Norm Peterson]

I am a bit confused( nothing new) So I would use the corner scales to wieght my car. then measure the ride hight, the wheel travel, droop(?)

I think what streetfytr68 is getting at is to use the corner weight scales in conjunction with ride height measurements to get wheel rates with a minimum amount of math (and measurement error) rather than having to measure and compute all the motion ratio stuff. This approach will also automatically include any rising or falling of the wheel rates that occurs due to geometry changes over suspension travel.

[Wheel Rate] = [Change in Scale Reading] / [Change in Ride Height]

with ride height measurements taken at the front and rear axle locations (think side view). Since fenderwell openings are convenient measuring points, you'd want to raise/lower both sides evenly (to avoid error introduced by right-to-left slope) before taking the scale readings.

Ideally, you'd also set the front scale pads on something like alignment-rack turnplates so that the lateral scrubbing of the front tires as the front end is raised (or lowered with additional weight) won't cause inaccuracies or other problems with the pads.

Norm, say if I went up to say 500-550 Lbs front and 250-300Lbs rears that would stiffin my handling.

What chicane67 said. Too much rear for that amount of front (and probably too much even with no rear sta-bar). Even 600 lb/in up front only wants something 150-ish in the rear, and I think a 200 rear might like 750 or so up front (and the numbers are with some trunk and rear seat load this time). Frequency-wise, those fall in the 1.1 - 1.2 range for the front and about 10% higher out back.

These numbers that I've been putting up are still based on lots of assumed data, so don't take them as gospel just yet. I'm probably guessing a little high on some and a bit low on others, but hopefully it all averages out reasonably close. FYI, here's a partial list of my spreadsheet's inputs/variable names:

CarWeight
Wheelbase
FrontSpringRate
FrontBumpPctCriticalDamping
FrontLoadedControlArmMotionRatio
FrontBallJointMotionRatio
FrontSpringAngle
FrontTireSpringRate
RightFrontUnsprungWeight
Speed
WeightFractionFront
PitchKpsquared/ab (see Help or Iterative Estimate)
RearSpringRate
RearBumpPctCriticalDamping
RearSpringMotionRatio
RearSpringAngle
RearTireSpringRate
RightRearUnsprungWeight
RearAxleWeight (enter zero if IRS)


Norm