Gang, I'd like to hear some discussion on spring rates, if you please. When I see coil overs listed in catalogs, the coils that fit over the shock always have spring rates identified (pounds / inch, I believe), and there are a wide variety of rates available. But when I see leaf springs or air bags advertised, from stock Model A replacements to bags that will hold up an old Caddy, there never seems to be a particular spring rate stated. As an aside, I once had an old race car builder tell me the best handling car had the softest springs necessary and that the suspension movement was controlled by the firmest shocks / anti-roll bars as needed to keep things stable. How would I do this on a home-built Model A for example? So, if I ordered an OEM replacement transverse springs or air bags for a hypothetical 2500 lb hot rod, how do I get the right rate? What is the relationship between the weight of the car at each corner and the spring rate needed? I'm not sure, but I doubt 625 lbs "spring" is needed at each corner of a 2500 lb car, eh? Or are they? So how do you determine the best springs / rates for any particular car weight? As far as I know, none of the T/A/B spring rates have ever been posted in catalogs. Or did I miss them? Or do you guys with leaf springs just keep adding or subtracting leaves until you get the ride quality you want, willy-nilly so to speak? And when you finally get the correct spring rates and are freaked because your car now rides too low or high? Does it piss you off that you couldn't identify / install the correct springs at the start of your project? As for air bags, I have them on my rod. At the start of the build I set the suspension up so that the shocks and bags were in the middle of their travel range. It seemed to be a logical starting point. Now that the car is done and rides ok, I'm wondering if I did it right. In practice, if I increase or decrease the air pressure to the bags, two things happen. The ride height changes and the "spring" rate or stiffness changes. So if I wanted to lower the car to make it handle better, I loose my spring rate, eh? I only raise it past center for speed bumps or steep driveways, and the ride becomes super harsh and jiggly. More air = more stiff, eh? Thanx in advance, Gary
I have been machinating on this very question for some time now. The only Air Ride car I have ridden in rode like a buckboard when up at highway height. I want to be able to adjust the ride height (slam it) on my '63 Bel Air but I didn't want it undrivable when down low. I am considering installing air cylinders that pull it down. I figure then I'll be riding at the same spring rate all of the time, not banging on the snubbers. Where am I wrong?
Well, these are really good questions, and ones that would take about three pages to try to explain. but, really briefly, springs (leaf and coil) are rated for both LOAD - which is the load the spring can carry at a specified height, and RATE - which is how much load it takes to deflect the spring. Load relates to the carrying height of the vehicle, and rate to the ride/handling characteristics. And, the spring specs must be converted to specs at the wheel, since suspension geometry affects them. Unfortunately, there is not a lot of knowedge out there, so most decisions must be made on guesswork. Air springs are even worse, because they are infinately variable, with rate dependent on bag pressure, and load dependant on VOLUME of air in the bag. There are a lot of equations that can help you get a handle on these things, but a lot too complicated to get into here. A VERY GOOD reference book - written in easy to understand and down to earth language - is "Ch***is Engineering" by Herb Adams (HP Books). Herb was a Pontiac ch***is engineer back in the day, and later went on to consult for numerous race teams. I would highly recommend this book if you want to get a handle on these concepts. ( I worked with Herb many years ago, but get no kick-backs....)
Excellent book, - page 29 calculating leaf spring rates = 1/3 page total and only two complete sentences. " Leaf spring rate is determined by the springs physical dimensions as shown in figure 5 - 4. Spring rate of a leaf spring can be approximated by this formula: Leaf spring RATE=( WN/12)X(1000T/L) cubed 3 W=width of leaves in inches N= number of leaves T=thickness of 1 leaf in inches L=length of spring in iinches 12 is a constant for all leaf springs. Thats it Page 31 spring selection All of the above is good background information, but in the end you have to make a decision on what springs to use in your car. understanding all the factors involved will help you make a decision as well as well as to rationalize the springs you are already using. The following are some of the factors That should be considered In choosing springs. Then list of paragraphs for racing final on street. Yep softest springs that will do the job will be best. Seams that unless you have a 4 position scale to individually weigh the corners is about guessing or going by what's been done before.
There is quite a science (and tons of Matlab, etc. models...) wrapped around ride-rate calculations, getting the basics set up then tuning front/rear rates from there to get what you want. I make no claims at expertise. The Herb Adams book is a start, also the indispensable Fred Puhn's 'How To Make Your Car Handle' and Carroll Smith's 'Tune To Win' though much of this is targeted at open-wheel racers (the chapter inspired by the '69 Penske Trans-Am season is ***led something like 'The Curious Case Of The Large Solid-Axle American Sedan') Or you can go jump in at the deep end, much of which began with the work of Maurice Olley: http://books.sae.org/book-r-206
Gang, Sorry for not getting back to this thread sooner.... I've been distracted. In the good news department, I was leafing thru a new Winters catalog and found a wonderful chart for torsion bars that cross references bar length, bar diameter and lever arm length to give spring rates. Wow. Now wouldn't it be wonderful ifn the leaf spring makers / sellers would also do that for ya? That way I could build a car with either coil overs, leafs or torsion bars and be able to select my components with some sort of premeditation. Still: 1. How does a back yard, garager, 1-800-will-it-fit rodder like me get leaf springs rated in the ball park for a new build even if the spring rate data was published? How do I know what rate is needed at each corner of the for a given / estimated weight car? That is, without of course, taking engineering cl***es or being able to easily understand all the date in the pubs referenced above? Does anyone at least actually know the spring rate of a stock Ford model A buggy spring, for example? 2. The only conclusion I've come to so far about buggy springs is that if it worked on the stock A sedan car and you hot rod it, everyone pretty much just uses the very same springs (old or repop) on the rodded car, regardless of any weight changes and no one seems to wonder or care about having correctly rated springs. Later, Gary
I have never seen a chart for leaf spring rates without all that mumbo-jumbo and calculations. Then they act like it is simple..well, they say, you just take the square of the hypotu... whenever some sales guy uses a sentence with words like 'all' or 'just' he is minimizing and likely doesn't know himself. I brought this subject up some time ago trying to calculate the attachment point for the rear spring on a modified and the most sage advice was to go and measure somebody elses'. I still think there is a 'deflection' chart for typical leafsprings, i wish somebody would post it. Thanks oj btw/ that herb adams book really is good - dated, but good. Isn't he the guy that designed the cragar ss wheel? Nice to have that on your resume' not to mention the royalties.
When I built my first 32 Ford, TCI recommended rear coil over 250 lb springs. ended up with 190 LBS. Had a front Monoleaf. ended up with an Eaton custom leaf. Gave Eaton the front and rear weight. He figured the rate I would need. The Speedway catalog in my opinion is way off in spring calculation. Ago
How did you know WHICH "bags" to put on your rod??? I'm just starting my 1930 1.5 ton chevy truck. It will sit on a 2x3x3/16 frame. I chose a 1970 Buick Stage 1 455, TH400, and a ford 9". Everything seems to be make/model specific out there. The front end set up was donated by a 1941 GMC 1/2 ton with a stock 4" drop axle. Any ideas there?
This would be a great discussion and valuable thread if this got 1/2 as much attention as a " what color..." or "rat-rod" thread gets.
31 Vicky with a Hemi Ha Ha Ha and I'm hoping it gets the attention it deserves. I'm looking forward to learning something worth while.
i might type some gibberish. You can look at a leaf spring and tell if it can give a soft ride. The more arch the smoother it will ride. Look at a 40 front spring compaired to a 32. The 40 has a lot more arch. I think ch***is engr's talk in inches of deflection rather than a specific spring rate or load. If you have 500 lb load and a 100 lb/ in spring, the deflecton would be 5 in. which would be a soft ride. If the spring was 250 lb/in , the deflection would be 2 in which would be very stiff. The spring rate for a front 32 spring is 150 lb/in/side. The front sprung weight is about 1000 lb or 500 lb/side. 500/150 is 3.33 in so has a medium to soft rate. The steel monoleaf springs are 500 lb/in rate/side so the deflection is 1 in so are extreamly stiff. The typical aftermarket springs run 300 lb/in/side. This info can be applied to rear springs and also coil overs by factoring the required increase in spring rate with angle. I run 120 lb coil over springs on my roadster.
If you don't want to guess, the best way is to put a set of scales between the axle and the ch***is where the spring is gonna attach. You want the body and ch***is weight only cause that is what you are holding up with the spring. The rear axle is unsprung weight and does not figure in.
You also have to take in consideration the angle that a coilover is mounted. I raced stock cars and modifieds for 15 years, It seems that my cars went faster with springs that were so far from what others were using....I have 200 lb CO springs on the back of my 46 Ford P/u, BUT they are mounted at a 30 degree angle. I should be using # 150 straight up. No calculations....thumb I have quarter elliptics in the front, number of leaves will change the rate, started with 4 (too soft and bottomed out) 5,(same) 6 (right height, too soft) now 7 and it gives a great ride at the right height. I used thrust rods to dampen the ride, no shocks.
33wire, you still need to know your SPRING RATE. how many pounds does it take to move the spring one inch....Coil springs are easy, leaf springs are harder to rate as you have to mount them solid. The spring company that sponsored my race cars in Omaha had a huge machine to rate leaf springs, but used a only a hydraulic press to rate coil springs. Length of spring, number of leaves, height of arch, width of leafs, location of clamps, free leaves, booster leaves, center pin location and mounting brackets, ALL matter when rating a leaf spring. When we got our rear leaf springs from the sponsor, he handed us 2 main leaves and 8 add on leaves plus the center bolts and said have fun...... we ended up using 4 leaves on the left rear and 2 on the right, it only took us 2 pratices to get it right.....
http://www.jalopyjournal.com/forum/showthread.php?t=588650 Here's an extremely handy little device to eliminate the guessing at weights. Thanks Andy
Hi Gary. I seem to have missed this thread. First thing to consider is if there is any leverage involved in the suspension. With solid axles this is usually not an issue, but most independent suspensions have some sort of leverage on the springs. That's easy enough to calculate, though, from measuring where various things attach to the control arms. You'll end up with a factor by which you'll have to multiply the spring rate. Some solid axle set-ups also feature leverage, e.g. where the springs attach to control arms rather than the axle itself. Then you have to consider the weight distribution of your car. The easiest way to get an idea of the spring rate you want is in terms of "droop". This is how far the wheels would drop if there were no weight on the springs (leaving aside things like droop stops that hold the wheels when the car is jacked up off the ground). It's an indication of different sorts of ride quality: you'd consider a droop of 5"-6" or more decidedly luxurious; less than about 2" is decidedly sporty. (This is a good, workable empirical approach: the rational equivalent would involve working from frequencies of oscillation, but the math can become quite complex. There one would aim for about 1.2Hz at the front and 1.4-1.5Hz at the back, depending on weight distribution, or lower if one can pull it off. It'd be interesting to do the calculation as a test after setting up one's ch***is using droop.) Let's say you've got a 2500lb car with 55:45 weight distribution, ifs featuring 1.8:1 leverage, and a conventional live axle; and you want a 4½" droop all round. Front spring rate: (2500 x 0.55 / 2) x 1.8 / 4.5 = 275lb/in Rear spring rate: (2500 x 0.45 / 2) / 4.5 = 125lb/in By explanation, there's 562½lbs on each rear wheel, so a 125lb/in spring will compress 4½" when subject to that weight. It's hard to calculate air bag spring rates, because there are so many variables, some of which are well nigh impossible to measure due to the bags' free-form shape. That is further complicated by the rising-rate nature of air as a spring medium - but that, rather than the facility to "lay frame", is one of the best reasons for using air as a spring medium. Air can be compressed, and as it compresses it resists compression, and thus is capable of working as a spring. But air cannot be compressed indefinitely, into zero volume. Therefore, the force required to reduce the bag's volume by a given amount keeps increasing as the volume decreases. That means that air springs change from a very soft rate at full droop to a very stiff rate at full bump. A few principles arise from this, which might guide one's choice of bags. Yes, more air means a stiffer rate. A bigger bag running a lower pressure will give a softer rate over the entire range of travel, and vice versa. But a bag near the lower end of its travel (i.e. almost empty) will give a sharper increase in rate than one near the upper end of travel (i.e. almost full), regardless of pressure. So Gary is right in reducing pressure/ride height in the quest for better handling, because although that results in softer springs, the springs will stiffen up very rapidly with any suspension travel. I think most bad bag installations are probably bad due to bags that are too small, and that allow too little travel, so that the entire travel is used up in normal driving. I'd recommend bags that have quite a bit of spare capacity over their advertised load ratings, and rather to use leverage so that they use only a part of their travel range.
Ned, Thanx for your inputs. I won't pretend to understand it all, like calc'ing leverage, but even if I manage to get to the answers in your hypothetical 2500 lb car example, I probably still can't order what I need from a hot rod shop or 1-800 houses like Speedway ifn they don't list the spring rates of their stuff. Grrrr. So... you can get a spring that fits, take it to a spring shop to have it tested and perhaps "tuned?" Or... we all know what the best route probably is, go for coil overs! Gary
Gnichols, you are correct about leaf spring rates, no one specifies the rate of their springs. It trial and error based upon the weight of your car. This is further complicated by the multiple differences in construction and motion ratios determined by the various suspension designs used by builders. You can make your own spring rater, to go to the race car swap met and see if you can find one of those old style wheel weight checking tools, they look like a tire jack that clips under the wheel lip, you push the handle down and a weight reads on the gauge. By using this tool and placing it on the arch of the spring and compressing the spring 1" you can determine the approximate rate of the spring from the gauge reading number. If you choose this method remember to place two pieces of plastic or thick polyethylene under the eyes of the spring to act as a slider. Sounds crude but it does work well for low buck occ***ional use or you can just purchase a leaf spring rater from intercomp for =/-$1000.
Thanx for your tips, ****. In the mean time... perhaps the answer to these questions would help? - what are the OEM spring rates of T / A / B / etc. leafs, front and rear? - what is the front / rear weight bias of those OEM vehicles? - how closely do the rates of repop springs match OEM specs? Then... compare that with your front / rear bias is to those OEM numbers and take a SWAG from that? Gary
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