It seems to me the whole exercise here is about aesthetics, or what it looks like. Performance, handling and road holding, are of far lesser importance. Strength and safety probably are of passing interest, but I have doubts about those too. So the aim here appears to be to create something with unique looks, not something that works especially well.
When your long slender beam axle supporting the entire vehicle weight right in the centre, snaps in the middle, things may become very clarified indeed. It is technically unsound and dangerous. Give it up.
Hey, I'd written you off as merely a scoffer because you hadn't said anything substantial. But now you've actually made a point. Thank you (this time I'm sincere). I looked up your profile and it says you're a retired engineer. You have my attention. How much weight will Henry Ford's axle take in the middle before it breaks? Do you have any numbers or are you drawing on your experience? I happen to have one in my garage. How can I measure it without breaking or bending it? A 1940 Ford has a lot more weight over its front axle than a front engine dragster. The engines are at opposite ends and the Ford has a body. I wish I knew exactly how much the axle would support. We could go back and forth with guesses all day. The whole reason for putting the spring in the middle was to separate the spring rate from the roll stiffness, so that when you adjusted one, it wouldn't effect the other. The aesthetics grew out of that. It's true aesthetics is a factor. If it wasn't, I'd weld a Corvette or BMW IFS to the front of it. "Give it up?" What is that, some sort of local lingo? I don't understand.
The problem is not how much force it will directly take to break. The problem is the high cyclic stress, particularly under tension in the lower part of the beam right at the middle. Cyclic stresses lead directly to metal fatigue, which always begin as microscopic surface cracks which then grow, very slowly at first, then progressively faster, leading to sudden catastrophic failure. Think of it like a wire coat hanger. A very strong body builder cannot snap a wire coat hanger just by pulling on it. But a small child can bend that wire coat hanger ten, maybe twenty times (with little effort) and it will then break for sure. So you build your front end , and it works fine. You drive over some pot holes, and it still looks fine. It may work for weeks, months, even years. Then one day totally without warning it will suddenly let go and crash your hot rod when you least expect it. Just remember, that seventy year old I beam axle with probably sub standard steel (by today"s standards), has already had a very long and hard life, and now you are asking it to do something way beyond what it was ever designed to do. Nothing terrifies serious racers or aerospace engineers more than metal fatigue. They crack test, and manaflux parts, polish thigs to a mirror finish, and throw things away after so many hours service as as routine maintenance procedure. Because they know, that what looks o/k can suddenly fail without warning. It is not how much force a part can stand before it breaks, but how many repeated stress cycles before it fatigues, and then breaks. What you are proposing to do is just asking for trouble. It will fail, maybe not immediately, but it will surely fail. I very strongly recommend you use two front springs, or the original transverse spring arrangement. The closer the load bearing ends are located to the king pins the better.
Another silly thought. If you want to build a sprung suspension with zero inherent roll stiffness, so you can incorporate your idea, (which I like) how about two interconnected air bags ? How do you insert vertical spaces between paragraphs in this Forum ? I do that as I type, but it all comes out as a solid block of ugly text.
I see what you're saying, but how do springs keep from fatiguing? I like your idea about the air bags. What do you think about using Monroe air shocks in place of the bags? Would you be concerned about using the 70 year old axle as it was designed?
By keeping the working stresses below accepted limits for the type of material and heat treatment for the particular spring. Top Fuel dragsters sometimes need to change the valve springs every single run. If they don't, they risk breakage, and then they lose an engine. Crappy aftermarket springs sometimes sag because they are made from inferior material that yields under load, or else they were badly designed in the first place. Springs like anything else need to be designed properly with the intended use in mind. Difficult to say. It might work in a very light vehicle, but these days air bags come in a variety of shapes and sizes, and may be a lot easier to apply. The air bag supplier can also provide a lot of technical specifications that may make deciding what to use a lot easier. No problem, as long as it is not obviously damaged in some way. Because the guys that designed it knew all about stresses and fatigue. Original factory parts will generally wear out long before they break, (if used in the application for which it was designed). They are designed that way. Very highly stressed parts like wheel studs and flywheel, and con rod bolts are built with sufficient design margin so they have virtually an infinite fatigue life {if properly torqued). Only hot rodders and racers stress, con rods, crankshafts, axles, gearboxes to the point where they fatigue and eventually break. And even then, it often takes many high rpm launches to break something.
For those who haven't read this entire boring thread, the purpose of the suspension is to allow height/ride change without roll rate change - all the while trying to look "traditional." Thank you, Warpspeed (and others) for taking the time to make some thoughtful and very useful replies. Unfortunately, I've squandered them to come up with this monstrosity: Basically, this: With these modifications: Make a pan hard bar in disguise out of the spring. Mount one side of it on the frame and the other to the axle. (Don't mount it in the middle.) Take the flex out of it by welding the leaves together on the backside. Take some of the arc out of it. Support the vehicle on Monroe air shocks. Connect them together with an air tube to let the air course back and forth. It should have the same effect as if a spring were mounted in the middle of the axle, but it transmits the load towards the ends where the axle where it's stronger. This resists gravity without affecting roll stiffness. (Thanks Warpspeed.) Monroe air-shocks are a little thicker than regular shocks, but they are closer to "traditional" then airbags. The I-beam axle is also the sway bar. It keeps the car from leaning in the corners. The torsional force in the axle is transmitted through the hairpins to the chassis. Reinforce the hair pins with sheet metal between the bars to keep them from bending when the axle twists. (Cut some stylish lightening holes in it for fun.) Any ideas for improvements, or reasons it won't work? (It's going on the front of a T-bucket type car.)
Very sneaky, it looks almost stock, but acts in a very different way to stock. I am not at all sure about welding the leaves of the spring together. That will vastly stiffen up the spring, as the leaves are supposed to act independently and slide over each other. Welding is not going to do anything good to the heat treatment of the spring steel either. Why not just use the standard front leaf spring as original, but instead of bolting it rigidly up under the front cross member in the centre, pivot the spring in the middle ? The spring is made to resist bending loads and to be supported in the middle, (that is what it does!! ) whereas the skinny front beam axle is not. If a centre pivot at the spring mounting does not appeal to you, a hidden rubber mount would effectively do the same thing. But it will need to be of a type that has very high lateral stiffness. Body roll is only going to be a very few degrees maximum anyway, so rubber should work fine to allow that roll movement and reduce front roll stiffness to effectively zero. You can then just use conventional fluid shock absorbers at the front in the usual way. The suspension will look completely conventional and traditional, but it will have just about zero front roll stiffness. Increasing the front roll stiffness back upwards with an antiroll bar will then be possible, but I would expect it may not turn into corners especially well, and have some transient handling problems, because of the very high front roll centre. But it will certainly be no worse than any other hot rod beam axle front end in that respect.
Coming up with new ways to do something is fun and challenging .But if this front end you want to builed fails it could be something that ends your life .I think it would be like driving a see saw until the center connection failed from fatigue. My 2 cents
As long as the rubber mounting is made completely captive it cannot seriously fail. Rubber is used in suspension parts just about everywhere in modern vehicle suspensions. It only becomes really dangerous if a rubber part can tear due to massive structural overload, and there is nothing else to hold the parts together. Designed with a little thought, cylindrical rubber suspension bushes with a through bolt are completely safe.
I am not at all sure about welding the leaves of the spring together. That will vastly stiffen up the spring, as the leaves are supposed to act independently and slide over each other. The spring is no longer being used as a spring. It's being used as a panhard bar. It's only there for looks. I could have just as easily used a straight tube but I want the appearance of the spring. I'd weld the spring leaves together to keep it from flexing so that it's a stiff panhard bar disguised as a spring. Why not just use the standard front leaf spring as original, but instead of bolting it rigidly up under the front cross member in the centre, pivot the spring in the middle? Because I want to raise and lower the front end as if it were on air bags, yet still keep the same roll rate. All the weight of the front end is supported by the air shocks. The spring is made to resist bending loads and to be supported in the middle, (that is what it does!!) whereas the skinny front beam axle is not. You changed my mind on putting the weight of the car in the middle of the skinny front beam. With all the weight being supported by the air shocks, it is supported at both ends of the beams as normal. An air tube between the shocks reduces the roll stiffness to zero. The roll stiffness is then controlled solely by the torsion of the axle. If a centre pivot at the spring mounting does not appeal to you, a hidden rubber mount would effectively do the same thing. That is a good idea to consider - especially if the air shocks can't support the entire weight of the front end. But the ad says they will support an additional load of 1200 pounds above the weight of a vehicle - should be overkill for the front of my car. Increasing the front roll stiffness back upwards with an antiroll bar will then be possible, I hope to save weight by using the torsion of the front axle as my roll bar. ...but I would expect it may not turn into corners especially well, and have some transient handling problems, because of the very high front roll centre. I don't understand why a high roll center is bad. I've tried to do my homework on this, but can't quite wrap my mind around it. It seems to me that a low roll center would make it easy to tip over, as if it were tall with a narrow base. A high roll center would make the body want to act like a porch swing. Wouldn't this have the effect of resisting body roll in a corner? Why is a high roll center bad? How does it hurt turning into corners and transient handling?
Your viewpoint is supported by a very low death rate among the Amish. I'm trying to be careful and smart, which is why I'm soliciting advice here and other places. I understand the appeal of old/antique/tradition. But there seems to be an inexplicable wide-spread resistance to NEW. I can kind of understand it a little on this message board because it is specifically for traditional rods. But I don't get the wide-spread resistance among most of mankind to anything new or different. Maybe I'm getting too philosophical and should get back to building something in the garage.
High roll centres have two main characteristics of interest. The first is that there will be much less body roll with high roll centres. What produces body roll is the height of the centre of gravity above the roll centre. The greater this distance the greater the roll couple produced, and that roll couple acting on the springs is what causes the body to lean in a corner. Now you might think that is a good thing. You could then build a car with zero body roll by placing the roll centres at the same height as the centre of gravity. The biggest disadvantage of having zero roll couple or body roll is that there is then no way to tune the suspension with either the suspension spring rate or antiroll bars. If the vehicle understeers or oversteers, there is absolutely nothing you can do to fix it. The lower the roll centre heights, the more sensitive the car will be to suspension tuning. The other big problem with high roll centres is jacking effect. The car will actually rise up on it's springs during cornering. (like a poll vaulter) This can cause the outside wheel to tuck under, and the vehicle can actually try to roll over before it slides sideways. Very scary. Modern road cars are built to have a fair bit of body roll, so the driver becomes frightened long before the car goes out of control. So roll centres are made fairly low. Race cars all have very low roll centres because they need to be predictable and the suspension needs to be tunable. With high roll centre cars like our 30's hot rods, they will never handle like a modern car. That is for many reasons, but the biggest difficulty is the high roll centres these cars always have, especially at the front. It is also why modern cars use independent suspensions, and the high performance ones use IRS at the back as well as the front. With IRS you can put the roll centre anywhere, even well below road level if you wished to do so. So you get to choose between building a traditional rod that handles like crap, or something with independent suspension at both ends that rides, stops, corners and steers like a sports car if you do it properly. That would really piss off the purists, but personally I like to drive my cars fast, and not just in a straight line. Not sit and look at them as a static art object. But that is just my own nutty character.
Oh, one thing I forgot to mention. With any beam axle, the NATURAL roll centre height is always at the height of the springs. The body rolls as one spring compresses, and the other relaxes, so the body normally pivots around spring height. This is the natural "roll centre". You can modify that by fitting a physical linkage to force the body to roll at some other point, instead of being decided by the spring mount height. But it must be a physical linkage, so how low you can mount that is determined by ground clearance. You obviously cannot fit a panhard bar one inch off the ground to get a one inch high roll centre. But there is a way to do it by building a "virtual" linkage to locate your beam axle. Google "Mumford linkage". A truly brilliant idea, but hardly practical at the front of a hot rod. But it would certainly work.
So if the entire front weight were supported with Monroe air shocks or air bags, the roll center would be centered between the tops of the shock towers. That would put it several inches above the frame. Ok, scratch that idea. Darn it! How about if you did everything as Mr. Ford intended, except the spring mount pivots - just as you were saying above. . That would place the roll center at the frame. But, what if you moved the pivot point below the frame? Your spring would be mounted to an up-side-down pyrimid with a pivot at the point. The frame would have a tab welded on the bottom to meet it. I'll try to draw another picture a little later. I'm typing on my phone right now.
wsdad Listen to the advice you are getting. Certain things cannot be changed somethings can be changed. You seem to want to hang your car from only air bags at the front. All cars use those in combination with springs. They are there to dampen out road vibration not to hold the weight of a car. Warpspeed has been giving you some VERY good helpful advice.
Except, change the tyre sizes, tyre pressures, amount of toe in, camber, move more mass on to the other axle, etc. I tend to work out my springs to give the require natural frequency to give the road holding and ride I want, if I then changed them to tweak the slip angle of the tyres I could only hope to end up worse than I started. Rather than use too stiff springs car makers have long grabbed for ever thicker sway bars as a fix for thier product's evils, while it's argueable that while such things can be usefull as a method of controlling the car's attitude, it's become very much a stock answer, rather than going back and re-assesing the reason for the ills. In many cases ultra low roll centres with lots of glass and a high mounted overhead cam engine have resulted in a relatively high C of G and hence a large roll couple which has to be resisted by mainly by the sway bars . However with a hot rod, chopped, and channeled, there is less glass, and heavy chassis the heavy stuff tends to be low down and hence the C of G can be fairly low, which should result in a much smaller roll couple, if it's small enough it could be resisted by the springs on one axle without problem ( I build trikes, so I can prove this works ! ), Certainly I have seen formula Vee's running zero roll stiffness rear ends, which means the roll couple is being resisted by the other axle alone. I note that they also run a relatively high rear roll centre, so again the roll couple is small, these don't seemed to have any problems getting round the bends. Passenger cars tend to be designed to have large amounts of nice safe understeer ( blame Ralph N for that one ) hence as you rightly point out Warpspeed they get very low roll centres and a monster anti roll bar accross the front , you can all but let the air out of the rear tyres and it's still going to understeer, plus the motor is often way out in front of the axle so it's got even more tendancy to understeer. But it's not the fastest way round a corner, it's just safe for the public to buy and drive. And it sure does not make what they use the universal solution, or even the right way of looking at the problem. I drive a Citroen BX station wagon as a hack, the suspension is done by using high presure hydraulic fluid (from an engine driven pump) to transmit the force to what are in effect remote mounted air bags and yes they are linked left to right !
Whatever "spring device" you use to support your beam axle, the natural body roll centre will go right through the centre of the springs. It will not be at the top of the shock tower, or at the lower attachment end , but half way in between. The trick here is to mount your spring (or springs) as low as possible on the beam axle. It will be below frame height with a transverse leaf, and this may be more difficult to achieve with long coil springs. Proper air bags can be fairly squat, which is also a step in the right direction mounting the whole "air spring" as low as possible on the beam axle. Long skinny air shocks have the unfortunate problem of being long.... Yes, you can modify the natural sprung roll centre by forcing the body to roll around a rigid fixed mechanical pivot point, or a virtual pivot point (in space) formed by some rigid linkages. Your idea of the upside down pyramid would work, but the reduction in ground clearance restricts how low you can place the pivot point. But the principle is sound. Getting the front roll centre as low as you can with your beam axle will be about the biggest single improvement you can make. Finding a way to do it that is both practical, aesthetically pleasing, and mechanically sound is something else.
All true !! But if your hot rod understeers badly, it would look a bit funny having the big tires on the front, and the skinny tires on the back to get neutral handling (just kiddin). Moving vehicle mass around is not that easy, usually the engine is where it is, because that is where it must go.. Being able to adjust roll stiffness at either end of the car is a very powerful suspension tuning tool, and that only works if there is reasonable roll couple available by having reasonably low roll centres. I agree with all your comments, suspension tuning is a nest of compromises, and we each have our own ideas about how best to do it, and what is most important. Indeed, they do go well, but they have made some pretty extreme compromises to defeat the massive front to rear weight distribution imbalance, and the tenancy towards terminal oversteer. Some of these cars even use a rear Z bar, a type of "anti" antiroll bar. A "Z" bar may be another suspension curiosity wsdad could look into further. It has vertical stiffness, but zero roll stiffness. Not suggesting it is a good idea, just something to ponder.
I don't think that one need ponder Z bars for too long, I don't see them having any use for a front axle. I think the point is that the three link design as proposed with the seperated vertical spring and roll spring has every chance of working, the only thing that this has perhapes shown is that it might be better if the spring were fixed to a lower point on the axle, and the radius rods moved up so they will be under tension during braking, which then means you just need a couple of stays to run from the lower points on the axle ends back to near the spring's fixed end to resist the axle trying to banana under braking and the job is a good 'un as they say. I still like the general principles of the idea, I can see some merit in the fact that it allows the axle to be able to sit way out in front, with the front weight of the car being taken on an attachment which is the length of a spring further back ( possibly close to the front motor mount, where the stress could be taken by the engine block Lotus style ! ) Would I do this with a 70 year old axle ? I don't know, I never have, I tend to think it's easier to make a tubular axle, because it means I can make the axle with the right king pin angle to suit the tyres, I have looked at old axles, but always rejected them on the grounds that it looked like a load of hard graft to alter. Plus they look kind of heavy, where as the tubular axles I have made come out at only a few pounds, I doubt they would last seventy years admittedly, but they work well enough for what is required. If any thing probably my tube axle slightly stiffer in the directions that matter. However I guess there is no reason why if you are prepaired to accept the stock king pin axis not being bang on the centre of the tyre contact, or play with the wheel offsets to move the tread to the right point ( if possible ? ) to get things right that way, why you should not use an old old axle.
What do you make of this? This guy says roll centers, as we have been discussing, aren't very important at all! He suggests a Force-Based roll center is what really affects roll. Roll Center - Myths and Reality Wm. C. Mitchell "...The Force Application Points (FAPs) and the Force-Based Roll Center are important. The Kinematic Roll Center is of little value. ...Load transfer, jacking forces and overturning moment are forces and moments, not kinematics. Thus we should address the problem through forces and moments rather than kinematics. This leads to the "Force-Based roll center". ...Myth #3: The distance from the kinematic roll center to the CG is the moment arm. [FONT=Times New Roman,Times New Roman]If you believe in the kinematic roll center it is easy to consider the distance to the CG as a moment arm. A long distance from KRC to CG produces lots of chassis roll. The problem is that this line is not perpendicular to the lateral force. The FAPs, defined as being under the CG, yield a moment arm perpendicular to the lateral force. The distance from FAP to CG is a valid moment arm. [/FONT]Myth #4 - The Chassis Rolls about the Roll Axis [FONT=Times New Roman,Times New Roman]The chassis [/FONT][FONT=Times New Roman,Times New Roman]moves [/FONT][FONT=Times New Roman,Times New Roman]in reaction to a lateral force: it does not roll about a point or axis. The movement includes chassis roll as well as vertical movement. The kinematic roll center concept clearly describes the roll yet neglects jacking force, which might be small for symmetric cases where both tires contribute equal lateral force. But for racing cars the majority of the lateral force comes from the outside tire. In some applications the inside tire may even be off the ground. Dixon writes " many authors introduce the roll-axis as an axis about which the vehicle actually rolls during cornering, the roll axis being the line joining the front and rear roll-centres. When a vehicle is actually moving on a road, the concept of a kinematic roll axis is difficult to justify in a precise way, especially for large lateral accelerations. Therefore the idea of the vehicle rolling about such an axis, although useful as a qualitative idea, should be treated rather cautiously, except in the special case of a stationary vehicle subject to loads in the laboratory." [1] " [/FONT] http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Vehicle Dynamics2007.pdf
Consider that any suspension system in use today applies the forces from driving and vehicle weight to the most outside corners of the chassis. Check out World of Outlaws sprint car front suspension for example. The chassis carries vehicle weight to the wheels that control vehicle direction. We want those wheels planted so that the car goes where we want them to. By attaching the spring as close to the wheel as possible handling is improved. Unfortunately the single center mounted spring is attached as far from the wheel as possible.
Roll centres are not simple things, as wsdad says, there are many aspects. But for us hot rodders, with a beam axle at each end of the vehicle, the biggest disadvantage of having high roll centres at both ends is that fitting antiroll bars will have very little effect on transferring dynamic cornering loads from one end of the vehicle to the other. In other words we will have a vehicle that has very low sensitivity to antiroll bar adjustment. I still have some very big reservations of feeding all the vertical loads into the centre point of a slim beam axle with a single central spring, and the continuous cyclic bending loads that will create in the axle.
If not being able to treak the slip angles by using sway bar addjustment is the biggest disadvantage of beam axles I would not worry too much, Given as I said before that there are a number of other factors which allow us to alter things. The load on the middle of the beam, I would be looking at testing the beam with double the static load and measuring the deflection before trying that one. I agree the idea of loading up that beam in the middle seems bad, but without knowing the weight of the car which is going to be applied to the beam and the stiffness of the beam it would be wrong to just asume that that is the case. If the car is nice and light then there is every chance that the beam is way stiff enough.
It will be a street-driven front engine dragster with a T-bucket body. Another way to look at is is a stretched t-bucket. Very similar to this: http://www.jalopyjournal.com/forum/showthread.php?t=238241 I was thinking this would be cool with a pointy cone shaped nose, rounded on the tip, with a single leaf spring touching the axle. Your eyes would follow the car's triangular shape from the wide rear wheels to the zoomies to the cone to the leaf spring. Mine would have to have shorter, stronger hairpins in order to keep it from becoming a tricycle. (The twist of the front axle acts like a sway bar).
Nice, So the load on the front beam is going to be fairly small ! Well bellow anything that is going to cause it to fail in any catastrophic maner, you might manage to bend the thing if you pop the thing up to high on the rears and then come down hard repeatedly, but with the motor that far back, the load on the axle is going to be way lower than what it was designed to take.
The Jalopy Journal
