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I-beam vs IFS - pros/cons

Discussion in 'The Hokey Ass Message Board' started by Texas Highlander Motorsports, Apr 28, 2010.

  1. Trying to do it right, but on a budget as all of us are.

    I have excellent mechanical / fabrication skills which is why I was considering the IFS.

    There is nothing wrong with the current I-beam other than the fact that the frame is 13" off the ground and I want it much lower - maybe if I can find a 4" or 5" drop I-beam that'll work along with some spring work - which is why I was 1st considering the IFS.

    The truck would be a daily driver - all blacktop, 30% twisty 2 lanes, 70% interstate 70-75 mph.

    I will be doing all of the work which is why I was considering the I-beam to save time and effort (somewhat:rolleyes:).

    Other mods: Pontiac 400/TH400 and narrowed Dana 70 dually (it is not currently a dually:cool:)

    tangible = real technical facts
    opinion = personal preferences

    I can see the technical benefits of IFS. I use to be a powered vehicle engineer for a suspension company. I have not had any experience with driving an I-beam vehicle, which is why I was asking those here that have had a lot of personal and in depth experience with one.
    Last edited: Apr 30, 2010
  2. I would rebuild it if it wasn't so high in the air. I could basically make it a gasser with the height that it is now in stock form.
  3. Great info...Thanks!!
  4. wsdad
    Joined: Dec 31, 2005
    Posts: 1,258


    Here's something else to consider if you are going to haul heavy loads with your truck.

    An IFS will tilt the tire out at the top and in at the bottom when the suspension squats under a heavy load. You'll be driving mostly on the outer edge of the tire, causing that edge to wear out faster. It's because of the short arm / long arm set up.


    They are designed that way on purpose to provide better grip with radial tires. Radials have a soft sidewall which "rolls under" just a little when you're going around a hard corner. When one side of the car is heavily loaded due to cornering, the tire tilt helps to compensate. However, when both sides are heavily loaded because you're hauling something heavy and you're going in a straight line, it just wears out your tires for you by driving on their edges.

    With an I-beam front suspension, the tires are always perfectly perpendicular to the pavement no matter how much the suspension squats under a load. That's one reason they use them on most 18 wheelers.


    Plus, if it has several leaf springs packed on top of one another (unlike the coil springs in the picture above), then the suspension is progressive. In other words, it's soft without a load and stiffens up as the load increases. The IRS's rarely do that.

    If you are planning on carrying heavy loads with your newly dually truck, I'd vote for the straight axle mostly because of the longer tire life, but also, it's a more rugged set up / less expensive / already engineered and got the bugs worked out / and of course the cool old truck factor.

    If you're not going to be carrying heavy loads very often, I'd probably go for the IFS due to less unsprung weight / smoother ride / better handling.

    The fact is, this is only my opinion, but it's based on fact (in my opinion).
  5. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,248


    The other way around isn't it?
    Because in your illustration your upper arm is already past its level point and any dropping of the suspension will cause the upper ball joint to pull inward.
    The lower is still level and with the additional length on the bottom the angle change will be minor compared to the top.
    But other than that I agree!!! :D
  6. bwahahaha, thanks for the refresher...:D
  7. The I-beam won't make it ride like a truck. How it is sprung will or won't. You can get them to ride fine but it's a lot of dicking around playing with springs.
  8. Ford-Man
    Joined: Apr 6, 2009
    Posts: 288


    You can also have your beam dropped. It costs a little, but with the mentioned spring work and a drop axle, you can get the lowered truck AND ride you want. My truck will be getting a 3.5" drop axle soon...hopefully. Contact Sid Drapel for details on cost for your dropped axle. Super nice guy to deal with. You could use that in combination to removed leaf springs and/or reversed spring eyes.
  9. atomickustom
    Joined: Aug 30, 2005
    Posts: 3,391


    The biggest disadvantage of a beam axle in the real world is that any time one wheel hits a bump it acts directly on BOTH wheels. (They are, well, connected to each other by a solid beam axle.)
    If you live in an area with a lot of potholes that is worth considering. As a friend of mine with a beam-axle Ford pickup once told me, "that thing finds EVERY bump in the road!"
    That is why IFS often ride smoother, not just shock and spring differences.
  10. jcmarz
    Joined: Jan 10, 2010
    Posts: 4,633

    from Chino, Ca

    He is looking for a profound answer, something real. Opinions are like A-holes. Everybody has one.
  11. One drives like a truck and the other drives like a car.

    Its all a mater of what you're after.
  12. ironhead68
    Joined: Feb 11, 2007
    Posts: 93


    Dana 70 dually? Wow...that's alot of rearend. (8-10 lug full floater?) Is this a 1/2 ton rated truck or something much larger? Maybe you're looking to haul/tow a good load.
    MII in front would seem to be mismatched.
  13. Heavy Flat Head
    Joined: Jan 16, 2007
    Posts: 30

    Heavy Flat Head


    Do not have much to say about the differences but you need to check this link out.<O:p></O:p>
    Over on one the ford site, one of the guys are using a Crown Vic cross member in older Ford trucks. The older dodge trucks frame width is with in the spread of the Ford trucks. The nice thing about this cross member (it bolts to the frame) it carries all suspensions and steering parts (rack and pinion). There are four bolts (two on each side) on the frame and two trailing arms to attach. Parts are easy and low cost and the weight of the Vic is about the same as the older trucks. I have been toying around about using this on my 70 Doge truck but the frame needs to be a little wider, a couple guys have done it, but more work. My thing about the Mustang front end is the amount of weight it was designed to carry.

    Oh yes and did I say the price, I have found them any where from around $300 to under a grand. Even when you add $500 (crown vic years are 03 to I think 07) for new parts it still is a lot less than a Mustang. Large disc and rack and pinion steering all in one compact package.

    If you can read through all the pages, lots of reading ,there is links to other sites that have used this upgrade in the older Dodge trucks. (46-60) <O:p></O:p>
    <O:p> </O:p>
    Heavy Flat Head.<O:p></O:p>
  14. I've been talking with Sid and I can only get 3" out of my axle due to the shock hole. He said that it would be around $300. I figure I can get another inch with removing some leafs.

    It is an 8 lug full floater. The truck is already a 1 ton. If I went MII, I would not enven consider it for any hauling. I doubt that I'll do much hauling anyways. I'm using the D70 because it was free.

    Thanks...I'll check it out.
  15. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532


    Tangible reasons, eh ?

    First thing is the narrow based king pin on a beam axle just cannot be as rigid, or as stable as a pair of wide based ball joints on a modern IFS.

    Second reason, the transverse leaf spring on a beam axle will produce a very high front roll center, which makes suspension tuning with an antiroll bar not really possible. It will handle at speed like any eighty year old car, and there is nothing you can do to fix that.

    The unsprung weight of a beam axle is considerably higher than an IFS. While that is less important with a beam axle in a Kenworth Truck, with two tons over the front axle, it will very seriously effect the road holding and ride of a much lighter vehicle.

    It really depends what your aims are.
    If you want a smooth ride, good handling and braking, and stability at speed, go IFS, radial tires, and disc brakes.
    If you want it to look really cool, ride like a truck, have some nasty instability, handle like crap, fit an I beam with drums, cross ply tires, and chrome absolutely everything.
    The hot rod purists will gasp at how great it looks when parked. They will absolutely marvel at all the chrome, and the strategic lightening holes.

    The admirers of your masterpiece will not have to drive it every day.

    Decide if you are building a work of art, or a vehicle that is fast, safe, and comfortable, and do your own thing.........
  16. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,218

    Ned Ludd

    In other words, the lateral stress on the king-pin bushes for any given moment will be greater with the beam. Fair enough. But the pieces are generally sized accordingly, and simpler.

    On the contrary. The popular belief that an early-Ford-style transverse leaf suspension rotates about the central spring mount is quite erroneous. Barring extreme roll angles, most roll is barely resisted by the spring but rather by the self-correcting geometry of the spring shackles, which generates a very low roll centre. The roll centre might at times be under the road: one of the shortcomings of the system is that the RC migrates a lot vertically. The degree to which the spring is a factor at all in roll stiffness depends on the spring rate: with a very stiff spring almost all of the roll stiffness comes from the geometry. That is why it is even possible to run these sorts of suspensions - however marginally - without any other lateral-locating device.

    But if I'm not mistaken the OP's truck has parallel leaves up front?

    What you say about the viability of an anti-roll bar is true, though, firstly because the vertical RC migration makes sizing the bar difficult, and secondly because the spring-base effects I mentioned before will require a heavier bar to achieve any given gain in roll stiffness, and consequently a greater damping conflict. The chances of a vaguely acceptable rate of damping simply not existing is greater, never mind an optimal rate.

    Add up the components and, at the very least as regards single-wheel movements (i.e. rotation about the RC) you've probably got the same unsprung mass as a slightly overdesigned MacPherson strut set-up. Though perhaps not quite in the case of a heavy-duty truck axle - even if much of that mass is relatively close to the RC.

    I'm not trying to knock you, Warpspeed: you're one of the guys on here I most often find myself agreeing with.

    I must repeat what I said about camber: no factory IFS is going to go from the typical baseline 100" effective swingarm to the sub-30" range that'll give useful negative camber gain within 1½" of travel. And camber thrust is camber thrust, regardless of tyre construction. If the wheel is effectively a cone segment rolling about its apex - due simply to its not being vertical - there will be camber thrust.

    Indeed, camber was one of the few means available pre-c.1935 to tune suspension, before a lot of suspension theory was properly developed.

    I still say the beam-axle concept has unexplored potential. I still say the best beam-axle set-up has yet to be devised; but I also say that it is unlikely to happen on the OP's truck, and therefore recommend to him a wishbone IFS.

    I'm wondering if Japanese light-truck torsion-bar IFS could be pressed into use without undue difficulty, as they're generally well sorted for the purpose, and quite sturdy. I got to thinking about them because they're much more common in my part of the world than Mustang IIs, which weren't available new here. If I'm not mistaken they're even on removable crossmembers except for the torsion bar rear mounts.

    Last edited: Jul 15, 2018
  17. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532


    Ned, a narrow based king pin, can never be as rigid and free of slop as something much wider based. It is not a question of size, but of rigidity and free play.

    The roll centre height of a sprung beam axle is determined by the location of the spring. One side compresses, the other relaxes. That determines the roll centre. The shackles do not vary much in length. But even if they did, a front roll centre located at axle height is NOT a low front roll centre.

    Most modern IFS cars have a front roll centre located around maybe two to three inches. Locating a transverse leaf spring with the shackles installed below the road level is obviously not even possible.

    The High unsprung mass is mostly made up of the I beam, leaf spring, and front bones. All of that must weigh far more, and does weigh far more than the very small typical pressed steel parts of a modern IFS.

    It appears that every car manufacturer on the planet has now abandoned beam axle front ends, and for very sound reasons. They are certainly not all totally wrong in doing so.
  18. The only thought that I would like to add here (and it has been said at least once before) is that light duty car style suspensions don't really belong on trucks. First off, trucks are simply heaver then the cars that the popular donor suspensions come from. Second, if you have a truck you WILL undoubtedly load it up with way more weight then the donor cars could ever be loaded with (everyone with a truck no matter how nice will eventually use it for what it's designed for, carrying stuff).

    The stock suspension was designed with a trucks use in mind, it will do it's job even if it is rough around the edges and might handle like a truck. Add disc brakes if you can and it will stop safer.

    If you do a IFS swap just think about it and make sure it is up to the task at hand. There are a few companies out there that make truck specific IFS and in my opinion it might be worth the extra coin to do it right and safe once. At least the extra consideration.

  19. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532


    Absolutely Jaysin.
    Trucks, road cars, and formula racing cars are all very different.
    They have all developed along very different lines, but each is excellent for the weight and speed of the vehicle.
  20. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,218

    Ned Ludd

    The shackles don't vary in length, but they do vary in angle. The strict determination of the roll centre with this set-up is more complex than it seems, as there are in a sense two roll centres. I submit that, for most roll movements, roll is resisted primarily by the self-correcting geometry of the trapezoidal figure represented by the axle structure, the vehicle structure, and the two shackles - which generates an instant centre at the intersection of the shackles' lines - which is near or below the road. Because the self-correcting action does generate resistance, some of the roll is resisted by the spring itself, and brings in a second roll centre half-way between the spring mount and the midpoint of a line connecting the spring eyes.

    The relative importance of each of these roll centres depends on the stiffness of the spring: if the spring were absolutely rigid the vehicle would not be rigid in roll but would roll about the instant centre defined by the shackles. The softer the spring, the more secondary roll you'll get, about the spring roll centre, the duplication of roll centres resulting in all kinds of spring-resisted motions all over the place. Barring a very stiff spring I'd say it's very hard to determine where the final effective roll centre is going to be in any situation. The thing becomes a three-link chain rather than a simple hinge.

    So your point that this arrangement is woefully inadequate is perfectly true, only for more complex reasons than you state.

    There are many reasons, sound and cynical in varying degrees. The problem with the doctrine of "progress" is that it fails to recognize the redefinition of the problem with every "advance". The paradox in all this is that one can see "progress" as the development of new problems ...

    Be that as it may. A major factor in the adoption especially of IFS is the understanding of the effect of polar moment of inertia on effective spring harmonics, and the implications of that for ride quality, round about the time of the Second World War. My Morris Minor is a prime example of that thinking: short wheelbase, engine well forward, and a fair amount of suspension travel.

    Another factor is the improvement in road quality brought about by increased public spending on roads infrastructure, as a result of economic patterns that required increased vehicular mobility. That made camber shenanigans at extremes of suspension travel rather unimportant, among other things.

    The resulting increase in vehicle traffic made a virtue of shortness, so that an empty space ahead of a beam axle became a definite liability. It made more sense to put stuff like the engine there, which is more easily achieved with IFS - especially as roads that allow higher speeds rule out higher centres of gravity.

    These factors have nothing much to do with roadholding dynamics, but did lead to widespread adoption of IFS; and consequently all the thinking and all the research and all the investment went into independent suspensions. As a result, the better IFS happened, and the better beam-axle didn't.

    Our situation requires many, short, heavy, mass-produced cars on good but crowded roads - and IFS. A situation that requires few, long, light, artisanal cars on bad but empty roads might find advantage in a better beam axle.
  21. Roger O'Dell
    Joined: Jan 21, 2008
    Posts: 1,143

    Roger O'Dell

    This is a thread that I enjoy, lots of good comments, especially from Ned Ludd. Ned I'd bet you work or worked in this area. Thx Roger
  22. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532


    Sorry, but on a buggy sprung beam axle, the body always rolls around the springs.
    It can only be forced to do otherwise by the geometry of any lateral locating linkages. In fact, that is exactly what an IFS provides.
    If you believe you can make your Rod either under steer, or over steer at will, by just slightly changing the shackle angles, I wish you luck. The spring shackles can point any way you like, but the spring still compresses on one side, and relaxes on the other when the body rolls on it's springs.
    And that defines the point around which the body rolls on the suspension.

    The vastly improved (and still improving) ride quality of modern day cars has a lot to do with the now pretty universal fitting of soft rubber isolators in all the suspension linkages that greatly reduce transmitted noise, vibration, and harshness.

    While I agree, pitching in a very short wheelbase car can be rather objectionable, and polar moment is definitely a factor with that.
    Smoothness and quietness on the other hand, rather than having a rattling vibrating bone jarring ride, has much to do with the deliberate soft compliance of rubber bushes between wheels and chassis in all three planes.
  23. krooser
    Joined: Jul 25, 2004
    Posts: 4,583


    My Peterbilt rides nice with the I beam axle...
  24. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532


    I am absolutely sure you are right.
    A model T Hot Rod would ride nice too, if it had two tons over the front axle.
  25. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,248


    Note to self:
    Tell wife the big block and blower is for the ride...;)

    Interesting post guys....
  26. Thanks for all of the excellent comments. Keep them coming as this is getting interesting!!
  27. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,218

    Ned Ludd

    As Griffin doesn't seem to mind our arguing about a front suspension set-up quite different to his ...

    What would keep the shackles symmetrical? Surely the system would find its equilibrium where the shackles' intersection sits on the line of the resultant force through the centre of gravity? And, as the shackles aren't parallel at rest but represent the angled sides of a trapezoid, the necessary distortion of the trapezoid to put the intersection there must involve roll: I estimate in a typical set-up there's scope for about 4½º of roll, unless the outside spring eye hits the perch.

    Now, this roll happens purely because the resultant force on the vehicle body acts at an angle from the vertical: it doesn't involve the spring at all. But: the shackle instant centre migration consistent with the sort of lateral acceleration an early Ford can pull only represents about 1½º of roll, and we know an early Ford rolls more than that at the limit, it's obvious there's something else going on: and that does involve the spring. There are two simultaneous rotations happening about two different centres.

    Oh no, the suspension does that all by itself! and that's the problem. The spring length changes as the spring flexes, and that changes the angle of the shackles, which causes the shackle instant centre to jump all over the place. Not only that, it varies the importance of the shackle instant centre in the whole system. The geometric basis of lateral weight transfer changes from moment to moment, and that renders the car horribly unpredictable, skittish, and woolly. And that is to say nothing about trying to determine an adequate damping rate for that lot.

    This is particularly true if the spring is soft, as the degree to which the spring is involved in the suspension's movement changes all the time. Austin Seven specials have, however, been made to handle fairly decently on very smooth race tracks by using well-nigh inflexible springs and rather long shackles. In fact, the buggy-sprung axle represents quite an elegant method of location up to the point where a floppy spring is introduced ...
    Last edited: Jul 15, 2018
  28. mustangsix
    Joined: Mar 7, 2005
    Posts: 1,297


    I'm still trying to wrap my head around what these do.....

    Attached Files:

  29. Antny
    Joined: Aug 19, 2009
    Posts: 1,071

    from Noo Yawk

    If you want the best ride and handling for the $, then a purpose-designed IFS system is THE answer. I say 'purpose designed' meaning; an IFS system that is designed for your vehicle, not a junk-yard swap. Although the latter can be made to work properly, you asked for "best ride and handling". So imo, a new (not a 30-year old junker) IFS, designed for your application, is the way to go.

    If you want to know how much better a truck rides and handles going from an I-beam to an IFS, I can tell you first hand; it's like going from driving an old truck, to driving a sports Seriously, there is no comparison. Just my own $0.02 from my own experience.
  30. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532


    Antny, yes indeed.
    Even the hard core 4WD people know that a rubber mounted IFS is vastly superior for their yuppie luxury "sports utility vehicle" on the bitumen. But for the the seriously crazy rock hopping off roaders, only a massive and enormously heavy front beam axle and diff, has the strength and indestructibility they demand.

    Anyhow, Ned.
    I can sort of see what you are getting at. That the body can swing from side to side and rock on the spring perches, rather than flex the actual transverse spring. But if you take both the spring, plus the spring perches combined, as the general location where the body moves with respect to the beam axle, all the body roll still occurs at roughly axle height (or above).
    And that is definitely not a low point of roll, with respect to the road.

    Now that problem can be fixed somewhat. by fitting a lateral axle locating linkage such as a panhard bar, watts linkage, or something similar. The body is than forced to roll around the physical linkage, instead if the springs. The big problem with doing this, particularly at the front, is that the linkage needs to be physically located where you want the new roll centre to be. And immediately you have a ground clearance problem trying to do that.
    The Mumford linkage has been developed to solve this exact problem with a beam axle, but it would be hideously ugly fitted to the front a traditional rod, and just not a practical solution. Even though it would probably work very well.

    Last edited: May 19, 2010

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