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Discussion Starter #1
I got some ribbing asking questions about IFS in a thread outside newbiedom saying all real wheeling is done with solid axles, I was curious when IFS has a point... since it was deemed good enough for the AM General Hummer H1, Pro-2, and Trophy Trucks.

I assume more wheel articulation being possible is part of it? How far can solid axles reasonably get for inches of travel anyways?

Are solid axles the best for rock crawling to keep scrapey differential bits up out of the way, but independant best for moving faster off road over less severe obstacles within-the-articulation-limit like barreling over a farmers field or down an old trail?
 

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In theory, IFS should be the superior suspension system, compared to a solid axle. The ability for each tire to move over terrain independently of each other is supposed to give them an advantage for better ride and control of the truck. But in practice, the IFS we end up with in a production truck typically falls short for several reasons, often related with costs. When cost is not an issue, it's hard to beat a well designed IFS. The H1 Hummer and a Trophy truck are good examples of this. The H1 was designed for the military and paid for by the unlimited resources of the American people. And you can clearly see the amount of engineering in the design, including geared hubs, an air supply fed thru the hubs, a heavy load capacity and significant sealing to resist water intrusion.
Trophy trucks benefit from the financial resources of their sponsors. They are designed for high speed over rough terrain and incorporate long wheel travel, heavy duty components and sometimes exotic materials to give them a competitive advantage.

A manufacturer could build a killer IFS under their trucks, like that of an H1 or a trophy truck, but that cost has to be passed on to the consumer, who is always looking for the best deal. So to keep their trucks cost competitive, manufacturers offer a suspension system that performs sufficiently enough to appease the general masses. Unfortunately, people like us are in the minority. Most people don't know or care about long wheel travel, or suspension articulation. They just want trucks that ride good, get decent fuel economy and don't need a step ladder to get into. These are features the manufacturers are currently building into their trucks, but these features affect the design of their suspensions in such a way that it doesn't appeal to those of us who want an off road capability in our trucks.

Take for example, fuel economy. One of the main reasons the 4x4 solid axle went to the wayside was that as an assembly, the solid axle was heavy. Current 4x4 IFS systems are designed to be as light as possible. They incorporate aluminum differential housings, and aluminum lower control arms, saving a few lbs. But these components aren't as durable. Aluminum diff housings are subject to blowing out and control arms are subject to fractures.

These days manufacturers are trying to make their trucks more appealing to a broader market. So to make entry and exiting easier for certain people, such as women and the elderly who are considering a truck, the manufacturers are lowering the ride height of their trucks, including 4x4s. IFS allows the ride height to be lower than a truck equipped with a solid axle. The problem of course is obvious, for an off road rig, we want more ride height.

Where does this leave us? Well considering the design perimeters that new trucks are built to, it's no wonder IFS typically falls short. The design is made to be as cheap and as light as possible. The current crop of IFS equipped trucks are not cheap or easy to lift. The components aren't designed to handle the additional stresses of larger tires and/or more powerful engines. They aren't durable enough to handle off road conditions such as mud, water, snow, running high speed terrain, or rock crawling. So our options are limited to either building a fully custom IFS of our own design, or swap to a tried and true solid axle which come in a far greater variety, are easily adaptable, and typically much more durable, thru and thru.

Ed
 

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Wow Ed, that was one of the most well written level one responses I have seen on the topic. Perfect for the newbie like me (newbie to 4x4, but engineering and marketing old-timer).

I suspect that per dollar spent the return is greater with regards to IFS as speeds increase (due to the reduced unsprung weight of IFS) and that at slower speeds, IFS dollars just don't return much.

Thanks
JB
 

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its a lot better on the road i guess. all the solid axle trucks i own/owned drive like ass down the street
 

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Discussion Starter #6
Thanks Ed, that's a pretty amazing response. ^O_O^ I think there should be a Reference Library of great posts on certain topics and that would be one of them.

So in a nutshell, the solid axle is alot closer to 'self contained' - if i'm upping the strength, other than maybe changing the spring pads there's not much else that absolutely needs to be altered. Whereas the moment I start modifying the IFS beyond stock design (like longer arms that get more articulation) all the little parts and upstream stresses start needing further upgrades, weight of components change possibly needing other upgrades, etc.

Does unsprung weight notably change fuel economy? I know weight in general does around town, just wondering if an IFS 4x4 would get better mileage towing for instance or in extended highway travel.
 

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In terms of weight reduction, Un-Sprung Mass and Rotating Mass reductions will net a greater return in terms of performance by an order of magnitude than removing weight from inside the cabin.

IIRC from my Honda Track/Autocross days, it was estimated that shaving 1kg from the wheels and tires was equivalent to removing 10kgs from the vehicle.
 

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its a lot better on the road i guess. all the solid axle trucks i own/owned drive like ass down the street
IFS will generally have the advantage in ride quality. Theres no denying that when both wheels can react to bumps independently, the ride can be quite smooth, however, thats not to say that a solid axle can't have a relatively smooth ride. With few exceptions, most trucks on the market, these days, have an independent suspension only at the front. Most trucks still use a solid rear axle.

Back in the days of the land yacht , those big Caddys, Lincoln's, Imperials, etc. all rode very smoothly, almost like they floated on a cushion of air, like a hover craft. All of them had IFS, but they also rode on solid rear axles. Some of todays trucks can offer a pretty smooth ride using the same basic set-up of IFS and solid rear. What makes a vehicle ride well isn't just the type of suspension used, but the springs that supports the vehicle. There were some trucks that had both front and rear solid also that did have a smoother ride, like some Land Rovers.

The solid axle has gotten a bad rap for ride for quite some time, but I don't always fault the type of axle being used. But the rest of the suspension supporting the axle may have been designed wrong. In stock form many of the older trucks that used solid axles also tended to have very little upward wheel travel, and the axle hit the bump stops frequently. The springs were sometimes too stiff for the expected load. Not only that, but if you went to a lift, some lift kit manufacturers used lift springs that were overly arched, or coils that were of a larger diameter. These things created a rough ride. OTOH, there's been a few IFS rigs that should have had a smooth ride but didn't.

Everything is relative, an IFS typically has the ride advantage, but solid axles are known to deliver relatively smooth rides too. IFS can also have a worse ride than it should too. I've driven IFS equipped trucks that rode like lumber wagons.


Thanks Ed, that's a pretty amazing response. ^O_O^ I think there should be a Reference Library of great posts on certain topics and that would be one of them.
Thank you

So in a nutshell, the solid axle is alot closer to 'self contained' - if i'm upping the strength, other than maybe changing the spring pads there's not much else that absolutely needs to be altered. Whereas the moment I start modifying the IFS beyond stock design (like longer arms that get more articulation) all the little parts and upstream stresses start needing further upgrades, weight of components change possibly needing other upgrades, etc.
I don't know if I can say that the solid axle is more 'self contained'. But I will say that, solid axles are generally very durable and have huge aftermarket support. The Dana family of axles in particular, is essentially the small block chevy of axles.
Every part of any Dana axle can be changed or altered, upgraded or simply replaced with a bigger model. Theres also huge support for other axle makes such as the popular Ford 9", the GM 14 Bolt, or Rockwells, so that you can build virtually anything using these axles as a base. Current IFS axles do not have no where near the support or adaptability of a solid axle. In the last few years some manufacturers have been coming up with better parts for these axles, but if you change a part somewhere, you can be pretty much on your own elsewhere in the system. And some parts simply don't exist yet, such as lock out hubs or a free spin system, to get rid of those cruddy wheel bearings. Nor are they economically adaptable.

Does unsprung weight notably change fuel economy? I know weight in general does around town, just wondering if an IFS 4x4 would get better mileage towing for instance or in extended highway travel.
In a nut shell unsprung weight can change fuel economy. The advantages are minimal but as you said reducing rotating mass will require less power to turn, thus saving fuel. In terms of suspension design, most manufacturers have gone to IFS because there is some weight savings that can be made. It's like front wheel drive cars. This design configuration has reduced weight because the axle and the tranny can be combined in a single, lighter package. Solid axles have to be heavy because the axle is designed not only to transmit power to the ground, but bear the weight of the truck, cargo and passengers. The IFS can be made lighter thus saving fuel because the axle only needs to transmit power to the ground, the vehicle, cargo and passengers are supported by the IFS suspension.

Ed
 

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IFS has lower unsprung weight as mentioned, plus the dynamics of a double A-ram geometry can can really control the suspension dynamics better than a solid axle. Anti-dive, camber/caster change, roll center change etc. You are more limited with solid axles, its hard to have camber with a solid axle.

So for suspension handling (high speed) IFS is better if you know what your are doing because there is a lot more variables.

IFS Limitation,
Strength for one both suspension joints and CVs but that has come a long way now just expensive. The joint at the knuckle need to be a ball go up down and turn just expensive for the good ones. Solid axles only need to turn.
Travel, it is hard to get a lot of travel, CV's only have so much usable angle. You also have to remember that steering and suspension travel get combined for a total angle, it is not as simple as add the two together but they do get added together.
That being said you need to limit both the suspension travel and steering, one reason IFS is not as good in the rocks. IFS cannot steer as well, not a big deal for going fast.

So suspension travel is limited in the down direction by CV angle (suspension/steering) and the width (Narrow diffs are expensive and less common). IFS is limited in the up direction by the diff hitting the ground.
H1's have portal axles so the diff is higher, this adds weight and complexity to the diffs.
Second reason there not as good int he rocks, with solid axles the diff goes up when one tire goes up basically the opposite with IFS.

IFS usually gains a little in visibility so you get some advantage in the rocks with that.

Most of those trophy trucks are only 2WD but have a crap load of HP to make up for it.
So they don't have to worry about CV angles and Diff Clearance.

So for the mass majority solid axle is still better than IFS, price/strength, complexity, ground clearance.
In my opinion for the top Ultra4 drivers doesn't really matter what they drive, a well tuned vehicle with a lot HPs that doesn't break will all run neck and neck.

But that is the coolest thing about this motor sport, what other sport allows such variety and they all are neck and neck.

Just my opinion but I hope this helps.
~Justin
 

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TTB axles found in the front of older Fords would be the best in between of both worlds. They are decently strong, have some aftermarket support & wheel decently. They can also go fast. Used parts are cheap, because of people SASing.

Ed
 
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