IFS, why isn't it better?

MtnRunner · **Joined:** Wed May 28, 2008 8:32 pm **Posts:** 89

My buddies and I are having a heated duscussion. Why is the straight axle more preferred by off-roaders? Doesn't military hummers use IFS? It looks like they do.

My buddies are convinced that IFS would be better. I tried looking around the forum on why exactly IFS isnt as good as the straight axle but came up with nothing. Just about how people would rather not have IFS in the jeep liberties.

Dont all the new Jeeps have IFS now?

JeepBumm68 · **Posted:** Mon Dec 21, 2009 10:32 am

The problem with IFS is that you are limited on flex. The CV's can only handle up to a certain amount of droop. I believe it is something like 30 degrees. Also when you put the IFS we have in the libbys on tires bigger than 33's you will destory either the CV or the Diff. The diff is another weak point. Prone to cracking. Of course IFS tech as come along ways. Their are many ways to strengthing the IFS to make it stronger.

New Cv shafts (beirfield's, JBA shafts). The Pumpkin really has no way of strenghting it. you could take a d44 or d60 housing cut it down and make it an IFS but that would cost more than just doing a SFA

I am sure i am missing things or could be wrong in a few things.

dgeist · **Posted:** Mon Dec 21, 2009 1:27 pm

With respect to HUMVEEs using IFS, they use it for a different reason. The purpose of that platform is not crawling over rocks and negotiating radical obstacles. It's transporting small amounts of personnel, escorting convoys, acting as a patrol vehicle, providing a stable platform for vehicle mounted armaments, etc. over a wide variety of terrain. There are marines units that still use Defenders (if I'm not mistaken) since the purpose there is not armament, but absolute mobility with a full load in ALL terrain.

If you've ever seen a H1 rock-crawl, you know it's too long, too wide, too heavy, and while good at swallowing up large bumps and looking impressive off-road, it can't negotiate tight obstacles since it's diff is in a fixed location (the military has tanks for the really nasty stuff. Why be elegant when you can be brutal. In war, elegant doesn't typically get you a victory.) I'd much rather be in an H1 than a TJ if someone was shooting at me, though. Different tools for different jobs.

That's from a non-military point of view, BTW.

Dan

ridenby · **Posted:** Mon Dec 21, 2009 1:49 pm

Shannon's car is IFS. I think this is going to be future of rock racing.http://www.youtube.com/watch?v=UHK2JjiPoBs&feature=related

sleeve84028 · **Posted:** Mon Dec 21, 2009 4:35 pm

The reason the KJ's IFS isn't all that great is because Chrylser cheapened it up before releasing the production version. Had the Jeep engineers actually been able to design the truck for off-roading the KJ IFS could have been a lot more capable. But such is life in the world of budgets.

The HMMWV is completely different. Again by design.

kayadog · **Posted:** Mon Dec 21, 2009 5:56 pm

Just about all modern off road race trucks (baja, Paris-Dakar etc) use long travel independent suspensions. Most competitive rock crawlers still use solid axles, but that is changing.

Here's a good picture of a Baja trophy truck with a long travel front and rear independent suspension.

http://www.fstmotorsports.com/jump.jpg

My solid front axle xj has much better articulation and is much easier to lift than my kj, but the kj rides much nicer on rough roads.

tjkj2002 · **Posted:** Mon Dec 21, 2009 8:57 pm

You can make IFS/IRS work very well for rock crawling,but a very high price tag which is in the neighborhood of $30,000 per end(starting cost).

SFA/SRA is known for more strength then IFS/IRS,and the cost factor makes it far cheaper then a IFS/IRS setup and you get more strength,ease of finding replacement parts,less complex,easier to maintain,and can get more/just as much flex.

JL Rockies · **Posted:** Mon Dec 21, 2009 11:57 pm

IFS is gangster.

nzoliver · **Posted:** Tue Dec 22, 2009 3:02 am

Howdy. This debate will go on forever!! Yes there are US Special forces using Land Rover Defenders. But, the independent suspension thing is something that be excellent if dealt with in a good manner - cue the Discovery 3. Ever seen one of these bush bashing? Amazing! The biggest drawback to them is the :2cents:

Will be interesting to see those new Jeep military items being built at the Egypt plant.

Truck on... :JEEPIN:

dgeist · **Posted:** Tue Dec 22, 2009 10:13 am

nzoliver wrote:

Truck on... :JEEPIN:

<grin> One thing we can all agree on is that the little bouncing jeep emoticon is pretty popular...

LibertyFever · **Posted:** Tue Dec 22, 2009 10:45 am

Everybody has made good points about IFS. Like anything in life there are Pros & Cons.

Myself I think IFS limits the size of tire you can run and in turn the amount of ground clearance. But there are times when that big shiny engine skidplate under your IFS lets you bounce or slid over obstacles on the trail.

I've shared a few trails with other IFS trucks, usually Toyota pickups. They do great even if they only have 33's.

InCommando · **Posted:** Tue Dec 22, 2009 11:32 am

A cost-effect bullet-proof IFS is the issue for a lower price point vehicle like the KJ.

There are plently of pics on here of KJ's with decent flex. The flex is one of those questions that the stock answer is "they don't do it well" when, to my eye, they have more articulation than given credit for, especially the OME stuff.

Lift: IFS's are more complicated to lift. This is true whether it is a KJ or a Tacoma.

Speaking of Tacomas: stock, they have had numerous IFS issues over the years and are very popular SAS 's because of it. Search "birfield." As does Nissan, as does GM, as does.... on down the line. I sold a set of FSJ axles to a guy for a SAS on a Frontier because his IFS kept popping.

The biggest issue with the KJ is that daimlerChrysler, like they did with so many things, put out a cheap design with the D30A being the biggest issue. But again, a serious factory upgrade would lead to a serious cost increase and that was not the market for which the KJ was designed.

The Humvee IFS/IRS design is different than the KJ in that, going with a huge IIRC, the axles go into an axle reduction gear box at each corner. This would mean that the axles could run at a much less severe angle and the torque on the shaft itself is moved from the axle to the parts past the reduction unit. Even if that is wrong, it is true that the Humvee diff is nothing magical as it uses the same differential that Jeep used in the Model 20 & 23 in Cj's, FSJ's, and some cars.

The original buyers of KJ's in the market segment they were designed for will rarely off-road them. Jeep couldn't care less about what owners ( many 2nd or 3rd) want to do with their KJ's several years after purchase.

Sorry. it was random IFs thought Tuesday.

tjkj2002 · **Posted:** Tue Dec 22, 2009 11:44 am

InCommando wrote:

The Humvee IFS/IRS design is different than the KJ in that, going with a huge IIRC, the axles go into an axle reduction gear box at each corner. This would mean that the axles could run at a much less severe angle and the torque on the shaft itself is moved from the axle to the parts past the reduction unit. Even if that is wrong, it is true that the Humvee diff is nothing magical as it uses the same differential that Jeep used in the Model 20 & 23 in Cj's, FSJ's, and some cars.

Yep the HMMWV(H1's) use a aluminum IFS/IRS AMC20 diff,the older ones with the turbo400 trans use 3.27 gears and the newer ones with the O/D trans use 3.54 gears.And yes they use a geared hub at each end to lessen the CV angles,they provide a extra 2:1 reduction.With the use of the geared hubs they need special gears to rotate in the reverse rotation(not the cut of the gears either,they are LP diffs).The brake rotors are also bolted right at the diff outputs,as seen in this link--http://flashoffroad.com/Maintenance/Differential/Diff_removal.htm.

I have also replaced many of those HMMWV's diffs,from broken cases to sheared output shafts,they do break but your talking about a vehicle with a curb weight from 5500lbs to well over 12,000lbs using the same diffs,geared hubs,and CV shafts.

TAHOE · **Posted:** Tue Dec 22, 2009 12:54 pm

My FSC IFS gives me a nice comfortable ride..... :BANANA:

Sucks though, 4" lift kit for my Tahoe would push $2000 to do it right and still not gain tons of ground clearance due to dropped front crossmember.
I think on emain reason for the IFS was to maintain a 4x4 option, but drop center of gravity while also giving better ride quality. Then come then birth of your family trucksters and SUV mall crawlers. I loved my Old K5 SFA blazer, but with a lift, it had rough ride and not as stable as my IFS Tahoe. Lot of the reasons were to cater to the masses.

JeepBumm68 · **Posted:** Tue Dec 22, 2009 1:00 pm

the Hmmv's pretty much use portal axles on the end. if a KJ a kj could be fitted with portal axles, they would be able to run 35's no problem and gain atleast 3 inchs of ground clearance. Biggest problem is portals are COINNNNNN.. and putting them on the fronts would be just most expensive than doing a SFA swap. The rear of a kj well flex madly with no problems.

FreedomKJ · **Posted:** Tue Dec 22, 2009 4:29 pm

One of the big reasons for selecting independent suspension over solid-axle suspension for a mostly road going vehicle (like the KJ) is ride quality. With IS you have significantly less unsprung weight than you do with SAS. Unsprung weight is basically everything under your springs. Tires, wheels, and on a solid axle, the entire axle assembly. The more unsprung weight you have, the harsher the ride.

Those Baja/SCORE/Trophy/Dakar trucks are IFS primarily because they are rear wheel drive only. They almost always, as in the above photo, have solid rear axles. When you have no halfshafts or front differential to worry about you can get enormous travel out of an IFS system. I can't find a picture at the moment but I've seen IFS setups on Baja trucks where the lower control arms actually cross eachother.

If you're really interested in long travel IFS on a KJ it might be worthwhile to contact Total Chaos Fabrication. They have IFS kits for Nissan and Toyota trucks that give 12+ inches of travel.

dieselenthusiast · **Posted:** Fri Dec 25, 2009 12:59 pm

MtnRunner wrote:

Dont all the new Jeeps have IFS now?

IIRC, only the Wrangler comes factory with a SFA. All the other jeeps have some type of IFS.

venturaII · **Posted:** Fri Dec 25, 2009 5:05 pm

Humvees use portal hubs to acheive the required ground clearance without having to resort to extreme driveshaft angles. But they are still saddled with short control arms and relatively short halfshafts, which really limits the amount of travel they have. Humvees are actually lousy rock crawlers - they only do as well as they do because they are locked when in Lo range. But the whole time, they are picking their tires up off the ground as they try to articulate.

KJs and other IFS production trucks have the same problem. Custom built off road racers with independant suspension have great travel because their control arms are as long as they can possibly be - they don't have to worry about passenger compartment intrusion or other chassis packaging requirements that a street vehicle is forced to deal with.

oldMountainMan · **Posted:** Mon Feb 08, 2010 3:13 pm

One disadvantage of an IFS as opposed to a SFA for wheeling that hasn't been mentioned is that with the IFS, when the suspension flexes, the front diff (already about the lowest point on a Liberty) gets lower, and more likely to scrape a rock, whereas on a SFA, the differential obviously stays at the same height relative to the wheels. If you're going over rocky terrain in a stock liberty, you want to take it nice and slow, because if you bounce it, that front diff (or the skid under it) is going to catch a rock.

XJMA · **Posted:** Tue Feb 09, 2010 1:32 pm

Solid axles are best for wheeling while IFS is best for on road ride comfort.

Solid axles are simple, can be made to flex to ridiculous extremes, and are strong. Also they're easy to upgrade and modify.

Independent suspension is very limited in terms of flex, and not so strong, and for the most part hard to upgrade. The exception is some of the new ridiculously expensive competition rigs but that's kind of an exception to the general rule.

IFS rigs with solid rear axles can generally do pretty well with bigger tires, but they just can't get the flex that a solid axle can. Wheels in contact with the ground, locker or not, are a good thing. Even if you have a locker, that tire in the air ain't helping you go forward.

Again, there are now some new ridiculously expensive competition rigs that use a special custom made independent suspension, but that's nothign like anything that you can buy at a dealership!!

There is a pic that says it all, but I can't find it. IT's the same vehicle with the same size tires on the same obstacle. The first pic with IFS it is lifting a tire way off the ground and can't make the obstacle. The second pic there is a solid axle and the tires are both touching (although the axle is close to 45 degrees, haha) and the truck made the obstacle. I'll try to find that pic!

IFS, why isn't it better?

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