IRS vs straight axle

Dirt Rider

Well-known member
This may have been touched on before, but every time I watch a video about over landing it strikes me how much the IRS vehicle wheels are off the ground due to a lack of articulation. I know the on road ride is nice and they handle better, but they seem to lose out in the traction department compared to straight axle vehicles. Also on side hills they will lean into it more and be more inclined to tip. Anyone know where the trade off is?
 

naks

Well-known member
Provided you have sufficient torque and traction control (because brake operated traction control effectively cuts your torque in half while it's active, due to gearing in the differentials) or locking differentials lifting a tire isn't generally a traction problem.
Limited articulation can be a stability problem but that is often possible to mitigate with careful line selection and willingness to get out and stack rocks or elect not to attempt certain routes and obstacles.
In the context of cross country travel on unimproved roads really it's not such a big deal in any case, either type of suspension has its strengths and weaknesses but they both work.

as an example:

 

MTVR

Well-known member
IRS is superior if engineered to be used off road. The problem is, it’s not engineered to be used off road. There’s very little that can be done aftermarket to improve it without completely re-engineering it.

This.

The stock IRS on our truck has 13" of travel, and the stock IFS on our truck has 16" of travel, because it was designed to use off road...
 

1stDeuce

Explorer
MTVR, off the top of my head (I'm not the OP...), Jeep Grand Cherokee 2011+, a bunch of LR's, Ford Expeditions... Pathfinders...

From my suspension and chassis engineering brain: Looking at similarly equipped stock-ish vehicles, one with a solid axle (Front or rear) is generally going to have quite a bit more articulation than IFS or IRS. This is because most of the vehicles we're discussing have around 8" of total travel vertically. (Obviously not an MTVR, but it's a bit outside this discussion...)

The reason a solid axle has more travel in roll is because the stops generally limit travel about 1' or so inboard of the tire position. As the axle articulates, the tire actually moves 10" or more becuase of the axle angle swinging it farther up or down. Independent suspension only moves 8" ever, it doesn't matter if it's "articulation" or straight jounce.

Also playing a part in the traction scenario is that the "wheel rate" (spring rate at the tire). With IFS it is always the same. With a solid axle, it is highest (IE, matches the actual rate of the coil or leaf spring) in straight compression of both sides at the same time, but the effictive rate at the tire in roll (articulation) is actually quite a bit lower than the spring rate. This is again because the spring is inboard of the tire by 1' or so, which means it is effectively acting on the tire through a 1' lever. This lower roll rate does a far better job of keeping weight distributed equally on the tires than IFS does.

Which takes us to mobility... In my experience with OE type vehicles, the idea that with lockers, traction control, etc, articulation doesn't matter is fairly incorrect... Even with lockers, if you're lifting tires, they're not providing traction. If you don't have more even ground contact pressure, it can be far more difficult to climb a ledge type event. With lockers or TC, you may still get through an obstacle, but better articulation and more even ground contact pressure get you through with far less drama or stress to vehicle components.

This is the dilemma of the adjustable height suspensions that can be found on LR's and Jeep GC's... You can make the vehicle higher for clearance, but this is at the expense of compliance because the droop travel of the suspension is reduced by however far the vehicle lifts itself. Reduced droop travel means you're lifting tires, teetering around, relying on TC or lockers, putting more stress on the tires that have traction, etc, rather than just driving through with all the tires on the ground. Therefore, if clearance isn't an issue, my experience has been that it's often better (ride, mobility, etc) to leave the suspension lower so the tires stay more evenly planted.

Hopefully this helps...
 

Smileyshaun

Observer
Doesnt matter if it’s ifs, irs solid axle with leafs or coils , 2” of travel or 20” of travel It matters more on the driver and traction. seen plenty of rigs with all sorts of travel And equipment that have drivers that can’t use it or they just always use it to show off but it never leaves the driveway .
 

billiebob

Well-known member
IRS is superior if engineered to be used off road. The problem is, it’s not engineered to be used off road. There’s very little that can be done aftermarket to improve it without completely re-engineering it.
THIS ^^^ nails it.

Independent Suspensions are built to offer more compliant, smoother travel on regular roads. They work well within the parameters they are designed for but Solid Axles offer far more options to upgrade or tune a suspension for travel far beyond those of a smooth, compliant ride.

PS, articulation is often over rated.
 
D

Deleted member 9101

Guest
My dads Navigator has IRS, thing rides like a car even when it's loaded down and pulling a travel trailer.
 

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