Why no IRS on big trucks?

[sportyaccordy]

Also, has any truck manufacturer made a system that would allow them to use a varying amount of springs on the back end depending on load? I.e., spring VTEC? Like one coil spring inside another or something?

I was on a construction job last week and we had a trench dug across the street... when trucks would pass over it the front suspension would be OK, but the back almost always crashed over it. I know this has to do with the springs being super stiff to accommodate various loads, but what (besides air springs) is being done to solve this problem? I'm almost certain better suspension design aids fuel economy, not to mention, it's much easier on our streets.

[93JC]

has any truck manufacturer made a system that would allow them to use a varying amount of springs on the back end depending on load? I.e., spring VTEC? Like one coil spring inside another or something?

[Eye of the Tiger]

Also, has any truck manufacturer made a system that would allow them to use a varying amount of springs on the back end depending on load? I.e., spring VTEC? Like one coil spring inside another or something?

I was on a construction job last week and we had a trench dug across the street... when trucks would pass over it the front suspension would be OK, but the back almost always crashed over it. I know this has to do with the springs being super stiff to accommodate various loads, but what (besides air springs) is being done to solve this problem? I'm almost certain better suspension design aids fuel economy, not to mention, it's much easier on our streets.

Spring VTEC? One spring inside another? Yeah, you pretty much answered your own question, because that's what airsprings are often used for.
And as far as IRS on big trucks, I don't think it's cost effective to put all the work into making an independant setup as durable as a solid axle already is.

[omicron]

Pfsht. Just wait a few months and buy an IRS Commodore SS-V ute with a 6.0 V8 and a six-speed manual.

[S204STi]

A solid rear axle (particularly of the full-floating kind) is still the strongest way to carry a load.  IRS has more weak points due to multiple load bearing members. With the coil or leaf spring attatched directly to the axle, the axle does all the work, being able to direct the load across its entire width to the wheels and to the ground, and the links (such as trailing arms, etc) just guide its travel. But with IRS one control arm with its ball joint and mounting points per side must bear all of the load, particularly when cornering (I suppose a sway bar helps, but the principle seems the same) and the overall design is not condusive to bearing heavy loads. 

However, my dad's old Hummer had IRS all around, and it had either a 3/4 or 1 Ton rating, I can't remember exactly which.  He also had a rear ball joint break on him.

Perhaps our resident engineers can correct me if I'm wrong about something.

[omicron]

2007 Falcon wagons and utes still have leaf springs which are distantly related to those of the 1960 Falcon, and apparently they still do a fine job.

[SVT666]

Solid axle is best way to haul heavy loads.  Besides IRS will hang lower and increase the weight of an already heavy truck.

[Tave]

And as far as IRS on big trucks, I don't think it's cost effective to put all the work into making an independant setup as durable as a solid axle already is.

That's my understanding of the situation as well.

However, my dad's old Hummer had IRS all around, and it had either a 3/4 or 1 Ton rating, I can't remember exactly which. He also had a rear ball joint break on him.

It's also a very expensive vehicle. As Nick pointed out, you can do it, but you've got to pay to play.

[GoCougs]

Cost vs. benefit, pure and simple.

The cost of a cushier ride and better handling dynamics of an IRS doesn't hold much sway with corporate customers.

Just with about any design, IRS could certainly be made to handle heavy loads and lots of abuse. In fact, it has been. Reference the HUMVEE already mentioned, or this monster, the US military's HEMTT:



Regarding "VTEC" suspension - that too has already been done: air bags are the current technology. Bose (the radio people) have developed an infinite spring rate setup using high-bandwidth linear motors in place of the springs.

[Tave]

Regarding "VTEC" suspension - that too has already been done: air bags are the current technology. Bose (the radio people) have developed an infinite spring rate setup using high-bandwidth linear motors in place of the springs.

I've heard about that. Wasn't it supposed to re-invent the tech when it came out? It's been awhile, what's going on with it?

[GoCougs]

I've heard about that. Wasn't it supposed to re-invent the tech when it came out? It's been awhile, what's going on with it?

I haven't heard anything about it in a while, either.

I suspect it is the same issue as IRS on a commerical truck: the cost vs. benefit equation isn't balancing.

[Michael Estorol]

you also have to bear in mind the extremely conservative mindset of the commercial vehicle buyer, which always tends to favour evolution over revolution.
I wouldn't want to be the first to adopt anything so radical; even if it was cheap and a salesman told me it was the best thing since sliced bread.

I happen to know (at very close hand) a company which caught a massive cold (in tens of thousands of pounds) thanks to being an early adopter of disc brakes on semitrailers about ten years ago...

[280Z Turbo]

I would think it also has something to do with getting too much negative camber when a heavy load is placed on the back of the truck. That would be bad for the tires.

[heelntoe]

I would think it also has something to do with getting too much negative camber when a heavy load is placed on the back of the truck. That would be bad for the tires.

i see that all the time on some of the smaller 'trucks' here. the manufacturer adds positive camber to compensate but that is just as bad for the tyres when there's no load at the back.

[GoCougs]

I would think it also has something to do with getting too much negative camber when a heavy load is placed on the back of the truck. That would be bad for the tires.

Whose to state that independent suspension has to be the double A-arm style?

[sportyaccordy]

Whose to state that independent suspension has to be the double A-arm style?

Camber will be an issue with any non-dependent suspension.

However, that can be solved by either using good air springs or supercharged gas level-loading shocks.

[Raghavan]

However, my dad's old Hummer had IRS all around, and it had either a 3/4 or 1 Ton rating, I can't remember exactly which.  He also had a rear ball joint break on him.

How can you have independent rear suspension in the front?

[GoCougs]

Camber will be an issue with any non-dependent suspension.

However, that can be solved by either using good air springs or supercharged gas level-loading shocks.

Camber seems to be an issue with any suspension; independent or otherwise.

A live axle may have zero camber when loaded, but that what about turning (or turning over a bump or uneven ground); especially when it's both a driving and steering axis.

The ideal I think would be actuator-controlled mechanism that controls camber based upon speed, turn radius, loading, etc. Getting the algorithm correct would be key, but then at that point is the cost and hassle worth the benefit. I think live axles and leaf springs are hear to stay for heavy duty trucks. Just get those drivers spring-air seats!

[S204STi]

Equal length upper and lower a-arms wouldn't necessary suffer any camber change during travel.? I suppose I overemphasized geometry issues in my first post, but if a manufacturer was to focus on towing ability rather than cornering ability with an IRS rear suspension they could do it, it would just be a compromise between vehicle dynamics and towing ability, which would beg the question of whether it was even worth it in the first place.

Also, a solid rear axle does not suffer camber change under load, nor does it change under cornering, which could be good or bad.

Edit: could be argued it technically does, since camber is the relationship of the vertical centerline of the tire to the centerline of the vehicle, so if the body rolls the relatioship changes.

[S204STi]

How can you have independent rear suspension in the front?

Oh cripes...

[Soup DeVille]

Camber seems to be an issue with any suspension; independent or otherwise.

A live axle may have zero camber when loaded, but that what about turning (or turning over a bump or uneven ground); especially when it's both a driving and steering axis.

The ideal I think would be actuator-controlled mechanism that controls camber based upon speed, turn radius, loading, etc. Getting the algorithm correct would be key, but then at that point is the cost and hassle worth the benefit. I think live axles and leaf springs are hear to stay for heavy duty trucks. Just get those drivers spring-air seats!

Most of them already have them.

[etypeJohn]

I think you don't see IRS on pickups for a variety of reasons, some of which have already been expressed.

Cost.  IRS costs more.
Complexity and weight. More U-joints or CV joints to wear and replace.  Additional parts and weight.
Need:  Customers probably haven't expressed a desire for IRS in a pickup.
Proven technology.  a solid rear axle and leaf or coil springs with locating links are stone simple, proven technology. 

[S204STi]

^^^^^^^^^^^

[Soup DeVille]

I don't think people who haven't ever taken apart a truck axle realize how heavy they already are. The differential carrier/ ring gear assembly probably weighs more than the average American V8. The axle shafts weigh over 100 pounds each. The entire axle assembly, with brakes, but sans wheels weighs a little over 1500 lbs. Thats a single axle.

Now imagine how much complexity and weight an IRS setup would add.


Then realize that all rigs operate under strict weight limits: and sacrificing 1000 lbs of caargo capacity in order to carry around 1000 lbs of truck simply isn't good business.

[sportyaccordy]

Camber seems to be an issue with any suspension; independent or otherwise.

A live axle may have zero camber when loaded, but that what about turning (or turning over a bump or uneven ground); especially when it's both a driving and steering axis.

The ideal I think would be actuator-controlled mechanism that controls camber based upon speed, turn radius, loading, etc. Getting the algorithm correct would be key, but then at that point is the cost and hassle worth the benefit. I think live axles and leaf springs are hear to stay for heavy duty trucks. Just get those drivers spring-air seats!

I've thought about this a lot... it would be awesome but really it's probably just as good for the manufacturers to design the camber curve to match how the car will lean under turning loads, as camber doesn't really affect performance much in a straight line.