Solid axle vs. IRS

[68sixspeed]

One thing that hurts a 4 link in roll is hard to picture at first, but the easiest way to describe it I've found is to consider the pinion angle change-- If you have some 'up' angle in the lower link as you would want to plant the tire, then as the suspension is compressed the bottom brackets move rearward sligtly and the top brackets move forward slightly making the pinion nose down under compression. If only one side of the car is being compressed in this situation the axel will just lock up in roll and be force to compress both sides evenly. (as one side of the car is trying to hold the pinion angle constant and the other is trying to move it up/down - trying to rotate the rear end housing.) Rubber bushings help this by allowing some stretch, but it's a length of bar issue, not a swivel/rotation issue with the rod end or bushing. To help this, you can set a true 4 link to more of a 4 bar setup by bringing the bars more into parrallel which minimizes pinion angle change and twisting of the axel tubes. (for example, I've heard of pro-street cars actually break rear end tube welds from the tube stress of street driving.) I hope that makes sense... -Dan

[pdq67]

I'mmmm Baaaack!!!

Well, AGAIN, has anybody ever thought about using the old Pontiac swing-axle transaxle by limiting the swing-axle's movements so they can't jack!!

Seem's I've read where I wanna say Smokey or maybe Jenkins used to race an early 'Vair up Pikes Peak and whipped some folks b/c he got the axle-jacking deal figured out!!

May be wrong on the guys, but still............ Early Ralph Nadar kid-killing 'Vair!!!

And if I was to create an IRS, I'd use a 9" pumpkin and top and bottom locating triangle/A-arm shaped, one-piece mono-leaf springs to locate the axle ends like the front suspension's A-arms and be done w/ all the Mickey Mouse crap used to locate it now!!

Except for the sway-bar and maybe a z-bar too...

pdq67

[maldo]

Originally Posted by toxicz28

This will refresh your memory.
Mona Lisa Vito: No, there's more! You see where the left tire mark goes up on the curb and the right tire mark stays flat and even? Well, the '64 Skylark had a solid rear axle, so when the left tire would go up on the curb, the right tire would tilt out and ride along its edge. But that didn't happen here. The tire mark stayed flat and even. This car had an independent rear suspension.

LOL LOL I can hear that in her NEW york voice she so hottttt

[BillyShope]

Originally Posted by MrQuick

Road racer street car would benifit more so than a drag street car.

Usually, but there's some misunderstanding here regarding dragstrip requirements.

The maximum effort...cornering OR tractive...from a tire pair is achieved with equal loading. While equal loading is essentially inherent with an IRS, the same is not true with a RWD car with beam axle. The driveshaft torque tends to unload the right rear.

So, why aren't there more IRS cars at the dragstrip? There are a couple of reasons. Durability was a problem with some of the earliest IRS units available to dragracers and, for this reason alone, most won't consider them.

But, probably the greater reason involves that which is called "hit" or "separation" on launch. In terms more common to this forum, dragracers feel they need more than 100% antisquat. While it's certainly true that such an arrangement provides additional initial loading of the rear tires, the dragracers seem to forget that "what goes up must come down."

So, with the design difficulties involved in achieving antisquat values even close to 100%, it's not difficult to understand why a Viper owner would succumb to dragstrip comments like: That car'll never be competitive with all that squat.

I wonder, however, how many Viper owners have never equalled their IRS 60 foot times after switching to a beam axle. I know of at least one IRS Viper that is doing very well.

It is possible, of course, to achieve equal rear tire loading on launch with a beam axle car, but some form of suspension asymmetry must be present to cancel driveshaft torque effects. My site is largely devoted to a discussion of these various forms of asymmetry. If you're interested in this sort of thing, I would direct you to Page 18, where there exists a spreadsheet for the setup of a 3link for equal rear tire loading. The spreadsheet allows the input of a desired antisquat percentage and has recently (within the last couple of days) been updated to take into account the unequal dynamic spring forces when antisquat is not 100%. (For this change, I had to assume that 40% of the roll stiffness is at the rear. If you really think you need more accuracy, you can copy the source for your own HTML file and change the .4 factor to something else. Considering the tolerances of the other parameters, I don't feel this is necessary.)
http://home.earthlink.net/~whshope
over 130,000 visitors

[van454]

This is an interesting discussion, I have been thinking about IRS for my 3rd gen Trans Am, and I was thinking C4, but maybe the Jag set-up is a better option? Has anyone looked at the 04-05 GTO set-up?

[calforniacuda]

Used C5 spindle, control arms, hubs, are pretty cheap. If you can live with stock track width the aluminum cradle for lower control arm mountings can be used. Suspension analyser program has free sample with location of control arm mounting points.

Not necessary to use C5 rear diff, 8.8 ford can be used or there are other good choices. Modified half shafts are needed. I'll try and post some pictures of my project

The cobra irs has some issues that can be easily fixed. The assembly is contained in a frame assembly that space permitting can be installed easier than most.

My limited experience has shown that a live axle is much easier to load and tune for drag strip, high traction situations. Irs would typically be used, not for straight line, but for road racing applications.

My 2 cents

[Ron S]

If you were never to use sticky tires,high HP motors or cars weighing over 3000 pounds I'd agree that the IRS set ups would work well.Most of the cars here are street cars,that get pounded on{lots of holeshots}I don't know of an affordable IRS that will be durable enough to put up with what I do to my cars.I'm sure you could spend thousands and thousands of dollars on good half shafts and beefed up center and still be spanked by a guy with a properly set up straight axle.I'm a firm beleiver in keep it simple stupid,durability always seems to go with that saying.Just my 2 cents.Ron

[wicked68]

dont be surprised if you get a lot of wheel hop with an irs

I have seen several late model cars running this that get nasty wheel hop under power in the first 200 feet.

[newby]

these open ended comparisons never work... there are too many possibilities.

for solid axle set ups there must be a dozen routes to go with varying degrees of fabrication required and their own pros-cons... from traditional 4 links to truck arm conversions to slapper bars to dse quadralink style set ups the list goes on and on.

for IRS you have just as many set ups... jag, ford mustang, crown vics and t birds, vettes, vipers, bmws, new gto's, porsche, custom 9", some newer suvs, home built and the list goes on.

when considering which route you want to go, you must decide on two things: how much do you want to spend and what do you HONESTLY plan to do with the car. if you plan to street drive and long trip the car, IRS without a doubt.

if you are not a member of your local scca club and the only venue you have for 100 miles is a drag strip then solid axle is probably the choice for you.

i would say that if you are north of 800ftlbs or your car is particularly heavy, solid axle all the way. however, if you have the money to build a reliable engine that puts out greater than 800ft/lbs of tq (which is coincidentally the max for most OEM IRS) then you probably have the money to modify any IRS to handle the power any way.

look at what the OEMs are doing. all of your 'top notch cars' are irs. the cars I most enjoy driving on the street all have IRS. Unless you live on the west coast, all roads are bumpy. I've had old ass solid axle cars that could pull a posted 45mph clover leaf on/off ramp at 60mph with no drama, but as soon as I hit a break in the pavement is was tokyo drift time. that craps gets old.

c4 irs and some others are different than say the c5/c6 that a previous poster bashed in that they are essentially 4 links. most ppl agree the gto and cobra rears make too many compromises and are not worth messing with. in europe people constantly rip the rears out of bmws for their own irs projects with great success. viper rears are pricey, vette rears will set you back around a grand, and most of your 'engineered' set ups will set you back at least that.

further more I would pay money to see ANY car launch well with 500+lbs/ft of tq and super hard super low profile run flat street tires that come stock on the vette... lack of traction does not constitute a design flaw.

if you want to get a real comparison your question needs to go more like: "...I have $2k to drop on getting a complete rear end in my car. I have a decent locker posi 12 bolt with drums under the car now, but I found a good deal on a dana 44 c4 rear. what are my options for the 12 bolt and which would you choose and why?"

	Detroit Speed Engineering Kyle & Stacy Tucker's Detroit Speed & Engineering

[Prechel]

i think a good question to ask would be who bought the 69 camaro from century 21 that had the c5 vette IRS swaped into it how did that turn out for the man who bought it what kinda problems did he run into if any

[Monte71]

i prefer live axle,but if IRS wasnt so expensive id prolly switch,and if it can handle 1200hp.

[Norm Peterson]

Originally Posted by newby

these open ended comparisons never work... there are too many possibilities.

<snip>

when considering which route you want to go, you must decide on two things: how much do you want to spend and what do you HONESTLY plan to do with the car. if you plan to street drive and long trip the car, IRS without a doubt.

if you are not a member of your local scca club and the only venue you have for 100 miles is a drag strip then solid axle is probably the choice for you.

I don't think the answer to the question of which suspension to use for street/trip/SCCA activity usage is anywhere near that absolute. For dragstrip duty it probably is.


Perhaps the questions for a typical PT car that should be asked are "by how much will the ride improve by swapping in IRS assuming that currently achievable stick axle suspension performance levels can at least be maintaind for each individual's specific driving?" and "after I get it done, will I think it was worth it?". Anybody else feel that a car that fails the second question will end up being sold?


Norm

[Jim Nilsen]

So far I am satisfied with my C4 IRS and front suspension. The car handles like a go kart. The corners have been really predictable and when it breaks loose it hooks right back up. The looks of the IRS is something you don't get with a straight axle. The ability to change camber is also a nice option although you can spend a lot of money and do the same with the right straight axle. I still haven't gotten on it hard enough to get wheel hop but we will see if it happens?

All in all it is what you like and then what the car likes for the way you have the rest of it setup, too much power and the IRS just isn't up to the task.

I built my rear frame so that I can put in a straight axle setup with ease and if I ever get too much power I may change it then. I paid $650 for my Dana 44 rear with the brakes and that is hard to beat.

[van454]

I have had a bit of look at the IRS from the VE commodore (pontiac G8) and it looks like a very well engineered set up. It is all on a nice sub frame too, shouldn't be too hard to fit one to another car. Maybe that would be a good option for my third gen T/A

[calforniacuda]

I think the cheapest and easiest IRS setup to put in another vehicle is from the Mustang Cobras. You can get a used entire system for under a $1000. There are lots of aftermarket kits to stop wheel hop. Anthing else needed like coilovers, bushings, sway bars, etc. The sytem fits into a cradel that was bolted into Mustang rear end.

[pdq67]

Reading right along.

pdq67

[Norm Peterson]

Originally Posted by calforniacuda

I think the cheapest and easiest IRS setup to put in another vehicle is from the Mustang Cobras. You can get a used entire system for under a $1000. There are lots of aftermarket kits to stop wheel hop. Anthing else needed like coilovers, bushings, sway bars, etc. The sytem fits into a cradel that was bolted into Mustang rear end.

At one point, I had a 3-D suspension spreadsheet that was based on this particular IRS. Damned if I can find it now, though.


Norm

[killer67]

Full chassis and independent is the way to go

[BillyShope]

Originally Posted by BillyShope

...and has recently (within the last couple of days) been updated to take into account the unequal dynamic spring forces when antisquat is not 100%.
http://home.earthlink.net/~whshope
over 130,000 visitors

Since "killer67" has revived this old thread, I have the opportunity to correct a comment I made earlier. Since, with complete cancellation of the driveshaft torque, there is no chassis roll, the relative values of front and rear roll stiffness have no effect and the spreadsheet change I mention above should not have been made. I have again corrected the spreadsheet on Page 18 and it is now in agreement with the spreadsheet on Page 40. In other words, if you calculate the distance forward to the instant center from Page 18 and input it into Page 40, you get the same results (i.e., identical angles for the symmetrical links.)
http://www.racetec.cc/shope

[Norm Peterson]

Originally Posted by killer67

Full chassis and independent is the way to go

If it were only that simple. Please read at least the first page of this topic.

"Full chassis" is kind of vague.

Anything as "one-off" as swapping an IRS into most any stick axle car is going to require some development. Unless, I guess, the car only gets used for easy cruising and/or static display at car shows and the IRS is simply a "bragging point".

At some risk of repeating myself within this thread, a well done stick asle suspension is better than in indifferently done IRS (and vastly better than a poorly done one).


Norm