Solid axle vs. IRS

[MoeBawlz]

Ive seen and read lots and lots of talk on the geometry of a solid axle. But what about IRS? obviously IRS have some advantages over a solid axle in the geometry or all the lemans race cars and many others would be using solid axles and not IRS.

What do you guys know of IRS geometry, as well as compairing it to solid axles in performace as well as practicality and ease of design and fabrication.

[MrQuick]

thats been a debate for years... it really depends on what the cars usage would be. Road racer street car would benifit more so than a drag street car.

Advantage for IRS, many, camber gain in turns, adjustable castor and toe and better contact patch... you saw "My cousin Vinny" right? Can't explain it better than that. LOL

[MoeBawlz]

seen it, dont remember it... been a while.

is setting up an IRS that much harder than a solid axle?

[Steve1968LS2]

An IRS is usaully stated to have less unsprung mass but it has much more sprung mass, it ends up mostly a wash.

Hard to get good anti-squat with and IRS.. many a new Cobra owner has ripped out the factory IRS and gone back to a live axle..

I would think an IRS system would be harder to install than say a three or four link equipped live axle. It really comes down to how you expect to drive the car.

[toxicz28]

seen it, dont remember it... been a while.

is setting up an IRS that much harder than a solid axle?

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.

[ProdigyCustoms]

Long story short, a chassis car with a good IRS set up like a C5 / C6 that uses upper and lower arms, properly set up, will be very hard to beat. But the new DSE 4 bar with the swivel bars, and the LD 3 link will put up a good fight.

For me, total hardcore...... Full chassis and independent.

[RobM]

On a perfectly smooth and level skid pad a solid axle will have just as much cornering power as and IRS with unequal length control arms. If the IRS isn’t set up right the solid axle will most likely be even better.
The down side of the solid axle designee is the second one tire hits a bump in a solid axle car it’s going to give a compromised contact patch to both tires. Even a car with a solid axles FRONT AND REAR (early nostalgia hot rod style I- beam double wish bone) technically would handle well on a flat smooth surface because the tires have no choice other than to use their entire contact patch, that’s technology from the early 1900's!. of coarse once the road gets bumpy this goes out the window.
I personally think IRS has more stuff to go wrong and is over rated. Id go with truck arm or a good three or even four link with a solid axle. Just remember the secret to handling is to keep the tires contact patch as large as possible. Suspension in a race car isn’t for comfort its to keep those tires planted. This is why a go-cart, a vehicle that is immune to body role for all intensive purposes handles so well. The bottom line is the tires are always in 100% contact with the road no matter what

[baz67]

This is why a go-cart, a vehicle that is immune to body role for all intensive purposes handles so well. The bottom line is the tires are always in 100% contact with the road no matter what

At least until roll stiffness lifts the inside front in a corner. Other than that I agree 100%.

I feel solid live axle with a link style suspension is the better choice for a PT aplication over a IRS. Yes, the IRS can be better. However, it is also more complicated to install and tune.

[boodlefoof]

Steve,

Where would the extra weight come from with the IRS? A solid rear is pretty heavy... I would think an IRS system would weigh less even with control arms and such. Or at least it would be close.

What about running a transaxle and eliminating the differential all together in an IRS system. Seems like this would save some weight too and shift some weight to directly over the rear tires.

[MrQuick]

Where would the extra weight come from with the IRS?

from the extra bracing,axles are pretty heavy,links and such, But like everything eles,can be made with lightweight material then you start getting expensive. Unless you make everything yourself.

[ProStreet R/T]

IMO when looking at something like this you need to seriously consider what you want the car to do, your budget, and mechanical ability.

For a comfortable street car that will handle extremely well IRS is great. You can use stock vette, cobra, or viper rear ends and have most of the components ready to go. But as was stated the overall weight is a bit more than a nice solid axle. Unsprung will be less, but more complicated to setup. Also consider what kind of HP you will put through it. To keep it reliable I put the limit right around 800ft/lbs, beyond that you start killing parts. I have seen many vipers literally rip the center diff section out of the car. It takes a good bit of bracing to support that kind of HP. But even before that you have to run a $1200 set of titanium half shafts to put the power to the wheels. It's far from an economical build.

For a reasonable price you can get a nice 9" built, 3 or 4 link, and when setup properly will handle well enough that 95% of us will never need anything more. Plus it will handle all the power you even dream to push through it. Even in the event you did break something, it's a $500 ring and pinion, not a $2000 pumpkin, and $1400 quaife diff.

Also do some research, contrary to popular belief you CAN run camber on a solid axle. I know it sounds crazy but it can be done with the right setup.

[David Pozzi]

The old IMSA GTO class allowed only stick axle rear suspension on cars like Camaro and Mustangs, but Corvettes could run independent of any kind, (quick change center section) or a solid axle. The DeAtley team with David Hobbs driving, switched from Camaros to Corvettes, they were winning with the Camaros, the Corvettes with independent suspension didn't work very well, they wore out the rear tires early in the race, traction out of corners late in the race was a huge concern for those cars. They eventually got the cars working better, enough to quit running the Camaros they had, but there was little difference between them on a race track.
Perhaps some other application/situation would show up differently.

A lighter race car like an open wheel forumla car, would have a very large proportional amount of unsprung weight which could become a problem, that's why lighter race cars benefit more from independent rear suspension. Also independent rear suspension has less anti-squat, so it works better on car with a rear weight bias.

[Norm Peterson]

Either a one-off IRS or one adapted from a significantly different car will be trickier to get set up such that you could/would take full advantage of the benefits. It's quite possible to be less comfortable sooner with an IRS, in which case you simply won't drive it as hard as the components would permit. You'd be both faster and more comfortable about it with a good stick axle setup.

boodlefoof - isn't that the C5/C6 solution? And others before it, such as the Porsche 928? Anyway, that approach trades off weight distribution improvements against polar moment of inertia increases. Arguably a good thing where driver capabilities are unknown (as is the case for the OEM's, their high performance offerings, and their targeted range of customers) or for high speed events. Not such a clear advantage at something like auto-x in a 3000+ lb car (might be course-dependent, involving specific slalom details).

Norm

[boodlefoof]

Norm,

Yeah, the Prosches have been using transaxles for awhile (the G50/50 is pretty popular amongst GT-40 kit car builders). The vettes seem to do it a little differently though... I think they have a separate differential mounted directly at the tail of the trans, whereas the transaxles I'm talking about (like the Porsche) integrate the diff inside the actual trans case. The axles then come out right in the middle of the trans as opposed to right behind it. I'm not sure of the weight difference between these two though.

[deuce_454]

ive adapted a jaguar IRS to my 32 roadster, its basically a dana 44 center section, inboard disc brakes, double wishbone rear suspension, although the upper "wishbone" doubles as a driveshaft, so it can sit as high under the frame as a stiff axle is, the unsprung weight is about 20 pounds per side and moving the anchorpoint for the longitudinal controllarm is no different than moving the intersection point of a 4 link...

as for titanium halfshafts... i shortened the oem halfshafts and used a solid 40mm OD chrome/vanadium steel bar... totally overkill but im sure ill be spinning the rims in the tires before i break one...

[slow4dr]

I am going back and forth between the two myself. I have been in contact with Mark @ LD and he is nearby so I'd love to go with their 3-link route. However I actually feel the IRS would be less expensive for me because I can do most of the fabrication myself.


IRS:
used C4 Vette IRS usually under $1000
Dana 44 upgrade $1000-$2000
sell the Dana 36 and recoup some $$$$
shortening control arms $cost of wire & gas
Shortening half shafts $cost of wire & gas
misc mounting hardware $100
The C4 already has decent brakes from the factory(with e-brake).
The only thing that could be interesting/costly is if the stock leaf spring & sway bar can't be modified & reused. Even if I added $1250(see below) it would still be less expensive than a bar set-up.

Total $3100 (worst case scenario)

3-link or 4-link:
reinforced 9" rearend $500
center section $500-$750
axles $500
watts link $500
Chromoly links for bars $300-$400
brakes $1000
Chromoly bars $200
shocks/springs $500-$1000
sway-bar $250

Total $5100 (worst case scenario)


I am an avid drag racer and I wouldn't run the IRS if I wasn't confident it could be made to 60 ft well. I know a lot of the Cobra guys swap to a solid axle but I also know some Supra/Viper guys pulling 1.3 short times with factory IRS's.

[deuce_454]

why not go with the jag unit, it is already a dana 44, albeit with 10 spline pinion and 9/16 ring gear bolts, but a dana 44 bolts right in (i know first hand).. i got mine for 500 bucks, locker, inboard disc brakes, e.brake, add cost of gas and wire for shortening and fabricating the mount and you are motoring.... a good friend of mine gets 1.5-1.6 sec 60 foot times with a procharged LT4 and 2600 stall in a 3000 pound 31 ford hot rod (with him in it) running a jag suspension and 275/60-15 drag radials (700R4, no transbrake, 3.54:1 rear ratio)

that is very decent in my book...

[deuce_454]

Sweet nova BTW, is that where you were considering IRS??

[slow4dr]

why not go with the jag unit, it is already a dana 44, albeit with 10 spline pinion and 9/16 ring gear bolts, but a dana 44 bolts right in (i know first hand).. i got mine for 500 bucks, locker, inboard disc brakes, e.brake, add cost of gas and wire for shortening and fabricating the mount and you are motoring.... a good friend of mine gets 1.5-1.6 sec 60 foot times with a procharged LT4 and 2600 stall in a 3000 pound 31 ford hot rod (with him in it) running a jag suspension and 275/60-15 drag radials (700R4, no transbrake, 3.54:1 rear ratio)

that is very decent in my book...

What is the typical track width on the jag rear? That could surely be another option and maybe one better suited for an early Nova.

Yes, this will be for the Nova in my sig.

[deuce_454]

the im almost sure that you will need to shorten it to fit under the nova, unless you stumble upon one from an e-type.. ill measure the unshortened one i have for spares tomorrow and post specs, but im sure the info is on the internet somwhere... but the jag unit is prety much what kugel has copied... take a look at http://www.cwiinc.com/ in the mean time...