Suspension Mythbusting: Solid Axle vs. IRS

frenchyd said:

An IRS can be lighter than a "stick" axle. But the right parts need to be selected.  The chief weight loss is the housing. Linkages and springs can be the same.  Plus you can  that the heavy disk and calipers and mount them inboard. Reduce unsprung weight.  

On that last point, you introduce some possibly insurmountable cooling problems by tucking the brakes up under the car. I have friends that raced XKEs and have gone to heroic lengths trying to get the rear bakes cool enough to function as intended - ducts, heat sinks bolted to the calipers. etc., etc.

Apparently the vintage racing club they were running with didn't allow a simple relocation of the calipers to the outside ends of the axles out by the wheels.

In reply to frenchyd :

The inboard brake heat can be dealt with.  Let me start by saying that's a lot of heat to put right next to the differential. So you have to deal with it or every race you'll be replacing seals and fluids. 
     The trick is to duct cool air in and.  AND hot air out.  With the disks fixed to the diff you can duct air right to the rotors/calipers.  However the ducts out need to be much bigger in diameter than  cool air ducts.  In order to Pick up heat that's radiated  as well.  Duct it out the truck lid.  That's the trick Air flowing through the area.   I use 4-6 inch truck exhaust tubing.  It makes a very neat look. 
Luckily, the rear brakes don't work as hard as the fronts do.  So you don't need to pick up that high pressure air at the fron.  You can pick it off NACA ducts in the rear widows.  Or the high pressure area under the car. ( or both)   

Nobody wants that crap!

Interesting article and comments.

My 65 Corvette roadster is probably unique and I'm pretty sure no one has another street car like it. All the running gear started out Chrysler.  A 1957 Chrysler 392 Hemi with dual Carter AFB's.  Originally a Chrysler 4 spd which I've replaced with a TEMEC 5 spd.  Then a Chrysler 8.75" solid axle, limited slip and 3.70 gears with after market disc brakes.  I've changed the rear suspension to 4 link and coil over shocks.  The weight is 48% on the rear, which is not to bad with that big Hemi up front.  This summer I'll be considering a rear sway bar after a little testing.

asphalt_gundam said:

I'd be interested in a very in depth article on solid axles. Even over on the pro touring side of things there's lots of argument about which suspension design is the way to go. 3 link (centered or offset) vs 4 link (parallel or triangulated) vs torque arm vs truck arm and watts link vs panhard bar. All have pros and cons, all can weigh different and vary in complexity. 

I think the only real setback that can't be gotten around in the IRS vs Solid is transfer of bump influence from one wheel to the other. I have a ford 9" rear now and it is much heavier than the stock GM 7.5 was. However...going to an aluminum center, light weight ring and pinion, light weight locker, aluminum daytona pinion support, drilled axles, light weight brakes, and a fabricated housing I've researched to be 45-50lbs of weight savings (still around an estimated 120-130lbs with out arms for an rear axle that can support 700HP+). Half of which is rotating weight. Floater style housings can be built with radius drive axles to have camber and toe. Then comes in the various pros/cons of each individual suspension design. A long arm 3 link and torque arm seem to be the top contenders for a track/auto x type car with the torque arm being easier to package in most cars. The other designs have proven that they can work well too. This is where rear steer comes in as something that can be tuned on a solid axle easily vs an IRS. 

I agree that IRS is the best performance option in a track setting but there are plenty of cars out there that don't have that as an option without massive work in cutting/welding/fabrication. I find it hard to believe that an IRS could be a viable option in the cost/work vs performance gain when starting with a factory solid axle car.

Back to the beginning of my post I would be very curious to see some back to back, real world (track, autox, and street) testing of IRS vs 3 link/panhard vs 3 link watts, vs 4 link pan and watts, vs torque arm in fully prepared scenarios on the same (or as close as possible) car(s). 

The only place you'd ever see that is maybe a factory with unlimited funds.  
      Pure performance oriented cars are all designed with IRS and the chassis is built accordingly. 
    The closest you will ever see is when the D type Jaguar near the end of its life was converted from a Straight axle to a IRS.   A one off with a tiny budget.  
 That engineer ( sorry his name escapes me )  drew up a design in 1957 that is the basis of every Jaguar from then until 2000.  
     And is still used with minor modifications today. 
    It's noteworthy that prior to the introduction of the Jaguar XKE. Racing sports cars were usually straight axle.   Following Jaguar in 1961 Corvette in 1963  most Sports cars went to IRS. 
 

Kolsen911 said:

Interesting article and comments.

My 65 Corvette roadster is probably unique and I'm pretty sure no one has another street car like it. All the running gear started out Chrysler.  A 1957 Chrysler 392 Hemi with dual Carter AFB's.  Originally a Chrysler 4 spd which I've replaced with a TEMEC 5 spd.  Then a Chrysler 8.75" solid axle, limited slip and 3.70 gears with after market disc brakes.  I've changed the rear suspension to 4 link and coil over shocks.  The weight is 48% on the rear, which is not to bad with that big Hemi up front.  This summer I'll be considering a rear sway bar after a little testing.

     Drag racing has different needs and trade offs than road racing.   That heavy Hemi  likely isn't that much heavier than a Chevy 454 of the same type ( iron block , Iron Heads ). 

Ok I may be getting old but yesterday I looked at an Exocet Miata kit car and I liked the idea of just taking something with a tried and tested setup that works. But then again I am very content with the way my stock Mazda RX-8 handles when driven fast. It just works, was cheep even new, has wide tires, revs like crazy and is just FUN to drive. 

Remember, I am old (but not slow) and my first car that I bought (with my money - $25) was a original Fiat 500 with no starter and very low compression so I could visit my girl friend in Hamburg that was three hours away from college in Germany. 

Bite me!  

my 84 alfa spider has a soild rear axle,, the pick up points at T bar from bodyshell to the top of the diff...  and 2 lower trailing arms from the bodyshell  to under the axle tube... i am planning to use a panhard link to the rear axle,, and design a tourqe arm rear suspension..i must design both of those.. the panhard will be easy.. got most of it made now.. but the tourqe arm part will be hard.. i plane to mount it to the tranny W/C t5.. has 3 mounting holes in the lower rear casing.. my rear roll center is at the top of the diff.. with the panhard system,, it should be at axle centreline.... the alfa spider, handles well enough.. but can be made better..i have moved my front roll centre from being underground to about 50 mm above the ground