IRS/IFS differences in setup and design?

[fe66comet]

I am looking into building my front and rear suspensions. First question is a single A arm Strange coil over or double a frame be better handling in a road race car? I have used the drag design before but only in a car with skinnies and slicks which made for no handling. Would the late Mustang k member design be a better choice? Next question is when I look at rear suspension IFS systems they are built with as long of links possible and usually mount to the differential housing, what is the advantage of this? Also I have notices that they are designed with camber built in under compession , I am guessing this keeps the tire flat as you articulate through your travel?

[machoneman]

Jon, this is a topic many experts have written upon over time. Herb Adam's books (a well known guru) on it are 90's dated but still sound. As a suggestion, hit the web, Amazon reviews in particular, and look closely at book reviews for sports car (not F-1) specific and road car matter.  The F-1 stuff is neat but DeDion and some other fairly exotic rear suspension set-ups really aren't germane to 3,000 lb corner carvers. SCCA literature is also a good source.

Btw, nobody uses a McPherson strut front unless it's by the rules such as "OEM suspension pickup points only". Double A arms are used by almost everyone, including F-1. 

I'd also look at Tom Gloy's  and Riley & Scott's chassis front suspension ideas. Gloy won Ford many a championship in the now defunct 5.0 liter Trans-Am classes with 600 hp V-8's in fairly heavy later model Fox and SN95 Mustangs. Rear wise, the very current Corvette (sorry!) IRS designs run in FIA/ European road racing (read: LeMans) could be a source of what's hot.

And, no kidding, the Web even has an IRS website!

http://irsforum.boardhost.com/viewtopic.php?id=263

http://irsforum.boardhost.com/viewforum.php?id=1



 

[fe66comet]

Thanks for the tips! I have been looking for info bit I have been having a hard time sorting through all the junk put there. Engineering info is hard to come by for IRS and I am not willing to pay 8 grand for a reworked Corvette kit. Heidts offers the rear IRS kit with an aluminum 9" housing and 31 spline generic limited slip, I think I could do better for less using a Currie track 9 IFS housing in 35 spline, Detroit locker. Sport an nodular iron gear case with Daytona big bearing pinnion support for 1800. I just need the knuckles off a ford F150 and hubs with custom # 30 CV shafts and the drive end of things are done. Then it is the hard part, making the suspension act right with some tubular A arms.

[machoneman]

More here:

http://www.fordmuscleforums.com/mustang-pages-1965-1973/492366-possible-turn-9-inch-rear-end-into-indpendent-rear-end.html

The 8.8 Ford unit under the SN95's is a self-contained IRS unit with sub-frame and has been used successfully under many a kit Cobra. Guess you know this.

[cammerfe]

Caroll Smith wrote several books about handling. They should all be available on line. Caroll was one of those who had input on Shelby's projects.

KS

[TomP]

Well my best advice is forget about hip modern buzzwords. A Comet already has "IFS" , most Fords have since 1949. All you are doing is getting something shinier, not better.

Considering there will be a slew of racers converting their new 2015 Mustangs IRS to solid axles and the new Challenger and Camaro racers already do that, converting a perfectly good leaf spring 9" car (like a championship winning GT350 has) to a heavier bulkier independent system is just for eyewash and has little practical value.

[TomP]

I guess further to that, i'd actually prefer struts if you were just changing for changes sake. It makes for more engine room and the camber change is an advantage when cornering. Look how many successful road racers use struts, most European and Japanese brands, the 80's and newer Mustangs do OK with it too.

IRS will probably make the car difficult handling rather than help. Ask anyone who crashed a Jag or Corvette when the back end got away from them.

[fe66comet]

The whole suspension system is outdated, heavy and not tunable for road race use. I am building a 95 Ranger for drag use, with a 9" 28 spline, 3.73 gear and Detroit locker. In front it is getting a Tremec 600 and a 400 cube Cleaver. Front suspension will be tubulay mustang II. Basic stuff for basic strait line use. If you look at serious road race use, full independent suspension is the way to go. Curry makes a fabricated 9" housing center section just for this purpose. An aluminum 8.8 will never hold up to a big cube high torque big block, a normal 8.8 only handles 350 HP. I need twice that HP with sticky tires and about 600 ft lbs of torque. The only rear drive axle shafts that will hold up will have to be custom #30 CV shafts in 35 spline with redrilled Ford truck hubs. The parts are redilly available from a vender that handles off rod racing parts for Ford F150 trucks.

[machoneman]

Yes, a 9" IRS rear would be better than the 8.8 but......

Road racing doesn't put the strain on a rear like drag racing would, as you know. As an example, on the west coast in particular, many true track-only 600+ HP FE Cobras actually run the Jaguar rear end with good luck. These are both original Cobras, few in number for obvious reasons, and a slew of high end (Contemporary and those South African Cobras, the maker's name who escapes me just now) and true kit-car Cobras.  Some are in historic races where driving is at say 7/10 ths or 8/10 ths. Still, others, and mainly those non-original Cobras, are beat to hell in real go-for-it 10/10 ths racing and the rears hardly ever break.  These on true sticky race tires on big name tracks.

Also, there are many drag-only 8.8's at near 1,000 hp that do live in drag racing,  See any 5.0 Mustang site where turbo'ed , blower'ed and NO2 injected 302-400+ Windsor engined  'Stangs regularl turn some truly amazing low e.t.s.  Soft-Loc clutches, expert clutch tuning and the sometimes 'Glide equipped cars admittedly help to lower the strain compared to the old days of non-slider clutches, for sure. Also, as the hp keeps going up, admittedly most eventually switch to the stronger 9".     

I do think the cheaper 8.8 would be more than sufficient unless you also plan to drop the hammer in true drag race fashion. JMO. 'Course the IRS 9" would be better but it will come as you know at a price. Would like to see your fabrication of the needed parts as the project progresses! 

Btw, here's a local Cobra running under the SCCA banner at Elkhart Lake. WI with a dyno'ed 660 HP detuned Roush 358 NASCAR Cup engine. Rear-end? A Jaguar based IRS ala' the original Cobras.

http://www.youtube.com/watch?v=95E9B6zPzuY
 
 

[TomP]

Perhaps we should ask just what sort of racing this is? It's something that allows independent rear ends on cars that don't normally have that and a leaf spring setup is no good for you... even with it's multi-championship pedigree. What you really need is a tube chassis, carbon fiber body race car.
 

[lovehamr]

Tom, I really don't know where you're coming from in your posts but the OP already stated several times that he wants the car set up for road racing and contrary to what you're saying a solid axle is a liability in that arena.  This is coming from a Boss302 (live axle) and Cobra (BDR w/IRS) owner, driver and open tracker.  As was stated earlier in this thread, the only reason that anyone road racing uses anything other that double A-arms front and rear is because they have to.  Why did GT350s run a solid rear axle?  Because they had to per SCCA production rules.  Why did Cobras run independent front and rear?  Because they could.  It's just a fact that a well designed IRS will outperform the best live axle on anything but a glass smooth surface.  Even then the most that the live axle can hope for is to equal the IRS.  Let the surface get bumpy at all and that all goes away because the IRS will let the tires stay in contact with the road better than the live axle every time thus improving lateral adhesion.

[lovehamr]

Jon, one book that I have made things much easier to understand when it came to roll centers, camber gain and loss and a slew of other things was this one:

http://www.amazon.com/Race-Car-Chassis-Construction-Powerpro/dp/0760302839

Maybe I just understand the writer's style but it helped me.

To your ideas on IRS fabrication using F150 parts, the biggest problem I see there is unsprung weight.  Those IFS spindles have to weigh a ton don't they?  One of the big precepts when discussing the advantages of an IRS is the reduction in unsprung weight which allows the ties to follow the surface irregularities more closely.  Have you thought about the IRS out of a TBird or even I think some Lincolns?  They'd have 8.8s but still might jump start you in the right direction.

Steve

[jimeast]

For oad racing,  IRS is the ultimate, but for example the spec racing FFR cars can use a solid axle rear end or an IRS.  For race tracks that are smooth, the difference might not be as big as you think.  Carroll Shelby tested an IRS for the Shelby in the 60's and supposedly had the same track times with a solid axle car.  I think an IRS in a Cobra for the streets is extremely beneficial due to short wheel base, very short drive shaft and the bumpy uneven roads we see.  For a Mustang, I think it's lees of an issue.  For the track or high end street performance, if you have not looked at Street or Track's 3-Link rear and Coil-Over front, it's a good place to start.  Opentracker Racing is a good place to look to see the vintage racing approach to handling improvements.

[fe66comet]

Currie makes the perfect center section but hubs and associated high hp components are still not there. The kits out there today claim 1000 HP tolerance but use a Thunderbird aluminum 8.8 center section and 28 spline axles. If you had a stick and over 600 HP I can't see it holding up. Even the expedition hub only is 30 at best.