The "Mustang II" IFS Real Info & Debate Thread

Discussion in 'Classic Mustang Specific Tech' started by reenmachine, Sep 17, 2007.

  1. There's always so much debate about the so-called "Mustang II" suspension systems that I figured we should have a thread all about it (STICKY?). Most of the information thrown around is false or unsubstantiated, but there are some good and valid points to be made on both sides. Everyone knows I'm a fan of a good MII IFS in certain applications, and I'll start the ball rolling with my feelings on the subject. It's certainly not the last word, and I'll say some things just as devil's advocate for the sake of the debate.

    There's bound to be some good action on the topic, but let's keep it clean and get some good info out there! I want to see arguments presented intelligently, and stories of structural failures should be first-hand or accompanied by evidence.


    Back in the day, drag racers wanted more room in the engine compartment for huge engines, so they started grafting the suspension system from the small Pinto-based Mustang II into the earlier cars. Street rodders wanted to get away from the non-independent I-beam front axle, so they did the same. It actually had great geometry and a compact package, but wasn't really up to the task of daily use in a larger car. Also, since every installation was custom, the quality of workmanship varied greatly. Long story short, in theory they were great, in practice they were so-so, and there were occasional failures.

    So, higher-end street rod shops started to design front suspension systems with the advantages of the Mustang II suspension (compact package, great geometry) but with custom-fabricated, extra-beefy (to use an engineering term), components. These are what evolved into the current "Mustang II" suspension package we see today. They don't share a single component with a Mustang II suspension, but they still carry the name due to their heritage I guess. There's no more strut rod, and no more conventional spring/shock combo. They're now a true SLA (Short/Long Arm) dual A-arm suspension system with coilovers, widely held to be one of the very best front suspension configurations for both ride and handling (see every other suspension thread on here).

    Since the resulting package is so compact, the shock towers can be removed from the car altogether. This causes some people to state that the front frame rails are now being loaded in a way contrary to how they were designed -- carrying the full weight of the front end. The claim is that the load is normally applied at the top of the shock tower and carried back to the firewall somehow. At first glance this seems to make sense, but it's only partially true. The load is indeed applied at the top of the shock tower, but the thin sheet metal between the shock tower and the firewall doesn't carry the load. The bracing that runs between the top of the shock towers and the center of the firewall carries a little, but it's mainly there to keep the shock towers from settling towards each other, since there's a large twisting force around the frame rails due to the application of the suspension force at such a distance. Leave that bracing off for a while and to replace it you'll need a hydraulic ram to spread the shock towers back apart.

    In reality, the suspension load applied to the top of the shock tower is carried by the shock tower down to -- you guessed it -- the frame rails. The load goes into the shock tower and the shock tower is mounted to the frame rail. The frame rails support the vast majority of the load regardless of whether the shock towers are present or not.

    Modern "Mustang II" suspension systems reinforce the frame rails with steel plate, and then tie the two sides together with a very substantial crossmember. There is much more load sharing between the two sides than with the stock suspension, and a much more rigid connection between the two sides.

    Due to the much higher performance of modern brakes over the stock ones, it's a good idea to reinforce the front end, regardless of the type of suspension, to better handle these loads. With shock towers, one uses heavier bracing between the towers and the cowl as well as bracing from one tower to the other. Without shock towers, it's good to run support from the front of the front frame rails up and back to the upper cowl. The two sides are already rigidly braced together by the crossmember.

    So, it could be argued that a good Mustang II-type suspension system is actually structurally superior to the stock configuration, since the loads are carried by the frame rails in a similar way and the two sides are connected in a far better way.

    As icing on the cake, you get a huge engine compartment for that big block, DOHC mod motor, or big-tube custom headers.

    As extra icing, you get front-steer rack & pinion steering so there's even more room for headers, clutch linkage, or whatever.

    And as the cherry on top, if you or a buddy are a good welder you can't beat the price!

    Go nuts.
  2. I couldn't agree more. Did I just see a grenade?
  3. Also having installed a R&C MII suspension I have some observations.

    Having cut apart nearly every piece of sheet metal on my fastback it is amazing how flimsy most of the metal is. The firewall is incapable of supporting any load. The cowl panel through the A pillars and roof is a more likely load path. The frame is of course only attached to the floorpan. When tied together the sheet metal provides enough structure to support normal use but not much more. Adding front torque boxes and subframe connectors makes for a very strong package. My fastback with the MII, torque boxes and subframe connectors is less flexible than my coupe was with stock suspenision, Monte Carlo bar and export brace. I haven't compared the two since I added torque boxes and subframe connectors to the coupe. Additional bracing from the frame to the cowl is probably a good idea and a cage is the ultimate for structual rigidity.

    As I mention in another thread I have been playing with Performance trends Suspension analyzier software. Included in the software are examples of both a 68 Mustang and a 78 MII. I also took measurements from my R&C MII to compare to the stock MII.

    The stock 68 is pretty bad in terms of camber gain and anti dive. The stock MII is better in both of those areas. However, the shorter "A" arms on the MII cause more rapid change. By lowering the upper control arms on the 68 the camber gain is better than the stock MII. The R&C and probably all of the other modern "MII" suspension have vastly different arrangements than the original MII but the geometry is similar. The UCA and LCA are true A arms and are mounted in poly bushings instead of rubber. I think the R&C kit allows for a greater range of alignment specs but I really hate the slotted A arm mounts to adjust alignment. They are prone to slipping (do not paint this area!).

    The original MII disc brakes were 9.5" but most of the new MII kits have 11" discs. Wilwood has a new spindle for use with MII suspension that is taller than the original spindle and has a 2" drop. The taller spindle works much like lowering the UCA and improves the camber curve. They claim other benefits but I haven't been able to substantiate them yet. This spindle also lets you upgrade to as large as 14" discs with 4 or 6 pot alum. calipers. On my 67 I plan to order the R&C kit without brakes or spindles and use the Wilwood parts.

    When considering price it is a bargain since most kits let you replace the whole front suspension, convert to front discs and add rack and pinion steering all at once. Compare that to one of the coil over conversions that use the factory pickup points. If you add up the coilover suspension, rack and pinion and disc brakes you're probably at $4k to $5k. The coilover kit may in fact be superior and is much easier to install but you still have the shock towers and you have spent an addition $1500-2500.

    As far as engine placement the MII actually gives you more flexibility in where you place the engine including lower and further back from stock. Yes, you do need to convert to a rear sump pan unless your are going modular.

    I have read about failures in the early days of the Heidt's and Fatman kits in hot rods. The failures were in the crossmember and not in the frame. The failures were due to braking loads on the lower control arms being transferred to a unboxed areas on the crossmember. Both companies have made changes to increase the strength of the crossmember.

    This conversion is not for everyone and not the ultimate Mustang suspension. But is a good suspension that does require some extra work and fabrication. I always have to say that one of the best handling cars I've ever ridden in was a Pinto that was modified for autocross. Those cars weren't really much lighter than our Mustangs either.
  4. Great observations and some of them reminded me of points I meant to touch on:

    I totally agree. I have found that as long as the contact area is left unpainted and the bolts are properly torqued they hold fine. The new Superide II from Heidt's does away with this system in favor of threaded Heim-type joints, eliminating any chance of the alignment adjustment slipping.

    Wilwood offers several kits to fit the Heidt's and R&C spindles, as do Baer and SSBC (and probably others).

    Another one of those things people claim that really gets under my skin is that installing a MII suspension requires that you move the engine up and forward. These very words even appear in a certain company's print ads. It is not true. I have personally compared before and after and measured even a small decrease in engine height with the MII. The engine doesn't move forward either -- a T-5 shifter comes right up through the stock hole just like it should.
  5. for a daily driver, the mustang ll suspension is not a bad choice. and yes the room afforded for installing a big block in a 65-66 stang is nice, but overall the stock mustang suspension with some tweaks is a superior design.
  6. I'm not claiming that the MII is the ultimate suspension design. It's a great design that's the perfect fit for some people and not for others. I'm simply countering the frequent spray that "MII sux yer car will brake in haf" whenever it gets mentioned.
  7. One gripe I have, which may or may not be related to the MII, is I have very little on center feel. I've got 4.25* caster and reduced the power assist but there is still very little road feedback and the steering doens't snap back to center. I may try reducing the assist a little more and perhaps the rack isn't "broken in" but I would like more feel.

    A little more suspension analysis shows slightly better (less) roll center migration, due to roll, on the 68 suspension with lowered UCAs when compared to the R&C MII but both are pretty good. With the Wilwood spindle the roll center on the MII moves very little but it goes from slightly left to slightly right of center when rolling from 1-5*. I'm not sure how that would affect handling.
  8. absolutely the only M-II suspension i would ever put in a mustang would be the Martz chassis kit that replaces the ENTIRE front subrame with beefier rails and the aformentioned support bars from the front of the frame rails to the firewall. i've ridden in a few cars that have had one of the other companies' kits installed and the cowl shake was worse than any Fox body vert i've ever ridden in because of chassis flex, maybe the newer kits have fixed some of this but i don't trust them anymore from the experiences i'v had with them.

    the stock shock towers do provide triangulation in the front end EVEN without the cowl braces in place, you can tell that just by looking at car in the junk yard that has been wrecked, very rarely does the damage ever extend past the shock tower. now, some may say that means nothing or has nothing to do with the suspension but it actually does, that same triangulation that keeps that car from crumpling up like a coke can applies to the suspension as well.

    i'm not saying that the stock mustang suspension is the be all end all for these cars but for my money a modified stock suspension is better than MOST of the M-II type stuff.

    personally i'd like to do the CPP front "mini-subframe" kit with a true lower A-arm, some Opentracker roller upper arms and some kind kind of coilover kit or Opentracker spring perches and some progressive rate springs.
  9. I'm running the MII suspension too

    from R&C; the coil-over package, very immpressed with the quality; also backed it up with a re-enforced front frame rail system.

    Aerospace components 12" disc brakes all the way around. I also bought R&C's bolt-on rear coil-over system, but's welded in.

    The car is back apart getting sealed and primed; all the components should be back on soon. I'll have pics then.

    btw: I dropped a 429 SCJ engine in there; lots of room.
  10. Compare a Mustang to a Mopar or GM - with the mustang the whole front end is all thin stampings and sheet metal, there isn't even a real crossmember, just a piece of tubing bolted in there. At least GM and Mopar have a thick, strong crossmember. And whatever carries the front suspension loads in the Mustang, be it the frame rails or the shock towers, it all runs into 18 gauge body sheet metal in the end.

    I'm convinced early Mustangs were never intended to last very long and probably never intended to do road course competition, drag racing or any other sort of high-stress stuff. They were cheap cars, plain and simple.

    Whatever front suspension you choose is going to be a compromise but, as stated earlier, the MII is a great choice for packaging, performance and budget reasons.
  11. That's why I cut out the front frame rails from the back of the shock towers forward and ran heavy gauge box tube back through the back half the the frame rail and into the sub frame connectors. It should really beef it up. Plus it has a matching front support;
  12. Is it possible to use a Fox/SN95 rack? They're similar in design and may have better ratios and/or steering feel. The width is 24.5" (inner tie rod pivot center-center).
  13. This is one of the biggest reason's I don't personally like them. It was absolutely annoying how bad it was. Maybe the car was a rattle trap to begin. I can't remember before the install.
    Another thing I really didn't like was turning radius. This was a heidts kit, so this is all I know. But a brand new Tundra has a better turning radius. IT was pretty bad. I realize for racing it isn't that big of a deal. But a 3 point turn on some of the things I had to do was just embarrassing.
  14. That's what it is, on the Heidt's kit at least.

  15. Pete, is yours a Fox mustang rack or a Fox T-Bird rack? i've heard that a lot of the newer design kits have a Fox body T-Bird rack, i think the Bird rack is a bit wider than mustang Rack, thereby giving a bit more turning radius
  16. I was the orginal owner of a 1975 302 Mach I. In that application and wheel base I always thought it was superior to the spring tower suspensions of the older cars. I could embarass the hell out of some very expensive Euro-Trash back then.

    MY ONLY problem with the Mustang-II in an early car is the Ackerman. Nobody who builds those systems I've spoken with over the years about that subject can give me any concrete information about exactly how they address the Ackerman. A few years back I spoke with Heidts and Fatman and both said "Ackerman isn't important".

    In Heidts brochure they even called the relationship between the upper/lower arms when viewed from the front end level with the center of the arms as an "unequal parallel-o-gram"....right...and all this time I thought it looked like a trap-a-zoid.

    After hearing things like that I kind of gave-up on them.

    If any of you know anybody who can supply that information, please post it here.
  17. OK...lets everyone attack this guy for telling THE TRUTH about how it was back then.
  18. I had a fox body vert = wet noodle. My fastback has no cowl shake. However, the subframe connectors and especially the torque boxes probably took care of that. I had the car on a rotissery and the torque boxes did more to stiffen the chassis than the subframe connectors.

    Turning radius isn't bad with my car either, although I haven't directly compared it to stock. I just know I can negotiate my tight circlular driveway without problem. I don't know what rack R&C uses.

    The front steer spindles make it hard to adjust for correct Ackerman. The steering arm does not angle far enough out due to interference concerns. The only way I've seen this addressed with the MII is with the rack location. I don't know if any of them have it worked out yet.
  19. If that's true then whoever you spoke to obviously didn't have a clue. Of course Ackerman is massively important or the car would be uncontrollable in turns. Every MII kit I've used seems to be correct from an Ackerman standpoint.

    These days at least, Heidt's actually has a pretty decent basic explanation of the geometry in the front of the catalog. It explains camber & caster change, toe angle, and bumpsteer with clear diagrams. They even get into steering rack selection and why the pivot points of the inner and outer tie rod ends need to be in specific locations.
  20. Nobody will, because he stated the facts, not opinions, in a clear and, more importantly, non-aggressive/combative way, an approach you don't seem to grasp.

    Everyone who knows Mustangs knows that they were a quickie rebody of a cheapo car, the Falcon. Mustangs were designed to maximize perceived style and value (and profit margin) while minimizing cost, not to tear up the road courses, etc.

    Simple mods can make them quite capable of doing so though.