Autocrossing or as some people call it "Killing Cones"

Classic Mopar Racing Forum

  1. BigWhip

    BigWhip Member

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    Aug 7, 2012
    I just saw were a member said that F body cars were not very good cars to autocross.

    Why not?

  2. NoCar340

    NoCar340 Well-Known Member

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    Dec 7, 2013
    Upper MI
    It's because the strut rod is also the torsion bar, so by necessity it doesn't function very well as a strut rod. There's simply not enough cushion at the forward pivot point, which is where the bar turns to go across the car (and becomes the torsion bar). On earlier, straight-bar cars the strut rod has enormous rubber cushions (isolators) at its forward anchor, which allow it to push and pull through its travel, keeping the lower control arm well triangulated. That keeps the steering knuckle straight through the range of wheel motion. On the FMJ suspension, there's simply not enough give at the strut-rod/T-bar pivot isolator, a necessity of the design, since allowing side loading of the torsionally stressed part of the bar could easily cause breakage. Chrysler tried to account for this by adding another rubber isolator at the control arm. Nice try, but it still doesn't allow for the strut-rod portion to act correctly: not only is it still not enough cushion, it also means the strut rod is "floating", meaning there's no solid triangulation point for the LCA. This causes the steering knuckle to go through all sorts of wacky gyrations depending on the wheel travel. Caster & camber both change markedly throughout the wheel travel, which results in handling that is funky at best and downright surprising at its worst. This doesn't happen on the longitudinal-bar cars since the strut rod is solidly bolted to the control arm, very close to the ball joint.

    Need more? Look at the length of the strut-rod portion of the FMJ bar. Now go look at any longitudinal-bar car, even an A-body. See how much longer the strut rod is? The longer strut rod travels through a much-wider arc, which means it's not "pulling" on the LCA nearly as much. Even with a similar amount of pivot cushion as an FMJ, the actions on the LCA and therefore the steering knuckle are far less severe.

    As if all that weren't enough, now throw all that onto a small mounting platform bolted to the car with four huge, double-sided bushings that allow the front frame rails plenty of room to flex independently. Suspension can't work correctly unless it's mounted to a solid platform, and in the case of the FMJ cars, it's simply not. "But wait!" you exclaim, "Look at the '73-up B-bodies! They have the same cushion setup!" and you'd be right. However, check out the enormous size of the K-member and its mounting points on the isolated B (and '79-'81 R) platform compared with the FMJ. It's a much more stable structure supporting a light-years better suspension design.

    Of course, there is also the matter of the "hockey-stick" FMJ bars moving weight (technically load) bias up and forward, which is the absolute wrong way to distribute anything near the front axle if you want the car to handle well.

    The FMJ design was considered so bad, a number of Chrysler suspension engineers actually quit their jobs in 1974-'75 rather than be associated with it.

    In both suspension designs, cars with polyurethane strut-rod isolators that get driven a lot wipe out the lower control arm bushings much more quickly compared to cars equipped with rubber. Something's gotta take all that stress and it's going to be the LCA bushing every time, which is why other than the manufacturers nobody recommends using polyurethane on the longitudinal-bar cars. I've never heard anything said about the FMJ cars, but very few people are concerned with putting a Band-Aid on a broken leg either. Experience tells me that while one might get the impression that their car handles better with poly bushings on the FMJ T-bars due to several factors (new parts, firmer ride, more road feel), a back-to-back comparison on an alignment rack would show the car will actually handle worse compared to new rubber parts since there's less give and the steering knuckle is now forced into positions the rubber wouldn't allow--in other words, you'll see even-more extreme changes in caster and camber. That means more corrections to make at the steering wheel.

    All that being said, if you spend enough time really pushing the limits, you can learn to drive any car correctly so that it will handle in ways that others wouldn't expect. I did things in my Mom's '84 Escort sedan that would make Ken Block poop his pants, and one of my best friends could probably turn Arie Luyendyck's hair white with a '78 short-wheelbase Chevy G-van. You work with what you've got. :icon_biggrin: You learn the faults and foibles and what to expect after a few trips into the weeds or ending up looking at where you've already been... but it's all worth it when one of the local SCCA hotshoes is screaming like a fag choirboy in the passenger seat of your '81 Tercel 5-speed that's held together with drywall screws, bungee cords and a prayer. :eusa_dance:
  3. brotherGood

    brotherGood Well-Known Member

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    Mar 31, 2013
    Urbana OH
    So, with the big demand for poly bushings, youre saying its best to not use them?

    I'll be rebuilding my entire k frame on a bench and replacing all at once, so everything will be new. But I'd like to get the best I can. Even with such a worn suspension, this was the best handling car I'd ever had (until the control arm broke). I haven't pushed it since because it now pulls hard to the left..granted, its been at the shop for 2 months waiting to get the brakes done..:angryfire:
  4. ramenth

    ramenth Well-Known Member

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    Feb 27, 2011
    Beaver Dams, NY
    Or you can find a good alignment shop and throw in about -5 degrees of camber and a few degrees of negative caster and get the cars where they really like to be.
  5. L98TPI

    L98TPI Active Member

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    Jan 2, 2012
    Lynchburg, Virginia
    Lets face it Chrysler cars were not great handlers out of the box compared to equivalent GM and Ford coil sprung cars. And are harder to modify into a good handlers (parts labor) than said other brands. We did have a Challenger back years ago that was an autocross beast. But it had NASCAR bars and leafs ($$$), poly and solid bushings, Lots of small things like gussets here and there. Fast steering box...and so on. It was not fun to drive on street other than a back road blast because it was so harsh. But it left at least one 911 guy cussing and throwing things. :laughing7:

    I suspect the lower bushes wear faster if poly torsion bar bushes are present, just because you can and will drive it harder. Nothing to do with the 'transferred load' from the now stiffer strut rod/torsion bar location. Just from cornering forces. I suppose its not a good idea to leave the lower bush rubber even if you are not looking to all out handling.

    Hell even Corvette suspension sucked out loud (did I say that out loud?) up until the C4 and got even better in the later 'C4s. And world class in the C5+

    Having owned and loved two '944s over the years I can attest to the fact you can get great handling and a ride that does not leave you knackered after a few hours behind the wheel.

    Other cars combined the lower control arm with another function. Capri's, which were pretty good handlers (had another friend that had a 2.8, quick car) had the sway bar that was also the strut rod. Skinny thing really, but enough to locate the control arm.