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Thread: Torsional Rigidity. Humor me

  1. #26
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    Quote Originally Posted by Amuro Ray View Post
    I can't imagine that the top isn't somehow reenforced with metal, has anyone seen exactly what the top is made of? Fiberglass isn't the most flexible, something must help it keep shape.
    Other than the nut-plates that the rear mounts bolt to, there's no metal in the hardtop. It's two thick layups glued togeather with structural adhesive. Having driven the race car and street car with a hardtop, I doubt it adds much to the structure, I still remember the squeaks from the street car.
    Last edited by Z3racer701; 02-10-2012 at 11:22 PM.

  2. #27
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    You probably should have ordered the coupe. BMW says the coupe is stiffer than the sedan in the E9X series (even with the non-optional fold-down seats of the coupe). I spend a lot of time in both, and I don't doubt. I think the reason is the identical wheelbase covered with more metal (sedans have a door, coupes have... metal). If you still doubt, go test drive some used examples and listen for interior rattles.

    This is a big departure from the E46 and E36, so your assumptions about rigidity are not unfounded.

    Fair disclosure: I'm on my 2nd 335i sedan, and often wish I'd ordered the coupe.

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    Last edited by rwalker; 02-11-2012 at 12:25 AM.

  3. #28
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    Quote Originally Posted by Amuro Ray View Post

    Has anyone considered making an "X brace" between the Roll bars to "connect" them? You have the roll over protection of the hoops, along with some of the benefits of a full roll bar?
    ^ this?

  4. #29
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    I wonder how much Randys work and a roll cage will change the feel of my Coupe?
    Good thread, great topic!


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  5. #30
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    I believe hz is the resonant frequency of the chassis, ie what vibration the car amplifies. humans are sensitive to some HZ more others. I bet it plays a role. Some cars I bet are more sensitive then others ie. the hz will travel more ie. vibrations are bigger then others. the more stiff the less hz amplified ie. less felt by the driver.

    So I believe HZ is not rigidity, its natural HZ of the car. The frequency the car vibrates at. A more rigid car is less likely to vibrate.imo

    btw. e39 1.82% as rigid as my e34 which = 13,100 mn/dg
    F10 = 37500nm.dg or e39 x 1.55%.
    Last edited by DearS; 04-17-2012 at 04:03 PM.

  6. #31
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  7. #32
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    Quote Originally Posted by Terry F. View Post
    My Pheaton = 37,000 . Only surpassed by the amazing Veyron at 60k

    Terry
    Haha, again though, the weight of the car comes into play. If your Phaeton is = 2x Ford Mustangs, you're actually lacking in the "chassis strength" category to this Great American Pony

    Great topic! I can positively state that a simple strut brace reduced the cowl shake in my car. Having not actually seen the way a hardtop mounts, I would think even something this simple would increase the rigidity of the chassis, though it obviously wouldn't be the 2.6x the coupe does. At the time, the coupe was SOOOO stiff due to the fact that the original design was the roady, hence a good amount of extra bracing, which was retained when the roof was added.

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  8. #33
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    I seriously doubt that the hard top makes any difference at all.

  9. #34
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    Quote Originally Posted by BimmerBreaker View Post
    Also, I find it hard to believe the Z3 is THAT soft of a frame. Maybe under 10,000N but 5600 is seriously like a mid-90s Buick... And some of those other BMW numbers are a little... off imo, X5 is nearly as rigid as the Pagani which has a carbon fiber monocoque frame?

    I did a little digging, after sifting through at least 20 pages with the same identical list as you posted, I did find this:

    "the M Coupé platform also boasts a significant functional advantage in terms of chassis stiffness with an unprecedented torsional stiffness of 32,000 Nm per degree, setting a new record in its segment and contributing not only to optimal handling dynamics but an exceptional level of passive safety"

    Which would put the Z3 at over 10,000NM which makes a lot more sense to me. And I have heard that the Z3M coupe is STILL the most rigid BMW chassis produced, and this would go along with that. Z3 is ~12,000

    And yes, theres a lot more to handling than just chassis stiffness, but a stiff chassis allows for far more predictable behaviour and can be tuned more to the limits.

    I try to looking for this phrase on google and all result are reported to 2006, so to the Z4 m coupè, as you can read on the list above.

    The Z3 chassis have a lot of problem into high performance uses, there is a lot of part to reinforce both the front end to the rear end.
    Also the Z3 coupe on hard track work have problem with differential subframe... do a search.

    The proof is that on Z3 chassis BMW don't have produced a race car, something that happened instead on Z4 both E86 and E89.
    Another proof is the crash test:
    here the diferences between the last 3 series
    E36 1 and 1/2, E46 4.
    the Z4 comes from E46, the Z3 came from E36...
    http://www.euroncap.com/en/ratings-r...eTechnologies=


    also the Z8, that have the double of z3, have a big problem on front end.

    the X5 is a SUV/off road car and weight 2 ton... the pagani weight 1.3 ton...

    The mcLaren F1 have 13.500 and it have one the first carbon monocoque chassis on a road car.
    There isn't any law that said if it's done of carbon is stiffer then an iron too.
    Last edited by frentz; 11-21-2016 at 12:09 AM.

  10. #35
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    I am well versed in the Z3's and am aware of their differential mount/trunkfloor problems, but Z3's having such problems with prolonged high-stress use does not mean they are weak frames, and the reason a race car was not developed from the Z3 coupe chassis was the lack of downforce and inability to implement a spoiler that would provide adequete downforce and be within race regulations.

    Z8 may be stiffer than the Z3 and yes it has problems, it is an aluminum frame... that is it's own problem.

    Torsional rigidity is just a number at the end of the day, a relatively meaningless technical specification that means little in the way of actual performance or handling abilities... the numbers I dug up for the Z3 may be wrong, but it's all a moot point anyways...

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    I never knew my Bugatti Veyron was so sloppy (60,000Nm/deg) but now that I am aware of why I can not drive the car properly on the track and through school zones I am selling it and getting that pos out of my garage.
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  12. #37
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    Quote Originally Posted by BimmerBreaker View Post
    I am well versed in the Z3's and am aware of their differential mount/trunkfloor problems, but Z3's having such problems with prolonged high-stress use does not mean they are weak frames [...]
    Torsional rigidity is just a number at the end of the day, a relatively meaningless technical specification that means little in the way of actual performance or handling abilities... the numbers I dug up for the Z3 may be wrong, but it's all a moot point anyways...
    It is what I said and i'm perfectly agree with you.
    the example is that mclaren F1, one of the best car ever made, have only 13500.
    on te moot point i don't agree, the data for Z4 coupè was an important result to BMW, and the source are clear.
    the Z3 coupè is 3 times stiffner than roadster, is a big improovment too.

    but is the list of Z3/ M3 E36 and E46 reinforcment kit:
    https://www.turnermotorsport.com/BMW...forcement-kits
    https://www.turnermotorsport.com/BMW...forcement-kits
    https://www.turnermotorsport.com/BMW...forcement-kits

    http://www.achillesmotorsports.com/B...sis-s/1481.htm

    and this is for Z4:
    https://www.turnermotorsport.com/BMW...forcement-kits
    http://www.achillesmotorsports.com/F...m-rk-f-e46.htm

    the Z4 also have a problem with front sway bar support with bigger bar for race use... nothing foundamental.

    Quote Originally Posted by BimmerBreaker View Post
    Z8 may be stiffer than the Z3 and yes it has problems, it is an aluminum frame... that is it's own problem.
    also all new lotus have alluminium chassis, and I do not think they ever had chassis problems.

  13. #38
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    Trolls need food too Randy

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  15. #40
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    Amazing how much misinformation/raw speculation masquerading as fact exists in the couple of threads on this topic that are out there. (Not targeting any of the people I’m quoting below, tbc) After reading this thread years ago and coming back across it now I figured I’d add to the mess.

    Quote Originally Posted by BimmerBreaker View Post
    It doesnt have to be structural, it connects body parts and transfers the load, hardtops DO help because it forces lateral forces to be transfered across the car through the hardtop instead of just being absorbed by the car and twisting it. The fact it isnt "structural" and can be removed easily has nothing to do with the fact it serves a purpose transfering loads.

    How is that different at all? In either case it is attached to the car through some latches, really makes no difference if its a hardtop built into the car or not, in either case its not firmly attached to the frame, the ONLY difference here is whether the car installs the top, or if you install the hardtop.

    Also, I find it hard to believe the Z3 is THAT soft of a frame. Maybe under 10,000N but 5600 is seriously like a mid-90s Buick... And some of those other BMW numbers are a little... off imo, X5 is nearly as rigid as the Pagani which has a carbon fiber monocoque frame?

    I did a little digging, after sifting through at least 20 pages with the same identical list as you posted, I did find this:

    "the M Coupé platform also boasts a significant functional advantage in terms of chassis stiffness with an unprecedented torsional stiffness of 32,000 Nm per degree, setting a new record in its segment and contributing not only to optimal handling dynamics but an exceptional level of passive safety"

    Which would put the Z3 at over 10,000NM which makes a lot more sense to me. And I have heard that the Z3M coupe is STILL the most rigid BMW chassis produced, and this would go along with that. Z3 is ~12,000

    And yes, theres a lot more to handling than just chassis stiffness, but a stiff chassis allows for far more predictable behaviour and can be tuned more to the limits.
    That’s correct the Fact that it’s not load bearing does not mean it has no effect, it adds another horizontal plane and ties together structural components. It’s not going to add nearly as much stiffness as a actual coupe’s fixed roof obviously, but that doesn’t mean the increase is negligible, it’s often quite appreciable, even if not noticeably impacting driving characteristics it can diminish body movements that otherwise are a pain to deal with.

    The quote you used refers to the Z4M Coupe, the Z3M coupes is 15,600nm and the roadster ~5500nm, and at the time of your post even the Z4m coupe was not The most rigid chassis they’d produced, much less the Z3. Regardless even the Z3 roadsters figure isn’t so bad, it’s not far off from miata’s. the c4 corvette was about 2200nm. The important thing is that it’s suspension is dialed in in such a way to minimize the effects of its stiffness and BMW did a decent job at that. The lotus elan is another vehicle that had terrible stiffness but you’d be hard pressed to know, unless you try and tweak the suspension. It’s no coincidence people lowering their Z3m roadsters give up far more than they gain in most cases.

    You’re quite correct otherwise, torsional stiffness is one component but it’s considered critical in the design phase because it can’t really be tuned out afterwards and increasing it isn’t so much hard as it is HEAVY. The stiffer the torsional rigidity the more effective the vehicles suspension is, it allows it to do its job, too soft and it effectively acts as a spring And is therefore the weakest component of the suspension at that time. There’s also much more too it, the reason it’s imoortant is largely due to the effect it has on load transfer in certain conditions. Having a stiffer torsional rigidity allows the suspension to be stiffer, otherwise you must use a softer suspension to compensate (which is ok in many applications but still). There is a point at which increasing stiffness is no longer beneficial for a particular vehicle, so the ultimate goal is to find a sweet spot between handling characteristics/stiffness/and weight. In this context the weight of the vehicle is irrelevant though, the torsional rigidity figure of a heavier vehicle is not somehow equal to the torsional rigidity of a lighter car, and a heavier car with equal torsional stiffness does not mean that car will have less capable driving characteristics/traction etc. and indeed it’s possible for the reverse to be true.

    Quote Originally Posted by Vinci View Post
    I seriously doubt that the hard top makes any difference at all. It isn't a structural piece and it wouldn't take much force to rip out the fasteners that mount it to the body.
    Go drive a Miata with the softop on the freeway, put the hardtop on, and observe the 65mph shudder that was previously present dissapear. Body torsional rigidity plays a large role in overall vehicle torsional rigidity and the hard top helps improve that, people do much more for far less perceptible improvements. That said it’s not going to have a significant difference on the handling/traction deficits.

  16. #41
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    The problem I have with saying the HT adds stiffness is the manner in which it attaches. The front is bolted on, but the back uses toggle bolts and a spring loaded foot that compresses as you tighten the toggle. It seems like it would only add the amount of stiffness it takes to compress the spring which is not a lot of force.

    I'm convinced that the chassis stiffness thing is an overblown issue for the Z3 and not something encountered in normal or even very aggressive street driving.

  17. #42
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    You know, there is such a thing as a "too stiff" chassis. It's cool that the Koenigsegg posts those crazy-high numbers, but I'm willing to bet it's not as comfortable a ride as something like a spaghetti-noodle-soft Cadillac.

    If you have theoretically-perfect structural rigidity, and you encounter a bump that's large enough to exceed the limit of your suspension's ability, then the strut is going to bottom-out and that corner of your car is going up in the air. Chassis flex can absorb some of that deflection and help keep your tires planted.

    A little chassis flex isn't a bad thing when you're driving your car in real-world conditions. It's not even a bad thing on most race tracks, as I can count on one hand the number of them I've been to that were smooth enough to allow a vehicle to work completely within the range of its suspension capabilities.

  18. #43
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    Lol sorry, but both the Z3 coupe and roady are about as stiff as a wet blanket in a bowl of soup. The coupe a little more rigid than the roady, but still twists and creaks everywhere, especially when pushed a little. Pretty sure there is jack all difference between the non M versions and the M versions of the chassis. But this isn't necessarily a bad thing and gives traction in some places that my Z4 just doesn't have. Its also the reason why its such a fun and forgiving car to drive. You can feel the line in the Z3, its about 4 meters wide, and in a very docile manner suggests you to back off a little if you step over it, that same line in my Z4 is about 3mm wide, and will try to kill you if you step over it.

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    Quote Originally Posted by Harvey Mushman View Post
    You know, there is such a thing as a "too stiff" chassis. It's cool that the Koenigsegg posts those crazy-high numbers, but I'm willing to bet it's not as comfortable a ride as something like a spaghetti-noodle-soft Cadillac.



    A little chassis flex isn't a bad thing when you're driving your car in real-world conditions. It's not even a bad thing on most race tracks, as I can count on one hand the number of them I've been to that were smooth enough to allow a vehicle to work completely within the range of its suspension capabilities.
    This!!! Spot on.
    Last edited by vanne; 03-21-2018 at 12:22 PM.

  19. #44
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    Quote Originally Posted by frentz View Post


    the Z4 also have a problem with front sway bar support with bigger bar for race use... nothing foundamental.
    Lots of Guys in the Z4M I know remove the rear Sway, completely. (track use only ofcourse) And yeah we have lots of issues with the front Sway ripping out. Most guys I know run front re-enforcement plates.

  20. #45
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    Quote Originally Posted by vanne View Post
    The coupe a little more rigid than the roady, but still twists and creaks everywhere, especially when pushed a little.
    Don't you think it is most likely the 20 year old plastic interior that is creaking rather than the chassis flexing. How did they sell these cars for $45k if they creaked like that due to chassis flex?

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