Ridgidity coupe vs roadster
- Thread starter Dreamer
- Start date
BMW Z4 Coupe: 32,000 Nm/degree
BMW Z4 Roadster: 14,500 Nm/degree
BMW Z4 Roadster: 14,500 Nm/degree
That's what I thought although that was based on what I'd read about the 3 series, which is very close. That said in it's own right the roadster is pretty good!
http://www.germancarzone.com/test-data/12334-list-torsional-rigidity.html
This was something that I found on the web so may not stand up in court! But they seem to have been quite thorough.
Just to add...
"But first the car. Adding a steel roof (it's so small there'd be no point in doing a carbon-fibre one as on the M6 to save weight) has doubled the torsional rigidity of the Z. The result is a car that's Porsche 911 stiff - which is very stiff indeed."
(from Channel 4 Driven)
http://www.germancarzone.com/test-data/12334-list-torsional-rigidity.html
This was something that I found on the web so may not stand up in court! But they seem to have been quite thorough.
Just to add...
"But first the car. Adding a steel roof (it's so small there'd be no point in doing a carbon-fibre one as on the M6 to save weight) has doubled the torsional rigidity of the Z. The result is a car that's Porsche 911 stiff - which is very stiff indeed."
(from Channel 4 Driven)
Interesting reading. I suppose you can only take there word for accuracy. Is torsional rigidity the whole story?
For instance a Lotus elise has lower value than a Z4 but is that less relevant as it weighs less? Surely you need extra factors in the equation to note how much difference these figures make?
For instance a Lotus elise has lower value than a Z4 but is that less relevant as it weighs less? Surely you need extra factors in the equation to note how much difference these figures make?
Dreamer said:
For your original question of how rigid the chassis is, then I believe so. However if you're talking about handling overall then there isn't one single factor that will win the day. To be honest looking numbers up on the internet it the limit of my knowledge here!
I did think it interesting though the difference between the European and US manufacturers listed.
Haha, good thread 
I'd ignore the figure. In practice does it make a difference when our suspension bushings, deviations on alignment settings, and so on, allow so much variance anyway?
Imagine our car at 1400kg loaded, thats about 350kg a corner, or 3500N.
Now imagine the left side of the car is pushed upwards at the front, and the suspension, dampers, and bushings take the full force they can, then transmit it to the chassis (lets say you drive up a kerb with the front left side). The result is about 7000N on the front left side, acting about a moment of 1.8m ish (right tyre contact patch)...
So that is 3500N/m of twist at the front of the car (if the other front side of the car would lift off the floor) The car wheelbase is about 2.5m, so 15,000Nm/deg gets us about 15,000/2.5m = 6000N/deg of twist on this car from front to rear.
All pretty rough here, pretty sure it's about right.
But that gives you about 6000/3500 of a degree of chassis twist, so about half a degree.
So now imagine that you are blatting down the road, and hit a lump. You can end up finding that you have more negative camber than you might want, meaning less grip, or more grip, and it all makes for an inconsistent delivery of tyre contact patch performance.
However, considering the relative stiffness of the springs, bushings, and tyres, you do have to ask how important the levels encountered may be.
All said and done I think it's pretty much irrelevant beyond a certain level on road cars with road car levels of grip and comfort.
If your car is retardedly stiffly suspended, but has a relatively low rigidity chassis, then you might start to notice, but then that car is flawed to start with.
I do question that a car developed from the start as a Roadster to be rigid enough for the desired suspension settings, then suffers so significantly next to the Coupe one.
Sticking a roof over a very stiff car will always make it much more stiff, but would you notice it? Half a degree vs quarter of a degree of twist?
On a road car, with road suspension, and road damping, with road tyres, and road tyre pressures and temps... you really would have to be some kind of special person to feel that quarter of a degree benefit in extreme cases over all that other springing/damping! (imho)
I think there would be an element of placebo effect at play here. If you then put the same driver in a roadster and "said" it had been strengthened and now had 30,000Nm/deg of torsional rigidity, we would have some saying they could feel the difference
Dave
I'd ignore the figure. In practice does it make a difference when our suspension bushings, deviations on alignment settings, and so on, allow so much variance anyway?
Imagine our car at 1400kg loaded, thats about 350kg a corner, or 3500N.
Now imagine the left side of the car is pushed upwards at the front, and the suspension, dampers, and bushings take the full force they can, then transmit it to the chassis (lets say you drive up a kerb with the front left side). The result is about 7000N on the front left side, acting about a moment of 1.8m ish (right tyre contact patch)...
So that is 3500N/m of twist at the front of the car (if the other front side of the car would lift off the floor) The car wheelbase is about 2.5m, so 15,000Nm/deg gets us about 15,000/2.5m = 6000N/deg of twist on this car from front to rear.
All pretty rough here, pretty sure it's about right.
But that gives you about 6000/3500 of a degree of chassis twist, so about half a degree.
So now imagine that you are blatting down the road, and hit a lump. You can end up finding that you have more negative camber than you might want, meaning less grip, or more grip, and it all makes for an inconsistent delivery of tyre contact patch performance.
However, considering the relative stiffness of the springs, bushings, and tyres, you do have to ask how important the levels encountered may be.
All said and done I think it's pretty much irrelevant beyond a certain level on road cars with road car levels of grip and comfort.
If your car is retardedly stiffly suspended, but has a relatively low rigidity chassis, then you might start to notice, but then that car is flawed to start with.
I do question that a car developed from the start as a Roadster to be rigid enough for the desired suspension settings, then suffers so significantly next to the Coupe one.
Sticking a roof over a very stiff car will always make it much more stiff, but would you notice it? Half a degree vs quarter of a degree of twist?
On a road car, with road suspension, and road damping, with road tyres, and road tyre pressures and temps... you really would have to be some kind of special person to feel that quarter of a degree benefit in extreme cases over all that other springing/damping! (imho)
I think there would be an element of placebo effect at play here. If you then put the same driver in a roadster and "said" it had been strengthened and now had 30,000Nm/deg of torsional rigidity, we would have some saying they could feel the difference
Dave
Mr Whippy said:I think there would be an element of placebo effect at play here. If you then put the same driver in a roadster and "said" it had been strengthened and now had 30,000Nm/deg of torsional rigidity, we would have some saying they could feel the difference
Dave
:lol: Yep seen that sort of thing plenty!
Great explanation Dave... I managed to understand something with is funamentally beyond me. How'd you know all that? Is this a work thing for you?
I don't claim to be an expert, but I try to understand it all because I enjoy simulating these things in a car simulator I've used for almost a decade now (tinkered along and learned as I went)
See here for context
http://www.z4-forum.com/forum/viewtopic.php?f=6&t=8490&p=203662&hilit=virtual+z4#p203662
What I pointed out was just over-simplified lots, and likely missing some core concepts, but generally you go stiffer for tyre contact patch control. If the Roadster was deemed stiff enough and the suspension bushings etc set up for it, then the Coupe won't benefit from it's rigidity hugely on top at all imho. Only if you literally turned it into a racing car for track attack, with rose jointed suspension with little deflection, and alignment settings were key, then that extra control in the chassis twist would be beneficial.
But on the road that 1/4 degree benefit to tyre contact patch in my example is so tiny relative to the movement in all the other parts, I'd question the ability to spot it. I can push my front wheel backwards and fowards against the rear bush of the front wishbone by about 15mm by hand! That kinematic alone will probably be enough to nullify the rigidity benefit of the Coupe for instance, in most driving on the road.
I'm glad BMW made the Coupe, and it's rigidity is impressive but is not the feature of the desired function, it is simply the feature of putting a roof over a body designed to be very rigid without a roof!
The faster rack and fatter rear arb probably make 99% of the difference people 'feel' but they will never say that because it's nice to think you are some driving adonis and attribute it to rigidity benefits that are likely impossible to detect on a road car.
http://www.germancarzone.com/test-data/12334-list-torsional-rigidity.html
I can't really see any pattern there between torsional rigidity and how good a car is to drive.
Dave
See here for context
http://www.z4-forum.com/forum/viewtopic.php?f=6&t=8490&p=203662&hilit=virtual+z4#p203662
What I pointed out was just over-simplified lots, and likely missing some core concepts, but generally you go stiffer for tyre contact patch control. If the Roadster was deemed stiff enough and the suspension bushings etc set up for it, then the Coupe won't benefit from it's rigidity hugely on top at all imho. Only if you literally turned it into a racing car for track attack, with rose jointed suspension with little deflection, and alignment settings were key, then that extra control in the chassis twist would be beneficial.
But on the road that 1/4 degree benefit to tyre contact patch in my example is so tiny relative to the movement in all the other parts, I'd question the ability to spot it. I can push my front wheel backwards and fowards against the rear bush of the front wishbone by about 15mm by hand! That kinematic alone will probably be enough to nullify the rigidity benefit of the Coupe for instance, in most driving on the road.
I'm glad BMW made the Coupe, and it's rigidity is impressive but is not the feature of the desired function, it is simply the feature of putting a roof over a body designed to be very rigid without a roof!
The faster rack and fatter rear arb probably make 99% of the difference people 'feel' but they will never say that because it's nice to think you are some driving adonis and attribute it to rigidity benefits that are likely impossible to detect on a road car.
http://www.germancarzone.com/test-data/12334-list-torsional-rigidity.html
I can't really see any pattern there between torsional rigidity and how good a car is to drive.
Dave
I can't argue with anything you wrote, and agree that the increased rigidity will make little difference to the handling of a road car, but I'm gonna have to disagree with this commentMr Whippy said:
You can and do notice the difference, not in the handling but simply in the way the car doesn't shudder over bumps. More accurately, it's not what you feel in the coupe it's just when you jump in something not as stiff you notice it where as before you probably wouldn't of.Mr Whippy said:I think there would be an element of placebo effect at play here. If you then put the same driver in a roadster and "said" it had been strengthened and now had 30,000Nm/deg of torsional rigidity, we would have some saying they could feel the difference
I owned the Z4MR with OEM strut tower brace for 19k miles, and have now owned and driven my Z4MC (also with OEM STB) for 14k miles.
The difference in ridigity between the roadster and the coupe is very noticible even in the streets and it's no placebo effect. The first clover-leaf on-ramp you take at high speed, you notice the added rigidity. The first man-hole cover you go over, you notice the added rigidity. Over rough roads, you notice the added rigidity in the coupe because the interior isn't rattling and squeaking as much.
You can pretty much toss all the theories out the window. The coupe is a much stiffer car and it's very noticible in the streets, and the added rigidity isn't only appreciated on the tracks. For two cars that are supposed to be ~90% the same, they actually feel very different and the handling characteristics feel very different also.
If anyone drives both the Z4MR and Z4MC and notice no difference, it's because he/she lacks the competence to discern the differences.
The difference in ridigity between the roadster and the coupe is very noticible even in the streets and it's no placebo effect. The first clover-leaf on-ramp you take at high speed, you notice the added rigidity. The first man-hole cover you go over, you notice the added rigidity. Over rough roads, you notice the added rigidity in the coupe because the interior isn't rattling and squeaking as much.
You can pretty much toss all the theories out the window. The coupe is a much stiffer car and it's very noticible in the streets, and the added rigidity isn't only appreciated on the tracks. For two cars that are supposed to be ~90% the same, they actually feel very different and the handling characteristics feel very different also.
If anyone drives both the Z4MR and Z4MC and notice no difference, it's because he/she lacks the competence to discern the differences.
GP20 said:
I would only make a side by side judgement myself if I had driven a Coupe and Roadster with the same rack and rear arb, to start making quantative views.
And I don't believe that it's not impossible to detect that something is different, things like the squeaks and shakes (not that I notice them much), but how much is the rigidity actually improving things anyway? Yes you notice a change, but what does it really mean?
Noticing a difference in two cars that have different steering racks, and different rear arbs, AND more chassis stiffness, doesn't mean all the benefits percieved and actual are down to the improved rigidity alone.
I'd notice a difference immidiately. I wouldn't class myself as good enough to say that ALL the difference I noticed was purely down to the increased rigidity though, because I honestly couldn't even tell you for sure considering the values seen here... http://www.germancarzone.com/test-data/12334-list-torsional-rigidity.html how stiff they actually *felt* when I have driven some of them.
Until you put me in a roadster with the same rack and rear arb I'd not know what performance benefits were due to the rigidity at all. Once you did, I'd probably still notice the difference, but how much that really matters would then be a totally new thing again! Is it simply minute degrees of benefit that you know exist and just pretend to realise despite the soggy road suspension and tyres masking any possible benefits beyond maybe 20,000Nm of torsional rigidity for such a car!?
If you sat me in a car, sent me round a track, and then asked me to guess the torsional rigidity value, I think I'd have some wildly different values to those seen above because it just isn't very indicative of much... it's easy however, to pretend you are special once you know what the figure is, and say that you can tell
It'd be cool to see the difference by either upgrading the roadster to coupe spec on arb/rack, and vice versa, and then drive all four cars side by side. I bet the arb and rack are massively beneficial for a more focussed car to start with! I bet a Coupe with the slower rack and softer rear arb would feel quite a bit duller!
Dave
Dave you're a mine of information! I almost understood some of it
Coupe is set up differently IIRC certainly on the 'M' with different steering geometry and thicker anti-roll bars. So the car will feel 'stiffer' on the road but not all due to the chassis although that will have an influence. The 'MC' was criticised for being too stiff by some reviewers but it certainly has it's devotees.
Coupe owners can say 'mine's stiffer than yours' to us roadster jelly owners- it makes up for the lack of sunroof/soft top on a hot sunny day
Coupe is set up differently IIRC certainly on the 'M' with different steering geometry and thicker anti-roll bars. So the car will feel 'stiffer' on the road but not all due to the chassis although that will have an influence. The 'MC' was criticised for being too stiff by some reviewers but it certainly has it's devotees.
Coupe owners can say 'mine's stiffer than yours' to us roadster jelly owners- it makes up for the lack of sunroof/soft top on a hot sunny day
Yep, the Coupe ultimately is superior for driving. I often think the rack is a tad too slow on the Z anyway, and they do seem setup for a bit too much understeer standard, so they are certainly things I'd change for the more driver focussed version!
But how much the chassis rigidity adds ontop of those changes is beyond me. I doubt BMW actually cared much that is was stiffer, hence the limit of their changes to take advantage of it.
Like I say, I think the stiffness is just a 'side effect' result of adding a roof. I doubt BMW went chasing figures or even re-tuning things because of it now being so much stiffer. I'm sure for instance that they could have added even more rigidity had they aimed for it, but I'm sure they went for styling and cost alone when doing the roof, knowing full well that the rigidity for the suspension setup they wanted (and had already setup) was more than enough!
Maybe one day we will find out if we get a Roadster owner fitting the Coupe rack and arb... would be an interesting comparison
That said, I'm sure most owners are not that drawn between one or the other for it to come down to that, I'm sure more choose one or the other for much more important reasons
Dave
But how much the chassis rigidity adds ontop of those changes is beyond me. I doubt BMW actually cared much that is was stiffer, hence the limit of their changes to take advantage of it.
Like I say, I think the stiffness is just a 'side effect' result of adding a roof. I doubt BMW went chasing figures or even re-tuning things because of it now being so much stiffer. I'm sure for instance that they could have added even more rigidity had they aimed for it, but I'm sure they went for styling and cost alone when doing the roof, knowing full well that the rigidity for the suspension setup they wanted (and had already setup) was more than enough!
Maybe one day we will find out if we get a Roadster owner fitting the Coupe rack and arb... would be an interesting comparison
That said, I'm sure most owners are not that drawn between one or the other for it to come down to that, I'm sure more choose one or the other for much more important reasons
Dave
Just my 2 cents.......the roof itself is not the primary reason for the rigidity change, although it adds a lot. Don't forget, with the coupe, you now have not only "A" pillars, but B and C as well. Those tied together with the roof, no matter how small, makes a more rigid car.
We coupe owners lost the roll hoops, but the bulkhead is still there and the B pillar comes out of the frame where the roll hoops would have-they tie into the roof instead of back down into the bulkhead.
We coupe owners lost the roll hoops, but the bulkhead is still there and the B pillar comes out of the frame where the roll hoops would have-they tie into the roof instead of back down into the bulkhead.
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