How does the centre of gravity of an E89 change with roof position?
- Thread starter B21
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Scottish Borders
Deepseaskateboard
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Attleborough, Norfolk
More weight over the back wheels will aid traction, but how much weight is that and is it really going to make much difference? In reality, a set of full suitcases will have much the same impact, or in your case Peter, the spare wheel.
All this of course is irrelevant as it is cancelled out by the Carphone warehouse you have installed in the cabin.
All this of course is irrelevant as it is cancelled out by the Carphone warehouse you have installed in the cabin.
North West Surrey
I would have thought it must lower the centre of gravity if the weight of the roof is sitting lower, and moving it rearwards must affect the front/rear balance.
A set of corner weight scales would answer the balance shift question if anyone has an E89 and access to some.
A set of corner weight scales would answer the balance shift question if anyone has an E89 and access to some.
This is a very good question, the roof approximately weighs about 120 kg with all the glass and hydraulics etcetera. So moving this from roof up to roof down moves the cars centre of gravity, both in the vertical and horizontal directions. If it drops 500 mm, then a proportion of that 120 kg will lower the CofG so lets be cautious and say 60% or 300 mm or 12" (I know you guys like to fantasise), this makes for better handling as I've discussed previously. For the 35i manual with it's near equal weight distribution, dropping the roof pushes that weight further back, this isn't necessarily bad, per se, there is greater traction/grip at the back from the wider wheels.
Mine's 12 inches, but I don't use it as a rule.
Boasting againMine's 12 inches, but I don't use it as a rule.
So..
The German Wiki for the E89 reckons that the E89 has a 30kg weight penalty on its roof compared to an E85..
I guess the E85 roof system was about 30kg too!
The E89 was a lot stiffer so most of its weight increase over the E85 comes from being stiffer …
Here’s a pic of three sets of ride heights
First is 4 corner ride height roof up
Second is 4 corner after FRH strut was given a two turn tweak to try and address cross axle ride height variation
Third is roof down..barely a mm and not consistent..
The conclusion on some pretty stiff springs is it’s in the noise
I suspect that static friction / stiffer springs means it’s not noticeable static..maybe dynamically different..
The German Wiki for the E89 reckons that the E89 has a 30kg weight penalty on its roof compared to an E85..
I guess the E85 roof system was about 30kg too!
The E89 was a lot stiffer so most of its weight increase over the E85 comes from being stiffer …
Here’s a pic of three sets of ride heights
First is 4 corner ride height roof up
Second is 4 corner after FRH strut was given a two turn tweak to try and address cross axle ride height variation
Third is roof down..barely a mm and not consistent..
The conclusion on some pretty stiff springs is it’s in the noise
I suspect that static friction / stiffer springs means it’s not noticeable static..maybe dynamically different..
The ‘car phone warehouse’ I think lie de minimus …..a few grams from the mr 12 volt, the Id4 motion, director head unit, speaker magnets, c/f trim..More weight over the back wheels will aid traction, but how much weight is that and is it really going to make much difference? In reality, a set of full suitcases will have much the same impact, or in your case Peter, the spare wheel.
All this of course is irrelevant as it is cancelled out by the Carphone warehouse you have installed in the cabin.
The killer is the 7.5” FMIC out front and the 326m front with a 235/35 19 tyre in the boot..
13.2 kg over probably 5kg for stock..
25kg of tyre and wheel in the back..
The great news is that’s it a massive increase in polar moment of inertia…so less snap oversteer compared to a Boxster!
Pendulum effect is fundamentally opposite to a concentrated polar moment of inertia …
There are at least four car layouts…
E89 type..heavy engine way out front..battery / passengers / battery at the back…net result ..a dumbbell effect …tends to be stable and can have both front end understeer and rear end oversteer..but needs a lot of provocation to initiate and predictable in how to manage ..high polar moment of inertia
Front engine / front wheel drive..mostly front end understeer pretty docile..more like a dart lower polar moment of inertia cog behind polar moment
Rear engine …prone to rear oversteer pendulum in effect lower polar moment of inertia but cog in front of the polar moment
Mid engine …mass centralised low polar moment, often with lower weight elsewhere..
Please read…
The Porsche Cayman is prone to snap oversteer primarily due to its mid-engine layout, which places the engine's weight between the axles, creating a high, sensitive polar moment of inertia. This configuration allows for high rear grip, but if that grip is lost, the rear end can swing out violently, especially if the driver abruptly lifts off the accelerator mid-corner, causing a rapid weight shift forward and unloading the rear tyres.
Key factors for this "snap" behavior include:
Mid-Engine Dynamics: The concentration of mass in the middle makes the car highly agile, but also sensitive to sudden, sharp driver inputs.
Weight Transfer (Lift-off Oversteer): Lifting off the throttle suddenly in a corner shifts weight to the front, removing necessary traction from the rear and triggering an instant spin.
Correcting Oversteer: If a driver overcorrects (steers too much) during an initial slide, the rear tyres can suddenly regain grip, causing the car to "snap" back in the opposite direction, sometimes leading to a "tank slapper".
Driver Sensitivity: Mid-engine cars like the Cayman reward smooth inputs and can be unforgiving of abrupt, jerky movements, distinguishing them from front-engine cars.
While modern electronic stability controls (PSM) in the Cayman drastically mitigate this risk, the underlying mechanical sensitivity remains, particularly when the system is disabled or driving at extreme limits.
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My lad has one of these fancy mountain bikes where the saddle goes up and down to maintain the best balance on undulating ground.
Using that theory @B21, you should put the roof up to go up a hill, then lower it before you come down the other side.
If nothing else, it'll keep @RobbiZ4 busy, fixing your roof.
Using that theory @B21, you should put the roof up to go up a hill, then lower it before you come down the other side.
If nothing else, it'll keep @RobbiZ4 busy, fixing your roof.
I’ve always wondered how much better an E89 would be with all the roof completely removed. Sadly we don’t have the weather for this.
My other idea was an E93 M3. They’re really cheap and would surely be massively improved without the several hundred kilos of roof mech and motors etc. I wonder if this has ever been done.
My other idea was an E93 M3. They’re really cheap and would surely be massively improved without the several hundred kilos of roof mech and motors etc. I wonder if this has ever been done.
I think people get too focused on the impact of the folding roof weight, it really doesn't make any noticeable difference IMO.
Same for the total weight of the car when compared to E85. E85 to E89 to G29 the weight difference is negligible, as is the impact of the weight and positioning of the roof on the E89.
Jump in an MX5 or Merc V250d and you'll feel the weight difference, but Z4 to Z4, or roof up to roof down, nope. E89 handling deficiencies are down to chassis components, not weight.
Same for the total weight of the car when compared to E85. E85 to E89 to G29 the weight difference is negligible, as is the impact of the weight and positioning of the roof on the E89.
Jump in an MX5 or Merc V250d and you'll feel the weight difference, but Z4 to Z4, or roof up to roof down, nope. E89 handling deficiencies are down to chassis components, not weight.
Intuitively it seems like a very good idea..however ..most of the roof parts up high are relatively light ..
If you look at a mass of say 1,600kg with very heavy masses in and around engine, diff, wheels etc the percentage change from say 30-50kg will not be a lot imho..certainly not discernible on a normal road…?
You didn’t respond to my point about adding weight to the rear of the E89 increases snap oversteer not reduce it.
You said Boxster, not Cayman.
Thanks for the oversteer lecture that I neither asked for nor needed as I understand it perfectly well, this ‘Peter clever, everyone else thick as mince’ attitude is getting out of hand.
Have you ever driven a Boxster…? Unless you’re being an absolute idiot they DON’T snap oversteer, I’ve had 3 btw.
Anything will let go given enough provocation, but to say a Boxster will let go before a E89 is pure nonsense, same as saying one can see how viewing cars in the mirror go handle and stop on a run, more nonsense. Is that what they teach you on advanced driving courses, to spend as much time as possible looking in the mirror at following vehicles rather than the road ahead..? Just curious…
