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Packaging compromises


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#1 sosidge

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Posted 16 February 2012 - 17:38

Given the choice between a car that was had the ideal weight distribution and was overweight, or one that had a non-ideal weight distribution and was on-weight, which would you choose?

And given the choice between a car with a low centre of gravity with the masses spread far from the CG, and a car with a higher CG but the masses grouped much more closely to it, which would you choose?

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#2 MatsNorway

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Posted 16 February 2012 - 19:40

non-ideal weight distribution and was on-weight,


Porsche 911 did/does allright..



a car with a low centre of gravity with the masses spread far from the CG


The lesser of two evils this one..

#3 gruntguru

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Posted 16 February 2012 - 21:59

Depends on the track, conditions, driver etc. There is no one answer to your question. Any one of the four evils you mention (weight, CGH, wt dist and polar MOI) can be compromised in favour of another, to better suit particular circumstances.

#4 Lee Nicolle

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Posted 17 February 2012 - 02:48

Porsche 911 did/does allright..





The lesser of two evils this one..

Porsche have done a very good job of making a bad package work very well. It just took several decades!

#5 cheapracer

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Posted 17 February 2012 - 04:30

B and A and the girl with the large attributes.

#6 DaveW

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Posted 17 February 2012 - 10:19

Given the choice between a car that was had the ideal weight distribution and was overweight, or one that had a non-ideal weight distribution and was on-weight, which would you choose?

In my view, tyres can be designed to work with any sensible c.g. position.

A (laterally) offset position is something else. One car I encountered, racing in a series that ran "success" ballast, was actually quicker with 50 Kg ballast than without - when the ballast was used to centre the c.g. position. Provides a rational for the offset drive line in some light weight vehicles with a non-central driver position (e.g Caterham).

And given the choice between a car with a low centre of gravity with the masses spread far from the CG, and a car with a higher CG but the masses grouped much more closely to it, which would you choose?

Depends upon the circumstances, as noted by GG. The result of This race provided an interesting example of the effect of yaw inertia. The R8 was quicker than the Panoz, courtesy of a low inertia & (probably) better aero. On the other hand the Panoz on the same tyres, similar c.g. position, but higher yaw inertia (front engine, & compensating moveables) was much quicker in the wet conditions at the start of the race, but was slower in the dry conditions that developed (a dfference of around 6 seconds, I recall). Probably mainly due to the low grip stability provided by the higher yaw inertia of the Panoz.

I can't comment on c.g. height, except to say that lower is usually massively better.



#7 NeilR

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Posted 17 February 2012 - 10:27

how low are cars CoG? I've calculated my projects Cog at slightly less than 400mm with a driver (note calculated - I accept it will be different by +/- 50mm), but how low do they typically go?

#8 Tony Matthews

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Posted 17 February 2012 - 12:07

Off the top of my head (not far from my own CoG), the 2000 F1 Ferrari had a CoG at 160mm.

#9 Kelpiecross

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Posted 17 February 2012 - 13:08

Off the top of my head (not far from my own CoG), the 2000 F1 Ferrari had a CoG at 160mm.


Measured from the ground or the bottom of the car?

#10 Wolf

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Posted 17 February 2012 - 13:22

I've never encountered height of CoG measured from anything else but ground, but still it wouldn't make much difference in case of F1 if it wasn't...

#11 Tony Matthews

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Posted 17 February 2012 - 13:34

Measured from the ground or the bottom of the car?

Top of my head. Oh, I see what you mean - ground.

Edited to say - now I will have to go and check...

I thought it was in the Ferrari F2000 book, but it doesn't seem to be. It maybe that Peter Wright told me, but it was not included in the text. Must be wonderful to have a fully-functioning memory...

Edited by Tony Matthews, 17 February 2012 - 14:17.


#12 munks

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Posted 18 February 2012 - 04:03

I've never encountered height of CoG measured from anything else but ground, but still it wouldn't make much difference in case of F1 if it wasn't...


If one's claim is 160mm, then the 20-40mm of static ride height would be significant. In my opinion, 160mm seems a tad low, even when measured from the bottom of the car. Also in F1, with or without driver and fluids would be important.

#13 gruntguru

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Posted 18 February 2012 - 04:17

American Stig had a COG greater than 160mm and that was lying on his back (on the ground). Measured from the ground or the bottom of the Stig - makes no difference.

#14 MatsNorway

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Posted 18 February 2012 - 10:34

Is CoG that important if it allready is mega low?

Im sure F1 favours low weight over low CoG.

When it comes to cars as in hatchbacks and touring cars i think having a low CoG means a lot.


http://www.youtube.c...VOklOIyxY#t=46s

#15 24gerrard

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Posted 18 February 2012 - 10:43

I have always had the feeling that a very low C of G prevents the transfer of weight onto the tyre contact patch.
Without high downforce surely the force goes mainly sidewise in corners reducing tyre grip?

#16 Wolf

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Posted 18 February 2012 - 11:19

I have always had the feeling that a very low C of G prevents the transfer of weight onto the tyre contact patch.
Without high downforce surely the force goes mainly sidewise in corners reducing tyre grip?


24Gerrard, higher CoG will cause bigger load transfer (which mighn't be a bad thing if it wasn't for tyre load sensitivity)... Lateral force will go sideways either way, but in case of higher CoG it will cause more (vertical) load transfer which in turn will decrease available grip on that pair of wheels. That's the way I see it.

#17 24gerrard

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Posted 18 February 2012 - 12:44

24Gerrard, higher CoG will cause bigger load transfer (which mighn't be a bad thing if it wasn't for tyre load sensitivity)... Lateral force will go sideways either way, but in case of higher CoG it will cause more (vertical) load transfer which in turn will decrease available grip on that pair of wheels. That's the way I see it.


Interesting debate Wolf.
I always found that lowering the C of G to a very low position on a saloon (powertrain mounting plus suspension) reduced the feel for the driver, without noticable decreases in lap times.
It acted much like a low polar moment of inertia and the break away became sudden and more violent, all else being equal of course.
The old front engined rear gearbox (Italian) layout with a relatively high C of G seemed to give the best 'feel', much like 4x4 rally cars.
Horses for courses?
Or maybe modern tyres, now there is a minefield.

#18 DaveW

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Posted 18 February 2012 - 13:51

I always found that lowering the C of G to a very low position on a saloon (powertrain mounting plus suspension) reduced the feel for the driver, without noticable decreases in lap times.


Probably depends on roll centre height....



#19 24gerrard

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Posted 18 February 2012 - 14:00

Probably depends on roll centre height....


The end result depends on both Dave.

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#20 bigleagueslider

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Posted 20 February 2012 - 01:31

With regards to the optimum packaging compromise for weight distribution or MOI, it depends upon how the car is to be driven. Looking at the two race car examples below, how important is yaw inertia or front/rear weight distribution?

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#21 gruntguru

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Posted 20 February 2012 - 03:14

:up: Touche` Slider.

#22 bigleagueslider

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Posted 20 February 2012 - 05:31

:up: Touche` Slider.


gruntguru,

I won't complicate the discussion by bringing up the subject of road race bikes. Where the rider actively and constantly adjusts the overall MOI using his body position, and where the F/R weight distribution can vary from 0% to 100% depending upon whether the rider is braking or accelerating.

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Regards,
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#23 pugfan

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Posted 29 February 2012 - 23:39

Is CoG that important if it allready is mega low?

Im sure F1 favours low weight over low CoG.

When it comes to cars as in hatchbacks and touring cars i think having a low CoG means a lot.


http://www.youtube.c...VOklOIyxY#t=46s


In a presentation from the chief chassis engineer for Williams, he stated that lots of effort is spent removing weight from the chassis only for it to be put into the ballast.

So low weight and low CoG are essentially the same thing for F1.

#24 carlt

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Posted 01 March 2012 - 09:43

Probably depends on roll centre height....


I usually strive to lower both COG and Roll Centre for Hillclimb saloon cars [this seems to allow very quick changes of direction without excessive yaw ]
The lower RC needs to be controlled with stronger Anti roll bars
As was pointed out earlier , 'horses for courses'
Karts seem to corner phenomenally well

#25 MatsNorway

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Posted 01 March 2012 - 12:00

In a presentation from the chief chassis engineer for Williams, he stated that lots of effort is spent removing weight from the chassis only for it to be put into the ballast.

So low weight and low CoG are essentially the same thing for F1.


IF you must CHOOSE what would it be is the question here. And by that i mean reaching the minimum weight limit in F1 OR having a low CG

Sorry.. did not get a lot of sleep this night..

Edited by MatsNorway, 01 March 2012 - 12:08.


#26 carlt

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Posted 01 March 2012 - 12:25

IF you must CHOOSE what would it be is the question here. And by that i mean reaching the minimum weight limit in F1 OR having a low CG

Sorry.. did not get a lot of sleep this night..


obviously DON'T choose at all UNTIL you've had some SLEEP

#27 bigleagueslider

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Posted 03 March 2012 - 03:16

IF you must CHOOSE what would it be is the question here. And by that i mean reaching the minimum weight limit in F1 OR having a low CG

Sorry.. did not get a lot of sleep this night..



MatsNorway,

Hope you got plenty of sleep.

The OP mentioned race cars, but not F1 specifically. While the question posed makes for interesting discussion, it is rather vague and thus has no simple answer. Descriptions like "low CG" or "masses spread far from the CG" are too subjective to make a decision.

All race car design is basically an exercise in compromise. The designer trades weight for strength, downforce for drag, performance for reliability, etc. All of these trades must also take into account various rules and regulations. The trade-off is further complicated with cars that generate significant aero downforce. Traction at each tire contact can sometimes be more affected by the aero CoP than the mass CoG or polar inertia.

Also, simply having a low chassis CoG may not be as important as having a consistent CoG throughout a race stint. For example, if an F1 car starts a race with a fuel load of over 120 lbs, but only has about 10 lbs left (in the bottom of the tank) on the lap before pitting, that makes for a noticeable change in weight, vertical CoG location, and polar MoI over the course of those laps.

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#28 Greg Locock

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Posted 03 March 2012 - 21:32

Given the choice between a car that was had the ideal weight distribution and was overweight, or one that had a non-ideal weight distribution and was on-weight, which would you choose?

And given the choice between a car with a low centre of gravity with the masses spread far from the CG, and a car with a higher CG but the masses grouped much more closely to it, which would you choose?

First one is difficult to answer, very much horses for courses. In the absurd case of a 0/100 weight distribution then the overweight 50/50 car would beat it around any significant corner. On the other hand a 49/51 car would beat a 50/50 car that was significantly heavier. I did run some sims in LapSim that seemed to show that once you were within 5% of the optimum F/R balance it was a fairly flat curve.

Second one is a bit easier, I think a low CG and high PMI would be much easier to set up and race. But again taken to extremes then anything might happen.





#29 bigleagueslider

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Posted 05 March 2012 - 04:58

.......In the absurd case of a 0/100 weight distribution.......


Greg Locock,

A vehicle with a 0/100 fore/aft weight distribution might not make for a good race car, but it might not be completely absurd. Think about the Segway scooter. It has the entire vehicle and passenger mass load on two coaxial wheels. It uses actively generated dynamic forces to balance those from accelerating, braking or cornering.

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#30 Warpspeed

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Posted 05 March 2012 - 08:55

Greg Locock,

A vehicle with a 0/100 fore/aft weight distribution might not make for a good race car, but it might not be completely absurd. Think about the Segway scooter. It has the entire vehicle and passenger mass load on two coaxial wheels. It uses actively generated dynamic forces to balance those from accelerating, braking or cornering.

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How many cornering G's can a Segway pull Greg?

Anyway, everyone knows a two wheel bike is an impossible abstract concept, it will just fall over.

Just as the aerodynamics and weight distribution of a bumble bee make flight an absolute impossibility.

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Edited by Warpspeed, 05 March 2012 - 09:02.


#31 desmo

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Posted 05 March 2012 - 15:50

Urban mythology:

http://www.sciencene...f_the_Bumblebee

#32 Greg Locock

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Posted 05 March 2012 - 20:46

Even the segway proves my point. If you had two segways coupled together, each of half the mass, on the same tires as a single segway, it would corner better, due to the vertical load sensitivity of the tire.

#33 pugfan

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Posted 05 March 2012 - 23:31

Even the segway proves my point. If you had two segways coupled together, each of half the mass, on the same tires as a single segway, it would corner better, due to the vertical load sensitivity of the tire.


A counter-argument could be that on the single segway the rider can more effectively counteract lateral load transfer and therefore be 'kinder' to the load sensitivity of the tyre...

#34 Greg Locock

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Posted 06 March 2012 - 00:39

A counter-argument could be that on the single segway the rider can more effectively counteract lateral load transfer and therefore be 'kinder' to the load sensitivity of the tyre...

O, is there a law against leaning on quad bikes?

#35 Catalina Park

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Posted 06 March 2012 - 09:59

O, is there a law against leaning on quad bikes?

On the early ones I used to lean out when cornering to get the inside rear tyre to slip so that the damn thing would turn. They started fitting diffs soon after that.

#36 Magoo

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Posted 06 March 2012 - 22:33

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#37 carlt

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Posted 06 March 2012 - 22:49

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or possibly 'how to package enough distance to avoid the shrapnel' ?

#38 GreenMachine

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Posted 07 March 2012 - 03:02

or possibly 'how to package enough distance to avoid the shrapnel' ?


or to cut down on radiation from the reactor ... :rotfl:

#39 Magoo

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Posted 07 March 2012 - 11:43

These days, when I see something like this my first impulse is to suspect it was photoshopped. This deal is real, though I have no idea what was the point of it.

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#40 gruntguru

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Posted 07 March 2012 - 12:19

A wheelie might be interesting.



#41 cheapracer

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Posted 07 March 2012 - 12:42

This deal is real, though I have no idea what was the point of it.


Quite obviously he wants to be first over the finishing line.


#42 giskard

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Posted 19 June 2012 - 03:32

QUOTE (sosidge @ Feb 16 2012, 18:38)
non-ideal weight distribution and was on-weight,

Porsche 911 did/does allright..


Could you expound? I thought a rearward weight bias is good for braking and acceleration. And F1 cars are rear weight biased, right?


#43 MatsNorway

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Posted 22 June 2012 - 09:53

QUOTE (sosidge @ Feb 16 2012, 18:38)
non-ideal weight distribution and was on-weight,



Could you expound? I thought a rearward weight bias is good for braking and acceleration. And F1 cars are rear weight biased, right?


Its a big topic.

It varies with grip, power, CG, wheelbase and more.

For a given set of rules on one spesific track there will be a optimum solution. Usually it will not be a rear mounted engine.

regarding the 911 the stories was probably related to the the tire sizes available/used. So the older porsches where possibly worse as they had more weight at the back and almost equal tire sizes.
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Rear heavy gives you a sluggish response at the back.

I don`t think it is a big topic on the modern 911s as they have bigger tires at the back and more weight at the front due to gizmos and safety dodads.

Besides most rulebooks are probably making rules so that a 911 can compete now.

The more serious Racing 911s does have even bigger tires at the back than their street legal versions. Probably related to them being free of AC, collision sones, stereo, this and that at the front. + more power.
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I am waiting for Porsche to mount their gear box at the midle of the car. And then run it back to the diff. Kinda like what the R35 does the other way.

Edited by MatsNorway, 22 June 2012 - 09:55.


#44 kikiturbo2

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Posted 22 June 2012 - 10:40

I much prefer their mid engind cars.. :)

#45 MatsNorway

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Posted 23 June 2012 - 07:01

I take the 911s any day.

But there is a reason they only have one model with rear engine.

#46 kikiturbo2

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Posted 23 June 2012 - 09:33

ok, I was provoking you a bit... gt3 is also on the top of my list... however I recently had the opportunity to drive the new 991 in various carrera and carrera S forms and also the whole new boxter lineup, on the sam day , and the same track, and it was interesting to see the differences... Boxter is just a significantly more balanced machine... Even with all the work done by porsche on the 911, it still can not escape from the dinamics of the big lump of metal hanging in the rear.. it is really noticable.... and you have to adjust your driving style to match..
For the record, base carrera with normal suspension and 7 speed manual was the pick of the lot.. :) As for boxter, S manual please.. :)

Edited by kikiturbo2, 23 June 2012 - 09:33.


#47 MatsNorway

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Posted 23 June 2012 - 18:38

Manual as in stick shift or those funny paddle shifters?

#48 kikiturbo2

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Posted 23 June 2012 - 19:24

stick shift, as in proper manual.. :) ...

other option is PDK twin clutch with flappy paddles. :)

#49 MatsNorway

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Posted 24 June 2012 - 08:11

Good to hear they still deliver with stick shift. But do you get that as a option in the GTs?
I think a stick shift will hold its price better.

Edited by MatsNorway, 24 June 2012 - 08:11.


#50 kikiturbo2

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Posted 24 June 2012 - 10:58

current info is that the next 991 gt3 will be PDK... which is a shame..

A well executed manual downshift brings me much more joy then half a second a lap pdk will bring. .:)