Lets talk weight

[ichitaka05]

Originally Posted by blur

Source?

Originally Posted by fatoni

dont remember but whats yours? i might be wrong but i seriously doubt a 20k car, likely with a steel roof is going to have a lower cg than things like the lfa or ariel atom

http://www.ft86club.com/forums/showthread.php?t=1717

As for low to the ground, Tada promises a “production car with the world’s lowest center of gravity.”

[SUB-FT86]

I would love this!!! 200hp@7000/160tq@4200, with 2500-2600 lbs priced at $22,500. That doesnt sound like im asking for a lot.

[ryude]

Originally Posted by fatoni

i understand that but you have to consider that a roof of a car is also at the very top of the car, the worst place for the weight to be. you call it added weight but but kind of omit the fact that roofs weigh something. on top of that roofs have to be roof shaped. added strutctures dont have to cover passengers so the can be shaped to put rigidity and weight first instead of an afterthought. how do you explain the elise? its plenty rigid and yet it doesnt have a structural roof. all that and it comes in at under 2000lbs. do you think that a fixed roof elise would drop 200 lbs?

I know you just want to argue for the sake of argument, but can we just say that we disagree and be adults?

[Dave-ROR]

Originally Posted by fatoni

i agree that most convertibles are heavier than coupes but my point is that most isnt good enough to say that if x convertible weighs y, z coupe should be lighter than x.

Sorry I wasn't trying to make the point that there's some specific rule of thumb or formula or anything for figuring that out...

[WingsofWar]

FD/FC rx7 has an incredibly low CoG as well with the Rotary engine being a very short engine and pushed back into the chassy. But Iv heard that the x-axis of the CoG could still have better improvement.

I could predict that the car could have a "perfect" CoG balance, but what im afraid of is, how does this affect the car?

Iv never driven a car with a perfectly low CoG..lowering a car via springs/coilovers only lowers the y-axis cog of the car right? but cars still suffers from front to rear and side to side weight transfer.

I think it will drive really good and stable at any forceful condition you put your car into, but i cant be to sure because i have no experience to that sort of sensation of a car that has near 0-central movement.

[LionZoo]

Originally Posted by fatoni

i understand that but you have to consider that a roof of a car is also at the very top of the car, the worst place for the weight to be. you call it added weight but but kind of omit the fact that roofs weigh something. on top of that roofs have to be roof shaped. added strutctures dont have to cover passengers so the can be shaped to put rigidity and weight first instead of an afterthought. how do you explain the elise? its plenty rigid and yet it doesnt have a structural roof. all that and it comes in at under 2000lbs. do you think that a fixed roof elise would drop 200 lbs?

The Elise is not a valid comparison. The Elise frame is an extruded aluminum tub chassis, similar in design to that used on certain racecars. The sides of the Elise tub are thick and high, which is why the door sill is also your armrest. The construction technique used for the Elise chassis is impractical for a high volume production car. The actual body panels of the Elise that forms the styling is non-load bearing. (FYI the Evora uses a much bigger version of the same chassis concept used in the Elise. However, the Evora has a stressed roof in the design for much greater rigidity.)

By contrast, the FR-S, the Miata, and the vast majority of high volume production cars use sheetmetal unibody construction. The body panels are load bearing and that includes the roof, if one exists. For an illustration of why roofs matter for chassis stiffness, just compare the stiffness of a shoebox with and without the lid on.

[blur]

Originally Posted by WingsofWar

Iv never driven a car with a perfectly low CoG..lowering a car via springs/coilovers only lowers the y-axis cog of the car right?

I *think* CoG refers to the the car chassis, regardless of height in suspension. So *in theory* a stock FR-S and a dropped FR-S would still have the same CoG, because it isn't dictated by the distance between the CoG and the ground.

Just a thought. Would love it if someone could enlighten me.

Oh and thanks ichi.

[WingsofWar]

Originally Posted by blur

I *think* CoG refers to the the car chassis, regardless of height in suspension. So *in theory* a stock FR-S and a dropped FR-S would still have the same CoG, because it isn't dictated by the distance between the CoG and the ground.

Just a thought. Would love it if someone could enlighten me.

Oh and thanks ichi.

oh? i always thought that CoG referred to similar to how Airplanes and Boats have a CoG frame formula for stability and balance. Which is why the boxter engine was chosen to power the car because of the CoG variable it added to improve the number closer to 0, compared to tall inline4 or v6 cylinder engines which have a horrible CoG.

The FD/FC rx7 had a low CoG, but the suspension/chassis height was just as high as any other production car. And it was my understanding that lowering the car improved the CoG.

[blur]

Originally Posted by WingsofWar

oh? i always thought that CoG referred to similar to how Airplanes and Boats have a CoG frame formula for stability and balance. Which is why the boxter engine was chosen to power the car because of the CoG variable it added to improve the number closer to 0, compared to tall inline4 or v6 cylinder engines which have a horrible CoG.

The FD/FC rx7 had a low CoG, but the suspension/chassis height was just as high as any other production car. And it was my understanding that lowering the car improved the CoG.

Yes both are true, but in the latter CoG is referred to as related to the ground. What I was saying is that the CoG he mentioned is referring to the chassis.

[WingsofWar]

Originally Posted by blur

Yes both are true, but in the latter CoG is referred to as related to the ground. What I was saying is that the CoG he mentioned is referring to the chassis.

Me too! i think the CoG mentioned is also chassis. Id love to see what kind of numbers the FT86 comes out to be via formula and compared to other cars with similar low cog numbers like the GC8 impreza and FC/FD rx7.

A quick google search let me pick this up from a physicsforms,

Center of Gravity (CG) is defined as that point about which, if the body were suspended from it, all parts of the body would have equilibrium ..i.e.. without tendency to rotate. it is the 3D balance point of the race car. all acceleration forces acting on a body can be considered to act through the CG of that body..we have the cg to be as low as we can get it.

Mass Centroid- is related to Cg ..sort of...ifin we slice the car into a series of sections, like a loaf of bread, each slice would have its own CG...if, in a side view, we draw a line connecting each sliced CG..we have the Mass Centroid Axis..not really a straight line but close to it.
it gives an indication of the distribution of the vehicles mass in the vertical plane.

Roll Center of the suspension is that point , in the traverse plane of the axles, about which sprung mass of that tend of the vehicle will roll under influence of centrifugal force. It is determined by drawing a line from the tire contact patch to a point in space that is the intersection of the upper and lower suspension arm linkage angles.

The CG and RC are located at different points. Usully the CG is many inches above the RC. CG can be change by moving " weight " around on the chassis.. The RC is changed by modifying how the upper and lower control arms mount to the chassis. Correct changes will improve the weight transfer to the rear tires thus improving traction.
lot of other stuff goin on here.. race car math of some is on my post in General Physics post " Race Car Physics" 31 Dec 2008

Finding the center of gravity height can be done in several ways, none of which are accomplished very easily and without some work. Presented here is the easiest method. The center of gravity height is calculated by weighing the car when level and then raising the car at least 10 inches at the rear and weighing the front again. Replace each shock absorber with a solid link to eliminate suspension travel

You need a set of electronic scales. You also need a set of blocks o eight inches tall. 10 inch is better.

First, weigh the car normally, then record the total weight (T) and the total number of pounds on the front tires-nose weight (N).

Jack up the rear of the car..CAREFULLY!! Then carefully place the blocks under the rear scales (you may find it useful to lock your rear axle). Now record your new nose weight (NI). Next, subtract your original nose weight (N) from your new, lifted nose weight (NI) and you will have the difference (Nd).

Figure it out using your calculator…

a. Multiply the nose difference (Nd) x 1660= (A)

b. Multiply the total weight (T) x 8 (the height of the blocks)=(B)

c. Divide your previous numbers, A by B, and this equals (C)

d. Add this number, C, to your axle centerline height or spindle center, and the number you obtain is your vertical center of gravity.






CGH= WB x FWc / TW x Tan of angle


Center of Gravity Height Formula


Definition of Variables

CGH - Center of Gravity Height
WB - Wheelbase (inches)
TW - Total weight
FW1 - Front weight LEVEL
FW2 - Front weight RAISED
FWc - FW2 - FW1 (change in weights)
HT - Height raised (inches)
Adj - Adjacent side (see below)
Tan q - Tangent of angle (see below)
CLF - Left Front tire circumference
CRF - Right Front tire circumference
C - (CLF + CRF) / 2 (average circumference)
r - Axle Height

[fatoni]

i dont mean to instigate but im trying to point out that just because a convertible weighs this much, the frs will weigh less

Originally Posted by LionZoo

The Elise is not a valid comparison. The Elise frame is an extruded aluminum tub chassis, similar in design to that used on certain racecars. The sides of the Elise tub are thick and high, which is why the door sill is also your armrest. The construction technique used for the Elise chassis is impractical for a high volume production car. The actual body panels of the Elise that forms the styling is non-load bearing. (FYI the Evora uses a much bigger version of the same chassis concept used in the Elise. However, the Evora has a stressed roof in the design for much greater rigidity.)

By contrast, the FR-S, the Miata, and the vast majority of high volume production cars use sheetmetal unibody construction. The body panels are load bearing and that includes the roof, if one exists. For an illustration of why roofs matter for chassis stiffness, just compare the stiffness of a shoebox with and without the lid on.

that is a little bit of a cop out. for starters we dont know if the frs is going to be a high volume car either. second, im just responding to the fact that people are saying convertibles are heavier than fixed roof cars. i pointed out that the elise doesnt follow the rule and then you say it doesnt count. i understand its a bit extreme but its still a car and it still doesnt have a structural roof. i could have used the previously mentioned z4 coupe vs vert to the same effect.

do i think convertibles are better performers than coupes? no. i do think that convertibles can be lighter than coupes. thats all im saying

[Dave-ROR]

Originally Posted by fatoni

i dont mean to instigate but im trying to point out that just because a convertible weighs this much, the frs will weigh less


that is a little bit of a cop out. for starters we dont know if the frs is going to be a high volume car either. second, im just responding to the fact that people are saying convertibles are heavier than fixed roof cars. i pointed out that the elise doesnt follow the rule and then you say it doesnt count. i understand its a bit extreme but its still a car and it still doesnt have a structural roof. i could have used the previously mentioned z4 coupe vs vert to the same effect.

do i think convertibles are better performers than coupes? no. i do think that convertibles can be lighter than coupes. thats all im saying

It's mostly invalid because it's NOT a convertible. It's a targa top car, which is completely different. It HAS a structure up top, unlike a convertible.

Yes a few (very few) roadsters can weigh less than the coupe versions, and that became more common recently as you lose a lot of airbags the coupe version requires, but it's still more common for coupes to weigh less, GENERALLY. It's also more common in cars designed as convertibles (or at least an intention that a lot of sales would be convertibles), so the coupe keeps a lot of that extra structure and gains the extra weight of the roofskin/bracing. Good for performance/stiffness of those coupes though

But again, the Elise doesn't apply simple because it's not a convertible.

[Spaceywilly]

Maybe it's better to think about "what do we want it to weigh" instead of "what will it weigh?" Since we know it will have 200hp, it's pretty easy to figure out lb/hp based on the weight. 3000lbs would give it the same lb/hp as a Miata and Mini Cooper S. 2750lbs would put it at the same lb/hp as a 2002 WRX. 2500lbs would put it at the same lb/hp as an Evo X. To get the same lb/hp as a Mustang V6 and 2009 STI, it will need to weigh 22,600lbs.

When you look at it that way, anything around 2500lbs would be amazing. Something like 2750 would not be too bad, and even 3000lbs would be not be unacceptable to me, since the Miata and MCS are both awesome cars to drive, even with around 15lbs/hp. Lb/hp generally gives you a good idea of "quickness" (it correlates pretty well with lap times, if you graph it), but there is no number that will tell you how fun a car is to drive.

[Dave-ROR]

I'd love 2500-2700, 2800 would be the most I'd consider acceptable...