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+--- Thread: center of gravity/mass (/showthread.php?tid=4471)
center of gravity/mass - bricktron - 04-02-2013
hi folks,
anyone know about how high up the center of mass is on a W123 with stock ride height? my wild guess is 500 - 600mm. it would be nice to know better because i'm interested in reducing it by using smaller tires (puhn again).
what would a 5 or 10 percent reduction achieve in terms of improved cornering?
RE: center of gravity/mass - raysorenson - 04-02-2013
Fun topic, but I have no idea. Smaller diameter tires are often used in autocross.
Sunroof delete would be a cool mod and moving the battery to the spare tire well would also help. I've also considered lowering the engine/trans but I don't think it can be lowered enough to be worth the effort.
You could also jack up the roll center by using concentric bushings in the F&R lower control arms.
RE: center of gravity/mass - Simpler=Better - 04-02-2013
The fuel tank stands out to me. Move it into the passenger footwells.
RE: center of gravity/mass - willbhere4u - 04-03-2013
on a 240d you could theoretically use all of the 300d parts ie engine mounts and trans crossmember and move the engine and trans back 4-5 inches
RE: center of gravity/mass - bricktron - 04-03-2013
replacing fuel tank with a container in the spare well is a great idea. by my math it's two or three times as heavy as the battery.
noone would bother doing these mods for a 5% decrease in weight transfer in cornering, right? i can't tell yet which formula would indicate the degree of improvement.
RE: center of gravity/mass - mbz123 - 04-05-2013
Agree with relocating the fuel tank. Probably not as much benefit as I would hope, but end goal is cumulative. Think about how much $$$$ you'd throw at trying to make a go-kart street legal? Sure makes my noodles ache!
If a tank with suitable dims made from lighter material could be sourced cheap enough (I know I know) you could relocate to where the spare tire well was before you eliminated that sheet metal. Lighter fuel tank, eliminated steel added to lower + further rearward position seems IMO one of the bigger adjustments possible (the other being sunroof rem.) Shoot, just tossing a wagon tank in there may be slight improvement. Throw your battery/ies to the sides a'la bimmeresque. Even + half percent would be fine with me. Cumulative, member? Seems to me more labor than cost involved here.
Now just waiting to stumble over an aluminum racing tank in some half-complete project gone bust at the parts yard. Hah! Always the dreamer I tell yas. Only aftermarket tanks I've ever seen were a couple aluminum WVO tanks pretty much same dims as stock with larger capacity.
Anyone know of an available simulation app that might calculate ballpark numbers? Without a full CAD style install that is.
MBZ123
RE: center of gravity/mass - Simpler=Better - 04-05-2013
I'm talking about putting a fuel tank in place of the rear seats. In 90s camrys the fuel tank is under the rear seat, that would be an easy /cheap retrofit.
Put the battery in the rear passenger footwell. That way your weight is low and in between the axles.
RE: center of gravity/mass - bricktron - 04-06-2013
i agree that a rear footwell tank might be even better, but the usability factor's not there vis-a-vis carrying one's family...
i read today that many front-engined, rear-wheel drive cars have their center of mass approximately at the height of the crankshaft. if i had the height of the center of the tank and the center of the spare well at hand, calculating the improvement should be easy.
battery to the back and to the right side sounds good too. is there any battery or pair of batteries that would fit better than a type 49 in that low recess?
RE: center of gravity/mass - raysorenson - 04-06-2013
I stuck 2 Optima red tops in the spare tire well. It's nice. It helped tame my high rear spring rates and the increase in traction is apparent. I used a Perko battery switch near the batteries in case I had to cut the power quickly.
Another thing about batteries is that they're a fixed ballast. You won't always have a full tank of fuel. I'll post a pick in my thread later.
RE: center of gravity/mass - bricktron - 04-08-2013
i'm a little concerned that moving the battery/ies back there will increase the polar moment at the same time as it lowers th center of gravity. this would make the car smoother on fast straightaways but more reluctant to corner, as i understand it... did you notice any difference in this regard, raysorenson?
RE: center of gravity/mass - raysorenson - 04-08-2013
If I'm not mistaken, there's zero polar inertia in a vertial line through the center of the rear axle. Replacing a 45# battery placed 8' in front of the rear axle with 60# of batteries 1.5' behind the rear axle should net a reduction in polar moment.
Playing devil's advocate and assuming zero net change in polar moment, moving mass from in front of the rear axle to behind the rear axle should reduce understeer when entering a corner and increase terminal oversteer when leaving the corner.
I haven't made enough of a change to be able to nail down any chassis behavior changes that one could point to a difference in polar inertia being the cause. I've got high rear spring rates to take cornering forces off of the outside front tire in my front-heavy car. It was causing issues like oversteer and loss of traction on the inside rear tire on corner exit. Putting batteries in the spare tire well helped this a lot.
RE: center of gravity/mass - raysorenson - 04-09-2013
I was wrong about the location of the Z axis. It can move forward with an increase in slip angle of the rear axle.
RE: center of gravity/mass - bricktron - 04-09-2013
ray, i think even with no slipping it takes more cornering force to turn a long weight than an equivalent short one (puhn calls this the "dumbbell" effect). this leads me to suspect the opposite, that it would increase understeer (commensurate with your experience, great!).
apparently the factor is the square of the distance to the center of gravity. imagine putting the battery on a twenty foot pole behind the car and how little you'd be able to turn then. now the trunk is not all that far from the center, but i still haven't got to the chapter about how to determine these locations by measurement.
thanks for all the comments!
RE: center of gravity/mass - Tmadia - 04-09-2013
I don't know if they would fit, but under the rear seats would be a great place for a sealed battery or batteries. That's where I put the one in my 1969 BMW 2002. It lowers the CG and moves it back a little, both of which should help.
The only downside is access is a bit of a pain.
RE: center of gravity/mass - sassparilla_kid - 04-09-2013
I say move the engine/transmission to the rear seat area, should help cornering a fair amount
RE: center of gravity/mass - larsalan - 04-09-2013
^I was going to say this same shit.
617 mid-engine rwd ftw.
RE: center of gravity/mass - sassparilla_kid - 04-09-2013
(04-09-2013, 05:57 PM)larsalan ^I was going to say this same shit.
617 mid-engine rwd ftw.
Imagine it in a coupe model
RE: center of gravity/mass - raysorenson - 04-10-2013
It's been done in a Fiesta and, of course, Jay Leno's got one.
![[Image: jayleno-video-w.jpg]](http://www.carsguide.com.au/images/uploads/jayleno-video-w.jpg)
RE: center of gravity/mass - bricktron - 11-17-2013
mbz123, afaik, weight transfer in a corner is proportional to the weight of the chassis * height of the center of gravity / track width between the wheels. therefore, a 5% reduction in weight, a 10% reduction in height, and a 1% increase in track width should mean a reduction to about 85% of previous weight transfer in the corners.