Effects of weight reduction?

scrabblegod · Post by **scrabblegod** » Mon Feb 05, 2007 11:16 pm

Ignoring variable such as aero drag and traction, are acceleration times directly related to weight and HP? ie:

Will a 150HP car weighing 1500lbs perform the same as a 300HP car weighing 3000lbs?
I guess this also assumes both are optimally geared for their rpm range.

If not, is there a generic formula to calculate the difference?

Thanks,

Gene

The Dark Side of Will · Mon Feb 05, 2007 11:51 pm

To the first order approximation, yes.

However, even with equivalent power/weight, the 1500# car will be able to come out of the hole faster than the 3000# car.

p8ntman442 · Post by **p8ntman442** » Tue Feb 06, 2007 11:33 am

F=MA so Acceleration = HP/Weight Both cars would acelerate at the same rate. However you have to factor in the force of friction which is the Coeficient of friction between the tires and ground, and multiply that by the normal force which is proportional to the weight. The heavier car would lose.

Pyrthian · Post by **Pyrthian** » Tue Feb 06, 2007 11:48 am

got nothing to back me up, but I have always heard a rough approximation that a car that weighs twice as much, needs 4 times the power to accelerate as quickly.

The Dark Side of Will · Tue Feb 06, 2007 11:48 am

There are a whole lot of small effects to take into consideration.

Rolling resistance is one of them.
Another is that rotating components can only be so light... equivalent flywheel weights would result in a bigger chunk of the small engine's power going to inertial parastic loss.

However, depending on the degree of optimisation, that could also be spun as favorable to the small engine, since the rotating components attached to the large engine will have to be bigger, stronger and heavier.

In any case, a lighter car will always out handle a heavier car at a similar level of development. A 1500# car would RADICALLY out handle a 3000# car.

The Dark Side of Will · Tue Feb 06, 2007 11:54 am

Pyrthian wrote:got nothing to back me up, but I have always heard a rough approximation that a car that weighs twice as much, needs 4 times the power to accelerate as quickly.

Not true.

Pyrthian · Post by **Pyrthian** » Tue Feb 06, 2007 12:11 pm

The Dark Side of Will wrote:
Pyrthian wrote:got nothing to back me up, but I have always heard a rough approximation that a car that weighs twice as much, needs 4 times the power to accelerate as quickly.
Not true.

yes, goto any 1/4 mile calulator that inputs weight & HP
its a straight line relationship - hp vs weight

The Dark Side of Will · Tue Feb 06, 2007 12:41 pm

If it were to need 4x the power at 2x the weight, that's not a linear (straight line) relationship. That's an X squared relationship.

Pyrthian · Post by **Pyrthian** » Tue Feb 06, 2007 1:23 pm

right - meant yes as in agreeing wit ya - I just hear that other statement alot.

The Dark Side of Will · Tue Feb 06, 2007 1:27 pm

ok. There ARE a lot of x^2 dependencies in pure acceleration, but power isn't one of them.

Pyrthian · Post by **Pyrthian** » Tue Feb 06, 2007 1:51 pm

right. if you look at the definition of torque, its directly/linearly linked to weight. and, hp is directly & linearly linked to HP's. thereby, twice the weight needs twice the HP - or - more accuratly - twice the torque. this is also all assuming many other things held equal. traction & power curve being biggies.

but, this is all mathmagic. I would expect if you took a 10 HP go-kart, doubled its weight & put in a 2nd 10hp motor - the lighter one would be faster.

Kohburn · Post by **Kohburn** » Tue Feb 06, 2007 3:19 pm

because traction is not linear - though it may be in a theoretical physics problem like they tend to set up in basic physics in college.

p8ntman442 · Post by **p8ntman442** » Tue Feb 06, 2007 5:03 pm

traction is linear, except when the tire squishes from the weight. Then you get more contact area, and it is no longer linear. Traction is a measurement of the force of friction between the tire and the road, I covered the foce of friction in my previous post.

Just to clarify these rules apply to weight and Wheel HP as variables while everything else remains constant.

The Dark Side of Will · Tue Feb 06, 2007 5:06 pm

Visco-elastic friction is NOT linear.

Coefficient of friction is greater at small normal force than large normal force. A tire can generate greater lateral g's on a light car than a heavy one (and thus more accelerative and braking traction as well).

The lighter car will launch harder for this reason.

scrabblegod · Post by **scrabblegod** » Wed Feb 07, 2007 8:39 pm

Thanks for the replies. I was without access for a day.

Several years ago, I started to build a light weight car. The car is a Locost, and is a replica of the late 50s Lotus 7 series car.

I got into Fieros and the project was pushed to the side.

I am going to get back on it. I was originaly going to use a Thunderbird Turbo Coupe as the donor.

Now that I work in the salvage yard, I have a lot more access to various parts.

I have a 93 Taurus SHO engine put back I am going to use. A rough calculation puts the final weight between 1300-1500 lbs.

The SHO is easily modded to 250HP NA

Add me at 230lbs and the fun should begin.

Gene

whipped · Post by **whipped** » Wed Feb 07, 2007 8:49 pm

scrabblegod wrote: I have a 93 Taurus SHO engine put back I am going to use. A rough calculation puts the final weight between 1300-1500 lbs.

The SHO is easily modded to 250HP NA

V6 or V8?

:la:

scrabblegod · Post by **scrabblegod** » Wed Feb 07, 2007 9:24 pm

whipped wrote: V6 or V8?

:la:

It is the V-6. I know where there is a V-8 that needs a head gasket, but the owner thinks he has the last one in the world.

Gene