Light Club
Text by Kevin Wells
Photo by Arnold Eugenio
Artwork provided by Nissan N.A. Press Archives
Everyone agrees that a lighter car is better in racing situations, but how
much horsepower or torque is weight loss really worth? To illustrate, we
are going to use a stock Nissan 350Z as a base and do linear acceleration
and power-to-weight ratio calculations to prove the theory. If you plan on
doing this to a different car (say, your own personal street and strip warrior)
you can replace the specs with your own and follow the calculations.
Vehicle Specs
HP 300 hp @ 6400rpm
TQ 260 lb/ft @ 4800rpm
Transmission Gear Ratios
01 : 3.794
02 : 2.324
03 : 1.624
04 : 1.271
05 : 1.000
06 : 0.794
Final : 3.538
Driveline efficiency : 80% (approximated)
Rear Tire Dimensions : 245/45-18
Rolling Radius : 1.11 ft.
Linear Acceleration : 0.299g |
If we can drop 100lbs from the net weight of our theoretical 350Z...
1076.54 thrust / 3500 lbs = 0.307g linear acceleration
Reverse the calculations and we find that it would take 272.2 lb/ft to
produce the same effect on our (originally) 3600 lb 350Z. The theoretical
torque gain from a 100 lb car diet would be 6.92 lb/ft in our case. So, if
we do a performance lightening of the virtual 350Z by reducing a target
weight of 200 lbs, we would have a 12.2 lb/ft theoretical gain in torque.
So, theoretically speaking, if you can drop 200 lbs off this car,
the equivalent power gain would be about 17.74hp and 12.2 lb/ft
of torque. While this may not seem like much, the numbers become
more significant when the gross vehicle weight including fuel and
driver starts at a lower number and when the starting horsepower
becomes significantly higher.
In other words, a 200 lb weight loss on a street-strip warrior that
puts down about 450HP and weighs in at under 2800 lbs with the
driver would provide more theoretical gains in power. The additional
upside to this situation is that this "free" power comes in the form of
reduction in overall mass moved so there is no additional strain (in
the form of increased actual power production) on the motor.
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Want to do these calculations for your own vehicle? Here are the formulas that you need.
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Rolling Radius
= tire section height + ½ rim size
ex : 245/45-18 tire on 18in rim
Find sidewall height
245mm x 45% = 110.25mm
Convert mm to inches
110.25mm / 25.4 = 4.34in
Calculate half rim width
18in / 2 = 9in
1/2 rim width + sidewall height = rolling radius
9.0in + 4.34in = 13.34in
Convert inches to feet
13.34in / 12 = 1.11 ft
Acceleration Thrust
ft/lbs x gear ratio x final drive x efficiency
| = |
final drive x efficiency |
| rolling radius |
ex : our virtual 350Z in 3rd gear at 4800rpm
260 lb/ft x 1.624 x 3.538 x 0.8
1.11
260 x 4.596
1.11
1194.96
1.11
= 1076.54
1076.54 lbs of thrust |
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Power to Weight Ratio
Another way to illustrate the benefits of overall vehicle weight reduction
is to calculate the power-to-weight ratio. This simple calculation can be
made by determining the weight of the vehicle (with fuel and driver) and
dividing by the maximum achievable horsepower. Moving and trucking
companies and recycling stations are good places to fi nd public weighing
scales if you don’t have access to a vehicle specific scale (keep in
mind that there is a larger margin of error with these types of scales.)
Of course, maximum horsepower can be measured on a chassis dynamometer.
ex: Our 300 hp, 3600 lb Virtual 350Z
3600 lbs / 300 hp = 12 lbs / hp
3500 lbs / 300 hp = 11.66 lbs / hp
| 1 hp |
x |
3600 lbs |
= 308.75 |
| 11.66 lbs |
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To produce the same power-to-weight ratio of the 100 lb lighter car versus the 3600 lb car, the heavier car would have to make 308.75HP.
100 lbs = 8.75 theoretical HP
Multiplied twice, 200 lbs
= 17.74 theoretical HP. |
