Quote:
Originally Posted by Nathan jones
Drilling brake disks/rotors or lathed for weight, has anybody done it themselves?
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For those interested, we can estimate how much we power we gain from weight reduction.
Let's use my 2009 Toyota Prius as an example. At highway speeds (65mph ~ 29m/s) the car needs around
10kW+ of power to maintain speed. Around
7kW of that is to overcome aero drag (unmodded),
3kW is rolling resistance (stock tires), and + is whatever else we have on. This is at least 250W (just the car circuitry being powered) but can easily be 2kW with AC cranking, radio blaring, headlights on, etc.
So right off the bat, we see that rolling resistance is a major factor in power consumption. Even in a highly optimized car like the Prius, it's a quarter to a third of the total. It's not uncommon in other vehicles for it to be
nearly half of the power loss even at cruising speeds. This is why low rolling resistance tires have such a profound impact on fuel economy.
Returning to the estimation, rolling resistance is mostly linear with weight (barring subtleties like heat and tire chemistry changes with use). The Prius is around 3,000 lbs, which means that for it each pound of weight reduction results in one watt less running power needed. This conversion will be different for each vehicle, but it's going to be around that ballpark. 1 lb = x Watts, where x is small.
So a few pounds here or there isn't significant, but serious changes are. I ripped out around 500 lbs from Champrius and it improved her mpg by almost 3%, as expected. And when I camp I add on +900 lbs of gear and I instead see a -7%+ worsening of FE.
Even if the weight you are trying to reduce is rotating, unsprung, etc. it has a small effect on power consumption. Of course, it has a huge effect on things like acceleration, handling, braking, and other performance aspects. But as far as fuel economy goes, it's still the same order of magnitude.
By the way, I just recently changed my headlight to 15W LEDs from 60W Halogens. Not only are they brighter and safer, that's the equivalent of dropping 90 lbs from the vehicle at nighttime. There's lots of ways to improve fuel economy, and by converting everything into the same scale (say power wattage) we can then rate how effective different approaches will be.
If you are riding the bleeding edge to reduce your brakes by 20 lbs... I'd say there are easier, safer ways to get an extra 20W of power in your vehicle.
