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New 02/06/2005
Last night, Speed had an episode of Sports Car Revolution. One of the
segments dealt with the Mugen Acura that they are modifying. I had not seen
the earlier episodes, but in this one, the installed bigger brakes and an
exhaust system.
They were expecting about 5 whp for the exhaust, and when they ran it, the
car lost 7 whp. They reinstalled the stock exhaust, and then lost 12 whp!
What they determined is that the larger brakes caused the net 7 whp loss, as
these were the only changes. I didn't catch which brakes they installed, but
they had bigger rotors, and although each corner's total brake weight
(unsprung) was reduced by 1.9#s (probably most of it in the caliper weight,
I would guess.....they didn't break out rotor/caliper weight differences),
the bigger diameter rotors added more rotating weight by moving the mass
farther from the center of the hub.
When modifying your car with either bigger brakes or heavier tires, it
"will" have an effect and a heavier wheel/tire comb will have a greater
detrimental effect on whp than larger rotors.
Ron Porter
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Absolutely right. Thus why serious drag racers don't worry
about huge wheel/tire combo's (unless it is lightweight drag
slicks) and they don't need or want big brakes.
On the road course though, big brakes and the stopping power
and ability to take and shed heat is worth more than the
rotating mass they add.
Anyone remember the aluminum disks that were on the Univ. of
Wisconsin's Aluminum Cow? That is the all aluminum Taurus
that had a SHO driveline (was a stick shift) that they made
into a hybrid award winning vehicle.
When I did the article for the SHO Registry way back when, I
saw the lightweight coil-over suspension they did, all the
custom made aluminum suspensions components and most
impressive were the brakes. These hybrid cars must be able
to stop as well as go, and they had competition style
brakes, but they came up with some really exotic disks.
These were a special aluminum alloy. They handed me a spare
disk and it was really light. Wish I had thought to weigh
it, but it was at least 1/2 the weight of a stock Gen III
style rotor. Then they handed me a big screwdriver and
hammer and challenged me to scratch the surface of the disk.
These disks probably cost about $500 or more each, and I
really tried, but could not scratch them. That is some
Aluminum alloy!
They might not take the extreme heat of real long distance
track events, but they were engineered to handle exactly
what they would be subjected to, and they worked. Should
have asked them what pads they were using!
Don Mallinson
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When @ Iowa State on the Formula SAE team, we made a similar call in
opting for aluminum rotors (water-jet cut 6061-T6). Yeah, they melted
a bit, but we had a bunch of extras made and designed the hub assembly
so they were easily changeable. In the short competitions, the melting
wasn't an issue. For the enduro, though, we changed over to steel in
an attempt to avoid a breakdown in the notoriously attritious (is that
a word?) race.
Matt Kennedy
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"What they determined is that the larger brakes caused the net 7 whp loss,
as these were the only changes."
I must be missing something here. The logic is that there's a "force
multiplier" effect with larger diameter rims/rotors/tires: Even if the total
weight of the larger-diameter combo is less that the pieces it replaces, more HP
will be required to rotate the larger-diameter assembly, because the weight is
located farther from the center of rotation.
That certainly holds true for ET, provided we assume the car is accelerating
all the way to the traps. Taken to an extreme it would make the car bog off
the line and when shifting.
But for HP at a constant RPM I can't figure where it would make the least
bit difference, even if you are measuring it at the wheels. Since the engine
isn't accelerating the spinning weight, no HP is being lost. (Although, if
you had rims with "turbine fan" spokes, the parasitic drag would kill ya. But
that's not a weight issue.)
Maybe it's as simple as when they increased the rolling diameter, they took
the engine out of the sweet spot of its power curve by changing the actual,
final drive ratio. It was running flat out but at a different RPM and just
not on the sweet spot anymore.
Cheers
Mark LaBarre
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As I understand it, running on a Dynojet is not a measure of constant rpm,
but rather a change over time.
Granted, if you let the car settle into a constant rpm & speed, things like
heavier wheels/tires/rotors/flywheels/etc won't matter as much, but this is
not a real-world example.
IMHO, this is one of the huge benefits of having chassis dynos, as it really
isn't very relevant as to what the engine puts out at the crank, but what
actually makes it to the tire contact patch.
Ron Porter