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· Dis Microphone Explodes
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Discussion Starter · #1 ·
Ok, my buddy and I are arguing about this.

My question is this; why is it that, for example, a 450 hp Ferrari, or 500hp Corvette Z06 would win in a very high speed roll on (ex. 160mph-190mph) against a 2005+ R1 when its power to weigh ratio is much less than the bike? Does someone have numbers or an accurate factual explanation regarding the drag coefficient/aerodynamics difference that illustrates why the cars have an advantage up top, regardless of the lesser power to weight ratio?

Thanks!

:thumbup
 

· arrivederci
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yeah. cars are wayyyy slipperier. you need a TON of power to plow through the air with sh*tty dc. bikes are like snowplows for aero.
 

· Humble Pastafarian
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Could not find any drag coefficient numbers for popular sport bikes in a quick search but the Z06 for example is 0.34 (actually higher than a base vette at 0.28 due to increased down force). Bike's drag coefficient will also vary greatly with ride position but like rocalotopus said we run into a brick wall at speed so far as aerodynamics go.
 

· Registered
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Because friction isn't dependendent on surface area, it is dependent on the angle. For example a 2 lb block of wood on a 30 degree incline would slide at the same speed as a 2 lb sheet of the same kind of wood. Even though the sheet has more surface area in contact with friction. You tend to think a bike is more aerodynamic b/c its smaller but that's not necessarily true
 

· Over doing it
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Ok, my buddy and I are arguing about this.

My question is this; why is it that, for example, a 450 hp Ferrari, or 500hp Corvette Z06 would win in a very high speed roll on (ex. 160mph-190mph) against a 2005+ R1 when its power to weigh ratio is much less than the bike? Does someone have numbers or an accurate factual explanation regarding the drag coefficient/aerodynamics difference that illustrates why the cars have an advantage up top, regardless of the lesser power to weight ratio?

Thanks!
Total drag (Dt) and drag coefficient (Cd) are two completely different but related things. Cd is equal to the coefficient of parasitic drag (Cdp) plus the coefficient of induced drag (Cdi). Cdi is minimal on motorcycles though so it can be effectively disregarded.

Total drag (in lbs) can be found by multipyling the Cd, dynamic pressure* (q), and the surface area (S).

* dynamic pressure is made up from the density ratio (local air density accounting for local altitude, temperature, and barometric pressure measured against standard density) multiplied by the air velocity in knots squared. The resultant of that is then divided by 295.

The formula looks like this:

Dt=Cd(q)S

As you can see drag is determined by five factors. Cd, surface area, local pressure, local temperature, and air speed.

As the above poster mentioned, high end sports cars generally have Cd numbers in the mid 0.3s. Motorcycles however have Cd numbers close to 1.0 depending on the model bike, size of rider, and riding position. In looking at the formula you should be able to deduce one major point. Drag is increasing exponentially with speed.

Plugging in some generic numbers show this better.

Variables: Standard atmosphere (sea level, 15 degree C, and a barometer of 29.92), Car at .35 Cd. Bike at 1.0 Cd. Car surface area at 19 sq. ft. Bike surface area at 7 sq. ft. (surface areas and Cds are approximate numbers).

50 kts

Bike Dt = 59.32 lbs

Car Dt = 56.35 lbs

100 kts:

Bike Dt = 237.3 lbs

Car Dt = 225.44 lbs

150 kts:

Bike Dt = 533.90 lbs

Car Dt = 507.20 lbs

Conclusion:

In the case of cars vs. bikes the bikes generally have triple the Cd but 1/3 the surface area so Dt remains about the same regardless of speed. Cars kick the crap out of bikes at high speeds because at high speeds the predominant factor is aerodynamics. As speeds increase the rate of acceleration decreases and the superior HP/weight ratio that gave the bike superior acceleration at low drag speeds cease to be the deciding factor. At high speeds where rates of acceleration are minimal you need horsepower to overcome the drag factors. As we have seen, aerodynamic drag is approximately equal so whoever has the most horsepower wins. :flex:
 

· #%^@$%^ Bike Thieves!
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very nice :)
 

· Dis Microphone Explodes
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Discussion Starter · #8 ·
Total drag (Dt) and drag coefficient (Cd) are two completely different but related things. Cd is equal to the coefficient of parasitic drag (Cdp) plus the coefficient of induced drag (Cdi). Cdi is minimal on motorcycles though so it can be effectively disregarded.

Total drag (in lbs) can be found by multipyling the Cd, dynamic pressure* (q), and the surface area (S).

* dynamic pressure is made up from the density ratio (local air density accounting for local altitude, temperature, and barometric pressure measured against standard density) multiplied by the air velocity in knots squared. The resultant of that is then divided by 295.

The formula looks like this:

Dt=Cd(q)S

As you can see drag is determined by five factors. Cd, surface area, local pressure, local temperature, and air speed.

As the above poster mentioned, high end sports cars generally have Cd numbers in the mid 0.3s. Motorcycles however have Cd numbers close to 1.0 depending on the model bike, size of rider, and riding position. In looking at the formula you should be able to deduce one major point. Drag is increasing exponentially with speed.

Plugging in some generic numbers show this better.

Variables: Standard atmosphere (sea level, 15 degree C, and a barometer of 29.92), Car at .35 Cd. Bike at 1.0 Cd. Car surface area at 19 sq. ft. Bike surface area at 7 sq. ft. (surface areas and Cds are approximate numbers).

50 kts

Bike Dt = 59.32 lbs

Car Dt = 56.35 lbs

100 kts:

Bike Dt = 237.3 lbs

Car Dt = 225.44 lbs

150 kts:

Bike Dt = 533.90 lbs

Car Dt = 507.20 lbs

Conclusion:

In the case of cars vs. bikes the bikes generally have triple the Cd but 1/3 the surface area so Dt remains about the same regardless of speed. Cars kick the crap out of bikes at high speeds because at high speeds the predominant factor is aerodynamics. As speeds increase the rate of acceleration decreases and the superior HP/weight ratio that gave the bike superior acceleration at low drag speeds cease to be the deciding factor. At high speeds where rates of acceleration are minimal you need horsepower to overcome the drag factors. As we have seen, aerodynamic drag is approximately equal so whoever has the most horsepower wins. :flex:
Awesome post man! Thanks.

Even after information like this, my buddy is still arguing for chrissakes. Haha.
 
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