DCSportbikes.net  
» Help Support .NET!
DCSportbikes Premier Membership for 25$ per year. Discounts! Click here for full information.

Now available in the .NET Shop:



Get your DCSBN Gear!
» Shoutbox
Sorry, only registered users have the ability to use our real-time shoutbox to chat with other members.

Register now, it's free!
» Online Users: 559
3 members and 556 guests
GRN96WS6, Slider, tonetone
Most users ever online was 4,519, September 2, 2015 at 03:26 AM.
Go Back   DCSportbikes.net > Sportbike Operation > Tips from the Experts

Reply
LinkBack Thread Tools
Rotating Weight vs. Non Rotating
Unread
  (#1)
Agitator
 
birdman's Avatar
 
Posts: 3,769
Join Date: October 23, 2005
Location: Fairfax, VA
Rotating Weight vs. Non Rotating - March 14, 2008, 06:27 AM

So, since this has been brought up, I created a spreadsheet (I can email it to you if you'd like) that figures out the "effective" weight loss associated with removing weight from rotating parts.

Now, the following is for an SV650 with a 45 tooth rear (smaller rear would decrease these numbers for everthing but the wheels and rear rotor), higher gear ratios (e.g. higher revving bikes like 600's would increase the numbers associated with anything but the wheel and rotor parts)

These are the effective weights of a given amount of weight on each part (e.g. if its 1.1, then removing 1lb of weight off that part would have the same effect on acceleration as removing 1.1lbs of non rotating weight)

Wheel 1.41 (assuming the weight comes mainly off the rim)
Rear Rotor 1.1
Front Rotor 1.15
Rear Sprocket 1.06
Front Sprocket 1.07
Clutch Basket 1.91 - 1.11 (depending on gear, 1.91 in 1st, 1.11 in 6th)
Clutch Plates 2.55 - 1.18 (depending on gear, 2.55 in 1st, 1.18 in 6th)
Engine 4.98 - 1.48 (depending on gear, 4.98 in 1st, 1.48 in 6th)

Note
These are based on estimated diameters and shape of parts, but should be close enough to make estimates as to where your $$ could be spent.

So, consider how much a lighter part costs, how much weight can you remove, and what effect that has.

Bear in mind, making the bike easier to lean is dependent on how much the parts weighed in the first place and many other factors (I'm working on those calcs)

So, lets say those fancy BST wheels drop 5lbs off each wheel and cost $3000 bucks...thats like taking 14lbs off the bike, or $200 a pound.

OR an aluminum clutch basket, aluminum clutch hub and lightweight flywheel ($1000 for parts for a duc, I don't have the equivalent for an SV) which removes ~4lbs off the clutch and 2lbs off the flywheel is like removing 7lbs off the bike in 6th, or 18lbs in 1st! (so about $142/lb in 6th or $55/lb in 1st

Lighten the crank (-2lbs) for $300 = 3-10lbs of effective weight loss

You can see where if you are building up a high performance bike, pay attention to the high speed rotating parts--thats where all the savings is!


Marc
CCS Amateur #851
2001 Ducati 850RS
2006 SV650 Racebike
2010 CBR1000RR

...and because sometimes you want to go fast, but need 4 wheels:
2009 Nissan GT-R
2003 Nissan 350Z (for sale, $11k OBO)
  Send a message via AIM to Send a message via AIM to birdman  
Reply With Quote
Unread
  (#2)
opossum puncher
 
zx6rfool's Avatar
 
Posts: 7,826
Join Date: June 9, 2003
Location: Winchester
March 14, 2008, 06:52 AM

Wouldnt the weight removed from the rims ratio be dependant on where the weight came from, ie hub vs spoke vs outer rim?


-
Attack Life! It's going to kill you anyway.

08 Z1000

Go Hokies!
  Send a message via AIM to Send a message via AIM to zx6rfool  
Reply With Quote
Unread
  (#3)
Que se jodan!!
 
Whisper's Avatar
 
Posts: 2,983
Join Date: May 17, 2006
Location: Land of the Bureaucrats
March 14, 2008, 06:56 AM

talk!!

When are we going to see the thread about power:weight ratio gain when a fat rider loses weight?????


Whisper
"Speak quietly and carry a big stick."
2014 BMW S1000RR
2006 GSXR 1000 - Street (Sold)
2005 GSXR 1000 - Track (Sold - I hate you, Paul)
2010 YZ250F -Motocross in Mexico (I still hate Yamahas)

Men are qualified for civil liberties in exact proportion to their disposition to put moral chains upon their appetites: in proportion as their love of justice is above their rapacity...Society cannot exist, unless a controlling power upon will and appetite be placed somewhere; and the less of it there is within, the more there must be without. It is ordained in the eternal constitution of things, that men of intemperate minds cannot be free. Their passions forge their fetters. ~Edmund Burke, Political Theorist and Philosopher


Some of us are lions, but even lions can be crushed under the elephant of bad management.
  Send a message via AIM to Send a message via AIM to Whisper  
Reply With Quote
Unread
  (#4)
B
It goes to 11.
 
B's Avatar
 
Posts: 16,806
Join Date: November 8, 2004
Location: Moet Chandon on a Schlitz budget
March 14, 2008, 07:56 AM

Well spoken Whisper... I will nearly guarantee that jogging your fat ass off will be the cheapest loss of weight (rotating or non-rotating) per dollar, availible.

So unless you're in perfect shape, or lazy, do a 2 miler three times a week before you start tearing into your engine


SV650s for SALE!!!
- 2007 SV650 Racebike-Superbike KWS/Thermosman suspension/Swenz bodywork/All GSXR Parts
- 2009 SV650 Streetbike Race blue with white stripe/No wrecks/fully faired with M4 full system

Shoot me a PM or talk to Nate (Nudist) if you're interested in purchase.
  Send a message via AIM to Send a message via AIM to B  
Reply With Quote
Unread
  (#5)
Agitator
 
birdman's Avatar
 
Posts: 3,769
Join Date: October 23, 2005
Location: Fairfax, VA
March 14, 2008, 12:19 PM

Quote:
Originally Posted by zx6rfool
Wouldnt the weight removed from the rims ratio be dependant on where the weight came from, ie hub vs spoke vs outer rim?
of course it matterrs a lot where the weight is removed from, in the above example I stated that the wheel weight was removed from the rim. The reason for this example is the ratio for hub weight is 1..the radius is so small the rotational part doesn't matter that much. As for the spokes, they are a very small fraction of the overall weight usually I'd doesn't matter much, but if they were a source of weight loss the ratio would be between the rim weight ratio and 1...so split the difference and that would be the answer


Marc
CCS Amateur #851
2001 Ducati 850RS
2006 SV650 Racebike
2010 CBR1000RR

...and because sometimes you want to go fast, but need 4 wheels:
2009 Nissan GT-R
2003 Nissan 350Z (for sale, $11k OBO)
  Send a message via AIM to Send a message via AIM to birdman  
Reply With Quote
Unread
  (#6)
GP Champ
 
shiphteey's Avatar
 
Posts: 3,259
Join Date: August 1, 2006
March 15, 2008, 06:48 PM

So, since this has been brought up, I created a spreadsheet (I can email it to you if you'd like) that figures out the "effective" weight loss associated with removing weight from rotating parts.

----I would be interested in that actually. I'll pm you my email addy in a bit.

Now, the following is for an SV650 with a 45 tooth rear (smaller rear would decrease these numbers for everthing but the wheels and rear rotor), higher gear ratios (e.g. higher revving bikes like 600's would increase the numbers associated with anything but the wheel and rotor parts)

----What is the significance in how high or low revving the motor is? Changing your gearing via sprockets is independent of internal gear ratios, let alone motor characteristics of higher revving bikes like 600s as you mentioned.

These are the effective weights of a given amount of weight on each part (e.g. if its 1.1, then removing 1lb of weight off that part would have the same effect on acceleration as removing 1.1lbs of non rotating weight)

Wheel 1.41 (assuming the weight comes mainly off the rim)
Rear Rotor 1.1
Front Rotor 1.15
Rear Sprocket 1.06
Front Sprocket 1.07
Clutch Basket 1.91 - 1.11 (depending on gear, 1.91 in 1st, 1.11 in 6th)
Clutch Plates 2.55 - 1.18 (depending on gear, 2.55 in 1st, 1.18 in 6th)
Engine 4.98 - 1.48 (depending on gear, 4.98 in 1st, 1.48 in 6th)

----Of course the changing of sprockets alone bring up frictional issues with the chain (smaller sprockets causing more binding up of the chain while simultaneously giving lesser rotating mass)...decisions decisions...

Note
These are based on estimated diameters and shape of parts, but should be close enough to make estimates as to where your $$ could be spent.

So, consider how much a lighter part costs, how much weight can you remove, and what effect that has.

Bear in mind, making the bike easier to lean is dependent on how much the parts weighed in the first place and many other factors (I'm working on those calcs)

So, lets say those fancy BST wheels drop 5lbs off each wheel and cost $3000 bucks...thats like taking 14lbs off the bike, or $200 a pound.

----Whereas on a motorcycle with significantly heavier wheels like the ZX12, I saw static weight savings of about 14 pounds with BST Carbon wheels over stock. The lighter motorcycles, and the newer motorcycles have come a LONG way! My Dymag Magnesium wheels for a ZX7R weighed about the same as the new generation of Yamaha and Suzuki wheels. Impressive to say the least although WHERE their weight is located is just as if not more important as you have alluded to earlier.

OR an aluminum clutch basket, aluminum clutch hub and lightweight flywheel ($1000 for parts for a duc, I don't have the equivalent for an SV) which removes ~4lbs off the clutch and 2lbs off the flywheel is like removing 7lbs off the bike in 6th, or 18lbs in 1st! (so about $142/lb in 6th or $55/lb in 1st

Lighten the crank (-2lbs) for $300 = 3-10lbs of effective weight loss

You can see where if you are building up a high performance bike, pay attention to the high speed rotating parts--thats where all the savings is!

----Birdman, something you didn't mention but is VERY important in my opinion to the discussion at hand is the thing that has the MOST effect outside of the motor in terms of rotating mass..given the testing I have done along with others in the game are tires. The right/lighter tire choice has even more impact, pound for pound, than wheel weight due to its placement (gyro effect) as well as its overall diameter. Changing the diameter in the rear as many racers already know changes its gearing, much like changing teeth on the rear sprocket. Factor in different compounds, different growth rates (a tire at 100 mph doesn't necessarily have the same diameter as it does at 200 mph) as well as DRASTICALLY different diameters among many of the manufacturers (I have measured a worn down the center 180/55 rate DOT tire that was noticably taller than another comparable tire from another manufacturer...."same size" and all that was brand spanking new).

Another thing I noticed about your numbers is that some of them seem a bit low. Some numbers I have seen have claimed the ratio closer to 4-5 times the effect depending again on here the weight was. In the real world while doing testing, for estimating purposes, if we go with the same ratio of wheel weight differences you mentioned earlier, that would put the Carbon wheels I have on at a relative static mass difference as about 20 pounds according to your figure, or about 3 gallons worth of gasoline. We have seen the wheels produce signicantly more "jump" when doing back to back testing swapping riders and doing roll-ons than the difference 20 static pounds make. We have even had a few guys buy brand new bearings (non ceramics) in stock and carbon wheels with back to back testing, even swapping wheels to isolate motor differences, rider differences, etc and in terms of ET (acceleration) and there was more than just the 20 pound variance your estimate would imply. The problem I feel your estimate introduces is that it doesn't take into effect start and stop speeds. The faster you go, the LESS the mass, whether rotating or static, matter...and give way to HP and eventually Aero. Now true, SV650s usually don't flirt with speeds significantly higher than 140 mph (relax SV die-hards, I know some of you go over 140 actual), which is when aeros really start coming in to play for all practical purposes of this discussion. FWIW though your wheel estimate DOES seem to corolate with MPH trap speeds seen on top end runs by a few of us. And in roll-ons, once speeds were underway and went well over the triple digit barrier the gains realized were less and less. That VARIANCE in acceleration is part of the bone I have to pick with your "effective weight" figures. Again, you did mention this was done for the SV, so the window may be smaller, but I figured it was noteworthy and relevant to the discussion. I'm glad you brought up the theoretical figures, but I think it is important to remember real world is what really matters. This is why even though I'm down on power given my safe tune and essentially stock motor I have put many turbo bikes in their place. My bike is only 30 lbs heavier than a gixxer 1000 with a pipe (a full 100 lbs lighter than a stock 12 with a full tank), and thats with a bottle. Factor in the lighter wheels and my jockey-ass and it all starts making sense really.

Once this post dies in a few days/weeks we should jump to the topic of friction! Fun stuff!

A.


Gemini Motorcycles (Sterling, VA)
www.GeminiMotorcycles.com

ZX14: 222 mph
ZX12: 229.6 mph
ZX10: 202.8 mph

ZX9: 177.6 mph
ZX6R: 180.8 mph
EX650: 130.7 mph

EX300: 114.7 mph
EX250: 122.4 mph
  Send a message via AIM to  
Reply With Quote
Unread
  (#7)
GP Racer
 
Musikkant's Avatar
 
Posts: 1,261
Join Date: March 31, 2006
Location: Few miles south of Tysons Corner
March 15, 2008, 08:10 PM

Quote:
Originally Posted by gixrB
Well spoken Whisper... I will nearly guarantee that jogging your fat ass off will be the cheapest loss of weight (rotating or non-rotating) per dollar, availible.

So unless you're in perfect shape, or lazy, do a 2 miler three times a week before you start tearing into your engine
You just had to go and offend my fat azz, didn't you, Ben!!


Celebrate freedom, honesty and excellence!
  Send a message via AIM to  
Reply With Quote
Unread
  (#8)
Agitator
 
birdman's Avatar
 
Posts: 3,769
Join Date: October 23, 2005
Location: Fairfax, VA
March 15, 2008, 09:16 PM

Ali brings up some really good points...I'll address and give more info when I can sit down at my laptop. So, more later


Marc
CCS Amateur #851
2001 Ducati 850RS
2006 SV650 Racebike
2010 CBR1000RR

...and because sometimes you want to go fast, but need 4 wheels:
2009 Nissan GT-R
2003 Nissan 350Z (for sale, $11k OBO)
  Send a message via AIM to Send a message via AIM to birdman  
Reply With Quote
Unread
  (#9)
GP Racer
 
Musikkant's Avatar
 
Posts: 1,261
Join Date: March 31, 2006
Location: Few miles south of Tysons Corner
March 15, 2008, 11:40 PM

This is very informative. Keep it coming!


Celebrate freedom, honesty and excellence!
  Send a message via AIM to  
Reply With Quote
Unread
  (#10)
Meh
 
Posts: 7,983
Join Date: October 1, 2002
March 16, 2008, 11:47 AM

The chain binding issue associated with smaller countershaft sprockets is a ofter overblown and altogether non-issue.

I am subscribing for some more information on weight though.
  Send a message via AIM to  
Reply With Quote
Unread
  (#11)
Agitator
 
birdman's Avatar
 
Posts: 3,769
Join Date: October 23, 2005
Location: Fairfax, VA
March 18, 2008, 07:25 AM

Quote:
Originally Posted by shiphteey
----What is the significance in how high or low revving the motor is? Changing your gearing via sprockets is independent of internal gear ratios, let alone motor characteristics of higher revving bikes like 600s as you mentioned.

----Of course the changing of sprockets alone bring up frictional issues with the chain (smaller sprockets causing more binding up of the chain while simultaneously giving lesser rotating mass)...decisions decisions...

----Whereas on a motorcycle with significantly heavier wheels like the ZX12, I saw static weight savings of about 14 pounds with BST Carbon wheels over stock. The lighter motorcycles, and the newer motorcycles have come a LONG way! My Dymag Magnesium wheels for a ZX7R weighed about the same as the new generation of Yamaha and Suzuki wheels. Impressive to say the least although WHERE their weight is located is just as if not more important as you have alluded to earlier.

----Birdman, something you didn't mention but is VERY important in my opinion to the discussion at hand is the thing that has the MOST effect outside of the motor in terms of rotating mass..given the testing I have done along with others in the game are tires. The right/lighter tire choice has even more impact, pound for pound, than wheel weight due to its placement (gyro effect) as well as its overall diameter. Changing the diameter in the rear as many racers already know changes its gearing, much like changing teeth on the rear sprocket. Factor in different compounds, different growth rates (a tire at 100 mph doesn't necessarily have the same diameter as it does at 200 mph) as well as DRASTICALLY different diameters among many of the manufacturers (I have measured a worn down the center 180/55 rate DOT tire that was noticably taller than another comparable tire from another manufacturer...."same size" and all that was brand spanking new).

Another thing I noticed about your numbers is that some of them seem a bit low. Some numbers I have seen have claimed the ratio closer to 4-5 times the effect depending again on here the weight was. In the real world while doing testing, for estimating purposes, if we go with the same ratio of wheel weight differences you mentioned earlier, that would put the Carbon wheels I have on at a relative static mass difference as about 20 pounds according to your figure, or about 3 gallons worth of gasoline. We have seen the wheels produce signicantly more "jump" when doing back to back testing swapping riders and doing roll-ons than the difference 20 static pounds make. We have even had a few guys buy brand new bearings (non ceramics) in stock and carbon wheels with back to back testing, even swapping wheels to isolate motor differences, rider differences, etc and in terms of ET (acceleration) and there was more than just the 20 pound variance your estimate would imply. The problem I feel your estimate introduces is that it doesn't take into effect start and stop speeds. The faster you go, the LESS the mass, whether rotating or static, matter...and give way to HP and eventually Aero. Now true, SV650s usually don't flirt with speeds significantly higher than 140 mph (relax SV die-hards, I know some of you go over 140 actual), which is when aeros really start coming in to play for all practical purposes of this discussion. FWIW though your wheel estimate DOES seem to corolate with MPH trap speeds seen on top end runs by a few of us. And in roll-ons, once speeds were underway and went well over the triple digit barrier the gains realized were less and less. That VARIANCE in acceleration is part of the bone I have to pick with your "effective weight" figures. Again, you did mention this was done for the SV, so the window may be smaller, but I figured it was noteworthy and relevant to the discussion. I'm glad you brought up the theoretical figures, but I think it is important to remember real world is what really matters. This is why even though I'm down on power given my safe tune and essentially stock motor I have put many turbo bikes in their place. My bike is only 30 lbs heavier than a gixxer 1000 with a pipe (a full 100 lbs lighter than a stock 12 with a full tank), and thats with a bottle. Factor in the lighter wheels and my jockey-ass and it all starts making sense really.

Once this post dies in a few days/weeks we should jump to the topic of friction! Fun stuff!

A.
Okay, I've done some confirmatory calculations using the spreadsheet.

One common thought is that for wheels, the factor is ~2. This would be true if the wheel was the diameter of the tire, however, since the wheel is not, and the rotating part is calculated with the square of the radius, the factor is reduced, leading to the 1.4 number. This correlates well with your tire statement. For a tire, assuming the mass were distributed evenly with respect to radius the factor is 1.72. In the real world, there is more tire surface at higher radius BUT a significant amount of the weight is the beads, so a value somewhere between 1.72 and 2 would be reasonable for a tire.

YES! there are massive differences in tire static weight--especially considering the ply material used (e.g. kevlar ply < steel ply) and the tread depth used (thin Q tires vs. street or rain tires for example)--the only way to compare on this is to actually weight the tires, anyone want to weigh a corresponding set of same sized tires?

Correct, above a certain speed aero's dominate, but that isn't the point of this post. Weight (well, more accurately inertia) and its linear vs. rotating component will always matter. What the aero effect does is reduce the "available" power able to be applied to increasing the kinetic energy of the bike.

For example, say the aerodynamically limited top speed of a bike is 170mph and it has say 140hp at the rear wheel. In that case, it takes 140hp to simply move the air out of the way at that speed meaning the available hp for acceleration is zero. Now, at half that speed (85mph) the drag power is less (drag power goes with the cube of the speed, because drag force is proportional to the square of the speed and power is proportional to force times the speed it is applied) or about 18hp, meaning the bike has 122hp available for acceleration.

So one can easily see that minor reductions in drag dominate the acceleration at drag dominated speeds--a 5% reduction in drag coefficient for the bike above increases the available hp @ 85mph by <1hp, but increases the available @ 170mph by 7hp. Now, decreasing the inertia by 5% at 85mph would increase the acceleration by roughly 5% but wouldn't increase the acceleration AT ALL at 170 because all of the available engine power is going to simply pushing the air out of the way of the bike.

Say you have two bikes with equal aero's, one with 200hp at the wheel and weighing 600lbs with rider, and one with 160hp at the wheel weighing only 400lbs. Now lets say the 160hp bike has an aero limited top speed of 180mph. The 200hp bike (assuming each is geared correctly to attain its top speed at the peak hp of the motor) will have an aero limited top speed of 194mph.

Now, lets say they both do a roll-on at 100mph. Both bikes will be expending ~27hp just to push the air out of the way at that speed (same aeros). The 160hp bike will have 133hp available, while the 200hp bike will have 173hp available (or about 30% more), however assuming they are the same model bike and are geared so that at 100mph they are both at peak power (or at the same relative power in their power-band), the inertia of the lighter bike is ~33% less, so even though its down on power, it will accelerate harder (~3% which is rather noticable) because the inertia is dominant. At lower roll-on speeds, the ligher bike will have even more of a jump, however, as the drag rises, the effective horsepower difference becomes more and more apparent since the acceleration per unit available power matters less than the available power ratio between the bikes.

Gearing

I made a generalization about 600's saying the have higher gear ratios. This is true, but not inherent to the motor size. Since the 600's have a higher rpm at which peak power occurs than larger motor'ed bikes, the overall gear ratio (motor to wheel) is higher to make this power available at lower speeds (the reason why current stock liter bikes go ~100mph in 1st, while 600's are much lower). Since the overall gear ratio is higher, and the inertia factor is proportional to the square of the gear ratio (e.g. 1 + a*x^2 where a is the inertia factor at wheel rotational speed and x is the ratio of component rotational speed compared to the road) mass reduction on internal engine components on a higher overall ratio is more apparent.


Marc
CCS Amateur #851
2001 Ducati 850RS
2006 SV650 Racebike
2010 CBR1000RR

...and because sometimes you want to go fast, but need 4 wheels:
2009 Nissan GT-R
2003 Nissan 350Z (for sale, $11k OBO)
  Send a message via AIM to Send a message via AIM to birdman  
Reply With Quote
Unread
  (#12)
GP Champ
 
shiphteey's Avatar
 
Posts: 3,259
Join Date: August 1, 2006
March 19, 2008, 07:45 PM

Okay, I've done some confirmatory calculations using the spreadsheet.

One common thought is that for wheels, the factor is ~2.

---Again, this factor seems artificially low but all I can report from is real world roll-ons. Seems to be even greater but I think rather than arguing over marginal figures, the common point is that rotating mass is more significant than static mass...and its signficance is largely dependent on WHERE in the rotation the mass lies.

This would be true if the wheel was the diameter of the tire, however, since the wheel is not, and the rotating part is calculated with the square of the radius, the factor is reduced, leading to the 1.4 number. This correlates well with your tire statement. For a tire, assuming the mass were distributed evenly with respect to radius the factor is 1.72. In the real world, there is more tire surface at higher radius BUT a significant amount of the weight is the beads, so a value somewhere between 1.72 and 2 would be reasonable for a tire.

YES! there are massive differences in tire static weight--especially considering the ply material used (e.g. kevlar ply < steel ply) and the tread depth used (thin Q tires vs. street or rain tires for example)--the only way to compare on this is to actually weight the tires, anyone want to weigh a corresponding set of same sized tires?

----LOL....I do this all the time...Rick & Dodge131 get sick of it actually! For example, Pirellis routinely weigh in less than there Dunlop counterparts, but Rick is always there to point out that they simply don't last from a roadracing perspective and don't feel as solid/stable as the Dunlops. Regardless, an off the wall comparision of a 140/90/16 inch Bridgestone tire for a cruiser weighed in at almost 17 lbs, a Pirelli 180/55 weighed in 5 lbs less, the Dunlop wasn't too far off either. Shinko 180/55 DOT legal drag tires are a good 2-3 lbs HEAVIER...but their increased contact patch means those with lots of HP or a stretched arm will put up a slightly quicker time at the strip but their MPH will be DOWN compared to the Pirelli or Dunlop. In my world though, it isn't just about weight, but height of the tire and whether or not it will spin in 2nd gear on spray...but I digress.

Correct, above a certain speed aero's dominate, but that isn't the point of this post.

---Granted.

Weight (well, more accurately inertia) and its linear vs. rotating component will always matter. What the aero effect does is reduce the "available" power able to be applied to increasing the kinetic energy of the bike.

For example, say the aerodynamically limited top speed of a bike is 170mph and it has say 140hp at the rear wheel. In that case, it takes 140hp to simply move the air out of the way at that speed meaning the available hp for acceleration is zero. Now, at half that speed (85mph) the drag power is less (drag power goes with the cube of the speed, because drag force is proportional to the square of the speed and power is proportional to force times the speed it is applied) or about 18hp, meaning the bike has 122hp available for acceleration.

----Sounds about right considering CBR1000RRs top out about 170 and put out about that RWHP. 18hp and 85 mph...not too far off the Ninja 250's 28 RHWP claim and 100ish mph flat out claim.

So one can easily see that minor reductions in drag dominate the acceleration at drag dominated speeds--a 5% reduction in drag coefficient for the bike above increases the available hp @ 85mph by <1hp, but increases the available @ 170mph by 7hp. Now, decreasing the inertia by 5% at 85mph would increase the acceleration by roughly 5% but wouldn't increase the acceleration AT ALL at 170 because all of the available engine power is going to simply pushing the air out of the way of the bike.

Say you have two bikes with equal aero's, one with 200hp at the wheel and weighing 600lbs with rider, and one with 160hp at the wheel weighing only 400lbs. Now lets say the 160hp bike has an aero limited top speed of 180mph. The 200hp bike (assuming each is geared correctly to attain its top speed at the peak hp of the motor) will have an aero limited top speed of 194mph.

Now, lets say they both do a roll-on at 100mph. Both bikes will be expending ~27hp just to push the air out of the way at that speed (same aeros). The 160hp bike will have 133hp available, while the 200hp bike will have 173hp available (or about 30% more), however assuming they are the same model bike and are geared so that at 100mph they are both at peak power (or at the same relative power in their power-band), the inertia of the lighter bike is ~33% less, so even though its down on power, it will accelerate harder (~3% which is rather noticable) because the inertia is dominant. At lower roll-on speeds, the ligher bike will have even more of a jump, however, as the drag rises, the effective horsepower difference becomes more and more apparent since the acceleration per unit available power matters less than the available power ratio between the bikes.

----Exactly. We see this all the time at the 1/4 mile vs 1 mile runs. The lightweight mods I do tend to make me jump out in roll-ons and at the strip against comparably equipped bikes...but in the Mile, where MPH (SPEED) is the key, not QUICKNESS/Acceeration/ET/Time at the lights at the drag strip. Once again though, in the real world, the guy that weighs 600 lbs with 40 more HP still usually ends up going slower at the Mile too....because its a LIMITED distance....unlike Bonneville it isn't a truely FLAT OUT run. Although MPH rules, acceleration in that one mile is extremely important. In an off the wall comparision....if we took a jockey on a pipe/pc3/filtered Busa or 12 and ran them against the "198 mph" Z06 Corvette....because the Corvette runs 2.5 seconds SLOWER in the 1/4 mile...it needs MILES and MILES and MILES and MILES to truely LOCK UP speeds with the 196 mph busa/12....up until it locks up MPH at 196 the bike is walking away. Miles...minutes later it finally reaches 196 (in theory at least). And to catch the bike it will only be traveling at MOST 2 mph faster....which means if the bike is 2 miles up it will take the slow lumbering cage 1 HOUR to catch up (assuming clear roads, enough fuel, etc). THIS is the reason why cars get raped in the real world, but once again, I digress. Point being while flat out MPH is important, so is acceleration through the gears and being able to put the power down.

Gearing

I made a generalization about 600's saying the have higher gear ratios. This is true, but not inherent to the motor size. Since the 600's have a higher rpm at which peak power occurs than larger motor'ed bikes, the overall gear ratio (motor to wheel) is higher to make this power available at lower speeds (the reason why current stock liter bikes go ~100mph in 1st, while 600's are much lower). Since the overall gear ratio is higher, and the inertia factor is proportional to the square of the gear ratio (e.g. 1 + a*x^2 where a is the inertia factor at wheel rotational speed and x is the ratio of component rotational speed compared to the road) mass reduction on internal engine components on a higher overall ratio is more apparent.

----Having ridden a lightened ZX12 motor (1287 with the WORKS done) that felt like a 2 stroke, I couldn't agree more. All my lightweight mods would take a back seat to his slighly larger bore and also potentially EQUALLY as important, his lightweight crank, knife rods....etc....it all adds up!


Gemini Motorcycles (Sterling, VA)
www.GeminiMotorcycles.com

ZX14: 222 mph
ZX12: 229.6 mph
ZX10: 202.8 mph

ZX9: 177.6 mph
ZX6R: 180.8 mph
EX650: 130.7 mph

EX300: 114.7 mph
EX250: 122.4 mph
  Send a message via AIM to  
Reply With Quote
Unread
  (#13)
Agitator
 
birdman's Avatar
 
Posts: 3,769
Join Date: October 23, 2005
Location: Fairfax, VA
March 20, 2008, 07:21 AM

A quick reason why the maximum for a wheel/tire would be 2.

Think about it this way, a point on the rim of the tire is moving at the same speed the tire is moving forward.

That way, the speed of that point on the rim of the tire is equal to the speed of the road (also equal to the speed at which the center of the wheel is moving forward), so the tire and the road aren't moving relative to one another (unless you have wheel-spin).

So, that point on the tire is moving backwards on the bottom and forwards on the top, so think of it as two parts of inertia--the first part is the whole wheel moving forward, the second is the individual point on the tire moving around in a circle. Since the two speeds are the same, you have at most, 2x the inertia.

From an energy perspective:
Linear kinetic energy = 1/2 * mass * velocity^2
Rotational kinetic energy = 1/2 * rotational inertia * (rim velocity / rim radius)^2

since the rotating inertia of a single point around an axis (or a very thin ring) = mass * radius^2, the rotational kinetic energy of a thin ring where the rim velocity is equal to the linear velocity of the whole ring relative to a fixed point = 1/2 * mass * rim velocity^2

since the two velocities are equal, the energy at at given velocity is twice the linear amount--hence the factor of two.


Ali, next week I'm gonna post a big thing about friction and we can discuss then! maybe an aero one after that? I'd like to discuss how some theories I have jive with your real world experience

t


Marc
CCS Amateur #851
2001 Ducati 850RS
2006 SV650 Racebike
2010 CBR1000RR

...and because sometimes you want to go fast, but need 4 wheels:
2009 Nissan GT-R
2003 Nissan 350Z (for sale, $11k OBO)
  Send a message via AIM to Send a message via AIM to birdman  
Reply With Quote
Reply


Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Powered by vBadvanced CMPS v3.2.3


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Friendly URLs by vBSEO 3.6.0
vBulletin Skin developed by: vBStyles.com
All logos and trademarks in this site are property of their respective owner. The comments are property of their posters, all the rest 2002-2010 by DCSportbikes.net. DCSportbikes.net is owned by End of Time Studios, LLC.