Wheels - What's 1 KG Weight Savings Equivalent To?
- Thread starter kenntona
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Re: Wheels - What's 1 KG Weight Savings Equivalent To?
Don't focus too much on rules, bests, formulaic ratios, because it is never so clear cut...it is specific to layout, model, setup. Mass can help or hurt, eg. too much mass in wheel, too much unsprung and rotating weight. Too little mass, too little wheel stiffness. Add mass in trunk, better ride, but more yaw inertia hurts speed of direction change. It's all tradeoff. Only with very specific targets and measurement can a change be determined to be better or not for sure.
ARBs just like suspension links are all a mix of sprung and unsprung depending on the kinematics of the system and at what point in the suspension travel you are measuring at.
In typical sedan type application, ARBs are usually around 20% unsprung and 80% sprung mass, at static ride height. Explanation on earlier thread http://www.bmw-sg.com/forums/suspen...r-anti-roll-bars-vs-stock-anti-roll-bars.html
kenntona;402778 said:
Don't focus too much on rules, bests, formulaic ratios, because it is never so clear cut...it is specific to layout, model, setup. Mass can help or hurt, eg. too much mass in wheel, too much unsprung and rotating weight. Too little mass, too little wheel stiffness. Add mass in trunk, better ride, but more yaw inertia hurts speed of direction change. It's all tradeoff. Only with very specific targets and measurement can a change be determined to be better or not for sure.
kenntona;402789 said:
ARBs just like suspension links are all a mix of sprung and unsprung depending on the kinematics of the system and at what point in the suspension travel you are measuring at.
In typical sedan type application, ARBs are usually around 20% unsprung and 80% sprung mass, at static ride height. Explanation on earlier thread http://www.bmw-sg.com/forums/suspen...r-anti-roll-bars-vs-stock-anti-roll-bars.html
rab
Well-Known Member
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
Was reading June 2009 issue of Excellence magazine (The magazine about Porsche) and found a reference which supprts your 1 to 6 ratio:
There is a feature story on P.49 about the Manthey Motors M600 - "a 997 GT2 taken to the next level". Manthey Motors are referred to as "one of the world's foremost Porsche tuners......a Nurburgring-based race shop".
In the article Manthey discusses the use of special rims, BBS Motorsport MM2, orderd from BBS Motosports to replace the factory wheels of the GT2 in order to reduce unsprung weight by 33 pounds for the 19" rims. He is quoted in the article as follows:
"Removing 33 pounds of unsprung weight at the wheels is equivalent to losing 198 pounds from the body of the car, as a factor of 1:6 has to be applied when the car is moving and that weight becomes mass. In fact, we are conservative with 1:6 ratio, as Porsche considers it to be 1:7."
33 pounds is about 15 kgs and 198 pounds is about 90 kgs. That's about the equivalent of permanently lugging around 4 Economy class suitcases in the boot (if you can fit them in a Bimmer boot).
kenntona;401596 said:
Was reading June 2009 issue of Excellence magazine (The magazine about Porsche) and found a reference which supprts your 1 to 6 ratio:
There is a feature story on P.49 about the Manthey Motors M600 - "a 997 GT2 taken to the next level". Manthey Motors are referred to as "one of the world's foremost Porsche tuners......a Nurburgring-based race shop".
In the article Manthey discusses the use of special rims, BBS Motorsport MM2, orderd from BBS Motosports to replace the factory wheels of the GT2 in order to reduce unsprung weight by 33 pounds for the 19" rims. He is quoted in the article as follows:
"Removing 33 pounds of unsprung weight at the wheels is equivalent to losing 198 pounds from the body of the car, as a factor of 1:6 has to be applied when the car is moving and that weight becomes mass. In fact, we are conservative with 1:6 ratio, as Porsche considers it to be 1:7."
33 pounds is about 15 kgs and 198 pounds is about 90 kgs. That's about the equivalent of permanently lugging around 4 Economy class suitcases in the boot (if you can fit them in a Bimmer boot).
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
Nope. Either something was lost in translation (as it often is), or it's just an empty statement that has never had its logic explained.. maybe because there is none.
" when the car is moving the weight becomes mass" is gibberish.. it's incorrect. When people say things like that it makes you question everything they say, or the translation. Mass at distance from center of rotation results in larger moments of inertia. Mass doesn't convert to mass and vice versa, going from static to dynamic or back... that's basic physics.
Generic benefit is also different from an exact calculation of inertia increase as it can include other factors like effect on ride and grip through mass and mass ratio changes, both sprung and unsprung frequencies and amplitudes. There cannot be a fixed ratio of 1:6 or 1:7 as a broad rule because the broadness will include variables like total vehicle mass, course type, suspension travel, tire type, damping ratios, etc. This list goes on to include thousands of variables.
Because someone says Porsche said something doesn't mean they actually said it. There's potential for misquote or misinterpretation. Big names often print rubbish in catalogs and press releases, but that's not because they don't know their stuff.. they're catering to the squirrels to make the sales. An employee or ex-employee of Porsche saying something doesn't mean that is Porsche's stand either. Until the head engineer or lead suspension engineer at Porsche says something explicitly and plainly that is inline with physics and is largely supported by peers, you should ignore formulaic rules. They almost never work unless the rule is for a very specifc range of conditions.
200 bucks to anyone that can explain logically, with basic calculations why 1:6 or 7 is true, and why 1:2 is inaccurate when in fact there are many situations in which 1:2 and 1:1.X is absolutely true. If 1:1.X - 1:2 are accurate in many cases, how can 1:6 or 7 be any kind of rule?
rab;435826 said:
Nope. Either something was lost in translation (as it often is), or it's just an empty statement that has never had its logic explained.. maybe because there is none.
" when the car is moving the weight becomes mass" is gibberish.. it's incorrect. When people say things like that it makes you question everything they say, or the translation. Mass at distance from center of rotation results in larger moments of inertia. Mass doesn't convert to mass and vice versa, going from static to dynamic or back... that's basic physics.
Generic benefit is also different from an exact calculation of inertia increase as it can include other factors like effect on ride and grip through mass and mass ratio changes, both sprung and unsprung frequencies and amplitudes. There cannot be a fixed ratio of 1:6 or 1:7 as a broad rule because the broadness will include variables like total vehicle mass, course type, suspension travel, tire type, damping ratios, etc. This list goes on to include thousands of variables.
Because someone says Porsche said something doesn't mean they actually said it. There's potential for misquote or misinterpretation. Big names often print rubbish in catalogs and press releases, but that's not because they don't know their stuff.. they're catering to the squirrels to make the sales. An employee or ex-employee of Porsche saying something doesn't mean that is Porsche's stand either. Until the head engineer or lead suspension engineer at Porsche says something explicitly and plainly that is inline with physics and is largely supported by peers, you should ignore formulaic rules. They almost never work unless the rule is for a very specifc range of conditions.
200 bucks to anyone that can explain logically, with basic calculations why 1:6 or 7 is true, and why 1:2 is inaccurate when in fact there are many situations in which 1:2 and 1:1.X is absolutely true. If 1:1.X - 1:2 are accurate in many cases, how can 1:6 or 7 be any kind of rule?
rab
Well-Known Member
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
You should write a "letter to the editor" to the magazine. It would be interesting to see what their response would be. This 1:6 ratio is being bandied around (it seems from a few sources) and could fall into the "urban myth" category.
It would also be interesting to see whether the quote from Manthey Motors is accurate as well as their reference to Porsche and if it is accurate, whether they can elaborate on the numbers from Manthey and Porsche.
They've printed it so they should be prepared to explain.
If you don't have access to the magazine, I can scan the feature article and send it to you so that you can write the letter appropriately.
The magazine is "Excellence - The magazine about Porsche", Issue number 174, June 2009.
Letters to the Editor can be sent to:
[email protected], or
Excellence, Letters to the Editor
P.O. Box 1529
Ross, California 94957
USA
Shaun;435837 said:
You should write a "letter to the editor" to the magazine. It would be interesting to see what their response would be. This 1:6 ratio is being bandied around (it seems from a few sources) and could fall into the "urban myth" category.
It would also be interesting to see whether the quote from Manthey Motors is accurate as well as their reference to Porsche and if it is accurate, whether they can elaborate on the numbers from Manthey and Porsche.
They've printed it so they should be prepared to explain.
If you don't have access to the magazine, I can scan the feature article and send it to you so that you can write the letter appropriately.
The magazine is "Excellence - The magazine about Porsche", Issue number 174, June 2009.
Letters to the Editor can be sent to:
[email protected], or
Excellence, Letters to the Editor
P.O. Box 1529
Ross, California 94957
USA
centurion
Well-Known Member
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
Shaun pls write the letter. I read the same issue of excellence. Was pretty shocked.
Here's the Excellence article:
Excellence Magazine
Anyway, the best test would be .... (because too many variables, need to do things the dumb way)
Do an acceleration test on stock wheels
Do an acceleration test on lightweight wheels
then load the centre of the car with lead weight progressively until acc time for lightweight wheels = acceleration time on stock wheels.
Ratio of lead weight total:difference in weight = the correct number.
For good measure, do a 0-200 km/h acceleration. Or, instead of acceleration test, do laptimes!
Uhhh .... Sport Compact Car did a test like this once (on Civic) and the ratio was 6:1. Should be more than 6:1 because the lead weight actually increase traction on the Civic.
Some good information:
Thread on RX8: Wheel Weight - RX8Club.com
SCC test: SCC Technical Assistance Program
Shaun pls write the letter. I read the same issue of excellence. Was pretty shocked.
Here's the Excellence article:
Excellence Magazine
Anyway, the best test would be .... (because too many variables, need to do things the dumb way)
Do an acceleration test on stock wheels
Do an acceleration test on lightweight wheels
then load the centre of the car with lead weight progressively until acc time for lightweight wheels = acceleration time on stock wheels.
Ratio of lead weight total:difference in weight = the correct number.
For good measure, do a 0-200 km/h acceleration. Or, instead of acceleration test, do laptimes!
Uhhh .... Sport Compact Car did a test like this once (on Civic) and the ratio was 6:1. Should be more than 6:1 because the lead weight actually increase traction on the Civic.
Some good information:
Thread on RX8: Wheel Weight - RX8Club.com
SCC test: SCC Technical Assistance Program
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
Starting from zero has it's own problems especially with big changes in wheel weight. To remove those variables it would have to be a rolling start from a non-traction limited speed.
Do you see any problems with the typical range of wheel weights and the variance in all other factors that stacks up? I see the latter overlapping the effect from wheels by far.
My current understanding has the absolute limit for pure long. accel. (say drag racing or the salt flats) at 1:2 (all wheel mass concentrated at the very circumference which is only theoretically possibly so reality will be quite a way below 2) - so wheel mass is ground speed + rotational surface speed (ground speed * 2) Anything beyond that must be to do with ride and handling and once you get into that, the range of possibilities is enormous and for certain 1:6 is not a rule. Just think of a rally car with a sprung mass of 700kg (very far from 1:2, 30" wheel travel, 150 inches/sec, so wheel mass and its ratio to sprung mass affects much else ). Now think of a smooth highway cruiser 7 series 1600 kg (close to 1:2, purer inline, ~10' travel, typical smooth road car sub 5 inches/second, wheel mass affects little else cept inline accel. )
===
Anyway I will try find time to put together a proper email and submit it.. would love to learn something new if it is real.
This is exactly what I mean. Where are the details of the test. What is the logic laid out that supports the results? Was it at least an A-B-A test?
Starting from zero has it's own problems especially with big changes in wheel weight. To remove those variables it would have to be a rolling start from a non-traction limited speed.
Do you see any problems with the typical range of wheel weights and the variance in all other factors that stacks up? I see the latter overlapping the effect from wheels by far.
My current understanding has the absolute limit for pure long. accel. (say drag racing or the salt flats) at 1:2 (all wheel mass concentrated at the very circumference which is only theoretically possibly so reality will be quite a way below 2) - so wheel mass is ground speed + rotational surface speed (ground speed * 2) Anything beyond that must be to do with ride and handling and once you get into that, the range of possibilities is enormous and for certain 1:6 is not a rule. Just think of a rally car with a sprung mass of 700kg (very far from 1:2, 30" wheel travel, 150 inches/sec, so wheel mass and its ratio to sprung mass affects much else ). Now think of a smooth highway cruiser 7 series 1600 kg (close to 1:2, purer inline, ~10' travel, typical smooth road car sub 5 inches/second, wheel mass affects little else cept inline accel. )
===
Anyway I will try find time to put together a proper email and submit it.. would love to learn something new if it is real.
bmwdorifto
Member
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
it also depends whether the weight saving is on the driving wheel or not. Higher factor for driving wheel.
it also depends whether the weight saving is on the driving wheel or not. Higher factor for driving wheel.
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
If you can explain clearly why you believe this, I will show why it is not true outside the very starting moments of an ET killing launch, and drift driving, neither of which form any considerable portion of drive time (if at all), for enthusiasts, especially locally.
bmwdorifto;438940 said:
If you can explain clearly why you believe this, I will show why it is not true outside the very starting moments of an ET killing launch, and drift driving, neither of which form any considerable portion of drive time (if at all), for enthusiasts, especially locally.
Re: Wheels - What's 1 KG Weight Savings Equivalent To?
A month ago I wrote politely and concisely to Excellence in 2 ways, 1 normal, and 1 through someone who knows senior staff there. 0 reply, not even an acknowledgement like "we've received your query and are investigating. Should have something in perhaps 2 months since we're very busy currently. We'll let you know when we have more information."
I'lll wait another couple months despite the negative signs so far, before writing Excellent off, because there's really only 1 of 3 possible responses:
1) Here is the reason for the 1:6 ratio... [ logical explanation ].
(this would be ideal)
2) We asked our sources but the reply was unsatisfactory.
(means Excellence just parrots lots of what they hear without even roughly understanding it, or quotes weak sources and doesn't verify what it writes, or that their source is not as authoritative/truthful as he appears and he just parrots stuff )
3) We asked our sources but there was no reply.
(means either Excellence and/or their sources are lazy and/or disinterested in finding out truth when they use tech blurbs here and there, but purely for effect, not understanding it, and not to pass true information)
rab;435959 said:
A month ago I wrote politely and concisely to Excellence in 2 ways, 1 normal, and 1 through someone who knows senior staff there. 0 reply, not even an acknowledgement like "we've received your query and are investigating. Should have something in perhaps 2 months since we're very busy currently. We'll let you know when we have more information."
I'lll wait another couple months despite the negative signs so far, before writing Excellent off, because there's really only 1 of 3 possible responses:
1) Here is the reason for the 1:6 ratio... [ logical explanation ].
(this would be ideal)
2) We asked our sources but the reply was unsatisfactory.
(means Excellence just parrots lots of what they hear without even roughly understanding it, or quotes weak sources and doesn't verify what it writes, or that their source is not as authoritative/truthful as he appears and he just parrots stuff )
3) We asked our sources but there was no reply.
(means either Excellence and/or their sources are lazy and/or disinterested in finding out truth when they use tech blurbs here and there, but purely for effect, not understanding it, and not to pass true information)
