Tony Posted October 24, 2006 Author Report Share Posted October 24, 2006 It's very subjective, the lower the Dynamic Index the more agile and lively the chassis tends to feel. It's a very subjective thing but it's powerful, history suggests that drivers emotionally identify cars with DI's between 0.90 and 1.10 as cars that handle well. This is because the car with a DI of 1.00 tends to handle in a linear and predictable fashion, so when at the grip limit tend to behave in a way that’s intuitive to the average driver therefore the required steering corrections or throttle/brake responses seem to come "naturally". The best technical summary I can give is to say that with the chassis at the grip limit, if you were to experience a variance in yaw demand from the front wheels (perhaps encountering a road feature that increases vertical load on the tire) this would "increases rear slip angle" if the DI is lower than 1.00 but the same action "reduces rear slip angle" if the DI is greater than 1.00.... I hope that makes sense Regarding family cars, most tend to be a little higher than 1.00 and tend to get a lot more so as they are loaded with passengers or luggage. You would be amazed how massively far out some cars are. For instance the classic VW beetle which believe it are not has a weight distribution not too far away from 50/50 actually has a DI of 0.48 and I think it's a well know fact that an un-modified one is just straight up dangerous when approaching the grip limit This is usually as a result of a project going to the design house before it goes to the dynamics engineers. I am grasping the formula but for me something is missing... A DI of 0.48 or 1.10 how is this grade manufactured... there is no maths? what decides the DI Quote Link to comment Share on other sites More sharing options...
Roger440 Posted October 24, 2006 Report Share Posted October 24, 2006 Good question! Quote Link to comment Share on other sites More sharing options...
jon Posted October 24, 2006 Report Share Posted October 24, 2006 I'm guessing i'm not the only one that's not come across this dynamic indexing in my travels then?! Quote Link to comment Share on other sites More sharing options...
scorps Posted October 24, 2006 Report Share Posted October 24, 2006 When was the D.I introduced to chassis design. Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted October 25, 2006 Report Share Posted October 25, 2006 Dynamic indexing or Inertia matching as it's sometimes known has been around almost as long as chassis have but it's value has only really ever been appreciated by those we tend to think of as the better chassis designers in the world, for instance both the E36 and E46 Bmw 3 series both have a DI of 1.00 (or as near as damn it) when you add an average weight driver. DI is calculated knowing the overall mass, the distance between the front axle and the centre of gravity and the distance from the rear axle to the centre of gravity. By knowing these lengths you can calculate the chassis "instant centre" and DI is an expression of the length between "the length between the rear axle and the centre of gravity" and "length between the instant centre and the centre of gravity" Please do bare in mind though that whilst having a good DI is really good news for a chassis it's not the be all and end all, you CAN fix a bad DI with enough chassis set-up Quote Link to comment Share on other sites More sharing options...
Tony Posted October 28, 2006 Author Report Share Posted October 28, 2006 Dynamic indexing or Inertia matching as it's sometimes known has been around almost as long as chassis have but it's value has only really ever been appreciated by those we tend to think of as the better chassis designers in the world, for instance both the E36 and E46 Bmw 3 series both have a DI of 1.00 (or as near as damn it) when you add an average weight driver. DI is calculated knowing the overall mass, the distance between the front axle and the centre of gravity and the distance from the rear axle to the centre of gravity. By knowing these lengths you can calculate the chassis "instant centre" and DI is an expression of the length between "the length between the rear axle and the centre of gravity" and "length between the instant centre and the centre of gravity" Please do bare in mind though that whilst having a good DI is really good news for a chassis it's not the be all and end all, you CAN fix a bad DI with enough chassis set-up Seemingly this is why so many owners seek perfection.... slowly-catchy-DI 1.00 Quote Link to comment Share on other sites More sharing options...
Tony Posted November 8, 2006 Author Report Share Posted November 8, 2006 Dynamic indexing or Inertia matching as it's sometimes known has been around almost as long as chassis have but it's value has only really ever been appreciated by those we tend to think of as the better chassis designers in the world, for instance both the E36 and E46 Bmw 3 series both have a DI of 1.00 (or as near as damn it) when you add an average weight driver. DI is calculated knowing the overall mass, the distance between the front axle and the centre of gravity and the distance from the rear axle to the centre of gravity. By knowing these lengths you can calculate the chassis "instant centre" and DI is an expression of the length between "the length between the rear axle and the centre of gravity" and "length between the instant centre and the centre of gravity" Please do bare in mind though that whilst having a good DI is really good news for a chassis it's not the be all and end all, you CAN fix a bad DI with enough chassis set-up Sam i am never negative and value any snippet of intelligent information that could transport our understanding above and beyond.... But i have an issue with this calculation.. 'WEIGHT'? I cannot see reference to the inertia... so the polar center... COG, suggesting the "instant centre" is not complete as a suggestion for the overall sum. Within the matrix where does the weight/inertia configure....Is there a KG addition to the sum? or should i be thinking about the corner weighting again? Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted November 9, 2006 Report Share Posted November 9, 2006 Good point Tony, when calculating a dynamic index overall chassis mass is indeed needed in order to factor-in yaw inertia. Quote Link to comment Share on other sites More sharing options...
Tony Posted November 9, 2006 Author Report Share Posted November 9, 2006 Good point Tony, when calculating a dynamic index overall chassis mass is indeed needed in order to factor-in yaw inertia. Before DI was a calculation my best attempts to explain/understand yaw was this http://www.wheels-inmotion.co.uk/forum/ind...p?showtopic=207 I feel you can read my frustration since there are obvious gaps within the indexing or indeed the understanding of the generation for the indexing.. Now the windscreen is squeaky clean and i can see things more clearly now ..... Still have one or two million questions though Sam, i think we need to meet real soon. Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted November 13, 2006 Report Share Posted November 13, 2006 It’s interesting reading and yep I think we should meet up soon I know what you mean though a lot of the principles and systems that under pin chassis dynamics aren’t at all intuitive. It’s great news that it’s all becoming clearer, and the good news is that there really is no voodoo involved in the subject that can catch you out it really is all just maths & science Quote Link to comment Share on other sites More sharing options...
Tony Posted November 16, 2006 Author Report Share Posted November 16, 2006 It’s interesting reading and yep I think we should meet up soon I know what you mean though a lot of the principles and systems that under pin chassis dynamics aren’t at all intuitive. It’s great news that it’s all becoming clearer, and the good news is that there really is no voodoo involved in the subject that can catch you out it really is all just maths & science 'Maths and physics' seems more realistic .... I think we could 'Yap on' about DI, weight transfer, polar yew/centre, and the ethics of pneumatic slip/trail angles until our noses bleed... I want to ask an honest question... If a car has an index of 1.00 then this surely is only a foundation for development not the actual end result? From a DI 1.00 enhancements relating to the intended requirements would then modify the DI + or - can we agree on this factor? Another question please, if a car has a DI of 1.00 and i marketed (example) Anti-roll bar up-grade for the intended car, would it be reasonable to advertise a DI benefit?... Like 'This product commands a + 2.0 DI' Quote Link to comment Share on other sites More sharing options...
jon Posted November 16, 2006 Report Share Posted November 16, 2006 My +2 DI beats your +1 orc. While you use the +10 health i release the +40 demon of banshi land and slay your delvian priest. Erm. Quote Link to comment Share on other sites More sharing options...
Tony Posted November 16, 2006 Author Report Share Posted November 16, 2006 My +2 DI beats your +1 orc. While you use the +10 health i release the +40 demon of banshi land and slay your delvian priest. Erm. Although if you were attentive the DI + ARB index does conclude - 15 seconds to your lap on the track.. or not? all the entrance codes are available at the intro Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted November 17, 2006 Report Share Posted November 17, 2006 DI is the mathematical explanation of the tyres (in axle pairs) ability to influence the actual chassis (i.e. the leverage the tyres actually have upon the chassis weight) this is then considered versus the way in which the chassis presents it’s tyres to the on-coming road during a yaw moment (turn). Antiroll, Springs and Shocks represent a very different chapter in chassis dynamics. The job of these components is to control the vertical load applied to any one tyre at any one time, this function whilst it's related in terms of the finished product these aspects are quite separate from DI and Inertia matching Quote Link to comment Share on other sites More sharing options...
Tony Posted November 17, 2006 Author Report Share Posted November 17, 2006 DI is the mathematical explanation of the tyres (in axle pairs) ability to influence the actual chassis (i.e. the leverage the tyres actually have upon the chassis weight) this is then considered versus the way in which the chassis presents it’s tyres to the on-coming road during a yaw moment (turn). Antiroll, Springs and Shocks represent a very different chapter in chassis dynamics. The job of these components is to control the vertical load applied to any one tyre at any one time, this function whilst it's related in terms of the finished product these aspects are quite separate from DI and Inertia matching Analyzing Quote Link to comment Share on other sites More sharing options...
Tony Posted November 29, 2006 Author Report Share Posted November 29, 2006 Dynamic indexing or Inertia matching as it's sometimes known has been around almost as long as chassis have but it's value has only really ever been appreciated by those we tend to think of as the better chassis designers in the world, for instance both the E36 and E46 Bmw 3 series both have a DI of 1.00 (or as near as damn it) when you add an average weight driver. DI is calculated knowing the overall mass, the distance between the front axle and the centre of gravity and the distance from the rear axle to the centre of gravity. By knowing these lengths you can calculate the chassis "instant centre" and DI is an expression of the length between "the length between the rear axle and the centre of gravity" and "length between the instant centre and the centre of gravity" Please do bare in mind though that whilst having a good DI is really good news for a chassis it's not the be all and end all, you CAN fix a bad DI with enough chassis set-up In you opinion Sam do you think the centre weighted block on the rear of this Mazda MK3 NC is an attempt to capture a more balanced Dynamic Index? Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted November 30, 2006 Report Share Posted November 30, 2006 Interesting one that, my first reaction would be to say yes. If you need to add ballast then that would be the ideal place to do it. BUT that weight is actually tiny! i'd be amazed if it even weighs one kilo, so I'm gonna guess that Mazda/Ford found something in either testing or simulation that they didn't like and this was a simple band-aid fix. Maybe the cross beem needed a little more mass to effect it's natural frequency and therefore the way it transfers movement or vibration to the bodyshell, what ever it is it's not obvious to me Quote Link to comment Share on other sites More sharing options...
Mat Posted November 30, 2006 Report Share Posted November 30, 2006 ive seen this effect a few times on different cars. a few years ago when i had my workshop in germany, i was doing some insurance work on an Audi s6, in the rear of the cars sills were iron blocks!! apparently there added to neutralise the weight distribution on the car, for lhd/rhd and trim levels the intresting thing came though when trying to order a replacement from Audi/vw... "What block........ ...?" was the response i got (albeit in German ) it was only when i spoke to Wolfsburg technical about them that the whole purpose came to light Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted November 30, 2006 Report Share Posted November 30, 2006 ive seen this effect a few times on different cars. a few years ago when i had my workshop in germany, i was doing some insurance work on an Audi s6, in the rear of the cars sills were iron blocks!! apparently there added to neutralise the weight distribution on the car, for lhd/rhd and trim levels the intresting thing came though when trying to order a replacement from Audi/vw... "What block........ ...?" was the response i got (albeit in German ) it was only when i spoke to Wolfsburg technical about them that the whole purpose came to light Now that I can kinda understand, but I guess those iron blocks had a more meaningful mass something like 20-30kg. Quote Link to comment Share on other sites More sharing options...
Mat Posted November 30, 2006 Report Share Posted November 30, 2006 ive seen this effect a few times on different cars. a few years ago when i had my workshop in germany, i was doing some insurance work on an Audi s6, in the rear of the cars sills were iron blocks!! apparently there added to neutralise the weight distribution on the car, for lhd/rhd and trim levels the intresting thing came though when trying to order a replacement from Audi/vw... "What block........ ...?" was the response i got (albeit in German ) it was only when i spoke to Wolfsburg technical about them that the whole purpose came to light Now that I can kinda understand, but I guess those iron blocks had a more meaningful mass something like 20-30kg. nope 2.5 kilo each there were 3 in 1 side and a 1 kilo block in the other sill Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted November 30, 2006 Report Share Posted November 30, 2006 Blimey i wouldn't of thought it was worth it, one shopping bag weighs more than that! Quote Link to comment Share on other sites More sharing options...
Tony Posted November 30, 2006 Author Report Share Posted November 30, 2006 Interesting..... I have a MK3 to set-up on Saturday so i will get some more images or if allowed remove the block and get an idea of weight. Quote Link to comment Share on other sites More sharing options...
jon Posted December 8, 2006 Report Share Posted December 8, 2006 Are we sure these aren't to stop harmonics in the member? The sw20 has a similar looking block on top of the rear subframe designed to help with vibrations or something. Mine's long gone of course. Quote Link to comment Share on other sites More sharing options...
Tony Posted December 11, 2006 Author Report Share Posted December 11, 2006 Dynamic indexing or Inertia matching as it's sometimes known has been around almost as long as chassis have but it's value has only really ever been appreciated by those we tend to think of as the better chassis designers in the world, for instance both the E36 and E46 Bmw 3 series both have a DI of 1.00 (or as near as damn it) when you add an average weight driver. DI is calculated knowing the overall mass, the distance between the front axle and the centre of gravity and the distance from the rear axle to the centre of gravity. By knowing these lengths you can calculate the chassis "instant centre" and DI is an expression of the length between "the length between the rear axle and the centre of gravity" and "length between the instant centre and the centre of gravity" Please do bare in mind though that whilst having a good DI is really good news for a chassis it's not the be all and end all, you CAN fix a bad DI with enough chassis set-up In you opinion Sam do you think the centre weighted block on the rear of this Mazda MK3 NC is an attempt to capture a more balanced Dynamic Index? I have set three NC's since this image and non of them had the (whatever it is)...... Also i was thinking? I wonder how the forces would react to the object if the car was mirror searched in London? It does look like a bomb Quote Link to comment Share on other sites More sharing options...
Tony Posted December 29, 2006 Author Report Share Posted December 29, 2006 The new TDi project car has a Dynamic Index of around 1.7, which is a very long way from ideal. I think we have a bit of a challenge with this project Can we play a theoretical wim game? I have a front engined rear wheel drive car with a poor manufactured index of 1.7, how would you suggest improving this? Quote Link to comment Share on other sites More sharing options...
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