Tony Posted January 23, 2009 Report Share Posted January 23, 2009 As for your last question...."Polar moment of inertia" Definition: The resistance of an object to rotational acceleration. When the mass of an object is distributed far from its axis of rotation, the object is said to have a high polar moment of inertia. When the mass distribution is close to the axis of rotation, it has a low polar moment of inertia. A mid-engined car has most of its mass within its wheelbase, contributing to a low polar moment of inertia, which, in turn, improves cornering turn-in. I would have thought a higher camber angle would have exagerated the phenominum ? Don't forget the MX5 camber position is added to the rear, whereas the Omega's is removed from the front. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 25, 2009 Author Report Share Posted January 25, 2009 Are we talking in the context of front end turn in ? On that basis, the Omega needs the front camber for turn-in where as the Mx5 can do without due to having an inherinetly more agile chassis ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 25, 2009 Report Share Posted January 25, 2009 Are we talking in the context of front end turn in ?On that basis, the Omega needs the front camber for turn-in where as the Mx5 can do without due to having an inherinetly more agile chassis ? Transition.. it's a lovely word. We did a bit about "polar moment of inertia" so this Omega is a good example of how transition needs the suspension to migrate and change the polar moment to oversteer the chassis, this transition is consistent with the cars dynamic index ratio. Maybe i need to pull Sam at TDi's white papers and explain this dynamic indexing? Quote Link to comment Share on other sites More sharing options...
CIH Posted January 25, 2009 Author Report Share Posted January 25, 2009 Perhaps how DI and Geometry compliment/dicatate one another ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 25, 2009 Report Share Posted January 25, 2009 Perhaps how DI and Geometry compliment/dicatate one another ? Here is Sam's paper, other papers i will email to you if needed in respect to Sam's wishes. ................................................................................ ........ When we talk about chassis dynamics we often talk about a car chassis as though it’s a singular thing, but actually when trying to think about technical aspects of either driving a chassis or it’s design that’s actually incorrect and unhelpful. The car chassis is in reality just a collective term for a group of independent mechanical sub-systems. Most people are of course just interested in the driving so are only concerned with the net result of all of the sub-systems performing together as one. The unfortunate effect is that this makes it quite hard for these people to explain themselves clearly when talking to technical engineers who are in place to change the chassis set-up to suit them. Understanding each and every one of the individual sub systems certainly does go a very long way to help in understanding exactly how a car chassis works, but understanding how all of these sub-systems interact with one another really is the Holy Grail of chassis dynamics. So if we begin to think of the chassis in fact just as a complicated mechanical system we can take a look at other mechanical systems in general and start to apply the universal sciences and truths involved in systems engineering. To avoid going into a great deal of depth talking about harmonics I think it’s best now if we can just except for the time being that systems have a natural frequency (actually they have more than one, that’s a discussion for another time) and that these natural frequencies of the system are determined by the systems stiffness. The primary frequency of a car chassis will normally be very low and measured in Hertz or Hz for short, this description is historically used in science to indicate cycles per second. Ok now for the bit we actually care about, the effect of the chassis’s primary natural frequency. In real life this frequency dictates the minimum level of chassis correction or inputs per second required from the driver in order for the car to stay on its intended path. For example: A Rolls Royce might typically be set-up with a natural frequency of around 0.65hz, meaning that you can be very lazy when driving this car and it will still follow it's prescribed path. A Mitsubishi Lancer EVO which is fairly typical of the aggressive sports saloon car market, might have a natural frequency of around 1.8hz meaning that the driver must give inputs at a rate of just under 2 adjustments per second, that’s quite a work rate. In the extreme a formula 1 car is typically set at 4.5-4.8hz frequency, that’s a MINIMUM of almost 5 adjustments per second! Interesting Points: • Formula 1 car chassis are tuned right up the to the discomfort threshold of the human driver. At frequency’s around or over 5Hz the ligatures holding many of our internal organs in place simply lose control and with high enough g-forces acting at these frequency’s our organs can collide with the inside of our skeletons causing serious long term damage, not to mention a great deal of pain. • Why do you think most sports road cars like the Mitsubishi Lancer Evo’s are released to the public market with a chassis tuned to no higher frequency than 1.8Hz? Quote Link to comment Share on other sites More sharing options...
CIH Posted January 26, 2009 Author Report Share Posted January 26, 2009 Because the brain of the average driver can't handle more than a couple of decisions a second and, if I recall, average drivers can only respond to certain stimulai where as better drivers can respond to things such as inner ear balance when driving at 10/10ths. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 26, 2009 Report Share Posted January 26, 2009 Most people can run at two or three Hzps any more than that the car becomes difficult to drive, when you tune the chassis your also tuning the frequency so adding/ removing the inputs p/s.... It's a balancing act you need to know because you can tune the best you ever have only to find the frequency doesn't suit the driver. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 27, 2009 Author Report Share Posted January 27, 2009 Where does the MX5 sit on this scale ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 27, 2009 Report Share Posted January 27, 2009 They have a near perfect DI of 1.0 so it's reasonable to assume the Hz would be around 1.5Hzps.... Official figures have not been released, or i haven't seen them. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 28, 2009 Author Report Share Posted January 28, 2009 How about something crazy like a TVR ? I presume they have quite a high Hzps ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 28, 2009 Report Share Posted January 28, 2009 How about something crazy like a TVR ? I presume they have quite a high Hzps ? Very much so.... Point is to know this.... When you come to tune this sort of chassis knowing the Hz is high allows you to ask the owner what he wants and you to tune accordingly. Even the ability to explain the Hz puts you in much greater understanding than the other tyre centres out there..... " Sir i think your chassis runs at about 5Hzps but with some tuning i should be able to reduce this to around 3Hzps". Quote Link to comment Share on other sites More sharing options...
CIH Posted January 29, 2009 Author Report Share Posted January 29, 2009 Are we talking in the context of front end turn in ?On that basis, the Omega needs the front camber for turn-in where as the Mx5 can do without due to having an inherinetly more agile chassis ? Transition.. it's a lovely word. We did a bit about "polar moment of inertia" so this Omega is a good example of how transition needs the suspension to migrate and change the polar moment to oversteer the chassis, this transition is consistent with the cars dynamic index ratio. Maybe i need to pull Sam at TDi's white papers and explain this dynamic indexing? When we talk about transition in what context ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 29, 2009 Report Share Posted January 29, 2009 The point where the steering yaw, inertia and weight interact/ articulate the chassis. This point is the holy grail of chassis dynamics where experience "imagines" this moment born from the drivers explanation of how the chassis is behaving, few modules could explorer this moment. Being reasonably well educated in the industry it's a fair bet an NSX would have a better DI than an Omega and the Hzps for the NSX would be high in comparison.... Having this knowledge it's very possible to tune the chassis to the customer. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 29, 2009 Author Report Share Posted January 29, 2009 Oh right I see now. So on our Omega I should try to imagine all that front camber movement, the car shifting it's weight on the suspension and everything all that entails ? Does a good DI go hand in hand with a high(er) Hzps ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 29, 2009 Report Share Posted January 29, 2009 Yes but the car becomes harder to drive..... Knowing this puts you in a much better position to understand the owners complaint... The Hzps is not normally noticed directly by the owner, more like they say the car is loose or over reactive, whereas in truth they are unable to comply with the cars Hz which forces them to blame the car. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 30, 2009 Author Report Share Posted January 30, 2009 It's rare for a car to leave our place fully sorted. Usually a combination of either no OEM adjustment (KF doesn't "do" aftermarket) or I'm not allowed to take a job to a point where I would believe it to be finished. So it's nice to have a car go out spot-on, even if it's just turning rear eccentrics and front TREs edit; oh and I've been pecking the head of our local Snap-On to source one of those BMW Strut-top camber force adjusters as lifting axle has been unpredictable and takes too long for fast-fit. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 30, 2009 Report Share Posted January 30, 2009 That's good work... I'm proud of you. Shame your so far away, i can "wheels raised" set the BMW front cambers in 10min, you must be missing a procedure somewhere so a day or two's training would help no end at the centre. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 30, 2009 Author Report Share Posted January 30, 2009 I don't mind the trip if you don't mind giving up your time. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 31, 2009 Report Share Posted January 31, 2009 I don't mind the trip if you don't mind giving up your time. Not at all.... Maybe you could come down on a Thursday, stay over at mine so you have Friday/ Saturday at wim. Quote Link to comment Share on other sites More sharing options...
CIH Posted February 1, 2009 Author Report Share Posted February 1, 2009 Tony that would be fantastic. I'll have to see what the situation is for getting time off at work. Quote Link to comment Share on other sites More sharing options...
Tony Posted February 1, 2009 Report Share Posted February 1, 2009 Tony that would be fantastic. I'll have to see what the situation is for getting time off at work. Make it happen... your welcome to stay at wim central (probably best) so we can have some larger talk and we can go from there Quote Link to comment Share on other sites More sharing options...
CIH Posted February 5, 2009 Author Report Share Posted February 5, 2009 Something that occured to me recently about lower/stiffer suspension with a larger tyre tread area; How to balance additional static camber (due to less dynamic gains) against excessive camber and violent tyre wear ? Is there a kind of middle ground or is that the choice between tyre preservation vs. a handling bias ? Quote Link to comment Share on other sites More sharing options...
Tony Posted February 5, 2009 Report Share Posted February 5, 2009 Most times it's down to experience and your understanding of the suspensions articulation, we can "pull down" the chassis and map the dynamic gains in 5mm increments, plotting camber and bump but most times i know the result before i've even done that. Quote Link to comment Share on other sites More sharing options...
CIH Posted February 8, 2009 Author Report Share Posted February 8, 2009 Had to abandon another axle lift as the rig insisted I should steer the wheels straight before finalising the lift. Which is great 'cos when you cancel you're left with a car in the air and a screen full of the Red Bars of Death. That was with a perfect Thrust angle and steer ahead. I discovered it didn't complain when I simply clicked "ok" without actually lifting the car. Well It doesn't want to display a Steer Ahead bar graph, either, which is annoying ie; I use to able to fill the screen with toe, total toe and steer ahead but it nowadays gives the choice for individual toe only. Really did my head in as the car (E46 3 series coupe) had (IMO) too much camber (-2 rear, -1.5 front). Quote Link to comment Share on other sites More sharing options...
Tony Posted February 8, 2009 Report Share Posted February 8, 2009 Can you please turn off or stop using "steer ahead" and set the system to "thrust" 4 wheel alignment.... "Steer ahead" as a angle only says something is wrong if the position is +-3', i want you to analyze a problem way before steer ahead say's so. The E46 seems very planted camber wise, has it been lowered? Quote Link to comment Share on other sites More sharing options...
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