will lowering improve handling?

[wheels-inmotion]

In a way, it's probably easier to begin with what DI isn't;

It isn't anything to do with your tyres (apart from what they weigh!)
It isn't anything to do with wheel geometry or alignment
It isn't anything to do with spring rates, suspension ratios or damper values
It isn't anything to do with suspension type or layout
It isn't anything to do with whether a car understeers or oversteers


It's all to do with where mass is on the vehicle, relative to the front and rear axles. The only way to change it is to move mass around - if you remove mass uniformly, you don't change DI, it's only changing the distribution that counts.

The calculation of DI comes down to an equation that looks a bit like K^2/ab.

k is the appropriate radius of gyration,
a is the longitudinal distance from the front axle to the c of g
b is the longitudinal distance from the c of g to the rear axle.

All of this can be projected into the plane of the road - there's no need to consider the height of anything, and so lowering a car can't change the DI significantly.

I say significantly, because there is a second order effect, because owing to the suspension geometry, the longitunal position of the wheel contacts will change by a tiny amount as the vehicle is lowered, but, this is a tiny effect compared with the lengths of a and b - even going from full bump to full rebound isn't going to change the product ab by much.

One non-mathematical way to consider DI is to imagine the car parked over a painted line on an ice rink. The line passes along the centreline of the car, directly under the centre of the front axle, and out, directly under the centre of the rear axle.

If you now imagine a hammer hitting the passenger side front hub, knocking the front of the car towards the driving side.

So, when the car comes to rest, a plumb bob that is dropped from the middle of the front axle will lie to the driver's side of the line in the ice.

If the DI is one, a similar plumb bob dropped from the middle of the rear axle will coincide with the line. In other words, a sudden change in sideforce (and hence slip angle) at the front axle results in no change at the rear axle.

For DI greater than 1, the rear axles plumb bob would be towards the driver's side of the line, and for less than 1, to the passenger side.

The only things missing from my rough description is that the car would really be travelling forwards.

----------------

This principle can also be applied in other planes.

For example, using the same formulation but using the radius of gyration in the pitch angle direction can tell you how the vehicle responds to a sudden bump on the front axle.

Another example - consider a driven live axle, and a simple beam dead axle, the two are kinematically similar, but, because of the weight of the diff in the middle, have a completely different DI. In this respect, having an beam axle with a DI of 1 is as close as you can get to independence with that set-up.

Nice "cut&past".... Why are you so aggressive on this topic?

We could "pants on" for years about chassis dynamics and no one reading would understand a bloody thing we are arguing about, nor would it contribute to the original post.

If you want to continue this "theoretical" debate, kinematic or not it would be polite to open another thread.

If you feel like going there then lets start from the beginning.... "Humans natural frequency"...... 1.7Hz ps as a base topic.

 

[Number_Cruncher]

To give one further, sporting example of something equivalent to DI, consider going in to bat for England.

When the ball speeds into the bat, you want the relationship between the impact point of the ball, the mass distribution of the bat, and where you grip the bat to have a "DI" of 1 - if you don't have this happy situation, you will feel a nasty sting from the bat!

The other technical term which is related to this phenomenon, if you want to google for it!, is "centre of percussion"

 

[Dieselman]

If you want to improve DI (which itself assumes that you have a means of finding what the DI actually is for your car - not trivial!), then you would need to move heavy items like the battery to the most advantageous position, [either closer to the c of g, or further away depending upon the original DI] and remove weight from other areas.

Always closer to the rotational centre otherwise the car will become unmanageable when on the limits.

The fact that a car has a 50-50 weight distribution isn't as important as the weight being between the axles.

Think VW Beetle as opposed to a Toyota MR2. The MR2 will slide gracefully, the Beetle will try to kill you.

 

[c240yaz]

In depth subjects on this forum always end up argumentative and disagreable. I for one have learned. You can also easily get BANNED.

Keep it simple and all will be happy !!

PS. Don't shoot the mesenger !

 

[wheels-inmotion]

No problem.. You want to come here to relax, not work...


Sure..

It won't happen....

Historically i get chased, then raped for information but due to time allocation i cannot afford to argue topics.

It's a real shame... I am not looking for custom whatsoever we are busy enough. The only reason i am here is from your link so i thought i would support it in your club.

I can transport papers from wim and satisfy those who are curious in chassis dynamics but i have little time available to explore totally new debates.

Saying that i will if they are genuine and beneficial to all concerned.

 

[Number_Cruncher]

>>Nice "cut&past"....

No, sorry, there's absolutely nothing that has been cut and pasted there. Alas it's part of my mental baggage, although I haven't used the formula I included for 12 years or so (so, it's entirely possible that I've mis-remembered it, and written it upside down!)

>>Why are you so aggressive on this topic?

I confess that I did bait a trap for you, sorry about that!, but, I was a bit concerned about the way you made the subject of lowering a car needlessly complex by introducing a concept that is as esoteric as dynamic index in post #6 - especially as it's virtually irrelevant to the OP's situation and question.

I'm sure that you do know your onions about wheel alignment, and I really don't intend to be aggressive. Although neither of us are at work, we do owe it to relatively technically naive posters not to mislead them or bamboozle them unecessarily. I felt OK to post a rather technical piece on DI, as both you and Dieselman can't be called technically naive.

Don't worry, I'm not currently reaching for my copy of Human Vibration by Griffin to pick over the finer points of human dynamic response!

 

[Bobby Dazzler]

Think VW Beetle as opposed to a Toyota MR2. The MR2 will slide gracefully, the Beetle will try to kill you.

The early (pre-facelift) SW20 MR2 (1989-) would try it's very best to kill you!!

It was very unpredictable at the limit. It seduced the driver, and all of a sudden let go when the driver wasn't expecting it.

The later facelift models had suspension, wheel, and tyre changes to make it less able creep up and surprise you. It would still try to kill you, but gave you a plenty ov notice before it went for the juggler, by being more progressive in it's approach to the limit.

You're right about sliding gracefully. The only time I've ever slid a car on the road was completely unintentional and outside the coolest local nightspot where people spilt out on to the streets.

The Yokohama tyres were a core part of the chassis improvements, and they'd been on back order for weeks. At the time I was doing 1000 miles or more a week, and so they became very slippy very quickly.

The Dazzler managed an unintentional slide for what felt like 200 yards past the cool people, Dukes of Hazard stylee. Not cool. I couldn't face going back for ages!!

 

[Dieselman]

The early (pre-facelift) SW20 MR2 (1989-) would try it's very best to kill you!!

It was very unpredictable at the limit. It seduced the driver, and all of a sudden let go when the driver wasn't expecting it.

The later facelift models had suspension, wheel, and tyre changes to make it less able creep up and surprise you. It would still try to kill you, but gave you a plenty ov notice before it went for the juggler, by being more progressive in it's approach to the limit.

You're right about sliding gracefully. The only time I've ever slid a car on the road was completely unintentional and outside the coolest local nightspot where people spilt out on to the streets.

I remember the early Mr2 issue, it was basically setup for the track and as you say had tremendous grip until the tyres let go completely.

well, if drivers couldn't handle it...

 

[Number_Cruncher]

>>Always closer to the rotational centre

Yes..., and no.

From purely a dynamic index point of view, you can go too far, and make the DI too low. As you move away from a DI of 1, you are effectively compromising how independant the front and rear axles are. With a DI of 1, a sudden change at the front axle isn't felt at all at the rear.

Of course, if the subject of the car losing grip is being discussed, there is lots more to it than just DI. To give the tyres maximum authority over the mass of the car, then, yes, reducing the mass and the radius of gyration are good things to do, but a compromised DI will affect the way that the vehicle and driver respond to sudden perturbations at the limit.

 

[wheels-inmotion]

>>Nice "cut&past"....

No, sorry, there's absolutely nothing that has been cut and pasted there. Alas it's part of my mental baggage, although I haven't used the formula I included for 12 years or so (so, it's entirely possible that I've mis-remembered it, and written it upside down!)

>>Why are you so aggressive on this topic?

I confess that I did bait a trap for you, sorry about that!, but, I was a bit concerned about the way you made the subject of lowering a car needlessly complex by introducing a concept that is as esoteric as dynamic index in post #6 - especially as it's virtually irrelevant to the OP's situation and question.

I'm sure that you do know your onions about wheel alignment, and I really don't intend to be aggressive. Although neither of us are at work, we do owe it to relatively technically naive posters not to mislead them or bamboozle them unecessarily. I felt OK to post a rather technical piece on DI, as both you and Dieselman can't be called technically naive.

Don't worry, I'm not currently reaching for my copy of Human Vibration by Griffin to pick over the finer points of human dynamic response!

Your a bad boy!.... DI and the chassis frequency is in direct correlation with the human natural frequency as you well know.

If i recall correctly 2.5Hz ps with a DI at 1.0 is a subliminal base... Change the DI or the Hz then the chassis needs to be calibrated to fit the human.

 

[Dieselman]

From purely a dynamic index point of view, you can go too far, and make the DI too low. As you move away from a DI of 1, you are effectively compromising how independant the front and rear axles are. With a DI of 1, a sudden change at the front axle isn't felt at all at the rear.

Of course, if the subject of the car losing grip is being discussed, there is lots more to it than just DI. To give the tyres maximum authority over the mass of the car, then, yes, reducing the mass and the radius of gyration are good things to do, but a compromised DI will affect the way that the vehicle and driver respond to sudden perturbations at the limit.

Not being funny here NumberCruncher, but why didn't you just post your information up relating to this topic as you abviously have a fairly in depth knowledge.
It seems a bit poor to have lured a helpful new member into an unneccessary argument.

I don't mean me there...

 

[c240yaz]

From a long time ago, I discovered this one.

Best way to disable a diesel on your worst enemy, is to tear up some newspaper, throw it in the tank via the filler neck.

Diesel starts and runs until a piece of the newspaper paper gets sucked against the lift pipe, causing a vacuum failure. Engine cuts out. Paper drops to the bottom of the tank.

Re start engine, cycle repeats itself, forever !

Unless you remove the tank, you cannot diagnose, or fix. Many a dollar has been spent trying to diagnose. Days, weeks, months. In some cases, never.

Many a complete fuel injection system has been replaced due to a mechanic, or team of techs trying to find out what the problem is.

Not a recommendation, just a bad experience.

 

[Number_Cruncher]

When I posted my first question, I wasn't particularly sure that there was no link between lowering a car and DI (the 12 year gap since I last gave it any serious thought!), and I wanted to see if there was something I had missed. I thought that the best way to approach it was to ask "naive" questions to avoid a direct confrontation.

Thinking about the subject since made it clear to me that there was no link - and apart from the early post #6, wheels-inmotion hasn't repeated that assertion, so, there is actually no argument.

 

[wheels-inmotion]

When I posted my first question, I wasn't particularly sure that there was no link between lowering a car and DI (the 12 year gap since I last gave it any serious thought!), and I wanted to see if there was something I had missed. I thought that the best way to approach it was to ask "naive" questions to avoid a direct confrontation.

Thinking about the subject since made it clear to me that there was no link - and apart from the early post #6, wheels-inmotion hasn't repeated that assertion, so, there is actually no argument.

I have no argument/'s whatsoever.

Facts are the lowering will not change the track/ wheel-base (well it will in small increments) but will probably change the uniform height front/ rear.... in turn this changes the polar centre then it must change the DI....

All things being true the chassis frequency must change and indeed this will impact on the humans natural frequency@ 1.7Hzps

In summary lowering will change all things above but unless the car is intended for DTCC who will notice?

 

[Number_Cruncher]

>>in turn this changes the polar centre

Ah!, perhaps there is a small technical disagreement here after all!

In lowering a car, you don't change the position of the centre of gravity in a front/rear sense. Also, you don't change the distribution of weight relative to the centre of gravity, and you don't change the longitudinal wheel positions significantly (any suspension geometry kinematics are tiny relative to the size of a or b).

Lowering a car doesn't make any significant difference to dynamic index, after all, it's just k^2/ab!, the height of the vehicle can't make it's way into that equation no matter how it might try! There's no frequency term there either.

 

[wheels-inmotion]

>>in turn this changes the polar centre

Ah!, perhaps there is a small technical disagreement here after all!

In lowering a car, you don't change the position of the centre of gravity in a front/rear sense. Also, you don't change the distribution of weight relative to the centre of gravity, and you don't change the longitudinal wheel positions significantly (any suspension geometry kinematics are tiny relative to the size of a or b).

Lowering a car doesn't make any significant difference to dynamic index, after all, it's just k^2/ab!, the height of the vehicle can't make it's way into that equation no matter how it might try! There's no frequency term there either.

Your good i'll give you that.....
I don't think our bantering will be of much value to most members since we are surfing the dark region of chassis dynamics but......

If the car is lowered then the coil rate will be different, so the chassis frequency will also change, this said the human frequency will need to adapt, historically the Hzps will rise.... Agreed?

 

[Number_Cruncher]

>>Agreed?

Yes, there can be a change in suspension frequency - it really depends upon how you acheive the lowering.

If, say, you change the coil spring rubber shims for thinner ones, then, there's very little change in suspension frequency.

If, on the other hand, you fit a coil spring with, say, fewer active turns of thicker wire, then, yes, suspension frequency will change.

One of the oddities of the MB front suspension layout is the large suspension ratio. As there's a squared relationship between wheel rate and spring rate, you need to make comparatively large changes in spring rate to have a serious effect on effective wheel rate.

 

[wheels-inmotion]

>>Agreed?

Yes, there can be a change in suspension frequency - it really depends upon how you acheive the lowering.

If, say, you change the coil spring rubber shims for thinner ones, then, there's very little change in suspension frequency.

If, on the other hand, you fit a coil spring with, say, fewer active turns of thicker wire, then, yes, suspension frequency will change.

One of the oddities of the MB front suspension layout is the large suspension ratio. As there's a squared relationship between wheel rate and spring rate, you need to make comparatively large changes in spring rate to have a serious effect on effective wheel rate.

Finally we have met in the middle.... Pint's on me, what ya having?

Interesting topic this chassis dynamics.... Maybe we should open up a debate somewhere in the club and give chassis calibration a right seeing to?

 

[Dieselman]

Feel free. This is good reading for me.

As long as you leave your handbags at home...

 

[wheels-inmotion]

Feel free. This is good reading for me.

As long as you leave your handbags at home...

Handbag indeed ... Fact is though the topic is a nose bleed of a read if the topic hijacks a thread... Maybe another thread would be polite.