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Mass and moment of inertia - steering


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#1 hansforum

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Posted 09 January 2012 - 23:03

Hi!

Can some tell which car would be faster around sharp corner, the car with greater mass and lower moment of inertia or car with lower mass and higher moment of inertia? Or which one would steer more easily through the corner? Generaly which one between these two is better design and why?


Thanks in advance, there will be more questions:)!

Edited by hansforum, 09 January 2012 - 23:06.


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#2 Greg Locock

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Posted 09 January 2012 - 23:48

Hi!

Can some tell which car would be faster around sharp corner, the car with greater mass and lower moment of inertia or car with lower mass and higher moment of inertia? Or which one would steer more easily through the corner? Generaly which one between these two is better design and why?


Thanks in advance, there will be more questions:)!

On the same tires the low mass car will corner faster, all things considered, in general. Low PMI is nice to have but is way less important than mass, and cg height.

#3 Wolf

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Posted 10 January 2012 - 00:47

As a small aside to the original question- I always felt (but never had 'formally' looked into it), but low PMI would be a bonus when turning, but might be even bigger disadvantage when the car tries to swap ends? Not only would the car be more sensitive to disturbances/imbalance, but also even more so harder to correct?

Reminds me of one of my pet peeves which is "stability". I've seen books wrritten on the subject (one even opened with long-winded poem, might have been a page or two long, as a motto promptly followed by definition of stability in Lyapunov space and whole lot of differential equations) without even mentioning there is something diametrically opposite to it (like say 'controlability') which might be of some use in a motor vehicle... Perfectly stable car is pretty easy to achive- just jack it up and rest it on on three points without wheels touching the ground. :p

#4 Greg Locock

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Posted 10 January 2012 - 01:38

Trouble is when you compare different cars you are comparing different tires etc, but yes I agree, my perception is that with some cars the back end slides, you have plenty of time to give it 'a dab of oppo' http://sniffpetrol.c...ory/troy-queef/ and/or throttle, and everybody is happy. Other cars the back end goes and it's all hallo fence posts.



#5 kikiturbo2

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Posted 10 January 2012 - 02:39

as is the traditional view of mid engine cars being tricky on the limit as opposed to conventional front engine / RWD with longish wheelbase which can hang the tail out all day long..

I think it it a bit more complicated than that.. :) but if I had to choose between the two in the original question, smaller mass wins..

Edited by kikiturbo2, 10 January 2012 - 02:39.


#6 Greg Locock

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Posted 10 January 2012 - 04:38

Of the various measures of a car's linear range steering response, PMI only really affects the yaw velocity delay time. At very low frequencies the car yaws in phase with the steering wheel motion. As you move the sw at a higher frequency the car lags, until at some point the car moves left as the sw moves right. Your choice of what phase lag is critical can then be used to define a delay time. This concept is based on the aerodynamic control theory side of things, and is related to wheelbase, cornering compliance of each axle, and PMI.

Generally small delay times are better.

Here's a test report. I wouldn't do this test this way, but I like step steer as an input for intuitive analysis.

http://driveability-...DE-B090601e.pdf

Edited by Greg Locock, 10 January 2012 - 04:47.


#7 kikiturbo2

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Posted 10 January 2012 - 08:28

There is one more thing that bugs me with respect to PMI and driving properties...
PMI is usually measured taking vehicle COG as a center point, is it not?

My point is that the vehicle doesn't usually rotate around the same point... In a normal turn in the point location is depending on front and rear suspension geometry.. But, when you get oversteer, all of a sudden you are rotating around a point somewhere between the front two wheels..

#8 saudoso

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Posted 10 January 2012 - 10:54

I guess you can aways assume it's spinning around COG. For modelling purposes, if it seems to be rotating around another point, you can consider it both rotating and translating.

#9 Kelpiecross

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Posted 10 January 2012 - 12:02

[quote name='Greg Locock' date='Jan 10 2012, 15:38' post='5474225']

Generally small delay times are better.

I presume the actual steering ratio of the car is important as well. With a really "quick" ratio (and power steering) it is easier to catch breakaways etc.
Back in the good old days of maybe five or six turns lock to lock it was pretty much impossible to catch anything.




#10 hansforum

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Posted 10 January 2012 - 12:14

Is there any good book or link that deal with that topic and car handling in general?



#11 hansforum

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Posted 10 January 2012 - 12:46

If moment of inertia is 20% lower does it mean that the tires will work 20% less during steering, or 20% lower mass means 20% easier on tires? Is there some rule of thumb if we change moment of inertia how does it change grip during cornering? I think that if we have two equal cars with equal masses and different moment of inertia the one with higher moment of inertia will have higher slip angle during cornering, tires are going to work harder to change direction or the car.


Do front tires work harder during cornering than rear tires? I ask this because in car magazines it often says that RWD is better because it separates steering from power? But I far as I know if the car have 50:50 weight distribution, during cornering it has same slip angles on front and rear tires so power on front wheels would have the same chance to indeuce understeer as power on rear wheels oversteer. Does it makes sense?

#12 jatwarks

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Posted 10 January 2012 - 12:55

I guess you can aways assume it's spinning around COG. For modelling purposes, if it seems to be rotating around another point, you can consider it both rotating and translating.

Having a front engined car with a forward CoG allowed driving "by the seat of the pants" as the driver, being behind the CoG, felt lateral movement from rotation much more, and so could control it from the feedback received.

In more recent designs, in which the driver is likely to be just ahead of CoG, that feedback is much reduced, reducing the available time in which to react.

I think that while low PMI is desirable, its effect on handling can be altered greatly by the position of CoG. The type of car being considered is important.

A lightweight car is generally better, but how would a lightweight car with its engine behind the rear wheels compare with an equivalently equiped but heavier car (within reason) with a mid-mounted engine ?

Starting with a clean sheet of paper is one thing, but how would you select an existing car, to be prepared for racing, from a variety of layouts that were close in standard performance terms?

If major changes in layout and weight were not allowed, which layout would offer the greatest scope for improvement ?

#13 kikiturbo2

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Posted 10 January 2012 - 13:00

Is there any good book or link that deal with that topic and car handling in general?


Milliken

#14 kikiturbo2

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Posted 10 January 2012 - 13:02

If moment of inertia is 20% lower does it mean that the tires will work 20% less during steering, or 20% lower mass means 20% easier on tires? Is there some rule of thumb if we change moment of inertia how does it change grip during cornering? I think that if we have two equal cars with equal masses and different moment of inertia the one with higher moment of inertia will have higher slip angle during cornering, tires are going to work harder to change direction or the car.


Do front tires work harder during cornering than rear tires? I ask this because in car magazines it often says that RWD is better because it separates steering from power? But I far as I know if the car have 50:50 weight distribution, during cornering it has same slip angles on front and rear tires so power on front wheels would have the same chance to indeuce understeer as power on rear wheels oversteer. Does it makes sense?



PMI will be most noticeable at turn in and fast direction changes... in steady state cornering much less so.



#15 kikiturbo2

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Posted 10 January 2012 - 13:05

Having a front engined car with a forward CoG allowed driving "by the seat of the pants" as the driver, being behind the CoG, felt lateral movement from rotation much more, and so could control it from the feedback received.

In more recent designs, in which the driver is likely to be just ahead of CoG, that feedback is much reduced, reducing the available time in which to react.

I think that while low PMI is desirable, its effect on handling can be altered greatly by the position of CoG. The type of car being considered is important.

A lightweight car is generally better, but how would a lightweight car with its engine behind the rear wheels compare with an equivalently equiped but heavier car (within reason) with a mid-mounted engine ?

Starting with a clean sheet of paper is one thing, but how would you select an existing car, to be prepared for racing, from a variety of layouts that were close in standard performance terms?

If major changes in layout and weight were not allowed, which layout would offer the greatest scope for improvement ?



I think you are forgetting one more thing, and that is the actual center of rotation... If you compare, for example, various FWD cars with completely different rear suspensions, you will see that, depending on the rear setup, they have vastly different feels and road behaviour.. French "hot hatches" with some degree of dynamic lateral and toe in flexibility on the rear will usually feel as if they rotate around an vertical axis between the front wheels, while cars with struits and more rigid toe in, will be more neutral...

#16 saudoso

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Posted 10 January 2012 - 13:51

If moment of inertia is 20% lower does it mean that the tires will work 20% less during steering, or 20% lower mass means 20% easier on tires? Is there some rule of thumb if we change moment of inertia how does it change grip during cornering? I think that if we have two equal cars with equal masses and different moment of inertia the one with higher moment of inertia will have higher slip angle during cornering, tires are going to work harder to change direction or the car.


Do front tires work harder during cornering than rear tires? I ask this because in car magazines it often says that RWD is better because it separates steering from power? But I far as I know if the car have 50:50 weight distribution, during cornering it has same slip angles on front and rear tires so power on front wheels would have the same chance to indeuce understeer as power on rear wheels oversteer. Does it makes sense?

When turning there are two independent components to be accounted for. One is the rotation of the car related to it's COG and the other is the rotation related to the center of the turn. The moment of inertia relates to the former, while the mass relates to the later. Keeping in mind mass also affects the moment of inertia.

I have no idea regarding how much each of the factors contribute to the total strain on the tires.

#17 Greg Locock

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Posted 10 January 2012 - 21:36

Is there any good book or link that deal with that topic and car handling in general?

The most commonly referred to is Milliken and Milliken, Race Car Vehicle Dynamics. That's about 80 bucks from amazon. It includes a reasonable amount of data, admittedly for rather old cars, so at least you know the sort of thing you should be measuring. Not surprisingly it is weak on ride, jounce bumpers, rebound bumpers and not especially good on damping. In my opinion it is the best intro as it mixes anecdote and maths and data up in a very readable fashion. On the other hand Carroll Smith books are also good, but by the time you've bought a couple of those books you might as well have bought RCVD.



#18 Greg Locock

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Posted 10 January 2012 - 21:42

If moment of inertia is 20% lower does it mean that the tires will work 20% less during steering, or 20% lower mass means 20% easier on tires? Is there some rule of thumb if we change moment of inertia how does it change grip during cornering? I think that if we have two equal cars with equal masses and different moment of inertia the one with higher moment of inertia will have higher slip angle during cornering, tires are going to work harder to change direction or the car.


Do front tires work harder during cornering than rear tires? I ask this because in car magazines it often says that RWD is better because it separates steering from power? But I far as I know if the car have 50:50 weight distribution, during cornering it has same slip angles on front and rear tires so power on front wheels would have the same chance to indeuce understeer as power on rear wheels oversteer. Does it makes sense?

Hard to answer because you are zipping around between steady state and manouvering.

PMI has virtually no effect on tire lateral forces a constant radius constant lat acc turn. On corner entry etc a high PMI car will have somewhat higher tire lateral forces than a low PMI car of the same mass.

Yes each axle will have the same slip angle in a constant radius turn if it is 50/50. assuming no throttle is being used.

#19 bigleagueslider

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Posted 11 January 2012 - 04:53

The car that is fastest through a corner is usually the one that can carry the most speed through the corner. A lower mass car might be able to brake later going into the corner and accelerate quicker out of the corner.