RE: Mark Hales on Technique: Moment of inertia

RE: Mark Hales on Technique: Moment of inertia

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Discussion

Kawasicki

5,841 posts

174 months

Sunday 30th July 2017
quotequote all
NJH said:
Mark asks an important question at the end of the article, important as it raises a point of basic physics often overlooked or missed. We are used to thinking of mass out the back swinging but of course in physics terms all it is, is mass away from the centre of mass of the car (I don't like using words like gravity as it will cause confusion in this context). Thus of course a great big lump of metal in the front of the car could create a moment of inertia and thus a front end swing (need a rotational axis behind the engine).

Thinking about the above question reminds me of one of my own cars and the number 1 way many people have crashed them other the years. The 944 is a perfect example of a dumbbell car, 200Kg engine sat bang over the front axle between the wheels, 55 Kg gearbox sat mostly behind the rear axle with a fuel tank sat next to it, when full, enough to double that mass. Its one of those cars which has a propensity to move about quite a bit when close to the limit, the back end will happily slip out a bit (sling) on the way into corners which is pretty exciting on track once you can learn to trust it. But it can't be taken more than a small amount probably 15 degrees or so I can't say for sure, attempts at large angles are very difficult to control IMHE. Over the years loads of guys on the forums or mail groups before that managed to catch the back end swing only to be bitten by the front end bitting back and fishtailing off into the ditch, a wall or whatever. Mate of mine did it and was adamant he had the steering pointing the right way but those front tyres were just skidding as the front of the car pointed off into some bushes after he had neatly caught the initial back end steep out (not so much power as going too fast in the wet). Has never happened to me but have heard enough stories to believe that maybe front end swing is a real thing.

One last point about inertia in general. People often say stuff about dialling out understeer whilst forgetting of course that you can't magically dial away a vehicles mass, if it has mass it will have inertia, and if it has inertia it will create understeer one way or another (inertia in basic physics means the car wants to continue going straight on). In practice what I believe these people are often really doing is forgetting basic physics and mucking up the cars setup with stuff like roll oversteer on turn in rather than driving the thing properly.
I have found the opposite to you, that a big inertia makes a car easier to drive...nothing seems to happen quickly. This has one disadvantage though, and that it is sometimes not easy to pick the moment that the car starts to recover from a slide, so some drivers leave the corrective lock on too long...with fishtailing consequences..and/or a hedge encounter

Max_Torque

13,349 posts

156 months

Sunday 30th July 2017
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AER said:
Kawasicki said:
That is not an 100% accurate statement.
Engineering is about useful approximations rather than pedantry.
Which is where i came in! I was as trying to generalise, and include both lateral and longitudinal weight transfer in my generalised statement, and the effects that drive weight transfer are mass, CofG height and Roll Centre heights. I was also pointing out that the dynamic movement of the Roll Centre Height can add significant non lineararities in the amount of weight transfer that occurs for any given loading scenario.

We can all cut and paste formulas till the cows come home, but lets be honest, this is a forum where being able to explain, in english, a good enough approximation of how things work is the name of the game ;-)


NJH

2,843 posts

148 months

Sunday 30th July 2017
quotequote all
Kawasicki said:
NJH said:
Mark asks an important question at the end of the article, important as it raises a point of basic physics often overlooked or missed. We are used to thinking of mass out the back swinging but of course in physics terms all it is, is mass away from the centre of mass of the car (I don't like using words like gravity as it will cause confusion in this context). Thus of course a great big lump of metal in the front of the car could create a moment of inertia and thus a front end swing (need a rotational axis behind the engine).

Thinking about the above question reminds me of one of my own cars and the number 1 way many people have crashed them other the years. The 944 is a perfect example of a dumbbell car, 200Kg engine sat bang over the front axle between the wheels, 55 Kg gearbox sat mostly behind the rear axle with a fuel tank sat next to it, when full, enough to double that mass. Its one of those cars which has a propensity to move about quite a bit when close to the limit, the back end will happily slip out a bit (sling) on the way into corners which is pretty exciting on track once you can learn to trust it. But it can't be taken more than a small amount probably 15 degrees or so I can't say for sure, attempts at large angles are very difficult to control IMHE. Over the years loads of guys on the forums or mail groups before that managed to catch the back end swing only to be bitten by the front end bitting back and fishtailing off into the ditch, a wall or whatever. Mate of mine did it and was adamant he had the steering pointing the right way but those front tyres were just skidding as the front of the car pointed off into some bushes after he had neatly caught the initial back end steep out (not so much power as going too fast in the wet). Has never happened to me but have heard enough stories to believe that maybe front end swing is a real thing.

One last point about inertia in general. People often say stuff about dialling out understeer whilst forgetting of course that you can't magically dial away a vehicles mass, if it has mass it will have inertia, and if it has inertia it will create understeer one way or another (inertia in basic physics means the car wants to continue going straight on). In practice what I believe these people are often really doing is forgetting basic physics and mucking up the cars setup with stuff like roll oversteer on turn in rather than driving the thing properly.
I have found the opposite to you, that a big inertia makes a car easier to drive...nothing seems to happen quickly. This has one disadvantage though, and that it is sometimes not easy to pick the moment that the car starts to recover from a slide, so some drivers leave the corrective lock on too long...with fishtailing consequences..and/or a hedge encounter
I think actually we are agreeing, the 944 example is a case in point in that the initial slip up to a degree is fairly comfortable to deal with but there is definitely a point where it becomes too big an angle and very difficult to deal with OR of course the recovery leading to potential fishtailing. If anything the initial ease with which the car can be put into small slip angles leads one into a false sense of security.

AER

1,002 posts

209 months

Monday 31st July 2017
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Max_Torque said:
Which is where i came in! I was as trying to generalise, and include both lateral and longitudinal weight transfer in my generalised statement..
To be fair, when I parse every word in your post carefully, I can begin to spot what you're on about. It's a pretty sophisticated point you're making in less characters than a tweet.

IMO, there are so many other factors, some of which you allude to but also toe changes, ARB stiffness, non-linear spring rates, travel ratios and damping that the roll centre part of it is exactly that - only a part of it.

It's no chance that cars have much higher roll stiffnesses these days and probably in part to eliminate excursions to the extremes of suspension travel, despite the technologies these days to model everything including multi-link suspension geometry to the nth degree. Of course with 600hp engines and what not, it's also a necessity due to the massively increased performance envelope. Alas in the process we've forgotten what ride quality is all about.

Edited by AER on Monday 31st July 02:30

dinkel

24,836 posts

197 months

Monday 31st July 2017
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Last week I drove 2 1977 Alfa Romeo Coda Tronca Spiders: one EU model on Solex and one converted US model, deleted the Spica in favour of Dell'Ortos.

The EU car has a flex in the body shell, which is OK. The US car was strenghtend and offered a more relaxed ride. But at a corner... the car was too stiff to neatly plant its front tires properly and make for a swift corner. I found it both odd and uncomfortable. One, because I am used to proper EU spec Spiders, and two: because the stiffness totally ruins the agility of the Spiders behaviour.

Excellent article and please: more of this!
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