RE: Mark Hales on Technique: Moment of inertia

RE: Mark Hales on Technique: Moment of inertia

Saturday 29th July 2017

Mark Hales on Technique: Moment of inertia

What powersliding a Lotus Esprit for Fast Lane has to do with weight distribution...



Last time, we looked at the effects of weight transfer on that most simple and uncluttered example of the mid-engined breed, the Formula Ford. The engine in the middle, driver just in front, all the major masses concentrated nicely towards the centre but with any slight excess towards the rear. That much helps with traction while the centrally located majority helps the car carry it all through the corners with the minimum of disturbance. It's worth sticking with this example for a bit longer because weight transfer and its effects are, to my mind, extremely important tools in the driver's toolbox, not least because they have a big effect on Old Motors which have less in the way of tyre grip or aerodynamic assistance.

Centralized mass moves more quickly...
Centralized mass moves more quickly...
If there is a downside to any centralization of mass, it's the fact that when it does exert an effect, it does so more quickly. It's sometimes described as the dumb bell effect which is a handy way of visualizing the distance between the major masses and the car's centre. The lump on the pole is the mass and the pole is the lever arm; swing it and see what the mass does - you can conduct your own research very easily. Take a house broom or something similar and grasp the handle near the besom and try and swivel the lot. Now grasp the handle about halfway up and see the difference. The official term for its effect is polar moment of inertia or movement around an axis.

A moment's thought will make it obvious that a mass of engine hanging right out the back - as it does in a Porsche 911 - will have a bigger leverage when it tries to swing right or left, than the engine of a Formula Ford which is not only lighter but much closer to the middle of the car. Let's pick a two-seater example of the Formula Ford concept, so you can come along for the ride. I'll go for the Lotus Esprit of the 1980s, a car which although sometimes undistinguished in terms of reliability was magnificent in handling balance.

Yes, it's the wrong year; hopefully you get the point!
Yes, it's the wrong year; hopefully you get the point!
I'll let the Lotus transport us back to the late 80s and Quarry Corner at Castle Combe where I was doing a feature for the late lamented Fast Lane magazine. We had on hand the latest version of the Esprit, a luminous yellow model powered by a 265hp version of the 2.2-litre leant-over turbo four and as usual, we were to do the skiddiest skiddy shots that could be seen anywhere because that was the magazine's trademark. At first, though, the Esprit proved unwilling to comply. Run the wrong way up the track (Combe wasn't as busy in those days and the track was ours for the day) turn it around so I could take a run, and pile over the crest at Avon Rise in third gear, brake sharply, heave the steering and whack on the power while the car was still unsettled. All I got in response though was power understeer which forced me to come back off the accelerator in order to avoid mowing down the snappers crouched or laying at the corner's exit. Health and safety? This was 1987...

You did get a bit of wag on the exit as the power reached its peak and the small amount of body roll loosened the inside rear tyre, but the initial spike lower down where the torque curve was at its fattest merely picked up the wedge nose and thrust it wide. This seemed odd at first, because on a normal quick lap where you weren't trying to provoke the car, the Esprit felt nicely sharp at the front, following the rush towards an apex with a slight touch of reassuring understeer, just enough that I felt comfortable taking some commitment into the corner. More importantly, I felt able to keep leaning on the front end while bringing the power on good and early without having to deal with sudden snappy oversteer. Since that was exactly what we were trying to achieve for the lens, on second thoughts the car's behaviour was consistent.

Later Ferrari spikier than lovely Lotus
Later Ferrari spikier than lovely Lotus
The eventual solution was to power into the corner with the car settled and the steering aimed at the mid-point and just back off suddenly as the car began to head in. Then immediately tread the power back. The Lotus obligingly swung its tail as the front end loaded up, then as the boost came back, seemed to go into slow motion, slowly and almost lazily increasing the angle of slither while the inside wheel smoked away some of the excess grunt. A couple of goes and I found it possible to let the castor clonk the steering against the opposite stop and adjust the angle of slither with more or less power. Having achieved it, from the corner of an eye, I saw Simon the Snapper on his feet and waving. "Yes, very good," he said, but it looks as if you've almost stopped, or more likely, it's the last frame before you finally spin," adding, "and you wouldn't want that, would you?" Er, no, but there didn't seem to be any halfway house. I thought about it all on the long trip home and concluded that braking savagely then turning hard probably overloaded the front tyres and compromised their ability to point just when I wanted it. Then there wasn't enough immediate power to overcome the grip of the rears because of the - admittedly minimal for the time - degree of turbo lag.

Maybe thinking technique. Maybe admiring the sun
Maybe thinking technique. Maybe admiring the sun
I've learnt a bit more of the theory since then and there are a couple of significant details; braking savagely does indeed add weight to the front wheels, but as soon as I came off the brake pedal, the weight also has time to ping back in the air before the turbo torque arrived, thereby creating exactly the opposite effect to the one I was seeking. The lift off, then plant it technique also adds weight to the fronts, but it gives it a fraction more time to settle and the tyres to respond (we'll get to the traction circle sometime in this series). The extra weight which has helped the front tyres in that regard has effectively been borrowed from the backs, which then have less to press them to the ground when the turbo boost arrives. Their ability to cope with the turbo's spike is suitably compromised. It's a fine line from the driving seat, but it does make sense in front of the keyboard.

The other interesting thing was that unless you made it all extreme with loads of power venting through a spinning inside wheel and loads of reverse lock, the Lotus would revert to gentle understeer, even when a slide had started. If you think about the details described above, you can see why that might be. Then as always, the question of the relevant ingredients and their arrangement has to be considered too. How much grip the tyres could offer - in those days there weren't any 30 profile 19-inchers - and how the geometry and spring rates had been configured. But, more years later than I care to consider, I now believe the most significant thing was a relatively light engine low down and sat right up against the bulkhead. That was a fundamental which made for a blisteringly quick track machine and an intimate and uncomplicated, yet astonishingly fast and reassuring road car. A much-missed classic in my opinion.

Gearbox across works better than underneath...
Gearbox across works better than underneath...
The Ferrari 360 which came a decade and a half later and bit me at Donington was more ready to do so, I believe because the masses were greater, even if the layout was similar. Later ones I'm told are better because the engineers did some tweaking in other areas but if the 360 could be awkward, the Testarossas and 512TRs which were the Esprit's contemporaries could be even more so. This I suspect was because their designers tried to combine the mid-engined ideal with the length of a V12 engine and ended up putting the gearbox underneath the engine. That pushed the weight high and required pretty stiff suspension to control it, a combination which made the cars fantastically unforgiving when they finally overcame the grip of huge tyres. All the speed of reaction that a lower polar moment is intended to achieve, compromised by the greater leverage of a mass relatively high up. It is why Ferrari later turned the gearbox across the car for the 1970s series of 12-cylinder 312T Grand Prix cars - the 'T' standing for Transversale. Optimal weight distribution is still one of the main reasons why Formula cars of today feature an engine in the middle. The other is packaging.

It might sound odd that the designer of a Formula One car should have to consider such things, but they do - along with the all-important aerodynamics. The engines are all the same size and so are the gearboxes, so the designers are stuck with these dimensions and if there is an optimum distance within the wheelbase (which everyone else will have arrived at) then a detail like a tall driver can make a big difference. The current Grand Prix car bears little or no relation to road cars but packaging is equally relevant in these too, because the designer has to consider at least two occupants and a place to put their luggage. But, let's return to the subject of Old Motors and the effects of weight distribution.

And we'll get on to this next time!
And we'll get on to this next time!
The more common layout for a great many older cars is the engine and gearbox ahead of the driver with the engine at the front of the car and the gearbox in the middle. This made sense to most manufacturers I suspect mainly because it was the fashion of the time; although there were those who thought different - like Andre Citroen - they weren't the norm. Now, before we look at the effect this has on weight distribution, just consider the effect it has on packaging - not to mention styling. The engine and gearbox take up space where the Esprit's boot is, and demand that the bonnet line is high enough to cover the engine rather than slope gently up to a low slung windscreen. The drive has to go from the gearbox to a back axle and so needs to pass through the cabin in between the occupants. The axle spans the space behind the occupants and eats into the luggage space in the boot, under whose floor is the obvious location for the fuel tank. Then you have to get the exhaust from the front of the car to the back and find room for the silencer. In its favour, a front mounted engine can be close to radiator and oil cooler where the essential air flow is easier to access.

There's a lot to consider before we delve yet further into the effects of mass on the car's dynamics, but until then, just try and visualize what difference a big lump up front will make when it swings when compared with the same lump sat behind you. Does it in fact, swing...? You'll doubtless have worked it out by then, but if not, answers will be forthcoming in the next epistle...

Read more from Mark on his website.

Motorsport images: LAT Photo

Author
Discussion

Debaser

Original Poster:

3,246 posts

189 months

Saturday 29th July 2017
quotequote all
Great article!

I've always tended to prefer the slower responses of a front-engined car over something mid-engined, at least on the road.

Max_Torque

12,143 posts

145 months

Saturday 29th July 2017
quotequote all
Worth noting that the ultimatly, dynamic weight transfer is driven by two factors (that don't cancel or scale directly)

1) Vehicle Acceleration (in any plane), after all F = MA

but critically

2) The difference height between the instantaneous roll centre height and the height of the vehicles Centre of Gravity. It is this "couple" that determines how much weight gets transferred from end to end, or side to side.


Back in the day, cars were light (er) and had high roll centres (because of simple suspension like beam axles etc. Then designers worked out they could lower the roll centre and improve certain parts of the dynamic behavior (too complex to cover quickly here). However, this generally mean't a larger total weight transfer to deal with, which mean't stiffer springs and anti-roll systems. Added to which, early multi-link suspension designed tended to be poorly controlled in-extremis. often the dynamic roll centre would suddenly plunge down several inches (sometimes feet!!) in the last few inches of suspension travel, which mean't, in-extremis the weight trasnfer was highly non linear. Add that to generally poor control of the tyre contact patch (castor, camber, toe etc) and you had a recipt for a car that could experience large and sudden changes in dynamic traction. Guaranteed to keep any driver on their toes...... ;-)


rockin

5,342 posts

173 months

Saturday 29th July 2017
quotequote all
Yes, mid-engine cars will spin like a top - if you provoke them. They always deserve excellent tyres and properly set up suspension. Don't expect the much-derided-on-PH nanny electronics to save you if you get seriously out of control because by then the laws of physics will have taken over.

Yes, a big engine hanging out in front of the car isn't a very good idea if you want to go round corners. Which is why Corvette has the engine behind the front axle and the transmission on the rear axle. Front-mid-engine. Again don't expect the much-derided-on-PH nanny electronics to save you if you get seriously out of control.

Scottie - NW

763 posts

161 months

Saturday 29th July 2017
quotequote all
Enjoy reading these articles on PH.

No doubt some keyboard expert will be along shortly to "have an alternate viewpoint" to put it kindly.

Kawasicki

5,088 posts

163 months

Saturday 29th July 2017
quotequote all
rockin said:
Yes, mid-engine cars will spin like a top - if you provoke them. They always deserve excellent tyres and properly set up suspension. Don't expect the much-derided-on-PH nanny electronics to save you if you get seriously out of control because by then the laws of physics will have taken over.

Yes, a big engine hanging out in front of the car isn't a very good idea if you want to go round corners. Which is why Corvette has the engine behind the front axle and the transmission on the rear axle. Front-mid-engine. Again don't expect the much-derided-on-PH nanny electronics to save you if you get seriously out of control.
I've gotten front and mid engined cars seriously out of control thousands of times and the stability control did an excellent job of stabilizing the car.

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Kawasicki

5,088 posts

163 months

Saturday 29th July 2017
quotequote all
Mid engined cars tend to be a little tricky to power oversteer because they are set up to understeer. If you try to be brutal with the car it will tend to either understeer deeply or flick into oversteer, and if it does oversteer it only gives the driver a small window of opportunity to then keep the car sliding by overpowering the rear tyres.

There is a much easier method.

Slow down in the corner, stay under the speed at which understeer is the clearly dominant balance, that is super important. Pick a gear, say third, and lift off while you are in the corner, you will hopefully feel a smidgen more front end grip, and obviously a smidgen less rear grip. At this point carefully turn in slightly more, the car will start to yaw/rotate very slowly inwards...as the car is yawing on a trailing throttle get back on the gas...you don't even need much. Of course you can floor it too, for huge oversteer.

Most good drivers have a problem oversteering mid engined cars because they are used to really leaning on the front tyres...they keep getting more understeer as they apply the throttle...they then get frustrated and start using the natural yaw frequency flick method to get the car sliding at the rear...which is pretty damn challenging to control, probably partly because of low yaw inertia mentioned in the article.

Edited by Kawasicki on Saturday 29th July 20:55

Cold

4,125 posts

18 months

Saturday 29th July 2017
quotequote all
//Zips up anorak so that only my nose is showing from beneath the elasticated hood - which is why I sound adenoidal//

Article said:
We had on hand the latest version of the Esprit, a luminous yellow model powered by a 265hp version of the 2.2-litre leant-over turbo four //
//This was 1987...
The 264 bhp Esprit SE was launched in May 1989. Cobwebs cause confusion - but give saddo nerds the chance to nitpick. hehe

Evilex

424 posts

32 months

Saturday 29th July 2017
quotequote all
Did I mis-read or misunderstand something?
Is there an assertion that the Testarossa (II)/ 512 TR/ 512 TR M cars are a V12?

They're flat 12s, of course. An evolution from the 512BB.

If the 'box is underneath, it'll still raise the engine and , consequently the centre-of-gravity. But less than a V would.

sege

168 posts

150 months

Sunday 30th July 2017
quotequote all
Evilex said:
Did I mis-read or misunderstand something?
Is there an assertion that the Testarossa (II)/ 512 TR/ 512 TR M cars are a V12?

They're flat 12s, of course. An evolution from the 512BB.

If the 'box is underneath, it'll still raise the engine and , consequently the centre-of-gravity. But less than a V would.
Wow nice pick up! It's not everyday that you will find fault with a Mark Hales article, although i doubt it is any more than a typo.

NDNDNDND

602 posts

111 months

Sunday 30th July 2017
quotequote all
sege said:
Evilex said:
Did I mis-read or misunderstand something?
Is there an assertion that the Testarossa (II)/ 512 TR/ 512 TR M cars are a V12?

They're flat 12s, of course. An evolution from the 512BB.

If the 'box is underneath, it'll still raise the engine and , consequently the centre-of-gravity. But less than a V would.
Wow nice pick up! It's not everyday that you will find fault with a Mark Hales article, although i doubt it is any more than a typo.
These engines are generally considered to be '180 degree' V12s, as their firing order replicates a conventional 'V'-configuration engine and they aren't laid out as a 'boxer' engine with the cylinders using separate crankpins.

Hasbeen

1,975 posts

149 months

Sunday 30th July 2017
quotequote all
I drove this, GT B Ferrari in a 6 hour sports car race.

It was the most dreadful handling car I have ever driven on the track, & possibly ever anywhere.

It had a great suspension, even inboard rear discs, but it also had a soft suspension designed for the American boulevard market. It exhibited all the above problems in spades.

In slower corners, up to about 80MPH it had complete rear end breakaway, the moment you turned the wheel.

It took about 30 laps of experimenting before I found a way of getting it round a corner. I had to come off the brakes at least 30 Ft before turning in, to give the dreadful thing time to get it's nose up, regather some composure, & get some weight back on the rear wheels.

Even then, I had trouble matching a Datsun Fairlady 2000, around the slower corners. It was driven by a bloke I raced Formula 1s against, so no dill himself.

AER

930 posts

198 months

Sunday 30th July 2017
quotequote all
Max_Torque said:
2) The difference height between the instantaneous roll centre height and the height of the vehicles Centre of Gravity. It is this "couple" that determines how much weight gets transferred from end to end, or side to side.
I disagree with this bit. Roll centres have zip to do with weigh transfer. It's entirely dictated by CG height above the tyre contact patch and track/wheelbase depending on whether roll or pitch

Kawasicki

5,088 posts

163 months

Sunday 30th July 2017
quotequote all
AER said:
Max_Torque said:
2) The difference height between the instantaneous roll centre height and the height of the vehicles Centre of Gravity. It is this "couple" that determines how much weight gets transferred from end to end, or side to side.
I disagree with this bit. Roll centres have zip to do with weigh transfer. It's entirely dictated by CG height above the tyre contact patch and track/wheelbase depending on whether roll or pitch
Have a read...

http://racingcardynamics.com/weight-transfer/


AER

930 posts

198 months

Sunday 30th July 2017
quotequote all
Kawasicki said:
AER said:
Max_Torque said:
2) The difference height between the instantaneous roll centre height and the height of the vehicles Centre of Gravity. It is this "couple" that determines how much weight gets transferred from end to end, or side to side.
I disagree with this bit. Roll centres have zip to do with weigh transfer. It's entirely dictated by CG height above the tyre contact patch and track/wheelbase depending on whether roll or pitch
Have a read...

http://racingcardynamics.com/weight-transfer/
Unless we're talking about 2CVs with laden roof racks, it is a very subtle secondary effect, mainly because the sum of Z_RC and h_s doesn't change dramatically and more-or-less sums to the same value as h in the end



Most of the value of this analysis is to demo that changing the front-rear roll stiffness bias can be used to influence how weight shift can be manipulated front-rear

Of course, the higher the sprung (and total) CG is, the more influence roll stiffness and the ratio thereof can have in the steady-state wheel loadings.

Mr2Mike

18,810 posts

183 months

Sunday 30th July 2017
quotequote all
Kawasicki said:
That basically backs up what AER is saying, total weight transfer is dominated by COG height and track width (for lateral transfer) which is why it's very difficult to change it once designed in.

Kawasicki

5,088 posts

163 months

Sunday 30th July 2017
quotequote all
Mr2Mike said:
Kawasicki said:
That basically backs up what AER is saying, total weight transfer is dominated by COG height and track width (for lateral transfer) which is why it's very difficult to change it once designed in.
AER said that roll centres have nothing (zipp) to do with weight transfer. That is not an 100% accurate statement.

AER

930 posts

198 months

Sunday 30th July 2017
quotequote all
Kawasicki said:
That is not an 100% accurate statement.
Engineering is about useful approximations rather than pedantry.

Kawasicki

5,088 posts

163 months

Sunday 30th July 2017
quotequote all
AER said:
Kawasicki said:
That is not an 100% accurate statement.
Engineering is about useful approximations rather than pedantry.
yep, but sometimes engineering is also about small details making the difference between a pass and a fail. I have tuned and tested high CofG vehicles to avoid roll over, roll centre height did have an influence...plus it was a tuneable parameter, it is tricky to change track width and CofG height.

NJH

2,798 posts

137 months

Sunday 30th July 2017
quotequote all
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.

Krobar

269 posts

35 months

Sunday 30th July 2017
quotequote all
Kawasicki said:
I've gotten front and mid engined cars seriously out of control thousands of times and the stability control did an excellent job of stabilizing the car.
I've fish tailed a mid engine car in the wet; those normally useful electronic aids did not help.