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Conundrums

Ifs, buts and maybes....

by Julian Edgar

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This article was first published in 2009.

Be interested in cars long enough and you’ll hear plenty of ‘self-evident truths’. Trouble is, I am not sure – whatever the books say – that many of these are in fact truths, let alone self-evident. So let’s have a look at ideas many people believe to be unambiguously true, but which often aren’t...

Bore / Stroke Ratio

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Heaps and heaps and heaps of people will tell you that a ‘long stroke’ engine will make plenty of torque. It’s self evident, you see – the longer the stroke, the greater the throw of the crankshaft for a given capacity. So the piston has more leverage over the crank – ipso facto, more torque.

Some journalists believe this to such an extent that in a road test they’ll first talk about the engine’s bore / stroke ratio – and what that apparently means. An ‘over-square’ engine is good for revs; a long-stroke engine is good for torque.

But then you’ll find people who believe just the opposite. The other day I was in a workshop that had apart an ancient Volkswagen Kombi engine. I was looking at the air-cooled flat four, the cylinder barrels removed and the pistons plain to see.

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“Wow”, I said, “look at how large the pistons are!”

“Yes,” said the workshop proprietor, “that’s why these engines make such good torque.”

And of course, for their capacity, these old Volkswagen engines are low revving, high torque designs.

But hold on, the VW engine uses a short-stroke, over-square format. And it makes good torque?

And if you look around, you’ll soon find plenty of engines that are short stroke with high torque, or long stroke with low torque. I figure that in fact as you reduce piston diameter, the force that can be brought to bear by the expanding combustion gases also decreases, offsetting the increase in stroke that applies more leverage.

I haven’t done the maths to prove that, but I strongly suspect that the relationship between torque and bore/stroke ratios is largely fanciful...

Wet Weather Handling

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Read any suspension textbook and the author will talk about lateral weight transfer in cornering. If it’s an in-depth book, they’ll describe the height of the front and rear roll centres, they’ll cover the suspension components that control roll stiffness (springs, dampers, anti-roll bars) and they’ll talk about wheel geometries.

But what they very rarely describe is wet weather handling.

Every race car driver knows that when the track turns wet, the adjustable anti-roll bars are set to softer rates. That is, the suspension is configured to deliberately allow more roll. Ask the driver why this works, and they’ll say something like “It gives the tyres better bite,” a statement that doesn’t really make a lot of sense – why wouldn’t that bite work better in dry conditions too?

But it’s true – on wet (and bumpy) roads, suspension that is softer in roll (and wheel rate) works far better.

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I’ve had an opportunity to asses this on a very different type of machine. On my recumbent, pedal-powered trike, I run long travel, very soft suspension. Plunging for the first time down a steep, wet, mountain road at night, I couldn’t believe how sure-footed the machine was. I can compare that to a commercial Greenspeed recumbent pedal trike, one that has suspension through only frame flex. In comparison, it’s as skittish as hell. But on a dry road, the Greenspeed is nimble and responsive, my suspension trike feeling lethargic and slow to respond.

It’s easy to come up with theories that explain why softer suspension works well in wet weather – and then it’s just as easy to come up with theories that show why this certainly cannot be the case!

But you can be sure that suspension companies that sell you stiffer springs and stiffer anti-roll bars never mention any of the trade-off – something to always keep in mind when sourcing suspension upgrades.

Bass and Treble

Listen to a nondescript car sound system and in many cases, you’ll say: “Hmm, needs better treble - and better bass as well.”

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But the odd thing is, if you improve just the bass – eg by using an add-on subwoofer, driven by an additional amp powered from the original head unit – you’ll often find that suddenly, the whole system sounds great: the missing treble has apparently appeared!

There’s no technical answer to the conundrum – the subwoofer isn’t slyly producing better treble. It’s just that your perception of the quality of sound is dramatically altered by better bass.

And there’s another, similar, trap to fall into. If you’re comparing different speakers, it’s very likely that their efficiencies will also vary. That is, for a given amplifier setting, the sound from one pair of speakers will be louder than from the other pair. And you know what? If the sound quality is actually similar, people will always suggest that the louder speakers are superior in quality to the quieter speakers!

Free-flow Intakes

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Free-flow intakes make sense – at maximum throttle, more air will get to the cylinders, better filling them and so resulting in an increase in potential power. Another advantage is that pumping losses are reduced – that’s the work the engine does in drawing in the air. And the more restrictive the intake, the greater the pumping losses.

But that’s at full throttle.

At part throttle, most of the pumping losses occur when the engine is trying to drag air past the partially closed throttle. The fact that there’s a free-flowing intake in front of that throttle should count for nearly nothing – after all, the throttle itself is making up nearly all the restriction!

So it makes perfect and logical sense to say that a free-flow intake should make no difference to cruise fuel economy. After all, with the throttle shut so far, where are the improved efficiencies to come from?

I don’t know – but I have seen it time and time again on a whole variety of different cars. A good intake system, one that decreases the restriction ahead of the turbo or throttle body, makes a measurable improvement to fuel economy...

Feedback

How a car feels to drive depends on a huge variety of factors. We sense the power of acceleration, the strength of the braking, the amount of body roll, and the loudness of noise.

But there are a lot more subtle signals than these. We also feel the weight of the steering, unconsciously assess the stiffness and length of throw of the gear lever, sense the heaviness of the brakes – and so on.

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Increase the weight of light power steering and you can be dead-set convinced that the car understeers less and has better stability at speed on narrow roads. Fit a steering wheel with a thicker rim and you’ll be certain that the steering is improved in ratio and feel. Install seats with better lateral support and you’ll reckon that not only is the steering better, but that the car rolls less when cornering!

And each of these is a personal thing: some people with small hands find that thick-rimmed steering wheels make them feel that they have less control – they can’t close their fingers around enough of the rim diameter. Others prefer a very soft brake pedal; still others like feather-light steering.

Many people who take an immediate dislike to a car do so because an unidentified aspect – like steering wheel rim thickness, or gear lever travel, or even rebound damping stiffness – is not to their liking. But ask them what they don’t like about the car and they won’t be able to tell you – they just don’t like it.

And the opposite also applies. To improve your car to make it better suit you is in part an exercise in getting these seldom thought about, almost-intangibles, right.

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In one of my cars, I lengthened the gear lever, so increasing throw but also reducing gear-change stiffness. In another, I lightened the power steering at slow speeds and made it radically heavier at high speeds. In another, I added a heap of side support padding in the seat back (but not much in the seat base), while in that same car I modified the engine management to provide much better throttle response at just a few per cent throttle – something many people would care nothing about.

So you can see what some regard as ‘self-evidently good’ (like short-shift gear levers!) others regard as anything but. Ah, the conundrums!

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