Common Automotive Misconceptions and Mistakes

[old-git]

Being the self styled resident pedant, I have noticed that there are many mistakes, untruths and misconceptions regularly stated on this forum.

I am sure that the writers are not aware of their errors or sloppiness so, in a hopefully lighthearted way, let's see if we can correct some of them so that we can all become better informed and sound less ignorant in the pub and on line

I'll start it off with my two favourites:

There is a difference between spoiler and wing. Learn what it is so you don't sound like an idiot when talking to those who do know the difference

In the UK, they are dampers. The spring is the shock! (unless you are American)

 

[MasterAuron]

Re: Common Automotive Misconceptions and Mitakes

I for one am guilty of calling them shocks despite knowing full well they are dampers. A habit I just can't get out of. "Springs and shocks" just rolls off the tongue so much easier...

 

[old-git]

Re: Common Automotive Misconceptions and Mitakes

I for one am guilty of calling them shocks despite knowing full well they are dampers. A habit I just can't get out of. "Springs and shocks" just rolls off the tongue so much easier...

Practice, practice, practice.

But at least you know what they should be called, so I will let you off

If you were to correct yourself, would you say springs and dampers or shocks and dampers, as both are technically correct (in the UK)? The latter would be more fun as it would confuse both the unwashed and the Americans

 

[Loz]

Can someone clarify catbacks and decats for me please, I get the two confused!

 

[pgarner]

de-cat is a pipe that does away with the cat

cat back is as said from where the cat is/was to the rear of the car

 

[Turboshaft]

"mag rims" for aluminum alloy wheels. i'm not that sure if this is used in uk though.

 

[old-git]

"mag rims" for aluminum alloy wheels. i'm not that sure if this is used in uk though.

Yes, it is.

 

[HDi fun]

The common misbelief that the number 5252 has anything whatsoever to do with bhp and torque.

Go and measure angular velocity in radians per second and torque in Kgfm. The number 5252 will not appear anywhere.

You simply cannot convert bhp figures to torque figures. They are different measurements and have different units.

It's like trying to convert kilograms to miles.

 

[pgarner]

goto disagree mate
2 top in google

http://www.nsxprime.com/FAQ/Miscellaneous/TorqueHPSpeed.htm
http://www.pumaracing.co.uk/POWER1.htm

both give the same reason for the 5252 (actually its not 5252 but very close to it depending on how far you take pi)

 

[HDi fun]

What about a Paxman Valenta V12 with max bhp delivered at 1200rpm?

How does that work then?

The 5252 is only a ratio. It has no units. Definitely not RPM.

Measure power in kW and revs in degrees per hour then? 5252 won't appear.

Your Pumaracing quote does actually state that this only applies in the units are, bhp, lbft and rpm.

 

[coupe]

well that went over my head.

 

[HDi fun]

goto disagree mate
2 top in google

That's why it's a commonly held misconception

 

[old-git]

I used to think that it was a straight conversion. However, when you compare NA and turbocharged engines with the same bhp but vastly different lb/ft figures it doesn't compute.

 

[HDi fun]

I used to think that it was a straight conversion. However, when you compare NA and turbocharged engines with the same bhp but vastly different lb/ft figures it doesn't compute.

Engine type makes no difference. Torque and RPM are easily measured. Power (the rate at which work is done; the rate at which energy is dissipated) can only be calculated - it's impossible to it measure directly.

5252 has no significance, no relevance at all to engine revs. It's only a number, a ratio used to calculate bhp from known angular velocity (measured in revolutions per minute) and torque (measured in lbft).

If you know the engine's angular velocity at the crank (in rpm) and the torque output (in lbft) , then power can be calculated. But it only applies for that engine at that time, at that angular velocity.

The 5252 number has no units. It's has nothing whatsoever to do with the crossing of power and torque curves.

 

[old-git]

Engine type makes no difference. Torque and RPM are easily measured. Power (the rate at which work is done; the rate at which energy is dissipated) can only be calculated - it's impossible to it measure directly.

5252 has no significance, no relevance at all to engine revs. It's only a number, a ratio used to calculate bhp from known angular velocity (measured in revolutions per minute) and torque (measured in lbft).

If you know the engine's angular velocity at the crank (in rpm) and the torque output (in lbft) , then power can be calculated. But it only applies for that engine at that time, at that angular velocity.

The 5252 number has no units. It's has nothing whatsoever to do with the crossing of power and torque curves.

Right, I have just picked what remains of my brain cell off of the floor

What I was saying was that, from my empiric observations of bhp/lbft graphs, there seems to me to be no correlation between the two figures. One engine can be 'producing' 200bhp and 150lb/ft whilst another can have more torque than bhp, especially big blocks.

I think I will stick with Rolls Royce's answer when asked about power - The power is adequate for the purpose. If you can run 9 second quarters it doesn't matter whether you have 200 or 500 bhp or lb/ft

 

[old-git]

Just thought of another one:

Wider tyres give you more grip because you have more rubber in contact with the road - wrong. The area of rubber in conact with the road stays almost the same regardless of the width or diameter of the tyres.

 

[HDi fun]

Just thought of another one:

Wider tyres give you more grip because you have more rubber in contact with the road - wrong. The area of rubber in conact with the road stays almost the same regardless of the width or diameter of the tyres.

Agreed. The shape of the contact patch is more important.

The total surface area (of the contact patch) changes slightly, very slightly even if silly wide tyres are fitted.

 

[dpsubaru]

Just thought of another one:

Wider tyres give you more grip because you have more rubber in contact with the road - wrong. The area of rubber in conact with the road stays almost the same regardless of the width or diameter of the tyres.

dont agree with this as surely depends on many variables including the amount of pressure in the tyres. and as most people are too lazy to check their pressures once a week for the 'average joe' who will be running a psi level much below the required one the tyre surface could have more contact with the road if it is wider? dont know if this reasoning is correct - no doubt the difference is negligible but its still a difference?

 

[indiaman]

Right, I have just picked what remains of my brain cell off of the floor

I think I will stick with Rolls Royce's answer when asked about power - The power is adequate for the purpose. If you can run 9 second quarters it doesn't matter whether you have 200 or 500 bhp or lb/ft

I agree, it is finally all about power to weight ratio.

 

[suzsx041]

dampers and springs ???? dosn't a damper dampen the shock therefore its a shocker and not everyone here is in the UK

 

[dpsubaru]

shock short for shock absorder and believe a reference to the whole damper/spring assembly as not much chance of absorption if no spring.

 

[HDi fun]

dampers and springs ???? dosn't a damper dampen the shock therefore its a shocker and not everyone here is in the UK

I've always seen it that the spring decouples the cars mass from the wheel and road, but has an unwanted side effect called oscillation.

The damper damps the oscillation.

Here's another myth busted:-

CARS IN AUSTRALIA DO NOT NEED VELCRO ON THE TYRES AND ROAD SURFACE TO KEEP THEM FROM FALLING OFF

 

[coupe]

the fuel pump was invented in austrailia as gravity feed dosn't work upside down.

 

[turbonutter69]

the fuel pump was invented in austrailia as gravity feed dosn't work upside down.

 

[obi_waynne]

Another one or two. Bigger exhausts reduce backpressure therefore increasing performance! Discuss...

Running a rich fuel mix makes more power.

The shock revelations that shocks are springs and not dampers was a shock and many are still in suspense over this one. And those that wing it referring to spoilers as wings are spoiling it for everyone!

 

[old-git]

dampers and springs ???? dosn't a damper dampen the shock therefore its a shocker and not everyone here is in the UK

The spring absorbs the shock, therefore it is the shock absorber. As HD says, the damper controls, or dampens, the oscillations of the spring.

If you still think that the damper acts as a shock absorber, try driving a car with the springs removed and see how much shock absorbing the dampers do -)

 

[old-git]

dont agree with this as surely depends on many variables including the amount of pressure in the tyres. and as most people are too lazy to check their pressures once a week for the 'average joe' who will be running a psi level much below the required one the tyre surface could have more contact with the road if it is wider? dont know if this reasoning is correct - no doubt the difference is negligible but its still a difference?

To do a proper comparison we have to assume that the correct tyre pressures are being used, otherwise the patch size will vary even on the same sized tyres

If you run incorrect tyre pressures you will spread the load over a greater area but unevenly. Lower pressures will result in the outer edges carrying more weight so they get hotter and wear more quickly. Higher pressures have the opposire effect and the middle of the tyre wears quicker.

If you managed to spread the load evenly it would not increase your grip as the total pressure available to press the tyre to the road surface hasn't changed, it has just been spread over a larger area so each square inch of tyre is pressing on the road with less force.

So, anyone who thinks they can run quicker times up the strip by reducing their tyre pressures are deluding themselves

Fitting wider tyres reduces the length and increases the width of the patch in contact with the road but its overall area remains the same. This is because it is still carrying the same weight and the weight of the car determines the tread patch area not the size of the tyre.

By reducing the length of the patch you are reducing the percentage of the tyre's circumference in contact with the road at any one time. This allows the tyre more time to cool on each revolution. If a tyre stays cooler softer rubber compounds can be used thereby improving grip.

This doesn't apply when considering drag tyres as these work in a slightly different way.

 

[dpsubaru]

og, thanks for the detailed explanation. one question in regards to the point made:
"By reducing the length of the patch you are reducing the percentage of the tyre's circumference in contact with the road at any one time. This allows the tyre more time to cool on each revolution. If a tyre stays cooler softer rubber compounds can be used thereby improving grip." - doesnt a hotter tyre stay stickier and therefore give more grip so their must be a certain trade off between width and grip?

 

[indiaman]

old-git's explanation is the best I've seen in years, thanks.

 

[Turboshaft]

Another one or two. Bigger exhausts reduce backpressure therefore increasing performance! Discuss...

Running a rich fuel mix makes more power.

Actually if the exhaust is too big it will reduce power. There is a reason why intake ports are slightly larger than the exhaust ones. It is more important to make sure that the flow in the exhaust is as smooth as possible (by removing the cat, and other components that might restrict the flow) rather than increase the diameter of the pipe.

One of the explanations for this is that a large exhaust will have a slower gas flow (since the discharge is equal to area times velocity of gas) therefore the gasses will have more time to cool inside the pipe. The colder the gas the heavier it is and the harder it will be pushed out.



Now about the AFR:
The ideal AFR ratio is 14.7 This means that one part of fuel needs 14.7 parts of air to burn completely.If the mixture is lean , it means that the AFR will be lower therefore not all the fuel will burn so there will be no gains in power, BUT unburnt fuel plays an important role in cooling the cylinders and preventing detonations, so if you don't want to bother with reducing compression ratio or installing a water injection system , this is a good alternative. In fact it has been used with great efficiency in the f1 engines during the turbo era.


And please be kind and excuse my bad English.

 

[old-git]

og, thanks for the detailed explanation. one question in regards to the point made:
"By reducing the length of the patch you are reducing the percentage of the tyre's circumference in contact with the road at any one time. This allows the tyre more time to cool on each revolution. If a tyre stays cooler softer rubber compounds can be used thereby improving grip." - doesnt a hotter tyre stay stickier and therefore give more grip so their must be a certain trade off between width and grip?

Sorry, I should have expanded on this. Tyres have an optimum temperature range. Outside of this, quite narrow range, they don't work properly. However, if a tyre gets too hot permanent changes occur to the rubber compound affecting performance and reducing the life of the tyre.

When people fit wider tyres and claim better handling, what they are experiencing is the effect of widening the wheel track, not improved grip due to tyre width (assuming tyre compounds are the same) They would get the same improvement by fitting wheel spacers

A wider tyre could, in theory, reduce the available grip due to it running too cold

Wider tyres, IMO, look better and this is the main valid reason for fitting them.

Tyre width is not the way to improve handling. The old Lotus Elan, built nearly 40 years ago, had 5 1/2" wheels with tyres to match. However, it will still run rings around any hot hatch that has just been 'improved' by lowering and wider tyres. This is because the genius who designed it knew what he was doing - most of us haven't got a clue

I could go on as this is a very complicated subject as grip is not just down to laboratory coefficients of friction.

 

[HDi fun]

In response to OG's post, yes, I agree, fitting wider tyres does not automatically give better lateral grip.

There are too many factors involved to make any sweeping statements but car makers are pretty canny when it comes to suspension, weight distribution, wheel diameter, sidewall stiffness, ambient temperature, intended use etc. amongst myriad other contributory factors.

It's not to say that a minor upgrade in wheel diameter isn't beneficial. Our ancient little Ibiza has 13" rims. YES! Tiny

THe problem here is that the choice of quality tyres is limited. Ideally I'd fit a set of modest 14" steel rims just to afford me a better choice of quality tyres.

But to fit 21" rims to it would be ridiculous. It would ride about as well as a trolley jack (ie. rigid) and probably handle no better than a trolley jack.

I haven't bothered simply because I have managed to find a decent Michelin tyre which does very good job in what is a runabout basic car.

Suspension tuning and tyre selection are always built on a set of compromises.