Hoodies

[magic torch]

slippydiff said:

As in small skinny tyres tend to aquaplane less than big fat uns ?

Yes, skinny tyres don't have so far for the water to travel across the width of the tyre.

Braking is largely determined by the co-effiecient of friction between the tyres and the road, and how quickly you can get the wheels to the ideal slippage to be able to stop.

Edited by magic torch on Tuesday 31st October 18:59

[BliarOut]

johnfm said:

Sorry guys - you are mistaking 'conventional wisdom' for 'cold, hard science'. In terms of stopping distance, -1g is pretty achievable by most cars. Contact patch has very little to do with it, as the retarding force acting at the tyre/road interface is a function of the coefficient of friction and the normal load. THe load is independent of contact area (this merely alters the pressure exerted by the the tyre on the road). Tyre compound and road surface are greatest influence. Though you would think that car mass would be really important too (as the kinetic energy of the vehicle is directly proportional to it), but a lighter car exerts less normal load onto the tyre - so less available grip.

Where Porsche brakes shine is repeated braking - ie, fade resistance. But the 'hoodie test' is just bad science. Sorry!

Bad science, bad science? How bally dare you. I'll have you know there was no science involved whatsoever, it was just good old fashioned hooliganism and damn good fun to boot

[doctorpepper]

sleep envy said:

if the hood doesn't cover your head you're not braking hard enough

Isnt there a law that requires them to be up all the time anyway??

zcacogp said:

johnfm said:

Just to be a boring science person, most cars are capable of the same braking g - its the tyre/road interface that is the limiting factor for braking g. If you look at most detailed car tests, braking times and distances are very, very similar for most performance cars...

I don't doubt you, but if this is the case, then what's the point of upgrading brakes?


Oli.

The correct reason is to disapate(sp?) heat more effectivly...

[speedlimit]

I thought a hoodie was a spooty 14 year old, did not know about the string. My wife who is American assumed it was a foreskin!
Must get out more.

[zcacogp]

johnfm said:

Tyre compound and road surface are greatest influence.

This is assuming that the weakest link in the braking process (/system) is the grip the tyre has on the road - i.e. your vehicle brakes can out-brake the grip that the tyre has.

Is this always the case? My experience with powerful cars and powerful brakes is non-existant (so this will be argued from a theoretical armchair-science point of view) but I would argue that in some cars this is not so. If you push the brake pedal as hard as you can at a given speed, and the vehicle does not skid (or the ABS does not cut in) then surely the brakes (pads/disks) are not exerting enough force to break the traction of the tyres, therefore there is more grip available, therefore the weakest part of the system is the disks/pads. If, however, you can make the ABS cut in at any speed then, conclusively, your disks/pads can produce more retardant force than the tyres can exert grip, and you are into the realm of needing to improve your tyres (or to drive on stickier roads.)

I can appreciate that things change when the system hots up - your tyres will become softer and grip better, and your pads and disks will heat up and work less well (usually, unless you have fancy exotic pad material). Therefore, the answer to the question 'Do I need better brakes or better tyres?' will be 'How hard to you drive the car?'

The hoodie test is simply a visual illustration of the joys of hard braking. Same thing happens if you have harnesses installed and there is no-one in the passenger seat - you can watch the harness pointing forward when braking hard.


Oli.

[anonymous-user]

johnfm said:

Sorry guys - you are mistaking 'conventional wisdom' for 'cold, hard science'....Where Porsche brakes shine is repeated braking - ie, fade resistance. But the 'hoodie test' is just bad science. Sorry!

completely agreed, sometime the lack of engineering knowlege on car forums is odd.
most people mistake initial bite for 'good' brakes.
the real difference between cars is tyre compound and ABS programming.
obviously brake feel and fade resistance are a different thing.

sleep envy said:

So what you are saying is the same car travelling at x mph will stop in the same distance regardsless if it has 165 section or 205 section front tyres?

very basicly yes. in practice no (but by a MUCH smaller margin than you'd expect).

the basic model of friction is that the friction force is equal to the force pressing down on the tyre times by the coefficient of friction. press hard on the desk and try and move your hand, now press harder and try again. so a heavy car presses down harder, so has more grip, but more mass to stop. reduce the contact patch and you increase the force per area pressing into the road - more grip per area but same grip over all.

in practice this falls over because tyres dont behave according to simple friction. they deform into the road (hence why greater than 1g cornering/braking is possible on soft compounds). its intuitive that a 335 section tyre will stop a car better than a bicycle tyre. but back to reality a 225 will only stop very marginally better than a 205.

a soft compound 225 on a porsche will always stop better than a crappy 195 taiwanese remould on a primara lurching all over the road. but it will not be by as much as you think - put the same soft compound on the primara and it will be very marginal.

Edited by francisb on Wednesday 1st November 11:50

[BliarOut]

francisb said:

johnfm said:

Sorry guys - you are mistaking 'conventional wisdom' for 'cold, hard science'. In terms of stopping distance, -1g is pretty achievable by most cars. Contact patch has very little to do with it, as the retarding force acting at the tyre/road interface is a function of the coefficient of friction and the normal load. THe load is independent of contact area (this merely alters the pressure exerted by the the tyre on the road). Tyre compound and road surface are greatest influence. Though you would think that car mass would be really important too (as the kinetic energy of the vehicle is directly proportional to it), but a lighter car exerts less normal load onto the tyre - so less available grip.

Where Porsche brakes shine is repeated braking - ie, fade resistance. But the 'hoodie test' is just bad science. Sorry!

completely agreed, sometime the lack of engineering knowlege on car forums is odd.
most people mistake initial bite for 'good' brakes.
the real difference between cars is tyre compound and ABS programming.
obviously brake feel and fade resistance are a different thing.

(although for completeness tyre/road friction is not the simple (Fr=mu x N) friction equation as the tyre deforms into the rough road - hence why >1g cornering/braking is possible on soft tyres)

Are you questioning my apprentiship????

It was a bit of FUN

[fidgits]

I personally think the 911 brakes are evil....

Picked up a rental car (US) a week or so ago - nearly went through the windscreen when i braked for the barrier to get out of the rental centre...

cue a week later - driving out of heathrow - pull up to roundabout and go for the brakes.... nothing happened!!!


Or possibly, over-servo'd crappy american brakes are evil and the 911's are angelic... im not sure...

[anonymous-user]

BliarOut said:

Are you questioning my apprentiship????

It was a bit of FUN

no. what aprentiship?

i know. my post was less fun than yours for which i appologise profusely.

[BliarOut]

francisb said:

BliarOut said:

Are you questioning my apprentiship????

It was a bit of FUN

no. what aprentiship?

i know. my post was less fun than yours for which i appologise profusely.

And I humbly accept

It was just an observation about how much fun you can have with Pork brakes on the limit...

I know what Fidg means about the servo effect.... I always nearly stood my over servo'd E30 on it's nose every time I swapped cars