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WhoseGeneration

4,090 posts

93 months

[news] 
Friday 4th May 2012 quote quote all
S. Gonzales Esq. said:
Could the 'new ones on the front' contributors please watch this video (skip to 1m40s if you're in a rush) and get back to us? Thanks.
Yes but, that was the USA, they only ever drive on straights don't they?

Zero 1

63 posts

30 months

[news] 
Friday 4th May 2012 quote quote all
S. Gonzales Esq. said:
Could the 'new ones on the front' contributors please watch this video (skip to 1m40s if you're in a rush) and get back to us? Thanks.
No disrespected intended with the comment I'm about to make sir (I'm being serious), but if you believe that you're terribly misinformed and you need to spend some time on the track with some professionals or attend a performance driving school.

Again, that video is aimed towards people who have a very low level of experience / understanding of driving physics.

http://www.youtube.com/watch?v=Pc8pawmfcQ0

Walter was driving a 911 GT3 on Pilot Cup Sports in this video...which are probably one of the worst tires front or rear a person can drive on in the rain. Combine that with rear engine which is extremely difficult to recover in a slide, and 400 + hp through the rear wheels and the people in the video you posted would have crashed into the guardrail in the first turn.

Take each of the corners he counter steers (oversteer) and replace them with understeer. He would have been off the grass or into the wall before the second passenger even hopped in for a ride.

JUST BECAUSE THE GENERAL PUBLIC DOES SOMETHING ONE WAY, DOES NOT MEAN IT IS THE CORRECT WAY TO DO IT.

FOR EXAMPLE:
-Here in California they are trying to run a series of ad campaigns encouraging motorists who use full synthetic oil to change their oil AFTER 15,000 MILES!

How many people here would go 15,000 miles between an oil change?
I attended a school with several teachers who were responsible for building Nascar Cup engines that won races for some of the biggest names in the industry. Now I really hate nascar and everything it stands for, but you couldn't pay these guys to change the oil in their daily drivers after 15,000 miles.

Whos advice would you take about your engine: A guy whos built cup car motors for the last 30 years or a group of politicians / environmentalists?

rolleyes







Edited by Zero 1 on Friday 4th May 03:54


Edited by Zero 1 on Friday 4th May 03:55

GravelBen

11,329 posts

116 months

[news] 
Friday 4th May 2012 quote quote all
It appears to be the same basic test as the 5th gear vid - smooth surface, constant water depth and gradually increase speed until you find the point where the tyre with less tread (and on the rear, less weight on it in this case) will aquaplane but the tyre with more tread won't.

For a more real-world test I would suggest finding a puddle, driving towards it at varying speeds and seeing if the car aquaplanes at (a)one end (b)both ends or (c)not at all. That form of test would at least let you identify how narrow that window of one-end-aquaplaning is. Possibly wrt depth 1mm difference in tyre tread might ~ 1mm of water depth able to be cleared, but how it varies with speed would be interesting.

To stir things up a bit more, if the water is beyond that depth window where only one end will aquaplane, the front end will always aquaplane first as it hits the puddle first.

Dave Hedgehog

7,311 posts

90 months

[news] 
Friday 4th May 2012 quote quote all
GravelBen said:
It appears to be the same basic test as the 5th gear vid - smooth surface, constant water depth and gradually increase speed until you find the point where the tyre with less tread (and on the rear, less weight on it in this case) will aquaplane but the tyre with more tread won't.

For a more real-world test I would suggest finding a puddle, driving towards it at varying speeds and seeing if the car aquaplanes at (a)one end (b)both ends or (c)not at all. That form of test would at least let you identify how narrow that window of one-end-aquaplaning is. Possibly wrt depth 1mm difference in tyre tread might ~ 1mm of water depth able to be cleared, but how it varies with speed would be interesting.

To stir things up a bit more, if the water is beyond that depth window where only one end will aquaplane, the front end will always aquaplane first as it hits the puddle first.
the reality is because of the nature of UK roads and speed limits there are very few tight fast corners (a few places where M ways join) the majority are gentle curves at the speed limit

where most people will have problems are hitting pools of water and emergency stopping (straight lined the vast majority of the time) where front tread depth is far more important and emergency avoidance where if you understeer you just wont be able to avoid the obstacle, and oversteer your gonna spin

crocodile tears

755 posts

32 months

[news] 
Friday 4th May 2012 quote quote all
its a difficult decision but if you're going to be 'pushing' the car (especially in the wet) be prepared for the rear of the car to step out with or without lifting off




- if you drive balls out - put the new tires on the front
(if you genuinely drive balls out you will be crapping yourself in no time)

- if you care more about safety - put the new tires on the back.
(no scary back end flying out and its easier to feel the limits of the front than the rear)

Edited by crocodile tears on Friday 4th May 12:55

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davepoth

23,357 posts

85 months

[news] 
Friday 4th May 2012 quote quote all
Zero 1 said:
Tire compound and tread levels affect braking more than any upgrade of rotors or pads. Think about it like this:

The front of a car is responsible for 70-80% of the load under heavy braking. If all that weight transfers to the front of the car and you have worn out tires at the front, and brand new rubber at the back, how well do you think the car is going to stop?
Gently.

Zero 1 said:
Now if you remember the original posters question was about a front wheel drive car, you are asking the front of the car on worn tires to handle the braking load, acceleration load (longitudinal) AND cornering loads (lateral). It is simply too much for the tire. Physics are physics.

The terms are coefficient of friction- Measure of the ratio at which a tire converts download to traction

and

Slip angle- difference between the direction the wheel rim is pointing and the direction the tire is traveling. This difference is reffered to as the slip angle. Tires have a range of slip angle where they deliver their maximum level of cornering traction.

Street tires tend to work over a wider range of slip angles and produce lower coefficients of friction. Slick tires (racing tires) tend to operate in a narrow band of slip angle and produce high coefficient of friction but drop off much faster once you exceed the limit.

Street tires tend to break away progressively so they are easier to control.

The bottom line is without tread you are asking the tire to do too much and you are exceeding it's capabilities of traction. There for you want more traction at the front ESPECIALLY on a FWD car where the tire is responsible not only for steering, but acceleration AND correcting the car once it has gone into an oversteer condition...
If the good tyres are on the back the car won't go into an oversteer condition unless provoked. Therefore your point it moot.

Dr Mike Oxgreen

1,704 posts

51 months

[news] 
Friday 4th May 2012 quote quote all
My thoughts are that it's a massive over-simplification to say that new tyres should always go on the back.

For me, it depends how much tread is remaining on the rear tyres. If the rears have still got a reasonable amount of meat on them (more than 4mm, for example) then I don't see the point of putting the better tyres on the rear - with reasonable meat on them, the rear tyres are not going to step out on you unless you really provoke them.

I wouldn't put new tyres on the front if my rears are getting near the end of their lives. In that case, I would consider swapping things around and putting new ones on the back until the others are due for replacement.

In a nutshell, for a FWD car I prefer to have the better tyres up front, but not if that results in a large difference between front and rear.

Zero 1

63 posts

30 months

[news] 
Friday 4th May 2012 quote quote all
davepoth said:
If the good tyres are on the back the car won't go into an oversteer condition unless provoked. Therefore your point it moot.
No car will oversteer unless provoked. At normal road speed it's pretty hard to make a car oversteer. My daily driver is a toyota truck with no weight at all on the back of the car. It will go from a push condition to snap oversteer in a heartbeat, but only when PROVOKED. But hey what do I know?

I'm speaking from over 10 years of racing experience across multiple different platforms and disciplines and am speaking on behalf of people who have raced professional and won multiple national championships and somehow my point is still not getting across. I live and drive in Los Angeles which is one of the busiest cities in the world when it comes to traffic.

A bit more sheep on pistonheads than I originally thought...



HustleRussell

6,462 posts

46 months

[news] 
Friday 4th May 2012 quote quote all
Zero 1, but how is all this talk about racing relevant in any way to a road car with road tyres?

davepoth

23,357 posts

85 months

[news] 
Friday 4th May 2012 quote quote all
Zero 1 said:
davepoth said:
If the good tyres are on the back the car won't go into an oversteer condition unless provoked. Therefore your point it moot.
No car will oversteer unless provoked. At normal road speed it's pretty hard to make a car oversteer. My daily driver is a toyota truck with no weight at all on the back of the car. It will go from a push condition to snap oversteer in a heartbeat, but only when PROVOKED. But hey what do I know?

I'm speaking from over 10 years of racing experience across multiple different platforms and disciplines and am speaking on behalf of people who have raced professional and won multiple national championships and somehow my point is still not getting across. I live and drive in Los Angeles which is one of the busiest cities in the world when it comes to traffic.

A bit more sheep on pistonheads than I originally thought...
You're still wrong. If the car loses grip, it loses grip. As a driving god you will probably save it with a leery drift, and then be kissed by many pretty girls. However, you are an aberration.

I know what to do if the back slides out because I've driven like an idiot in snowy car parks, and done other such "hoonage"; I'm not a drift king by any stretch but I know what oversteer feels like and can at least have a go at correcting it. That probably puts me into a very small subset of the population.

For the 99% however, the first thing that happens when the rear end of the car slides is blind panic. They will likely attempt an emergency stop (clutch and brake to the floor), which is almost certainly the worst thing to do when the back is about to swing round, and applying opposite lock is not on the driving test of most nations so will not be a natural reaction.

I'll reiterate the point again, in bold. The natural reaction of the vast majority of the population if any of the wheels of a car lose traction is to brake and continue to steer into the corner. With the good tyres on the back the slide will be on the front wheels, and braking and steering will not make the slide any worse, and as the back wheels still have purchase they will be still slowing the car down until such point as the lateral G reduces sufficiently for the front wheels to regain traction.

With the good tyres on the front the slide will be on the back wheels, and the braking and steering input that 99% of people will use will make the slide worse.

Even if the slide isn't recoverable, it's preferable to hit whatever you are about to hit head-on (understeering into a tree) rather than side or rear on (oversteering into a tree), since that's where the best impact protection is on a car.

BertBert

8,289 posts

97 months

[news] 
Friday 4th May 2012 quote quote all
Zero 1 said:
Take each of the corners he counter steers (oversteer) and replace them with understeer. He would have been off the grass or into the wall before the second passenger even hopped in for a ride.
I think you are barking up the wrong tree. Just because Walt is a driving genius and can handle oversteer doesn't make an argument that oversteer is the safer way on the road. You are not thinking about what the recommenders of better tyres on the rear are saying. They are saying that "it is safer to put the better tyres on the rear". Not faster round a track in the wet with WR at the wheel, but safer.

I can think of a few reasons why this might be. There are going to be situations of cataclysmic loss of grip and situations of marginal loss of grip. I think that safer is going to be getting to marginal loss of grip and slowing down as you realise that you lost grip. So understeer is a very good way indeed of giving that warning. As I said before, people can easily deal with that situation as they lift off and the car sorts itself out without any counter steer needed. That has to be "safer".

Even on track what happens? You know this as you are an expert. If when you push, the front starts washing out, that limits your speed and you just go slower (even though it's an affront). If you are slipping at the front, you just ease off and it sorts itself out. So on track a car with understeer characteristics is safer.

BB

GravelBen

11,329 posts

116 months

[news] 
Friday 4th May 2012 quote quote all
Obviously (as reiterated by the last twop posts) the blanket 'on the back' advice is fine for Joe Average Numpty-driver who doesn't want to think about driving, but a competent driver should be able to make their own decision without being subjected to a mass bleat of expectation to do the same. Personally I will continue to put tyres on the end that gives the best results.

Edited by GravelBen on Friday 4th May 23:48

Zero 1

63 posts

30 months

[news] 
Saturday 5th May 2012 quote quote all
davepoth said:
You're still wrong. If the car loses grip, it loses grip. As a driving god you will probably save it with a leery drift, and then be kissed by many pretty girls. However, you are an aberration.

I know what to do if the back slides out because I've driven like an idiot in snowy car parks, and done other such "hoonage"; I'm not a drift king by any stretch but I know what oversteer feels like and can at least have a go at correcting it. That probably puts me into a very small subset of the population.

For the 99% however, the first thing that happens when the rear end of the car slides is blind panic. They will likely attempt an emergency stop (clutch and brake to the floor), which is almost certainly the worst thing to do when the back is about to swing round, and applying opposite lock is not on the driving test of most nations so will not be a natural reaction.

I'll reiterate the point again, in bold. The natural reaction of the vast majority of the population if any of the wheels of a car lose traction is to brake and continue to steer into the corner. With the good tyres on the back the slide will be on the front wheels, and braking and steering will not make the slide any worse, and as the back wheels still have purchase they will be still slowing the car down until such point as the lateral G reduces sufficiently for the front wheels to regain traction.

With the good tyres on the front the slide will be on the back wheels, and the braking and steering input that 99% of people will use will make the slide worse.

Even if the slide isn't recoverable, it's preferable to hit whatever you are about to hit head-on (understeering into a tree) rather than side or rear on (oversteering into a tree), since that's where the best impact protection is on a car.
I'm in agreement with you here 100% I was just trying to make the point that just because a majority of people do something it doesn't mean that is the correct way.

I do think that driver training in the US (not sure about the UK?) is completely ridiculous and we should be more like Finland. If everyone knew how to handle oversteer I wonder what the fatality rate on roads would be reduced by.

Just trying to give an alternate point of view from experience to consider.

Zero 1

63 posts

30 months

[news] 
Saturday 5th May 2012 quote quote all
crocodile tears said:
its a difficult decision but if you're going to be 'pushing' the car (especially in the wet) be prepared for the rear of the car to step out with or without lifting off




- if you drive balls out - put the new tires on the front
(if you genuinely drive balls out you will be crapping yourself in no time)

- if you care more about safety - put the new tires on the back.
(no scary back end flying out and its easier to feel the limits of the front than the rear)

Edited by crocodile tears on Friday 4th May 12:55
Thats a good way to put it. I tend to be the kind of person where if it rains I'm firing up my 125cc kart to race around the block. I used to go up in the mountains when it would rain to practice drifting in my Miata and collect telemetry.

For the average road user I would still advise putting them on the front though since on fwd your steering AND drive wheels need the grip, but if your not used to driving hard I think it's more mental than anything.

If were talking about aqua planing then really you should just replace all of your tires. A few hundred of your British pounds are worth more than putting anyones life at risk any day.





Nigel_O

Original Poster:

398 posts

105 months

[news] 
Saturday 5th May 2012 quote quote all
GravelBen said:
Obviously (as reiterated by the last twop posts) the blanket 'on the back' advice is fine for Joe Average Numpty-driver who doesn't want to think about driving, but a competent driver should be able to make their own decision without being subjected to a mass bleat of expectation to do the same. Personally I will continue to put tyres on the end that gives the best results.
YAY! - it would appear that after 6 pages of "deliberation", we have our answer (which is what I thought all along, but as the subject sparked a similar discussion elsewhere, I thought it would be entertaining and informative to repeat it on PH)

So - I'll continue to do what I've done for years - when a pair of tyres wear out, I'll simply replace them on whatever axle they came from, in the (relatively) safe knowledge that I can handle a bit of lift-off oversteer and any amount of understeer (which with 400+bhp through the front wheels, I tend to get a fair bit of in the wet...)

AnotherGareth

165 posts

60 months

[news] 
Monday 7th May 2012 quote quote all
Nigel_O said:
I can handle a bit of lift-off oversteer
What will you be doing if lift-off isn't the cause of the oversteer?

powerstroke

2,819 posts

46 months

[news] 
Monday 7th May 2012 quote quote all
AnotherGareth said:
Nigel_O said:
I can handle a bit of lift-off oversteer
What will you be doing if lift-off isn't the cause of the oversteer?
I would guess turning the steering wheel thingy in the direction of travelscratchchin

Nigel_O

Original Poster:

398 posts

105 months

[news] 
Monday 7th May 2012 quote quote all
AnotherGareth said:
What will you be doing if lift-off isn't the cause of the oversteer?
Unlikely, in a FWD car.

Zero 1

63 posts

30 months

[news] 
Tuesday 8th May 2012 quote quote all
Nigel_O said:
Unlikely, in a FWD car.
"OmG WhY ArE YoU TAlKinG abOut OvArStuRRR?" spin

Nigel_O

Original Poster:

398 posts

105 months

[news] 
Tuesday 8th May 2012 quote quote all
Zero 1 said:
"OmG WhY ArE YoU TAlKinG abOut OvArStuRRR?" spin
er.....

I repeat a point I've made before - most 'ordinary' modern cars (even RWD and 4WD) are set up to understeer and simply cannot oversteer unless they are seriously provoked or are dangerously / intentionally defective.

The only way to get a FWD car to oversteer for ANY reason other than lift-off is to have rear tyres that are almost slick or geometry that's seriously skewed (such as weird toe-out angles, barmy spring rates or over-high tyre pressures)

Yes, I know that FWD race cars such as BTCC are set up to have a 'flighty' rear end, but this is by design as the drivers are generally highly skilled and able to handle a bit of oversteer. Even so, I'm struggling to recall an instance of a FWD BTCC car oversterring without reason (usually because of a nudge or drifting off-line).

My own FWD car is set up to be fairly pointy (uprated rear ARB, increased spring & damper rates, increased negative camber on the front, 2.2 turns lock-to-lock rack) and it was already regarded as a fairly good FWD chassis. However, I cannot get it to oversteer without provocation and never when on the power. In the wet, I reckon the only way that new tyres on the front and worn tyres on the rear would cause oversteer is in standing water and with the rears well below the legal limit (which can hardly be the fault of putting the new tyres on the front)
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