Safe Speed Forums

You're very kind!

(but remember some of it could well be bo11ox)! Stuff I've picked up at work over the years but never actually bothered to verify - although it sounds plausible to me)!

yes let me know how that goes won't you

rear tyre's are the only thing stopping the back end swinging out ant any point in time.
if you lose pressure on the rear you lose that lateral stability and you'll suddenly find yourself in something like continuous oversteer.

the easiest demo for this is run a toy car down a book/desk/plank..... lock the front wheels with blue tack it keeps going straight.
lock the rear wheels with blue tack and it swaps ends.

Soz ed but have you got that the right way around?

I haven't got a toy car but in my mind's eye if you lock the rear wheels it would stay straight whereas if you lock the front the back end would try and catch up with the front. Locking the front wheels would have a similar effect to if you were pushing a pencil across the table it would try and move to the left or right.

If I yank the handbrake on it stops in a straight line unless I'm deliberately trying a handbrake turn.

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

yes thanks

Yes, Ed is quite correct, I remember doing this thirty years ago in a physics lesson.

Have you ever seen the chavs using McDonald's trays under the rear wheels? http://www.youtube.com/watch?v=DCqlJUhQhp4

It is down to the difference between static and sliding friction, a rolling tyre is using static[*] friction and a skidding tyre sliding friction which is lower. Think about it, if a locked tyre had more grip why do we have abs?

[*]I know it is not static friction in the classical sense, however for this argument it is close enough.

ETA - watch this one http://www.youtube.com/watch?v=aiUzOSILcBw&feature=related particularly what happens when he reverses about 17 seconds in.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

As a youth I worked on a dairy farm. Dairy farms have scraper tractors and scraper tractors scrape $hit off concrete, it is slippy and the machinery normally beaten within an inch of it's life.

Cue stunts . with independent rear brakes unlocked, proceed down the yard a a fair old lick. Now stamp hard on ONE brake pedal to lock the wheel. All of the drive now goes to the driven wheel and speed doubles , but the tractor wants to go sideways. Now simply steer into the skid to hold a straight line.

If I have a blow out on the back of my car, I still have drive, steering and brakes, but she will wriggle about at bit. I have riden 3 or more miles home from town with a flat on the back of my little bike, the back end was all over the place, but it largely went were it was pointed and I got home. My car as twice the number of wheels, and won't fall over.

I also once went to move a newish tractor, while moving it it became appart that something was a-foot. A tooth had come adrift in the front hub and was locking the planetary gears causing the wheel to lock. I could drag it backwards, but not push it forwards.

Blimey Toltec! Tray drifting in a physics lesson? Wish I'd gone to your school!

As with the others, I have to agree with Ed - although in running the car down a slope, are you you're "cheating" a bit in that the tractive force is coming from the centre of gravity - part way along the wheelbase? A typical front wheel drive Chavmobile in McDonalds' carpark would be "pulling" from the contact patches of it's front tyres, so might stay straight for a BIT longer.

The trouble, Tone, is that I think what you say would probably work IF:

1. the ground was perfectly smooth, level and of uniform grip.
2. the tractive force from the front was always great enough to "yank" the tail along behind it.
3. the tractive force could be completely evenly applied to both front contact patches (i.e. the car didn't have a diff).

As soon as you lift off (or the engine runs out of enough power to keep accelerating the thing, or one front wheel gets a smidgin more "bite" than the other one), the back end will overtake you. I think it's the same reason why people always THINK that if a front wheel drive car lets go at the back end for any reason, they can accelerate and it will just "pull" the car straight again. It never seems to work like that! Obviously, once the back end HAS started to move sideways - even by a small amount, you've had it!

Sorry for being a doubting Thomas, although you have given me hope that I haven’t gone completely mad yet Mole.

If we are talking about going in a straight line, then surely if the rear wheels are slowed or locked, (basically any braking force from the back), then the car isn’t going to veer off course - but that can’t be said of the front.


This is why I use the analogy of pushing a pencil, the equivilent of a large 'finger' pushing the front of the car. If I draw a straight line on a table and align the pencil with it and push with my finger without trying to correct the direction, (finger rigidly following the line just as front braking would in effect), it would try to veer off. That would be the same reaction as the front wheels on a car trying to decelerate as tyre differences and road irregularities etc. give the left or right an advantage which will start to turn the car, would it not?

Similarly, if the pencil was moving with a little engine of its own and you grabbed the rear of it, the equivalent of braking or locking the rear wheels, the pencil would simply come to a stop in a straight line. (Hec I've done it in cars often enough so I know that's correct, I think, I hope ).

Have the laws of physics changed. I know I stand to lose face here but I’m just trying to understand.

Infamy infamy! You’ve all got it infamy.

_________________
The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

But that's not in a straight line; that's effectively a hand brake turn What would happen if it was rear wheel drive and the trays were under the front tyres?


If they did that test with them on the back in a straight line, which is what ed was saying I think, you would simply come to a stop.

I've got no more toys to throw out my pram now

_________________
The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

hmmmm i'm not putting this very well.. and don't have much time to do so right now (if only lucy was here).

yes my assumption was if you'd had a blow out of any kind you might want to slow the vehicle down in a controlled manner, not rely on continually accelerating the thing where you wanted it to do (FWD)!

the toy car example is about cornering stiffness... or reacting lateral / sideways movement.. which a locked or blown tyre is severely compromised in doing.
if a front is blown it cannot provide any sideways force (not in the normal way anyhow), so the car won't turn just carry straight on.
if a rear is blown similarly it cannot provide any sideways force (which usually resists the back end from stepping out) so the back end is free to move around as it pleases.

as with all these things, sure if you know whats coming and react correctly maybe just maybe the less conventional answer is better.
but at the end of the day even as an experienced and trained driver i have little confidence that on a routine journey i'd be ready and able to catch the unexpected.... hence i'll take the easier to control option any day.

When slowed, yes, the car stays in a straight line. This is because the rear tyres still have some grip. When locked, they loose grip - and they can't loose it fore and aft but retain it side-to-side - they are either gripping or they're not!


OK, why would it veer off? If the table is flat and level, the surface has uniform grip and your finger really is pushing in a straight line...?

I think the answer is that the table is not infinitely flat and level and your finger can't push in a straight line. The pencil is inherently unstable - bit like trying to balance it on it's point vertically on your finger. In theory, if you kept your finger still, it would stay there. In practice, it falls off.

In both cases, the most absolutely minute external force is enough to start a change of direction, and once it starts going out of "true", you get a very rapidly increasing torque trying to turn it more so. Assume the whole side of the pencil is uniformly in contact with the table. The resistive force against which you're pushing acts through it's centre of gravity - which will be (discounting the pointy bit!) half way along its length. Assume that your finger is acting straight and true - in a manner it could never hope to do in real life. Something (a microscopic lump on the table, a bit of grease or polish on its surface, etc) causes the pencil to deflect sideways by a minute amount. In so doing, it's centre of gravity isn't exactly in line with the force your finger is exerting any more. As the pencil goes further out of line, that distance between the direction of the force your finger is exerting and the line paralell to it passing through the centre of gravity of the pencil gets bigger, so the effect becomes more pronounced. It VERY quickly runs away in a "vicious circle" and the pencil spins. It IS possible to keep it going in a straight line by constantly correcting the direction in which you're pushing (like a space rocket getting pushed by its motors).



Yes!


OK, we assume that te pencil is "front wheel drive"? It's engine is pulling it along and you apply a retarding force to the back. Yes, it will stop in a straight line, but that's because you're anchored to the ground independently of the pencil. OK, so let's put the front of the pencil on wheels and the back of it dragging along the desk to simulate your retarding force. If both front wheels pull EXACTLY together and evenly AND if the surface of the desk doesn't change (or the surface of the pencil in contact with it), I'd expect it to go in a straight line (as you have managed to do in a car). After all, the rear wheels don't have NO grip, just "reduced" grip. If you keep accelerating, I think there's every chance the car WILL move in a straight line (all other conditions being equal). It's being pulled from the front hard enough for the resistance provided by the car's inertia acting through it's centre of gravity, somewhere behind the front wheels, to keep the two forces more or less one behind the other. As soon as you start to lift off though (or worse, decelerate), or EVEN worse, brake, the situation is reversed and it ends up working like your pencil getting pushed from behind.

No they haven't. But one of the brilliant things about vehicle dynamics is that it's all dictated by a relativley few simple laws, several hundred years old, and yet as soon as you combine more than about two or three factors, it blossoms into staggeringly complext problems that sometimes even quite powerful computers struggle with!

Infamy infamy! You’ve all got it infamy. [/quote]

Of all the arguments I've heard on the subject, this is the only one which makes sense.

The tread is there for one thing only, shifting water. Most of the time the legal minimum is quite sufficient, the only time you would want more is going through deep standing water at speed in which case you are likely to be going straight, once the front wheels have gone through there isn't much for the rears to do.

All arguments about understeer/oversteer only apply to the track.

Get yourself a bicycle, find some damp grass, figure out which is the back brake, get some speed up, yank on the back brake so the rear wheel locks, see what happens. (let go of the back brake before you come off). Make sure it's the back brake you pull, you don't want to do what I did not knowing that most of the world has their brakes the wrong way round.

Which is the right way? And why cycle transmission on the right?

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

I can do that, on my motorbike too, as a way of turning 180 degrees. But I wouldn’t want to try locking the front wheel because I’m sure I would fall off a lot quicker which is why I would want the better tyres and grip on the front every time. Ask any biker which end he would rather start to lose grip and he’ll go with the back every time; it’s more recoverable. If the front starts to go the first thing you know about it is when you’re bum is on the ground, (Been there, done it – ouch!) This only goes to prove my point.


Even in the McD video he’s actually doing a controlled turn but how much control would he have if it were the front end? I would say none at all. This is my point and that’s the only point I was making. If you are talking about going around a bend instead of a straight line I would still rather have the front gripping more than the back for the same reason.

With the pencil analogy, I was likening it to a car because similar forces are at work. i.e. it does start to go off course because there are so many factors and imperfections influencing the pencil. Theoretically, if you had a perfect balance then like Mole said the pencil wouldn’t go off course but it is precisely because nothing is perfect, least of all with cars, that if the front locks up a car will become so unstable. In a car you have poor distribution of weight for a start, (driver only or passengers, the weight of the driver etc.),. So if the front locks up it’s only a question of time before it will start to turn. If the back locks up the car will only try and turn if you are on a bend, like a handbrake turn, but you have more chance of recovering it IMHO.

If you think about a rocket which is powered from the rear it is inherently unstable but there are electronics and mechanics constantly correcting to keep it vertical. If the power could somehow come from the front it would be more stable. (Look at me trying to talk rocket science )

My cycle front brake is on the right. Do other countries have it on the left then? It seems logical to me to have it on the right but that’s probably because my motorbike front brake is in the same place.

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

UK cycles tend to have the front brake lever on the right. Other countries (Americas for example) seem to have them the opposite way around which can cause confusion! Motorbikes seem to always have the front brake lever on the right, as the left lever is the clutch.

There still seems to be a bit of debate as to whether new tires should go on the front or on the back. Personally I can only think of a handful of times when I've lost traction on the rear wheels and all these were during heavy braking. The rear end stepped out of line but it wasn't very hard to correct the fishtailing and keep the car moving in the direction I wanted, despite not expecting it, nor having a great deal of skid correction practice. I can think of many times when I've lost traction on the front wheels either under acceleration or braking. Controlling the car's trajectory (not always the same as the direction the car is pointing) wasn't so easy in these cases.. and slightly counter intuitive. (note this all relates to FWD cars)

I saw some arguments outside this forum for putting new tires on the rear, stating that the limits of traction could be more easily sensed through the front wheels/steering rather than the rear. If you're driving within the traction limits of the front tires then the rear tires should easily be able to cope and knowing the traction limits of the poorer set of tires is pretty useful.

As partially worn tires still grip well, I guess it is more about the differential grip between the front and the back. Which end of the car needs more grip and why? Does it matter if the car is FWD or RWD? Possible food for thought: why do motorbikes almost always have a larger rear tire than front tyre? Same goes for most RWD cars. The larger tire presumably offering more grip.

I still haven't come across a compelling reason to put them on one end as opposed to the other.

In the 70s, and before, the rear tyre on motorbikes was larger although not by much and I always assumed this was because if you are carrying a pillion you have a much greater weight in proportion to the bike and rider over the back wheel.

But my feeling is it’s as much aesthetic and practical these days with many race bikes having provision for only one rider but still using an elephant tyre on the back. It is logical to assume the larger the tyre the more rubber will be in contact with the road which would help with both acceleration and breaking of course. Therefore I guess that's the reason in part or whole for why motorbike tyres have grown. If you think of drag bikes and cars they always have big back wheels for big grip. (You would get more grip with an increase in either circumference or width but it's a trade off between sheer size and weight versus gain and cost).

Tyre technology has moved on with bikes using dual compounds etc. but you raise a good point, and one which I think adds more weight, literally, to the argument that they should be on the front for both bikes and cars and maybe herein lies the answer you're looking for..

Under hard breaking the ratio of retardation, so I’m told, is approx 80:20 front:back on motorbikes. I don’t know how much it is for cars but I would imagine the same situation exists although to a far lesser degree.

So if the majority of stopping power is coming from the front where better to put new tyres, especially on wet roads?

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

I'm intrigued! Why so? Don't cars under or oversteer on roads?

I think it's more than that. I'm sure I've seen big bikes stopping hard with their rear wheel barely touching the ground? If so, that's 100 : 0 front:rear! Certainly cars can manage better than 80% braking on the front axle when they're really standing on the anchors. Bikes have a very high centre of gravity, so they transfer more weight to the front under braking than a car would (proportionately). I think they have big back tyres because they can only brake with their front wheels whereas they can brake and accelerate with their rears and their power to weight ratios are such that they need all the rubber they can get on the road!

I think the advice to put the better tyres on the back is because most drivers find that underster is easier to control than oversteer. I know I do! When Joe Average realies that his car isn't going where he's pointing it (front OR rear) his primary instinct is to lift off. Doing that in a car that is understeering is exaclty the right thing to do to tighten its line. Doing it in a car that is overstering can have you going backwards into the scenery. More advanced drivers always moan about the inherent understeer in most road cars and say they would prefer something more neutral. I think manufacturers deliberately build in quite a lot of natural understeer to most "ordinary" cars and they make it as progressive as they can. Some front wheel drives (particualrly hot hatches of the 80s and 90s) used to have a bit of lift-off oversteer to make people feel like heroes but for most of the people most of the time, understeer is easier and more natural to control. Besides, if all else fails and you ARE going to "fall off", it's better to hit the scenery going forwards than backwards or sideways as the bulk of he car's protective systems are designed to work that way!

I absolutely don't doubt that there are drivers out there who can control overster very well, but I think the number of drivers who THINK they can actually far outweighs the number that actually can - especially in a panic situation! I can sit in a powerful rear wheel drive car and do doughnuts and "party tricks" all day in controlled conditions, but I wouldn't trust myself to catch the tail when it unexpectedly steps out half way round a wet motorway slip road at 70!

Yes but the tread depth, provided it's over the legal minimum isn't a factor.