Continental brakes - Wrong way round

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tatanab
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Re: Continental brakes - Wrong way round

Postby tatanab » 5 Oct 2018, 7:07am

Mick F wrote:I'd be braking very hard or very suddenly on the rear to get a lock-up.

I have 8 machines, of which only 3 have rear brakes. When I get on one of those it is usual for me to temporarily lock the back wheel when braking for the first few occasions, until I recalibrate myself.

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meic
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Re: Continental brakes - Wrong way round

Postby meic » 5 Oct 2018, 7:13am

However it can't be over-stressed that the front brake is the one that wil stop you the most quickly and therefore might well save your life.

I have to do an energetic stop about once every 5,000 miles. Normally I have worn out a set of pads before that happens.
It is anticipation which saves your life. Good observation means that brakes are mostly used to reduce speed in a planned way, often a set of pads on my bike get worn out without ever experiencing an emergency stop. So like your customer it is my back pads which wear out quicker.
Yma o Hyd

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Mick F
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Re: Continental brakes - Wrong way round

Postby Mick F » 5 Oct 2018, 8:11am

+1
Utterly correct.



Also, the quickest way of stopping is using BOTH BRAKES.
iandusud wrote: ........ it can't be over-stressed that the front brake is the one that will stop you the most quickly and therefore might well save your life.
Mick F. Cornwall

gxaustin
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Re: Continental brakes - Wrong way round

Postby gxaustin » 5 Oct 2018, 9:32am

I don't know about the 1950s but by the late 70s plenty of kids could do what would later be termed as an 'endo' either by design or by misfortune.


Put on brakes, move centre of gravity forward off saddle, when front wheel stops the rear of the bike lifts. Put foot down quick. Actually that's maybe where the fear of going over the handlebars comes from?
Put on brakes, remain seated, bike stops under control.
Stand over bike while stopped, apply front brake hard, push handlebars away - rear of bike lifts. That's how I change gear when I stop in the wrong ratio.
Simple physics

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mjr
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Re: Continental brakes - Wrong way round

Postby mjr » 5 Oct 2018, 9:34am

meic wrote:I have to do an energetic stop about once every 5,000 miles. Normally I have worn out a set of pads before that happens.

Indeed. Hard stops should be rare if you ride well. The back brake is on the inside because of controlling speed, not emergency stops.
MJR, mostly pedalling 3-speed roadsters. KL+West Norfolk BUG incl social easy rides http://www.klwnbug.co.uk
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Mick F
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Re: Continental brakes - Wrong way round

Postby Mick F » 5 Oct 2018, 3:05pm

Riding Mercian today, I was considering frame geometry and centres of gravity.

My bum sits right at the back of the saddle. I have long arms. My bike has short chainstays.
These three facts have my COG quite a way back so there's a good bit of weight on the rear wheel and not so much on the front wheel.

Slam on the brakes and I stop PDQ and there's no real danger of doing a header.
Mick F. Cornwall

Brucey
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Re: Continental brakes - Wrong way round

Postby Brucey » 5 Oct 2018, 3:56pm

if you use the front brake hard enough, one of two things will happen; either the rear wheel will lift or the front wheel will lock up. Which happens first depends on what your grip is like and whether your weight distribution is forwards or backwards.

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Chris Jeggo
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Re: Continental brakes - Wrong way round

Postby Chris Jeggo » 5 Oct 2018, 4:17pm

Brucey wrote:... depends on what your grip is like ...

cheers

Grip between tyre and road, that is. Nothing to do with how strong your hand is.

Takes me way back to school-days maths/physics problems on friction - "does it topple or slide?"

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Mick F
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Re: Continental brakes - Wrong way round

Postby Mick F » 5 Oct 2018, 4:37pm

Yes, grip between tyre and road.
No doubt whatsoever.

I can pull hard enough on the levers thank you.
Mick F. Cornwall

iandusud
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Re: Continental brakes - Wrong way round

Postby iandusud » 5 Oct 2018, 4:41pm

meic wrote:Good observation means that brakes are mostly used to reduce speed in a planned way,


I wouldn't argue with that but that doesn't preclude the blind idiot who decides that you are invisible and pulls out straight in front of you.

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Re: Continental brakes - Wrong way round

Postby iandusud » 5 Oct 2018, 4:49pm

Mick F wrote:Also, the quickest way of stopping is using BOTH BRAKES.


I wouldn't argue with that and I do use both brakes when braking hard. However the harder you brake the more weight is transferred to the front wheel and the less grip there is between the rear tyre and the road surface. So under very hard braking the rear brake becomes less effective.

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Mick F
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Re: Continental brakes - Wrong way round

Postby Mick F » 5 Oct 2018, 4:53pm

Depends on your speed.
Faster you go, the more the rear brake will retard the speed.

............... and also is influenced by weight distribution.
Mick F. Cornwall

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Chris Jeggo
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Re: Continental brakes - Wrong way round

Postby Chris Jeggo » 5 Oct 2018, 7:32pm

To slow a vehicle requires a horizontal force applied at the centre of gravity (CG). The actual braking force is applied at the tyres, thus generating a couple, or turning moment. In normal braking the vehicle does not topple because an opposing couple is generated by weight transference, the load (vertical load, implied rather than explicitly stated in what follows) on the front wheel increases while that on the back decreases.

Now let's do a thought experiment, which is easier than doing it out on the road. You are travelling at some speed and wish to slow down. Apply the back brake without locking the back wheel. There are two results - deceleration and some weight transference. Now hold that back brake setting and apply and gradually increase front braking. Both deceleration and weight transference increase. While the back wheel is still rolling the force its tyre exerts on the road remains constant, because we are holding constant the back brake setting. However, as the load on the back wheel decreases we will eventually reach the point where the back wheel slides, when rear wheel load multiplied by coefficient of friction (CF) reduces below rear wheel braking force. From this point on, as front braking continues to increase, weight transference continues to increase, rear wheel load continues to decrease, so rear wheel braking force, being CF times load, continues to decrease.

We still haven't stopped, so what happens next? That depends on the answer to the "topple or slide" question. For a bike (not recumbent) with decent tyres on a dry, decent road surface the answer is "topple". This happens when weight transference reaches its limit, the point where the rear wheel load reduces to zero, and therefore also the point at which the rear wheel braking force (CF times load) becomes zero. At this point the back brake has ceased to have any effect, so might as well not be on.

So in theory, and in "topple" conditions but not in "slide" conditions, maximum braking is achieved by front brake alone. But can the theoretical maximum be achieved in practice? Only to the extent that a cyclist can match the performance of a well designed anti-lock braking system, and the ABS servo loop can have a faster reaction time than the human.
If the front braking is less than maximum then clearly some braking can be achieved by the back wheel, but the total will be less than the maximum.

But maybe the "slide" condition is more interesting, because a front wheel skid is greatly to be avoided in that it is very likely to have you off pretty quickly. Just before the front wheel locks there is still some load on the back wheel so further braking can be obtained. The maximum braking force obtainable equals total vehicle weight times CF, but the necessary front-rear distribution of braking force to achieve this depends on the CF because it depends on the braking couple which depends on the total braking force. When the CF is on the point of being large enough to produce toppling rather than sliding then the situation is as above. The other extreme is black ice having a tiny CF, when the maximum achievable braking force is so small that to a first approximation weight transference can be ignored and the front-rear braking distribution needs to match the front-rear weight distribution.

The theory above does not describe the real-world situation exactly. I have ignored the difference between static and sliding friction, which are distinguished in school-level friction theory, and neither tyres nor road surfaces behave exactly like 'classical' friction materials. But even so, classical theory is a good guide to what happens in practice, and when you reach the limit you will get either toppling or sliding. If you correctly judge that the braking limit is due to toppling and you reckon you can react to the rear wheel lifting quickly enough to avoid an 'imperial crowner' then you should use front brake only to stop as quickly as possible. In other conditions, depending on how risk-averse you are, you can achieve more braking with two brakes than one, although in many conditions the contribution of the rear brake will be but a fraction of that of the front brake.

Brucey
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Re: Continental brakes - Wrong way round

Postby Brucey » 5 Oct 2018, 7:45pm

Mick F wrote:Depends on your speed.
Faster you go, the more the rear brake will retard the speed.


in relation to what? The maximum rate of deceleration (in m/s^2) possible with the rear brake is constant regardless of speed. 'O' level physics, that.

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Mick F
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Re: Continental brakes - Wrong way round

Postby Mick F » 5 Oct 2018, 8:00pm

30mph.
Squeeze the front brake as hard as you can without touching the rear brake.

What happens?
You slow down, but you don't do a header.
You will slow down more if you use both brakes.
Mick F. Cornwall