Pressure in tyres 700 x 25

desweller

ElectronShepherd wrote:

lostboysaint wrote:

The bit about the ton weight and the lack of understanding that the most efficient braking would STILL be to have the front brake powered to the point of lifting the back wheel is just astounding.

I would say that the idea that the front wheel will have sufficient traction to lift the rear wheel at all (in the hypothetical case of the 1 tonne weight) is just astounding.

I stand by what I said: In cases where the weight on the back wheel (be it a mythical 1 tonne weight or just a big rider who likes his pies and is sat firmly on the saddle) is too much to be lifted by the available traction from the front wheel, the rear wheel will be in contact with the ground regardless of the braking from the front wheel, and in those cases, braking the rear wheel too will reduce the stopping time.

I agree that in some limited circumstances (e.g. lightweight rider, dry conditions, grippy tyres and tarmac), you can get minimum stopping distance just using the front brake. However, it simply isn't the case in all circumstances, even in the dry, which is what Sheldon Brown is saying.

The CoG of a bicycle is so high and so close to the front wheel that they cannot easily be traction limited. The weight transfer has a big moment arm around the contact patch. If the bike is not lifting the back wheel then you can exert more retarding load with the front, unless the brakes themselves are not working well enough.

As a real world example, I weigh 95kg. My touring bike weighs 17kg dry, 35kg fully loaded (front and rear panniers + water bag). Analogous to your big-guy-on-a-bike scenario at 130kg all-up, with weight pretty heavily biased to the rear. It's equipped with V-brakes, not anything swish like MRS's discs, but I can still get the rear wheel off the ground in an emergency stop.

It's a bit hairy though, all the extra weight over the back wheel wants to overtake the front; a lot harder to control, so I don't do it unless I have to.

Where it does start to give up is slowing down on long descents; I can get brake fade. However I have never lost front tyre traction in the dry on this bike.

meanredspider

The only time I use the rear brake is when I'm riding on a surface with compromised grip (ice, gravel etc) or a stupidly steep descent (normally on the MTB)

ai_1

meanredspider wrote:

The only time I use the rear brake is when I'm riding on a surface with compromised grip (ice, gravel etc) or a stupidly steep descent (normally on the MTB)

Ditto

ai_1

ElectronShepherd wrote:

lostboysaint wrote:

The bit about the ton weight and the lack of understanding that the most efficient braking would STILL be to have the front brake powered to the point of lifting the back wheel is just astounding.

I would say that the idea that the front wheel will have sufficient traction to lift the rear wheel at all (in the hypothetical case of the 1 tonne weight) is just astounding.

I stand by what I said: In cases where the weight on the back wheel (be it a mythical 1 tonne weight or just a big rider who likes his pies and is sat firmly on the saddle) is too much to be lifted by the available traction from the front wheel, the rear wheel will be in contact with the ground regardless of the braking from the front wheel, and in those cases, braking the rear wheel too will reduce the stopping time.

I agree that in some limited circumstances (e.g. lightweight rider, dry conditions, grippy tyres and tarmac), you can get minimum stopping distance just using the front brake. However, it simply isn't the case in all circumstances, even in the dry, which is what Sheldon Brown is saying.

You're overcomplicating this. Your tonne on the back example is irrelevant. We're either assuming sufficient traction or not. With that assumption the theory is sound. Without it there are exceptions. I don't think anyone is denying that - including Sheldon. What is it specifically that you disagree with?

lostboysaint

ElectronShepherd wrote:

lostboysaint wrote:

You are wrong then!

Which bit? About the lifting the rear wheel?

I'll give you another clue - next time you're running, try slowing down by only weighting your back foot.

ElectronShepherd

Nope - you've lost me. No idea what running has to do with it.

So, a direct question for you - it's nice and easy - has a one word yes or no answer...

Do you think that braking using only the front wheel of a bike will always have the ability to lift the rear wheel, regardless of how much weight is over the rear wheel?

Bobbinogs

...and, more importantly, exactly what has all this to do with tyre pressure :?

lostboysaint

ElectronShepherd wrote:

Nope - you've lost me. No idea what running has to do with it.

So, a direct question for you - it's nice and easy - has a one word yes or no answer...

Do you think that braking using only the front wheel of a bike will always have the ability to lift the rear wheel, regardless of how much weight is over the rear wheel?

It gives you the perfect demonstration of weight transfer and why you put your resistance through your front leg to slow down.

Too many variables for a one word answer and you're still barking up completely the wrong tree. As I said, why not run along and do some research on the subject. (Ever wondered by a Harley still has more front braking power than back despite having a lardy ghey rider sat firmly over the rear?)

Anyway, I'm done now. You're either a troll or genuinely not very bright and not willing to learn. In either case I'm clearly wasting my time with you.

Oh, and to the OP - you still want to be looking at a LOT lower pressures than some of the people on this thread are suggesting. 100psi in a 25mm for someone around 80kg is just ridiculous.

desweller

ElectronShepherd wrote:

Nope - you've lost me. No idea what running has to do with it.

So, a direct question for you - it's nice and easy - has a one word yes or no answer...

Do you think that braking using only the front wheel of a bike will always have the ability to lift the rear wheel, regardless of how much weight is over the rear wheel?

Have you done any calcs to determine the boundary condition that you're talking about? Or are you just being a pedantic ass for the sake of it?

Obviously there are scenarios where Sheldon's description is not correct; e.g. a car can lock its front wheels without risking going end over tip because the CoG is lower and further back.

But for the vast majority of bicycles it is a valid statement.

ai_1

ElectronShepherd wrote:

Nope - you've lost me. No idea what running has to do with it.

So, a direct question for you - it's nice and easy - has a one word yes or no answer...

Do you think that braking using only the front wheel of a bike will always have the ability to lift the rear wheel, regardless of how much weight is over the rear wheel?

Stating you only want a one word answer does not make it reasonable. The question is phrased in such a way as to make it pointless. Either that's intentional and you are evading the points that have been made or you don't understand the points made and think this is the question at issue.

ElectronShepherd

Oooh, touched a nerve there, didn't I? Two insults in as many replies...

It's an internet forum, people. It's only a bit of fun - no need for name calling just because you don't agree with someone!

My view is that the maximum total kinetic energy loss rate from both wheels (braking is essentially the conversion of kinetic energy into heat) will be greater than the maximum kinetic energy loss rate possible from the front wheel alone, so that when the front braking is reduced enough to keep the rear wheel on the ground so that the rear wheel can aid in slowing down, you slow down more quickly. Yes, there will be a weight transfer favouring the front, so more energy will be dissipated by the front wheel than the rear, and that's why a Harley has larger discs on the front, to more efficiently lose the extra heat. That doesn't mean the rear brake is useless though.

Having said that - very happy to read up on it as suggested - got any useful links, lostboysaint? Perfectly happy to find out I'm wrong and admit it, but at the moment, I don't think I am.

ai_1

ElectronShepherd wrote:

...My view is that the maximum total kinetic energy loss rate from both wheels (braking is essentially the conversion of kinetic energy into heat) will be greater than the maximum kinetic energy loss rate possible from the front wheel alone, so that when the front braking is reduced enough to keep the rear wheel on the ground so that the rear wheel can aid in slowing down, you slow down more quickly....

Okay, I think I follow your thinking, however there is a flaw in the logic which hopefully I can explain clearly:
If you reduce front braking enough to keep the rear on the ground you can then provide braking with the rear as you say, however, you can never apply as much breaking with both wheels as that will just unload the back wheel again. The rear wheel does not lift because the front wheel is doing the braking. It lifts because the bike is decelerating due to a force applied at the ground rather than a force in line with the centre of mass. It is irrelevant which wheel provides the braking force. So, if bike rotation is due to deceleration and centre of mass then the maximum safe deceleration for any given bike occurs when the back wheel is just about to lift. At this point the rear wheel is incapable of contributing to braking since it has no traction. Letting off the front brake just reduces the rate of deceleration. You can't compensate by braking with the rear because it will always skid before you end up with a higher deceleration than the front wheel only case.

If you used a parachute attached somewhere just below the saddle for braking then the bike would not pitch forward. However using wheel brakes, it will pitch forward and thus, if traction is not an issue, maximum braking will be achieved with the rear wheel on the point of lifting and the front wheel doing all the braking.

meanredspider

I'd add to this to say kinetic energy loss is only relevant if it's a limiting factor (which might be the case if you were doing a long alpine decent on rim-braked carbon wheels). In most cases the brakes are more than capable of dealing with the energy.

Just draw yourself a picture of the forces involved and it will be immediately apparent what's going on.

cycleclinic

A few ago after a decent I entered sudbury and the car I front started to slow I braked the tension rear wheel lifted the car braked some more I had to brake more but all I had as the front brake using the rear would not have helped. I ended up teetering on the front wheel and just avoided the embarrassing tip over. I use the rear brake with light braking on the front sometimes to avoid unsettling the bike for example in a bend but most of the time I use the front to scrub speed as the rear brake almost does diddly squat.

The maximum braking force is entirely dependent on the value of the reaction force on each wheel. The higher the reaction force the higher the value of rolling friction. given the rolling friction force cause the wheel to rotate as it pointing backwards and we'll the wheel can rovate so the rolling friction force becomes a moment to cause rotation, braking creates a force to oppose this. When the braking force which acts to reduce the angular velocity of the wheel (bbraking force is a moment which want to rotate wheel also) the amount of braking force is obviously related to the reaction force on the wheel. When you brake weight transfers to the front. The more the braking moment is the bigger the weight transfer. This can make the back brake useless in the extreme but for the most part the back brake is of limited use. 90% of your braking effort is done by the front brake. Bringing kinetic energy into this is not helpful as it confuses matters without answering the question or providing an explanation. Sorry for the physics.

rolf_f

meanredspider wrote:

You can certainly lift a rear wheel through braking - I've done it plenty of times on my disc-braked Volagi. I weigh 90+kg. Of course you can minimise the effect of this by putting more and more weight over the rear wheel but, ultimately (assuming you don't end up with the centre of gravity behind both wheels - effectively giving two front wheels and no rear wheels - or at ground level) weight transfer leads to the same conclusion. I've never managed to lock the front wheel on a firm dry surface.

Indeed - and I've managed to do the same on a rim braked road bike - though I'm under 2/3rds the weight of MRS. On that basis, that my MTB (disc braked) has a little bit shorter braking distance I've attributed to the fact that the bike itself is substantially heavier hence a bit more effective braking force can be applied before the rear wheel unweights.

andy_wrx

I mainly use the rear brake to settle the bike, rather than stop,
eg in Italy a year or two ago I went from full sun into deep shadow and straight into a pothole, the bike went into a shimmy.
I grabbed just a momentary bit of rear brake to settle the bike...
...and it was lucky it was a momentary bit, as it was a hire bike with the brakes the European 'wrong way round' and so I'd actually just braked the front (despite trying all week to re-educate my brain as to which hand was which brake...)
- oops ! Still I'm still alive to tell the tale.

Anyway, Sheldon made this point with 'front brake left or right ?', saying that he preferred his bikes set-up with front brake right as in UK not as normal in US, saying it was his dominant hand, being right-handed.
He suggested that the idea of having the front brake controlled by your weaker hand was based on the crazy assumption that if you were to brake too hard with the front you would lock the front wheel and go over the bars, which you could only do by hitting a log or big kerb or something, certainly never by locking the wheel.

He then went on to locking front vs back...

meanredspider

Rolf F wrote:

Indeed - and I've managed to do the same on a rim braked road bike - though I'm under 2/3rds the weight of MRS. On that basis, that my MTB (disc braked) has a little bit shorter braking distance I've attributed to the fact that the bike itself is substantially heavier hence a bit more effective braking force can be applied before the rear wheel unweights.

Part of the reason for the MTB result may be down to the front suspension. There's a saying in engineering that "the greatest load is carried by the stiffest member". The front forks on the MTB will absorb some of the instantaneous weight transfer meaning that more load will remain on the rear wheel - certainly up until the point the forks begin to bottom out (at which point you'll pitch right over the bars :shock: )