Cancellara's Descent

Monty Dog

IME the trick to going fast downhill is keeping it smooth - there's barely enough rubber on the road and grip so marginal at that speed that the entire notion of 'scrubbing' your front tyre at that speed will only result in expensive dentistry. Beside, you get far more effect from using your body as a wind brake

frenchfighter

andrewgturnbull wrote:

markos1963 wrote:

Just the same as on a motorbike, the process of leaning puts the angle of the tyre into a postion that it causes more friction and it loses energy. In fact you can see this when driving a car to a certain extent, drive up to a constant radius bend and drive around it at a constant speed you'll find you have to increase power/revs to maintain that speed, then drive around it but without increasing revs or power and see the speed drop(obviously wind resistance influences the result a bit). Its not an excuse to neglect braking altogether but if you need just a bit then it saves on brakes and rims etc.

Hi there.

With respect, I'm calling bs on that one... A bicycle tyre is round in profile, not square like a car tyre - so where is the increase in friction?

Cheers, Andy

As far as I understand it, if you lean your bike there is more of the tyre in contact with the road. The greater the surface area touching the road, thre greater the friction and therefore it slows you down. To what extent your speed is lowered I have no idea.

markos1963

andrewgturnbull wrote:

markos1963 wrote:

Just the same as on a motorbike, the process of leaning puts the angle of the tyre into a postion that it causes more friction and it loses energy. In fact you can see this when driving a car to a certain extent, drive up to a constant radius bend and drive around it at a constant speed you'll find you have to increase power/revs to maintain that speed, then drive around it but without increasing revs or power and see the speed drop(obviously wind resistance influences the result a bit). Its not an excuse to neglect braking altogether but if you need just a bit then it saves on brakes and rims etc.

Hi there.

With respect, I'm calling bs on that one... A bicycle tyre is round in profile, not square like a car tyre - so where is the increase in friction?

Cheers, Andy

I'm using a car as an analogy, as I oringinally said its only small amounts of speed you can lose doing this so don't go flying into a hairpin expecting to stop! The part of a bike tyre that is in contact when upright is the thinnest and when cranked over naturally more tyre comes into contact, increasing friction.

Oh and by the way a tyre isn't round in profile its more cone shaped

andrewgturnbull

markos1963 wrote:

andrewgturnbull wrote:

markos1963 wrote:

Just the same as on a motorbike, the process of leaning puts the angle of the tyre into a postion that it causes more friction and it loses energy. In fact you can see this when driving a car to a certain extent, drive up to a constant radius bend and drive around it at a constant speed you'll find you have to increase power/revs to maintain that speed, then drive around it but without increasing revs or power and see the speed drop(obviously wind resistance influences the result a bit). Its not an excuse to neglect braking altogether but if you need just a bit then it saves on brakes and rims etc.

Hi there.

With respect, I'm calling bs on that one... A bicycle tyre is round in profile, not square like a car tyre - so where is the increase in friction?

Cheers, Andy

I'm using a car as an analogy, as I oringinally said its only small amounts of speed you can lose doing this so don't go flying into a hairpin expecting to stop! The part of a bike tyre that is in contact when upright is the thinnest and when cranked over naturally more tyre comes into contact, increasing friction.

Oh and by the way a tyre isn't round in profile its more cone shaped

Hi there.

I don't mean to be pedantic... but why should the contact patch increase as you turn, when your weight and the tyre pressure remain constant? The shape of the patch may change with the tyre profile, but I don't see how the area can?

My tyres are pretty round if you just look at the profile of the bit that touches the ground and ignore the sidewalls.

I might well be wrong, please feel free to explain why I am!

I'd also like to point something else out: as you lean into a corner your body gets closer to the ground i.e. it loses potential energy. Where does this energy go? This actually accelerates you (acceleration can mean change of direction as well as speed). The effect is small, but you can feel it if you corner at low speeds without braking.

Cheers, Andy

Ps If Cancelara's descent was so fast, how come a cameraman standing on the back of a motorbike with huge panniers could keep up? Paulo Savoldelli he's not!

Mettan

Angus444 wrote:

Bravo Fabian!! Great piece of film!!

The quality of the roads are a big factor......here in Angus, we must have the worst road surfaces in Britain. !

Agreed - road surface is vital.

markos1963

OK, I went out and had a look at my tyres on my bike, Michelin Lithiums, pretty average sort of race tyre. The centre contact area is only a few mm in width, the sidewalls where the herringbone pattern is, is up to 3 times the width. When I canted the wheel over nearly all of that herringbone comes into contact with the surface. I'm not so sure if bike tyre manufacturers do as much as motorbike tyres in terms of carcass construction but you normally get deformation when lent over and this also increases the contact patch.

Now I may be getting my physics confused here but I thought that a moving object has kinetic energy not potential, potential energy is the weight of a cyclist at the top of a descent and it changes to kinetic when he moves off(just as the electricity in a battery is potential until a circuit is formed) As you lean into a corner your head moves closer to the turning point and it feels faster, I doubt that you actually get faster as I haven't seen an increase in speed as I have leant over on motorbikes or cycles usually the opposite ie scrubbing.

Oh and one more point your weight increases as you go around a corner due to centifrugal forces

andrewgturnbull

andrewgturnbull wrote:

... but why should the contact patch increase as you turn, when your weight and the tyre pressure remain constant? The shape of the patch may change with the tyre profile, but I don't see how the area can?

markos1963

andrewgturnbull wrote:

andrewgturnbull wrote:

... but why should the contact patch increase as you turn, when your weight and the tyre pressure remain constant? The shape of the patch may change with the tyre profile, but I don't see how the area can?

The length of the contact patch is similar but the width is increased, with the extra pressures(centrifugal) it will defrom more and get even wider so increasing area. Don't forget Andrew most cycle race tyres are designed to offer as little resistance as possible when upright but to give grip in the corners. Granted some makes of tyre will offer little scrubbing effect but all will give a bit. I have no way of knowing what Fabian's are like and I doubt if he is consciously doing it, he's one of those natural riders who seem to do things on feel alone.

andrewgturnbull

Hi there.

Can't be bothered to dance around this anymore and continue correcting your physics...

In my opinion, bicycle tyres do not scrub of any speed as you turn into a corner.

Cheers, Andy

Jez mon

I've thought about the whole lean over at slow speeds and tbh it makes sense. When you lean over your c of g, will get lower, and hey presto, you lose g.p.e. so if energy is to be conserved, you have to gain energy somewhere else. (Probably using all the wrong words here but nevermind!)

However, when you move in a straight line and go from riding on the tops to riding on the drops, you would also lose gpe, however, i'm not quite sure where it would go. Please someone enlighten me because it's annoying me!

knedlicky

Term1te wrote:

Awesome, I was reading the live feed on the TdF website and they said he was over 90 kmh in places! Puts my descent of the Galibier last weekend to total shame.

I’d guess he was only at 80 km/hr max, sometimes under 50 km/hr. Good to watch, though, as you get you see more or less the whole descent. But he’s only competent and confident (not perfect, as someone posted).

knedlicky

Not sure about some of the ’physics’ being discussed here (scrubbing, etc) but when turning one doesn’t just lift the inner pedal to prevent it catching the road surface but also to redistribute the weight, so the force is increased downwards and is less to the side. One also does this with the head and shoulders if less perceptibly - you can see Cancellara do this, now and again, pretty noticeably at the bend at 55 secs and when he’s just over the 4 mins mark, but also other times.

This is so the wheel isn’t pushed sideways, which happens if you lean too much. Therein lies the loss of speed and control due to false distribution of body mass and where the resultant forces go. They then act to the side and not in the direction of ‘down’ or forward when your bike faces downhill. If you get it more right, your weight distribution is more effective in the line of direction when you come out of the curve, so you speed faster away. It’s the same basic principle as in downhill skiing.

Not keen on the scrubbing theory.

mmitchell88

andrewgturnbull wrote:

Ps If Cancelara's descent was so fast, how come a cameraman standing on the back of a motorbike with huge panniers could keep up?

About 150+bhp should do it.

Firecrakka

The tyres are made with a different rubber compound on the sidewall to "increase friction".
As friction is required for grip while cornering. The physics you seek are in the tyre compounds, not the angles, contact patch and surface area etc.
Hard compounds while upright with even weight distribution.
Soft compounds for grip, when weight shifting and centrifical force come into play.
Without the added friction in the corner you would simply slide out.
Friction slows us down, but its byproduct is grip, cornering is where we use it to our advantage.

robthehungrymonkey

Turning definitely does slow you down. Some ammount of force is used to change direction, assuming everything else is equal that reduction in forward momentum will mean a reduction in speed.

Now, i didn't explain that very well, but hopefully the gist has been got.

A tyre will wear quickly if turning, rather than rolling purely straight, this extra force on the tyre is coming from somewhere...

hamstrich

I'm pretty sure that the Earth goes around the Sun without slowing down.

When you're turning a corner, you're effectively in circular motion, so it's angular momentum you should be concerned with. Yes, as you turn you're constantly chainging direction, and yes you will experience a reduction in momentum in the original direction you were travelling, but that's simply because you're going less fast in the original direction! Your overall speed isn't affected one bit...

robthehungrymonkey wrote:

Turning definitely does slow you down. Some ammount of force is used to change direction, assuming everything else is equal that reduction in forward momentum will mean a reduction in speed.

Now, i didn't explain that very well, but hopefully the gist has been got.

A tyre will wear quickly if turning, rather than rolling purely straight, this extra force on the tyre is coming from somewhere...

robthehungrymonkey

hamstrich wrote:

I'm pretty sure that the Earth goes around the Sun without slowing down.

When you're turning a corner, you're effectively in circular motion, so it's angular momentum you should be concerned with. Yes, as you turn you're constantly chainging direction, and yes you will experience a reduction in momentum in the original direction you were travelling, but that's simply because you're going less fast in the original direction! Your overall speed isn't affected one bit...

Dude, I'm afraid you're just wrong.

When turning, the tyre friction increases overcoming the opposite force. This causes the change in direction. I might not be explaing it that well (i'm no physics or mechanics teacher).

When a train goes round a bend the same things happen (and there is much less friction there) it pushes against the outside rail changing it's original velocity, resulting in a different direction at a lower speed. As energy was used pushing against the rail.

The rules in space are the same, but they don't have friction or any other interferring factors. The sun/earth just have gravity and centrifugal force.

fletch8928

physics blah, compound blah, friction blah.
It was a good descent to watch, he is obviously very good at it and has belief that his machine is going to go where he points it. the conditions were perfect for the camaraman and he made a great job of recording a very good rider going down a slope fast. dont we all wish we had a descent like that?

robthehungrymonkey

fletch8928 wrote:

dont we all wish we had a descent like that?

I've forwarded a link to a friend who went with me up (and down) beachy head on saturday... we both commented how much we were bricking it on the way down!

This makes me feel :oops:

Excellent find by the way, i was trying to explain it to my girlfriend last night!

jswba

I liked it when he had a wee while they were changing his wheel too.

El Gordo

robthehungrymonkey wrote:

hamstrich wrote:

I'm pretty sure that the Earth goes around the Sun without slowing down.

When you're turning a corner, you're effectively in circular motion, so it's angular momentum you should be concerned with. Yes, as you turn you're constantly chainging direction, and yes you will experience a reduction in momentum in the original direction you were travelling, but that's simply because you're going less fast in the original direction! Your overall speed isn't affected one bit...

Dude, I'm afraid you're just wrong.

When turning, the tyre friction increases overcoming the opposite force. This causes the change in direction. I might not be explaing it that well (i'm no physics or mechanics teacher).

When a train goes round a bend the same things happen (and there is much less friction there) it pushes against the outside rail changing it's original velocity, resulting in a different direction at a lower speed. As energy was used pushing against the rail.

The rules in space are the same, but they don't have friction or any other interferring factors. The sun/earth just have gravity and centrifugal force.

And the gold star goes to the hungry monkey. Cornering requires acceleration (as in a change in velocity), that requires energy which can only come from your speed (since your mass is not about to change).

It is however going to be a pretty minimal drop in speed since your mass is quite low so as a means of controlling my speed I'll stick to my brakes.

Amazing decending though. We we're only inches from needing to have a discussion about the physics of his head bouncing off the cliff face at 50mph.

all show no go

So here is the definitive guide to what happens to your rubber hoops when you go round a bloody corner!
I think you'll find some kind of answer at the bottom of page 16......

http://books.google.co.uk/books?id=msdT ... t&resnum=7

markos1963

robthehungrymonkey wrote:

hamstrich wrote:

I'm pretty sure that the Earth goes around the Sun without slowing down.

When you're turning a corner, you're effectively in circular motion, so it's angular momentum you should be concerned with. Yes, as you turn you're constantly chainging direction, and yes you will experience a reduction in momentum in the original direction you were travelling, but that's simply because you're going less fast in the original direction! Your overall speed isn't affected one bit...

Dude, I'm afraid you're just wrong.

When turning, the tyre friction increases overcoming the opposite force. This causes the change in direction. I might not be explaing it that well (i'm no physics or mechanics teacher).



When a train goes round a bend the same things happen (and there is much less friction there) it pushes against the outside rail changing it's original velocity, resulting in a different direction at a lower speed. As energy was used pushing against the rail.

The rules in space are the same, but they don't have friction or any other interferring factors. The sun/earth just have gravity and centrifugal force.

As A train driver I would say that a train doesn't touch the outer rail as we corner unless the curve is very tight(less than 5 chains radius) Train wheels have a special profile that acts in two different ways. Firstly trains don't have a differential on the drive wheels so the wheel profile allows the outer wheel to rotate slightly on its axsis and secondlyit makes the ride quality better for passengers with the wheel nestleing down in the track ,in theory you could take the flanges off a wheel and it would still track normally around most bends.

Eurostar

Obvious, innit? The centrifugal force is trying to push the tyre across the road. So of course the friction's going to be much greater than when the tyre is upright.

synchronicity

Just watching it now... I think 4:10 - 4:20 is the best demonstration of his skills.

Along with where he narrowly misses the car at 3:34...