Fast descents

robthehungrymonkey

Well, i hit 38mph coming off the downs in Winchester at the weekend sitting upright to act as an air brake... and i was terrified! lol. Scanning the road like mad for any possible bumps or potholes.

Road bikes still scare me on decents, especially when you don't know the road.

robthehungrymonkey

ded wrote:

...since their downhill force is greater than the skinny one...

Sorry, I should know the answer to this, but why?

nasahapley

Nuggs wrote:

Jazza1969 wrote:

It's a matter of gravity of course, but a laden tourer or old heavy racer (or rider ) might well go faster downhill than that ultra light carbon machine...

I think Newton might have something to say about that...

I reckon he'd say this:

"You are of course correct, the terminal velocity of a body which is not in freefall, but is subject to air resistance, is proportional to the square root of its mass. It follows that, ceteris paribus (Newton would've liked a bit of Latin!) a heavier bike/rider will reach a higher terminal velocity than lighter one. And don't talk to me about Galileo dropping stuff from that tower either, proper amateur stuff that was".

mostly harmless

robthehungrymonkey wrote:

Sorry, I should know the answer to this, but why?

Force down the slope is equal to mass * acceleration due to gravity * sine of slope angle.

Therefore a bigger force for bigger riders. Air resistance is what deterimines final speed though.

whyamihere

A larger rider on a laden tourer with panniers will have much, much more air resistance than a racing whippet down on the drops with a skinny carbon bike though. I reckon the whippet would have a higher maximum, it just may take a bit more pedalling to beat the weight difference under acceleration.

dennisn

whyamihere wrote:

A larger rider on a laden tourer with panniers will have much, much more air resistance than a racing whippet down on the drops with a skinny carbon bike though. I reckon the whippet would have a higher maximum, it just may take a bit more pedalling to beat the weight difference under acceleration.

I'm not so sure. I'm overweight(to put it nicely) and I seem to have very little trouble
passing those "racing whippets" on the big downhills. Then again maybe I have a bit
less in the brains department. Sure is fun though. If not downright scary at times.
As for using the brakes I can usually knock off 5 or 10 MPH simply by sitting up in the saddle.

Dennis Noward

NWLondoner

My fastest downhill is approx 32mph. I could have maybe pushed harder but i had black cab up my ass and could see a parked BMW in front of the approaching roundabout.

London Taxi, BMW, Roundabout :shock: I took this as a sign and slowed down.

dennisn

In the end, even if you know pretty much every inch of the hill and have ridden it
dozens of times there is still that little patch of gravel or sand or hot tar or animal
or flat tire or whatever that can take you down before you can say "oh sh*t", let alone
formulate a plan, in your head, to deal with the "problem". For those of you riding clinchers I would urge you to remember how quickly a flat tire can come off the rim,
on level roads, at slow speeds, let alone on a high speed run down a curvy mountain.
Yeeeeeeeeehaaaaaaaaaaaaaa

Dennis Noward

gondwanaman

1st post but can vouch for the slowing effect of staying upright on descents. On relatively windless days I hit 34 point something coming down bumblebee lane on my mtb hunched over and as a comparison the following day I sat up with arms outstretched (silly I guess) and hit just over 27. MPH that is.

markos1963

46 mph on the hill down into Cromer whilst overtaking a car that had just overtaken me!
My main thought on descending is know your road, most peoples fears come from a lack of knowledge. If you know the bends,surfaces,junctions etc you can make an informed descision on what to do(then just go for it anyway ). I have ridden motorbikes for 30 years and I suppose it helps as you get used to speed whilst sitting on a 2 wheeler. All those of you who get jittery at 30+ ought to try 175+(I can't tell you where) on an open road

robthehungrymonkey

mostly harmless wrote:

robthehungrymonkey wrote:

Sorry, I should know the answer to this, but why?

Force down the slope is equal to mass * acceleration due to gravity * sine of slope angle.

Therefore a bigger force for bigger riders. Air resistance is what deterimines final speed though.

But force is not the same as speed, and acceleration as you stated there is constant (9.81m/s squared if i remember rightly*sine of slope angle) and irrelevant to mass.

:oops:

nasahapley

robthehungrymonkey wrote:

mostly harmless wrote:

robthehungrymonkey wrote:

Sorry, I should know the answer to this, but why?

Force down the slope is equal to mass * acceleration due to gravity * sine of slope angle.

Therefore a bigger force for bigger riders. Air resistance is what deterimines final speed though.

But force is not the same as speed, and acceleration as you stated there is constant (9.81m/s squared if i remember rightly*sine of slope angle) and irrelevant to mass.

:oops:

RobTHM, I think the confusion arises here because you are thinking of gravity as an acceleration rather than a force. There's exactly the same gravitational force acting on you (9.81 Newtons per kilo), whether you are sitting reading this or have just jumped out of a plane, but the accelerations are very different!

Using Newton's famous equation Force = mass x acceleration, for a body rolling down a smooth slope and subject to air resistance, the equation can be rewritten (taking A as the angle of the slope from the horizontal):

mass x acceleration =

mass x g x sin A (the gravitational force acting parallel to the slope), minus

k (a constant) x velocity squared (the opposing force due to air resistance).

or ma = mgsinA - kv^2

Re-arrange and you get a = gsinA - (k/m)*v^2

From this you can see that if you have two bikes/riders with the same aerodynamic properties (i.e. same value of k), already freewheeling at a given velocity down a slope, the heavier one will accelerate at a greater rate. If they started at the top of the slope at v=0, then the instant they started rolling they would be accelerating at the same rate (gsinA), but as soon as velocity is non-zero the extra mass of the heavier rider helps him to overcome air resistance and so he accelerates at a slightly greater rate (and will eventually reach a higher terminal velocity). The cumulative effect over a long distance (such as an alpine pass), could be fairly significant.

Sorry to waffle on, it's a slow day at work!

robthehungrymonkey

nasahapley wrote:

RobTHM, I think the confusion arises here because you are thinking of gravity as an acceleration rather than a force. There's exactly the same gravitational force acting on you (9.81 Newtons per kilo), whether you are sitting reading this or have just jumped out of a plane, but the accelerations are very different!

Using Newton's famous equation Force = mass x acceleration, for a body rolling down a smooth slope and subject to air resistance, the equation can be rewritten (taking A as the angle of the slope from the horizontal):

mass x acceleration =

mass x g x sin A (the gravitational force acting parallel to the slope), minus

k (a constant) x velocity squared (the opposing force due to air resistance).

or ma = mgsinA - kv^2

Re-arrange and you get a = gsinA - (k/m)*v^2

From this you can see that if you have two bikes/riders with the same aerodynamic properties (i.e. same value of k), already freewheeling at a given velocity down a slope, the heavier one will accelerate at a greater rate. If they started at the top of the slope at v=0, then the instant they started rolling they would be accelerating at the same rate (gsinA), but as soon as velocity is non-zero the extra mass of the heavier rider helps him to overcome air resistance and so he accelerates at a slightly greater rate (and will eventually reach a higher terminal velocity). The cumulative effect over a long distance (such as an alpine pass), could be fairly significant.

Sorry to waffle on, it's a slow day at work!

That's some good quality waffling.

Now I (think i) understand. The extra force helps to overcome the air resistance.
I feel better now. Though, i really should have known that without the need of an explanation!

ded

Beat me to it nasahapley! Have a look at:

http://www.sportsci.org/encyc/cyclingupdown/cyclingupdown.html#downhill

if you really want to hurt your head with physics! But this bit is probably the best summary for humans (not physicists)...

www.sportsci.org wrote:

Scaling reveals that larger cyclists have a greater ratio of mass to frontal area. They therefore descend hills faster as a consequence of purely physical, not physiological, laws. Since the larger cyclist has a greater mass, gravity acts on him or her with a greater force than it does on a smaller cyclist. ... While the larger cyclist also has a greater absolute frontal area than the smaller cyclist, the difference is not as great as that for their masses.

dennisn

ded wrote:

Beat me to it nasahapley! Have a look at:

http://www.sportsci.org/encyc/cyclingupdown/cyclingupdown.html#downhill

if you really want to hurt your head with physics! But this bit is probably the best summary for humans (not physicists)...

www.sportsci.org wrote:

Scaling reveals that larger cyclists have a greater ratio of mass to frontal area. They therefore descend hills faster as a consequence of purely physical, not physiological, laws. Since the larger cyclist has a greater mass, gravity acts on him or her with a greater force than it does on a smaller cyclist. ... While the larger cyclist also has a greater absolute frontal area than the smaller cyclist, the difference is not as great as that for their masses.

I knew there was an advantage to being "massive" or should I say "having a bit of extra mass"?

Dennis Noward

sub55

well in that case i must by the biggest bloke on here then
[img][/img]
thats 68.4 mph, now that was pretty quick

Nuggs

From fast descents to a discussion about physics. I love this place.

Jazza1969

A fly or other insect colliding with your face is much sorer at 40mph than at 25mph. They don't tell you that on road safety campaigns.

STEFANOS4784

"A fly or other insect colliding with your face is much sorer at 40mph than at 25mph. They don't tell you that on road safety campaigns"

"well in that case i must by the biggest bloke on here then

thats 68.4 mph, now that was pretty quick"
:shock:
You must be joking