desending

jimmypippa

phy2sll2 wrote:

airbag wrote:

From a standstill, they both accelerate at the same rate, but the full can will accelerate for longer)

^ This.

They will initially accelerate at the same rate until resistive force becomes significant compared to forwards force, at which point the acceleration of both will begin to decay, faster for the lighter body than the heavier since resistive force for both will be the same (as proportional to viscosity, velocity and surface area) but forwards force will be larger for the more massive body.

The lighter body will reach terminal velocity earlier.

EDIT: I should have added: but that velocity will be lower than for the heavier body.

Except for the cans of coke which would be affected by rolling inertia. Mass on the rims of wheels is harder to accelerate than on the axles.

First.Aspect

Sketchley wrote:

The best way ive had it explained to that potential energy at top of the hill is greater the greater the height of the hill and the greater the mass. Forces acting against accelerations are to donwith rolling resistance which if tyres correctly inflated ahould be about equal, and wind restistance which all depends on how small you can make the front end, here the difference between the light and heavy rider is smaller thab the diference between potential enrgy. Hence heavy riders are faster downhill......

Its physics, Jim, but not as we know it.

Drfabulous0

I have selflessly given over my afternoon to the pursuit of learning, I had to alter the experiment slightly because I didn't have a ramp and I didn't want my beers all shook up. I have discovered that it is faster to roll a joint when I have eight full cans than when I have eight empty cans, therefore I conclude that full cans roll faster. I advise the OP to always carry a six pack on descents to increase the overall weight, then drink them before climbing back up.

This experiment has been conducted in a purely scientific manner and the results are therefore irrefutable.

sketchley

Genius...

itboffin

JG is excluded from all conversations, he's a freak of nature and must not be encouraged even my tactics of out drinking hom failed, although I hands down will the drink alnight and wake up earliest badge.

This is my only badge and it looks a lot like this :-(

itboffin

Heres a completely useless fact I've always been a fairly good climber, a few years ago I decided to climb to
12 pins in the west of Ireland and drink a can of guineas at the top of each, this meant carrying a case to the top of the first etc etc needless to say logic kicked in after the first and I just drank them all and wobbled back down.

To be sure to be sure

rick_chasey

Never found my megre 55 kilos to be a hindrance on all but the straghtest descents.

meanredspider

All of this is down to a balance of forces.

ITB - most likely down to wind but cold air makes a significant difference and poorer rolling resistance does too. Even a flappy jacket influences it. Your weight loss won't makes as much difference as the other things.

As for cornering, it's the same thing except in reverse. In this case you're accelerating a mass through an arc and mass is working against you (think about how much effort it takes to swing a small hammer versus a heavy one). Lighter riders can corner better than heavier ones.

(I knew 25 years of engineering would come in useful at some point....)

sketchley

Yes but us fatties would have got to the corner quicker and will accelerate out of it quicker.....

Lot and lots of variables, but being a loony is the biggest advantage of the lot

meanredspider

Sketchley wrote:

Yes but us fatties would have got to the corner quicker and will accelerate out of it quicker.....

Lot and lots of variables, but being a loony is the biggest advantage of the lot

Yes - it obviously depends a huge amount on the frequency and type of corner. Us larger guys might accelerate out of a corner quicker but will have to go slower around the corner.

I seem to be reasonably quick down hills - partly due to weight and partly due to suicidal tendencies. Motorsport helps a lot with cornering as the principles are identical: it's not how fast you enter the corner but how fast you leave it.

EKE_38BPM

So a formerly fat bloke has lost 5.7 kg and is now merely overweight.

I don't think that ITB would have bought new clothing to go with his new svelte(ish) figure and was probably wearing the same jacket for both downhill runs. New ITB's clothing would have been more flappy and much less aerodynamic resulting in a slower descent, but I don't think it would have been a ~25km/h difference.

Probably the faster/fatter ITB run was tail wind assisted or the slower/svelter ITB run was head wind impeded. Drink a beer or two and don't worry about it or put the weight back on and descend faster as per Sketchley's explanation.

rick_chasey

Sketchley wrote:

Yes but us fatties would have got to the corner quicker and will accelerate out of it quicker.....

Lot and lots of variables, but being a loony is the biggest advantage of the lot

Won't necessarily accelerate quicker.

In fact. Probably not.

meanredspider

Well - look at it this way. Air resistance squares with speed. Without any air resistance, the 50kg cyclist would accelerate at the same speed as the heavy 100kg cyclist. So, at low speeds IN AIR, the two would accelerate at similar rates, but the higher the speed, the bigger the difference in the balance of forces and the greater the advantage of the heavier cyclist and the quicker he'd accelerate.

This DOES assume a few things that are reasonable in this instance:
The bikes are broadly similar (similar size wheel, bars etc)
The heavier rider doesn't have a frontal area proportional to his mass (a 100kg rider doesn't have twice the frontal area of a 50kg rider - this would only be the case if we were a standard thickness).

I think these are the two key differences why heavier riders go quicker down a straight hill. And, riding with my quick 55kg mate, we know that they do.

roger_merriman

I like descending though I want to live so not many that i'll let the bike off the leash.

Taking one of the hills nr my folks place 3miles 1000ft drop even freewheeling due to my weight i'm far from last, 6-7 mins down but this is where the light weights shine is going up a good time is sub 30mins and folks can loose a lot of time going up or gain.

itboffin

Despite a nasty cold headwind I still managed a respectable 40mph on the descent this morning, I'm ignoring the fact my avg going up was about 9 mph

Still a win in my books

Daz555

Drfabulous0 wrote:

Get back to school, an object with more mass does not fall faster under gravity. Unless you have significantly reduced your frontal area and therefore the air resistance it doesn't make one bit of difference.

Air resistance is the key. A balloon and a bowling ball will fall at the same rate in a vacuum. However when resistance is to be considered then the stronger force of attraction for the bowling ball allows it to push more air out of the way and therefore reach a higher terminal velocity - which is why bowling balls fall quicker than balloons here in the real world.

On a bike going downhill the rider's mass will therefore play a part.

Personally I'd rather be quicker uphill - defo need to lose a few kilos.