Rodion R

Take of the Campy bits. That should slow you down a little.

nlerner

https://www.physicsclassroom.com/clas...laws/u2l3a.cfm

RavingManiac

Fill your tires with helium, that will even things out.

jettore

Ok, then there are so many variables involved no one can answer this. The only real test is to start at the top of a hill at the exact same speed as the other rider and coast down. See who wins. I'm gonna bet with both riders in an aero position the CF bike wins.

One thing though, you should not be riding up behind random riders and following so close that there is any chance your wheels can touch.

aixaix 

There would be no speed at all. Bicycles don't work without gravity. (Jets do. That's why rockets can accelerate in space.)

Gravity acts on all things equally. Fellow named Galileo figured this out about 500 years ago. All objects fall at the same rate. The gravity that is pulling you down a hill is acting on every other rider, big and small, light and heavy, the same. What varies is rolling resistance and wind resistance: that's it.
Think about free fall in a vacuum: no friction, no wind resistance. Everything falls at the same rate, accelerating 32ft per second per second. Add atmosphere and you introduce wind resistance, which slows everything down, but differently, depending mainly on shape. Add an inclined plane such as a hill, and you have yet another variable, friction against the slope, which affects different objects quite differently.
You may be more aerodynamic than other people, and your bike may roll better than any other, but I assure you, gravity isn't pulling you down the hill any faster than anybody else.

RobbieTunes

I've been riding both carbon and steel bikes for years. All I have to go on is my experience.

1-Hubs matter.
Maybe it's psychological, but I've tested several, and my ball bearing hubs roll farther, even on level ground.
I have a 1000' straight/level course, and I timed my "slowdown" from 20mph on several bikes.
My 600 and DA hubs roll simply farther, after I stop pedaling, than my modern wheels.

2-Weight matters. I've always outrolled my riding partner, who is all of 125 and rides carbon.
However, on my Kestrel, I don't, and that's with "spin forever" Eurus wheels...actually makes me mad.
I shouldn't be mad. Following a 26-year old triathlete female is generally a pleasant experience.

3-Aero helps, but I still roll a lot faster on my Cinelli than I do on my Kestrel. Noticeably.
Today, I rode a Bianchi I'm repairing, and I was wearing ankle weights, and the downhills were pretty darn fun.
On my same 1000' calibrated stretch, disc wheels and aero helmet keep my momentum from 20mph going longer.

The OP's downhill experience is most likely due to weight, then perhaps hubs.
I've never been a fan of modern wheels on the downhill, mainly for noise.
I can, and do, pass the group, going downhill, not pedaling, and 2 or 3 guys follow me most of the time.
The rest can, and do pass me on the uphill, but not often, because I go ahead and pull the uphill.
Then, of course, I'm exhausted, move to the back and reconsider my fitness sport.

vettracer

Nope, the heavier bike/rider combination will win.

AlbertaBeef

Gravity itself is the acceleration and it's constant for both ... BUT my mass is making me faster... it's part of the equation. It's been quite a while since I took physics, but mass is most certainly part of the equation. Mine is substantially more than a climbing specialist. Air resistance affects him more than me... And thus I go downhill faster...

Net force = mass x acceleration. More mass on a downhill = more force. More force with less resistance (more mass is less-impacted by air resistance) = faster rider.

AlbertaBeef

+1. I'm not sure why people even argue this.

Gonzo Bob

Earth's core is magnetic and the magnetic force is pulling your steel bike down

loneviking61

Mmmm, almost but not quite. Gravity is a constant (and if that ever changes, we are in deep Kimchee!). But, there is resistance from the air all around us, and rolling resistance from the contact of the tires with the ground. Your mass is great enough to overcome more of that resistance than the 'other guy' with the carbon fiber bike. Less resistance, more forward momentum---ergo, you go faster. Your friend on the carbon bike can lay down on the aero bars and reduce some of his wind resistance, but he still doesn't have enough mass to reduce things enough to catch you.

On the flip side, going uphill that mass hurts you. Gravity is still pulling on you and your friend. There's still resistance from the air and the road. But, your friend with the lighter bike is able to overcome that resistance by pedalling hard. You, on your steel bike, can't pedal hard enough to match the lower resistance that your friend on the carbon fiber bike has going uphill.

loneviking61

jimblairo

What does a lugged steel bike have to do with going faster than a carbon bike? Absolutely nothing.

It is going faster b/c it is attracted to the earth by its magnetic field.

RobbieTunes

Lugged steel bikes are cooler.

End of discussion.

sailorbenjamin

Umm... Would it have anything to do with the stiffness of a CF or Aluminium frame or fork? The steel absorbs the little bumps and things and keeps going while the stiffer materials are slowed by every little bit of resistance they come across.
Just a thought. Can't say it's based on anything.

haaseg

I think the term everyone is looking for is "inertia". Inertia is affected by mass. The more inertia you have, the less affected you are by resistance. Think about the amount of breaking power that would be required to stop a loaded freight truck going 55mph vs a Fiat going 55mph. The heavier bike/rider going downhilll is going to be less affected by road and wind resistance.

sailorbenjamin

This whole conversation reminds me of the time a couple of years ago when I forgot to pay the gravity bill. Everything was up in the air for a week before the check finally cleared and they sent a guy to turn it back on.

repechage

I like that.

Near my house is a long steady straight downhill section. I catch lots of guys. I just lost a good amount of weight, like 10% of my mass, so gravity is not pulling that mass any longer at 32 feet per second squared. Still catch them. I think contributing factors are resistance related. By habit I coast with my knees in, cranks at 3-9 o'clock, head looking forward, arms in, sometimes on the tops of the bars. It is all for better aero. Tubulars help too.

By they way, the all but new new SRAM stuff is really loud, I think it has to do with how the cassette is designed, its like a bell. Fulcrum wheels are also often noisy. Many of these new bikes do resonate a lot over bumps.

zandoval 

p=mv ...Its just a simple matter of momentum and velocity - Here's a good link for simple details - I just love how bicycles give us a better understanding of truly important physics...

Lets also not forget I=mrr ...Mass moment of inertia overcoming street surface resistance is why the heavy guy gets to the bottom faster...

https://www.physicsclassroom.com/class/momentum/u4l1a.cfm

Rodion R

Ignoring air resistance and friction, acceleration due to gravity is independent of mass. All objects will accelerate towards the earth's core at the same rate of 9.8m/s^2
https://www.haverford.edu/educ/knight...ccelarator.htm

DannoXYZ 

In the real-world, we do have air-resistance. Which is related your cross-sectional area, multiplied by Cd. On a downhill above +45mph where you can't apply much more power by pedaling, most of the power to overcome this air-resistance is from mg and 1/2mv^2 the higher your mass, the higher your kinetic energy is to oversome the air-resistance.

I used to do hill-repeats on a 3.5 mile hill in Santa Barbara. I'd bring up a back-pack and stuff it full of rocks at the top so I can do the downhill faster. Ended up with about 20-30 lbs of the stuff. Easily gained 5-7mph on the straights (+50mph, up from 45mph without the rocks) and chopped off 10-15 seconds from the downhill time. Above 35-40lbs of rocks, it really hurt my back, increased braking distances and slowed cornering-speeds too much.

Ed in Toronto

Thanks for the replies everyone, I was worried that this thread might degrade into a steel vs. carbon melee, but it didn’t and you guys have given me some interesting ideas, and some great funny replies too.


From the replies, there are 2 ideas that have me thinking. The first is drafting, next time I’m in a following position, I’ll go off to the side to see what effect this has. Maybe the other bike will even start pulling away, I’m not very aerodynamic when riding, usually I’m wearing blue jeans and a tee shirt. I’m not a cyclist, I just ride a bike.


The second idea that’s got me thinking is that all things being equal, both bikers have to deal with wind resistance equally, but if one rider weights a lot more, they wind might have less of an effect because his weight is pushing him through the air. Don’t know how to test this out, but I’ll keep it in mind when I’m going downhill with other riders. I’ll be more aware of how much the other rider weighs.

jolly_ross

leftthread understands I think.

If you and the bike weigh twice as much - then there is twice the force acting between you and the planet true - BUT - it has twice the inertia to overcome. Inertia cancels out weight. Exactly. Always.

Wind resistance and rolling resistance matter - but unless you have something really wrong they're not going to vary a great deal between one person and another. By far the greatest influence on your speed of real world descending is your bike-handling skills. If you stay off the brakes and corner that bit faster then you'll be a long way ahead.

Unless of course you overdo it and crash, it's a fine line.

CardiacKid

I am no physicist, so I can't cite the correct rules, but I believe everyone is hung up on the wrong rules. The Pinewood Derby Rule of Physics says "All other things being equal, the heavier car wins" For those unfamiliar with the Pinewood Derby, it is an annual competition for Cub Scouts. They make their own small wooden cars and race them against other Scouts down a ramp. There is a weight limit of 5 oz. I was in charge of the competition in my sons Pack for three years. I observed over and over that a car that weighed 4.9 oz had no chance of winning.
In that same regard, I have seen over and over skinny people pass me on the uphill and me fly by them on the downhill. Everybody agrees reduced weight is very important to climbing, but cant seem to agree the opposite is also true.

DiabloScott

All things being equal, the heaviest car would have the highest terminal velocity, but that doesn't mean it would win a short race down a ramp.