I'm a beginner rider and I haven't had much experience riding in a group but I've realized that when coasting on even small declines, I tend to speed up faster than others. I often have to use the brakes on a downhill to match speeds with other riders. I was just wondering what kind of factors go into downhill speeds. Am I just in a more aerodynamic position? I'm usually up on my hoods and not tucked in just like the other people I ride with.

I have a Jamis Xenith Comp 2007 which is a pretty light bike. But I'm not lighter than the other riders. In fact, I'm at 180 lbs.

This is kind of related but does anyone draw a correlation with matchbox car races? Race wooden cars down a straight track by starting both of them at the same height? We did this in our senior year of college and had some design criteria (frontal area, min/max weight, and materials). It seemed that the more forward and lower the weight distribution, the faster the wooden car.

Just a fun question to ask. What are your comments?

You go down hill faster because you are gravitationally enhanced.

Haha... ouch. Just say it! Say that I'm fat!

Well, I'm 5'9, about 163 lbs. Did a ride on Saturday with a nice descent. I tucked into as aero a position as I could achieve. I managed about 41.1 without pedaling. A guy came by me at I'm guessing approximately 45-ish plus. Couldn't tell you his height, but I'm guessing the weight was 275-ish. Our bikes probably were of similar weight.

Haha wow does weight seriously play such a factor on downhill...

No wonder everybody passes by me on downhills... I just scaled in at 126! With shoes!

I weigh 130 lbs and pass ppl on decents quite often by tucking into an aero position in drops with elbows and knees in

I am not heavy about 145 and in one century noticed that I descended faster than bulkier and taller folks. I do not have ceramic hubs but was running some old Zipps. I also noticed I was in the drops and slightly aero than most folks. I am sure there were animals that could have pedaled like crazy or have Cervelos with newer Carbon wheels and would passed me, but compared to the general population -- I was descending faster than most even though I am lighter. Maybe the aero position, the wheels or the orbea frame

Same bike, same tyres, same aero position, same hill, different wheels (Ksrium ES from Bontrager Race) gave me a 5kmh difference (10%).

It's friction. You against the air, tyre against the tarmac, hubs against the axle etc.

Weight minus air drag determines your downhill speed. The best downhillers are usually the worst uphillers. It's the only reason the sprinters can make it to the finish to contest the sprint on a hilly course.

Cojones. Perceived size thereof.

It's long been known (like, a couple millennia) that two bodies of differing masses will fall at the same rate if air resistance isn't a factor.

I was catching & passing on downhills this weekend, too (only to nearly die on the uphills ). Could it have been the aerodynamic fairing effect of my energy keg (gut)? Or the tire pressures, or the slightly aero wheels, or did I just pedal longer at the top, or..?

It is.

Two masses of the same x-section area at the same speed will have the same amount of air resistance. Mass X gravitational acceleration gives you a component of force down the hill. Subtract the two and you get the net force. Net force/mass gives you acceleration. Heavier people at high speed tend to get the better of lighter people with this equation when going down hill.

A feather with it's stem filled with lead will fall faster than a feather without the extra weight.

Yeah, it's the age old experiment of two equal masses in a vacuum falling that we saw in grade school science class... it gets more interesting in real life though.

I think rollin is on to something... it's air resistance (drag), rolling resistance (tire friction), and rolling resistance (hubs friction). I think there's more though. Angular momentum and how fast you can get up to speed (acceleration) will undoubtedly affect your top speed.

For example if two different wheelsets have the same weight but one had more weight concentrated towards the center of the wheels, those wheels would have less moment of inertia (angular mass). They will probably get up to speed quicker than if the weight was distributed towards the outside of the wheel.

I don't know what's different about my bike though. I have Shimano Wheels (WH-R500) with Vittoria Zaffiro tires. Or it could be just as simple as I'm usually drafting behind someone and I'll gain speed quicker due to lower air resistance. It just surprises me that this happens to a great extent even when I'm on a shallow grade side by side with my slightly lighter friend who rides a 2006 Jamis Quest.

Heavier people will also roll out longer than lighter people after the downhill has ended because of their greater momentum.

So racers get load up with extra water bottles on top of the hill?

Yes... actually. Feed zones are usually near the top of climbs (on the backside, not on the way up). And many racers (tri folk are notorious for this, apparently) dump their water on the way up the hill.

I weigh 210lbs and downhill a loud sonic boom can be heard.

https://en.wikipedia.org/wiki/Terminal_velocity

Notice that in formula, your terminal velocity increases as square root of your mass/cross section area.
If you assume your mass is proportional to your volume, your cross section is proportional to the square of the cube root of your mass. So your terminal velocity is proportional to the sixth root of your mass, so higher mass will give you a higher terminal velocity.

I descend faster now, at 150lbs, then when I did at 165lbs.

Better technique then. I'll bet that after cycling 15 lbs off your body, you are more flexible, get in the drops better, and are just generally more comfortable on your bike and take better lines through curves and use your brakes less. You also probably are stronger and accelerate faster (with your pedals) at the top of the hill.

All things that will negate the 15 lbs of lost weight.

the guys in my group ride are 30 to 50 pounds lighter then me at 218lbs...
on the descents I will win handily.. on the flats i can pull for much longer...
on the hills I need a crane to get me over them...
and yes after a descent I will roll longer then a lighter rider if we have similar equipment and both are in a similar aero position... but I think that is only because my speed is higher at the base of the descent.

Interesting. Never thought of it that way.

You know of Isaac Newton, right? Here's the thing: Gravity acts in proportion to mass. That means, more mass has more gravity acting on it. However, the laws of momentum / inertia are such that more mass requires more force to accelerate it. It turns out that in a vacuum (no wind resistance and no friction), it is easy to determine that the force of gravity on mass increases with mass at exactly the same rate that the force required to accelerate an object increase. In other words, heavier objects do NOT fall faster than lighter objects, in a vacuum (ie, a frictionless environment).

So eliminate the notion that gravity acts upon you differently than someone else because of your weight. Well, technically it does, but momentum and inertia cancel it out.

However, what could be different is friction. And by friction, there are several things to consider: Air friction (aerodynamics), the friction in your hubs, friction through tire deflection, and friction of the tire on the road surface.

So.... if you're passing on the downhill without pedaling, you may be more aerodynamic than them, or your hubs may be better, or your wheels/tires may offer less rolling resistance.

Or it's also possible that you're choosing a better line on your descent. A skier may slow his descent by choosing a line that follows an S pattern down the mountain. Switchbacks reduce the grade percentage, and thus create a slower descent. It's possible that you're pointing yourself straight down the mountain, cutting corners out of it, whereas others are weaving just enough to either increase the distance they have to travel, or effectively reduce the degree of descent.

All these things are pretty minor except for rolling resistance or aerodynamics. If you're passing them, it's probably a combination of these two characteristics.

I also seem to be "gravitationally enhanced." I think it's a matter of density. That is, folks who are both skinny and heavy seem to have an easier time going fast down a hill.

Gravity is only important on a short straight hill. On any decent sized hill, the fastest one down is the one who loses the least speed in the switchbacks. Many big guys slow down way to much going in to a corner and accelerate too slowly coming out.

Aerodynamics is also very important. Many big guys cannot bend over and put their chin on the stem like skinny guys. If you can make yourself low and skinny, you can out accelerate heavier guys with sloppy technique.