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Downhill speed - is it really weight dependent?(11 posts)

Downhill speed - is it really weight dependent?Matno
Mar 14, 2003 9:16 AM
I thought I remembered from physics class (way back when) that two balls of different weights rolling down a hill will still reach the bottom at the same time, but that doesn't seem to translate well to the real world. As a lightweight rider (140) I am almost always the last person down the hill in a group ride. I can be pedaling as hard as I can in my 53/12 gear while bigger guys are practically coasting next to me and passing me at the same time! I guess it's all fair in the end since I can get UP the hills faster, but I'd sure like to know if there are any tricks to going faster downhill besides picking up a few rocks from the side of the road!

Incidentally, I have aerobars, and the guys who dust me on the downhills don't. Doesn't seem to matter one way or the other. Same thing used to happen on mountain bikes when we'd hit the bottom of the trail and ride the road down the canyon. Very annoying.
acceleration, momentum nmcollinsc
Mar 14, 2003 9:19 AM
i didn't do very well in physics but..._rt_
Mar 14, 2003 9:30 AM
incline plane problems were the one thing i actually did understand. if i recall correctly (which i'm probably not since i took university physics in 1987) because the pull of gravity is the same on both objects (9.8 m/sec squared, i think) but the friction is different, given sufficient distance both will reach the bottom of the hill at the same time. however, if the hill is not sufficiently long then the larger one will reach the bottom before smaller one.

as a very light weight rider (100 lbs) i share your problem. no matter how aero i get i always end up losing ground on descents (unless, of course, i find a nice big person to draft off of!). part of what i've learned to do is to assume a mtb position on longer descents (low & off the back of the saddle) this seems to help me keep my speed. other people swear by putting their weight forward but that doesn't seem to work for me. the other thing i've learned is to know that i will never be able to get a gap in a race if i take off on a descent that leads out onto a flat. ;-)

the last thing that has helped me (besides knowing never to touch the brakes) was getting a different bike. my OCLV seems to descend better than my bianchi. something to do with geometry (since i am using the same wheels), but that goes beyond my limited understanding of physics!

Did I imagine "inclined plane" vs. "free fall"?cory
Mar 14, 2003 9:34 AM
Two objects of different weight in free fall will drop at the same rate, but I think (?) I remember that when balls of different weights roll down an inclined plane, the heavy one wins.
For sure, that's true when two bicycles roll down the hill into Reno: My weight varies from about 225-240, and the guys I ride with are mostly 160-190. They can all outclimb me, but without exception, every time, I outcoast them. Holds true when we switch rides, holds true on road or mountain bikes, holds true in Nevada or California. Often I pass them when I'm just kicking back and they're pedaling.
Yeah, but would it hold true below the equator?Matno
Mar 14, 2003 9:50 AM

I used to drive my roommate crazy when I'd outcoast him on the flats (I kept my bike well-tuned, he pretty much bought his and rode it without any maintenance for years). Now I know how he must have felt.

But like you said, when it comes to hills, you can switch bikes and it doesn't matter. Kind of stinks. My bro-in-law did a race last week (he's an awesome mountain biker, but this was his first race on a road bike), and being close to the same weight as I am, he got killed on all the downhills. He made up for it on the climbs, but said it was frustrating to get passed on the downhills by the same people he easily dusted going up! I guess life is all about compromise! Maybe I'll just accept that I'm light and enjoy it. Besides, the time differences on climbs are bigger than the time differences on descents. Now if I could get in good enough shape to actually ENTER a race... 8^)
Did I imagine "inclined plane" vs. "free fall"?juanteal
Mar 14, 2003 10:18 AM
Two objects of different weight in free fall will drop at the same rate *only in a vacuum.*

When you factor in the atmosphere, bowling balls fall much faster than feathers. Hard to believe, but in a vacuum they fall at the same rate.

Same rules apply for rolling stuff down hill. Heavier = faster (unless you're on the face of the moon!)
Mar 14, 2003 9:52 AM
In a vacuum on a bike with no friction a 140 lb rider and a 210 lb rider would coast down the hill at the same speed. The force of gravity is 1.5 times higher on the 210 lb rider, but the inertia of the big rider is also 1.5 times higher, so the acceleration is the same for both riders. In reality, there is wind resistance. The force of gravity increases in proportion to mass, but the wind resistance increases in proportion to area. A 210 lb rider has 50% more mass than a 140 lb rider, but only about 30% more body area. So, the lighter rider has higher wind resistance relative to gravitational force compared to a heavier rider. However, the lighter rider has a greater advantage going up hill than the heavy rider has going down hill. The only "trick" going down hill is to get as aero as possible by minimizing the area exposed to the wind.
Wind resistance.TWD
Mar 14, 2003 10:26 AM
Oh boy,

The rusty gears in my mind are starting to grind to life.

I think that might be true in a vacuum, but you're forgetting about the forces exerted by wind resistance.

Newton's first law of motion states something to the effect of bodies in motion stay in motion and bodies at rest stay at rest. What this means is that an object will stay at the same velocity unless some force (or combination of forces) act on it to speed it up or slow it down.

Newton's second law states that force = mass x acceleration. So, assuming two riders of different mass coast down a hill, the heavier rider will be exerting alot more force (since gravity is a constant for both riders)against the force of wind resistance which is the largest force slowing you down. So, the force of the wind has more of an effect on the lighter rider since his smaller mass doesn't exert as much force to counteract the wind. The heavier rider will go faster.

This is oversimplified since it neglect rolling resistance and the aerodynamics of each rider, but you get the picture.

So go eat some donuts. You'll get fatter....errr.... I mean faster on the downhills.
Mar 14, 2003 10:52 AM
All I know is I pass most of my biking buddies on the downhills, and get killed on the climbs. I definitely could lose a few pounds. What gets to me about skinny little climbing dudes is when they coast down the hills when leading a paceline. The downhills is where us fat guys pick up our average speeds. There is one exception to this rule -- when descending on curvy mountain roads. Descending on mountain roads has as much to do with nerves and handling skill as it does with weight. Just ask JS Haiku and MikeP about our ride to the top of Brasstown Bald in North Georgia last fall. According to our weights, I should have been the fastest one down the mountain, but I was hitting my brakes so often that I negated any advantage. I also was shaking like a leaf, partly from nearly freezing to death and partly from sheer terror.
not to mention...Fredrico
Mar 14, 2003 1:43 PM
Your heavier inertial mass would have peeled you off the corners, while the lighter guys could more easily re-direct their weight into the turns(?) Another physics question, at any rate.
re: Downhill speed - is it really weight dependent?smooty
Mar 15, 2003 6:21 AM
I weigh 200lbs and I once hit 67mph comming down Reeds gap off the blue ridge parkway in Virgina.
just in case you wanted to know..........