yorick76

i'm an average cyclist -- two good rides a week and I seem to pass more often than not on uphills, and can stick with the pack on flats, but on any downhill, my bike just does not keep up.

I'm lighter than some of my riding partners, but my bike is heavier (I ride a cro-moly). I don't get what the problem is, maybe I'm just not pushing hard enough early in the downhill?!

I picked up new Mavic Open Pros - so the bearings are relatively new.

What else could be slowing me or my bike on the downhill....I'd prefer to blame the bike.

thanks for any input.

fmw

Blaming the bike is the wrong answer. Sorry.

duckliondog

Yeah, a heavy bike is only heavy by 5 lbs or so. A heavy rider is heavy by, well, a lot more. I'm 130 and I suck downhill, actually, that's what I do when I go downhill, I suck wheels. Don't blame your bike, improve your tactics. Learn to do a full aero tuck. Get your arms narrow and your belly nearly on the top tube. You bike is fine.

'nother

++ aero position, angle of attack on corners, etc.

I'm frequently out with guys heavier than me who are pedaling like mad in their highest gear, but not in an aero position and who brake hard going into turns. I just tuck in, very little pedaling, fly right on past 'em. It takes a while to build up the skills and confidence, though. Couple of years ago I was the one getting passed

roadfix

Perhaps you're simply not pedalling hard enough during your descents.

NM-NewRoadie

I vote buy a new bike. forget all these "skills" and such, just UPGRADE! :-)

broomhandle

or just not use your brakes.

yorick76

well I'm sure the bike isn't all of the problem but a good wheelset has some effect?!

any physics majors out there -- what is the effect of weight on downhill acceleration?



I definitely could hone the downhill skills, but even in a settin where I start at the top of a hill w/ a few riders and we don't pedal I'm very soon a few bike lengths behind.

roadfix

then gain a few pounds...

fmw

I remember the demonstration a physics professor made in one of our college classes for non-majors. He put a quarter and a down feather in a 5 goot long glass tube and evacuated it. Then he flipped the tube end over end and the quarter and feather both reached the other side of the tube at the same time. The point was that air resistance is the issue, not the pull of gravity. Acceleration in a vacuum happens at 32 ft per second per second at sea level, regardless of the mass of the object.

Weight isn't the issue for descending on a bicycle. Air resistance is. To a small extent you could say that wheel bearings bear on the issue (friction) but not much. If you want to coast faster gaining weight won't help. But getting more aerodynamic will. If you are descending under power, then applying more power is the issue just like it is for climbing.

You can't blame it on the bike.

godspiral

I think downgrade is more accurate

get the heaviest wheels you can find, and hire someone to drive a light bike at the bottom of each hill.

When coasting, my $100 mtb with 1 knobby tire screems past $3000 bikes

roadfix

I'm going to have to disagree with you here. Big riders with huge frontal areas creating more air resistance are able to descend (coast) noticably faster than lighter/skinny aero riders. I understand even a slight air resistance can slow down a light weight rider. I know aerodynamics come into play at a certain point here but I think weight plays a major role when descending. Of course when you put these two riders on a mountain top on the Moon they'll both descend at the same rate.
One reason why tandems quickly pick up some frightening speeds on descents.

fmw

I'm not a physicist. I'm just relaying something I learned in a classroom a long time ago.

Fred

fuerein

I found this:

<a href="https://www.sciforums.com/showthread.php?t=39234"> Do heavier objects fall faster?</a>

That ought to provide all sorts of fun math arguing both sides of the coin for you.

vw addict

are your hubs too tight? I loosen my hubs to the point of loose, and tighten them a 1/16th turn. I cruise past everyone on the downhills.

roadfix

I understand, but physics aside, I'm sure we've all experienced riding downhill that heavier riders tend to pick up more speed than lighter riders...

cooker

You missed the point of the lesson. Weight is a huge issue. In a vacuum they fall at the same rate since there is no air resistance, but as soon as you add air, the heavier object falls way faster. So if the OP gained weight he would overcome the air resistance way better than a light person.

urbanknight

Sorry, YOU missed the point of the lesson. Drop a pebble and a rock from the same height and they will reach the ground at the SAME exact time. The acceleration rate for all objects affected only by gravity is 9.8 m/s/s (meters per second per second). The feather does not fall more slowly because it is lighter, rather because it has a large wind resistance, which is why it falls at the same rate in the tube but not in air. A grain of sand weighs less than a feather, but I promise you it will reach the ground first! High school physics, sorry to say.

Now with that set aside, weight does play a small role, but it's not quantity as much as placement. If you place as much of your weight as low and forward as you can, you will descend more quickly. This seems to account for only about 0.5 to 1 mph according to my meager experiments on a bike (literally shifting back and fourth and watching the speedo change, so take that as you will). Air resistance is also reduced in that position, and I promise you sitting up and riding on the tops reduces speed more than a few mph, so I would agree that air resistance is a major factor here. I think the only reason heavier people descend more quickly is that theis weight can be shifted around more readily without sacrificing stability. I noticed that while I was terrible descending at 120 lb, I pass fellow club members with more "aero" bikes now that I'm 160 lb.

Your bearings, on the other hand, will account for something like 0.1 mph. I was overly proud when I found a front hub that spun (free spinning in my hand for multiple tests) for three times as long as any of my teammates' wheels (Sachs-Maillard sealed bearing 28h from the early 90s for those who care) even against Record, DT Hugi, Dura Ace, White Industries, etc. but I still descended like crap. As long as the bearings aren't being held back, they're a minimal concern. Fix the rider.

Darn, I was going to post a pic of those steel Hed wheels, but their website was changed

urbanknight

btw one thing I don't know how to test is if that descending rate could be afected by the fact that bicyclists are not in freefall, rather rolling on an incline, so inertia might have something to do with it. Eh?

miyata610

LOL...

and don't forget that a heavier body will have a greater Potential Energy at the top of the hill. This is the stuff that powers a coasting bike down a hill, so the more PE you have the more you will accelerate and the faster you will go.

Period.

dalmore

After air resistance, rolling resistance seems to be the next big thing to tackle. Folks here have mentioned getting more aero but not much has been mentioned on rolling resistance in this thread so I'll throw it out there. Perhaps play with the air pressure in your tires a little. Some of the guides I've been reading suggest running your tires a little below max pressure is the optimium for low rolling resistance. I would not have expected that - seems counterintuitive to me. But air is free so it seems worth a shot.


Here's one of those guides.

gwhunt23

Why heavier riders move faster- it's all in the momentum.

In physics, momentum=mass*velocity (for now, just think of velocity and speed as the same thing.) Momentum is literally defined as intertia, the tendency for an object to resist change in its current path and speed.

So, a heavier rider with more mass will have more momentum than a less massive rider with the same velocity. Thus, the heavier rider with more momentum will not have his velocity (speed) affected by air resistance, potholes, and friction as much as the lightweight rider.

waterrockets 

Rolling resistance goes up though... but yeah momentum

My racing buddies and I used to do "coasting intervals" down rolling hills near Boulder when we couldn't muster the motivation for real intervals. At 185, I won a lot

It's pretty fun to coast up into a draft, pull over and pass, and then the other guy does the same. You can get going pretty fast with a long enough hill and good timing.

Avalanche325

Talking about free-falling bodies at all is WAY off track. Unless you miss a turn.

We are talking about rolling resistance on an inclined plane plus (or actually minus) wind resistance. Rolling resistance on an inclined plane is a vector equation that takes into account three forces, acceleration due to gravity, normal force (perpendicular to the plane) and rolling resistance. The rolling resistance portion takes mass into account. So this favors the heavier rider. All of that is just for a straight flat surface at an angle.

Then, momentum comes into effect getting through turns and over small bumps. Momentum is directly proportional to mass, so this favors the heavier rider.

The wind resistance part is a parasitic loss. You could almost take that out for low speeds. Or if both riders can get the same aero tuck and have the same frontal area, regardless of weight. Chances are that the heavier rider has a larger frontal area, so this favors the smaller rider, but this will be a very small differance in a tuck. This is also a smaller portion of the equation until you get to very high speeds.

Gearing can make a differance. A 53-11 of course will get you going faster than a 52-12 before you have to let nature take over. It will also determine if you can accelerate out of a turn.

Skills, skills, skills. Braking, accelerating, and "the line". Read a performance driving or motorcycle book for some insight. If you have to learn about the line, you will never really be great at it, but you can improve.

I am a bigger rider (195lbs) and I blow by people going down. Going up, I just blow. So, small guys, gain your time going up and increase your skills going down.

Light weight is a bigger advantage over all. Here is why. Lets say you and I are doing a 5 mile 1000 ft climb and then coming back down. If you are 2mph faster going up and I am 2mph faster going down, you will win. The reason for this is that there is more time spent going up, so your average speed for the 10 miles is higher.

waterrockets 

No, momentum applies, as the acceleration of gravity adds to it, as does the mass. The more momentum you have, the more you fight the breeze.

Frontal area increases proportionally to the square root of the rider's mass (well volume, but assuming even distribution...). So a rider that weighs 2x only has 41% more frontal area, but twice the momentum...