3 ms·

Also you reclaim the energy on the downhill. Mass doesn't matter as much as people used to think. (Hence no modern bikes being made out of drillium.) The one t

by kqr 4y ago

Also you reclaim the energy on the downhill. Mass doesn't matter as much as people used to think. (Hence no modern bikes being made out of drillium.) The one thing where I suspect mass might matter is in the wheels. It's a huge difference between getting a heavy wheel spinning and a light one, and at least for commuting/recreational usage, most of that will be bled off as heat anyway.

rhn_mk1 4y ago
You reclaim it downhill if you book it. Casual riders seem to brake after reaching a certain speed, so they will send that energy to heat the brakes. Plus, the unfun of getting up the hill is probably more important than the fun going down, as it takes longer :)
- DangitBobby 4y ago
  Unless you have a stop sign, traffic signal, or sharp curve at the bottom.
fluoridation 4y ago
On the other hand, a heavy wheel will also hold more angular momentum and so will better resist drag, at the cost of more difficult acceleration, braking, and stiffer but more stable steering.
- roelschroeven 4y ago
  A heavier wheel, just like a heavier bike in general, doesn't "better resist drag". Yes, it makes the bike slow down slower, just as it makes it harder to speed up the bike. When people say heavier bikes and/or wheels "resist drag better", it sounds like all that extra mass somehow magically makes it so that you need less power on the pedals to maintain speed. That is simply wrong.
  - fluoridation 4y ago
    No, it's not wrong. Given the same cross-section and initial speed, a heavier object moving through a fluid over a certain time will decelerate less than a lighter object, and so will require less energy to bring it up to the speed it had before decelerating, because the difference in speed will be less. For the same reason, you would not ride while dragging an open umbrella behind yourself, but you'd have no problem putting a metal cube of the same mass on your stem. It's the ratio of cross-section and mass.
    - roelschroeven 4y ago
      > a heavier object moving through a fluid over a certain time will decelerate less than a lighter object True > and so will require less energy to bring it up to the speed it had before decelerating, because the difference in speed will be less False The higher inertia of the heavier object resisted the deceleration, and that same higher inertia will resist acceleration too in exactly the same way. You've gained nothing at all. > For the same reason, you would not ride while dragging an open umbrella behind yourself, but you'd have no problem putting a metal cube of the same mass on your stem. It's the ratio of cross-section and mass. Now you're talking about aerodynamics which is a whole different matter. For aerodynamics, cross-section is of course a major factor (not the ratio of cross-section and mass!). That's pretty obvious, but has nothing to do with a discussion about mass. (Ratio of cross-section and mass is relevant for things in free fall, because there gravity is the relevant force. Completely different from a vehicle on a flat road.)
- aqfamnzc 4y ago
  Drag will exert a constant force regardless of mass. So heavier wheels may allow you to coast farther, but at constant speed they don't affect the drag component.
  - fluoridation 4y ago
    I didn't say greater mass changes drag, I said it resists drag better. As you say, the force remains constant, so with constant force and greater mass, acceleration gets a lesser absolute value.
jjav 4y ago
> Also you reclaim the energy on the downhill. What does this mean? A downhill gives your legs a rest if you coast but it's not going to restore all the energy you burned going up the hill. And if you're riding sportily (not even a race necessarily), you'll still be pedaling the same amount of watts downhill. You'll just be going faster for the same amount of muscle power. > Mass doesn't matter as much as people used to think. (Hence no modern bikes being made out of drillium.) Modern bikes are getting lighter and lighter. > The one thing where I suspect mass might matter is in the wheels. Yes, in terms of ROI by unit weight, lighter wheels are the best place to start.
- elektronaut 4y ago
  > Modern bikes are getting lighter and lighter. The trend over the last few years have been disk brakes, aerodynamic features and wider tires, all of which come with a weight penalty. Some brands are now starting to converge their aero and lightweight offerings, though.
dtgriscom 4y ago
TIL: drillium, https://en.wikipedia.org/wiki/Lightening_holes https://en.wikipedia.org/wiki/Lightening_holes
roelschroeven 4y ago
> Also you reclaim the energy on the downhill. Not really. Bikes don't have a battery to store reclaimed energy, if that would even be a thing. Maybe you go a bit faster on the downhill; I wouldn't call that reclaiming energy. > Mass doesn't matter as much as people used to think. On a flat road at a constant speed, mass doesn't matter (except for the slightly increased rolling resistance). With speed changes (in situations with sharp corners for example, or if you want to accelerate fast to escape from the bunch or to catch someone else) or in hilly/mountainous areas, mass does matter. For recreational use, spending lots of money for shaving of a few grams here and there is not worth the effort, that's true. But a few kilograms more or less do make a difference. > Hence no modern bikes being made out of drillium. Modern top-of-the-line road bikes for climbing are already as light as allowed by the UCI. It makes no sense to drill holes in such a bike: you wouldn't be allowed to use it in a race.
- fluoridation 4y ago
  The flaw in that reasoning is that it assumes that the only conceivable use for a lightweight bike is to compete in a UCI-sanctioned event.
  - roelschroeven 4y ago
    An unstated assumption in my comment was that situations where light weight is most desirable are always UCI-sanctioned events. I admit that that view is probably too narrow.
- burnished 4y ago
  Kinetic energy is still energy, you store it going up hill and reclaim it going down.
  - DangitBobby 4y ago
    Unless there is a stop sign, traffic signal, or sharp curve at the bottom before you've gotten to spend all that energy you worked so hard to build up. It's just not a guarantee that you can go full tilt down the gigantic hill you just climbed.
andreareina 4y ago
Modern bikes don’t go below a certain weight because of the UCI minimum. If you look at non-UCI hill climb events you’ll see some crazy stuff.
- indymike 4y ago
  This. When I used to ride (80s and 90s), a lot of people would try to shave weight and you saw some crazy stuff to get the bike down to 13-15 pounds. Bikes would be dangerously fragile because people would drill stuff and make wheels with fewer and fewer spokes. 17-19 lbs was really where the balance was between safe and light... and having equipment fail cost you a race really sucked... and people were getting hurt.

by kqr 4y ago

rhn_mk1 4y ago

You reclaim it downhill if you book it. Casual riders seem to brake after reaching a certain speed, so they will send that energy to heat the brakes. Plus, the unfun of getting up the hill is probably more important than the fun going down, as it takes longer :)

DangitBobby 4y ago
Unless you have a stop sign, traffic signal, or sharp curve at the bottom.

fluoridation 4y ago

On the other hand, a heavy wheel will also hold more angular momentum and so will better resist drag, at the cost of more difficult acceleration, braking, and stiffer but more stable steering.

roelschroeven 4y ago
A heavier wheel, just like a heavier bike in general, doesn't "better resist drag". Yes, it makes the bike slow down slower, just as it makes it harder to speed up the bike. When people say heavier bikes and/or wheels "resist drag better", it sounds like all that extra mass somehow magically makes it so that you need less power on the pedals to maintain speed. That is simply wrong.
- fluoridation 4y ago
  No, it's not wrong. Given the same cross-section and initial speed, a heavier object moving through a fluid over a certain time will decelerate less than a lighter object, and so will require less energy to bring it up to the speed it had before decelerating, because the difference in speed will be less. For the same reason, you would not ride while dragging an open umbrella behind yourself, but you'd have no problem putting a metal cube of the same mass on your stem. It's the ratio of cross-section and mass.
  - roelschroeven 4y ago
    > a heavier object moving through a fluid over a certain time will decelerate less than a lighter object True > and so will require less energy to bring it up to the speed it had before decelerating, because the difference in speed will be less False The higher inertia of the heavier object resisted the deceleration, and that same higher inertia will resist acceleration too in exactly the same way. You've gained nothing at all. > For the same reason, you would not ride while dragging an open umbrella behind yourself, but you'd have no problem putting a metal cube of the same mass on your stem. It's the ratio of cross-section and mass. Now you're talking about aerodynamics which is a whole different matter. For aerodynamics, cross-section is of course a major factor (not the ratio of cross-section and mass!). That's pretty obvious, but has nothing to do with a discussion about mass. (Ratio of cross-section and mass is relevant for things in free fall, because there gravity is the relevant force. Completely different from a vehicle on a flat road.)
aqfamnzc 4y ago
Drag will exert a constant force regardless of mass. So heavier wheels may allow you to coast farther, but at constant speed they don't affect the drag component.
- fluoridation 4y ago
  I didn't say greater mass changes drag, I said it resists drag better. As you say, the force remains constant, so with constant force and greater mass, acceleration gets a lesser absolute value.

jjav 4y ago

> Also you reclaim the energy on the downhill. What does this mean? A downhill gives your legs a rest if you coast but it's not going to restore all the energy you burned going up the hill. And if you're riding sportily (not even a race necessarily), you'll still be pedaling the same amount of watts downhill. You'll just be going faster for the same amount of muscle power. > Mass doesn't matter as much as people used to think. (Hence no modern bikes being made out of drillium.) Modern bikes are getting lighter and lighter. > The one thing where I suspect mass might matter is in the wheels. Yes, in terms of ROI by unit weight, lighter wheels are the best place to start.

elektronaut 4y ago
> Modern bikes are getting lighter and lighter. The trend over the last few years have been disk brakes, aerodynamic features and wider tires, all of which come with a weight penalty. Some brands are now starting to converge their aero and lightweight offerings, though.

dtgriscom 4y ago

TIL: drillium, https://en.wikipedia.org/wiki/Lightening_holes https://en.wikipedia.org/wiki/Lightening_holes

roelschroeven 4y ago

> Also you reclaim the energy on the downhill. Not really. Bikes don't have a battery to store reclaimed energy, if that would even be a thing. Maybe you go a bit faster on the downhill; I wouldn't call that reclaiming energy. > Mass doesn't matter as much as people used to think. On a flat road at a constant speed, mass doesn't matter (except for the slightly increased rolling resistance). With speed changes (in situations with sharp corners for example, or if you want to accelerate fast to escape from the bunch or to catch someone else) or in hilly/mountainous areas, mass does matter. For recreational use, spending lots of money for shaving of a few grams here and there is not worth the effort, that's true. But a few kilograms more or less do make a difference. > Hence no modern bikes being made out of drillium. Modern top-of-the-line road bikes for climbing are already as light as allowed by the UCI. It makes no sense to drill holes in such a bike: you wouldn't be allowed to use it in a race.

fluoridation 4y ago
The flaw in that reasoning is that it assumes that the only conceivable use for a lightweight bike is to compete in a UCI-sanctioned event.
- roelschroeven 4y ago
  An unstated assumption in my comment was that situations where light weight is most desirable are always UCI-sanctioned events. I admit that that view is probably too narrow.
burnished 4y ago
Kinetic energy is still energy, you store it going up hill and reclaim it going down.
- DangitBobby 4y ago
  Unless there is a stop sign, traffic signal, or sharp curve at the bottom before you've gotten to spend all that energy you worked so hard to build up. It's just not a guarantee that you can go full tilt down the gigantic hill you just climbed.

andreareina 4y ago

Modern bikes don’t go below a certain weight because of the UCI minimum. If you look at non-UCI hill climb events you’ll see some crazy stuff.

indymike 4y ago
This. When I used to ride (80s and 90s), a lot of people would try to shave weight and you saw some crazy stuff to get the bike down to 13-15 pounds. Bikes would be dangerously fragile because people would drill stuff and make wheels with fewer and fewer spokes. 17-19 lbs was really where the balance was between safe and light... and having equipment fail cost you a race really sucked... and people were getting hurt.