What is the truth behind bike weight? Does it really help THAT much?



ScienceIsCool

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Jun 25, 2006
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Crankyfeet said:
You don't need to do an experiment. It's simple physics. Doesn't SOMEONE around here have some knowledge of physics/mechanics.... :rolleyes:

It is the same reason why a guy gets a better start in a 100 meter sprint by pushing off rigid starting blocks as opposed to the same guy pushing off foam rubber blocks. Any deformation of the frame is work absorbed that is not being transmitted via the chain to the rear wheel.
Yeah, I've worked out the numbers here in this forum a few times. The difference between a super stiff frame and a "noodly" frame (using measured frame stiffness) was less than 0.05% of Power. Or something like that. I can't be bothered to run the numbers again or look it up. So maybe 0.2 Watts at a big effort of 400 Watts.

In other words, the zipper on your jersey will probably have a bigger effect...

John Swanson
www.bikephysics.com
 

Crankyfeet

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Jun 5, 2007
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ScienceIsCool said:
Yeah, I've worked out the numbers here in this forum a few times. The difference between a super stiff frame and a "noodly" frame (using measured frame stiffness) was less than 0.05% of Power. Or something like that. I can't be bothered to run the numbers again or look it up. So maybe 0.2 Watts at a big effort of 400 Watts.

In other words, the zipper on your jersey will probably have a bigger effect...

John Swanson
www.bikephysics.com
You've worked out the numbers in this forum? Theoretically? Or you've done experiments?
 

Crankyfeet

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sogood said:
It was interesting. I feel there is a flaw in his thinking on the physics of BB stiffness. The pedal stroke is a not a uniform application of force. The frame/BB absorbs the energy when pedal force is at the maximum of its cycle (around 2 o'clock to 4 o'clock) and then "springs back" most likely when the force is least (around the 12 o'clock and 6 o'clock positions). At the 12 and 6 position, the frame returning its "spring" will be absorbed by lifting the rider's weight back up... but will not neccessarily be going into the drivetrain. You can jump up and down on pedals as much as you like if they're in the 12 and 6 o'clock positions and not much force is going to go into the drivetrain.

Of course the physics of when the frame responds (springs back) is dependent on the properties of the frame. It will occur at some point after maximum applied force though.

Also, the tendency of a rear wheel to slip sideways when out of the saddle and sprinting is compensated for by tilting the frame from side to side IMO. Something you see sprinters do.
 

gemship

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Sep 19, 2006
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dhk2 said:
Believe you're confusing negativity with realism, and missing the fact that many of us who are responding are already riding lightweight, mid-to-high end equipment.

A 7 lb lighter bike is certainly going to make you faster on steep climbs, since speed at a given watt output is almost proportional to total weight for low-speed climbing. Saving say 20 seconds on a steep 10 minute climb is significant to many of us, although the reasons many vary. Expect to see a speed increase of 0.2 mph on your computer, not 2-3 mph, and you won't be disappointed.

Thankyou my thoughts are your thoughts. Seems realism is a stick in the mud.
 

gemship

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Sep 19, 2006
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By the way I'm very excited about the Cervelo. The damage ringed in at 5300 dollars. This also included a pair of Specialized S works cycle shoes that are so light and riding shorts and a Those feel great by the way, wish I had them sooner. oh yeah and a computer, my first so now I dont have to use my watch. With pedals the bike tipped the scales at 15.7 pounds. :eek:
 

Camilo

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Apr 5, 2007
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Crankyfeet said:
You don't need to do an experiment. It's simple physics. Doesn't SOMEONE around here have some knowledge of physics/mechanics.... :rolleyes:

It is the same reason why a guy gets a better start in a 100 meter sprint by pushing off rigid starting blocks as opposed to the same guy pushing off foam rubber blocks. Any deformation of the frame is work absorbed that is not being transmitted via the chain to the rear wheel.

Also.. if you want to do an easy test to see the effects of stiffness.... just rig up a really flexible framed bike. One made of plastic, but strong enough to take the weight. Then go for a sprint. Then see if you notice any difference in your speed.
I'm not a physicist nor a strong cyclist, but my WAG is that it is a miniscule, immaterial issue until two strong riders are sprinting the last 100 meters of an important (to them) event, and a 1/2 meter difference is significant. That is 1/2 of 1 percent, right? And then, the psychological thing is just as important as measurable physical differences.

Myself? I'd like to think that it's worth it if it helps me keep up with my buddies on a hilly afternoon ride, but really, I don't care if they are 20 meters ahead at the top of a hill, they almost always would slow down enough to catch, or I'd be able to push 3 more strokes before coasting down the next hill, then I'd catch up.
 

Crankyfeet

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Camilo said:
I'm not a physicist nor a strong cyclist, but my WAG is that it is a miniscule, immaterial issue until two strong riders are sprinting the last 100 meters of an important (to them) event, and a 1/2 meter difference is significant. That is 1/2 of 1 percent, right? And then, the psychological thing is just as important as measurable physical differences.

Myself? I'd like to think that it's worth it if it helps me keep up with my buddies on a hilly afternoon ride, but really, I don't care if they are 20 meters ahead at the top of a hill, they almost always would slow down enough to catch, or I'd be able to push 3 more strokes before coasting down the next hill, then I'd catch up.
I am just interested to get to the truth in the matter. I would like to know how this half of one percent figure was determined.
 

kakman

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Dec 3, 2005
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Crankyfeet said:
I am just interested to get to the truth in the matter. I would like to know how this half of one percent figure was determined.
I think he's referring to half a metre over a 100 metre sprint.

/k
 

kakman

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Dec 3, 2005
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sogood said:
Within that thread, the bike in question was a Cervelo Team Soloist. So the arguments were primarily with aero benefits. Hilarious.
I didn't see the thread but it seems there are regular claims about speed improvement with new bikes. There is, of course, the new bike placebo affect, but there's also the issue of fit and geometry.

My anecdotal evidence shows speed increases of around 2-3 kph when I ride the drops rather than the hoods. It's quite possible the person in question was seeing improvements, but they may have been more to do with a better position on the bike that anything else. The Soloist is very aero (and stiff) and may account for a small improvement but, as Alientaor said, the aero effect of the bike pales into insignificance compared with the rider, wheels, helmet etc. I think it's far more likely the benefits, if real, came from improved position.

Like most of us, I'm in no position to disprove what was said, so we need to keep an open mind about the possible reasons behind it.

Could this only be achieved on this bike? Possibly, but probably not.

Could it have been achieved with the previous bike? Probably, but possibly not.

Are they happy with the bike and its perceived performance? Guess that's all that matters.

As I said in my only other post on the matter, lightness is critically important to me only when lifting the bike onto the roof rack.

/k
 

JohnO

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Jul 5, 2003
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On the other hand, super stiff frames and wheels tend to hammer you while you're riding, especially if the road isn't perfectly smooth, so there's fatigue to the rider to consider as well.

For most of us, it's just personal jewelry. Pretty cool jewelry, though. I look at it as an investment in my health. If I want to decorate it with the odd bit of carbon or titanium, so what?
 

sogood

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Aug 24, 2006
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JohnO said:
On the other hand, super stiff frames and wheels tend to hammer you while you're riding, especially if the road isn't perfectly smooth, so there's fatigue to the rider to consider as well.
Not if you carefully selected one of those "Laterally-stiff-vertically-compliant™" CF frame. :p
 

TheDarkLord

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Dec 24, 2007
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kakman said:
I didn't see the thread but it seems there are regular claims about speed improvement with new bikes. There is, of course, the new bike placebo affect, but there's also the issue of fit and geometry.

My anecdotal evidence shows speed increases of around 2-3 kph when I ride the drops rather than the hoods. It's quite possible the person in question was seeing improvements, but they may have been more to do with a better position on the bike that anything else. The Soloist is very aero (and stiff) and may account for a small improvement but, as Alientaor said, the aero effect of the bike pales into insignificance compared with the rider, wheels, helmet etc. I think it's far more likely the benefits, if real, came from improved position.
I haven't read the thread, but I think you are misunderstanding alienator here. A significant contribution to a reduction in speed comes from the air-resistance, and thus a more aerodynamic position should give you a significant increase in speed. More so with the rider's position compared to wheels, helmet, etc. This is because the surface area that is exposed to a direct head-wind is decreased most with a more aerodynamic position of a rider as opposed to smaller gains with helmet, wheels, etc. So, riding on drops should give you a nice boost in speed. But I guess you cannot climb hills by riding on drops...
 

kakman

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Dec 3, 2005
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TheDarkLord said:
I haven't read the thread, but I think you are misunderstanding alienator here. A significant contribution to a reduction in speed comes from the air-resistance, and thus a more aerodynamic position should give you a significant increase in speed. More so with the rider's position compared to wheels, helmet, etc. This is because the surface area that is exposed to a direct head-wind is decreased most with a more aerodynamic position of a rider as opposed to smaller gains with helmet, wheels, etc. So, riding on drops should give you a nice boost in speed. But I guess you cannot climb hills by riding on drops...
Um, that's exactly what I said. Look for the bit where I say rider position probably accounted for the gains and the aerodynamics of the bike are small compared to helmet, wheels etc. :)

/k
 

TheDarkLord

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Dec 24, 2007
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kakman said:
Um, that's exactly what I said. Look for the bit where I say rider position probably accounted for the gains and the aerodynamics of the bike are small compared to helmet, wheels etc. :)

/k
oh, ok. Sorry if I misread your post...
 

JTE83

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Jan 28, 2004
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oldbobcat said:
I propose a more scientific study--10 mile time trial on a rolling course, out and back, three times on each bike, one ride a day, with identical tires, shoes, and pedals, identical warm-up, and no panniers on the OCR.

I am working temporarily in Houston, TX - on the Space Shuttle. So I only have 3 of my bikes with me here, the rest of my 11 bikes are in Chicago. So I can't do a comparison test of all 7 of my road bikes, 6 of them having powermeters.

I'm spending more time on my weekends wrenching on my Honda Civic HX modifying it to be a dream ride. The HX is a classic fuel efficient car desired by hypermilers, and I'm spending lots of web time figuring out the best fuel efficient techniques and modifications to my car. Last tank was 43.87974 mpg, which is way better than any modern 2008 non hybrid fuel efficient car out there. So now I got a new hobby! - Hypermiling my car!
 

alienator

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Jun 10, 2004
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JTE83 said:
I am working temporarily in Houston, TX - on the Space Shuttle. So I only have 3 of my bikes with me here, the rest of my 11 bikes are in Chicago. So I can't do a comparison test of all 7 of my road bikes, 6 of them having powermeters.

I'm spending more time on my weekends wrenching on my Honda Civic HX modifying it to be a dream ride. The HX is a classic fuel efficient car desired by hypermilers, and I'm spending lots of web time figuring out the best fuel efficient techniques and modifications to my car. Last tank was 43.87974 mpg, which is way better than any modern 2008 non hybrid fuel efficient car out there. So now I got a new hobby! - Hypermiling my car!

  1. What's that got to do with bike weight?
  2. Wow. Saving gas by not riding a bike. Hmmm. That almost comes close to nearly making sense.
  3. Do you have a grasp of that whole significant digits thing? I'm willing to bet real money that your calculation isn't reliable beyond 1/100.....and that's a generous offer. Such ignorance with calculations really makes me fear for the shuttle program.
 

Crankyfeet

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alienator said:
Do you have a grasp of that whole significant digits thing? I'm willing to bet real money that your calculation isn't reliable beyond 1/100.....and that's a generous offer. Such ignorance with calculations really makes me fear for the shuttle program.
I think its fair to go to 5 decimal places. 1/100,000 of a mile is a little over half an inch a gallon. That's around 8 inches on a full tank which could be the difference between just making it to the pump or your car's fuel tank inlet being tantalisingly just out of reach of the nozzle... :D
 

Phill P

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Jul 9, 2006
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I think the point to signifacant figures is the accuracy of your measurement.

If was 5% incorrect (5% incorrect is not too much to expect from a car), and the pump was 1% wrong, the quoting to 5 dp is way over kill. I'd say there would be error even in the 2nd dp.

but we digress.....
 

sogood

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Aug 24, 2006
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Crankyfeet said:
I think its fair to go to 5 decimal places. 1/100,000 of a mile is a little over half an inch a gallon. That's around 8 inches on a full tank which could be the difference between just making it to the pump or your car's fuel tank inlet being tantalisingly just out of reach of the nozzle... :D
ROTFL! We really have some great talents here. :D
 

TheDarkLord

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Dec 24, 2007
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alienator said:
  1. Do you have a grasp of that whole significant digits thing? I'm willing to bet real money that your calculation isn't reliable beyond 1/100.....and that's a generous offer. Such ignorance with calculations really makes me fear for the shuttle program.
You know, at least one NASA mission has failed because of a mix-up between different units. Actually, I would say 1/100 is very generous...
 

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