# General Cycling Discussion - Dead weight v/s rotating weight

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View Full Version : Dead weight v/s rotating weight

danr
04-15-03, 06:44 PM
OK, I understand the whole rotating weight concept with bicycle wheels. If you have tires that are, lets say, 10g heavier than a previous set of tires, that rotating weight will feel like 100g or so (just an example). In essence, bicycle wheels is where you would really like to save weight.

But what about dead weight? Lets say I have a steel frame that is 2 pounds heavier than an aluminum frame. Is there a difference in performance, besides the fact that you're just carrying around 2 extra pounds?

Just curious.:confused:

Chi
04-15-03, 07:56 PM
I would think that

rotational weight = the rotational acceleration * the mass of the rotating object

At a stable velocity with no outside forces acting on your system (biker+bike), theoretically, you need no force to move the bike! Of course, you have friction working against you, so you'd have to factor some friction as a coefficient into those two formulas.

Someone check my physics?

Hope that makes a little sense ...

DnvrFox
04-15-03, 08:10 PM
For all you would ever want to know about wheels and drag and inertia:

http://www.analyticcycling.com/WheelsConcept_Page.html

Most folks I hvae read who truly know what they are talking about feel that advantages due to weight differences in wheels are highly overrated when compared with other variables.

Go to rec.bicycles.tech for more discussion about such things

froze
04-15-03, 08:36 PM
Well that analytical cycling site is fine and dandy BUT you also have to take into consideration reliablity VS performance. If you are strictly racing than you may have to sacrifice reliability to gain a performance advantage. However if your not racing than a good 28, 32 or 36 hole spoke rim are the most reliable and you should use more spokes in the rear since that is where most of the weight and stresses are place. Tandems and heavy touring bikes use 40 and 48 spoke rims-why? for reliability. If your a heavy guy, over 200lbs than I would go 36 in rear and 32 in front otherwise 32 in rear and 28 in front should be fine.

But overall the most gain from weight reduction is in rotational mass. I use 36 spoke rims (only because back when I bought my first race bike in 79 high strength wheels were not around and the common belief was the stiffer 36 spoked wheel would flex less thus transmitting more power and I still use the Suntour Superbe hubs from that period), I did realize years later that 36 was an overkill especially with better modern rims so when I bought my Torelli Master Series rims, I laced them with DT Revolution spokes to cut down on rotational weight. These spokes are almost as light a TI spokes without the expense or constant adjusting.

Also there is a spoke company called Saniens (I think that's the name) make a superstrong bladed spoke that fits non slotted hubs-in other words hubs with round spoke holes, that slice through the air turbulance with ease.

danr
04-15-03, 10:02 PM
Originally posted by DnvrFox
For all you would ever want to know about wheels and drag and inertia:

http://www.analyticcycling.com/WheelsConcept_Page.html

Most folks I hvae read who truly know what they are talking about feel that advantages due to weight differences in wheels are highly overrated when compared with other variables.

Go to rec.bicycles.tech for more discussion about such things

Thanks for the links. I previously searched for info, but I couldn't find any.

I do understand that there are other factors besides weight that go into bikes and wheels. I just don't see the difference between 2 extra pounds of frame weight, and 2 extra pounds of body fat (unless you're a serious racer, where every little bit counts)? To me, the whole weight thing just seemed overrated.

Once again, thanks to all for the replies.

:beer:

D*Alex
04-16-03, 06:46 AM
Well, it may be a bit over-rated, but there are a few things that are agredd upon:
1) Every extra gram of rotating mass affects acceleration.
2) Every extra gram of total mass required more work to move and to raise up (as in climbing a hill).
3) Every extra gram of mass on a bike is extra unsprung mass, which has a negative effect on the ride of the bike.
Everything else is just a matter of degree. You want to be fast/smooth? How much are you prepared to spend?

Richard Cranium
04-16-03, 07:37 AM
Go ahead - get those new Titanium water bottles -- just don't fill 'em.

Not to bore anyone with such an "off-topic" note, but the underlying concepts related to inertia and mass are "grossly magnified" by those cyclists that fail to develop a "smooth-pedaling-style." (especially the heavy ones)

One might conclude that "good form" always trumps good equipment.....

Jim311
04-16-03, 09:30 AM
From what I've heard, adding one pound of rotational weight is about the same as adding 3 pounds of stationary weight. It sounds about right, too.

danr
04-16-03, 01:58 PM
Originally posted by D*Alex
Well, it may be a bit over-rated, but there are a few things that are agredd upon... ...How much are you prepared to spend?

Or, how much weight are you prepared to lose? I can stand to lose about 5 - 10 lbs. I'd rather lose that weight than spend \$1000 on lighter parts.

danr
04-16-03, 02:04 PM
Originally posted by Richard Cranium
Go ahead - get those new Titanium water bottles -- just don't fill 'em.

Not to bore anyone with such an "off-topic" note, but the underlying concepts related to inertia and mass are "grossly magnified" by those cyclists that fail to develop a "smooth-pedaling-style." (especially the heavy ones)

One might conclude that "good form" always trumps good equipment.....

Totally agree. The lightest and fastest bike doesn't mean anything if you don't train and work on technique.

Another reason why I posted this is because I ride a little faster on my "heavy" steel road bike than I did on my lightweight aluminum bike. Once again, I know other factors besides weight come into play, but we're talking an extra 4 +/- 1 pounds. Plus, I have 700x25 tires on my steel bike, as opposed to 700x23 tires on the old aluminum bike.

As far as physics, doesn't more weight equal more momentum? Could the momentum of the spinning wheels and heavier bike offset the benefits of a lighter weight bike and wheels?

Am I overanalyzing?

D*Alex
04-16-03, 02:10 PM
Yes, you are.

danr
04-16-03, 02:13 PM
Originally posted by D*Alex
Yes, you are.

OK, I'll just shut up and ride.

:beer:

John E
04-16-03, 03:47 PM
For slow, smooth, constant-rate in-the-saddle hill climbing, the effort required is roughly proportional to the combined weight of rider, bicycle, and all attached accessores. In this case, an extra kg or two of bicycle weight is negligible for the nonracer. I'll keep my 10kg Bianchi, thank you!

For the out-of-saddle climber who tosses the bike from side to side while holding him/herself relatively upright, frame, saddle, and handlebar mass become somewhat more noticeable.

For acceleration, every gram of mass in the tyre or rim holds one back as much as two grams in the frame, because the tyre's angular intertia is approximately equal to its linear inertia. Many cyclists forget that reduced-spoke wheels need to have heavier rims for a given level of reliability. Thus, the sole advantage of a reduced-spoke wheel is improved aerodynamics, and this difference becomes significant only at relatively high speeds.

I still strongly believe the recreational, touring, or transportation cyclist is better off with good old-fashioned 32- (or more) spoke wheels. The impact of weight is over-rated, since some 85 percent of one's cycling effort is used to overcome wind drag.

hayneda
04-16-03, 04:03 PM
As an engineer, I can tell you the hype about weight is HIGHLY overrated.

There are two factors concerning weight:

1) Lifting it while climbing.

2) Accelerating it.

For just cruising down the road, weight it almost irrelevant. Aero drag is the primary force you have to overcome, followed by a significantly lower frictional force due to rolling resistance of tires, bearings, etc.

As far as rotating vs. non-rotating mass, rotating mass is only a factor (above non-rotating mass) during acceleration (and deceleration). So, it's really only significant for those who need sprint performance (racers). As far as climbing is concerned, lifting an extra pound on the wheels is no different from lifting an extra pound on the frame.

So, if you really want to improve your performance, don't eat that second taco and loss 10 lbs off the engine. The performance gain will be 10 times that of new wheels and will cost you nothing.

Dave

danr
04-16-03, 06:20 PM
Originally posted by hayneda

So, if you really want to improve your performance, don't eat that second taco and loss 10 lbs off the engine. The performance gain will be 10 times that of new wheels and will cost you nothing.

Dave

Nice taco reference.;)

Unfortunately, there are a few things worth living for in life. Instead, I'll just take a diet drink with my taco.

DnvrFox
04-16-03, 09:00 PM
Originally posted by hayneda
As an engineer, I can tell you the hype about weight is HIGHLY overrated.

There are two factors concerning weight:

1) Lifting it while climbing.

2) Accelerating it.

For just cruising down the road, weight it almost irrelevant. Aero drag is the primary force you have to overcome, followed by a significantly lower frictional force due to rolling resistance of tires, bearings, etc.

As far as rotating vs. non-rotating mass, rotating mass is only a factor (above non-rotating mass) during acceleration (and deceleration). So, it's really only significant for those who need sprint performance (racers). As far as climbing is concerned, lifting an extra pound on the wheels is no different from lifting an extra pound on the frame.

So, if you really want to improve your performance, don't eat that second taco and loss 10 lbs off the engine. The performance gain will be 10 times that of new wheels and will cost you nothing.

Dave

Thanks for stating the reallity more clearly than I have ever seen it stated before.

Richard Cranium
04-16-03, 09:26 PM
That's a very silly way to think about it. My point about smoothness of pedal stroke was supposed to elucidate the fact that there is significant loss or "lack of even, or constant" momentum during any cycling.

If bikes were powered by motors instead "bi-peds" weight would be much less important. Even heavy legs don't work as well as light ones....

The "three-to-one thing" is only a factor of overcoming inertia during acceleration. But never mind.

ComPH
04-16-03, 11:05 PM
It's been some time since I thought of physics, but isn't that true that everytime you are riding up hill or against the wind, you are overcoming an opposing force? In that case, the forces you are creating by pedaling, in order to overcome the friction and gravity is Force=mass x acceleration. That would mean that even if you are riding at a constant speed, you are actually accelerating against decelerating forces while the mass is more or less constant. That would mean that you are actually accelerating more/less at all times. Of course these forces become much more significant when the hill is steep and/or the headwind is strong, neverheless you are constantly accelerating the bike against these decelerating forces, so that rotating mass is more significant practically at all times....

hayneda
04-17-03, 09:35 AM
Originally posted by ComPH
It's been some time since I thought of physics, but isn't that true that everytime you are riding up hill or against the wind, you are overcoming an opposing force? In that case, the forces you are creating by pedaling, in order to overcome the friction and gravity is Force=mass x acceleration. That would mean that even if you are riding at a constant speed, you are actually accelerating against decelerating forces while the mass is more or less constant. That would mean that you are actually accelerating more/less at all times. Of course these forces become much more significant when the hill is steep and/or the headwind is strong, neverheless you are constantly accelerating the bike against these decelerating forces, so that rotating mass is more significant practically at all times....

You on the right tract, but no that's not quite correct. When riding uphill at a constant speed, the work you are doing to lift the weight of the system (bike and rider) is mgh, where m is the mass, g is the gravitational acceleration, and h is the height through which you lift it. When I say acceleration with respect to the bike, I mean you are going from a slower to faster speed, i.e. you are increasing your velocity.

When you accelerate (speed up), you must put energy into two things beyond that needed for a constant speed for the given conditions (whether you are riding up and hill or on the level). First, you must accelerate the mass--good 'ol F=ma. That includes the mass of the bike, wheels and rider. This is called rectilliner acceleration or increasing the kinetic energy (linear momentum). Second, you must also provide for the rotational acceleration of the wheels--this is above the energy required just to accelerate the wheels in the direction of travel. This is called angular acceleration or increasing the angular momentum of the wheels.

For example, if you pick up a rock and throw it, while it's flying through the air it has kinetic energy--to slow it requires a force (drag or gravity). If you tie a string onto it and whirl it around in a circle, it has angular momentum and it takes energy to either speed up or slow down the rotation. Of course, when you let go of the string, the rotation energy is then converted into kinetic energy as the rock flies off in the straight path.

You can think of it this way: the wheels are kinda a double whammy cause you gotta both accelerate them forward, as well as "spin them up." But unless you're racing . . .

Climbing is simply lifting all the mass up through the earth's gravitational field. The more mass or the higher or faster you lift it, the more work is required.

Hope this helps,
Dave
Who in addition to being a rocket scientist dweep, also rides his bike

DnvrFox
04-17-03, 10:04 AM
it has angular momentum and it takes energy to either speed up or slow down the rotation.

It also takes energy to keep it at a constant speed, due to resistance of air and the pull of gravity.

hayneda
04-17-03, 10:21 AM
Originally posted by DnvrFox
It also takes energy to keep it at a constant speed, due to resistance of air and the pull of gravity.

For air resistance, yes. For gravity, only if you're going uphill. But when going uphill, assuming you are not accelerating to a faster speed, the rotational speed of the wheels do not change, so the only factor is total system mass. So, whether you have an extra pound on the wheels, in the frame, or around your middle, the force required of you to lift that while climbing is the same.

Dave

DnvrFox
04-17-03, 02:30 PM
Originally posted by hayneda
For air resistance, yes. For gravity, only if you're going uphill. But when going uphill, assuming you are not accelerating to a faster speed, the rotational speed of the wheels do not change, so the only factor is total system mass. So, whether you have an extra pound on the wheels, in the frame, or around your middle, the force required of you to lift that while climbing is the same.

Dave

I was referring to the rock on the string. In a vacuum, without any force applied, it would slow down and stop spinning eventually.

hayneda
04-17-03, 03:44 PM
Originally posted by DnvrFox
I was referring to the rock on the string. In a vacuum, without any force applied, it would slow down and stop spinning eventually.

Only due to the very small amount of friction of the string--sorta like the wheel bearings. If it is spinning in a vacuum, and there was no "bearing" friction, it would spin forever (regardless of it's orientation w.r.t. gravity). Of course, there will always be some friction as we can't make a perfect bearing. As for the string, it would have "bearing" friction at the point where it was held as it flexed and/or twisted from rotation.

Dave

ComPH
04-17-03, 04:52 PM
hayneda, I think I get your point. Unless I race and need to accelerate extremely fast, the rotational acceleration is overrated, (unless I like to build a flywheel and coast a bit), in which case paying for high end rotating components what appears to be their weight worth in gold is non-economical. Of course super racers get sponsored and get the components for free anyway. For someone like myself is better to go for reliability and comfort.

RiPHRaPH
04-17-03, 04:57 PM
these arguments as well as any physics calculations assume 'all other things equal' ......and we all know that in life there is no such thing... this whole discussion just bakes my noodle.

i'll just spend my time training to overcome any minimal weight improvements. if we can all agree that weight doesn't have that great effect on speed or acceleration on level ground....and a heavier weight is advantageous for downhill speed....then doesn't theweight argument for the uphill portion of our rides become largely.....negligible?!

and are we discussing weight on an uphill in the context of an endurance ride or a short burst? i'll take the heavier, quad busting rider regardless of rotational or dead weight anytime over a light weight tour rider for an 'uphill snapshot'. then we are talking about energy produced again.

danr
04-17-03, 08:47 PM
Thanks for the replies. I definitely learned something here.

Originally, I thought wheel weight was critical. After reading posted links and finding a little info on my own, I learned that wheel weight and aerodynamics do not matter as much as I thought they did.

Once again, it definitely matters if you're a racer, where every little bit counts. However, for my purposes of solo riding and occasional group riding, I feel better about sticking to my \$170 wheelset.

ChristoE
09-01-05, 05:06 AM
This is obviously a very old topic, but since I am fairly new to the sport of cycling (mountain biking) - it is something to look at.

I understand all the arguments, for and against the idea that ligther wheels makes a difference. It is also understood that acceleration is where rotational weight has the biggest influence.

Still I want to add the following, in mountain biking, the only constant thing is change. So, you are acceleraing and decelerating all the time, the weight of the wheels in that case must make a difference - the harder it is to accelerate the rotation of the wheel, the more enery is required. That implies that your legs must provide more.

What I am saying is: assuming that lighter wheels will accelerate better when the same force is applied. If you do a 40km race once, with heavy wheels and once with lighter wheels you should be able to do the race faster with the lighter wheels. SO, to say that lighter wheels will not really make is difference is NOT true.

jbrians
09-02-05, 05:53 AM
And since we are looking at old threads.....

Change is the constant in road riding too. You just don't always notice that you are changing.
Every time there is an outside influence on your wheels, eg. a bump in the road, the lighter the wheel is, the intertia will resist the change less. Energy from the bump will be transferred into the frame as you'd expect and from there into you. But you are better able to absorb that energy. The bike can only dampen it or transmit it. If it's being dampenend, who knows where it's going to?
IMO, a lighter wheelset is more responsive and easier to ride than a heavy one.

sanseng
11-03-08, 01:39 PM
As the friction between road and wheel is dependent on total downward force on the road, a lighter frame, wheel, or rider definitely helps whatever be the condition. But whether it is relatively significant or not is the question.
For normal biking for limited period, other factors are more important lke riding comfort, finish & color, tyre pressure, bearing, etc., mass reduction may not be that much affordable as they use costly superior material mostly.
For racing in stadium, on custom-built tracks, everything is important that reduces friction like light frame, well designed frames, low friction bearings, aerodynamic helmets and everything, low weight wheels and spoke design, body-hugging suit of smooth material, etc.
For some cases like cross-country racing or such, a rugged frame and special tyre, etc. is more important. Air resistance and mass reduction are not so much important in this case, I feel.
I think the distribution of mass over the frame is also important in case of balance and performance, not only the mass. But again, in high speed with a lighter frame and wheel, u have to have the right technique to ride.

PaulRivers
11-04-08, 03:32 PM
The last time I read through a thread like this, everyone seemed to agree that rotating mass on the outside of the wheel was twice as important as mass on the frame. Strange how now lots of people are saying 3 times as important...the twice as important theory was that the weight traveled twice as far (1x horizontally, 1x up and down with the wheel) which made a certain amount of sense to me.

Every time there's a thread like this, someone writes a post about how rotational mass "doesn't matter" which is think is absurd - obviously something that you're pushing forward + up and down is going to take more work to move than something that you're just pushing forward.

That being said, it's all about a cost/performance balance. The high end Mavic Ksyrium ES wheelset that came with my bike costs about \$1,000 and is roughly 1500 grams (so I've read) for both wheels. A set of Mavic Open Pro wheels, which everyone seems to recommend on the bike forums, costs about \$300 for a wheelset and weighs about 2000 grams for the entire wheelset. That's 500 grams difference.

So let's be optimistic about rotational weight and say that it matters 3 times as much as non-rotational weight. According to google's unit conversion tool, 500 grams is rougly 1.1 lbs, and 3x500 grams = 1500 grams, and 1500 grams is roughly 3.3 lbs. So the question is - is it worth it paying an extra \$700, which is over 300% more, to lose 3.3 lbs off your bike?

As everyone has said, unless you're racing or doing a lot of riding with people with really high end bikes (just to even things out for you), probably not. On the other hand, maybe you're Donald Trump and \$700 is pocket-change for you, in which case spending the money is just not a big deal. :-)

huhenio
11-08-08, 10:53 AM
I won't bother with physics, but the 32mm tires I just got on my 48x16 motobecane messenger are a hell of a lot harder to turn.

On the other hand, when I commute on my 18 pound fuji ... I FLY!!