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Wheel weight vs. aerodynamics

Discussion on bikes, and whatever...

Re: Wheel weight vs. aerodynamics

Postby BMANX » Mon Apr 02, 2012 12:26 am

They are not mutually exclusive and I think this is where people see the tree and not the forest. Even on a dead flat course the lighter bike will be faster than the heavier bike if the aerodynamics were the same. Aerodynamics save minutes while weight saves seconds. You have to look at the complete package. Let’s face it as well, most professional riders are running tubular wheel sets that are lighter option to begin with. So by our account they would be faster on clinchers as both Zipp and ENVE say their tubular and clinchers offer almost identical aerodynamics. Just one is heavier than the other (there are other reasons to run tubulars as well).
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Re: Wheel weight vs. aerodynamics

Postby Matteo » Mon Apr 02, 2012 3:06 am

I am not sure I agree that lighter will always be faster than heavier. Note that I am not saying that the opposite is true. The following is very basic, so maybe one of the engineers can chime in. There is a frictional force between road and tired, which creates a torque that slows the wheel down. Assuming that the frictional force is the same for a light and a heavy wheel*, the torque will be the same. As torque = (moment of inertia) x (angular acceleration), the heavier wheel (larger moment of inertia except for some strange heavy hubs) will experience a smaller acceleration (which here is negative, so a deceleration). In other words, it "holds it speed" better. This is why Sosenka apparently used a 3.2kg rear wheel for his hour record.

Of course this has to be balanced with the need to accelerate the bike on a real course (not much acceleration going on in the hour record once he got going), which is easier for the light wheel. But that would be my point: in the end lighter could still be faster than heavier, but it is not immediately obvious to me.

*This is of course debatable, especially since in a simple picture the frictional force is directly proportional to the weight on the wheel. But this is complete weight, so a change in wheel weight is a smaller effect than the effect on the moment of inertia. So my "educated guess" is that angular deceleration is still smaller for the heavier wheel.
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Re: Wheel weight vs. aerodynamics

Postby BMANX » Mon Apr 02, 2012 3:23 am

Too many broad statements in this post. There is a large difference between indoor cycling in a very control environment vs. any cycling that happens outside.

So I will stand by my comment for anything happening outside.
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Re: Wheel weight vs. aerodynamics

Postby Ypsylon » Mon Apr 02, 2012 6:59 am

Yeah, let captain obvious chime in here for a second and say that faster=faster.

Leaving CRR out of the picture for a moment, my gut tells me I'm with BMANX on this one for general riding/racing. Especially if you don't even know how the race is gonna go and if you have to attack several times to make a break-away stick, etc.

My rudymentary knowledge of physics really makes me wonder how the holding speed thing is supposed to work. This might be a reductio ad absurdum, but if your wheel was heavy enough, at some point you don't have to pedal at all to (almost) keep your speed. But, you have to get the system up to that speed, which takes a lot of energy. With speed at start line < speed at finish line, I just don't see how you'd benefit.

It could be true that for a perfectly flat or downhill TT, which starts on a ramp, like they do at the TdF, a heavier wheel might be faster.
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Re: Wheel weight vs. aerodynamics

Postby Matteo » Mon Apr 02, 2012 1:04 pm

First, let me say that I agree that in almost all cases lighter wheels will be the better choice. I was simply trying to give some basic explanation for why Fibre-Lyte might have a point about heavier wheels having an advantage in specific circumstances.

As I wrote above, my post was very basic and therefore broad, and I would be more than happy to learn more of the physics details. But per BMANX's post above, we had eliminated inclines and declines ("on a dead flat course") as well as aerodynamics ("if the aerodynamics were the same"), and I tried to focus on the remaining effects. I do believe there is a "flywheel effect," but as stated above I agree that it has to be balanced with the need for accelerations. I don't think it is as easy as final speed > initial speed, otherwise Sosenka probably would not have used the heavy wheel. (In that case, there is a "one time investment" to get the wheel up to speed, but you would have to integrate the effect of deceleration over the whole ride). But on a "real" course the repeated accelerations will probably negate any positive effect from a heavy wheel, but it would be interesting to see whether there is a "realistic" course, such as Fibre-Lyte's "flat, fast, time trial," for which the two wheels come out even. Wasn't there a prologue at the Giro a few years back that was basically a ~3km straight, flat course?
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Re: Wheel weight vs. aerodynamics

Postby Ypsylon » Mon Apr 02, 2012 1:50 pm

That's funny, I was thinking about that prologue while typing my response this morning. Cippo was wearing his pinkish musle skinsuit that day.

Do we agree that rim weight only matters when there is an acceleration?

I just don't see where the energy you put into the system should come back out, if you don't stop pedaling before the line.

I could be wrong, but I believe that heavier rims will need more total energy.

The only possibility the heavy rims could come out on top would be that the human body somehow has an easier time providing that energy.

I don't know what the reasoning for the flywheel effect was, maybe with the fixed gear it was supposed to help him overcome a dead spot in his pedaling motion. Just because one guy did it, doesn't mean it was a good idea in general.
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Re: Wheel weight vs. aerodynamics

Postby Matteo » Mon Apr 02, 2012 2:11 pm

Oh yes, the muscle skinsuit :D

First, let me address your reductio ad absurdum (which I missed in my previous reply). Unfortunately, that does not work. One of the assumptions in my previous post was that the frictional force was the same for light and heavy wheels. I tried to explain in the "footnote" that that seemed not completely unreasonable to me, as naively the frictional force is proportional to the total weight. A change in wheel weight will be a small relative change in overall weight of rider+bike+wheels, but will have a relatively larger effect on the moment of inertia. Once you send your wheel weight to infinity, the frictional force will blow up as well. So the reasoning about torque etc does not work any more. Well, that and (as you noted) the fact that you would need an infinite amount of energy to get things moving in the first place.

I want to agree that rim weight only matters when there is an acceleration, but I think what I am taking away from this discussion is that (a) my understanding of basic physics is not that great and/or (warning, captain obvious) (b) the "real life" situation of a wheel is very complicated.

So yes, heavy rims require more total energy to get up to a given speed. In the ideal (intro physics textbook) case, once it is going at a constant speed, there is no more work required to keep them going. But we do have all kinds of retarding forces, and as I was trying to think through there might be an advantage to a heavy wheel in how the system of rider+bike+wheels respond to the retarding forces. (And just to repeat, I agree that in almost all cases the disadvantages outweigh the potential advantages.)

And while one guy doing something does not mean that it is a good idea, I would say that more people have similar experiences. After all, Fibre-Lyte brought up the idea in his post, and I have seen all kinds of discussions about how heavy wheels "hold their speed" better. Not that a discussion among cyclists has anything to do with reality, I give you that :D
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Re: Wheel weight vs. aerodynamics

Postby Fibre-Lyte » Mon Apr 02, 2012 10:33 pm

Ok, I'm going to come back in to this one. I really should stop posting then leaving :grin:

I think that in the real world you have to think about all aspects of a given race. The basics are that the heavier a rim the longer it takes to slow down from its rotational velocity. If you think simplistically a 2 ton pickup will take longer to stop than a 1 ton car with the same brakes. With a wheel, the only deceleration without brakes is the friction with the road (discounting air resistance). I could get all scientific and I will if anyone wants me to at this point but I'll refrain for now. What you have to weigh up is (literally) whether a lighter wheels reduced weight and increased accelerability outweighs a heavier wheels potential momentum benefits. If weight was the be all and end all, wouldn't the disc wheel manufacturers all be trying to get their weights down to the Lightweight disc standard. Yet many discs are very heavy and for time trialling very popular. In the UK there is a at least one down hill tt that I know of (heavier weight going down hill = faster until you get to a certain speed) and there are many flat tt's. Saying that, there is a local tt that is a hilly tt and my first choice of wheel would be what Bman suggests, yet many still opt for discs, possibly for aerodynamic benefits, but is that really necessary going up hill?

I'm not trying to suggest that heavier is always better, or that I'd disregard lighter. What I am trying to suggest is that the type of wheel needed varies for every rider given the type of riding and the available terrain. I still like my Lew 43mm rims and wouldn't swap them for anything (ok, may be I would) so personal choice is a factor as well.

Incidentally, pro's run tubs for other reasons than weight or aerodynamics. Being able to pump the tyres to pressures exceeding what a clincher can handle would be a case in point. Using what they are given and not what they choose would be the other factor :grin:
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Re: Wheel weight vs. aerodynamics

Postby Matteo » Tue Apr 03, 2012 12:32 am

Fibre-Lyte wrote:[...] I could get all scientific and I will if anyone wants me to at this point but I'll refrain for now. [...]

Actually, I would very much appreciate that if you have the time and are willing to invest it. I would also be interested in an explanation for this:
Fibre-Lyte wrote:(heavier weight going down hill = faster until you get to a certain speed)
Edited: Never mind, I have an idea for this one.

But maybe all of this belongs in a different thread?
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Re: Wheel weight vs. aerodynamics

Postby howardjd » Sun May 20, 2012 2:38 am

Hi how are ya'll doing my name is Jason and am new to the forum. I'm also very intrigued with the principles of wheel inertia and cycling performance. After reading about Sosenkas 3.2 kilo wheel I decided to do some testing of my own. My intial testing involved doing trials of air inflation vrs water inflation. The results were very interesting. Even on a gently rolling course I got slightly faster results with the water inflation, around 20 sec over a 20min ride. A key aspect I noticed with the heavier wheels had nothing to do with linear motion but rather the lateral motions. Becuase of the higher angular momentum the wheels had decreased side to side lateral motions during riding. This included slip angles for the front wheel, rocking back and forth over the contact points, and sliding laterally on the road. The bike was just plain very stable and hence this is where alot of benefits come in. It may not be that in perfect form riding the heavier wheels were any faster, but that they allowed for more efficient riding for the less than perfect human rider. In windy conditions the water wheels performed much better in cross winds than with lighter. I was so inspired by my research that I used a pair of spoked wheels to make a 7 pound and 12 pound disk. The 7 pound rode awesome and I even claimed a ameteur time trial record at 28mph for 10miles at the Lowes Motor Speedway time trial series in NC. The 7 pound disk was made with a very lumpy rough surface and I took it to the windtunnel were I got some pretty good results. Due to injury I have yet to test the 12 pounder which I have dubbed the air hammer. There really is something to riding heavy, particualry for flat courses, gently rolling courses, courses with long shallow grades, and rollacoaster courses with series of step hills low in magnitude like say <20 feet. The key point is that is that the heavy wheels set you up in a system that has much more angular momentum resutling in a more rigid ride allowing for riding straight with ease. It also seems advantageous for stronger riders at it allows more efficient riding with a large power flucuation in your power profile. As a 190 pound athlete with a background in weightlifting I found riding heavy to be of much benefit on the appropiate courses.
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Re: Wheel weight vs. aerodynamics

Postby gregorythomas » Sun May 20, 2012 5:13 am

with respect to rolling momentum and the benefits of a heavier wheel, there aren't any. the energy in always equals the energy out + frictional losses. momentum only affects acceleration (both positive and negative).
the potential benefits of a heavy wheel with respect to allowing the rider to ride straighter, this is a new and very interesting approach. i forget the actual figures, but i remember reading that turning a couple degrees increases rolling friction something like 16%. howardjd's testing is enlightening.
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Re: Wheel weight vs. aerodynamics

Postby Ypsylon » Mon May 21, 2012 6:20 pm

That's indeed very interesting, I had never thought about not going straight, but I'd believe that's a good point.

It'd really depend on the course and the rider a lot, but if it's flat and pretty straight there's not much too lose going with heavier wheels.
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Re: Wheel weight vs. aerodynamics

Postby pawnii » Mon May 28, 2012 4:49 pm

I wonder why deeper rims in the back isn't more popular on the road.
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Re: Wheel weight vs. aerodynamics

Postby BMANX » Mon May 28, 2012 5:11 pm

I think they will become more popular when the manufacturer offers wheels like ENVE and Reynolds seem to be doing now. 3/4 or what ever Reynolds calls their offerings now.

Good idea to run a little deeper on the back.
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Re: Wheel weight vs. aerodynamics

Postby Cyco » Sun Jun 03, 2012 10:40 am

Mavic used to make a rear disc (the Comete +-) that had 12 spots close to the outside that allowed you to add weights to keep your momentum on flat courses, and remove some or all of them when the course required more accelerations.

They didn't last on the market very long.

I have also ridden one of their original all aluminium track discs (just under 3kgs), and whilst it took some serious effort to get going I can see why it would be popular for hour record type riding. Ripped your legs off trying to stop it.
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