RCMoeur 

Brings back memories of my first century (mumblesomething) years ago on a Schwinn Varsity. Those heavy steel wheels certainly had a flywheel effect... but trying to add energy back into that system during the final 10 miles after slowdowns was to say the least a challenge.

RCMoeur 

:::raises hand:::

Within 2 blocks of my house are streets with 17+% grades (yes, in flat Phoenix), so I'm blessed with convenient opportunities for short intense hill intervals.

Unfortunately, my body is not as receptive to these opportunities as it was in my younger days. I'll do 3-4 good intense 100% effort intervals, and suddenly it's not just my lungs reminding me of my exercise-induced asthma (yes, I use an inhaler before and during), but a very queasy belly and other unwelcome side effects. And this is with ample recovery between efforts. Someday I might figure it out...

Getting back on topic: my racing bike (old Raleigh Pro Mk IV) has shallow-section aero rims for minimum weight with some 'aeroness', and my "fast but not racing fast" bike has deep aero rims. The lighter rims consistently 'feel' faster, but the difference is very minor.

rsbob 

When it is windy here, I run a 35mm in front and a 60 in back. If seriously windy - 35s front and rear. No wind, 60s all around.

Jughed

I’m finding all sorts of 40mm carbon fiber wheels for the same price as the hunts. But they are used. Wheels that had 1800-2k original MSRP….


Whats the thoughts on used carbon, from a place like the Pro’s Closet?

tomato coupe

Don't buy anything from the Pros Closet -- they're capitalists.

Fredo76

The "flywheel effect" incorporated into cycling models used on the internet to "prove" this or that may be the stupidest simplifying assumption that I've ever seen. OTOH, I did work in the computer field for 30 years, so maybe not. But it is pretty damn dumb. I can't believe how many people have fallen for it, because... COMPUTER!!!

asgelle

How is including relevant physics a simplifying assumption?

Fredo76

Because the physics isn't actually relevant. The modelers state that rotating weight at the circumference is equal in effect to twice the static weight at the center - fine. But then they simply assume that you get that energy back, as if there were energy stored up in a separate flywheel that you could recover later. But there is no separate flywheel to store the energy, and such energy stored in the actual wheels is lost anytime they slow down, whether from coasting or braking, so you can't get it back in a purposeful way.

asgelle

You've been looking at the wrong models. https://www.researchgate.net/publica..._Cycling_Power (and can't kinetic energy of the system be thought of as stored energy available to be converted to potential energy or heat?)

Fredo76

Boy, they really don't want anyone to cut and paste from their research! So, I typed it in by hand:

Changes in Kinetic Energy. The kinetic energy at the beginning and end of each road cycling trial was based on the velocity recorded by the SRM at those time points.

They are not even attempting to measure loss and gain of kinetic energy along the course, they just assume that it all evens out!

It seems that they have a decent model for estimating the power needed by a given rider on a given course at given speeds. To use their model to claim that weight off the wheels is no more beneficial than weight off of static bits, or off the rider, as I have seen on several web videos, is daft.

Maelochs

I am pretty physics-dim but @Fredo76 seems to have the facts here. A bike wheel is not a flywheel in the way a car flywheel is because the wheel itself is braked---the energy isn't stored to be recovered when the car coasts, but is scrubbed off by the brakes.If you habitually pedaled and then coasted, you could experience a flywheel effect, I guess .... but with aero drag on the bike and rider being a huge force by comparison to a spinning disc of less than a kilogram ....

Possibly if you put solid lead wheels on a bike and released it down a steep mountain, it would store enough momentum to roll a bit up the next ..... right? isnt that sort of the claim here?

As for rotating vs. static mass .... we are talking wheel sets at 1500 g? With tires and all, a kilogram? a few hundred grams difference in wheel weight equals an equally tiny difference in force .... and that is all damped down by 100 kg or more of bike and rider .....

I like this article (https://www.cyclist.co.uk/in-depth/w...tweight-wheels) which seems to say that if you climb for an hour on climb, you would save an enormous amount of time --FIFTEEN SECONDS---using aero instead of lightweight rims.

I don't know, folks. When I think of losing 15 seconds over the length of an hour;s climb ... I am so freaking tired I just don't care. But for those who do, there it is.

elcruxio

Reading all this made me think about my fatbike. The studded tires on it are 1700 grams. Each.

I guess there's a flywheel effect or slowdown in acceleration. I don't really notice it.

RChung

Um, no, look at equation 12. They do estimate the loss and gain of KE. The variable "I" captures the moment of inertia of the wheels.

That's true, their model doesn't account for braking -- but it turns out we can usually spot braking events (even brief "feathering" of the brakes) in the data because the decelerations are much much *much* larger than the usual coasting decelerations. Their model works quite well when we exclude braking events.

PeteHski

The only time you truly lose the flywheel energy is when actively braking. When coasting you are actually making use of that kinetic energy.

The only dumb thing is assuming that the people developing the models are dumb. In any case it appears that they are modelling wheel inertia anyway.

If you are racing in crits with lots of rapid accelerations and heavy braking, then lightweight wheels make sense. Also for steep climbing wheel static weight is quite important, but their inertia is negligible. For everything else aero dominates and wheel weight and inertia is inconsequential.

Eric F 

I'm not very sharp with physics math, but I'm pretty sensitive to what I feel on the road.

I don't think anyone implied that you could store energy to be used after deceleration with a bicycle wheel. My previous comment about the "flywheel effect" and heavier aero wheels was related to how things felt after the wheels had been accelerated to higher speeds - it felt like they were almost pulling me along. We're taking about speeds in the 25-40+mph range. Accelerating them was a bit less responsive than lighter wheels, but on certain crit courses where the last laps are pretty flat out and you're coasting through corners, not braking, these wheels worked really well for me. On a long finishing straight winding them up to 40+ was exciting.

Conversely, I used a set of Zipp 303s for a short time. For sprinting, I hated them. Their lightness felt like I didn't have a stable platform to work against.

PeteHski

What you say is in line with the physics. The heavier wheels really are “pulling” you along with their inertia at high speed. When coasting they would carry their speed longer too. The downside of course is that you have to use more energy to accelerate them up to speed. It is only during acceleration that they effectively double their static mass.

Eric F 

That lines up with my sensations.

rsbob 

Interesting subject. Yesterday I rode my 50 mile 2,400’ course with my heavier 60MM Aeros versus the 32s I used over the summer. My subjective analysis was that I was a bit slower on the multiple 5-10% climbs, but really cruised on the flats. My summer average speed was 15.7MPH but yesterday’s was 17MPH. (Yeah one whole MPH - but over 3 hours…) No PRs (or thirds) on climbs but a few on the flats. I know there are other variables, like wind, rest, energy/motivation - something to prove, but I went out with the intention of taking it easy. My average heart rate was virtually identical- but 3 BPM faster in the Summer (heat?). My unscientific real-world conclusion, larger aero wheels do help with average speed on a mixed terrain IF that is your goal. I’m keeping them.

Eric F 

Your previous comment about selecting different depth wheels depending on the route or wind conditions makes a lot of sense to me, based on my own experiences. It sounds like you proved it with your recent ride, as well.

Fredo76

My post does read that way, doesn't it? I did not mean to malign the modelers themselves and I apologize. What I am objecting to is misuse of their model.

Yes, but the simplifying assumption lies in only taking two measurements in the 'road cycling trial', at the start and the end. I assume that works fine for the modeler's purposes. But, in my opinion, using the model and its trials to assert that changes in the kinetic energy of the wheels along the way can be safely ignored when making statements about the merits of light wheels, is a mistake.

Maelochs

Remember when riding a bike was simple fun?

:d :d :d


Yeah ... I understand that for some folks, doing 14th-order derivatives and such is also fun ,.,.. Personally I just want an S-works Aethos and I will sacrifice all those lost watts to aero. Sue me in conceptual math and physics of mental cycling court. (If it will help me get a lighter sentence, I will install 37-mm wheels.)

RChung

Well, their model allows you to calculate the change in KE as frequently as the data allow but in their particular experiment, they knew they weren't using the brakes during the middle of their test intervals, and their unit of analysis was the test interval, so they didn't need to calculate KE at each point in-between. In their particular experiment, all they needed was the start and end speeds.

venturi95

Well there's the fly in the ointment, right there. Climbing on a 7% grade is faster on heavier aero wheels? Yeah, right, and I'm yur Irish uncle.

rsbob 

Very nice to meet you. Never knew I had relatives in Ireland.

Maelochs

Depends how quickly you climb, I guess.

https://www.cyclist.co.uk/in-depth/w...tweight-wheels

“So let’s take that into the real world. Zipp’s 202 Firecrest clinchers are its shallow lightweight option at 1,450g, while its 808s are the super aero choice at 1,885g, a weight penalty of 435g.

According to our data-modelling, that means they would still be the better choice at gradients of up to around 5%, in line with Quan’s statement earlier.

But all our calculations thus far effectively assume that any gradient is constant and that the road is straight, when in reality neither is the case.

Having to negotiate bends and changes in pace introduce a new variable into the equation: inertia, which is roughly a calculation of mass multiplied by the distance to the wheel’s centre.

Wheel maker Mavic has studied the impact of inertia on wheels and found that the higher the weight at the rim (a la aero wheels), the greater the inertia.

‘That’s important because inertia influences how quickly the wheel reacts,’ says Mavic research engineer Maxime Brunard. ‘If speed is varying greatly from one moment to the next, you want low inertia. If you ride at a constant speed, high inertia is fine.’

In short, if confronted with short, punchy climbs, go shallow. For longer gradients that require one speed, go aero.”