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Comment by semi-extrinsic | Hacker News Reader

> whose performance bottlenecks are imposed by the bike instead of themselves

I'm a fluid dynamics researcher, and I'll tell you the money these people spend on "aero" and "weight saving" and "higher drivetrain efficiency" stuff is 99.9% snake oil. Unless you are among the top 1000 road bike racers in the world.

Increasing your exercise load by half an hour per week, or spending a little time understanding and implementing aerodynamically efficient body posture or other forms of technique, will both yield improvements that are several orders of magnitude higher than the marginal improvements achieved by spending $$$ on "aero" and carbon fiber.

I've seen detailed calculations of rider performance that indicate a 500 g saving in frame weight (which is huge), over the course of a 120 km race with 1200 m climb, will save you a whopping total of 15 seconds on ~4 hours of race time.

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I'm an engineer and a biker. If you only look at the numbers or try to run simplistic calculations, it's easy to think that bikers are just deluding themselves.

But there's more to it than just numbers on paper, and there's more to biking than just riding in a straight line from point A to point B. Carbon fiber frames aren't just about saving weight. The Carbon Fiber has different flexibility and damping properties that result in a different feel, for example.

My engineer friends make fun of me for spending $1K on a set of carbon fiber wheels, until I let them ride the bike. It only takes a few pedal strokes to feel how the reduced rotational inertia translates to faster acceleration. The advantages are even more significant when jumping or throwing the bike around, because the bike's moment of inertia is reduced significantly.

Carbon fiber handlebars don't save much weight, but the damping properties make for a minor reduction in fatigue. Carbon fiber Derailleur arms will again only save a few grams, but they're more likely to spring back from impacts than metal pieces that will simply bend. More expensive shifters have better bearings and cables that make for more precise shifts, meaning less fiddling over time and less mental effort to make sure I'm only changing 2 gears instead of 3.

The list goes on and on. Casual observers think we're just being silly and fooling ourselves with expensive parts. Meanwhile, I'm still buying the expensive stuff, enjoying it, and having zero regrets. At this point I just don't talk about it with any non-bikers because so many of them think they've got it all figured out.

> I'm an engineer and a biker. If you only look at the numbers or try to run simplistic calculations, it's easy to think that bikers are just deluding themselves.

Bikes are all about diminishing returns.

If you spend $500, you get a bike that's probably twice as good as a $250 department store bike.

Spend $1000, and you get a bike that's half again as good as that $500 bike.

Drop 2 large and you get a bike that's 25% better than the $1k bike.

etc etc until you are paying $15,000 for a bike that's 0.5% better than a $10,000 bike.

We all find what part of the curve we want to participate in.

Personally, I think $4500 for a really nice looking eBike with some fantastic add-ons is a decent deal.

> Bikes are all about diminishing returns.

Pretty much everything is about diminishing returns though, so nothing special about bikes.

For instance I can (and have) build a Miata race car for $5K to do good laptimes at the race track. Or I can spend over $100K on a car to go a handful seconds faster per lap.

> Pretty much everything is about diminishing returns though, so nothing special about bikes.

Mostly truth. Some things more than others.

if you do the math you can quickly prove to yourself that reduced inertia isn’t doing anything for you. you then either have to conclude that newtonian mechanics does not apply to bikes or perception can’t be trusted!

if you do a blind experiment, where you don’t know the wheels under you, or a third party adds weights to the rim you can demonstrate this. be sure to use the same tires, at the same pressure, as tires and pressures make massive differences in comfort feel and efficiency.

source: me, long time triathlete, cat3 bike racer, coach of a professional cyclist and state tt record holder, gear nerd who has done lots of power meter experiments and spreadsheets

> if you do the math you can quickly prove to yourself that reduced inertia isn’t doing anything for you.

This is exactly what I was talking about. People like to assume that a simple math equation tells the whole story, while ignoring the fact that bikers do more than just exist as rigid attachments to a bike that travels in straight lines.

I can do the math, but I can also swap wheels in a matter of minutes and feel the very obvious difference.

I'm talking about 29" mountain bike wheels, where the mass and moment of inertia is significantly more than what you see on 26" road bikes. It's palpable.

Maneuvering the bike in the air is another place where the mass difference is noticeable. Dropping 500g doesn't sound like much when you think about holding 500g in your hand, but it's a different story when that mass is on the ends of a 48" long wheelbase and you're trying to maneuver it in the 0.5s that you're in the air.

If you're just riding something like your tri bike in a straight line down a smooth asphalt road at a constant velocity then it obviously doesn't matter, though.

> I can do the math, but I can also swap wheels in a matter of minutes and feel the very obvious difference.

You’re not necessarily wrong, but I will point out that high end audiophiles will often say the same thing about their absurdly priced equipment, and then refuse to participate in a double blind test.

You meant 26 inch mtb bikes? 700c road wheels are also 29 inches

Done it. The place I most notice lightweight wheels are when climbing slowly on steep mountain bike stuff. Climbing like this is not a steady application of power like steady riding on the road; it's a series of small accelerations. When doing this lighter wheels are incredibly noticeable.

You also probably won't see this in your PM data unless you're getting data on partial pedal strokes.

More steady power stuff like on the road, climbing fast, etc? I agree that lower wheel weight just isn't something you'll notice. (Very good evidence for this is riding a fatbike on a fast rolling dirt road ride vs. climbing very steep fire roads.)

> if you do the math you can quickly prove to yourself that reduced inertia isn’t doing anything for you. you then either have to conclude that newtonian mechanics does not apply to bikes or perception can’t be trusted!

This is only true if you measure only along the forward/reverse axis.

Cyclists stand up, and rocking the bike sideways is easier with lighter wheels.

On mountain bikes there are even more cases.

If the pack accelerates out of a corner faster than me due to reduced inertia i will be off the back and can do the rest of the race on my own. That's something i have some experience with, unfortunately. If its a sprint i will lose the sprint. This is all normal mechanics. The only situation it doesn't do anything is in a flat time trial which starts with everyone already at the speed they will hold for the race, which doesn't have any change in road or wind conditions nor any corners. I doubt a race like that exists. maybe the triatlon comes closest because of the very long distances involved.

I'm a mountain biker, and I also ride fatbikes in the winter. There's a couple other reasons for carbon:

- Carbon fiber is a worse conductor of heat than aluminum. Use some carbon fiber bars, foam grips, and either carbon brake levers or aluminum ones with heatshrink tape over them and your hands will stay warmer in the cold. When it's REALLY cold and you're using pogies (grip area covers) this is particularly important as it means less heat wicking away from your hands.

- In my experience the sidewalls on carbon rims are a LOT stronger than aluminum. Sure, it's possible to break carbon, but it takes a lot more force than it does to dent or crease aluminum. I've ridden carbon rims hard on seriously rough trails, hitting rims on things which have dented my aluminum rims, and the rims are just fine.

- When building bicycle wheels one of the usual steps is truing the wheel, or adjusting the tension on each spoke to pull the rim into being straight and eliminating wobble. This is not a thing with carbon rims. With carbon rims they are SO stiff that instead you bring all the spokes up in tension evenly and then maintain this even tension and ensure the wheel is straight (no radial or lateral wobble). Unlike aluminum rims, changing the tension on just one, or even a handful, of spokes won't pull that part of the rim in whatever direction. The rims are just that stiff, and a well-built even-tensioned carbon wheel will last a long time.

- It's also possible to do far more complicated forming with carbon fiber than one can do with metal, particularly at reasonable weights. Having a very low stand-over height (good for technical mountain biking) with carbon is no big deal; just some engineering. With aluminum it requires a lot more material, which can add pounds (noticeable amounts) of weight.

I'm a fairly serious mountain biker and I’m not sold on Carbon Wheels. I’ve seen plenty of them get destroyed on the trail where an aluminum model would probably have been ok. I will admit to having a carbon frame, but I’m now looking at some aluminum ones as the price difference is hard to ignore for about 2.5 kgs of weight.

I'm a fairly serious mountain biker as well, and I'm wholly sold on carbon rims. In particular, I'm waiting on a set of rims to be delivered to rebuild some I9 Hydra Trail S aluminum-rimmed wheels with carbon.

I've been riding on carbon rims on both hard tail XC bikes and my full suspension trail bike for six years. I've also had aluminum rims on my single speed and hard tail trail bike. I can't count the number of times I've hit the carbon rim and had it be just fine, but all of my aluminum rims have some degree of denting from much lesser riding.

Between the bikes with carbon wheels there's about 12,000 miles of trail riding and they've been just fine.

After I dented the rim on the I9s and had a spoke break I decided a good winter project would be rebuilding the wheels to carbon rims because my previous experiences with them have been so good.

I think the primary benefits of carbon are stronger sidewalls (less likely to dent) and that they cannot be bent and thus can't go out of true (unless the wheel is built wrong). Yes, they can be broken, but it takes a LOT more than it would for a comparable aluminum rim.

(For reference, the rims I've beat on so hard are Light Bicycle RM29C07 and I'm replacing the stock rims on the I9 wheels with AM930.)

Good carbon wheels are incredibly strong. Carbon has the advantage of flexing where aluminum would deform.

You can watch Danny macAskill try to destroy bare carbon wheels (no tires) on concrete here: https://www.youtube.com/watch?v=VfjjiHGuHoc

I assume some of the horror stories come from people buying cheap carbon wheels or early models before they had layups and molds figured out.

If you're really concerned, add something like a cushcore to the inside of the tire for additional damping.

Your complaints are common, though. I've all but given on up trying to discuss carbon wheels with people in person because there are so many preconceived notions out there. It was the same way 5-10 years ago with everyone avoiding carbon handlebars for one reason or another.

As everything carbon-fiber, a lot depends on the quality of the build.

Look for "Danny MacAskill Tests Santa Cruz Reserve Carbon Wheels": he really tries to destroy those rims, and it takes him doing ridiculous things to do so. Aluminum rims would have cracked/ bent much earlier.

> reduced rotational inertia translates to faster acceleration

\aside why not smaller (radius) wheels? Use higher gearing, and accept the bumpiness.

Though for touring aot commuting, there's something pleasant about higher inertia.

Larger wheels roll over obstacles better. That's why most mountain bikes are 27.5" or 29", which is about as big as you can go without causing too many other compromises.

Bumpiness isn't just a comfort issue. Your body ends up absorbing a lot of the up and down motion, which drains your forward momentum.

> I'm a fluid dynamics researcher, and I'll tell you the money these people spend on "aero" and "weight saving" and "higher drivetrain efficiency" stuff is 99.9% snake oil. Unless you are among the top 1000 road bike racers in the world.

So, biking audiophilia.

At least weight saving is easily measurable and indisputable. Audiophilia focuses mostly on areas that are not so easily quantifiable.

I've heard audiophilia derided as the practice of listening to the noise instead of the music, sand that was enough for me to get off the train and never think twice.

Carbon frames are worth it for the damping factor alone if you ride for more than an hour a week IMHO.

>spending a little time understanding and implementing aerodynamically efficient body posture

You might be interested in what we're up to [0], get in touch if it interests you.

[0] https://bodyrocket.cc

Ooo, nice. This is quite clever - still will require insight and effort from the rider, but it should be less of a shot in the dark, if it works. I'm not in your target market personally, but I like the approach.

Are you going all out with triaxial load in the seat post, or uniaxial (posterior)?

You're asking the right sort of question :)

Seatpost is actually one of the simpler sensors, without giving too much away it's sensing force in 4 different directions.

Is the 4th direction time?

Typically it would be a rotation (or torque, if we're talking load force/torque cells).

Limited in what I can say, but you're not wrong :)

I don't understand why you need to measure anything but instantaneous crank torque, speed, and (for completeness) pitch angle. Integrating power in vs acceleration out, the difference is loss. What am I missing? Does bad posture waste energy before it reaches the crank?

In an ideal world with zero friction, zero wind, constant air density and no other losses that would be true. There are a few outfits using this approach, so we are the outliers (being the only ones doing it via direct force measurement, for which we have the patent).

The difference between energy in and work out is, exactly, friction, wind, and "other losses", so you have not said anything meaningful. That is your right, but then why answer at all?

Depends if you want to answer “what” or “why”.

I am involved in the distribution of very expensive bicycle components. The demand is massive. From my commercial experience, this sort of stuff has lots of similarities with other branded goods (bags, watches, designer clothes). It probably is built to a higher standard than mass-merchandise, but people buy it for more reasons than the tiny theoretical improvement in performance. I reckon more people buy a Rolex as a signaling device than because it more closely to Swiss Chronometer accuracy standards. I think the same for lots of bike stuff. Not really a value-judgement (I'm thankful for my customers) but in many cases, it's not a pure performance-per-dollar value calculation that they're making.

In theory, theory and practice are the same. In practice, they are not.

Your post is what theory looks like when people forget the difference. Your theory leaves out tons of variables, including the condition of the road, and the differences in how materials deform under pressure. I've seen the stats you're referring to, and that's what it looks like in a lab with perfect conditions.

Go ride a $400 bike and a nice carbon fiber bike. The difference is much more substantial than 15 seconds per 4 hours. It's less fatiguing, the pedals feel stiffer (cheap bikes feel mushy in the pedals), my speedometer has a higher average speed, it's easier to climb hills (flat surfaces like in the lab are rare on real world streets--flat roads are almost never 0% grade), etc.

Sometimes you need to put down excel and actually do the thing you're theorizing about.

My understanding is that weight saved on anything that rotates has an even bigger impact, but that doesn't change what you've said. The vast majority of posers bragging about the 500 grams they've saved in parts are carrying an extra 10 pounds on their own frame.

the rotating weight thing is mostly myth, while true that rotating mass costs more to accelerate, cyclists don’t accelerate hard enough for it to amount to anything substantive. source: i did the math in excel

The TdF pros disagree with you. Yes they can be traditionalist and have "pack mentality", but these people spend literally 6-10 hours on a bike straight.

In particular, when you are bonking and trying to keep up with a group or get up a mountain, you become PAINFULLY aware of the deficiencies in your equipment. An extra pound on your bike when trying to keep up with your group up a 8% grade will feel like you're carrying an anvil.

What do you mean by “accelerate hard enough?” Can I see your math?

Cycling is huge here in Belgium, definitely as a hobby.

A semi-professional once told me how crazy it is that people with a beer belly spend a huge amout of money on their bike, just to get a few grams off. A proper diet could save them a few kilos for free.

aero changes can be quite substantive even for amateurs. on the order of a minute or so per 40k tt

aero equipment and position and training are all orthogonal so it isn’t a question of picking just one.

My brother used to run a aftermarket Ducati performance parts company. “Racers” would swap out every bolt on the bike from steel to titanium (which is actually a terrible idea for a high end sports bike), yet leave the 15 lbs “toolbox” (which was a useless joke of a toolset) under the seat. Also, there’s a reason most jockeys (for horses) are tiny...

"Ride up grades, not buy upgrades"

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