[loeg]

It's interesting that trackies in the 70s were trying to reduce weight that much. I don't think it's perceived as especially advantageous these days. The high-ish end track bike I'm assembling now will be a little over 8 kg (almost 18 lb). We also race much bigger gears (typically 95-110 gear inches in mass start racing, bigger for sprinting) than mentioned in the article (72 gear inches). The position that is considered aerodynamic is also much different -- there is much less focus on getting that insanely low, and instead the focus is on being narrow and getting the forearms parallel with the ground.

[hackingonempty]

It largely is the streetlight effect: we all have or can easily get tools to measure weight, we all have significant experience with weight, etc... Aerodynamics are much more difficult, especially in the 1970s where you can't just do some CFD simulations on your computer. There also weren't cheap solid state strain gauges to outfit wind tunnels or the bike drivetrain. Since only tiny aero gains are available without banned aerodynamic devices, there isn't much optimization you can do and need sensitive tools.

[cenamus]

Maybe that's just mostly due to the UCI regulations? No point to develop a fully fitted 5kg bike if it has to be 7kg anyways I suppose

[jdietrich]

Most competition bikes weigh significantly more than the 6.8kg limit, because weight just doesn't matter very much. A lot of state-of-the-art road and track bikes weigh around 8kg.

On the flat, weight only affects you during accelerations - at a steady speed, it has no significant impact on performance. Aerodynamic drag and rolling resistance are constantly sapping away power, so features that reduce these losses are nearly always worthwhile even if they increase weight. Even on a moderately hilly road stage, aero trumps weight by a considerable margin; on the track, weight is almost entirely irrelevant, particularly in longer events.

A lot of riders like the feel of a lightweight bike, a lot of them believe that light bikes are faster, but that's only true on exceptionally steep stages or hill climbs.

[cenamus]

So it only matters on basically every classic non time trial race?

[Tostino]

Did you ignore the part where GP was talking about the tradeoffs made, and how aero improvements are almost always worth the extra weight?

It's about optimizing the whole system, not just one part of it.

[matsemann]

With the speeds the pro's climb these mountains, aero is still a big deal. For you and me slogging along it's almost all weight that matters. But not for the pros.

[david-gpu]

Keep in mind that arodynamic drag is not a function of ground speed, but airspeed. If you ride slowly in a relatively windy place, your airspeed can easily be twice of your ground speed.

That said, given how many of us are overweight, worrying about a couple of kilos on the bike is funny talk anyway.

[eftpotrm]

I remember hearing Callum Skinner talk about the British track team preparing for the Olympics, and how the biggest problem they had was strength - I remember the number 2400 for their best track sprinters, I forget if that was in watts or newtons but either way it's a massive force and they were snapping frames.

The discipline of cycling that's the most weight-motivated is hill climbing. Track cycling really doesn't have that as an issue, and definitely does have a materials strength issue, so I'm not shocked they're not building to a weight limit.

[loeg]

2400W is plausible for a world class track sprinter.

[ndsipa_pomu]

Unless you can hide away a 2kg battery/motor combo. (Just joking really as they do scan for hidden motors nowadays)

[bluGill]

If you have a 5kg bike you can put the extra 2kgwhere you want it. this can make a difference in a race.

[loeg]

If it mattered, though, every track bike would be exactly 6.8 kg -- and they aren't.

[Gualdrapo]

Not sure if you know about Merckx's bike for his hour record - they did many interesting modifications to it aiming to reduce its weight

[loeg]

Yeah, Sheldon mentions it in this article. I don't think those mods helped. Weight has no impact on sustained speed.

[hackingonempty]

Rolling resistance increases linearly with weight, so it does have some impact.

[loeg]

Ok. Almost no impact. Worth keeping in mind we're talking about a system weight of like 180 lb vs at most 185 lb here (Merckx was ~163 lb), for a relative difference of up to 3%.

[xpe]

> Weight has no impact on sustained speed.

On a nice track, assuming a perfectly smooth surface and zero elevation change, I'm willing to accept the effect may not matter enough to care. But introduce even just a little bumpiness or some elevation change (perhaps in the track curves), and it might matter for someone pursuing the hour record.

[ndsipa_pomu]

Surface irregularities (bumpiness) are the reason why lower pressure tyres are now preferred on bikes. The idea is that a very rigid tyre will deflect the bike and rider up and down which wastes some energy/momentum as well as fatiguing the rider, whereas a lower pressure tyre can absorb those irregularities and "roll-over" the bumps. This is part of the reason that recent thinking has moved from skinny high pressure tyres, to wider medium pressure tyres. (Wider tyres will tend to roll quicker than a thin tyre at the same pressure - something to do with how the contact patch deforms the rubber).

However, cycling tracks are designed to be very smooth which is why high pressure tyres are still used there.

[xpe]

Any bump results in some energy transfer. In the case of small enough bumps and tires at ideal pressures, most energy is returned, but not all. These losses accumulate. The question is "how much does it add up to?" This is why I recommend using the phrase "negligible effect" instead of "no effect".

[loeg]

You're not going up and down the track during an hour record. Just doing laps at the bottom (zero elevation change). Track surfaces aim to be very smooth in general.

[xpe]

> You're not going up and down the track during an hour record.

Here the English language obscures the physics. Sure, the black line on the track is at a constant elevation. But the tire's point of contact is different from the system's center of mass (CoM). CoM is key here. When a rider tilts in the turns, the CoM lowers. In the straights, it raises. So, you _are_ going up and down during the hour record.

The question now becomes: how much effect does this elevation change have?

It is one thing to be aware of the effect, run the calculations, and find the result is negligible. Has anyone done this? That would be an interesting analysis, and I'd like to see it.

With this in mind, I will make another claim: for a particular rider, there is an ideal line around a velodrome that would minimize center-of-mass elevation change. This line would be faster than the current black line. How much faster? This would be a fun simulation problem.

Another interesting connection: center of mass and bicycling explains why pumping works on a BMX track, a pump track, a trail, and so on. (There are other mainstream explanations, but I think the CoM explanation is the most elegant.)

[dahart]

It’s an interesting question & thought experiment! To the degree that it even matters compared to all the other bigger forces, I would put money on riders naturally adjusting for CoM changes by riding slightly higher during the turns; I’d bet they already take approximately the ideal racing line you suggest. I just watched a couple of velodrome rides on YouTube, and it does seem like riders are often closer to the red line during the turn and the black line during the straightaway, statistically, but it’s noisy and would need to be measured.

The CoM’s elevation change on a velodrome track is due to roll rotation around the direction of travel, not to climb & descent. You can’t pedal harder to recover from a lean, so this is a different kind of up and down than straight line elevation changes. It makes sense that work is being done somehow if the CoM moves up and down, but the turns come with necessary changes to the higher moments of inertia anyway that flattening the CoM elevation doesn’t change. I’d speculate that the ideal CoM line might not be flat, in the presence of mandatory high speed banked turns; the fastest line and the line minimizing CoM elevation change might be two different lines. Do also keep in mind that on a velodrome track, a higher elevation line is a slightly larger radius turn & longer travel path. It’s also possible that trying to compensate for CoM elevation change adds as much time as it saves.

[matsemann]

Wouldn't you mainly get the energy back? Like, you use some going up the wall, and then gain it all back going lower.

[kevin_thibedeau]

You slow down incrementally between every power stroke.

[sudosysgen]

A heavier bike would slow down less. At the end of the day the energy is lost to friction.

[kevin_thibedeau]

Wind resistance is the primary force slowing you down at speeds where riders care about such things. The tiny micro-accelerations after every dead zone add up in spent calories when you have a more massive bike+rider system.

[sudosysgen]

That's my point exactly, wind resistance is the principal loss of energy.

The microaccelerations in the dead zone will be lesser in magnitude when you have a heavier bike plus system, but it will be more calories per m/s to recoup - while a lighter system will accelerate easier but have more acceleration to do. In the end the dead zone acceleration are calorically going to be exactly the same no matter the weight.

[stevage]

Yeah I was curious about the weight thing too. The hour record is probably the event in which weight matters the least, since there's no acceleration and no hills.