The utility cyclist wrote:Stating the extra weight doesn't make any difference is your opinion, it makes a difference, it makes a difference when cycling, when lifting, ordinary folk that don't cycle much DO notice the difference between a heavy Dutch bike and one that isn't, it's noticeable especially when you add in the draggy dynamo wheel and heavy duty tyres which pinch even more of your effort.
15 Watts of differential in energy absorbed by heavy duty tyres alone is a significant factor when your average joe/people that only ride short distances are maybe producing 100Watts.
It doesn't work like that though....
I'm not sure precisely what tyres you are proposing for your lightweight hybrid - presumably NOT Continental GPs or whatever.
At any rate when it comes to tyres it is a compromise between puncture resistance, longevity and speed.
Let's say you have 37-622 Schwalbe Marathon Greenguards on your Dutch bike.
The coefficient of rolling resistance at 60psi is 0.00638
https://www.bicyclerollingresistance.co ... athon-2015
And then you have some sort of lightweight hybrid with Grand Prix 4000s II, to use a fairly extreme example (the Marathon treads are 3x thicker, and the sidewalls twice as thick). The CRR at 95psi is around 0.00365. https://www.bicyclerollingresistance.co ... i-23-25-28
In terms of the efficiency of hubs vs. derailleurs, for lower power outputs (80W), they are similar. http://www.ihpva.org/HParchive/PDF/hp52-2001.pdf
In real conditions, a hub gear system will be more efficient than a typical rusty poorly maintained derailleur system.
An efficient dynamo hub costs almost nothing when switched off, and 5W @ 100W, switched on.
So let's plug in these numbers:
60kg rider, 11kg lightweight hybrid
CdA = 0.5 m^2 (fairly upright)
CRR = 0.00365
Drivetrain loss: 10% (best case, with well maintained system)
80W = 12.9mph, on a flat road
80W goes to:
15W = rolling resistance (GP4000s)
8W= drivetrain (derailleur)
58W = wind resistance
60kg rider, 17kg Dutch bike
CdA = 0.6 m^2 (very upright)
CRR = 0.00638 (Marathons)
Drivetrain loss: 15% (worst case, with light on)
80W = 11.0mph
Where does the 80W go?:
24W= rolling resistance (Marathons)
13W = hub gear drivetrain + dynamo
43W = wind resistance
Firstly, very heavy weight tyres are not absorbing an extra 15W at low speeds. Rolling resistance is proportional to speed; the numbers from bicyclerollingresistance are measured at 18mph. At lower speeds, rolling resistance is less. At 11mph, 43W is spent on fighting wind resistance. If we were to switch to short-lived, puncture-prone racing tyres on the same bike we'd gain just 0.7mph.
Obviously, then NOT fitting heavy duty tyres on a bike for someone producing only 80 or 100 watts is very stupid, because there's a good chance they'll struggle with mending a puncture and, well, the extra 9W you can buy them by reducing the practicality of their bicycle is eaten up by a 1 kph speed increase, which nobody would notice.
When going DOWN a hill, heavy tyres aren't significant either - here it's air resistance versus gravity. A very upright and draggy bike (and I have one) will not accelerate very much, while a crouched aero bike will accelerate quickly.
Going UP hill, heavy tyres are less of an issue than on the flat - at 100W/60kg/5% hill, you will do 4.3mph on a upright heavy (17kg) Dutch bike with Marathons + lights on, and 5.2mph on a lightweight 11kg hybrid with GP4000's. Faster, but hardly supersonic in comparison.
On a short journey, speeds aren't in any case particularly relevant.
Over some longer journey they become slightly more so, but as per the above:
(a) weight isn't as significant as it seems, because even in the rather generous case towards the lightweight bike, of a 60kg rider and a 11kg vs. 17kg bike, the heavy bike only adds 10% more weight to the system, which means a loss of speed of less than 10%, which really is not a big deal.
(b) wind resistance is significant in cycling, except when cycling uphill very slowly, but it isn't all that relevant to our 80W cyclist, since they don't produce enough power to get up to significant speeds and hence have to battle significant wind resistance, so an upright stance is not an impediment to their progress because at slow speeds not much power is required in the first place.
(c) high-quality heavy-weight tyres are better than lightweight puncture-prone ones unless you want to eek out the maximum performance from your bike. For example, for an racing bike (CdA 0.39m^2) with 95% efficiency drivetrain, the Marathons would be at 19.1mph at 200W on the flat, and the GP4000s 19.8mph. Not a huge difference, but if you are racing then why not. Plainly for more practical applications, lightweight tyres are not a good choice.
(d) derailleur systems are VERY efficient at athletic outputs of say 300W with well-chosen gears: a 44/16 derailleur gear (with a 44/32/22 crankset & 12-34 cassette) is given as 97.2% efficient at 300W. The 32/16 at 80W, however, is only 90.9% efficient, and worse again in typical conditions.
I would note also the numbers for efficiency for the small cog: 22/12 is 91.8% efficient at 150W, 32/12 is 91.0% efficient, and 44/12 is 93.3% efficient. I assume modern 11t cogs are even worse.
The point at which these considerations change is when you need to cover relatively long distances, in which case significant speed improvements are possible IF you produce more power than these numbers AND can get into a more aero position.