PBA wrote: I'm sure this is entirely correct.
it contains some approximations but it at least contains terms that you can grapple with rather than things you can't so easily.
Mick's test though, takes him from a standstill at the top of a hill to a standstill at some point along a flat section. I think the change in potential energy is therefore entirely converted to losses.
You are correct although the expression is intended to describe what happens during the acceleration phase, since Mick has noticed the speed discrepancy. What happens after that is all the KE terms then turn into losses like you say and (at any given speed) a machine with large heavy wheels ought to have an advantage.
At the low speeds achieved, I guess that aero losses wouldn't be massive and the difference from one bike to another could be ignored?
ideally yes but in reality the Moulton will be different; aerodynamically speaking the frameset is a disaster (it reminds me of a stringbag biplane) but then again there is probably a 'win' in that the small wheels are thought to be less draggy. Air is pretty thick and sticky stuff really; the airspeed doesn't need to be that high before it can be a really significant loss.
However in Mick's tests the thing that worries me most is the variation in wind speed from day to day, hour to hour, minute to minute. Ideally you would take both bikes and run then at the same time or many times over, back to back.
Inertia developed in getting the wheels turning then serves to propel the bike forward once it's on the flat?
yes, but if that were the main effect here you would expect the Mercian to accelerate more slowly and then to roll further, rather than to accelerate faster
and roll further.
Does R then require further breaking down?
well, lumped into that are all the things that are not 'aero losses'; so somewhere in R are terms that describe
- the drag in the wheel bearings
- the drag in the freewheel
- the suspension losses
- the drag in the tyres
etc.
the first two ought to be insignificant but the second two won't be. Devising tests that measure these things accurately and realistically is very difficult; if it were easy we'd all be doing it!
FWIW I have occasionally tried doing rolling tests at low speed on an almost flat (perhaps fractionally downhill) road. Me and my chum ride alongside and then freewheel together when we think our speeds are exactly equal. The slightest error in speed between the two bikes completely messes up the test. After a hundred yards we have both slowed considerably and there is usually a clear difference between the distance travelled by both bikes. We then swap bikes and do the same thing again. I can report that the usual outcome is (if the two bikes are similar) that I roll further than my chum, because I'm heavier (but about the same height). We usually choose an evening condition where there is no wind or just a breath of following wind.
The road isn't super bumpy but it isn't perfectly smooth either and it seems that the exact line taken can affect the results a little. The other thing that appears to happen is that at certain speeds there is more of a difference in the ways the bikes roll than at others, which may indicate that the road surface has bumps in it that do or don't resonate with either bike and affect the suspension losses.
Surprising results in these tests include that (after rolling 100 yards at less than 5mph) my carrier bike (which is on Vittoria Hypers and has an SA IGH) is usually within two yards of my chum's carbon road bike (which is on GP4000s in 25mm). Once on the move properly (10mph or more) it is very clear that the carrier bike is hard work; this is because of the aero drag of the bike and the very upright riding position. Rolling resistance-wise they are more similar.
I wonder if Mick could get a chum to help in a similar way?
cheers
