Thoglette wrote:cj7hawk wrote:
A typical 49cc motor will be around 200w on a typical bicycle when geared with typical reduction ratios, with additional power limited by the speed of the motor and wind resistance.
Power is power. Whether you've set yourself up to access it is pretty irrelevant.
I can't think of a more "typical" 50cc motor than the ubiquitous Honda
GHX-50 stationary engine. Which provides 1 to 2 hp across it's entire power band. That's 3 to 6 times 200W
And, without looking closely, the photos above look a lot like the 10hp (7,500w)
DeNardis race engines with off road cycle kit. Also 50cc
As noted in the various other threads, one has to try pretty hard to limit a 50cc motor to 1hp. 10cc four stroke at 5000 rpm would be more like it.
Hi Thoglette,
Your misunderstanding here is there's no such thing as a 2hp engine, and there's no engine around that can produce 2hp across it's entire band. That's just not possible. The RPM of the engine is the primary contributor to HP. This is key to understanding what I'm saying.
So when they say an engine can produce 2hp, they mean that within it's over-power envelope, it can hit 2hp at 9000 rpm or something like that. This means that it's about 1.5 maximum
real HP at that speed. And if that's at 9000 rpm, then at 2000 rpm, it's going to be around 330W output. This is likely reflected in the real engineering data for that engine if you ever look it up.
Now we're down to 330W. Let's assume that we lose about 25% due to poor gearing and reduction and clutch issues. We're down to about 247W now, at the wheel. Let's also factor in that it's a low-quality chinese engine and losses in the dyno at bicycle speeds due to measurement errors, and there's a real chance that a 50cc engine will give a result under 200W.
Really.
Now, if it's geared so that at 2000 rpm it's doing about 25 kph, then in real-world terms, that means that air resistance which increases at the square of velocity will immediately overcome RPM acceleration, which increases at the sum of velocity. Sure, maybe the dyno will show about 700 to 800w, but the argument is that this is in a stationary position, and there's one factor here that hasn't been taken into.
Movement.
Watts, as a unit of movement against force show that you can't measure real-world watts on a stationary dyno, because not all the forces are present.
So, what I said was absolutely correct, and could be verified by any engineer. It's not difficult - it just means that those presenting the argument need to be aware of all the real world factors.
On top of that, DoT, who make the rules, have stated that a reasonable test for a bicycle is that, if it runs in two directions on a straight level path, and the average top speed is calculated, and it's around 25kph ( but not exceeding 30 kph ) then that is a reasonable test for real world conditions... And it's so very easy to do as well.
At this point, it's a bona-fide 200W bike, no matter what the dyno says. Legally so too.
So, as I mentioned, there's a chance the guy will be lucky. But also, as my calculations have shown, it's borderline, so there's also a chance that he will be over. And if the dyno comes over, he will most likely plead guilty anyway, whether he was guilty or not.
For what it's worth, the above test I describe is pretty much how Traffic Police confirmed their view of it yesterday too.
Regards
David