Good day. I followed a link to the Sur-Ron page and noticed this image:
I noticed that the torque & power lines crossed at roughly 1500 rpm. I thought, "Hmm, that's interesting. I'm used to the imperial formula of (lb.ft. * rpm)/5252 = hp, but I now realise I don't know the corresponding SI formula that would be more useful in the electric world. Let's see if I can recreate it." And of course, I'm having trouble doing so.
First I just subbed in the torque & power units with their conversion constants:
1 lb.ft. = 1.356Nm
1 hp = 0.7457kW
(1.356Nm * rpm)/5252 = 0.7457kW
Then I just started dividing out constants to pare the 5252 down to the new crossover RPM, where metric torque & power should coincide:
(1.356Nm * rpm)/3916 = kW
(Nm * rpm)/2888 = kW
My 2888 crossover point would seem to be about double what the Sur-Ron graph shows. When I try a sanity check using their chart, I get this:
(24.5Nm * 1500rpm)/2888 = 12.73kW, versus the roughly 3500 watts that they show.
What am I missing? Everything on their graph seems to be zero-indexed, so it's not like I'm being fooled by misleading advertising or anything. Did I make a stupid math error or something? I tried reverse engineering the imperial formula to see where the 5252 comes from, knowing that 1 hp = 550 lb.ft./s, but I was having trouble when I went to discombobulate "rpm" -- uhh, 2*pi radians per 60s, or what? I was lost. lol I feel like I'm getting stupid as I age. Can anybody help me here?
Thanks,
Jim "JimmyJazz" Jones
Calgary, Alberta, Canada
I noticed that the torque & power lines crossed at roughly 1500 rpm. I thought, "Hmm, that's interesting. I'm used to the imperial formula of (lb.ft. * rpm)/5252 = hp, but I now realise I don't know the corresponding SI formula that would be more useful in the electric world. Let's see if I can recreate it." And of course, I'm having trouble doing so.
First I just subbed in the torque & power units with their conversion constants:
1 lb.ft. = 1.356Nm
1 hp = 0.7457kW
(1.356Nm * rpm)/5252 = 0.7457kW
Then I just started dividing out constants to pare the 5252 down to the new crossover RPM, where metric torque & power should coincide:
(1.356Nm * rpm)/3916 = kW
(Nm * rpm)/2888 = kW
My 2888 crossover point would seem to be about double what the Sur-Ron graph shows. When I try a sanity check using their chart, I get this:
(24.5Nm * 1500rpm)/2888 = 12.73kW, versus the roughly 3500 watts that they show.
What am I missing? Everything on their graph seems to be zero-indexed, so it's not like I'm being fooled by misleading advertising or anything. Did I make a stupid math error or something? I tried reverse engineering the imperial formula to see where the 5252 comes from, knowing that 1 hp = 550 lb.ft./s, but I was having trouble when I went to discombobulate "rpm" -- uhh, 2*pi radians per 60s, or what? I was lost. lol I feel like I'm getting stupid as I age. Can anybody help me here?
Thanks,
Jim "JimmyJazz" Jones
Calgary, Alberta, Canada
