Re: How do you measure SPEED on skates?

Hi all,

FNGuy wrote:
> Besides the timed average around a measured course, or a buddy on a bike 
> with an expensive speedo, how do you know how fast is fast?  I haven't seen 
> whiz-bang Doppler gizmos at the skate shops...

I have used my GPS, a Garmin GPS 12, to measure distance and speed. The 
GPS measures in 0,1 km increments, and the accuracy is rated at +- 1 
km/h. Checking a known distance with an ordinary watch makes me believe 
it is quite accurate.

This summer I saw a sports shop selling the Sigma Inline Computer at a 
very low price, the shop trying to get rid of it, so I thought what the 
hell, I'll buy it. It is basically the BC500 bicycle computer mounted on 
a thick base containing the inline computer. All mounts on the boot. A 
magnet is driven in between the wheel spokes and the sensor is fastened 
with one of the wheel bolts. Some washers are furnished to get the 
spacing correct. The sensor is a standard reed relay and it is 
recommended to attach it to the second from front bolt. A standard mono 
jack plug is used to connect the reed relay to the cimputer base. I 
glued the cable to my frame and boot to guard it a little.  I used two 
washers to get the space correct on my old Rollerblades. I can hear the 
sensor clicking open/closed when I spin the wheel still, I might not 
when I'm 70! :-)

One then inputs a wheel circumference and the inline computer is 
supposed to take the diagonal path the wheel travels, the wheel stopping 
in the air and wheel wear in to account. Pretty impressive I suppose.

On small trips of 10 km or so the GPS and the Inline Computer show about 
the same. This Autumn I skated with friends in Denmark and we did a day 
trip of 55 km. The speed difference was usually +- 1 km/h often in 
favour of the GPS, and at the end of the day the GPS showed 55 km while 
the inline computer showed 52,something km. I sort of trust the GPS a 
little more as it is more expensive :-) and the error margins are stated 
in the hand book, but the inline computer is probably not more that a 
couple of percent in error. I am not sure of how good it is at wheel 
wear, it probably has some emperical formula built in.

GPS Pros: Gives a bit more info as max speed, avg speed, direction 
(helpful around midnight when going home on dark country paths in 
unknown cities!)

GPS cons: Bigger, heavier,  (4x AA batteries) sometimes a speed spike, 
in Denmark a spike of 63 km/h was registered, not very realistic. I have 
no problem with buildings here in Europe so far, but it might be a 
problem in Manhatten, not getting enough satellites in view.

Inline computer pros: Small, light, battery life is probably one year at 
least.

Inline computer cons: Geeky, nerdy, hopeless, doesn't get points from 
young aggro inliners! :-) Sensor/cable close to ground clutter.

BTW, I killed the sensor in a Amsterdam FNS this autumn. Something hit 
it while I was skating and cracked the glass reed relay casing and bent 
the steel plate a little. I didn't notice the knock at least. The reed 
relay is a glass component soldered to a cable and hot glued in a 
plastic housing screwed to a thickish metal plate, the metal plate being 
screwed to the boot. I'll probably buy a new reed relay and solder it 
myself, rather than getting a new sensor from Sigma, probably expensive. 
Reed relays are first year electronics, are cheap and found at i.e. RS, 
so 'anybody' can do the repair.

A GPS is nicer to have, more all-round usage I think. But it is a hassle 
to carry around. The skate computer is nice for beginners, or speed 
skating on the road, or for logging km skated. On a track one can use 
pen and paper and a stopwatch, and aggressive inlining would kill it on 
the first grind...

Hope this helps, and YMMV... :-)

br Franklin
(Oslo, Norway)