Dash computer update.

Arch

Married to Night Train
Location
Salford, UK
wel,, I opted for the Cateye long fitting kit and an Enduro 8, and it fitted nicely - just right in fact, not a lot of extra wire at all - just enough to cope with the handlebar movement when steering on full lock. Had to use a lot of shim to get the sensor and magnet close enough together, but it seems to be registering.

Just need to double check the circumference setting, the kojaks are a size not listed in the setting up sheet (20 x 1.35 - anyone know the circ off hand? I'm not bothered enough to care about distortion when I'm sitting on it), and I forgot to take a tape measure to the lock up.

And I've got an idea of how I might just get away without a rack, and make my own side pods. I've got one small rucksack that fits in the space, need to find one to match the other side. Might just manage to hold lightly packed gear for a hotel tour - providing I really can pack light. Off out now to see if I can find a suitable bag.
 

mcshroom

Bionic Subsonic
This guy found his to be 1505

http://www.bikeforums.net/showthread.php?582304-Programming-computer...

Hope that helps
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
I guestimated my cycle computer using:
ETRO number + (2 x Tyre width) * Pi,
then checked it against my GPS (Etrex Legend HCX) on a series of day rides last summer.

Findings:
  • Cruising flat and straight roads at speed, the computer and GPS agreed to within 0.1%. (20 mile average)
  • Ambling curvy roads, the GPS reading was often down by ~ 2% compared to the computer. (15 mile average)
  • Blasting steep but straight descents, the GPS was down by .97% (14% gradient average. 1 mile.)
  • Climbing steep and winding ascents, the GPS was down by 3%~4%. (12%. 1 mile)
  • On a really stupid steep and winding ascent, the GPS was down by over 8% (14% average, 3/4 mile)
  • On straight but slow sections (traffic) GPS was up by 0.9% (15 mile average).

In some cases, I repeated the same sections of road (regular commutes, training loops, etc) in opposite directions or at different speeds, and got contrasting readings each time.

Conclusions
  • The GPS and cycle computer broadly agree.
  • Weaving around the road can cause the GPS to under read slightly.
  • Weaving a lot can cause the GPS to under read significantly.
  • The GPS disregards any distance travelled on the vertical axis (Pythagoras)
  • When moving slowly, imprecision in the GPS location will pad the mileage a little.
  • As the jitter and imprecision work in opposing directions, they tend to balance out for normal riding.
  • The imprecision and loss of altitude info will compound on a steep weaving climb.
  • The cycle computer is a good indicator of how far the bike has actually travelled.

Slow weaving (from above)
. ___ . . . . . .___
./ . . . . . ./ . . . . . .___Cycle computer (tracks front wheel)
/ . . . . . ./ . . . . . ./
X------X------X-------X------X----GPS track (fails to notice jinks)
. . . . . ./ . . . . . ./ . . . .
. . . . .__/ . . . . . __/ . . . .




Big descent (side view)


.
.
. .
. .
. . .
. . . ____ Cycle computer (traces hypotenuse)
x---------X GPS (measures adjacent side)


On the whole, the difference was pretty negligible, and I soon dumped the computer. As my legs got stronger, I found the readings from the GPS improved considerably.

YMMV. Mine certainly did :becool:
 
Location
Midlands
Arallsopp -excellent - it is exactly my experience - but to that I would add - bike computer measures how far the bike has moved on the surface of the earth - GPS measures how far you have travelled on a theoretical spheroid/projection - in most cases there is not enough differenceto worry about

I still like to have both - the bike computer so that I can keep an eye on my speed - hammering into a hair is not a terribly good idea on a heavy bike - and so that I can view things on the GPS like the altitude screens, stopwatch etc seperately
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
Agreed. In my findings, the theoretical spheroid projection shares a topology with huge swathes of East Anglia, and disagrees vigorously with Kent :biggrin:

As with you, I'm happy to ignore the error and enjoy the ride. The only time its ever bugged me was coming back from Brighton a few weeks back. If the steep section of the Devils Dyke road descent is over 14%, the max speed I recorded on the GPS (47.98 mph) should very probably be north of 50. :becool:
 
Location
Midlands
Ditto - also the other reason for keeping the computer is that if you get on a very steep bit going down and accelerate quickly there is a little bit of a lag in the GPS registering the sudden increase in speed - again handy to be able to see the computer when the corner looms up

you might gather that I like to be able to brake late into corners - OK on a stiff bike
 

redddraggon

Blondie
Location
North Wales
So say if you have a 705, and use the GSC 10 cadence/speed sensor which was set up on the flat straight road. Will the Garmin still read the horizontal distance measure by GPS or the number of wheel revolutions determined by the GSC 10?
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
LOL. Assuming 7000ft variance in height (as a right angled triangle) I think you're looking at an additional 15.46 yards. :wacko:

I dunno. Math wasn't my strong point.
 
OP
Arch

Arch

Married to Night Train
Location
Salford, UK
squeaker said:
Using what :tongue: 3.142 x (406 + 70) = 1495 on my abacus. I'd suggest that 148cm would be nearer the actual rolling circumference, unless you pump your tires up to the max....
HTH

PS: Keep us posted (with pics) on the bag front :smile:
Hmmm. Confusion. Mine are (at least I'm fairly sure, and it's 2 mile trip to go and check, so I won't just now) 37-406, not 35-406. That's the same as that chap in the link mcshroom gave.

Use 37, and the maths works out to 1508.

And someone on Velo Vision said he runs his computer on marathon racers at 1530, so that the kojaks should be 'a little bit less'.

I'll double check the tyre next time I'm at the lock up. I have a small rucksack to try out as a sidepod, and need to get up and try it for size this week, so that I can go back and get a matching one if it fits ok. Then it's a matter of playing about with a lot of spare buckles and the rucksack straps. If I'm really clever I might be able to work it so that I can carry one or both sides, depending on need.
 
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