A groovy little bicycle computer project

[pwillikers]

I put a bicycle computer on my s last night. It seems to work fine and I kinda like it so I thought I'd share...

It's a SIGMA SPORT BC800. I bought it from Bike Nashbar for $13. It will display speeds up to 183 MPH. It mounts to the s bar with a rubber oring like it was made for it. I just had to loosen the allen pinch bolt on the right bar, slide the mastercylinder assy. out abit, wrap the oring on and slide the assy back. My bars are mounted on top of the top triple clamp but I'm reasonably certain it'll mount just as well on bars below the triple clamp. It is angled perfectly for viewing.

The wheel sensor was a bit different. I had to lengthen the wire from the sensor to the computer, just cut and solder, blah blah blah. You know the drill. I fooled around with several mount schemes and settled (for now) on the arrangement pictured. I used this amazingly strong double sticky tape that I found somewhere to hold the magnet to the wheel. It works (and is ASTM tested up to the Vmax indicated on the computer in the pictures) but it looks dorky and appears vulnerable to being knocked off. I'll be happy to hear of alternatives from the peanut gallery.

Have at it.

Oh yeah... For calibration purposes, I calculated the circumference of the front tire to be 1890 mm. (which when corrected for mph is 1174. you'll understand what this is all about when you read the instructions.) I haven't had it on long enough to do a gnat's ass validation of this calibration. When the speedo reads 70 mph, the computer reads 66 mph which is within reason.

[moto one]

I guessing that you dont trust the stock speedo? Mine seems a bit optamistic.

[CarlosT]

Borrow a friend's GPS and you'll see the real speed.

[pwillikers]

I'm sure the speedo is optimistic. I installed it so I'd know how fast I was really going and for the geek factor entertainment value of the computer functions:
Vavg
Vmax
Trip time
Trip dist

[Sirus]

I saw an ad for a GPS that is out, or coming out, made by Timex. The watch acts as a heart monitor as well as speedometer, odometer stopwatch etc.. maybe more $. But hey, all you need to do put it on your wrist and go. You could watch your heart rate too and see if its proportional to your speed ;-)

Sirus
2003S

[rob_wijhenke]

He, PeeWee, look at the left handlebar:


Have been using these for a couple of years now, great asset!
I have noticed my speedo is optimistic at low speeds and gets pessimistic the faster you go! Hence I corrected the circumference to be 1960 which give me at 200+ kmh a more or less accurate reading. According to a Dutch magazine which tested the S 2 years back or so, the stock speedo is not too far off compared to many other bikes. I believe 100 kmh was 98 real kms.

BTW, you can calculate the real circumference at any given speed by using a formula I lost. Anybody has it? Had to do with (force*speed)/material strength or whatever it was.

[Eron Flory]

Calculating the actual circumference of the tire at speed would require knowing a lot more about the tire than you are likely to be able to find out without some serious testing resources.

If you really want to get geeky about this, remember that as the bike is leaned over the indicated speed will become more optimistic because the tire is no longer contacting the ground at the largest radius. Corning loads will flatten out the contact patch, further reducing the radius from contact patch to axle.

Eron

PS, I think the top speed function is cool. People always ask me how fast I went at a certain track, but looking at the speedo just as you are grabbing the brakes at the end of the staight is a good way to spoil a great day.

When I fitted one of these little goodies I measured the front wheel circumference with a tape measure so I got it as close as possible. You could just as easily run a piece of string round it and measure that. I also recalibrated the bikes speedo and they both tally now to within around 1mph across the whole speed range. The pic shows a top speed run I did with luggage to see how it pulled loaded up and also where I have fitted it now that my speedo is accurate. I found the speedo to be 10% optimistic as standard so at a true 100mph it would show 110 and at top speed it would be way over 150 which isn't actually possible due to the rev limiter.

[Rapid Dog]

Neat-o...but I don't get it...

[JerryTreeXprt]

How about just putting a mark on the sidewall and one on the ground as a starting point, roll the bike forward (or reverse, whatever you're into) until the tire does one whole rotation, mark that spot on the ground and measure the distance from point A to point B. About as accurate as you'll get without lasers. As far as the speed being off because of the smaller circumference when leaning over, the regular speedo has to deal with that slight inaccuracy, but you've got to measure from somewhere so the centerline of the tire seems like the best choice.

[Trevor Rossack]

When I installed the Sigma on my S that is the method I used to calibrate the unit. A day or so later I was lucky enough to come across a speed indicating unit in my local area (because of heavy penalties for being 3kph over limit, this allowed drivers to "calibrate" their true speed) and found it to be reading 1kph fast at 80 and 100kph. This was immediately rectified.

Cheers
TREVOR

[rob_wijhenke]

Andy, when does your limiter kick in? stock 8250 or chipped 8500+ ? (mine +/- 8700) I don't know how much the theoretical difference than is but it must be good for couple of kmhs.

About calibration: I corrected and confirmed the settings of the digi speedo by measuring the distance travelled at various speeds 100, 150 kmh by checking the actual distance between hectometer poles aside the road. Every road has them here so that's a pretty good way especially if you take a stretch of let's say 5 or 10km. You have to make sure to keep your line to minimize aberration.

[pwillikers]

Did any of you guys come up with a better mounting scheme for the wheel sensor than I?

Rob, I'm using the Laser chip which cuts at 8500rpm which I reckon would give a theoretical top speed of 151mph. You'd probably have to be well tucked in though and take the mirrors off!

[bcdaum]

Paul,

You might try mounting the sensor to the lower brake mount and zip-tying the magnet to where the brake disc mounts to the wheel. It is a really tight fit, but that was how I did it on my Sportster a few years back. It did work really well.

Bob

[pwillikers]

The wheel magnet took flight at 109 mph. So much for my super double extra sticky foam tape. On to plan B what ever that will turn out to be .

[pwillikers]

Rob, I wonder how much the tire changes diameter due to centrifugal force between 10 and 200 kph? Could that account for the spedo's change from optimism to pessimism? Ever watch fuel dragster's slicks get really big in burnouts? I've heard it's like a continuously variable transmission as they increase in speed. Makes sense. Although, I don't much like thinking about the forces on tires at high speed.

[markjenn]

On my R6, I was able to zip-tie the magnet to a brake rotor carrier. This is with the pickup zip-tied to the inside of a fork leg. I'm sure it is Ok, but I would prefer not attaching the sensor to a hydraulic line.

Here's a tip for getting the circumference. Put a spot of oil on your tire, roll it across the driveway and measure the average distance between the spots. Not much oil, of course.

Last note: The o-rings they give you for attaching the display base plate to your handlebar suck. They'll give up within a year. I've got a 2nd one on there now, of the Home Depot variety, and it seems to be holding up better.

- Mark

[Rootboy]

This is how i mounted my magnet....I did a search here and Tominator posted the following link...I think it comes from IBMWR...good luck....

http://www.hal.varese.it/~alexb/sigma.html

I used the back out of a fridge magnet and hot glued it to the rim.