Pushbike Computer for the 1100GS

Mick Fagan

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This is an article that I wrote for my local BMW club. After some prompting by Squidbrain suggesting that he too would like to read the said article, and, that I should insert it here.

Consider it inserted!!

All prices in Australian Dollars in Melbourne.

For quite some time, I’ve been toying with the idea of fitting a pushbike speedometer to my motorbike. Well, I have done that, and it reminds me of a Confucius saying, “Man who has two clocks, never knows correct time”.

The pushbike speedometer computer that I eventually purchased is called, a Sigma BC800 manufactured in Germany. This was bought for two features, which I thought were important and/or desired.

The first feature was a requirement, what top speed the unit would measure to. Most pushbike computers only measure up to 99Kph or 99Mph. In the case of the Sigma BC800 it has a top speed of 300 Kph as this is definitely within the range of my bike, no problem.

The second feature that this particular Sigma computer has, is the ability to track two wheel sizes. In pushbike land it is relatively normal for one to have a road bike and a mountain bike and this computer would allow me to have the dirt tyres as bike 1 and road tyres as bike 2.

Sigma is also the only manufacturer of these computers that I could find, which sells extra handlebar mount sensor kits. In fact, the kit is called a rear wheel sensor kit. It’s extra long, so that it can go from the handlebars to the bicycle rear wheel. This is a decided advantage for fitting to a motorcycle due to the fork legs being a lot further away from the handlebars, compared to a pushbike that is.

One thing that appeared to be a problem was fitting the sensor to a fork and the positioning of a magnet somewhere on the front wheel, which would trip the sensor. I had, over the last year, looked at various methods and decided that simplicity was to be the overriding factor in mounting this computer. To that end, I looked around for a smallish Rare Earth Magnet. The best I found was at Jaycar Electronics, in the form of a 4 pack of 10mm dia x 3mm thick Nickel coated magnets for ten dollars. Rare Earth Magnets are extremely powerful in the magnetic department compared to normal magnets, that’s why I used one. It would trip the pick-up sensor far more easily than the weaker supplied OEM unit, I believed.

I then took one of the magnets, placed it on the head of one of the bolts that hold the R/H disc. The bolt is steel, so the magnet stuck there like a Limpet Mine. I then positioned the pick-up unit as close as possible, and, couldn’t believe my luck. I noted that it looked as though it was going to sit perfectly in position, once secured to the lower part of the fork leg slider. Spinning the front wheel with the computer attached to the handlebar mount and with the sensor handheld, the speedometer worked a treat. Leaving the magnet on top of the Disc retaining bolt, I then Araldited it to the top of the bolt head, it shouldn’t be hard to remove, I hope.

The pick-up sensor was then held in place by using thin cable ties instead of the rubber bands supplied. I then cable tied the sensor wire to the fork slider and positioned the computer mount on the handlebars alongside the twist grip. As the unit is designed to go on pushbike handlebars, fitting to the GS handlebars took about 1 minute.

Then came time for the obligatory test ride, which had me wondering just what I was going to see. At this point in time I was running Continental TKC80 tyres, these are the dirt tyres for the 1100/1150 GS bikes. I know from experience, that the speedo reading changes slightly from a new TKC80 to a rather worn set. As the 1100GS has an unbelievably inaccurate speedometer, with the general consensus being that it is close to 8-9% out (reading high) I took to the road.

The circumference of the front wheel had been measured with new TKC80 tyres at two thousand and fourteen millimetres, the computer had been given this knowledge, and the sensors were perfectly aligned with the magnet attached to the Disc bolt.

My first impression as I rode down my street at very low speeds, was that the speedo readings jumped a bit. That is, one second it was reading 17 Kph then it went straight to 33 Kph as I accelerated. If I held any steady speed, the reading settled down quickly and remained in alignment with the bike. However, sharp braking or acceleration at low speeds meant that the computer wasn’t able to follow as fast as I thought it would. I then reasoned that if I was able to put 2 magnets on the front wheel, equally spaced, tell the computer that the front wheel circumference was half of what it really was, then, the response times at low speeds should be 50% quicker. At higher speeds there is very slight a lag, which iis almost unoticeable.

Meanwhile, travelling at faster speeds, it was obvious that there was a major discrepancy between the BMW speedo and the pushbike one. The BMW speedo was reading equal at or near 50Kph but took off, quite markedly, at 80Kph. In fact, the needle was closer to 90 than 80 when the pushbike speedo showed 80Kph. 100Kph indicated on the pushbike speedo, was very close to 107 – 109 on the BMW one. This correlates to what I think is correct from experience, overhead radar speed indicators on the freeways, and, good old gut feeling. It doesn’t of course mean that it is more correct than the original speedo, but I believed that it was.

Now, I didn’t really purchase and fit the pushbike computer just to get a truer reading of speed attained, there were a couple of other functions that I lusted after.

The trip distance function, is identical to the trip distance meter already fitted to the BMW speedo, so this will now be used to calculate parts of a trip that are critical, eg: - 17 Klm turn half left, another 5 Klm turn hard right, leaving the original trip meter to be retained to give me the big picture on fuel range.

Trip time function, gives you the actual time you are travelling only. It does not include any time that you are stopped. Therefore, it really indicates just how long you are moving forward. Most people would be quite surprised, at just how slow one travels, I know, I am.

Average speed function, gives you the actual average speed you are travelling at. This is a very good way of working out just how far you will be able to travel in a given time, allowing one to plan a trip and it’s stops, with the knowledge that your goals should be achievable. This was the function that I desired the most.

Maximum speed function, gives you the maximum speed attained since the trip meter was last set to zero. In today’s world of the speed camera, dense traffic and similar restraints, this is almost a non-requirement, but it is nice to know sometimes, eh!

Sigma, manufacture a range of pushbike computers, they range from the BC400/800 and higher ended Targa models. The 400 model is the same as the 800 except it does not have the ability to run two wheel sizes. If you wish to have the cheapest form of pushbike computer, this is the one. You more than likely, will have to extend the sensor twin core cable by about 300 –400mm but that should be easy. Alternatively you could purchase the rear wheel sensor, which is longer and it should go straight on, at least in the case of my 1100GS it did. Part number is 0332.

Cost of the 400 is about $50 the 800 is $60 and the rear wheel sensor kit is $35. Other models go up close to $120, but really, the extra functions of the higher priced computers which include pedal revolutions a minute and things like that, aren’t useable on a motorcycle.

Very few pushbike shops seem to stock the German Sigma computers, however Cecil Walker in Elizabeth St. Melbourne and Lawrencia Cycles in Glenferrie Rd. Hawthorn are stockists. I ended up buying the computer in the City and the rear wheel sensor in Hawthorn.

One overriding factor to me, for purchasing the BC800, was the fact that the speedometer figures are 10mm in height, compared to all the other Sigma models having 8mm height figures! These 2mm larger figures made a difference on a pushbike that I rode, so I figured that on a motorbike, the larger figures would be far easier to read, especially as the unit was going to be further away from my eyes.

I have now had the bike computer fitted for a few months and have concluded that it is everything that I expected and slightly more. I have deduced that the speedometer on my particular bike, is reading about 7% high at a genuine 100 and 80Kph when fitted with Continental TKC80 dirt tyres. This was tested by a simple stopwatch test over a flying half kilometre at these two speeds. I know that it isn’t that accurate a test, but it’s close enough for the intended purpose of not getting a speeding fine.

The odometer reading on the Sigma computer is reading about 5% less than the BMW one, which means that for every ten thousand kilometres that the machine travels, it is actually doing five hundred less. I’m getting to think that maybe BMW has a vested interest in having the service intervals come up sooner, rather than later.

The one rather annoying, but liveable shortcoming, is that the unit is not lit at all, therefore when travelling at night one has to resort to checking out the speedo as one passes under lights, this, is a surprisingly easy thing to do.

Mick
 
Roger, yes I have seen that in the catalogues, but the problem is that I don't have that much room on the bars left.

I run a tyre bead breaker which is attached to the bars along the crossover brace and running along the triple clamp.

I work on the keep it simple approach, where less is more. That way when I step off in the sticks, I won't be pulling things off left right and centre.

I once in the seventies, was running a two way radio on the bike when I stepped off. To put it bluntly, I spent about one hour out of two, pulling off aerial cables and the like before I got to the broken bike bits. I decided that having lots of little things can be a bit of a problem in the bush.

It's a good idea though, thanks for the thought.

Mick.
 
Steve, I looked at the Panoram bicycle computer, but it was to my way of thinking too large to comfortably and unobtrusively sit on the bars.

It also has the disadvantage of not being able to measure above 190 Km/h. I know that sounds preposterous from someone who owns a bike with a top speed of 195 Km/h however when I visit the Northern Territory where there is no speed limits on the open road, I would like to give the bike it's head, even if it's only for a miniscule blast.

I purchased the extra long mount as I wished to use the normal mount for another pushbike that I have. It was one of those things that just worked out better that way, otherwise I would've cut and extended the wires.

Mick.
 
Yes please Mick. I tryed to send you my email address by private mail but was prevented from doing so! Could you post a picture on this thread?
Thanks:)
 


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