5 Jan 2015

5th January 2015 A faster bike computer??

Monday 5th 41F, 5C, heavy overcast, almost still. Had a quiet walk up to the woods and back. Grey but dry. The spray tracks are back to sticky mud again.

I reset the Sigma to 2100 [instead of 2135] and it registered 17.6 miles to the GPS' 15. Is the Sigma working properly but the GPS is not? Why would it [the Sigma]  not register distance correctly using the wheel setting number recommended by Sigma themselves in their very own instructions? The Sigma is showing that I am cruising at 20 mph quite regularly. This seems rather optimistic compared with my previous computers.

On the Cateye my average cruising speed was only 16mph. None of this makes any sense at all. Have I crossed into a parallel universe? It can't be due to any confusion between kph and mph. Can it? The instructions sheet clearly shows both mph and kph match for the wheel settings. Besides the m/k conversion factor is 1.6. Which is a huge difference! Not just a couple of mph exaggeration.

I often wondered why I would max out at 30mph on descents on the Cateye. Quite regardless of steepness and tail wind I struggled to get beyond 30mph. It was almost like an unbreakable barrier. It is too long ago to remember which exact wheel circumference I was using on the Cateye. Perhaps I adjusted it to match the GPS logger? That might make sense. Though why the GPS logger would be so wrong is still odd. What I really need is a known distance to check the accuracy of the Sigma. But where would I get the reference distance? Google Earth? Some other online mapping service? Hmm. Sheldon Brown shows two different figures for Sigma which have nothing to do with each other. Nor does either number [for 700x25] match the recommendations in the Sigma instructions.[Mph settings in all cases] Now I have just found a different figure on Sigmas' website FAQ!  Okay: Think of a number. Don't tell me what it is... Abracadabra...Your number is... 2046? What about 2105? Could it be 2135? Best of three? Alright then...it has to be 2146? None of the above? Welcome to the Twilight Zone. 8>{

Tuesday 6th 39F, 4C. Incredibly heavy overcast, slightly misty. Very, very dark! Even at 8.45 9.00am it it is still not remotely what can be called daylight. Walked for an hour and half through the woods and back. I am exploring different tracks and firebreaks to try and avoid the very wet conditions on my regular routes.

Further research suggests that GPS can easily have an error of 1-2%. Depending on the equipment involved, of course. I have heard that it [GPS] is poor at detecting speed on hills. Presumably because it is relatively inaccurate at measuring altitude compared with linear distance along the ground. Some amateur YT videos suggest foolishly high speeds attained for long descents using GPS bike computers as speedometers.

With normal chainsets it would be completely impossible to continue pedaling usefully at the claimed [high] speeds. Yet the YT perps can be seen pedaling when in fact they would need literally hundreds of rpm in cadence to "catch up" with their claimed road speed without spinning out. No sign of the huge chainwheels required. Nor mention of the tiny rear sprocket to go with it. Matt Weaver used 90t x 9t gear in his HPV streamliner for 65-70mph.

I tried a 68T TA chainwheel once in my distant youth and spun out on 68 x 13 [141"] on one of the long descents into Bath. Climbing that same hill first on 68t x 26t [71"] was quite a struggle. It was fortunate that I had toe-clips and straps because body weight alone would not have moved me forwards more than a few yards uphill!

It is funny to see top racing cyclists trying to pedal while descending mountain passes with their usual gearing. A "touring" top gear of 50x12 gives only 40mph at 120rpm. A very unlikely 60x11 gives 50mph at 117 rpm cadence. You don't see many chainrings of that size in the T de F!  While the rather more commonplace 54 x 11 racing bike gearing gives only 45mph @ 117rpm. 50mph takes 130 rpm. 55mph takes 143 rpm. While 60mph requires 156 rpm. Which is just possible for a fit rider.

Unusually heavy vehicular traffic and wet conditions have completely spoilt the familiar tracks. These are normally leaf-covered with little sign of human activity. I prefer the sense of exploring the untrodden as I plod my solitary path.

I have seen just over 150 rpm myself, briefly in a low gear on the flat, while testing my top cadence just for fun. I used to be able to pedal very fast when I was a skinny teenager but had no cadence meter to prove it. I ran 44x28 fixed for a few days. Just to see what it felt like. 20mph required 163rpm. Good fun descending the local hills!

[Edit the figures in the boxes to find new ratios.]

The Sigma Data Center webpage is still down for repair. Though I seem to have downloaded an evaluation copy from their website I assume this isn't the full Monty. Reset the Sigma to 2105. Rode to the shops. 16-20mph going with a tailwind. 8-12mph coming back straight into a cold headwind. My legs were aching. Which is unusual these days. I passed a spot where some drunken nutter had completely lost control by overshooting a corner. The grass bank on the opposite side of the road was gauged. Not only in one place but in three places spread over 100 yards! 14 miles. [I think]

Wednesday 7th 37F, 3C, almost still, clear sky waiting for sunrise, the constant stench of dark grey smoke from our westerly neighbour burning demolition waste for fuel. Perhaps this is how they can afford several horses, dogs and luxury cars? It may also explain why my chest is bunged up all winter.

Here the unspoiled, sunlit beech woods invite exploration. 

Despite my shortness of breath I enjoyed a pleasant walk up through the woods in bright sunshine. Though it was not so much fun being sprayed by speeding, [GPS rat-running] juggernauts on the road to get to my exit. Signs of further drunks losing control on the corners and taking to the verge on the opposite side of the road. There was a small flock of [Thrush-like] Redwings sharing a huge lawn with the local blackbirds. A sparrow was taking used bedding from a bird nesting box. With light winds and sunshine it could be a good day for a ride.

Rode to Fåborg with a fairly gentle side wind. Going quite well cruising at 16-20mph. Wandered about at my destination taking a few photos in the cold wind and then headed home again. Now I was pushing against a head crosswind. Fortunately traffic was light as I rode the appalling cycle path past the Fåborg harbour and then the long climb up from Millinge to Jordløse. Being able to use the road, instead of the 4th class, 3rd World cycle lane, must surely be well above my pay grade. Probably worthy of  prosecution for undermining the authority of somebody really important. Somebody like a drug addled, habitual drunk in a rust bucket with no license, insurance or road tax. I passed under several birds of prey perching on lampposts. Times must be really hard in the state of Denmark. The Sigma [with a 2105 wheel setting] and the GPS agreed within half a mile today. Returned at dusk with all my lights flashing. 54 miles.

Click on any image for an enlargement.

No comments:

Post a Comment