16/09/2013

Gear Ratios

Now most people who buys a bicycle when they look at the specs, they look at the No. of gears it has. I know as I do it too, whether it has 3, 5, 6, 15, 18 or more. But do you actually look into the details of the gear ratios, again I don't. All you want is the 1st gear to easily to pull away from stand still & go up the gears gradually to increase the speed with less effort, again that's all I expected too.

So as most know I rebuilt a bike called 'The Beast', it has 18 gears. 3 at the front & 6 at the back, rather old school in tech as is a 1 piece crank so haven't really touched it.


But on my rides due to my increase of speeds I can achieve in each gear as my health improves, I have noticed an odd thing when I change from 6th to 7th & 12th to 13th. Every time I do this I have to change up a gear on the front chainwheel, when going to 7th & 13th I feel like I'm actually going down a gear or 2 not up. So I found it easier to just go from 6th to 12th & then straight to 18th & miss all the other gears.

Now I know this can't be right, so after a few weeks of this I decided its time to sit down & work out why. So to start with I counted all the gears on the front & rear, then work out the gear ratios etc using an app I installed on my mobile. Here's the results:


Now looking at this, its just a load of numbers & showing it in this layout its not easy to see a pattern, but I could see something strange in the gear ratios. So I did a table to put them in gear order, as shown to make it easier to see:


Now it made it clear, I was right in how the gears were handling. When I go from 6th to 7th, I do indeed go down in gear as in by gear ratios. 7th gear ratio is in between 2nd & 3rd & same happens with 12th to 13th, where 13th gear ratio is between 8th & 9th gear. The chart below shows it even clearer...


So what doe this mean, well it means my gear ratios are wrong. What can I do, that is something I have to sit & work out as I havent found any chainwheels to fit this old 1 piece crank & probably means an upgrade of the bottom bracket & find the right chainwheels to get a steady increase of the gear ratios without any dips. After all any dip in the chart like shown above will affect performance of the riding.

I know this hasn't shown any "how to's" or "fix it's", but is an interesting observation which I thought I would share. It's something I have now learnt & can use in the building of my new bike 'Roadrunner' & now know to not just take the gearing on a bike for granted.

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