Burtty's Blogs

Greetings All, 

Thanks to Geelong Cycling Glub Web Master, Simon I have graduated to the world of the "Blog" and let me tell you I am barely able to type with excitement. This is an astute move by Simon as I now have a place to air my world view independent of him.

Simon has struggled with the various contributions I have made to the Web Site, understandably as he no doubt felt the need to distance himself from the thoughts expressed.  Clearly this is why Simon is "The Web" Master.

I will regurgitate some of my previous material and place any race report or observations on the "cycling condition" generally as well as pushing my own particular barrows. I have quite a few.

Might I say from the beginning my aim is to entertain, amuse and possibly educate, never to offend. Any jibes of a personal nature will be carefully considered dependent on my assessment of "skin thickness". For example, it will always be "Open Season" on "PJ", Junior members will get the "Softly Softly" treatment.

So, to my first contribution, a "Treatise" on gearing, entirely my interpretation from discussion with knowledgeable cyclists and experience. I was moved to print several years ago by a question from a cycling associate with his own ideas and research, this is my response.

Conversion of “Gear Ratios” to Kilometres per Hour.

Dear Reader,

This treatise was composed in response to a question relating speed, KPH to whatever “gear’ the rider happened to be in. The answer is neither straight forward nor simple as the following testifies. Apologies for my sense of humour if any, it is the only one I have.

1. Calculate the “gear” in inches.

(Universally understood language of cycling.) Origins dating back to the Penny-farthing, that unwieldy machine, the front wheel diameter  dictating it’s gearing. Bigger wheel equals faster, inner leg length not withstanding, smaller wheel equals easier climbing. Think of a child’s tricycle. As the child grows in height and strength the pedals of  a larger diameter front wheel can be reached and pushed.

2. Divide “sprocket” (Small gear at rear wheel in “cluster” of 9 now for the self respecting, 10 or 11.) into “chain ring” (Typically 53 and 39 teeth at “Crank”)

So, 53 divided by 11 = 4.8

3. Multiply 4.8 x 27 (pre 700c wheel) = 130.1

Conversationally: “I was rid’n downhill, wind up me clacker push’n a 130”.

A cyclist need go no further, understanding is complete.

Now for the “café” Set.

The number “130” represents the diameter in inches of a wheel that would travel as far in one revolution as a bicycle in one pedal rotation riding 53/11. So for a penny-farthing to be “geared” identically to a modern “safety bicycle” being ridden with the 53 / 11 gear combination it, the penny farthing needs a front wheel 130 inches or 3.25 meters in diameter, clearly an impossible steed for human use.

Multiply 130.1 by Pi, 3.142 (approx) = 408.7 the distance in inches travelled per pedal revolution.

Divide 408.7 by 39.3 (inches per meter approx) =10.4m

Multiply 10.4 x 80 (assumed cadence) = 832.05m per minute.

Multiply 832.05 x 60 (minutes in 1 hour) = 49.9 KPH.

Therefore, pedalling 53/11 at 80 rpm you will be travelling at very nearly 50KPH, eg. Grubb Rd southbound in a decent northerly.

Robbie can pedal at 120 rpm so in favorable conditions may do 74.8 KPH.

Applying the same calculations for the 39/23 (no self respecting cyclist would contemplate a lower ratio) again at a cadence of 80 RPM we would be pedalling a ”45.8”  (rounded up to “46”) at 17.5 KPH. Recommended use, cliff climbing and pulling mallee roots.

If per chance you venture to a track cycling meeting you will now be able to join the conversation regarding gear choice with knowledge and confidence as inches remain the gear description of choice in this environment having been faithfully handed down through the generations.

Shamefully this tradition is lost on the “Nouveau Cyclist”, not so the “Hard Men” of racing.

John Burtt.                  (Corrections welcome)

Written by :

John Burtt

 

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Re gear ratios - your knowledge never ceases to amaze me, if only I could afford the membership fee. New club clothing design excellent. Congratulations to designer.

Moose , November 09, 2009

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