Double or Triple chain rings ?

[4F]

Ah, the 'because I say so' defence much beloved of parents when telling children what to do

Asking for an explanation and demanding proof are two different things, although if you cannot provide an answer I accept they appear the same.

What has Santa got to do with the weight of my bike, and what gearing does his sleigh use from a standing start on a 10% pitched roof?

Tsk SteveB, everyone knows he is called Father Christmas and not Santa

 

[Fab Foodie]

Its your choice whether you believe or not.

Personally, I don't believe Jesus was the 'Son of God',

I don't put up a Christmas tree and don't go to any of their ceremonies.


About my 'mystical' "psychobabble". Maybe it shouldn't be in the 'Beginners' section because I had been riding for eight years before I was indoctrinated into the inner circle of cycling witchcraft - when I was fourteen.
Sorry Newbies - If you are the type who DEMAND proof, remember next December 25th is just another ordinary day.

Jimbo, this is not about what you do or don't believe in.
This is about giving advice to beginners. If you're gonna give us pearls of widom they need to be reasonably accurate or demonstratable. So far what you've written makes no sense, that's why we've asked for a better explaination, not simply what you've previoulsy stated in Bold letters. You might even be right, but you cannot discern that from what you've written.

These are still good questions:
OK, some questions.

Why reciprocate by 25?
Why multiply by 1000?
Why should 40" be OK for all riders?
Where does a 10% gradient fit into this?
Why does the weight of the rider not make a difference?
Why does the fitness of the rider not make a difference?
If I lose 2 stone will I still need a 40" gear?


Can I ever be indoctrinated into the inner circle of witchcraft? I've got 30 odd years of experience.

December the 25th's not just an ordinary day, it's a public holiday (Oh and for Christians it's a religious day, and for non-christians it's a day to give presents for no particular reason and for retailers to make a lot of cash)

 

[jimboalee]

Explanation.

It just happens to fit.

Many. many moons ago, before 531 was introduced, bike designers and Internal Hub Gear manufacturers noticed there was a 'dead simple' equation to approximate the gear length required to ride a bike up a hill.

There is the Sturmey Archer 3 speed hub. Its No. 2 ratio is a straight through 1:1 ratio. On a 25lb bike, when this is geared to be the equivalent to JK Starley's 52" gear, 1st ratio, at 0.75% was 38".
The 'dead simple' equation said 40".
"If lowest ratio is less than the result of the equation, the bike was a runner".

For touring, Sturmey introduced the 4 speed hub, the FW with a 66.66% 1st and 3rd was 1:1.
Really serious tourists would arrange 1:1 to be LESS THAN 50" and have a double chainset and tensioner.

This is, of course, all history and I'm surprised to learn I am the only one on CC who has heared of the equation.

Even the great Sheldon Brown doesn't give any advice on gearing for bike weight. All he says is :-

"If you had a perfect bicycle, with an infinite number of gears, you would always be pedaling at the same cadence, with the same amount of resistance to the pedals. Of course, the bike would go slower uphill, and faster downhill, but your legs would not know the difference."

Can I emphasise the last phrase 'your legs would not know the difference'.

Go to the lengths of calculating the power requiremnts to ride a bike, and you will see the 10% gradient power in the 'correct gear' is very similar to the power requirement at about 18 mph.

My predecessors in the 'Witchcraft', who must have been inspired fellows, found the equation worked.
Instead of going through the labourious task of calculating the Power requirement for a specific bike, the equation was quick and gave a unbelievably close figure of gear length for the bike's weight.

 

[tyred]

None the less, I enjoy Jimbo's posts on the subject of gearing. I might not fully understand exactly where he is coming from all the time or I'm not sure how much of it really applies to my own cycling but I'm sure he's giving advice based on years of experience.

I studied electronic engineering at college. One of the lecturers was around retirement age and gave many unusual formulae for working out voltage drops around a circuit and whatever. These were not found in any text book, he never explained where he got them from, but they always gave the correct answer and were easier to use than the "proper" formulae. The following year, with a different lecturer for the same subject, our new lecturer insisted we stop using all these short cuts.

 

[Fab Foodie]

Explanation.

1. It just happens to fit.

2. This is, of course, all history and I'm surprised to learn I am the only one on CC who has heared of the equation.

3. Even the great Sheldon Brown doesn't give any advice on gearing for bike weight.

1. It just happens to fit does not meake it right or meaningful, you can correlate many things but it does not mean they are connected.

2. There may be a reason for this.

3. See answer 2.

 

[yello]

FF, I think tyred's explanation above is useful for getting a handle on jimbo's methodology. Your questions to jimbo are arguably in a different language. You're perhaps asking him to answer your questions in your language.

Anyone that's ever done any kind of translation work will tell you it's not always possible to provide an exact, or even adequate translation. In the absence of learning the mother-tongue (as it were), you'll just have to appreciate the beauty and take on trust!

 

[jimboalee]

Another 'Pearl of wisdom'.

JK Starley, when he was developing his Rover Safety Bike, had it on a William Froude water brake dynamometer to assess it's power requirements.

A single speed 52" gear required approx 100 Watts to ride at 12.5 mph. Remember the thing didn't have mudguards and was quite heavy so had an advantageous Inertia.
Acceleration and hill climbing weren't that good.

I've got my Apollo gent's upright geared to Starley's 52" in middle, and 38" in first, and it rides like a blessed dream. I comfortably rode the route of the Warwickshire Wanderer 100km Audax on it without any bother at all.

I geared my GF's bike to the theory. She's not a cyclist by any stretch of the imagination, and she rode 30 miles round ( not over ) Bredon Hill, nr Eckington in an afternoon.

I wouldn't - believe me - allow my relationship to collapse due to a bike with the wrong gearing.

I could sit here all day recounting historic references, but the simple answer to everybody's question is:-

Work it out for yourself, find a 10% hill and give it a trial.
You may find Sheldon's "your legs would not know the difference" could quite well be true.

 

[jimboalee]

Yet another 'Pearl of wisdom'.

To all newbies who are looking to buy a bike.

If you see a bike that at first looks 'racey' and is a bargain. Check the gearing.

There are still cheapos out there that have 42 tooth small rings and a 25 largest sprocket. The bike might be made of Hi-Ten and probably weighs in at over 25lb.

42 x 25 on a 700C tyre is 45".

The "mysterious" equation says 40" for 25lb.

DO NOT, repeat, DO NOT buy that bike. It won't climb. It will feel like someone has grabbed the back of your saddle when you try any gradient steeper than 10%.

Even if you retrofit a 38 ring, it won't be good enough.
Go further and fit a 28 sprocket and its getting close.

But don't buy it because it will be a struggle to carry full water bottles.

I dare say FF, et al, will agree with this.

 

[Steve B]

OK, I think we are now getting somewhere nearer the source of the equation. however it needs an important caveat at the start. It would be better phrased as follows:

Newbie: I am an average rider, what gears should I be looking for on a bike to get me comfortably up a 10% gradient.

LBS: Well sir, for an average cyclist we recommend a 40" lowest gear, as we find most of our average fitness riders can manage a 10% gradient with this gear length.

Newbie: But if I get fitter or thinner will I need the same gears?

LBS: No sir, you will hopefully not need to drop down to the 40" gearing, though we recommend you start with it.

Newbie: I like the look of that top end Carbon race bike Lance Armstrong used to ride - can you sell me one so I can be like him?

LBS: Well it has a 52" lowest gear sir, you will need to be fit to climb on it, but its your money and if you really want it, even though it may not be the best bike for you, we will sell it you rather than lose a sale.

two weeks later....

Newbie: Hey cyclechat members, how can I learn to climb? Got myself a new bike and boy am I unfit over hills.

The point I am making is that the original formula cannot be in any way a definitive formula to suit every rider on every climb at every fitness level. What it can be is a good starting point for the average bod who wants to start riding. It arrives at an answer of 40", and that answer suits most riders who are starting out. How it arrives at the answer is still unclear, and its application to new riders is fine, but to all riders? Nope, not a chance.

 

[4F]

Steve B, keep going. This is becoming a cracking thread.

 

[Ant]

Very interesting stuff I must say.

It's inspired me to try and work out the gearing on my new bike using the 'Sheldon Brown' calculator, but I'm having a bit of an issue with wheel size.

They're Rodi Airline Evo 700c with 31mm rims

Not sure what that translates to on the calculator. Do I select 700X32 as the closest?

I've never really put much thought in to wheel size before

 

[MacB]

SteveB, excellent summation, basically Jim's equations assume certain variables. His response to the question, 'what if the rider in question falls short of that variable?', is that any rider needs to aim to reach that standard as a minimum. There is sense in that but a beginner using it as a buying guide would not work.

Personal experience has seen my lowest gear requirement on my commute go from 31 inches to 44 inches. This is a good improvement on my part but did involve several hundred miles of riding. Had I started out with a low of 40 inches, I'd of been walking some hills, getting pretty disheartened and may have quit altogether.

Ignoring rider weight is another error, I'm 21lbs less than my starting weight, that must have a say in my ability to not go below 44 inches now.

 

[rich p]

The point I am making is that the original formula cannot be in any way a definitive formula to suit every rider on every climb at every fitness level. What it can be is a good starting point for the average bod who wants to start riding. It arrives at an answer of 40", and that answer suits most riders who are starting out. How it arrives at the answer is still unclear, and its application to new riders is fine, but to all riders? Nope, not a chance.

Excellent post, Steve. I think I tried to make that point on post no.40 but I love these tennis match threads

 

[jimboalee]

OK, I think we are now getting somewhere nearer the source of the equation. however it needs an important caveat at the start. It would be better phrased as follows:

Newbie: I am an average rider, what gears should I be looking for on a bike to get me comfortably up a 10% gradient.

LBS: Well sir, for an average cyclist we recommend a 40" lowest gear, as we find most of our average fitness riders can manage a 10% gradient with this gear length.

Newbie: But if I get fitter or thinner will I need the same gears?

LBS: No sir, you will hopefully not need to drop down to the 40" gearing, though we recommend you start with it.

Newbie: I like the look of that top end Carbon race bike Lance Armstrong used to ride - can you sell me one so I can be like him?

LBS: Well it has a 52" lowest gear sir, you will need to be fit to climb on it, but its your money and if you really want it, even though it may not be the best bike for you, we will sell it you rather than lose a sale.

two weeks later....

Newbie: Hey cyclechat members, how can I learn to climb? Got myself a new bike and boy am I unfit over hills.

The point I am making is that the original formula cannot be in any way a definitive formula to suit every rider on every climb at every fitness level. What it can be is a good starting point for the average bod who wants to start riding. It arrives at an answer of 40", and that answer suits most riders who are starting out. How it arrives at the answer is still unclear, and its application to new riders is fine, but to all riders? Nope, not a chance.

Newbie: I am an average rider, what gears should I be looking for on a bike to get me comfortably up a 10% gradient.

LBS: Well sir, most Off the Peg bikes these days have low enough gearing for a chap like you to get up a 10% gradient.

Newbie: I like the look of that top end Carbon race bike Lance Armstrong used to ride - can you sell me one so I can be like him?

LBS: Certainly. It's a fine engineered machine which weighs just over the Legal limit. Its for serious competition, but if you want one, we can measure you up and order one your size.

Newbie: Hey cyclechat members, how can I learn to climb? Got myself a new bike and boy am I unfit over hills.

CC respondant: You lucky bastard. Wish I could afford one of those bikes.



BTW, the 40" you mention is for a bike ( including all extras ) that weighs 25lb, adjust accordingly.

 

[jimboalee]

This thread has turned into

"What gearing do I buy to get me up a hill?"

Which is a very common question from new cyclists.

A good LBS man will have a good idea based on the cyclist's answer to a couple of questions:-

Do you get exhausted walking up three flights of stairs?

Have you ever ridden up a steep hill before?


These are, of course, Red Herrings.

The LBS man knows from experience the appropriate gearing for a bike. He is not going to tell any Joe who comes into his shop his trade secrets.

As I have implicated, today's bikes are geared accordingly and new cyclists have nothing to worry about.

Much earlier in this thread, the subject turned from 'Double or triple' to 'that doesn't matter, its the gearing that matters', so I contributed with a piece of 'folklore' about choosing the lowest gear your bike MUST have. Not the absolute lowest, but the gear the bike should have in its line-up.

Now working from first principles, estimating the speed a cyclist climbs hills is one of the most difficult questions to answer. Try it, I've worked myself round in circles trying to devise some calculations.
The non-determinable factor is the rider's mental attitude on the day.


Assume a 75 cadence for climbing a steep one. Plot a kW vs kmh curve. Determine the power requiement for 30 kmh. Split this between horizontal and vertical progression for various gradients and back-calculate the horizontal element to kmh.

Assosiate the kmh for each gradient to 75 cadence and calculate the gear ratio that is applicable.

Hey presto.

Why 75 cadence?
Why 30 kmh?

You don't think I'm going to tell 'trade secrets' do you?