Gear Ratios

[Over50Newbie]

Hi,

Can someone help me figure out how to find out the gear ratios on my bike?

I have a 2005 Terry Classic - it is the one with the smaller wheel on the front.

I have 3 chainrings on the front and 9 cogs on the back.

Do I really have to count the teeth on each one to figure it out?

And do I have to measure the size of my back wheel?

Aren't there charts that are already done from the manufacturer that will tell me this?

I looked in my manual and it wasn't there, and I searched online at the Terry site, but I also couldn't find it there.

I am confused.

Basically, I just want to know which gear is lower compared to another gear. For example, when I'm in my 2 in front, 1 in back gear, it feels easier to climb a hill than when I'm in my 1 in front, 3 in back gear. But that could also just be my imagination.

Am I making any sense at all?

Any help at all would be greatly appreciated.

Thanks,

Lynette

[OakLeaf]

There are specs on Terry's website for an '02 Classic (look under "Bikes On Sale") but not an archive. Do you know whether they changed the bike between '02 and '05?

The number of teeth is stamped on the cogs, but they can be hard or impossible to read while they're on the bike. It'll probably be easiest to see the small chainring and, if you don't have a pie plate, the largest cog on the cassette. It shouldn't take you that long to count though.

You only have to measure the run-out of your tire if you're super obsessive. Tire and wheel size doesn't affect the ratios anyway, it's a constant in your equations. Top gear is top gear whether your wheel is 24" or 700c.

But really, unless you're racing and it's super important, just shift by feel. If you're not in a gear where you're in danger of cross-chaining, then shift the rear derailleur only. If you're in, say, your largest chainring and the middle or "low" end of your freewheel cogs and you want to downshift, then downshift the front and upshift by a couple in the back until your cadence and the amount of effort you're expending feels right. The cogs on the cassette are your close ratio shifts, the chainrings give you ranges. No one shifts back and forth constantly between the front and the rear to get an exact sequential progression of gears. It's not necessary and you'd waste a lot of momentum shifting.

[pinkbikes]

There are some useful discussions about gearing on this thread here if you need general info about gearing.

http://forums.teamestrogen.com/showt...t=30141&page=2

But if you are looking to find out where your overlap is, then I just did a little spreadsheet with the chainring teeth down the left hand side and the cog teeth across the top. You probably know this, but the usual way to express gears (well the old fashioned way) is in "gear inches" and the formula is:

Wheel diameter (in inches)xChainring teeth/Cassette teeth

Generally a 100inch gear is considered to be a big gear and hard to push, 90 pretty much a common top, 80 a speed gear, 60-70inches most often used. 40-50 for hills. Below 40inches would be for very hilly terrain and for carrying heavy loads. So sayeth "Richard's Bicycle Book" circa 1984! Lots of things on bikes have changed since then but probably not basic human physiology and how we propel a bike!!

The ratios spreadsheet for my tandem looks like this and it is easy to identify the overlap zones (the shaded bits) you are trying to find. Btw I just used 27" because I don't really care about the tiny error in converting from 700c!

As you can see from this, using say the smallest ring on the front (30teeth) and the 4th gear (19teeth) on the back is a 42.6inch gear. Whereas using the middle ring on the front (39 teeth) and the 1st gear (25teeth) on the back is indeed just ever so slightly easier at 42.1inches. Too small a difference to call but that's the point at which the ratios start to overlap, which is what you were looking for I think?

[Grog]

First, the number of teeth should be imprinted on each cog at the back. You might need to clean your bike to be able to read them (mine tend to be a bit too dirty to be legible).

Basically, I just want to know which gear is lower compared to another gear. For example, when I'm in my 2 in front, 1 in back gear, it feels easier to climb a hill than when I'm in my 1 in front, 3 in back gear. But that could also just be my imagination.

This is totally possible depending on the gear ratios. But really I'd suggest just doing it by feel. You can be pretty sure that your smallest chainring in the front combined with the biggest cog at the back will be your easiest gear, though.

[Eden]

There is a really good gear ratio calculator on Sheldon Brown's web site
http://www.sheldonbrown.com/gears/

[pinkbikes]

It doesn't matter if you have two different-sized wheels. Only the driven wheel (ie the rear wheel) counts! The front wheel is just a passenger.

[Eden]

Yeah - that makes sense.

[Over50Newbie]

Ha! It was my imagnation. My 1-3 gear is actually easier than my 2-1 gear.

With all of your help, I figured it out. Thank you, ladies!!

I think that I did it right - I used the size of my back wheel (which is actually 700c, but I also used 27" )

It was so much fun to do, that I did it for both of my bikes. My Terry Classic and my Trek Navigator 300 (which has a tire size of 26").

Here they are:



I am so stoked to actually see it in table format - it makes everything so clear to me!

What do you think, do they look correct?

Thanks again,

Lynette

[wildeny]

There is a really good gear ratio calculator on Sheldon Brown's web site
http://www.sheldonbrown.com/gears/

Sheldon's gain ratio takes the crank length into account. I think it's better than gear inch. Besides, it is dimensionless.

[pinkbikes]

Ha! It was my imagnation. My 1-3 gear is actually easier than my 2-1 gear.

Nope - that imagination gets you sometimes! Good old mathematics always tells the truth! Although you can get statistics to lie sometimes!!

I think that I did it right - I used the size of my back wheel (which is actually 700c, but I also used 27" )

It was so much fun to do, that I did it for both of my bikes. My Terry Classic and my Trek Navigator 300 (which has a tire size of 26").


I am so stoked to actually see it in table format - it makes everything so clear to me!

What do you think, do they look correct?

I also did the same (maths geek) so I have a lovely spreadsheet with the ratio table for each of my bikes (27" and 26" wheels). I did a quick check of the ratios we had in common and yours are spot on. So I think you get a gold star for that one! You will never need to wonder again.

[pinkbikes]

Sheldon's gain ratio takes the crank length into account. I think it's better than gear inch. Besides, it is dimensionless.

Sheldon's formula takes crank arm length into account and works on gain ratio, which gives the added benefit of taking into account the greater/lesser lever arm of longer/shorter cranks. Strictly speaking it sort of introduces a "feel meter" for two sets of identical gearing with different crank lengths.

I find this is only a useful add-on if you are using it compare perceived difficulty of the same gearing ratios on bikes with different lengths of crank arm. Which is of course completely valid and very useful, especially if you are thinking of changing your crank arm length and want to know how it may make your existing gearing "feel" compared to how it used to.

But if you have your set of gears already and you are already set up with a suitable crank arm length for your anatomy (or even have multiple bikes with the same length of crank arms) then all it adds is mathematical complexity and it obscures the straight comparison of the actual gear ratios on different bikes.

Don't get me wrong - Sheldon was an absolute genius and his formula has valid uses - but really most of the world talks gear inches when comparing apples with apples (even in Europe and Oz where metric is next to Godliness! Gear centimetres???).

I'm not sure why anybody would prefer a "dimensionless" answer because frankly a dimension is much more meaningful in outright terms. A gain ratio of 6.6 means very little unless you are comparing it to a gain ratio of 8.4 or 3.1. Whereas a 100inch gear is a gear where you will travel pi x 100inches for each time you turn your pedals through one rotation. And hell - that is a pretty long way whatever length your crank arm is!!!

So..... for the life of me I have always pondered why they didn't just slap the damned pi into the formula there and make the dimension of gear inches actually mean something more representative - like how many inches you will travel for the one full crank rotation (maths geek - quite the waste of time really to ponder these things)?

I guess it is just that the numbers would have been so much larger and the pi is only a constant anyway, so why bother? Hmm - still a waste of time to ponder.....

[Over50Newbie]

Woo-hoo! Thanks PinkBike - I'm so glad that I did it right.

And, once again, thank you to everyone that responded to my question. I love that we all help each other out on this site.

Lynette

[wildeny]

I'm not sure why anybody would prefer a "dimensionless" answer because frankly a dimension is much more meaningful in outright terms.

A dimensionless quantity is more universal. No matter where you live, you get the same gain ratio from the same configuration.

In US/UK, you use gear inch: gear size = Wheel diameter (in inch) x Chainring teeth/Cassette teeth
but people in other places may use gear meter, which is the "roll-out" distance (or gear centimeter )

Gain ratio is about leverage. More precisely, the torque produce by your feet vs the torque by the rolling friction. The minimum torque you need to apply is equal to the torque by the rolling friction.

Torque = force * lever arm, which leads us to the minimum force you apply:

F_c = gain ratio * F_r [c = cyclist, r = rolling friction]

For the same rolling friction, the higher the gain ratio, the more forceful your pedaling. (or just consider F_r = 1)

This combines with the cadence is related to your power (that's what the power meter measures).

Gear inch/Gear meter tells you how far you can go when pedal one turn, while gain ratio tells you how much effort(work) you make(do).

However, no matter which method you use for gear size, all of them don't consider the effect from the tire (surface roughness, pressure).

[pinkbikes]

A dimensionless quantity is more universal. No matter where you live, you get the same gain ratio from the same configuration.

In US/UK, you use gear inch: gear size = Wheel diameter (in inch) x Chainring teeth/Cassette teeth
but people in other places may use gear meter, which is the "roll-out" distance (or gear centimeter )

Gain ratio is about leverage. More precisely, the torque produce by your feet vs the torque by the rolling friction. Two should be equal to each other under the ideal condition.

Torque = force * lever arm, which leads us to

F_c = gain ratio * F_r [c = cyclist, r = rolling friction]

For the same rolling friction, the higher the gain ratio, the more forceful your pedaling. (or just consider F_r = 1)

This combines with the cadence is related to your power (that's what the power meter measures).

Gear inch/Gear meter tells you how far you can go when pedal one turn, while gain ratio tells you how much effort(work) you make(do).

However, no matter which method you use for gear size, all of them don't consider the effect from the tire (surface roughness, pressure).

As an engineer I am pretty comfortable with the physics of torque. But I think you may be a little confused in your explanation here. In ideal conditions I think you would certainly NOT want the torque created by your feet and the torque created by the rolling friction to be equal, or you would not overcome the rolling friction and you wouldn't go anywhere!

You then give a formula where you insert the gain ratio so that the torque produced by the cyclist is equal to the gain ratio multiplied by the torque from rolling friction. So what you say there is that in ideal cases they are NOT equal? Which is it?

Also I think that you may be a little confused - gear inch/metre does not tell you how far one revolution takes you because the formula does not involve pi, which is required to develop the circumference of the wheel from the diameter.

Wildeny, I don't disagree with you that Sheldon's formula is great and I am quite comfortable with the physics involved in its many applications. But the OP just wanted to know which gear was higher, not compare different rolilng frictions, surfaces etc, so gear inches is just a simpler method of doing what she wanted without all the rocket science!

[wildeny]

pinkbikes, I do not disagree with your reason for using gear inch. As you already pointed out.

My previous reply was to explain the dimensionless quantity and the principle behind the gain ratio.

In ideal conditions I think you would certainly NOT want the torque created by your feet and the torque created by the rolling friction to be equal, or you would not overcome the rolling friction and you wouldn't go anywhere!

You then give a formula where you insert the gain ratio so that the torque produced by the cyclist is equal to the gain ratio multiplied by the torque from rolling friction. So what you say there is that in ideal cases they are NOT equal? Which is it?

Thanks for pointing it out. I should say that that is the minimum force you need to apply so that the wheels can start to turn. The minimum force is just to let the object remain at its status (if it's at rest, it's still at rest; if it's moving, it still moves with the same speed).

Also I think that you may be a little confused - gear inch/metre does not tell you how far one revolution takes you because the formula does not involve pi, which is required to develop the circumference of the wheel from the diameter.

I checked the definitions in "The Long Distance Cyclists’ Handbook" by Simon Doughty before writing the previous post. There it said the gear size in meters is "development", i.e. the "roll-out" distance. Also in Sheldon's page. Maybe I shouldn't use the term of gear meter here.

Even though gear inch does not include "pi" in the formula, it is based on the same idea without multiplying "pi". (imo, neglecting the multiplication of pi is jut to make the calculation easier)

What I'm trying to say is that gain ratio is not more complicate than gear inch. Their formulas only differ by the crank length, although the thinking is slightly different:

Gear inch[/Gear meter] tells you how far you can go when pedal one turn, while gain ratio tells you how much effort(work) you make(do).

That difference is important when comparing the gear inches from different bikes. Of course, for someone who always uses the same crank length, he/she doesn't need to consider the effect from the crank length, as you already mentioned. But, why not try to understand the effect of the crank length in gear size and know how to quantify it? [so that one can understand why his/her taller fellow can pedal with ease by the same gear inch]
(Note: the crank length is related to bike fitting too)

My intention in my first post was only to point out Sheldon's gain ratio. I'm sorry if my following explanations confuse people or make people feel more uncomfortable with gain ratio.