Gear Percentage Calculation?

Jezston

Über Member
Location
London
Hi, trying to compare the gears on my kona dew to some hub gears, however the hubs are measured in percentages, and my kona's in teeth.

How to I work out the 'percentage' from the teeth?
 

MacB

Lover of things that come in 3's
same starting point, so go by the actual teeth, front and rear to give you the gear inches for the direct ratio. Hub gears will then be a %age based on that. You just multiply the direct ratio by the %age variance, ie the I-9 is from .542 to 1.844, so if your basic calc returns 51.8 inches then bottom would be 28.1 and top 95.5.
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
Jezston said:
Sorry, how to I go from teeth to inches?
Work out the ratio for any one given gear as "Teeth up front / Teeth at back". (eg 52/11 = 4.72)

Measure your wheel size in inches. (eg. 26")

Multiply one by the other. (4.72 x 26 = 122.9)
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
Good lord. Just worked out my Furai (when the gears work) has gear inch ranges of 19.4 to 105.9. (Metres development of 1.5 to 8.4). I think its time to fit that bigger chain ring.
 

tyred

Legendary Member
Location
Ireland
On a slightly related note, does anyone use a Sturmey Archer XRD8? I do fancy the idea of converting my Peugeot to hub gears at some point in the future. I have spare AW hubs but would like a wider gear range so I homed in on the XRD8 to look at the spec and it seems that bottom gear is a 1:1 drive. Surely this is crazy and would require a chainwheel and rear sprocket of the same size to give a sensible gear range?
 

MacB

Lover of things that come in 3's
tyred said:
On a slightly related note, does anyone use a Sturmey Archer XRD8? I do fancy the idea of converting my Peugeot to hub gears at some point in the future. I have spare AW hubs but would like a wider gear range so I homed in on the XRD8 to look at the spec and it seems that bottom gear is a 1:1 drive. Surely this is crazy and would require a chainwheel and rear sprocket of the same size to give a sensible gear range?
these two sites are worth a read:-

http://www.hubgear.net/
http://hubstripping.wordpress.com/internal-gear-hub-review/

the SA 8 speed does come in for some criticism around having bottom as direct drive. I think claims are that efficiency drops as you move further from this gear. As most riding would be done in higher gears this makes some sense. But there is a good arguement for their variation in gear steps, ie having big changes at either end of the range. This means that the middle 6 gears are nicely spaced at 13% intervals for general riding. But you have a bailout gear at either end for the extremes.

One of the criticisms levelled at Rohloff is the equal steps, it's also thought a boon by some. What it does mean is getting the gears right at one end of the range can often mean a compromise at the other end. for example getting the low gears right for laden, hilly tours, can mean a very limited choice in the general riding gears.
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
tyred said:
...it seems that bottom gear is a 1:1 drive. Surely this is crazy and would require a chainwheel and rear sprocket of the same size to give a sensible gear range?
Depends on what the top gear ratio is, and what kind of gear inch you'd consider sensible at the low end. I can barely spin fast enough (and frequently don't manage) to keep my bike upright at 19.4" gear inches.

A good target range for me is 25" at the bottom and around 110" at the top. 25" is enough to pull me through 15% hill starts on my loaded commuter, and 110" is good for 35mph sprints. Above that, I spin out and have to convince gravity to lend a hand.
 

MacB

Lover of things that come in 3's
arallsopp said:
Depends on what the top gear ratio is, and what kind of gear inch you'd consider sensible at the low end. I can barely spin fast enough (and frequently don't manage) to keep my bike upright at 19.4" gear inches.

A good target range for me is 25" at the bottom and around 110" at the top. 25" is enough to pull me through 15% hill starts on my loaded commuter, and 110" is good for 35mph sprints. Above that, I spin out and have to convince gravity to lend a hand.
can't be done on the 8 speed, a low of 25" would give a top of 76", 1 to 3.05 being the range.
 

tyred

Legendary Member
Location
Ireland
I would be aiming for something in the low to mid 30s as a bottom gear, so with a 27" wheel, it is not quite a 1:1 ratio between crank and rear sprocket but pretty close to it. I could survive with a low of 40-44" but if I was going to do that, I may as well use the three speed after all as it was a few extra gears at the lower end was what I was looking for.
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
Suspect this is why the only hub gear bike I have is a dual drive. 'Bents demand a fairly wide range (or at least do with my legs). There's no standing up on the pedals to help at low speeds, and the top end is normally some way north of an upright.

Add in silly wheel sizes, and you're quickly into gearing hell. :evil:
 

MacB

Lover of things that come in 3's
GrasB said:
Could be done with a double at the front but difficult due to 1:1 being the lowest ratio.
true but that starts to defeat the simplicity aspect of a hub gear. It really does depend on how low you want the range to go. As the gear steps are in %ages it can tend to bunch too many similar gears together at the bottom of a range. In this respect SA had a reasonable idea but could have done with another big step, or two, at the bottom, currently it goes:-

28% then 5 at 13% then a final jump at 28% again - so you could have a range of 34 to 103 inches but the middle six would be 43 to 80. I'm not sure how the planetary gears work in the SA but a better spread would be given by 28% twice at the bottom and then climbing in 15% increments after that.

This is where I find the Rohloff disappointing, it's 14 equally spaced gears. the most useable setups seem to end up with too many close gears at the bottom. If you avoid this then you end up with too high a low and a top that's silly big.

I'm looking forward to seeing how the Shimano Alfine 11 speed stacks up, it should have a coup,e of graded %age steps to avoid the Rohloff style bunching, as does the current I-9. It has 11 gears covering 409%, compared to Rohloff 14 over 526%, SRAM I-9 9 over 340% and the 8 speed offerings over approx 305%.
 
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