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Old 04-22-10, 11:55 AM   #1
prtyich
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Gear ratios

What does it mean that the Dahon Speed TT has a gear ratio from 30" to 131", and the Vitesse P18 from 31" to 96"?

Why is there such a large discrepancy on the top end - is that because the TT has 27 gears and the P18 has only 18?

If so, does that mean that the TT is a far faster bike since, the last 9 gears are harder to pedal, but will drive the bike faster?
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Old 04-22-10, 01:28 PM   #2
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It is not directly related to the number of gears -- if you put on a big enough chainring you could achieve higher gears with a few number of gears. It gives a number by which one can describe how many times the wheel will spin for each revolution of the crank. Technically, a gear inch tells you something about the equivalent wheel size on a penny farthing.

http://www.sheldonbrown.com/gloss_g.html#gearinch

http://en.wikipedia.org/wiki/Gear_inches

http://www.sheldonbrown.com/gears/
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Old 04-22-10, 05:03 PM   #3
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My brain hurts from reading the Wiki article...can you build a rocket after comprehending that article?
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Old 04-22-10, 06:12 PM   #4
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It is rather simple - here is my crack at supplying a simple answer:

1. Gearing is the ratio of the number of teeth of the front chainring and the rear cogs:

Ratio=NT(front)/NT(rear); the Vitesse has 9 rear cogs so has 9 of those ratios, hence a gearing "range" of 31"- 96". (Wheel diameter in inches is multiplied with simple cog teeth ratios to yield inches.)

2. The TT has higher gears because it has an internal geared hub, which supplies additional step-up and step-down gears compared to the Vitesse; so

Ratio1 = 1.33*NT(front)/NT(rear);
Ratio2 = 1.0*NT(front)/NT(rear);
Ratio3 = 0.75*NT(front)/NT(rear)

Each of these Ratios come in 9 values, hence 27 gears.
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Old 04-23-10, 09:56 AM   #5
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My brain hurts from reading the Wiki article...can you build a rocket after comprehending that article?
Sorry. That is the reason I gave the penny farthing example. If you change the size of the wheel, then one revolution of the crank will lead to a different distance travelled. A bigger wheel -- i.e., a gear inch increase -- leads to more distance travelled with a single revolution of the crank.

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Old 04-23-10, 10:58 AM   #6
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What does it mean that the Dahon Speed TT has a gear ratio from 30" to 131", and the Vitesse P18 from 31" to 96"?
It heads the Speed TT will go faster, but both climb hills about the same

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Why is there such a large discrepancy on the top end - is that because the TT has 27 gears and the P18 has only 18?
Partly. Though you can design a large range or more gear inches with less total number of gears. However typically more gears means more options both for climbing and for speed.

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If so, does that mean that the TT is a far faster bike since, the last 9 gears are harder to pedal, but will drive the bike faster?
[/quote]

Pretty much. Depends on your ability to pedal that, riding position, drag, etc as well. But all things equal more gear inches=faster bike to a point.
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Old 04-23-10, 11:24 AM   #7
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It heads the Speed TT will go faster, but both climb hills about the same/Partly. Though you can design a large range or more gear inches with less total number of gears. However typically more gears means more options both for climbing and for speed./Pretty much. Depends on your ability to pedal that, riding position, drag, etc as well. But all things equal more gear inches=faster bike to a point.
Since the TT ratio is about 30% larger, that would mean a person must pedal 30% more on the Vitesse to travel the same distance - or 30% faster to move at the same speed - as the TT?

In a 30-mile ride, that would be ALOT more pedalling, i.e., far more stress on a cyclist riding the Vitesse than the TT.
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Old 04-23-10, 11:33 AM   #8
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But all things equal more gear inches=faster bike to a point.
But fewer gear inches mean you can pedal faster = faster bike to a point.

The truth is that the fastest bike has the gear which strikes a balance between how fast the rider can pedal and how hard the rider can pedal. So you can't say that either higher gears are faster or lower gears are faster because it depends if "higher" is closer to the perfect gear or further away from it.

So to answer the OPs question, the TT is faster because it has a wider range of gears (which makes it more likely to find the right gear) but not necessarily because it has higher gears.

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Old 04-23-10, 11:54 AM   #9
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In a 30-mile ride, that would be ALOT more pedalling, i.e., far more stress on a cyclist riding the Vitesse than the TT.
Not necessarily. Would you rather pick up a 1 pound package a thousand times or a 1000-pound package once?

Anyway, you have walked into an old argument in cycling. At the moment, "sport cyclists" appear to favor high cadences -- I average 90 RPM -- which translates to the higher gearing on the TT having little value. But YMMV.
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Old 04-23-10, 12:14 PM   #10
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What does it mean that the Dahon Speed TT has a gear ratio from 30" to 131", and the Vitesse P18 from 31" to 96"?
To answer your question directly: Most riders pedal best with a pedal cadence within a certain range. A good rule of thumb average is 90 cycles per minute but more experienced riders often pedal faster, novices slower. Whatever your preferred cadence, the higher gear means you will go faster but correspondingly have to push harder. At those speeds the main thing you are pushing against is wind. For a cadence of 90, gear numbers of 96 and 131 correspond to speeds about 24 and 35. The wind drag at 35 is twice what it is at 24. Unless you are going downhill or have a strong tailwind you won't be able to maintain a good cadence in the higher gear. Which means you will use it very rarely if at all. Of course, you could use the 131 to go 24 but your pedal cadence would be lower than most riders would recommend or find efficient.

On the other hand, you may use 96 quite often. So the only other question is this: When you aren't using the 131, what gears will be available below it that you can use? If it has the same number of gears to cover a broader range, the spacing between them must be different somewhere. It may not matter at all because you tolerate enough range in your cadence that you don't need so fine a difference from one gear to the next. Or maybe the shift sequence covers enough gears but isn't convenient or smooth. Or maybe the Vitesse P18 just has fewer gears.

I'm not a racer and not a strong rider by a racer's standards. I rarely use a gear as high as 90 though there are times when one could be useful (as opposed to necessary). Never in my life have encountered a situation where a gear over 100 was absolutely necessary.

In any case, 131 looks like a bigger number, doesn't it?
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Old 04-23-10, 12:34 PM   #11
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Not necessarily.
Anyway, you have walked into an old argument in cycling. At the moment, "sport cyclists" appear to favor high cadences -- I average 90 RPM -- which translates to the higher gearing on the TT having little value. But YMMV.
I know what you mean .. This year I've started riding a two speed Sachs coaster brake bike for my recent 20mile after work loops, with the same group I rode with last year (rolling hills with about 1000' of elevation change) .. absolute best time I recorded last year (towards the end of the season) on any of my bikes was a loop average speed of 16.3mph on my Pacific Reach Road (18 speed .. 30-117 gear inch) .. these rides are not races, just fitness enhancers ... last night I averaged 15.5mph on my 2 speed 54 to 74 g/i bike.. and this is early on in the year... I was surprised ..
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Old 04-26-10, 10:39 AM   #12
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My speed d7 has about 92 gear inches. I'm not a particularly in shape guy, nor do I have clip ins, or any claim to being an "expert" cyclist.

92 gear inches is comfortable on flat ground, or a slight decline for me. If I get to a downhill of any significance I quickly find myself reaching too fast of a cadence pretty quickly. For me the 131 inches would mean that I would still be able to pedal on the downhill. 96 gear iches would mean that I'm geared okay for flat land.

If you're like me you won't need the higher gearing unless you live in a hilly area. Even then its not really needed. Its just nice to have.
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Old 04-26-10, 06:15 PM   #13
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Sorry. That is the reason I gave the penny farthing example. If you change the size of the wheel, then one revolution of the crank will lead to a different distance travelled. A bigger wheel -- i.e., a gear inch increase -- leads to more distance travelled with a single revolution of the crank.

Comparison to an ordinary or penny farthing is entirely appropriate. The gear inch concept was invented way back then to convince people who were used to riding the large wheeled bikes that a "safety cycle" with small wheels could be geared to give them the same performance. Every cyclist of that day was familiar with the effects of wheel size so the gear inch equivalent was a perfect way to get the point across. These days gear inches are just numbers to those of us who are not familiar with large wheel cycles but they are perfectly useful numbers even though we don't have the tight experiential bond to wheel size that existed back then.

It is also correct that the large gear inches are useful mostly for downhill runs. With a top end in the mid 90's you just have to coast down hills that are very steep. People who want to pedal as hard going down a hill as up it will frequently need those high gears in areas with steep hills. If you are content to coast, you do not need the high gears and might wish that you could have lower gears. In fact you can, just change out the chainring to a smaller one and you can shift the range downward. The ratio between the lowest and highest gear will remain constant but you can set the low gear where ever you want it within reason. If this model had only the rear cassette and a fixed speed hub then the total range in gears would be the largest cassette gear divided by the smallest. With the rear hub available you multiply that range by the largest hub ratio divided by the smallest. That is why this model has more total range than models that lack the Dual Drive rear hub. Nothing new here, I'm just reinforcing what has already been said.

Now you can get the same effect from a front derailleur as you get from the Dual Drive hub. When you have an FD you multiply the range of the cassette by the largest chainring divided by the smallest chainring to get the total range. The Vitesse has an FD in fact but it has only two chainrings that must be fairly closely spaced since it has very little more range than Dahon's 7 and 8 speed models which have neither the FD nor the DD hub.

If you want to supersize your gear inch range you would start with an 8 speed model and put a Schlumpf High Speed Drive on it. The HSD will multiply the range of the cassette by 2.5 and a Dahon standard 11-32 cassette would give you 16-118 gear inches. If that is not enough you could put a Capreo rear hub on the bike and modify a Capreo cassette to 9-34 teeth, giving you 15-144 gear inches!!

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Old 04-26-10, 09:05 PM   #14
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most people in SS/FG, who only get one gear, use something around 65~75GI for just cruising along.

one downside to having a wide gear ratio, especially if you have fewer gears to choose from, is the jump between the gears. Sometimes you can be caught between gears, where the one you're in will feel too low, but the next one up is too high.
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Old 04-26-10, 10:42 PM   #15
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... If you are content to coast, you do not need the high gears and might wish that you could have lower gears ...
This is exactly my case. I geared my 3x9 DualDrive low, 17.1 to 92.5 GI according to Sheldon.

When I roll downhill at >20MPH, I don't pedal and just let gravity do its thing. On the other hand, the low gear allows me to climb those long and steep (for me) hills that I could not do otherwise.

Kam

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Old 05-01-10, 12:47 PM   #16
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What does it mean that the Dahon Speed TT has a gear ratio from 30" to 131", and the Vitesse P18 from 31" to 96"?

Why is there such a large discrepancy on the top end - is that because the TT has 27 gears and the P18 has only 18?

If so, does that mean that the TT is a far faster bike since, the last 9 gears are harder to pedal, but will drive the bike faster?
It means the Dahon has high gears that you'll never need and the Vitesse has a useful top gear.
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