Bicycle Gears, Gear Inches, Etc...

Crank arm length

You are correct But I must add,,

It would have a direct effect on a perceived cadence as In a relation to
felt knee pressure and applied torque loads on joints..

Shorter or longer crank arms would in that aspect become part of the
final 'Roll Out' of the bike per a given cadence.

I will continue to contemplate this on the 'Tree Of Woe' for quite some time.

The Page one Dragline systems down here In Florida's phosphate mines have a bucket that could hold TWO school buses.
The boom at the end accelerates from still to a speed of 55 MPH then back to zero on Its approximate 180 degree ark of travel.
The measured speed only half way out on the boom is, half that.

So think of the crank arm length as the first gear ratio your engine (Legs) encounter as they produce power (watts)

If you are clipped in you are (Should be) producing power during the full 360 degree crank rotation..

Crank length, longer/shorter, Chaining's, larger/smaller 48teeth/52teeth,,,,,,,,,,,

Follow me ?

 

I believe that the key word here is perceived. Of the three variables in your statement, the only one that can be measured while riding is cadence.

Gary

 

Thanks

Gary

 

Finally got it right

After owning four velomobiles and forty three other Recumbent Bikes and Trikes, I have used essentially every gearing system available including Rolhoff hubs, Schlumpf drives, Alfine hubs, 7,8,9,10 and 11 speed cassettes and every crank length and chainring combo from touring triples to racing doubles etc.
The lowest gearing I ever had was on my Sinner Mango sport Velomobile which had the 14 speed Rolhoff and the schlumpf mountain drive with a 34 tooth chainring. After tons of experimenting, this is what I ride now that I have a bad hip and lower back etc:
My 2015 scorpion FS26 has an FSA light full carbon crank with a 22/34/42 triple, KMC hollow pin chain and a SRAM pg950 11-36 rear cassette . My 2014 Catrike expedition has the same setup except with a 24/34/44 FSA light triple carbon crank.
My counterpoint presto has a vintage 1995 stock Shimano triple touring crank with 39/52/60 chainrings paired to a shimano 12-34 7 speed cassette and a Sachs three speed internal hub. It's got 63 speeds and has a super low climbing gear and a ridiculously high downhill and flat gear. All three bikes use 170 crank arms which are correct for my 5'10" frame!
I live in the mountains of Northern NJ where there are some extremely steep climbs everywhere. In my experience, internal hubs other than the classic sachs three speed have a lot of internal drag and the five sclumpf drives I have had all had noticeable drag.
My former 2013 scorpion fs26 had the Rolhoff 14 speed hub, schlumpf speed drive and the Dynamo hub lighting system. There was so much drag from all that. I had trouble riding it! The internal hubs are also insanely heavy although essentially maintenance free. That's my experience over the past 26 years riding recumbents exclusively.
Back in the 80's I rode from NY to Main and back and then across the USA the first time. I did it on a crappy 10 speed Walmart bike! Two years later I crossed the USA again on a much more advanced Centurion 10 speed and the following year did it again on a modified GT mountain bike. Following that I raced 14 time trials for the Olympic cycling team on yet another ten speed so go figure! I was dropped in the final round by the way! Those Zitslian fellows were just too strong for me!

 

Thanks!

Indeed traveling 3899 miles on a crappy Walmart bike across the USA in 1986 was quite an accomplishment but what was interesting about that choice of equipment, you guessed it- no mechanical problems other than a loose pedal bearing which I managed to tighten! No clipless pedals, biking shoes or padded spandex! Just a teenager (I did my first crossing with AYH (American Youth Hostels) when I was sixteen years old between 11th and 12th grades. The following year I did it with Bikecentennial and then a few years later with Adventure cycling which used to be Bikecentennial.
A lot of people over on BROL ask me what I would ride for my fourth crossing and without hesitation I would say my new scorpion FS26. I would not even use a trailer but would load the panniers.
I look at my recumbents in my garage now and still cannot believe I crossed the USA three times on an upright! Like most of us I had no idea about recumbents back then. Believe it or not I had my first tecumbent which I bought directly from Dick Ryan (A long wheelbase vanguard) in 1993 but was afraid to use it for my last crossing in 1994 because I dicnt trust the under seat steering God setup as I had problems with it. I'm sure it would have been fine! Duh!

 

I just wanted to post a note that not all calculators are created equal. For example, Sheldon's calculator for some wheels will not produce the same results as others. For example the 20" calculation is about 7% different than the basic equation (Fr/Rr*WheelDiam). As I recall, the 700 is dead on. The reason may be that he actually measured some wheels and use that number instead of the published diameter.

The difference may not be significant depending on the calculation or use. The trouble comes when someone says they have a 88GI and you calculate the same drivetrain on a different bike and you get 95GI. The difference is always larger on the high end.

I noticed this when I was using Mike Sherman's calculator (I like the visual) home.earthlink.net/~mike.sherman/shift.html But it does not calculate the internally geared hubs (such as NuVinci) so I was using Sheldon's calculator. I noticed that I was not getting the same results for the same setup.

So, when comparing, use the same calculator for all the options. When comparing your numbers to others, be aware there may be differences.

 

Not only are not all calculators are created equal, but neither are wheels/tires.
Since 20 inch wheels were mentioned, I'll pick on them first. A 20 X 1.75 is not the same a 20 X 1 3/4. Actually there are 4 different rim sizes (diameter) that are called 20 inch. Looking at their ISO Bead Seat Diameter they are

• 451
• 440
• 419
• 406

To make things more complicated there are 7 different 26 inch wheels when looking at the ISO sizes.

So how is one to calculate gear inches? (In the following examples I'm going to use measurements from my trike [36 for the front and 11 for the rear and 20 inch wheel/tire] and the calculator on the computer. A spread sheet or desk top calculator could be used also)

First if you are wanting a close estimate then use the following
Front tooth count / rear tooth count * wheel diameter in inches (36/11*20 = 65.454545454545454545454545454545 or 65.45)

If you want more accurate results then you need to do a roll out of the rear wheel and get the circumference of the tire.

• If you measure the roll out in mm then divide that number by 25.4 (1565 mm / 25.4 = 61.614173228346456692913385826772 inches) We can round that to 61.61417 or 61.61 for what is needed.
• With the circumference in inches we need to calculate the diameter of the wheel/tire (Circumference / PI = diameter). Using the computers calculator the result is 19.612400467623322872307329895904. Rounding that to 19.6
• The final calculation is Front tooth count / rear tooth count * wheel diameter in inches. Using 36 for the front and 11 for the rear, the formula becomes 36/11*19.6 = 64.15 gear inches

For an even more accurate measurement you can sit on the bike/trike during the roll out, but is that really needed?

Of course if you don't want to do a roll out you could just lay a tape measure across the wheel/tire and eye ball the diameter.

I hope I have not muddied the water too much.

Gary

 

Accuracy really depends on the application. If one was designing a bicycle computer accuracy would be really important. But if one is simply comparing multiple drivetrain options for their bike/trike/quad/etc, then accuracy is not as important and the relative difference is important. A few GI +/- error applied equally to all is in the noise.

For example...I have a Catrike Road and the upper 95GI is not high enough for my riding. As such I'm considering different options. I created a spreadsheet to compare a couple options and since I use the same rule/equation for all, they are easily compared. Had I used multiple online calc's the results may have different methods and as such the results may be artificially skewed.

BTW ... here is the equation I use:

GI = FrRing/RrRing x Wheel diameter x HubRatio

HubRatio = 1.0 for a standard geared hub. But for the NuVinci N380 it is .5 on the low end to 1.9 on the high end. Other IGH's have different numbers.

As pointed out, 20" is not necessarily 20". But since I use 20" as a constant for all options, It is consistent and close enough.

Like the phrase ... "measure it with a micrometer, mark it with chalk and cut it with a torch"

 

I looked at Sheldon Brown's calculator's code and about 2/3rd's down there is a table of nominal and actual size. Here is the section for 20" tires

<option value="20.15"> 32-451 /20 x 1 3/8 </option>
<option value="19.90"> 28-451/20 x 1 1/8 </option>
<option value="18.68"> 20 X 1.75 / 44-406 / BMX </option>
<option value="18.43"> 20 X 1.25 / 32-406 </option>

You can look for yourself to find the tire size that interests you . Right click on page and then select "View Source".

So what does this mean to the user?
- When someone quotes a GI remember that the devil is in the accuracy of the diameter value (either entered or selected depending on the tool).
- When using GI to select a drivetrain, use one tool or method for all options calculations so that the answers can be compared.

 

What kind of cadence do you ride with? 95 GI at 90 rpm is over 24 mph. Might not be enough if you like to pedal when you're going down hills, but for riding on the flats it should be plenty unless you're a really strong rider or a use a low cadence.

 

I'm not sure about my actual cadences since I don't have that on the computer. I did check once via the watch and my preferred cadence is 70+ in the flats. But that so much depends on the day (me, the weather and the cycling spirits)

There are times when I pedal much faster and that tends to be down hills and from a down hill run into the flats. Or when riding with a noticcable tailwind The trouble with the low high end 95GI (20" nominal) is that you cannot gain on downhills what you lost going up the hill so that the avg speed suffers. And to my point above, if you use 18.5" as the wheel diameter (which is close to reality) vs the 20" nominal indication, then you have an 87GI resulting in a 23mph *90cadence.

But the issue also begs the question of how much pressure you can apply to the pedals at a given cadence. Applying zero pressure to the pedals at any given cadence results in no work applied to the speed of the bike and only energy consumed moving your legs. You might as well coast. As your speed increases, with a low Gi top, the faster the cadence and the harder to apply any pressure. And at some point the body's coordination makes it difficult to effectively apply work to the bike. What I have found that works is pulse pedaling. Given the trike's GI, at around 28MPH I pedal really fast for a few pedal rotations and stop and then repeat. This avoids the bike's and my instability issues related to too much cadence.

But this is a diversion from the original intent of the thread and my comment that not all calculators are created equal so be careful comparing results from different calculators and methods.