Saturday, March 1, 2008

Gearing Completed

A few days ago the rear cassette was changed form the stock 11-32 SRAM PG-970 to an 11-34 SRAM PG-970. This may not seem like much but in gear ratios, it is quite a bit on the bottom end.

Remember that the last change was to replace the 30T chainring with the 24T chainring. That accounted for a 20% gear reduction. The change from 32T to 34T on the rear cassette represents another 6%. The end result is that PhattKatt now has a low end of 13.2 gear inches which is considered very low... low enough that most 2 wheel'd cycles would wobble and fall over. 3 wheels are great!

Wikipedia has a quick reference for what gear inches really means HERE

In short, gear inches shows what the virtual size of the wheel would be in terms of diameter. In PhattKatt's case, 13.2 gear inches means it is like pedaling a wheel with pedals attached to its axles and the wheel would be 13.2" in diameter. It also means that each full turn of the pedal at this ratio is 41.5" of forward motion [PI*D].

Here is PhattKatt's final gear inch chart:

Please note the crossed out ratio's. There is a limitation to the rear derailleur known as Chain Wrap Capacity as stated before. This particular rear derailleur has a chain wrap capacity of 45T [T=teeth]. This is typical for long cage derailleurs. In PhattKatt's case, we have now exceeded the chain wrap capacity by 6 teeth. To calculate this, note the difference between the cassette's smallest and largest gears and the chainrings smallest and largest gears. PhattKatt has a cassette of 11T to 34T, or a 23 tooth difference and the chainrings are 24T and 52T, or a 28 tooth difference. Add the two and you get 23+28=51 teeth... now subtract the 45T capacity and you get 6 teeth over the recommended capacity for the SRAM X-7 rear derailleur.

Take a look at what happens at the rear derailleur as we go through the gears:
[click on the image to go to the animation]

The other *mystery* to rear derailleurs appears to be chain length sizing. This is specifically important to recumbents because most recumbents allow you to adjust the placement of the pedals via the boom. Changing the boom will affect the chain length, and therefore also affects the rear derailleur significantly. Maybe the next part of this post will make it clear why this is so.

You know those fine print rice paper pages that are often included with your new bike bling? They have microscopic words on them and little caricatures that kinda look like bike stuffs... and sometimes you can recognize certain words that look like "WARNING"... "CAUTION"... and "DO NOT!"... but you are to excited to ever bother to find out what they mean? Well, this is one of those explanations.

When sizing the chain on a recumbent bike you MUST understand the ramifications of doing this wrong. Errors here could mean catastrophic failure up to and including serious injury. You don't have to take my word for it as many others have posted similar warnings which often go unheeded. You've now been warned again... so let it be said... Follow the manufacturer's instructions! ....and you can always find them online so you can enlarge the fine type to a legible level. Here we will discuss the SRAM Rear Derailleur instructions as they are applicable to PhattKatt.

SRAM states that when sizing the chain for the rear derailleur, you must size the chain by wrapping the chain around the large chainring and the large cassette gear and bypassing the rear derailleur. You want the chain to be taught when doing this. Now add 2 links to the chain. 2 links means 1"... one outer and one inner link. This will provide sufficient deflection for routing the chain through the rear derailleur and still have the full chain wrap capacity as your rear derailleur was rated for. Failure to follow these requirements, and allowing for a situation where the chain would be to short for the large/large gear combination can and will cause drive system failure which can have catastrophic effects up to and including injury! It is way to easy to brain-fart that bad combination and the drive system is all to happy to try to oblige your control command into this self destructive behaviours. Just don't do it!

Below is a picture of how the derailleur looks on PhattKatt in the 3 different chainrings while on the largest cassette gear:

In short, always make sure your chain is long enough! When adjusting the boom for a friend, resist the temptation to lengthen the boom unless you are willing to dial out the top [largest] chainring form the front derailleur. This is easy to do with the limit screw on the front derailleur.

Also note that I have exceeded the manufacturer's recommended Chain Wrap Capacity with the gearing I've chosen to accept on PhattKatt. Doing this means you have some understanding and control over the way your trike is being used. I would NEVER recommend anyone do such modifications without careful consideration and understanding the ramifications of doing so. I personally feel that on PhattKatt, this is an acceptable risk.

If lowering your gearing is needed on your trike, consider replacing your crankset to remain within the manufacturer's derailleur's chain wrap capacity. A simple solution to lowering your gearing for climbing is to purchase a crankset with 22T/34T/44T chainrings. This will provide the appropriate chain wrap capacity with your 11-34 cassette.

Until next time, keep the rubber side down.

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