Third chainring advice

[jorgemartin]

There's this pulley/wheel thingy. You can see it on this picture under the seat just in front of the rear shock. http://www.onbikex.de/images/bikTifID_25295_2000_2000.jpg

 

[RecordAceFromNew]

An interesting feature/issue/problem (depending on how you look at it) that will arise switching to a 53/39, is that suddenly you will find that you have lost 18 ratios - the reason being that ratios of adjacent gears for the hub and chainset will all be the same (136% or 73%), so up or down either when available will give you exactly the same gears.

With 20x1.25 tyres a 39T/34T/low will give you only 4.2mph at 90rpm at the pedals, some might feel it is better off walking...

 

[jorgemartin]

Given that the (medium cage) derailleur has a capacity of 37t, I could have a 53-46-39 crankset and I wouldn't loose 18 ratios.

 

[RecordAceFromNew]

By adding a 39T ring, instead of adding 27 (3x9) you would be adding 9 ratios, because 18 are duplicated. However I believe the actual loss of ratios by swapping rings is less because even the existing arrangement has many similar if not exactly the same ratios.

Going triple, apart from needing to get a triple chainset (or a tripleizer and longer bb which I don't think I would recommend), you probably need a matching bb, you might also find your existing front mech unsuitable for shifting a triple.

Are you anticipating the 36 (2x9x3-18) ratios insufficient?

 

[jorgemartin]

Okay, I get now how the ratio duplication works. My only concern is: will a 39t chainring improve the steep uphill business over a 46t? Probably so but I guess the only way to know is to do it. On the other hand, because the derailleur's capacity is 37t (53-39+34-11=37), I couldn't possibly use a smaller chainring, so 39 is my only choice.

 

[Angelfishsolo]

Okay, I get now how the ratio duplication works. My only concern is: will a 39t chainring improve the steep uphill business over a 46t? Probably so but I guess the only way to know is to do it. On the other hand, because the derailleur's capacity is 37t (53-39+34-11=37), I couldn't possibly use a smaller chainring, so 39 is my only choice.

Yes a 39T over a 46t will aid you hill climbing.

 

[jorgemartin]

I've checked SRAM's website and it says this about the X9 9 speed rear derailleur: "Aluminum long/medium-cage/composit short cage" ... So I guess that means that the X9 is a short cage after all and the teeth capacity will be reduced. This information doesn't appear in the 2010 technical manual document...

 

[RecordAceFromNew]

I've checked SRAM's website and it says this about the X9 9 speed rear derailleur: "Aluminum long/medium-cage/composit short cage" ... So I guess that means that the X9 is a short cage after all and the teeth capacity will be reduced. This information doesn't appear in the 2010 technical manual document...

I didn't even know they come as short cage... It is probably best to compare length (with the 10 sp specification here) and see if it is near 55mm.

 

[jorgemartin]

I measured the cage at 55 mm. I presume that the chainring capacity will be 30t. If I use a 39t-53t front with a 11-34t cassette (37t total), and I'm careful to use the smaller front chainring with the three or four biggest rear cogs, would that be okay?

 

[RecordAceFromNew]

Can't see it from your link above but you were probably talking about something like this?

 

[jorgemartin]

I was referring to the cage of the X9 rear derailleur... How would I use this Alfine chain tensioner? By the way, are there any recumbent specific features (such as an idler or the length of the chain, which is around 3.8 metres) that influence this whole cassette teeth capacity?

 

[RecordAceFromNew]

I was referring to the cage of the X9 rear derailleur... How would I use this Alfine chain tensioner? By the way, are there any recumbent specific features (such as an idler or the length of the chain, which is around 3.8 metres) that influence this whole cassette teeth capacity?

I must firstly declare that I know nothing about recumbents, but I believe tensioners are often needed/desired because of the exceptional length of the chain? If so I would have thought because of that pushing the rear mech's wrap capacity will exacerbate the problem, hence something like below is common/desirable?

 

[jorgemartin]

Hmm, I wonder if the idler on my bike, while it doesn't look like the one on the picture, could deal with this problem. According to Sheldon Brown:

An idler is a pulley or roller that does not produce any mechanical advantage, nor transmit power to a shaft. Idlers are used to lead a chain around a bend (as in short wheelbase recumbents), or to take up slack in a drive chain