Thursday, August 16, 2018

24t on My 700

I now use a 52t/39t/24t chainring combo with a 36/11 cassette. Running this setup on a road crankset requires a little more thought and care then a traditional combination of gears. It's common for trikes to put on a 26t small chainring with a 50t large chaining. This allows the long derailleur cage to maintain good tension on the chain throughout the range of gears. But when you want to get the lowest gear possible from your 36/11 cassette by using a 24t chainring while still using a 52t large chainring for high speeds requires a little more effort. Of course you can use internally geared hubs and cranksets but I prefer the simplicity, weight and ease of maintenance of the traditional derailleur systems.

In regards to chain tension you'll find that going small/small (using the small chainring and small cog on the cassette) leaves you with a sloppy chain. Not recommended. To avoid this problem simply avoid shifting the chain to the last 4 smallest cogs on the cassette when using the 24t chainring. You'll find the gear ratios that you sacrifice (highlighted in yellow) while using the 24t are duplicated when shifting to the 39t chainring. Just a different combination of cassette cogs when using the 39t chainring (see chart).


Chain on 6th largest cog and the 24t chainring. Chain tension is still good.

Chain on the smallest 11t cog and the 24t chainring. Chain tension is very loose.


Setting up the 24t chainring on the crankset takes a little thought. Going from the 39t to the 24t is a big drop. First I always use a granny guard to keep the chain from bouncing off the chainring and ending up on the bottom bracket when shifting down while riding on rough roads. Also when shifting down you don't want the chain ending up jammed between the 39t and the small 24t chainrings. With the 24t there is more room for the chain to find it's way between these two chainrings. There is little room for error when adjusting the spacing between the 24t and 39t chainrings. I keep a 6 inch piece of chain on my work bench to check the spacing between the granny guard and the 24t and also between the 24t and the 39t chainrings before reinstalling the crankset.

The granny guard provides peace of mind that the chain will not bounce off when shifting.


Different angle looking at the spacing between chainrings.

On my Catrike 700 another issue comes up. The return tube is offset just enough that once the 24t chainring is installed the return chain slightly rubs on the edge of the ramps on the 39t FSA chainring. The rubbing is so slight it would probably not cause a problem but knowing that anything is doing what it's not suppose to do is enough of a reason for me to correct it. Luckily it was easily resolved, I ground down the top edge of the ramps without compromising shifting.


Offset of the chain tube. 

Edge of the ramp on the 39t chainring ground down just enough to keep the chain from rubbing.


Now climbing the Central Coast Wine Country should be a little easier while not sacrificing downhill speed.






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