Folding Bikes - NEED Help Please with my Rear Derailleur

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vmaniqui
07-02-09, 03:03 AM
hi all,

i am still in the process of upgrading my dahon boardwalk d7 and encountered some issues with my rear derailleur. honestly i need an expert advise. after experimenting with so many RDs and hangers i think i finally found the right RD and hanger that i improvised to fit my dahon. once i installed the cable here's what happened. on the first picture the gear is at the lowest and on the last picture the gear is at the highest. i had a shimano double chainring crankset. am i doing something wrong or i just need a longer chain. appreciate all the help. as you can see from the last picture, it looks like the RD is pulled all the way to the front. do i need to get a longer chain ? or something else.

thanks again.
vic

wim
07-02-09, 03:56 AM
If the chain does not fit on the biggest chainring and biggest sprocket, you need a bigger chain indeed. However, this might not be your only problem.
A derailleur also serves to take up the slack in your chain. You have a large sprocket in the back, and a double chainring. Your derailleur might not be able to take up all the slack, calculate the difference between the biggest and smallest combination of chainring+sprocket. example: 53+28 and 14+39 --> difference is 81 - 53 = 28. A short cage can take up less slack than a midcage or long cage derailleur. What do you have? You should be able to find the answer online too. You can also check if there is a gap of about minimum 1-2 centimeters between the upper pulley of the RD and the largest sprocket.
If I remember correctly, the RD and shifter on some dahons work the 'opposite way' = high-low/ low-high. Check that too, although this should not be your problem (yet).
vmaniqui
07-03-09, 03:07 AM
example: 53+28 and 14+39 --> difference is 81 - 53 = 28. A short cage can take up less slack than a midcage or long cage derailleur. What do you have?

i have a 52/39T Shimano 105 with Hollowtech II BB, my RD is a Shimano Alivio Long Cage, and my cogs are 11-30T. i re-checked my chain and it is the right length. up until 12:30am yesterday and today - i was working on it trying to figure out what the problem is. although it will shift to all gears normally. just the extreme pull of the RD when on 52T and 30T got me worried.

thanks,
vic