Chain too short = BLAM-O in big-big

[pdmtong]

FYI: sh$#^t can happen
Had an unusual JRA yesterday which has implications for folks running short cage RD's when maybe they should be using medium or long cage RDs

So I am doing a parking lot test ride on a used full-sus mtb I was thinking about buying and slowly pedaling <5mph. I shift into the front so it's going into the big-big and then BLAM-O, the rear hanger tears off and the RD is now completely twisted and lodged betwen the cassette and spokes. One spoke is bent, the RD is toast. Lucky I was barely pedalling so nothing worse. Appears the chain was sized too short for big-big without accounting for a slight stretch when this particular supension gets loaded.

Point here...always heard this could happen, now I know it can happen and if I was going faster on a road bike and mis-shifted into big-big with too short a chain I would think the RD twisting itself into the rear wheel would have caused a pretty abrupt crash.

BTW, I don't need a reminder on big-big no-no/chainline...this was a test and all I was looking for was proper indexing across all the combinations.

So there you are...happy wednesday!

[gdw]

Bummer. Was it your bike or did you trash someone elses rig?

[mgm777]

FYI: sh$#^t can happen
Had an unusual JRA yesterday which has implications for folks running short cage RD's when maybe they should be using medium or long cage RDs

So I am doing a parking lot test ride on a used full-sus mtb and slowly pedaling <5mph. I shift into the front so it's going into the big-big and then BLAM-O, the rear hanger tears off and the RD is now completely twisted and lodged betwen the cassette and spokes. One spoke is bent, the RD is toast. Lucky I was barely pedalling so nothing worse. Appears the chain was sized too short for big-big without accounting for a slight stretch when this particular supension gets loaded.

Point here...always heard this could happen, now I know it can happen and if I was going faster on a road bike and mis-shifted into big-big with too short a chain I would think the RD twisting itself into the rear wheel would have caused a pretty abrupt crash.

BTW, I don't need a reminder on big-big no-no/chainline...this was a test and all I was looking for was proper indexing across all the combinations.

So there you are...happy wednesday!


You're not alone. First ride after a LBS performed drivetrain clean and new chain, My riding buddy was JRA on his Madone (DA7800). While climbing a hill, he mistakenly shifted big-big. Ripped his rear der, the hanger, and carbon tube supporting the structure clean off.

Be careful out there.

[fierte_poser]

poop :crap:

[pdmtong]

fortunately wasn't my bike.
unfortunately was planing to buy the bike.

once it gets fixed seller (trustworthy guy) and I will reconnect. there wasn't anything underhand here...and I dont want thread drift into that tiring buyer/seller relationship obligation discussion. my only point in posting was that while we should never ride big-big, sometimes you might forget and if you do and the chain is too short...it wont be pretty.

[RPS]

FYI: sh$#^t can happen
Had an unusual JRA yesterday which has implications for folks running short cage RD's when maybe they should be using medium or long cage RDs
Given this choice (not that I recommend using the wrong derailleur) wouldn't it be safer to install the right length chain to handle the big-big and then try to remember NOT to shift to the smallest cogs while in the little ring?

Glad you were not hurt.

[gdw]

I'm didn't mean to imply that anything underhanded occurred. I feel sorry for the guy but unfortunately damage like that can happen when you set up a drivetrain so that it won't handle a big/big combo. It's better that things went wrong in the parking lot than out on the trail.

[pdmtong]

my apologies...i def wasn't trying to suggest an underhanded implication on your part...i added that for clarification - not as a comment to you.

[peanutgallery]

Always been an issue with a short cage on a triple set-up. Years and years ago, when I first started riding I picked up an Eddy B book. In there among the wacky old school training tips (like eat horse for dinner - go fast in the morning) were some tech tips. One that struck me was that Eddie B suggested keeping the chain as long as possible, particularly so on a time trial bike. His thinking was the weight and momentum of the chain spinning would be enough to allow the rider to push a slightly bigger gear or turn it over a little faster. I was naive, but I have used the idea ever since. Every chain gets the same treatment, measured in little to little with the chain barely grazing the rear der. cage. Works great on doubles, worked great on triples until someone invented a 34.

Glad to hear you and the seller are still on speaking terms

[GuyGadois]

Sheldon Brown speaks of chain length :hello:: http://sheldonbrown.com/derailer-adjustment.html#chain

also, Park Tools gives the same instructions, just a little more detail: http://www.parktool.com/repair/readhowto.asp?id=26

I used to measure the length of the chain installed through the RD. Not anymore!

[RPS]

I used to measure the length of the chain installed through the RD. Not anymore!Is there really that much difference when the rear derailleur has the right capacity? Both work just about the same IMO, although I prefer the longest chain possible so that I can change to a larger cassette without having to replace the chain.

In reading the links it seems these may be alternate methods for when derailleur capacity is not adequate/correct.

If the derailleur's capacity is not adequate (like when using a double rear derailleur on a triple, or when using an extra-small granny chainring) then the safest way to proceed is by big-big to ensure adequate chain length. If so, whether to thread through the derailleur or not seems more about convenience than anything else -- end point is basically the same.

On the other hand, if the derailleur is sized properly and you cut the chain based on using an 11-21 cassette by using the big-big and then decide to install a 12-27 you will need to add links or a new chain. That seems too limiting to me.

[Samster]

****!