New chain + old cassette = useless 1st gear

[RebornRider]

I've certainly heard and read about this, but today was my first experience. I cleaned the bike, cleaned the new chain, and installed and lubed the new chain. I thought the cassette had some life left. Wrong! The first climb resulted in fairly loud and violent skipped teeth in first gear. Rather than further abuse the new chain, I turned around and rode home.

I replaced the chain+cassette together about 1200 miles ago according to my Garmin ride logs. The chain failed the 0.75 chain checker with wiggle room, so it was well past its prime.

Chain is XX1, cassette is XG1150.

Lesson learned!

 

[Mikerb]

By first gear I assume you mean the biggest cog??

When you fit a new chain to a part worn cassette you need to reset the B Tension.....especially as far as the lowest gears a re concerned. The chain wrap is at its greatest on those larger cogs so I doubt the chain skip was due to a worn cassette.........that is far more likely to happen on the smallest cogs especially if you use them a lot.
A new chain has less side to side flex than a chain that is worn in, so if the gears were well indexed and the B tension setting caused no problem before the chain change it could well be that more chain gap (B Tension adjustment) was required to prevent the chain indexing too far into the inside of the cassette on the 2 largest cogs.

 

[RebornRider]

By first gear I assume you mean the biggest cog??

When you fit a new chain to a part worn cassette you need to reset the B Tension.

Yes, the largest cog. Blame my 55 years of being a car guy for me assuming that y'all say the largest cog is first gear.

I don't see why replacing a worn chain with a new chain would change B tension. Looks to me as though that setting is completely independent of the chain. Please explain.

BTW, I'm very fussy about shifting performance, so I keep after all the adjustments.

Edit to say that I just checked with the SRAM gauge and it looks spot on to me.

Also recorded a video of how the gap changes with suspension compression because I set my gap with nobody on the saddle. The gap opens a smidge when I bounce on the bike, but the change in gap is near zero between me off the bike and static compression with me on the bike.

 

[Mikerb]

Yes, the largest cog. Blame my 55 years of being a car guy for me assuming that y'all say the largest cog is first gear.

I don't see why replacing a worn chain with a new chain would change B tension. Looks to me as though that setting is completely independent of the chain. Please explain.

BTW, I'm very fussy about shifting performance, so I keep after all the adjustments.

Edit to say that I just checked with the SRAM gauge and it looks spot on to me.

Also recorded a video of how the gap changes with suspension compression because I set my gap with nobody on the saddle. The gap opens a smidge when I bounce on the bike, but the change in gap is near zero between me off the bike and static compression with me on the bike.

The chain gap setting does 2 things. Firstly it ensures the chain does not jam against the largest cog, and secondly it provides the right amount of movement of the chain by the guide wheel ( upper jockey) to change from one cog to another. The chain gap grows progressively as the derailleur moves down the cassette to the smallest cogs. A fully broken in chain starts to develop more sideways flex whereas a new chain is quite stiff in terms of sideways flex. So as a chain wears that sideways flex can get to the stage where the guide wheel struggles to change gear at the outboard end ( small cogs) of the cassette where the chain gap is greatest...............and that can be cured by closing the chain gap provided it still avoids jamming on the largest cog. The reverse of that is also true. A new chain with much less side to side flex does not need such a close chain gap and can ( as I suggested a bove) actually lead to the guide wheel pushing the chain fractionally too far when the chain gap is small...ie changing up to the largest cogs. So although the chain gap has a pre determined setting ( when measured at shock SAG) that is likely only totally applicable with a new cassette and chain.

ps you should see a difference in the chain gap when comparing rider plus bike SAG compared to bike weight SAG only. It is small if the shock is relatively short travel ( 140mm), comparatively bigger if the shock is longer travel ( e.g 170mm)

 

[RebornRider]

FWIW, the new cassette solved the problem. I didn't change the gap (B tension) cuz the SRAM gauge said it was correct as found.

Agree that gap affects shifting. I had no issue with shifting yesterday with the new chain + old sprocket.

As mentioned, I recorded a close-up video of the gap in first gear with me off the bike, sitting on the saddle, and finally bouncing on the pedals. As expected, it is smallest with no rider and grows with additional suspension travel. The difference between no rider and me sitting on the saddle was minuscule.

 

[Mikerb]

FWIW, the new cassette solved the problem. I didn't change the gap (B tension) cuz the SRAM gauge said it was correct as found.

Agree that gap affects shifting. I had no issue with shifting yesterday with the new chain + old sprocket.

As mentioned, I recorded a close-up video of the gap in first gear with me off the bike, sitting on the saddle, and finally bouncing on the pedals. As expected, it is smallest with no rider and grows with additional suspension travel. The difference between no rider and me sitting on the saddle was minuscule.

setting SAG is best done in the ready position, fully kitted out for riding, and measured a fter the shock has been cycled a few times ( gets rid of any initial stiction and warms it up.......as it would be when riding).

 

[Binhill1]

I've certainly heard and read about this, but today was my first experience. I cleaned the bike, cleaned the new chain, and installed and lubed the new chain. I thought the cassette had some life left. Wrong! The first climb resulted in fairly loud and violent skipped teeth in first gear. Rather than further abuse the new chain, I turned around and rode home.

I replaced the chain+cassette together about 1200 miles ago according to my Garmin ride logs. The chain failed the 0.75 chain checker with wiggle room, so it was well past its prime.

Chain is XX1, cassette is XG1150.

Lesson learned!

If you take it easy the chain will bed in to the old cassette . It will jump on smaller sprockets . Once apon a time you could reverse the sprocket and get more out of it alas no more.

 

[steve_sordy]

When that happens, I put the old chain back on and run with that until shifting becomes a pain. Then change the cassette and chain together. Then keep an eye out for the front ring to fail!

 

[Binhill1]

Mean it will jump on small rings on new chain till it bed's in then all is ok . Works for me .